Natural gas migration and accumulation model and favorable exploration targets in Ordovician dolomite in Jingxi,Ordos Basin

Abstract

The Ordovician dolomite reservoir in Ma5⁵–Ma5¹⁰ sub-members in Jingxi in Ordos Basin is a newly discovered field with multiple natural gas pools. The gas accumulation patterns of the reservoir are unclear. Considering the geological background, the genesis, migration, and accumulation of natural gas in Jingxi were studied systematically, and favorable exploration targets were predicted. Natural gas in Ma5⁵–Ma5¹⁰ sub-members is a mixture of Upper Paleozoic and Ordovician products. The Upper Paleozoic coaliferous gas was mainly expulsed downward through the hydrocarbon-providing window where the coal-bearing source rocks made contact with the dolomite reservoirs. The gas then migrated from west to east and accumulated under the condition of lithology variation. The Ordovician petroliferous gas mainly migrated from bottom to top through fractures and mixed with the coaliferous gas in Ma5⁵–Ma5¹⁰ sub-members. The natural gas reservoir formation model was summarized as the migration of gas over a short distance and partial charging into the dolomite reservoirs from the Late Triassic to Middle Jurassic, and the migration of gas over a long distance and massive charging into the dolomite reservoirs during the Late Cretaceous. Ma5⁵ and Ma5⁶ sub-members are the focus of further exploration, and petroliferous gas in Ma5⁷–Ma5¹⁰ sub-members deserves attention. The dolomite reservoirs of the hydrocarbon-providing windows and the east of these locations are the favorable exploration targets.

Keywords

Natural gas genesis natural gas migration favorable gas exploration targets dolomite reservoir Ordovician Ordos Basin

Introduction

In spite of numerous studies and the many models available, the conditions and mechanism of natural gas accumulation in carbonates are still debatable (Borjigen et al., 2014; Tissot and Welte, 1978). Natural gas migration and accumulation patterns and mechanisms, which are considered to be influenced by many factors such as natural fractures, accumulation time, and gas components (Chen et al., 2017; Cumella and Scheevel, 2008; Schoell, 1980; Tang et al., 2000), are crucial for studying gas pool formation and for predicting favorable exploration targets.

Natural gas genesis is a long-term complicated problem, including the thermal maturation of organic matter, microbial generation, biodegradation, thermochemical sulfate reduction, and gas mixing. Although the geochemical characteristics and origin of gas have been intensively studied (Burruss and Laughrey, 2010; Rodrigue and Philp, 2010; Schoell, 1980; Sun et al., 2009; Tang et al., 2000; Wu et al., 2017), the processes of gas accumulation when complex geological processes are involved are still unclear. Because of the complexity of natural gas accumulation, a variety of studies on natural-gas-charging models, accumulation time, and migration characteristics should be considered simultaneously to ensure the reliability of the results. Many marine carbonates in some areas were almost unaffected by tectonic movements, and oil and gas reservoirs were seldom damaged (Halbouty, 2003; Mann et al., 2003; Zhu et al., 2015). However, the marine carbonates in Central China, especially, those in Ordos Basin, have complicated geological characteristics and gas accumulation processes.

The Ordos Basin is the biggest gas-rich basin in China and comprises multiple giant gas fields, such as Jingbian, Jingxi, Sulige, Daniudi, Yulin, and Wushenqi gas fields (Hu et al., 2007; Liu et al., 2012; Wu et al., 2017). The Jingxi gas field is an important and newly discovered prospecting area comprising lower Paleozoic products (Yang and Bao, 2011; Yang et al., 2013). The discovery of multiple gas-rich regions covering Block S345 (namely Block S203 in the oil field enterprise), Block S127, and Block T18 (namely Block T15 in the oil field enterprise) indicates that the Ordovician dolomite reservoirs have a great exploration potential in Jingxi. Although the Jingbian gas field with weathering crust reservoirs in Ma5¹–M5¹⁰ sub-members has been extensively researched (He et al., 2005; Ma et al., 2011; Wang et al., 2009), few convincing conclusions were drawn about natural gas generation and accumulation characteristics in the Ma5⁵–Ma5¹⁰ sub-members in Jingxi, thus limiting the gas exploration activity. In this study, the genesis, migration, and accumulation of the natural gas in Ma5⁵–Ma5¹⁰ sub-members in Jingxi were researched. Moreover, favorable exploration targets in the Ordovician dolomite reservoirs were predicted. This study is important for understanding gas migration and accumulation pattern.

Geological setting

The Ordos Basin in the Western Block of the North China Craton is an intraplate cratonic basin covering an area of 3.7 × 10⁵ km², extending across five provinces, namely, Shaanxi, Gansu, Ningxia, Inner Mongolia, and Shanxi. The Ordos Basin comprises six structural sub-units, namely, the Yimeng Uplift, Weibei Uplift, Western Fold-Thrust Belt, Tianhuan Depression, Jinxi Flexural-Fold Belt, and Yishan Slope (Luo and He, 2008). The Jingxi area is in the west part of the Yishan Slope and is located between the central uplift belt and the Jingbian gas field (Figure 1). During the dispositional stage of Majiagou Formation in early Ordovician, Jingxi was in a typical epicontinental sedimentary environment, and sedimentary carbonate interbedded with gypsum-bearing carbonate developed here under the influence of periodic transgression and regression. The Ma5 Member of Majiagou Formation can be divided into 10 sub-members from Ma5¹⁰ to Ma5¹ from bottom to top. In Ma5⁵, Ma5⁷, and Ma5⁹ sub-members, the following rocks developed: dolomite, limy dolomite, muddy dolomite, and partial gypsum-bearing dolomite, all of which are attributed to the short half-cycle of transgression accompanied by contemporaneous to penecontemporaneous dolomitization during the depositional stage (Liu et al., 2017). In Ma5⁶, Ma5⁸, and Ma5¹⁰ sub-members, rocks similar to those in Ma5⁵, Ma5⁷, and Ma5⁹ sub-members but with more gypsum-bearing dolomite were developed (Figure 2).

