Discovery of a shoal-controlled karst dolomite reservoir in the Middle Permian Qixia Formation,northwestern Sichuan Basin,Southwest China

Abstract

The dolomites of the Middle Permian Qixia Formation have been important targets of natural gas exploration in the Sichuan Basin for decades. However, more and more exploration and research indicate that the formation of the reservoir might be related to karstification. To testify this hypothesis, we conduct comprehensive outcrop, core, and logging analyses based on a case study in the representative northwestern Sichuan Basin, which has obtained exploration breakthroughs recently. Results show that the Qixia dolomite reservoirs are mainly developed within fine-crystalline dolomites formed by a series of diagenetic modifications, which can be further divided into three types according to the macro- and micro-occurrences of dolomites: euhedral-subhedral crystalline dolomites in the quasi-stratiform karst system (mean porosity and permeability is 3.51% and 3.11 mD, respectively), euhedral-subhedral crystalline dolomites in the leopard porphyritic karst system (mean porosity and permeability is 3.36% and 1.22 mD, respectively), and allotriomorphic mosaic crystalline dolomites with residual parent rock fabrics (mean porosity and permeability is 0.94% and 0.92 mD, respectively). Their reservoir qualities decrease along the order. The formation mechanism of the reservoir is shoal-controlled karst. The preservation of residual intergranular pores within the thin-layer grainstones of shoal facies provides favorable channels for karst water. In the vadose zone, the heterogeneous dissolution within grainstones leads to the formation of leopard porphyritic dissolution features. In the phreatic zone, the karst water flowing along the stratiform grainstones results in the formation of quasi-stratiform dissolution features. The karst system is filled with loose carbonate sands and gravels, whose reservoir properties are far superior to parent rocks, and they can provide migration channels for the hydrothermal fluids with rich Mg²⁺ in the burial stage. The replacement of hydrothermal fluid results in the redistribution of pores and vugs of inter-fillings within karst system and the formation of intercrystalline pores and residual vugs, but the reservoir space of parent rocks keeps the same as the original condition. Therefore, the exploration of the Qixia dolomite reservoir should be changed to shoal-controlled karst.

Keywords

Shoal facies-controlled karst hydrothermal dolomitization dolomite reservoir Middle Permian Qixia Formation Sichuan Basin

Introduction

Carbonates play important roles in global petroleum exploration, and karstification and dolomization, as common diagenesis in such rocks, have significant implications for the formation of high-quality reservoirs through diagenetic alteration (Braithwaite et al., 2004; James and Choquette, 1988; Moore and Wade, 2013). As a consequence, karstification and dolomization have attracted continued attention from petroleum geologists for decades (Cao et al., 2015; Conliffe et al., 2012; Davies and Smith, 2006; Feng et al., 2013; Lü et al., 2011; Wei et al., 2015). The Middle Permian Qixia Formation in the Sichuan Basin of southwestern China is one of representative cases (Tian et al., 2014; Wang et al., 2013).

The Middle Permian Qixia Formation dolomites in the Sichuan Basin of southwestern China have attracted significant attention due to the important role in the natural gas exploration in the basin (Chen, 2009). However, their formation mechanisms and distribution rules have been controversial for decades. The present main view is that they are pore and cave type reservoirs, with wide and stable distribution in the northwestern Sichuan Basin (Xu et al., 2005; Zeng et al., 2010). The reservoir formation is thought to be closely related with dolomitization (Tian et al., 2014), including the mixed water dolomitization, burial dolomitization, basalt leaching dolomitization, and hydrothermal dolomitization (Chen, 1989; He and Feng, 1996; Jin and Feng, 1999; Wang et al., 2014; Wang and Jin, 1997; Zhang, 1982). Among them hydrothermal dolomitization is the present overwhelming understanding (Chen et al., 2013; Shu et al., 2012).

