Karst paleotopography on top of the Dengying Formation and petroleum geological significance in the Central Sichuan Basin,China

Abstract

Reconstructing karst palaeogeomorphology is a useful approach to identifying target areas for oil and gas exploration. This study used the elevation method to reconstruct the karst palaeogeomorphology at the top of the 4th member (Z₂dn⁴) of the Dengying Formation (Z₂dn) in the Central Sichuan Basin based on 2D and 3D seismic and drilling data. The bottom of the Longwangmiao Formation and the top of the 2nd member (Z₂dn²) of the Z₂dn were optimized as the upper and lower base levels, respectively. The reconstructed palaeogeomorphology was divided into five types – slopes, erosional peneplains, monadnocks, domes and depressions – according to their morphology, scope and gradient, allowing inference of the palaeohydrological conditions. Slopes were subdivided into gentle (<1.2°) and steep (≥ 1.2°; maximum gradient around 16°). Reservoirs on steep slopes and monadnocks are the most developed, owing to their considerable hydraulic head differences, strong palaeohydrodynamics, and intense erosion and incision. Steep slopes with more developed karst porosities are banded and immediately adjoin a regional hydrocarbon generation centre. Hydrocarbons generated in this centre tend to migrate laterally and become entrapped in porous slopes and their vicinities. Consequently, hydrocarbons move upward along steep slopes and, preferentially, slopes and monadnocks. Thus, the steep slope zone and its vicinity (including some monadnocks, gentle slopes and peneplains) are the most favourable areas for exploration of hydrocarbon reservoirs and should be the primary targets.

Keywords

Palaeogeomorphology eogenetic karstification Dengying Formation Central Sichuan Basin hydrocarbon accumulation

Introduction

Carbonates are widely distributed soluble rocks with the potential to form high-quality reservoirs under the influence of karstification (Agosta et al., 2010; Cross et al., 2021; Florea and Vacher, 2006; Liu et al., 2019). Carbonate petroleum reserves account for more than half of global petroleum reserves (Firme et al., 2021; Su et al., 2021; Zhao et al., 2014) and are closely related to palaeokarstification (Araújo et al., 2021; Liang and Hou, 2021; Liu et al., 2019). Some 20–30% of global oil and gas resources are related to disconformities (Burchette, 1996; Fritz et al., 1993). The Ediacaran Dengying Formation (Z₂dn) of the Sichuan Basin, China, represents a set of deposits marked by a regional disconformity at the top of the unit (Li et al., 2019). The Dengying deposits are affected by this disconformity and represent an important reservoir related to the development of a weathering crust (Luo et al., 2017; Zhou et al., 2020). The weathering interface at the top of the Dengying deposits implies disconformable contact with the good source rocks of the lower Cambrian Maidiping Formation (Є₁m) and Qiongzhusi Formation (Є₁q), thereby forming an excellent source–reservoir combination (Qu et al., 2020). In recent years, petroleum exploration in the Z₂dn of the Gaoshiti–Moxi area of the Central Sichuan paleo-uplift has made great breakthroughs (Liu et al., 2015; Xie et al., 2019). By the end of 2018, the total proven reserves of natural gas were 1 × 10¹² m³, and cumulative gas production reached 4.19 × 10¹⁰ m³ (Li et al., 2020), demonstrating the good potential of these Neoproterozoic carbonate rocks. Therefore, this area has become a new exploration focus for the Sichuan Basin and provides an important example of deep–ultradeep carbonate petroleum exploration.

The main karstification type forming vugs and caverns in the Z₂dn⁴ remains debated. There is a general consensus that the whole Sichuan Basin was uplifted by episode II of the Tongwan Movement and experienced long-term meteoric freshwater leaching and dissolution (Hao et al., 2017; Luo et al., 2015; Wei et al., 2016; Yang et al., 2016; Zhou et al., 2016). To comprehend the main types of karstification in the Z₂dn⁴, understanding the related eogenetic and epigenetic karst processes is necessary. Eogenetic karst refers to karstification in early diagenetic stage, which is characterized by high initial matrix porosity and permeability in the rock. Reported examples are mainly located in small isolated carbonate islands and exposed portions of attached carbonate platforms (Florea and Vacher, 2006, 2010). Epigenetic karst refers to late diagenetic karstification, which features much lower initial matrix porosity and permeability with stabilized minerals, and is generally located in hinterlands (Tang et al., 2013). Because of the presence of a widespread disconformity at the top of the Z₂dn⁴, many scientists suppose that eogenetic/epigenetic karstification has caused the development of reservoirs (Che et al., 2019; Yang et al., 2014a; Yang et al., 2014b). However, due to prominent differences in reservoir volume and commercial gas productivity between platform margins and interiors (Jin et al., 2017), epigenetic karstification is not applicable since deep burial before the epigenetic stage would have reduced these differences between all parts of the platform (Tan et al., 2015). The karst products of the Z₂dn⁴ in the Central Sichuan Basin differ distinctly from those of the Ordovician in the Tarim Basin. In other words, they are seldom related to the development of fractures and crevices but are closely related to lithofacies, especially microbial and granular rocks, and geographical position (Jin et al., 2017). On the basis of the karst features of the Z₂dn⁴, the eogenetic karst theory has been applied to interpret these karst phenomena (Hou et al., 2021; Jin et al., 2017, 2020; Tan et al., 2015) and a coupled pattern has been established. The eogenetic karst principle can interpret the differentiation of karst porosities in platform margins and interiors more clearly. A burial karstification pattern has been proposed to explain the difference in karst development between the platform margin and interior of the Z₂dn⁴ in the Northern Sichuan Basin (Li et al., 2019). It occurs some distance away from the Mianyang-Changning intracratonic sag that has thicker source rocks, which reduces the dissolution capability of organic acids due to the high migration distance. Hence, both eogenetic and burial karstification seem reasonable. However, in the burial stage, the fluids produced in the closed diagenetic environment are much less abundant than those produced in the open atmospheric environment, and a net increase in porosity attributed to reservoirs is more likely to stem from the eogenetic stage rather than the burial stage. Therefore, it has been suggested that burial karstification is simply a kind of reworking and adjustment of pre-existing reservoirs (Tang et al., 2013; Zhou et al., 2020).

The key factor controlling the development and distribution of karst reservoirs is karst palaeogeomorphology. Thus, the distributions of such reservoirs are mostly predicted based on the reconstruction of palaeogeomorphology (Li et al., 2013). Karst topography is a result of the interaction between multiple karstification events and soluble rocks under complex diagenetic conditions (Albertão et al., 2011; James and Choquette, 1984). The consequent morphology and distribution are important characteristics of the degree of karstification development (Yan et al., 2016). Typical karst topography is essentially correlated with surface and subsurface dissolution and subsequent solution-induced collapse; hence, topography and hydrology are closely correlated (Ford and Williams, 2007; Waele et al., 2009) and the morphological relief depends largely on variations in hydraulic potential. Karstification of the palaeogeomorphology controls the development and position of reservoirs and determines the formation and distribution of secondary carbonate reservoirs (Hou et al., 2020; Loucks et al., 2004; Ronchi et al., 2010; Sayago et al., 2012; Zhu et al., 2019). Recent exploration and research show that the main factors controlling karst reservoirs in the Gaoshiti–Moxi area of the Central Sichuan Basin are sedimentary facies and karstification, with the latter being key to the formation of high-quality reservoirs (Duan et al., 2019; Shan et al., 2017; Wen et al., 2017). Hence, karst geomorphology was reconstructed and classified using the topographical framework of paleokarst disconformities. This can be used to analyse the relationship between palaeogeomorphology and reservoirs, and associate specific palaeogeomorphological units with potential hydrocarbon reservoirs, thereby effectively predicting the distribution of karst reservoirs in the Z₂dn⁴ within the Central Sichuan Basin.

Geological setting and stratigraphy

The Sichuan Basin is located in Southwest China and covers an area of approximately 2.6 × 10⁵ km². It is a typical large petroliferous basin that covers the Precambrian Yangtze Craton (Liu et al., 2021). More than 20 hydrocarbon-bearing strata are contained in the entire basin, among which the upper Sinian (Ediacaran) Z₂dn contains the oldest set of marine sedimentary carbonate reservoirs (Wei et al., 2005, 2016). The Central Sichuan Basin is located in the eastern part of the Leshan–Longnvsi palaeo-uplift (Figure 1) and covers an area of approximately 3.54 × 10⁴ km². This palaeo-uplift experienced a stage of initial uplift in the middle and late Sinian, a stage of denudation at the end of the Silurian, and a tectonic adjustment stage in the Hercynian, IndoSinian, Yanshanian and Himalayan periods that shaped the present structural features (Jiang et al., 2014; Li et al., 2013; Xu et al., 2012). The Leshan–Longnvsi palaeo-uplift is characterized by early formation, a wide range of effects and long duration (Luo et al., 2015; Yang et al., 2016; Zhou et al., 2016). Its long-term, multicycle, pulsating, complex evolution has exerted a significant impact on hydrocarbon accumulation in the Sinian Z₂dn (Du et al., 2014; Jiang et al., 2014; Yang et al., 2016).

Figure 1.

Tectonics of the Sichuan Basin and stratigraphy of the Z₂dn⁴. Numbers within diamonds represent the locations of (1) a high and steep structural belt in the Eastern Sichuan Basin; (2) a less steep structural belt in the Southern Sichuan Basin; (3) a less steep structural belt in the Southwest Sichuan Basin; (4) a high and steep structural belt in the Western Sichuan Basin; (5) a low and gentle structural belt in the Northern Sichuan Basin; and (6) a gentle structural belt in the Central Sichuan Basin.

The Z₂dn⁴ of the Central Sichuan Basin constitutes the uppermost portion of the Z₂dn and is predominately composed of carbonate platform microbial and micritic dolomites. The member has a thickness of 200–380 m and is unconformable with the overlying Є₁m or Є₁q and conformable with the underlying third member of the Z₂dn (i.e. Z₂dn³; Figure 1). At the end of the Z₂dn's deposition, affected by episode II of the Tongwan Movement, the carbonate platform was exposed above the sea level and eroded and leached by atmospheric freshwater, forming a widespread disconformity between the Sinian and Cambrian strata (Yang et al., 2014a), as well as a set of high-quality karst weathering crust reservoirs of mound-shoal complexes on top of the Z₂dn⁴ (Wang et al., 2014; Wei et al., 2016). Development of the karst system was largely influenced by the palaeoclimate; that is, recharge by meteoric freshwater. Warm temperatures contributed to the carbonate dissolution process because the rate constants of carbonate mineral dissolution increase with temperature. This produces more CO₂ for acid fluid formation due to the oxidation of more organic materials (Moore and Wade, 2013). Tropical humid and warm environments supply abundant and frequent precipitation that recharges karst fluids, which facilitates the generation of karstification and karst products. Metastable carbonate minerals are also rapidly stabilized and transformed into stable minerals under such climates (Morse and Mackenzie, 1990). In the Sinian, with the break-up of the Rodinia supercontinent, the South China Block dispersed as a small block around 30° N and South China (including the Sichuan Basin) was surrounded by a tropical ocean (Hoffman and Li, 2009). The palaeoclimate was warm and wet, providing enough meteoric freshwater for later karstification after tectonic uplift during Tongwan episode II. On account of the wide range of exposure of the carbonate platform in the Yangtze Block, massive amounts of organic material from the seafloor oxidized to produce large amounts of CO₂. This enhanced the partial pressure of CO₂ in the atmosphere and accelerated chemical weathering of crustal rocks in the Yangtze Block. Thus, considerable terrigenous inputs generated from extensive karstification shed into the oceans via meteoric freshwater and negative excursion of δ¹³C evidently occurred across the Sinian and Cambrian boundary (Li et al., 2017). The increased CO₂ partial pressure would have also elevated atmospheric temperatures, further facilitating chemical weathering (Beaugrand et al., 2015; Jaraula et al., 2013; Joachimski et al., 2012) due to greater precipitation and freshwater acidity. Karstification of the Z₂dn⁴ in the upper Yangtze Block would also have intensified due to the favourable palaeoclimatic conditions.

Materials and methods

Research on karst palaeogeomorphology in relation to petroleum geology has primarily focused on its control of hydrocarbon accumulation for the prediction of oil and gas accumulation zones to guide exploration (Zhao et al., 2017). Currently, commonly used methods for restoring karst palaeogeomorphology include the residual thickness and impression methods (Jin et al., 2017; Liu et al., 2015; Li et al., 2013; Wang et al., 2014).

In the past decade, there have been several reconstructions of the karst palaeogeomorphology at the top of the Sinian strata in the Gaoshiti–Moxi area of the Central Sichuan Basin. The use of residual thickness and impression methods and their combination has achieved some results (Jin et al., 2017; Li et al., 2013; Liu et al., 2015; Wang et al., 2014). Nonetheless, the accuracy of the reconstructed palaeotopographies is greatly affected by the disadvantages of the impression and residual thickness methods (Yan et al., 2016). Interpretations of the reconstructed karst palaeogeomorphology at the top of the Sinian in Central Sichuan basin, by means of residual- thickness method, come to be inconsistent with the gas-testing productivity (Jin et al., 2017). On the seismic section of residual-thickness method, crossing the Mianyang-changning intracratonic sag and the adjacent carbonate platform, evident onlaps that indicate the slope-break belt occurred mistakenly in the platform interior, demonstrating disagreement of the residual-thickness paleogeomorphology with objective reality and inapplicability of the method for this interface (Jin et al., 2017). In addition, division of the four members of the Dengying Formation can be carried out efficiently only in the range of the Sichuan Basin as lithological differences; for the periphery of the basin, character of the four-member division for the Dengying Formation is no longer practicable since sedimentary variations (Li et al., 2018). Thus, it is not feasible for the study area to implement the residual-thickness method for paleogeomorphological reconstruction simply because of basin-scope geomorphological inconsistency brought about by usage of thickness of the Z₂dn⁴. The key for impression method is the selection of the upper base level, different choices of which will definitely lead to distinct results. Serving the bottom boundary of the Longwangmiao Formation (Є₁l) as the upper base level, the reconstructed paleogeomorphological units in the Gaoshiti-Moxi area was divided into highland (or tableland) and gentle slopes dipping to the northwest, revealing the main westward discharge direction of runoffs (Li et al., 2018; Liu et al., 2020; Zhu et al., 2018). In contrast, the karst landform restored in the same region, making use of the top boundary of the Є₁l as the upper base level, was interpreted with analogous mode from Marche, Italy as tablelands and transitional slope (Jin et al., 2017). Regarding the flooding surface inside the Canglangpu Formation (Є₁c) as the upper base level, employing the impression method, result of the reconstructed paleogeomorphology of the Gaoshiti-Moxi area manifested a larger range of tableland whereas the gentle slope area was much limited (Yan et al., 2020).

Using drilling data from more than 40 wells and 2D and 3D seismic data, the elevation/double boundary method (Yan et al., 2016; Zhao et al., 2018) (hereafter referred to as the elevation method) was employed to reconstruct the karst palaeogeomorphology at the top of the Sinian strata in the Central Sichuan Basin. A number of achievements have been obtained by using this method to reconstruct the karst palaeogeomorphology of the middle Ordovician Majiagou Formation in the Ordos Basin (Yan et al., 2016; Zhao et al., 2018).

The elevation method is similar to the impression method in principle. The key is to select the superstrata that fill and level the karst weathering crust surface to characterize the highs and lows of the palaeogeomorphology. The major difference between these methods is that, in the elevation method, the moldic thickness of the original superstrata is converted into elevation data according to a lower base level. After levelling off the upper base level, the lower base level that is as close as possible to the disconformity should also be selected. The selected lower base level should represent the closest structural surface. A reference level with an elevation of 0 m is set from the lowest point of this lower base level. Compared with the impression method, the elevation method can quantitatively characterize the topographic relief of the disconformity as it incorporates the thickness of the weathered stratum itself. In contrast, the impression method can only depict the topographic relief (high or low) based on the moldic thickness of overlying strata, and the relief quantities are totally controlled by the selection of the upper base level. The specific steps of this method are as follows:

Selection of the upper base level is the key to palaeogeomorphological reconstruction. Its main principles are (Yan et al., 2016): (1) It should fill and level the underlying weathering disconformity; (2) the selected upper base level must be isochronous and roughly correspond with a transgressive surface or maximum flooding surface; and (3) the distance between the upper base level and weathering disconformity (i.e. the moldic thickness) should be minimized. The closer the distance, the more detailed the description of the geomorphological features of the weathering crust; furthermore, the adverse effect of later tectonics on the accuracy of the reconstruction can be minimized. (4) The selected upper base level can be identified and precisely traced in seismic profiles. The selection of a lower base level should meet the requirements of (3) and (4) as much as possible.

After flattening the upper base level, the lowest structural position of the lower base level is found (namely, the point of maximum thickness between the upper and lower base levels); then, a flat surface can be extended (i.e. a horizontal reference plane with an elevation of 0 m from this point).

The elevations (H_E) of corresponding points between the horizontal reference plane and weathering disconformity can be calculated using the maximum thickness between upper and lower base levels (H_TM) minus the thickness between upper base level and disconformity (H_M), then plotted. An equation that summarizes the operation is given as follows:

H_{E} = H_{TM} - H_{M}

where H_E is the elevation, H_TM is the maximum thickness between the upper and lower base levels, and H_M is the moldic thickness. The elevation calculation model is displayed in Figure 2.

Then, time-domain data of the upper and lower base levels and the disconformity are acquired from interpretation of the three horizons using drilling data and 2D and 3D seismic data. Corresponding depth-domain data are obtained through time-depth conversion according to calculated interval velocities. The thickness from the upper base level to the weathering surface (moldic thickness), the thickness from the weathering surface to the lower base level (residual thickness) and the total thickness between the upper and lower base levels are obtained by subtracting the depths of the three interfaces. Subsequently, the maximum thickness is subtracted from the moldic thickness and total thickness within the research area, and the palaeogeomorphological elevations are obtained.

Figure 2.

Principle of palaeogeomorphological reconstruction via the elevation method (modified from Yan et al., 2016; Zhao et al., 2018). The moldic thickness (H_M) is the thickness of the superstrata filling the disconformity, where the upper base level represents a horizontal plane at the top of the superstrata. The lower base level represents the closest structural surface to the disconformity, and the horizontal reference plane is a horizontal plane extending from the lowest structural point of the lower base level. Thus, the elevation (H_E) represents the thickness between the disconformity and lower base level, while the maximum thickness (H_TM) represents the thickness between the upper base level and horizontal reference plane. The palaeogeomorphology can thus be characterized by the H_E data of the disconformity (i.e. H_TM − H_M).