According to the theory of hydrothermal dolomitization, the dolomite reservoirs should be located around deep faults. However, more and more exploration results reveal that high production is not always associated with faults, suggesting possible new reservoir origins. Recently, some studies demonstrated that there exists an unconformity on the top of the Qixia Formation in the Sichuan Basin, and the unconformity is subjected to meteoric water karstification (Wang et al., 2013). The distribution of high-quality dolomite reservoirs in the outcrops and cores indicate the relationship with dolomitization of fillings in the karst system (Tan et al., 2015). Therefore, the formation of the Qixia dolomite reservoirs in the Sichuan Basin might be related to karst. The alteration of karst on the reservoirs might result in the complexity of petrography and geochemistry of the dolomites, and further leading to the difficulty of data interpretation and contentious views.

To testify this hypothesis, based on a case study in the northwestern Sichuan Basin where natural gas exploration in the Qixia dolomite reservoirs achieves important breakthroughs recently, we investigate the characteristics and address the formation mechanisms of the Qixia dolomite reservoirs. The results are significant to the future oil and gas exploration, and can also improve our understanding of the geological setting of the Sichuan Basin during the Middle Permian.

Geological setting

The study area is in the northwestern Sichuan Basin with an area of 5000 km², municipally covering Guangyuan, Jiange, Wangcang, and Cangxi areas and tectonically in the transition zone of Northern Sichuan Gentle Fault and Fold Belt, Longmen Mountain Fault and Fold Belt, and Micang Mountain Uplift Belt (Figure 1). The Permian Qixia Formation is 70 to 130 m thick, and it can be divided into two members. The first member of the Qixia Formation (Qi 1) mainly consists of micritic rocks, while the second member (Qi 2) is mainly featured by thin-medium interbedding of granular rocks and micritic rocks. Thus, it appears that there is a complete transgressive-regressive cycle within the Qixia Formation (Figure 2), which might be formed on the platform margin (Hu et al., 2010), or on the gentle carbonate slope (Zhao et al., 2012).

Figure 1.

Location of the study area and generalized stratigraphy of the studied Middle Permian Qixia Formation.

Figure 2.

Stratigraphic column of the Lower Permian Liangshan Formation to Middle Permian Lower Qixia Formation in the study area. Based on the results in the Liaojiapo Section. See Figure 1 for the location of the section.

Based on outcrop section and core observations, combined with regional paleogeographic background and seismic constraints of formation thickness, it is found that there was a carbonate platform sedimentary system adjacent to the continent during the deposition of the Qixia Formation, with the northern barrier-lagoon-tidal flat system gradually changing into southern open platform (Figure 1). The widely distributed intra-platform shoal and barrier beach is the foundation for the development of the dolomite reservoir (Figure 1).

In terms of lithology, the underlying Liangshan Formation calcareous siltstone gradually changes into the Qixia Formation silty limestone (Figure 2), which suggests the retreating terrigenous sediment supply due to continuous transgression, especially from the Motianling–Hannan–Niushan ancient continents (Geology and Mineral Resources Bureau of Sichuan Province, 1991). As for the Qi 2 deposition, Huang et al. (2004) argued that it should correspond to an important shoal-forming period, and granular rocks should be dominant, interbedded with some micritic rocks. However, according to the statistics of eight outcrop sections and four coring wells in the study area, 90% of the single shoal in Qi 2 have thicknesses less than 1.5 m. Thus, in terms of thickness scales, there is a huge difference between the shoal and that of platform margin, which indicates that deposits in the study area should not be the products of platform margin environment, whereas flat to open platform environments (Zhao et al., 2012).

Data and methods

The Qixia Formation in this study is featured by great burial depths (more than 6000 m) in general and thus has high drilling costs. As a result, only six wells have been drilled to the Qixia Formation in the study area, all of which are located along the piedmont of the Longmen Mountain and Micang Mountain (Figure 1). They are all coring wells, with an accumulative coring length of 500 m, providing invaluable materials for this study. Meanwhile, to compensate the relatively limited coring wells, we investigate 14 outcrop sections along the basin margin.

A total of 280 samples are selected for detailed petrological studies, and 133 representative samples with different lithologies are implemented with physical property tests and mercury injection analysis. Porosity was measured on a JS100007 Helium Porosimeter and permeability was measured using an A-10133 gas permeameter. The average capillary radius were calculated using the data collected on an AutoPore IV 9505 automatic high pressure mercury porosimeter following standard industry methods in Chuanxibei Gas Field Branch Company.