Results

Selection of base levels

With regard to the reconstruction of the karst palaeogeomorphology at the top of the Z₂dn⁴ in the Central Sichuan Basin, two base levels were selected: the upper base level, which was the bottom boundary of the Є₁l, and the lower base level, being the top of the Z₂dn² (Figure 3). The reasons for this selection are as follows.

Figure 3.

Selection of the upper and lower base levels (modified from Liu et al., 2015). The interface between the Є₁l and the Є₁c was selected as the upper base level, and the interface between the 3rd and 2nd members of Z₂dn was selected as the lower base level. The numbers in this figure show the drilling depths of each horizon of each well (in metres). The ‘Maximum thickness point’ represents the lowest structural points of the Z₂dn³ and Z₂dn⁴ in the section, respectively. For the section location, see Figure 1.

The lower Cambrian successions of the Є₁m, Є₁q and Є₁c stacked on top of the Z₂dn in the Sichuan Basin represent a conformable group formed by transgression–regression cycles. They largely fill up and level the palaeogeomorphology at the top of the Z₂dn. The variation in the thickness of the Є₁l is relatively low horizontally, indicating that the Sichuan Basin's tectonic activity had stabilized when the Є₁l was deposited (Yang et al., 2016). The bottom boundary of the Є₁l has a transitional lithologic interface of clastic rocks on top of the Є₁c with carbonates at the bottom of the Є₁l. Lithofacies and logging curves are categorized by abrupt variations between the Є₁c and Є₁l. A bed of purplish-red sandy mudstone (known as the lower red bed) is stably distributed on top of the Є₁c regionally, exhibiting relatively strong reflection in seismic sections (Figure 4). Consequently, the bottom boundary of the Є₁l can be distinguished and traced. Together with 2D and 3D seismic data from the Central Sichuan Basin, the bottom boundary of the Є₁l is optimized as the upper base level for reconstruction of the karst palaeogeomorphology at the top of the Z₂dn.

Figure 4.

Seismic reflection profile of the upper and lower base levels. The bottom boundary of the Є₁l and top boundary of the Z₂dn² are the upper and lower base levels, respectively, due to their approach to the top boundary of the Z₂dn (the disconformity) and comparatively strong and more easily traceable reflections.

The lower base level was selected using the same principle used for the upper base level. The interface between the bottom of the Z₂dn³ and top of the Z₂dn² represents broadly distributed across the entire basin. The top of the Z₂dn² is a zonal disconformity that is isochronous. Moreover, the Z₂dn³ is one of the marker layers in the region and chiefly consists of fine-grained clastic rocks that notably contrast lithologically and electrically with the dolomites from the superjacent Z₂dn⁴ and subjacent Z₂dn². The bottom boundary is a weathering surface of episode I of the Tongwan Movement, which is traceable by its relatively obvious reflection in the seismic sections (Figure 4). For this reason, it is used as the lower base level.

Classification of the palaeogeomorphological unit

The karst palaeogeomorphology at the top of the Sinian (Figure 5) was plotted on the basis of the difference between the maximum and moldic thicknesses of the Central Sichuan Basin. The palaeogeomorphology of the study area is roughly composed of two parts – bottom land and high land – which are separated by a slope. The high land is the main body of the Z₂dn⁴ platform.

Figure 5.

Palaeogeomorphology of the disconformity at the top of the Z₂dn in the Central Sichuan Basin. The maximum elevation of this basin approaches 1500 m in the central-eastern part, while the lowest part lies in the northwestern corner of the map. The palaeogeomorphology exhibits lowland features in the western and southern parts of the basin. The middle and southeastern parts are at the highest elevation, while the northern part is stepped with a progressively decreasing elevation northward.

Note that there is a steep slope separating the bottom land and high land in the west. The bottom land is a trough called the Mianyang-Changning intracratonic sag, which is the result of various geological processes, such as rifting and erosion (Li et al., 2015; Liu et al., 2016).

Following the reconstruction of the palaeogeomorphology at the top of the Z₂dn⁴ in the Central Sichuan Basin, a slope map was plotted in the light of a digital elevation model (DEM; Figure 6) to characterize the gradient distribution, which contributed to the subsequent geomorphic classification. The maximum slope in this basin lies in the west and reaches almost 16°, representing a steep slope, and is represented by a thick, bright band along the line between wells g19-gk1-m22 (Figure 6). The overall terrain of the dark-blue area is flat with a rather low slope and is separated into pieces by green bands, exhibiting separation of plains by slopes. Green rings of varying size indicate closed circular side slopes (including monadnock and depression side slopes). This variety of traits demonstrates the diversity of palaeogeomorphological units. The karst palaeogeomorphology at the top of the Sinian in the Central Sichuan Basin can be divided into five units: erosional peneplains, karst slopes, monadnocks, domes, and depressions, among which the slopes can be further subdivided into steep and gentle (Table 1). It is clear that gentle slopes occupy most of the area and the peneplains and domes are bounded by gentle slopes, while monadnocks and depressions (except the large one in the south) are mostly bounded by peneplains (Figure 7), which may imply geomorphogenetic relations among the palaeogeomorphological units. Multi-stage peneplains and their bounding gentle slopes present stepped features (Figure 8), indicating the pulsing descent of an erosional basis. The slope rimming the carbonate platform in the west is visually highest in gradient and, thus, is categorised as steep. According to the statistics of the slope gradients, we found that an inclination of 1.2° is suitable for dividing steep and gentle slopes. The slopes (blue-green – green areas) inside the platform are gentle, being <1.2° in gradient.

Figure 6.

Gradient map of the palaeogeomorphology of the disconformity at the top of the Z₂dn⁴. The maximum gradient at the top of the Sinian in the Central Sichuan Basin is approximately 16°. The warmer colours indicate slopes, including steep, gentle and side slopes. The flat regions contain peneplains, depression bottoms and the tops of monadnocks and domes (dark blue).

Figure 7.

Classification of palaeogeomorphological units associated with the top of the Z₂dn⁴. The slopes (particularly gentle slopes) occupy most of the area of the Central Sichuan Basin. Steep slope rims occur around the Z₂dn⁴ in the west, while gentle slopes prevail in the largest part bounding the flat peneplain. The principal part of the depression is located in the south and the rest of the small depressions are scattered evenly within the peneplain. The monadnocks and domes are surrounded by peneplains and gentle slopes. There are fewer of them than there are depressions, and they are scattered and encircled by peneplains and gentle slopes.

Figure 8.

Schematic 3D model of the palaeogeomorphology at the top of the Z₂dn.

Table 1.

Classification of palaeogeomorphological units of the top of the Z₂dn in the Central Sichuan Basin.

Palaeogeomorphological unit	Classification criteria		Geomorphogenesis
Erosional peneplain (1100 < He < 1380 m)	Even topography eroded close to the erosional basis, gradient <0.2°		Karstification
Monadnock (1100 < He < 1380 m)	Residual mound/hump surrounded by peneplains and gentle slopes, bottom diameter <20 km, bottom diameter/height ≤ 1, with a closed side slope		Karstification and erosion
Dome (1350 < He < 1500 m)	Large-scale residual hills analogous to monadnock, bottom diameter > 20 km, with a closed side slope		Karstification and erosion
Karst Slope (650 < He < 1450 m)	Steep slope	Transitional zone between the main discharging area and the platform, gradient ≥ 1.2°, not closed	Karstification and erosion
Karst Slope (650 < He < 1450 m)	Gentle slope	Transitional zones between peneplains, 0.2° < gradient < 1.2°, not closed	Karstification and erosion
Depression (1120 < He < 1380 m)	Local discharging lowlands, with relatively flat bottoms and side slopes		Karstification and precipitation

Discussion

Factors determining reservoirs

Control of lithofacies distribution

Different lithofacies distributions gave rise to initial differences in porosity in the reservoirs of the Z₂dn⁴. High-quality reservoirs (> 5% porosity) are more prone to develop in microbial rocks, which form mounds and shoals at platform margins. Despite continuous porosity losses in the burial stage due to compaction, rigid frameworks of mound-shoal complexes preserved the porosity more effectively than inter-mound deposits owing to better capability of anti-compaction (Figure 9). Thus, microbial rocks represent preferential conduits for karst fluids after the shallow burial stage. Subsequent internal dissolution of mound-shoal complexes further increased the porosity. Therefore, lithofacies, particularly microbial rocks in mound-shoal complexes, tend to form superior reservoir (> 5% porosity) formation.

Figure 9.

Histogram of the porosity of different sedimentary subfacies in the Central Sichuan Basin.

The distribution and development of mound-shoal complexes are also vital and related to potential eogenetic karst and reservoir formation. Mound-shoal complexes rim the platform margin, while some patch mounds and shoals are scattered across the inner platform. The complexes are obviously distributed wider in the north than in the middle and south (Figure 10(a)). The distribution density of vugs and the proportion of large vugs in the platform margin mound-shoal complexes evidently outnumber those of the inner platform (Figures 10(b) and (c)). At the platform margin, microbial and granular dolomites are the main reservoir-forming bedrocks. Dolomitic thrombolites, dolomitic stromatolites and dolarenites are the most favourable lithofacies (Figure 11), indicating strong control of the development and distribution of reservoir space by sedimentary and lithological facies. Microcrystalline dolomite also has a relatively high porosity (Figure 11).

Figure 10.

(a) Sedimentary facies distribution (modified from Xu et al., 2021). (b) Quantity of vugs per metre and (c) proportion of different-diameter vugs in single wells at the platform margin and inner platform (data cited from Tian et al., 2020).

Figure 11.

Porosity of different lithofacies. Grey diamonds in (a) indicate porosity outliers and green triangles represent the mean porosity of each lithofacies.

Control of palaeogeomorphology and karstification

The control of reservoirs by karst palaeogeomorphology is mostly embodied in dissolution mechanisms and patterns of different palaeogeomorphology units, which depend on differences in palaeohydrological conditions and materials. These eventually lead to differences in karstification intensity, giving rise to distinct types and developments of diverse palaeogeomorphology reservoirs.

Different palaeogeomorphological units possess diverse karst hydrodynamic conditions (Han et al., 2019). The recharge, runoff and discharge conditions of karst hydrodynamic systems also differ (White, 1988, 2002). Dissimilarities in diagenetic material affect the way karst water moves and, subsequently, the intensity and characteristics of karstification (Jin et al., 2020).

Considerable differences occur in hydraulic head, flow velocity, flow potential energy and flow occurrence, resulting in more intense dissolution and erosion on steep karst slopes that produce honeycomb-like vugs (Figure 12(a)), large crevices (Figures 12(b) and 13(a)) and laminoid vugs (Figures 12(c) and 13(b)), and solution collapse in dolomite breccias. Dissolution is so intense in steep slopes that the first-stage marine fibrous cements rarely preserved well. Framework voids of microbial rocks and other dissolution vugs are cemented by dolomites during the shallow burial stage and filled by solid bitumen residue in the subsequent hydrocarbon charging progress (Figure 14(a) and (b)). Also, there are ‘clean’ dissolution pores without bitumen (Figure 14(c)). Gentle karst slopes acting as transitions between erosional peneplains are formed by periodic base-level descent induced by enhanced stream incision on antecedent erosional peneplains. Karstification and erosion in gentle slopes are not as intense as in steep slopes, probably due to lower flow rates, weaker hydraulic potential and longer water retention times. However, the karst products do resemble those in steep slopes. Solution fractures (Figure 12(d)), crevices (Figures 12(e) and 13(c)) and small solution vugs (Figures 12(f) and 13(d)) are typical features in gentle slopes. Solution fractures are mostly filled by later-burial-stage dolomites, silica and solid bitumen (Figure 14(d)). Crevices are usually filled by mud, dolomites and vadose silts (Figure 14(e)). Although the overall gradients of gentle slopes are small, sophisticated solution pore-fracture systems can still form. Solution pores are more developed in gentle slopes than in peneplains and depressions. Marine cements in gentle slopes are rare and solid bitumen fillings are common (Figure 14(f)).

Figure 12.

(a) Honeycomb-like vugs (from a steep karst slope; Well M109; elevation 5110.17 m). (b) Crevices, filled by broken bedrock, dolomite and argillite (steep karst slope; Well GK1; 5028.35 m). (c) Dissolution vugs, densely packed horizontally (steep karst slope; Well GK1; 5150.58 m). (d) Solution fractures and related dissolution-expanding vugs (gentle karst slope; Well G1; 4983.75 m). (e) Crevice, filled by mud, bitumen and other clay minerals (gentle karst slope; Well G18; 5140.58–5140.77 m). (f) Millimetric dissolution vugs, packed along laminae (gentle karst slope; Well G18; 5134.2 m). (g) Mottled karst feature, formed by infiltration and dissolution of eogenetic karst fluid and impregnation of bitumen in matrix porosities. The black mottles contrast with the bedrock distinctly (gentle karst slope; Well AP1; 5062.65 m). (h) Honeycomb-like dissolution vugs (peneplain; Well M9; 5034.66–5034.83 m). (i) Millimetric dissolution vugs, packed in laminae (peneplain; Well G2; 5012.8–5012.86 m). (j) Dissolution fractures (depression; Well M17; 5076.5–5076.62 m). (k) Millimetric dissolution vugs, packed along microbial laminae (depression; Well M8; 5114.54–5114.64 m).

Figure 13.

Fullbore Formation MicroImager (FMI) image features of karstification in different palaeogeomorphological units of the Z₂dn⁴. (a) Crevice, presenting as a nearly symmetrical dark stripe (Well G7). (b) Dense laminoid vugs, packed along the layers, presenting as dark patches and ovals (Well M22). (c) Intermittent dark stripes, indicating small crevices (Well G1). (d) Honeycomb-like vugs, presenting as scattered dark spots (Well G1). (e) Sparse vugs, presenting as dark spots and fine lines (Well M12). (f) Very dense honeycomb-like vugs, presenting as irregular dark clots and patches. Note that the red clots stand for interconnected vugs and the green clots stand for isolated vugs (Well J1). (g) Metric-sized cave, presenting as the dark shaded interval in the image. Note that the light shaded area in the cave indicates breccias with high resistivity (Well G8). (h) Subvertical dissolution fractures, presenting as high-angle bent symmetric dark stripes (Well GT2). The dark shaded areas in FMI images indicate low electrical resistivity, whereas light shaded areas indicate high resistivity (denoting bedrock). The numbers denote well depths.

Figure 14.

Microscopic karst features in different palaeogeomorphological units. (a) Dolomitic stromatolite, where framework voids are half-filled by bladed dolomite and solid bitumen; marine cements are absent (steep slope; Well GK1; 5149.58 m; plane-polarized light). (b) Microbial micritic dolomite, dissolution pores half-filled by solid bitumen and a few bladed dolomites (steep slope; Well GK1; 5145.1 m; plane-polarized light). (c) Dolarenite with dissolution pores rimmed by a few cement residuals (steep slope; Well M109; 5107.43 m; plane-polarized light). (d) Microbial micritic dolostone; bedrock separated by solution fractures presenting the peculiarity of pseudo-breccia; solution fractures are filled by micro-to-fine-crystalline dolomite, silica and solid bitumen (gentle slope; Well G1; 4987 m; cross-polarized light). (e) Micro-to-fine-crystalline dolostone; the crevice is filled by vadose silts (gentle slope; Well G1; 4966.06 m; cross-polarized light). (f) Micritic to micro-crystalline dolostone; solution pores are interconnected by extremely fine throats and are half-filled by certain residual dolomite cements and solid bitumen (gentle slope; Well G18; 5134.64 m; plane-polarized light). (g) Dolomitic stromatolite; laminae-parallel dissolution vugs are filled by silica (peneplain; Well AP1; 5062 m; cross-polarized light). (h) Coarse crystalline dolostone; bedding-parallel dissolution pores are filled by fine-crystalline dolomite and solid bitumen (peneplain; well AP1; 5053 m; plane-polarized light). (i) Dolomitic stromatolite; developed laminae-parallel dissolution pores are filled by fine-crystalline dolomite and solid bitumen, while a few isolated pores remain unfilled (peneplain; Well G2; 5013.99 m; plane-polarized light). (j) Granular boundstone; the crevice is filled by one darker and one lighter layer of vadose silts (depression; Well M17; 5067 m; plane-polarized light). (k) Micritic-to-micro-crystalline dolostone; the void is filled by the middle-lower vadose silts and the upper fine-to-meso-crystalline dolomites, presenting as geopetal fabric (depression; Well M17; 5077.4 m; plane-polarized light). (l) Coarse crystalline dolostone; dolomites are replaced by silica and the irregular dissolution pores are filled by silica and solid bitumen (depression; Well M8; 5156.13 m; cross-polarized light).

In the erosion cycle, land uplifted above sea level is eventually eroded to close to the erosional datum plane, and peneplain morphology progressively appears and approaches sea level during this process (Ford and Williams, 2007). The erosional peneplain presents as flat terrain with gradients <0.2° bounded by a series of slopes. A stepped geomorphology consisting of peneplains and gentle slopes indicates a pulsing and descending process of multistage erosion. A lack of hydraulic potential associated with flat terrain and long distances from discharging areas lead to weak incision, making peneplains major runoff areas. Karst fluids generally move horizontally although some move downward and percolate through structural fractures or the matrix pores of porous dolomites locally, forming a typical mottled dissolution system (Figure 12(g)), relatively sparse vugs (Figure 12(h)) and millimetric vugs arranged horizontally (Figures 12(i) and 13(e)). The solution fractures in peneplains are less developed. First-stage marine cements are almost absent and many of the open and interconnected porosities are frequently hatched by silica (Figure 14(g)) and solid bitumen (Figures 14(h) and (j)), such that the effective porosity is reduced.

Monadnocks are residual mound landforms that have not been eroded to the erosional datum plane in a peneplain. They have a side slope that is generally <20° and a shape generated by stream incision. The height difference between the top and the base does not exceed 200 m. The side slopes of monadnocks are comparable to slopes. They are repeatedly eroded by water flow and possess the characteristics of a large (centimetric-decimetric) karst vuggy system (Figure 13(f)) and metric-sized cave (Figure 13(g)). Although there are no drilling data, karst characteristics and dome products can be inferred from their analogues, scilicet karst gentle slopes and monadnocks, because domes, monadnocks and gentle slopes are all characterized by slopes (Table 1). Yet, domes are located almost entirely in inner platforms, the lithofacies of which differ from the monadnocks at platform margins. Accordingly, the karst products of domes should be analogous to those of gentle karst slopes situated at inner platforms.