Basic characteristics of reservoir rocks

General characteristics

According to the macroscopic and microscopic observations as well as physical property analysis of outcrops and cores, it is found that the overall average porosity and permeability of dolomites is much higher than that of limestones (Table 1). Such dolomites are developed mainly in the middle and upper parts of the Qixia Formation within shoal facies, and can be classified into three types, namely leopard porphyritic dolomites, dolomites in quasi-stratiform karst system, and dolomites with residual parent rock fabrics. The porosity and permeability significantly varies with dolomite types (Table 1).

Table 1.

Physical property of the Qixia Formation in the northwestern Sichuan Basin based on core analysis.

Properties Lithologies		Porosity (%)			Sample number	Permeability (mD)			Sample number
Properties Lithologies		Average	Maximum	Minimum	Sample number	Average	Maximum	Minimum	Sample number
Dolomite	Leopard porphyritic dolomite or dolomite limestone	3.36	8.83	0.94	43	1.2219816	31.4	0.000253	39
	Dolomite in quasi-stratiform karst system	3.51	13.38	0.72	26	3.1060597	28.8	0.000268	20
	Dolomite from residual parent rocks	0.94	2.87	0.60	11	0.9160933	4	0.00189	6
Limestone	Dolomite limestone	0.77	2.01	0.32	4	2.0009	4	0.00180	2
	Micrite	0.35	0.39	0.31	2
	Grainstone	0.55	2.60	0.18	28	1.6357107	17.7	0.000543	21

Leopard porphyritic dolomites

Leopard porphyritic dolomites as well as some limestones are mainly developed in the upper part of the Qixia Formation, and are associated with grainstones. Dark and light patches can be observed. Dark ones are dolomites (Figure 3), and undolomitized plastic limestone breccia can be seen in the larger dark patches (Figure 3(d)). In contrast, dolomitization degree significantly varies within light patches (Figure 3), such as the nearly undolomitized ones in Chejiaba, Changjiagou, and Liaojiapo Sections (Figure 3(a)–(c), (g)), and the completely dolomitized ones in well K2 (Figures 3(h)–(j)).

Figure 3.

Macroscopic (outcrop and core observations) and microscopic (thin-section observations) features of the leopard porphyritic dolomites (limestones) in the Qixia Formation (Qi 2 member), northwestern Sichuan Basin. See Figure 1 for the location of the outcrops and well. (a) Leopard porphyritic dolomite in the Chejiaba Section; (b) close-up of the corresponding red dashed box in (a); (c) leopard porphyritic dolomite in the Changjianggou Section; (d) close-up of the corresponding red dashed box in (a); (e) microscopic feature of the leopard porphyritic dolomite in (b); (f) Microscopic feature of the matrix limestone (sparry bioclast limestone) in (b); (g) leopard porphyritic dolomite in the Liaojiapo Section; (h) leopard porphyritic dolomite in well K 2, 2414.07 m, core; (i) leopard porphyritic dolomite in well K 2, 2413.08 m, core; (j) porosity contrast between non-leopard porphyritic matrix and leopard porphyritic dolomite, well K 2, 2413.96 m, core; (k) microscopic feature of the leopard porphyritic dolomite in (j); (l) microscopic feature of the non-leopard porphyritic matrix in (j).

From the macroscopic perspective in core observations, it is found that dark dolomite patches in the leopard porphyritic dolomite are mainly featured by well-developed pinholes and small residual vugs, which are, however, not developed in the light patches (Figure 3(b), (h), (j)). From the microscopic perspective in thin-section observations, it is found that dark and light patches are obviously different. In dark patches, there are mainly idiomorphic-hypidiomorphic crystalline dolomites, with occasionally-observed micrites and silty dolomite fillings (Figure 3(e), (k)). The reservoir spaces are mainly intergranular pores and dissolution pores (Figure 3(e), (k)), with some micro pores between fine-crystalline fragment fillings and some unevenly-distributed small residual vugs. In contrast, for the dolomitized light patches, reservoir spaces are controlled by the fabrics of parent rocks, such as the allotriomorphic mosaic occurrence due to densification and dolomitization of parent rocks where there are extremely little intercrystalline pores (Figure 3(l)). In the undolomitized light patches, the lithologies are mainly sparite bioclast limestones and micrite bioclast limestones; they are generally tight, and pores are not observed under the microscope (Figure 3(f)). These reflect strong dolomite reservoir heterogeneity.