A depression is a local or regional discharging area receiving runoff and groundwater from the surrounding elevated area. It has side slopes that are commonly <0.5° and are a type of sunken terrain, in contrast to monadnocks. Erosion and dissolution of karst water passing through the erosional peneplains and slopes make the karst water itself reach saturation and then slow down to gradually gather and precipitate in depressions. Dissolution fractures and millimetric pores can be found in the cores (Figure 12(j)). Somewhat millimetric dissolution vugs (Figure 12(k)) can form in microbial laminae, dolomitic stromatolites and clots under a greater gradient, while karstification in rocks without microbial components is not very developed. Depressions are dominated by carbonate and even siliceous cementation and precipitation, with fewer and less developed pores (Figure 13(h)) and a high pore-filling degree. Vadose silts are preserved well (Figure 14(j)) in depressions due to their low-lying terrain. Certain pores are filled by vadose silts and dolomite cements form geopetal fabrics (Figure 14(k)). Developed dissolution pores can be found in some favourable lithofacies of depression side slopes but are basically filled by dolomite, silica and solid bitumen (Figure 14(l)).

According to logging data and based on the principles of karst hydrogeology, Figure 14 was plotted to show the hydrological and hydraulic conditions of two cross-sections of Central Sichuan Basin (Figure 15). Discharge of groundwater is mainly diffused towards the north and west by gravity, which is consistent with the major dip direction of the slopes according to the palaeogeomorphology. Vadose and upper phreatic zones are the most developed and deep phreatic zones can only be found in the north of the basin, while epikarst zones are rare. Metric-sized caves are well developed in the vadose and upper phreatic zones, indicating prolonged karstification and sufficient recharge. Vugs are predominantly distributed in the vadose zone and upper part of the upper vadose zone, which are adjacent to the western steep slope, implying lithological and karstification differences between the platform margin and interior. Fractures are distributed relatively randomly but sloping terrain is more developed, which may reveal an influence of gradient on fractures, especially vadose fractures and crevices.

Figure 15.

Hydrological conditions of the Z₂dn⁴ of the Central Sichuan Basin. For the profile location, see Figure 5.

Reservoirs are most developed in steep karst slopes where erosion and karstification are rather intense. The reservoir physical parameters, gas testing productivities, reservoir thickness and slope gradients of boreholes in the Z₂dn⁴ are listed in Table 2. In the ten boreholes on steep slopes, the average porosity is 2.79%, the average permeability is 0.294 mD, the average gas testing productivity is 78.88 × 10⁴ m³/d, the average reservoir thickness is 162.7 m, and the average gradient is approximately 2.03°. The main reservoir volumes on the steep slope are centimetric-decimetric vugs (Figures 12(a), (b), (d) and 13(b)). In the nine boreholes on gentle slopes, the average porosity is 2.08%, the average permeability is 0.482 mD, the average gas testing productivity is 17.56 × 10⁴ m³/d, the average reservoir thickness is 73.2 m, and the average gradient is approximately 0.58°. The average permeability of gentle slopes is higher than that of steep slopes, which seems implausible. The lack of permeability data for steep slopes may explain this.

Table 2.

Statistics of reservoir capabilities, gas testing productivities and gradients in different palaeogeomorphological units.

Palaeogeomorphology		Well	aPOR	aPERM	GTP	TOR	SG	Palaeogeomorphology	Well	aPOR	aPERM	GTP	TOR	SG
Karst slope	Steep slope	gk1	2.55	0.334	0.7	184.7	1.602	Erosional peneplain	g2	2.22	0.015	91.19	154.6	0.093
		g3	2.49	0.356	95.76	208.5	1.211		g10	1.58	0.593	45.46	144.4	0.046
		g6	2.82		209.64	195.7	1.299		g11			2.2	100.3	0.133
		g7	2.3	0.4	105.65	136.3	2.141		g12			22.31
		g19			8	175.8	1.819		g20	1.93	0.333	Water Zone	124.5	0.053
		g102			62.63				m9	1.8	0.039	0.51	186.2	0.012
		m47			Dry Layer	149.4	4.652		m12	1.64		3.21	94.5	0.075
		m22			105.6	152.5	2.845		m18			3.85
		c1	3.1	0.086		91.6	1.879		m19	1.7	0.108	12.82	103.6	0.064
		pt1*	3.48		121.98	264	1.763		m103			14.87
	Gentle slope	g1	2.45	1.84	36	134.3	0.984		m105			24.46
		g9	2.55	0.232	126.66	249.5	0.629		m21	2.21	0.102		33.2	0.042
		g18	2.41	0.474	24.07	147	0.326		ap1	1.99	0.291	Dry layer	89.1	0.204
		g21	1.16	0.092	Dry layer	31.5	0.425		m39			Dry layer		0.037
		m11			8.28	67.7	0.295		m10	1.62	0.083	0.19	131	0.076
		m51			2.1		0.169	depression	m8	1.62	0.028	6.69	75.8	0.485
		m52	2.15	0.203		68.3	1.108		m23			6.37	47.9	0.426
		p1	1.87		Water Zone	37.9	0.854		nj				33.3	0.226
		hs1*	1.84	0.053	5.67	228	0.861		m17			2.13		0.22
monadnock		j1	3.2			194.8	0.974		gt2			Water Zone	39	0.343
monadnock		g8			76.72	138.9	0.326	monadnock	g103			15.65

aPOR: average porosity (%); aPERM: average permeability (mD); GTP: gas testing productivity (10⁴ m³/d), TOR: thickness of reservoir (m); SG: slope gradient (°).

The karst water incisions on erosional peneplains are relatively weak. Reservoir volumes lack large vugs and predominantly have millimetric vugs (Figures 12(f) and 13(e)). Porosity diameter and vugs’ density are distinctly minor and lower than those of the slopes. The statistics of 15 boreholes on the erosional peneplains show that the average porosity is 1.85%, the average permeability is 0.196 mD, the average gas testing productivity is 15.71 × 10⁴ m³/d (which approaches that of gentle karst slopes), the average reservoir thickness is 116.1 m, and the average gradient is only 0.08°.

There are only three boreholes drilled through monadnocks. Nevertheless, the average gas testing productivity of these three wells was 46.19 × 10⁴ m³/d and the average reservoir thickness was 166.9 m. Among the three wells, well J1 had truly well-developed karst vugs (Figure 13(f)), while well G8 had metric-sized caves (Figure 13(g)). The average porosity was 3.2%, which is greater than those of the erosional peneplains and gentle slopes.

In the classical evolution pattern of karst geomorphology, depressions can be regarded as the onset of a new peneplain. However, the remnant depressions surrounded by peneplains (Figures 7 and 8) ceased in the process of forming next-stage peneplains. Packed dolines may be developed in the flat bottoms of depressions, leading to their collapse to form breccias and mechanical stowing. As a regional discharging area, large amounts of mechanical stowing brought by frequent karst fluids may block the porosities and prevent further karstification; thus, reservoir volumes may not be developed. The porosity of the depression in well M8 was only 1.62% and its average permeability was just 0.028 mD. The average gas testing productivity of the five wells was 5.06 × 10⁴ m³/d, the average reservoir thickness was only 49 m, and the average slope was 0.34°.

Slopes have clearly higher porosity than peneplains and depressions, which may be due to sedimentary and lithological differences. Yet, the permeability of the palaeogeomorphological units still differed distinctly, even undergoing burial alterations and structural modifications for hundreds of million years. Karstification impacts are non-negligible in causing permeability differences amongst the palaeogeomorphological units. Pores are less interconnected and their throats are occluded by multi-stage pore fillings like dolomites, silica and bitumen (Figures 14(b), (f)-(i) and (l)); hence, fractures and vugs are now the main providers of permeability. Hydrological and hydrodynamic differences between palaeogeomorphological units notably influence the development of dissolution fractures and vugs created by karstification, provoking differential development and permeability of dissolution fractures and vugs among slopes, peneplains, monadnocks and depressions. Dissolution fractures and vugs are more developed in slopes than other units and their permeability is also greater, which can be ascribed to greater hydraulic head differences and more intense incision and erosion of karst fluids.

From the perspective of their contributions to reservoirs, the units can be ranked in order as: steep slope, monadnock, gentle slope, erosional peneplain, and depression. Diverse palaeogeomorphological units vary greatly in porosity, permeability and gas testing productivity, affecting the development of reservoirs by controlling the intensity of dissolution and erosion.

Between the vicinity of the steep karst slope and the inner platform, the reservoir qualities of the Z₂dn⁴ become progressively poorer. The scope of eogenetic karstification differs as a consequence of karst hydrodynamics and fluid properties. Reservoirs in steep karst slopes and their vicinities develop throughout the member, while an overwhelming majority of those in the inner platform is distributed simply in the middle and upper parts of the Z₂dn⁴, which has typical eogenetic karst features (Figure 16).

Figure 16.

Reservoir correlations of wells selected from different palaeogeomorphological units. The number and total thickness of reservoirs tend to gradually decrease with increasing distance from the western steep slope. The bottom boundary of a reservoir can indicate the lower limit of karstification (shown by the green line). Vertical differences in reservoir development in the wells imply differences in karstification type and intensity. For the profile location, see the black line in Figure 5.

Impact of palaeogeomorphology on hydrocarbon accumulation

The natural gas in the Z₂dn in Central Sichuan Basin is derived from different source rocks (Zheng et al., 2021). Some geochemical proxies were used to analyse the gas-source correlations. A rhenium–osmium isotopic geochronometer applied to the study of solid bitumen in the Z₂dn in the Ziyang-Weiyuan area showed Re–Os isochron ages of 414 ± 44 Ma with an initial ¹⁸⁷Os/¹⁸⁸Os (Osi) of 1.36 ± 0.19. This indicates that the oil generation timing was in the Early Devonian, which is consistent with the range of ¹⁸⁷Os/¹⁸⁸Os values of the Є₁q (Shi et al., 2020a). U-Pb isotopic ages and clumped isotopic (Δ47) temperatures of fine-medium crystalline dolomite cements and hydrocarbon inclusions that were contemporaneous with hydrocarbon generation in the Central Sichuan Basin yielded an age of 416 ± 23 Ma and temperature of 84–120 °C. Meanwhile, the burial depth of Є₁q was 1000 m shallower than that of Z₂dn, leading to lower temperatures of 70–100 °C that already exceeded the hydrocarbon generation threshold (Shen et al., 2021). Mn/Fe ratios and a 10Mn-Fe-P ternary diagram of bitumen from Z₂dn and possible source members exhibited the best affinity between the Z₂dn and Є₁q in the Ziyang-weiyuan area (Shi et al., 2020b). The compositions and contents of hydrocarbon gases in the Central Sichuan Basin are similar to those of the adjacent Ziyang-weiyuan area but with higher CO₂ and lower N₂ concentrations, while the δ¹³C₂ value in the Central Sichuan Basin is evidently higher, indicating the genesis of deep carbonate metamorphism and thermochemical sulfate reduction (Zheng et al., 2021). Bitumen biomarkers were characterized by the similarity of the high C₂₉ sterane abundances of mudstone from Є₁q and microbial dolomite from Z₂dn. The sterane distribution of reservoir bitumen in the Z₂dn resembles that of source rocks in both the Є₁q and Z₂dn. All of this evidence suggests that the hydrocarbons in reservoirs were derived from mud-shale of the Є₁q and microbial dolomite from Z₂dn (Zheng et al., 2021). Thus, the main source origin of the Z₂dn in both the Central Sichuan Basin and neighbouring Ziyang-weiyuan area is mud-shale of the Cambrian Є₁q.

Shale of the Є1q in Sichuan Basin is with high TOC content of 1.85 wt% (Total Organic Carbon) and Ro of 2.53% (vitrinite reflectance) on average (Li et al., 2021). The established hydrocarbon generation and expulsion model indicates the maximum of generation intensity for shales of the Є1q nowadays reaches 32 × 10⁶ t/km², and that the maximum of expulsion intensity is 18 × 10⁶ t/km², suggesting the huge and excellent hydrocarbon generation and expulsion potential of shales of the Є1q (Li et al., 2021). And negative δ¹³C_org values in the bottom section of the Є1q is −32.91 ‰ on average, showing a high degree of anoxia of ambient waters (Wang et al., 2015). Whereas in the top section of the Є1q, the negative δ¹³C_org values are higher, implying a l dysoxic–oxic sedimentary circumstance (Gao et al., 2021; Wang et al., 2020). This may illustrate the environmental variation through whole piling-up process of the Є1q. In the duration of transgression immediately after the eogenetic karstification, subjecting to the karst landform, the deep-water shelf attained a greater quantity of organic-matter-bearing sediments than the shallow-water shelf; not only because of the larger depositional thickness, but also due to nutrients from hydrothermal upwelling, that made the primary productivity higher; and that the anoxic surroundings favoured preservation of these organic matters in deep-water shales (Li et al., 2022). All these come to that the hydrocarbon generation potential is higher in deep-water shelf than in the shallow-water shelf. Hence, shale of the Є1q in Mianyang-changning intracratonic sag (deep-water shelf) is indeed the better source rock for the Z₂dn⁴ with remarkable hydrocarbon-generating potential than that of the shallow-water shelf overlying the Z₂dn⁴.

The organic-rich shale of the lower Cambrian Є₁q in the Central Sichuan Basin is the major hydrocarbon source of the underlying Z₂dn⁴ (Zheng et al., 2021; Zeng and Guo, 2015). Its sediment is thick, particularly combined with the residual Є₁m in the Mianyang–Changning Intracratonic Sag in the west, which constitutes the regional hydrocarbon generation centre where the overall thickness of source rocks exceeds 400 m (Figure 17). Dark shale and siltstone of the Deng III member subjacent to the Z₂dn⁴ have the potential for hydrocarbon generation. Therefore, the Z₂dn⁴ in the Central Sichuan Basin is encircled by source rocks on three sides and has the characteristics of ‘upper generation and lower accumulation’, ‘side generation and lateral accumulation’ and ‘lower generation and upper accumulation’.

Figure 17.

Source rock thickness variation in the Є₁m and Є₁q in the Sichuan Basin. Note that the thickest source rock region lies in the Mianyang-Changning intracranial sag, which is adjacent to the Central Sichuan Basin.

In the ‘upper generation and lower accumulation’ pattern, the conditions required for downward migration of hydrocarbons in overlying source rocks are not easily met. Only when the overlying source sequence pressure surpasses that of the underlying reservoirs are oil and gas driven downward (Cong et al., 2016; Fu et al., 2017). Therefore, reservoirs formed by a downward migration pattern are generally small in scale.

In contrast, the ‘side generation and lateral accumulation’ and ‘lower generation and upper accumulation’ patterns accord with the principles of hydrocarbon migration. The thicker source rocks in the western part of the steep karst slope supply substantial hydrocarbons that continue to move upward along the slope to the inner platform, passing through a monadnock, erosional peneplain and gentle slope. The migrating capability of the hydrocarbons decreases with increasing distance (due to friction, capillary pressure, etc.). This leads to more hydrocarbons being entrapped in the steep karst slope belt and adjacent areas than in the inner platform (Song et al., 2021). Surely, the heterogeneity of the primary and karst vugs of the platform margin and interior also hinders hydrocarbon migration. Due to sealing and blocking by the mudstone and shale of the overlying Є₁q and the impact of intra-reservoir heterogeneity, hydrocarbons first accumulate on the steep karst slope and in its vicinity (especially in high areas such as monadnocks), with more developed karst vugs occurring in the course of subsequent oil and gas charging. However, in the erosional peneplain, gentle karst slope and dome in the inner platform, only a small quantity of oil and gas accumulates in local high areas. Hydrocarbons in depressions escape with ease, such that beneficial oil and gas reservoirs do not form readily (Figure 18).

Figure 18.

Hydrocarbon migration and accumulation in the Z₂dn⁴ in the Central Sichuan Basin. Hydrocarbons in this basin are predominately generated in the regional centre of hydrocarbon generation, which has thicker source rocks, and migrate laterally to the porous dolomites in the adjacent Z₂dn⁴. Only a small number of hydrocarbons impregnate and migrate to the Z₂dn⁴ from the overlying and underlying source rocks.

Conclusions

The palaeogeomorphology of the top of the Sinian strata in the Central Sichuan Basin was reconstructed by using the elevation method. In light of morphology, distribution range, calculation of gradients and palaeohydrological conditions, the reconstructed palaeogeomorphology is divided into karst slopes, erosional peneplains, monadnocks, domes and depressions. And karst slopes can be further subdivided into gentle slopes (<1.2°) and steep slopes (> 1.2°).

Giving the credit to the most intense karst hydrodynamics, stream incision, mixing-water karstification involving with fresh water and seawater, the steep karst slope and its vicinities (including part of peneplain, gentle slopes and monadnocks), which are characterized by much developed large-scale honeycomb-like and laminoid vugs, bear the most developed reservoirs of the Central Sichuan Basin. And that the platform margin, with more porous mound-shoal complexes, is superimposed with the steep karst slope and its vicinities. This fact endowed the steep karst slope and its vicinities with excellent diagenetic foundation for both the after eogenetic and burial karstifications. In addition, abuttal of the steep karst slope and its vicinities to the regional hydrocarbon generation centre made it apt for oil and gas to be accumulated and trapped preferentially.

With respect to the other palaeogeomorphological units, reservoirs of which are less developed for the inadequacy of efficient porosity and permeability brought about by the prevailingly and dominantly non-advantageous lithofacies such as micritic dolomite, for the comparative weakness of karstification with sole fresh water recharge and limited initial seepage and flow channels ascribing to the less porous lithofacies, for the small amount of oil and gas charging imputing to the farness from the regional hydrocarbon-generating centre. It follows that steep karst slope and its vicinities should be considered the most favourable exploration targets.

Footnotes

Acknowledgements

The authors thank the Exploration and Development Research Institute of PetroChina Southwest Oil and Gas Field for their support, plentiful data and permission to use and publish that data. Special thanks are extended to Dr Wang Wenzhi and Dr Huang Zisang for their enlightening and insightful suggestions and help, which have made this paper better.

Authors contribution

X.Y., F.H. and S.P.: Investigation. X.W. and Y.Y.: Project administration. W.W. and H.Z.: Resources. L.W.: Supervision. Z.Z.: Writing – original draft. All authors have read and agreed to the published version of the manuscript.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the Major Science and Technology Project of PetroChina Co., Ltd., National Science and Technology Major Project (grant number 2016E-0602, 2016ZX05004-005).

ORCID iDs

Zikun Zhou

References

Agosta

Alessandroni

Antonellini

, et al. (2010) From fractures to flow: A field-based quantitative analysis of an outcropping carbonate reservoir. Tectonophysics 490(3–4): 197–213.