The main throat type of the Leopard porphyritic dolomite is necking throat, and the pore-throat configuration is relatively good. It is demonstrated by mercury injection parameters that both the displacement pressure and the median saturation pressure are high, while the mercury injection curve presents fine-middle skewness characteristics, indicating poor sorting (Figure 4(a)). The average porosity of 43 samples is 3.36%, with the maximum and minimum of 8.83% and 0.94%, respectively. The average permeability is about 1.22 × 10⁻³ µm². It can be seen from the porosity–permeability correlation diagram that there are three types of sample points with certain positive correlations (Figure 5(a)). The first type presents low porosity and low permeability characteristics, representing the bedrock area with light non-leopard porphyritic fabrics. The second type displays medium porosity and medium permeability characteristics, representing dark patches with leopard-porphyritic fabrics. The third type shows low porosity and high permeability characteristics, reflecting the influence of micro fractures. Therefore, it appears that the physical properties of samples vary with the sampling locations. There is strong heterogeneity within such kind of reservoir rocks on a small scale, but relatively good lateral and longitudinal continuity in the dark patches on a large scale (Figure 3(a)).

Figure 4.

Curves showing the mercury injection of the Qixia dolomites in the northwestern Sichuan Basin. (a) Leopard porphyritic dolomites (limestones), well K 2, 2413.73 m; (b) the dolomite in the quasi-stratiform karst cave, well K 2, 2447.59 m; (c) the dolomite with residual parent rock fabrics, well K 2, 2427.67 m.

Figure 5.

Porosity–permeability correlation of the Qixia dolomites in the northwestern Sichuan Basin. (a) Leopard porphyritic dolomites (limestones); (b) the dolomite in the quasi-stratiform karst cave; (c) the dolomite with residual parent rock fabrics.

Dolomites in the quasi-stratiform karst caves

Such kinds of dolomites are mainly formed in the middle and upper parts of the Qixia Formation, just underlying the “leopard porphyritic dolomites”. Through careful observations of the Chejiaba Section, three layers of quasi-stratiform dolomites are recognized. The single dolomite layer has a stable occurrence horizontally in general, but it still transverses through layers and sharply changes to limestones. The layers are separated by tight micrites, and developed dolomites are found to connect different layers vertically (Figure 6(a), (b)). The second layer of the karst cave is filled with breccia (Figure 6(b)). Corresponding with this, laminated pores and vugs layers and typical cave breccias were observed (Figure 6(c), (d)), which is verified by imaging logging (Figure 6(e)).

Figure 6.

Macroscopic (outcrop, core and image logging observations) features of the Qixia dolomites in the quasi-stratiform karst caves, northwestern Sichuan Basin. See Figure 1 for the location of the outcrops and well. (a) The first-layer quasi-stratiform cave on top, Chejiaba outcrop, the bottom of the Qi 2 member, (1) the dolomite in quasi-stratiform cave, (2) compacted limestone; (b) quasi-stratiform caves of the second and third layers on bottom, and the second one is filled with breccias, Chejiaba Section, the upper Qi 1 member, (1) the dolomite in quasi-stratiform cave, (2) compacted limestone; (c) caves and loose fillings, well K 2, 2426.71 m, core, upper Qi1 member; (d) Breccias in cave, well K 2, 2438.11 m, core, upper Qi 1 member; (e) image logging feature of quasi-stratiform dissolution pores and caves, well K 2.