Albertão

Mulder

Eschard

(2011) Impact of salt-related paleotopography on the distribution of turbidite reservoirs: Evidence from well-seismic analyses and structural reconstructions in the Brazilian offshore. Marine and Petroleum Geology 28(5): 1023–1046.

Araújo

REB

Bruna

Rustichelli

, et al. (2021) Structural and sedimentary discontinuities control the generation of karst dissolution cavities in a carbonate sequence, potiguar basin, Brazil. Marine and Petroleum Geology 123: 104753.

Beaugrand

Edwards

Raybaud

, et al. (2015) Future vulnerability of marine biodiversity compared with contemporary and past changes. Nature Climate Change 5(7): 695–701.

Burchette

(1996) Unconformities and porosity in carbonate strata. Marine and Petroleum Geology 13(5): 596–597.

Che

Tan

Deng

, et al. (2019) The characteristics and controlling factors of facies-controlled coastal eogenetic karst: Insights from the fourth member of neoproterozoic Dengying formation, central Sichuan Basin, China. Carbonates and Evaporites 34(4): 1771–1783.

Cong

Zhao

Liu

, et al. (2016) Conditions of oil-gas downward migration in vertical and lateral and their differences in accumulation laws. Journal of China University of Mining & Technology 45(05): 951–957. (in Chinese with English Abstract).

Cross

Veen

Al-Enezi

, et al. (2021) Seismic geomorphology of karst in cretaceous to early cenozoic carbonates of north Kuwait. Marine and Petroleum Geology 128(3): 104947.

Zou

, et al. (2014) Theoretical and technical innovations in strategic discovery of a giant gas field in Cambrian Longwangmiao formation of central Sichuan paleo-uplift, Sichuan Basin. Petroleum Exploration and Development 41(3): 268–277.

10.

Duan

Dai

, et al. (2019) Reservoir characteristics and their controlling factors of the fourth member of upper Sinian Dengying Fm in the northern Sichuan Basin. Natural Gas Industry 39(07): 9–20. (in Chinese with English Abstract).

11.

Firme

Quevedo

Roehl

, et al. (2021) Mechanical behavior of carbonate reservoirs with single karst cavities. Geomechanics for Energy and the Environment 25: 100209.

12.

Florea

Vacher

(2006) Springflow hydrographs: Eogenetic vs. Telogenetic Karst Ground Water 44(3): 352–361.

13.

Florea

Vacher

(2010) Eogenetic Karst hydrology: Insights from the 2004 hurricanes, peninsular Florida. Ground Water 45(4): 439–446.

14.

Ford

Williams

(2007) Karst Hydrogeology and Geomorphology. Chichester: Wiley.

15.

Fritz

Wilson

Yorewicz

(1993) Paleokarst related hydrocarbon reservoirs. Tulsa: SEMP Core Workshop No.18.

16.

Yang

(2017) A method forecasting distribution areas of fault transporting oil-gas migration and its application. Acta Sedimentologica Sinica 35(03): 592–599. (in Chinese with English Abstract).

17.

Gao

Lash

, et al. (2021) Stratigraphic framework, redox history, and organic matter accumulation of an early Cambrian intraplatfrom basin on the Yangtze Platform, South China. Marine and Petroleum Geology 130: 105095.

18.

Han

Lin

Wei

, et al. (2019) Paleogeomorphology reconstruction and the controlling effects of paleogeomorphology on karst reservoirs: A case study of an ordovician-aged section in Tahe oilfield, Tarim Basin, China. Carbonates and Evaporites 34(1): 31–44.

19.

Hao

Yang

Wang

, et al. (2017) Supergene karstification in the Sinian Dengying Formation, Sichuan Basin. Sedimentary Geology and Tethyan Geology 37(01): 48–54. (in Chinese with English Abstract).

20.

Hoffman

(2009) A palaeogeographic context for Neoproterozoic glaciation. Palaeogeography Palaeoclimatology Palaeoecology 277(3–4): 158–172.

21.

Hou

Fan

Henderson

, et al. (2020) Dynamic paleogeographic reconstructions of the Wuchiapingian stage (Lopingian, Late Permian) for the South China block. Palaeogeography, Palaeoclimatology, Palaeoecology 546: 109667.

22.

Hou

Yang

, et al. (2021) Characteristics and formation of Sinian(Ediacaran)carbonate karstic reservoirs in Dengying Formation in Sichuan basin, China. Petroleum Research 6(2): 14.

23.

James

Choquette

(1984) Diagenesis 9: Limestones—the meteoric diagenetic environment. Geoscience Canada 11(4): 162–194.

24.

Jaraula

Grice

Twitchett

, et al. (2013) Elevated pCO2 leading to Late Triassic extinction, persistent photic zone euxinia, and rising sea levels. Geology 41: 955–958.

25.

Jiang

Wang

, et al. (2014) Tectonic evolution of the Leshan-Longnvsi Paleo-uplift and reservoir formation of Neoproterozoic Sinian gas. Natural Gas Geoscience 25(2): 192–200. (in Chinese with English Abstract).

26.

Jin

Tan

Tong

, et al. (2017) Karst paleogeomorphology of the fourth member of Sinian Dengying Formation in Gaoshiti-Moxi area, Sichuan Basin, SW China: Reconstruction and geological significance. Petroleum Exploration and Development 44(1): 58–68.

27.

Jin

Zhu

, et al. (2020) Characteristics and main controlling factors of reservoirs in the fourth member of Sinian Dengying Formation in Yuanba and its peripheral area, northeastern Sichuan Basin, SW China. Petroleum Exploration and Development 47(6): 1172–1182.

28.

Joachimski

Lai

Shen

, et al. (2012) Climate warming in the latest Permian and the Permian-Triassic mass extinction. Geology 40(3): 195–198.

29.

Wen

(2013) Paleogeography and tectonic-depositional environment evolution of the Late Sinian in Sichuan Basin and adjacent areas. Journal of Paleogeography 15(02): 231–245. (in Chinese with English abstract).

30.

Liu

, et al. (2015) Formation and evolution of Weiyuan-Anyue extension-erosion groove in Sinian system, Sichuan Basin. Petroleum Exploration and Development 42(1): 26–33.

31.

Zheng

Zhao

(2017) Geochemical evidence from marine carbonate for enhanced terrigenous input into seawater during the Ediacaran-Cambrian transition in South China. Precambrian Research 291: 83–97.

32.

Pan

Yan

, et al. (2018) Karst paleogeomorphology features of the fourth member of Dengying Formation in Sichuan Basin and the significance of oil and gas exploration. Geological Science and Technology Information 37(3): 191–195. (in Chinese with English Abstract).

33.

Wang

Feng

, et al. (2019) Reservoir characteristics and genetic differences between the second and fourth members of Sinian Dengying Formation in Northern Sichuan Basin and its surrounding areas. Petroleum Exploration and Development 46(1): 54–66.

34.

Xue

Cheng

, et al. (2020) Progress and development directions of deep oil and gas exploration and development in China. China Petroleum Exploration 25(01): 45–57. (in Chinese with English Abstract).

35.

Pang

, et al. (2021) Hydrocarbon generation and expulsion characteristics of the lower Cambrian Qiongzhusi shale in the Sichuan Basin, Central China: Implications for conventional and unconventional natural gas resource potential. Journal of Petroleum Science and Engineering 204: 108610.

36.

Liu

Song

, et al. (2022) Organic matter enrichment due to high primary productivity in the deep-water shelf: Insights from the lower Cambrian Qiongzhusi shales of the central Sichuan Basin, SW China. Journal of Asian Earth Sciences 239: 105417.

37.

Liang

Hou

(2021) Fluids flow behaviors of nitrogen and foam-assisted nitrogen floods in 2d visual fault-karst carbonate reservoir physical models. Journal of Petroleum Science and Engineering 200(8): 108286.

38.

Liu

Luo

Tan

, et al. (2015) Reconstruction of paleokarst geomorphology of Sinian Dengying Formation in Sichuan Basin and its significance, SW China. Petroleum Exploration and Development 42(3): 311–322.

39.

Liu

Wang

Sun

, et al. (2016) Control of intracratonic sags on the hydrocarbon accumulations in the marine strata across the Sichuan Basin, China. Journal of Chengdu University of Technology 43(01): 1–23. (in Chinese with English Abstract).

40.

Liu

Tan

, et al. (2019) Eogenetic Karst in interbedded carbonates and evaporites and its impact on hydrocarbon reservoir: A new case from middle Triassic Leikoupo formation in Sichuan Basin, Southwest China. Journal of Earth Science 30(5): 908–923.

41.

Liu

Dan

Luo

, et al. (2020) Characterization of karst paleo-geomorphology and the paleo-water system on the top of the 4^th member of the Dengying Formation in the Gaoshiti area, Sichuan basin. Carsologica Sinica 39(2): 206–214. (in Chinese with English Abstract).

42.

Liu

Yang

Deng

, et al. (2021) Tectonic evolution of the Sichuan Basin, Southwest China. Earth-Science Reviews 213: 103470.

43.

Loucks

Mescher

McMechan

(2004) Three-dimensional architecture of a coalesced, collapsed-paleocave system in the Lower Ordovician Ellenburger Group, central Texas. AAPG Bulletin 88(5): 545–564.

44.

Luo

Wang

, et al. (2015) Formation mechanism of the Sinian natural gas reservoir in the Leshan-Longnvsi Paleo-uplift, Sichuan Basin. Natural Gas Geoscience 26(3): 444–455. (in Chinese with English Abstract).

45.

Luo

Yang

Luo

, et al. (2017) Controlling factors of Dengying Formation reservoirs in the central Sichuan paleo-uplift. Petroleum Research 2(1): 54–63.

46.

Moore

Wade

(2013) Carbonate Reservoirs — Porosity and Diagenesis in a Sequence Stratigraphic Framework. Amsterdam: Elsevier Academic Press.

47.

Morse

Mackenzie

(1990) Geochemistry of Sedimentary Carbonates. New York: Elsevier Scientific.

48.

Dong

, et al. (2020) Development and distribution rules of the main Neoproterozoic source and reservoir strata in the Yangtze block, southern China. Precambrian Research 350: 105915.

49.

Ronchi

Ortenzi

Borromeo

(2010) Depositional setting and diagenetic processes and their impact on the reservoir quality in the late Visean–Bashkirian Kashagan carbonate platform (Pre-Caspian Basin, Kazakhstan). AAPG Bulletin 94(9): 1313–1348.

50.

Sayago

Lucia

Mutti

, et al. (2012) Characterization of a deeply buried paleokarst terrain in the Loppa High using core data and multiattribute seismic facies classification. AAPG Bulletin 96(10): 1843–1866.

51.

Shan

Zhang

, et al. (2017) Characteristics of dolomite karstic reservoir in the Sinian Dengying Formation, Sichuan Basin. Petroleum Research 2(1): 13–24.

52.

Shen

Zhao

, et al. (2021) The dating and temperature measurement technologies for carbonate minerals and their application in hydrocarbon accumulation research in the paleouplift in central Sichuan Basin, SW China. Petroleum Exploration and Development 48(03): 555–568.

53.

Shi

Cao

David

, et al. (2020a) Hydrocarbon evolution of the over-mature Sinian Dengying reservoir of the neoproterozoic Sichuan Basin, China: Insights from Re–Os geochronology. Marine and Petroleum Geology 122: 104726.

54.

Shi

Cao

Luo

, et al. (2020b) Major elements trace hydrocarbon sources in over-mature petroleum systems: Insights from the Sinian Sichuan Basin, China. Precambrian Research 343: 105726.

55.

Song

Liu

Luo

, et al. (2021) Logging evaluation of solid bitumen in tight carbonate in deepburied and ultra-deep-buried strata of the central Sichuan Basin. Acta Sedimentologica Sinica 39(01): 197–211. (in Chinese with English Abstract).

56.

Wang

Yang

, et al. (2021) Hydrothermal alteration and hydrocarbon accumulations in ultra-deep carbonate reservoirs along a strike-slip fault system, Tarim Basin, NW China. Journal of Petroleum Science and Engineering 203(10): 108605.

57.

Tan

Xiao

Chen

, et al. (2015) New advance and enlightenment of eogenetic karstification. Journal of Paleogeography 17(04): 441–456. (in Chinese with English Abstract).

58.

Tang

, et al. (2013) Prediction of weathering paleokarst reservoirs by combining paleokarst landform with unconformity: A case study of Sinian Dengying Formation in Leshan − Longnvsi paleo-uplift, Sichuan Basin. Petroleum Exploration and Development 40(06): 674–681.

59.

Tian

Peng

Wang

, et al. (2020) Analysis of reservoir difference and controlling factors between the platform margin and the inner area of the fourth member of Sinian Dengying Formation in anyue gas field, central Sichuan. Natural Gas Geoscience 31(09): 1225–1238. (in Chinese with English Abstract).

60.

Waele

Plan

Audra

(2009) Recent developments in surface and subsurface karst geomorphology: An introduction. Geomorphology 106(1–2): 1–8.

61.

Wang

Zou

Dong

, et al. (2015) Multiple controls on the paleoenvironment of the Early Cambrian marine black shales in The Sichuan Basin, SW China: Geochemical and organic carbon isotopic evidence. Marine and Petroleum Geology 66: 660–672.

62.

Wang

Jiang

Wang

, et al. (2014) Paleogeomorphology formed during Tongwan tectonization in Sichuan Basin and its significance for hydrocarbon accumulation. Petroleum Exploration and Development 41(3): 305–312.

63.

Wang

Tian

, et al. (2020) Climate–ocean control on the depositional watermass conditions and organic matter enrichment in lower Cambrian black shale in the upper Yangtze platform. Marine and Petroleum Geology 120: 104570.

64.

Wei

G Q

Liu

Zhang

, et al. (2005) The exploration region and natural gas accmulation in Sichuan Basin. Natural Gas Geoscience 4: 437–442. (in Chinese with English Abstract).

65.

Wei

Yang

Xie

, et al. (2016) Formation conditions, accumulation models and exploration direction of large-scale gas fields in Sinian-Cambrian, Sichuan Basin, China. Natural Gas Geoscience 26(05): 785–795.

66.

Wen

Wang

Zhang

, et al. (2017) Classification of Sinian Dengying Formation and sedimentary evolution mechanism of Gaoshiti-Moxi area in Central Sichuan Basin. Acta Petrologica Sinica 33(4): 1285–1294. (in Chinese with English Abstract).

67.

White

(1988) Geomorphology and Hydrology of Karst Terrains. Oxford: Oxford Press.

68.

White

(2002) Karst hydrology: Recent developments and open questions. Engineering Geology 65(2–3): 85–105.

69.

Xie

Deng

, et al. (2019) Characterization of unique natural gas flow in fracture-vuggy carbonate reservoir: A case study on Dengying carbonate reservoir in China. Journal of Petroleum Science and Engineering 182: 106243.

70.

Wei

Jia

, et al. (2012) Tectonic evolution of the Leshan-Longnüsi paleo-uplift and its control on gas accumulation in the Sinian strata, Sichuan Basin. Petroleum Exploration and Development 39(4): 406–416.

71.

Shen

Yang

, et al. (2021) New understandings and potential of sinian–lower paleozoic natural gas exploration in the central sichuan paleo-uplift, Sichuan Basin. Natural Gas Industry B 8: 105–113.

72.

Yan

, et al. (2016) Paleotopography reconstruction method and its controlling effect on fluid distribution: A case study of the gas reservoir evaluation stage in Gaoqiao, Ordos Basin. Acta Petrolei Sinica 37(12): 1483–1494. (in Chinese with English Abstract).

73.

Yan

Peng

Xia

, et al. (2020) Distribution features of ancient karst landform in the fourth member of the Dengying Formation in the Gaoshiti-Moxi region and its guiding significance for gas reservoir development. Acta Petrolei Sinica 41(6): 658–670. ,752 (in Chinese with English Abstract).

74.

Yang

Wei

Zhao

, et al. (2014a) Characteristics and distribution of karst reservoirs in the Sinian Dengying Fm Sichuan Basin. Natural Gas Industry. (in Chinese with English abstract).

75.

Yang

Huang

Zhang

, et al. (2014b) Features and geologic signifcances of the top Sinian karst landform before the Cambrian deposition in the Sichuan Basin. Natural Gas Industry 34: 38–43. (in Chinese with English abstract).

76.

Yang

Wen

Luo

, et al. (2016) Hydrocarbon accumulation of Sinian natural gas reservoirs, Leshan-Longnüsi paleohigh, Sichuan Basin, SW China. Petroleum Exploration and Development 43(2): 197–207.

77.

Zeng

Guo

(2015) Enrichment of shale gas in different Strata in Sichuan Basin and its periphery-the examples of the Cambrian Qiongzhusi formation and the Silurian Longmaxi Formation. Energy Exploration & Exploitation 33(3): 277–298.

78.

Zhao

Liu

, et al. (2014) Petroleum geological features and exploration prospect of deep marine carbonate rocks in China onshore: A further discussion. Natural Gas Industry B 1(1): 14–23.

79.

Zhao

Wang

Feng

(2017) Review on paleomorphologic reconstruction methods in oil and gas fields. Journal of Earth Sciences and Environment 39(04): 516–529. (in Chinese with English Abstract).

80.

Zhao

Liu

Shi

, et al. (2018) A new interpretation mode for pre-carboniferous paleo-geomorphology of the Jingbian gas field, Ordos Basin. Natural Gas Industry 38(04): 53–58. (in Chinese with English Abstract).

81.

Zheng

Pang

Luo

, et al. (2021) Geochemical characteristics, genetic types, and source of natural gas in the Sinian Dengying Formation, Sichuan Basin, China. Journal of Petroleum Science and Engineering 199: 108341.

82.

Zhou

Wang

Yin

, et al. (2016) Characteristics and genesis of the (Sinian) Dengying Formation reservoir in Central Sichuan, China. Journal of Natural Gas Science and Engineering 29: 311–321.

83.

Zhou

Yang

, et al. (2020) Characteristics and controlling factors of dolomite karst reservoirs of the Sinian Dengying Formation, central Sichuan Basin, southwestern China. Precambrian Research 343: 105708.

84.

Zhu

Shen

Zeng

, et al. (2018) The application of paleogeomorphy restoration to the study of karst weathering crust reservoir:A case from the fourth member of Dengying Formation in Moxi area, Sichuan Basin. Marine Origin Petroleum Geology 23(04): 87–95. (in Chinese with English Abstract).

85.

Zhu

Wang

Zhang

, et al. (2019) Characteristics of paleogeomorphology and Paleokarstification and the impact on natural gas accumulation: A case study of upper assemblage of Majiagou formation in central sulige gas field, ordos Basin, China. Carbonates and Evaporites 34(4): 1353–1366.