This type of dolomite is mainly composed of filling breccia in the quasi-stratiform karst caves as well as dolomitized carbonate sands. From the macroscopic perspective in core observations, it is found that the reservoir space mainly consists of developed pinholes and small residual vugs (Figures 6(c) and 7(a), (b)). Pinholes are mainly found in the dolomitized carbonate sands, and they are not developed in the dolomitized breccia (Figure 6(d) and 7(a), (b)). From the microscopic perspective in thin-section observations, medium fine crystalline dolomites are dominant between breccia, with highly idiomorphic features as well as dirty center and bright edge (Figure 7(c), (e), (g)). The main reservoir spaces are intercrystalline pores and intercrystalline dissolution pores (Figure 7(c), (e), (g)). Similar to the reservoir space of the light dolomitized patches in the leopard porphyritic dolomite that of dolomitized breccia is also controlled by the parent rocks (Figure 7(d), (e)).

Figure 7.

Macroscopic (core observations) and microscopic (thin-section observations) features of the Qixia dolomites in the quasi-stratiform karst caves, northwestern Sichuan Basin. See Figure 1 for the location of the well. (a) Breccia and pinhole-like loose fillings in quasi-stratiform cave, well K 2, 2441 m, core, Qi 1 member; (b) breccia, pinhole-like loose fillings and residual caves in quasi-stratiform cave, well K 2, 2444.79 m, core, Qi 1 member; (c) microscopic feature of C area in (a), crystalline dolomite; (d) microscopic feature of (d) area in (a), residual grain dolomite; (e) microscopic feature of (e) area in (b), breccia is compacted and its composition is allotriomorphic and mosaic crystalline dolomites, and inter-breccia composition is idiomorphic-hypidiomorphic crystalline dolomites with pinhole-like feature; (f) porosity contrast between the matrix dolomites of quasi-stratiform cave periphery and the dolomites of fillings inside the cave, well K 2, 2452.23 m, Qi 1 member; (g) microscopic feature of G area in (f), idiomorphic-hypidiomorphic crystalline dolomites; (h) microscopic feature of (h) area in (f), residual grain dolomite.

Throats in inter-breccia dolomites are mainly sheet-shaped, with high pore-throat coordination numbers and good pore interconnectedness. They are featured by the lowest displacement pressure (less than 0.1 MPa), the lowest median saturation pressure (less than 1 MPa), and coarse skewness indicating good sorting (Figure 4(b)). The average porosity of 26 samples is 3.51%, with the maximum and the minimum of 13.38% and 0.72%, respectively. The average permeability is about 3.11 × 10⁻³ µm². It can be seen from the porosity-permeability correlation diagram (Figure 5(b)) that there are three types of sample points with strong positive correlations in general. The first type presents medium-high porosity and medium-high permeability characteristics, representing the idiomorphic crystalline dolomites with inter-breccia pinholes. The second type displays low porosity and low permeability characteristics, representing the samples that change from breccia crystalline dolomites to inter-breccia crystalline dolomites. The third type shows low porosity and high permeability characteristics, reflecting the influences of fractures.

Dolomites residual from parent rock

Such kind of dolomites are common in the Qixia Formation of the study area, and generally occur in four forms, namely the completely dolomitized light part in the leopard porphyritic dolomites (Figure 3(j), (l)), dolomites along the rim of quasi-stratiform karst caves (Figure 7(f), (h)), dolomitized breccia in the karst system (Figure 7(a), (b), (d), (e)), and dolomitized parent rock near fissures and stylolites (Figure 8(a)–(d)).

Figure 8.

Macroscopic (outcrop and core observations) and microscopic (thin-section observations) features of the Qixia dolomites from parent rock, northwestern Sichuan Basin. See Figure 1 for the location of the outcrops and wells. (a) Dolomitization along the stylolites, well K 1, 4228.50 m, core, Qi 1 member; (b) dolomitization along bedding planes (indicated by arrows), the color of dolomite is isabelline, Changjianggou Section, Qi 1 member; (c) close-up of the red dashed box in (b), isabelline dolomite along high-angle small fissure; (d) porosity contrast between the matrix limestones and dolomites formed after dolomitization, well K 1, 4225.24 m, Qi 1 member; (e) microscopic feature of E area in (d), bioclast micrite; (f) microscopic feature of (f) area in (d), allotriomorphic and mosaic crystalline dolomites.

It is identified under the microscope that such kind of dolomites are mainly allotriomorphic, mosaic, medium-fine crystalline (Figures 3(l), 7(e) and 8(f)). The reservoir space is mainly intercrystalline pores (Figures 3(l), 8(f)). Phantom fabrics of grains from the parent rock can be occasionally observed (Figure 7(d), (h)). The reservoir space after dolomitization is mainly residual intergranular (dissolved) pores (Figure 7(d), (h)), which well inherit characteristics of the parent rocks (Figure 8(e), (f)).

Throats of this type of dolomite are mainly tube-shaped, with low pore-throat coordination numbers. Mercury injection parameters are characterized by high displacement pressure, high median pressure, and no obvious flat segments in the curve, which indicate small pore-throat radium, poor connectivity, and strong heterogeneity (Figure 4(c)). The average porosity of 11 samples is 0.94%, with the maximum and the minimum of 2.87% and 0.60%, respectively. The average permeability is about 0.92 × 10⁻³ µm². It can be seen from the porosity-permeability correlation diagram that there are two types of sample points, respectively representing the bedrock area with low porosity and permeability and the micro fracture-controlled area with low porosity and medium permeability (Figure 5(c)).

To sum up, there are differences among three types of dolomites above in terms of reservoir space types, pore-throat structures and physical properties. Physical properties decrease in the order of idiomorphic-hypidiomorphic crystalline dolomites in quasi-stratiform karst system, idiomorphic-hypidiomorphic crystalline dolomites in leopard porphyritic karst system, and allotriomorphic mosaic crystalline dolomites with residual parent rock fabrics. Note that all the three kinds of dolomites above are influenced by (micro) fractures to varying degrees, which are significant for the reservoir connectivity. Therefore, the reservoir types of the Qixia Formation in the study area should be (fracture-) pore and cave type.

Formation mechanisms and control factors of high-quality reservoir

Based on the above results, we conclude that the Qixia reservoir in the study area is apparently influenced by karst. The reservoir origin can be termed shoal-controlled karst, considering all the geologic elements that related to the formation of reservoir. This can be elucidated from three facets as below.

Sedimentary facies

The development of karst in the study area is closely related to sedimentary facies and associated rock types, i.e. dolomites. For example, it is identified in the Chejiaba Section through macroscopic and microscopic observations that the matrixes in leopard porphyritic and quasi-stratiform karst systems are both sparry grainstones, while those in multi-stratiform karst system are wackstones (Figure 9). However, in the Qingfengxia Section where restricted lagoon sedimentary deposits are dominant, the overall tight lithology leads to the undeveloped dolomitization and karst, and thus the undeveloped reservoirs.

Figure 9.

Shoal-controlled karst model for the formation of the Qixia dolomite reservoir in the northwestern Sichuan Basin. Hydrothermal dolomitization also impacts.

Tan et al. (2015) and Wang et al. (2013) proposed that the large-scale regression during the late Qixia period resulted in a large-scale and relatively long exposure of early-formed rocks, which then suffered karstification. Intergranular and intragranular pores in high-energy grainstones got well preserved due to the weak compaction and cementation, and thus there are highly permeable zones within rocks favorable for the infiltration of karst water (Jin et al., 2014). Intergranular pores can be dissolved and filled. However, pores in micrites are very limited due to the strong compaction and cementation, and only some matrix micro pores are developed. But the poor percolation condition makes it difficult for fluids to flow through, and thus karstification is not intense. There are only some vertical karst channels connecting different karst systems in local areas.

Therefore, it can be concluded that sedimentary facies (i.e. shoal) controls the karst, while the superposed dolomitization contributes to high-quality dolomite reservoirs, including above-mentioned leopard porphyritic dolomites and dolomites in quasi-stratiform karst system. It is the shoal that lays the material foundation for the formation of high-quality reservoirs.

Shoal-controlled karstification

As mentioned earlier, influenced by the regression and large-scale and relatively long exposure, the Qixia Formation in the study area suffered karstification during the early diagenesis stage (Tan et al., 2015). Different lithologic facies, vertical karst zonation, and hydrodynamic conditions lead to different karst features in different intervals of the Qixia Formation (Figure 9).

In the vadose zone in the upper part of the Qixia Formation, karst water initially flows through intergranular pores in high permeability layers under the influence of lithological facies, and then infiltrates significantly due to the gravity, which leads to incomplete dissolution along the horizontal bedding. Pores are gradually dissolved and form three-dimensional networks in the areas with fast water flow rates and strong dissolution (Martin et al., 2006). The networks expanded by dissolution and the matrix preserved are mixed, presenting leopard porphyritic morphology on the cross section. Such kind of dissolution is thought to be related to high porosity and high permeability karst systems, and also regarded as one of the most typical indicators of eogenetic karst (Baceta et al., 2007; Tan et al., 2015). Because the grainstones with weak diagenesis during the early diagenesis stage are vulnerable to discretion due to the influence of karst water (Meyers, 1988; Moore et al., 2010), leopard porphyritic pores and caves are generally loosely filled by in-situ or external carbonate sands. But they still have good reservoir properties, which are beneficial to the later modification of burial dissolution and hydrothermal dolomitization, thus forming the distinctive leopard porphyritic dolomites (limestones) in the upper part of the Qixia Formation.

In the phreatic zone in the middle and upper parts of the Qixia Formation, karst water still dissolves the rocks along high permeability layers, and it can dissolve the bottom thin but tight water-resisting layer, thus forming the vertical channels via the dissolution fractures. Karst water can further influence the underlying high permeability layers. However, as the Qi 1 member is overall tight, and the water-resisting layer is thick, the karst water stays in the middle and upper parts of the Qixia Formation for quite a long time. Moreover, runoff is dominant in the phreatic zone, which leads to the relatively complete dissolution along layers, and thus the quasi-stratiform karst system (Xiao et al., 2016). The dissolution pores and caves loosely filled by carbonate sands and those unfilled commonly constitute the migration channels for hydrothermal fluids, and they form the dolomite reservoirs in the quasi-stratiform karst system.

In conclusion, shoal is the most favorable facies belt. The reservoir space consisting of dissolution pores and caves as well as residual pores and vugs with loosely filling carbonate sands provides favorable conditions and places for the late dolomitization, which results in the present leopard porphyritic dolomites (limestones) and dolomites in quasi-stratiform karst system. Therefore, shoal-controlled karstification is the basic cause and key to the reservoir formation.

Hydrothermal dolomitization

As reviewed above, the Qixia dolomite reservoirs in the Sichuan Basin have been believed to be related to hydrothermal in origin (Jiang et al., 2014). This is also found in this study although the dominant control is shoal-controlled karst as discussed above (Figure 10).

Figure 10.

Macroscopic (outcrop observations) and microscopic (thin-section observations) features showing hydrothermal dolomitization in the Qixia Formation, northwestern Sichuan Basin. See Figure 1 for the location of the outcrops and well. (a) Dissolution channel across strata is filled with carbonaceous mud and dolomite, Banzhucun Section, Qi 2 member; (b) Saddle dolomite inside cave, Chejiaba Section, the bottom of the Qi 2 member; (c) close-up of the white dashed box in (a), leopard porphyritic dolomite limestone; (d) the transition zone between dolomite and limestone, well K 1, 4216.73 m, Qi1 member.

In the late Middle Permian, the E'mei taphrogenic movement gradually reached the maximum (Luo et al., 1988), associated with intense volcanism and abnormal mantle activity (He et al., 2006), which led to the entrance of high temperature dolomitized fluids into the Qixia Formation along deep faults (Jiang et al., 2014). As a result, the dolomites are mainly distributed along fluid conducting channels, including shoal-controlled karst systems (Figure 10(a)–(c)) and stylolites (Figure 8(a)), bedding planes (Figure 8(b)) and fissures (Figure 8(c)). Differentiated dolomitization took place along the early formed channels, and “baking” characteristics can be found in the contact between limestones and dolomites under the microscope (Figure 10(d)), which indicates that hydrothermal fluid flew through favorable channels and dolomitized surrounding rocks.

Taking the formation of the leopard porphyritic dolomites in the Chejiaba and Liaojiapo Sections as examples, it is found that hydrothermal fluids flew through the loose fillings and residual pores and vugs in the leopard porphyritic system, dolomitizing fillings, and surrounding rocks. However, the tight matrix in the non-leopard porphyritic system greatly limited the hydrothermal dolomitization. As for the fact that dolomites are dominant in both the leopard porphyritic and non-leopard porphyritic systems (e.g. well K 2), we infer that the parent rocks might have relatively high porosity and permeability so that fillings or matrix in the karst system were influenced by hydrothermal fluids and evenly dolomitized. Moreover, it should definitely be related to the variation of the amount of hydrothermal fluid supply and the concentration of Mg²⁺ in different areas (Chen et al., 2013; Shu et al., 2012).

Not all the dolomites can act as good reservoirs. The porosity of the matrix limestone unaffected by karstification is not higher than that of the corresponding dolomite (Figure 8(d)), with an increase in porosity only of 0.12–1.04% (8 pair sample data). Originally tight matrix limestone is still tight after dolomitization, and the limited free space leads to the allotriomorphic mosaic characteristics of metasomatic dolomite (Figure 7(f)). However, karstification effects on matrix granular limestone and dolomitization can increase the porosity by 11.08% (Figures 3(j) and 7(f)). Because there are enough free spaces in the loose carbonate sand fillings, metasomatic dolomite presents hypidiomorphic-idiomorphic characteristics (Figures 3(k) and 7(g)), and the reservoir properties of dolomitized matrix are mainly controlled by parent rocks (Figures 3(l) and 7(h)). Therefore, the shoal-controlled karstification should be the key to reservoir modification.

In contrast, the hydrothermal dolomitization in the late Middle Permian and early Late Permian might only contribute to slight increase in porosity (Jiang et al., 2014). Metasomatism results in intergranular (dissolved) pores and vugs through redistribution of pores and vugs in the fillings in the karst cave system, while the reservoir space changes slightly after the dolomitization. Therefore, the dolomitization process might be a volume-constant metasomatic process (Machel, 2004), and its contribution to reservoir may be mainly reflected in the pressure-resisting effect that is conducive to early pore preservation under high temperature and high pressure environment (Ehrenberg, 2004; Sun, 1995). This is featured in two facets. First, reservoir properties of dolomites with residual parent rock fabrics, although worse than those in the karst system, are much higher than those of grainstones (Table 1), and sutures in grainstones are highly developed. Second, although the rocks surrounding dissolution pores and vugs share some static formation pressure, the pressure-resisting rock matrix formed by dolomitization of loose fillings enables the pores and vugs to be preserved to the largest degree, which provides space and channel for adjustment of later burial fluids (Pan et al., 2012).

In conclusion, the exploration of the Qixia Formation in the study area should not pay too much attention to the hydrothermal dolomitization as previously, but to highlight the shoal-controlled karstification.

Conclusions

The dolomite reservoir of the Middle Qixia Formation in the northwestern Sichuan Basin is shoal-controlled karst in origin, which consists of idiomorphic-hypidiomorphic crystalline dolomites in quasi-stratiform karst system, idiomorphic-hypidiomorphic crystalline dolomites in leopard porphyritic karst system, and allotriomorphic mosaic crystalline dolomites with residual parent rock fabrics. Among them, the first type has the best reservoir properties.

The preserved early residual intergranular pores within shoal facies grainstones provide favorable channels for karst water, and thus lay the material foundation for the karstification. The shoal-controlled karstification during the early diagenesis stage is the key to reservoir optimization, and it determines the distribution pattern of high-quality reservoir space. The Qixia Formation dolomites are mostly developed in the areas with hydrothermal fluid conducting channels, and the layers are not stable.

The exploration of the Qixia Formation should highlight the crucial role of shoal-controlled karstification, rather than pay too much attention to the hydrothermal dolomitization as previously. It is a new case of dolomite reservoirs of shoal-controlled karst origin in China.

Footnotes

Acknowledgements

We thank PetroChina Southwest Oil and Gas Field Company for providing data and for granting permission to publish this work.

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: the National Natural Science Foundation of China (Grant No. 41602148).

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