Figure 1.

(a) Location of Ordos Basin, (b) structural location of Jingxi, and (c) strata and gas-water distribution of Ma5⁵–Ma5¹⁰ sub-members in Jingxi (modified after Liu and Jiang, 2017).

Figure 2.

Stratigraphic column from Ma4 Member to Benxi group and distribution of source, reservoir and caprock in the Jingxi area (modified after Liu and Jiang, 2017).

The Caledonian movement in the late Ordovician resulted in uplifting and 150 million years of weathering and erosion of the basin (Su et al., 2008). The study area is close to the central uplift. Therefore, the area has suffered serious erosion, resulting in the absence of the strata from Upper Ordovician to Lower Carboniferous. The residual strata exposed become older from the karst depression to the central uplift (Zhao et al., 2012). During the Late Carboniferous, the basin gradually changed from an epicontinental basin to an intracontinental basin, and a set of clastic rock formations were deposited in the study area. These formations were unconformably in contact with the underlying Majiagou Formation. During the Late Cretaceous to Neogene, the basin suffered structural inversion because of the effects of Late Yanshanian movement and Himalayan movement, resulting that the strata in the study area changed from west high–east low to east high–west low.

Characteristics of source rocks and natural gas genesis

Characteristics of source rocks

According to the analysis of the contact relationships between source rocks and reservoirs in Jingxi, the source rocks that are in contact with the Majiagou Formation reservoirs comprise Permo-Carboniferous coal-bearing source rocks and Ordovician carbonate rocks (Cheng et al., 2007; He et al., 2005; Sun et al., 2017; Yang et al., 2011). Because of undeveloped faults in the Majiagou Formation, the natural gas here cannot migrate over a long distance vertically. Thus, the genesis of natural gas in Majiagou Formation is closely related to these two sets of source rocks.

Being paralic sedimentary, Permo-Carboniferous source rocks contain coal-bed and dark mudstone. The thickness of the coal-bed ranges from 4 to 16 m, while that of the dark mudstone ranges from 60 to 100 m. Moreover, the total organic content (TOC) of the coal-bed ranges from 67.8% to 74.7%, while that of the dark mudstone is between 2.17% and 3.26%. The kerogen type of Permo-Carboniferous source rocks is dominated by type III with minor type II₂. Moreover, the source rocks have high thermal evolution and reach the gas-generation threshold.

The source rocks of Ordovician Majiagou Formation have a thickness of 10–125 m. The number of samples with TOC greater than 0.3% only account for 22.39%. The kerogen types are dominated by type I with minor types II₁ and II₂. Moreover, the thermal evolution degree of the Ordovician carbonate source rocks is also high, similar to that of the Permo-Carboniferous source rocks.

Natural gas genesis

Natural gas component

The natural gas in the sandstone reservoirs of Shihezi, Taiyuan, and Shanxi Formations of the Upper Paleozoic is widely distributed in the Ordos Basin. Methane is the dominant component, and the dry coefficients range from 0.8719 to 0.9950. The natural gas in Ma5¹–Ma5⁴ sub-members of the Ordovician Majiagou Formation is mainly distributed in the weathering crust reservoirs in Jingbian gas field. Similarly, the gas is dominated by methane, but the dry coefficient is higher than that of the Upper Paleozoic gas with values ranging from 0.8874 to 0.9967. Ma5⁵–Ma5¹⁰ sub-members in Jingxi gas field contain higher methane content than Ma5¹–Ma5⁴ sub-members and the Upper Paleozoic, and the dry coefficients for Ma5⁵–Ma5¹⁰ sub-members range from 0.9333 to 0.9989. Moreover, the dry coefficients of Ma5⁷–Ma5¹⁰ sub-members in Jingxi are slightly higher than that of Ma5⁵–M5⁶ sub-members (Figure 3).

Figure 3.

Dry coefficient of natural gas in different formations.

Carbon isotope of natural gas

A comparison of the carbon isotopes of natural gas (Figure 4) shows that the carbon isotopes of methane, ethane, and propane in the natural gas in Ma5⁵–Ma5¹⁰ sub-members in Jingxi are close to those of the gas in Ma5¹–Ma5⁴ sub-members and in Shihezi, Taiyuan, and Shanxi Formations of the Upper Paleozoic in the Ordos Basin on the whole. This similarity in the carbon isotopes indicates similar gas genesis. In previous studies, the natural gases in Ma5¹–Ma5⁴ sub-members and in the Shihezi, Taiyuan, and Shanxi Formations of the Upper Paleozoic in the Ordos Basin were considered to be typical coaliferous gas from Permo-Carboniferous coal-bearing source rocks (Dai and Xia, 1999; Hu et al., 2007; Yang et al., 2009). That is, the natural gas in Ma5⁵–Ma5¹⁰ sub-members were mainly generated from the Permo-Carboniferous coal-bearing source rocks. However, in fact, the carbon isotopes of methane and ethane in the natural gas in Ma5⁵–Ma5¹⁰ sub-members are slightly lighter than that of methane and ethane in the natural gas in Ma5¹–Ma5⁴ sub-members and the Upper Paleozoic. Considering the distribution relationship between source rocks and reservoirs and the characteristics of source rocks, the natural gas in Ma5⁵–Ma5¹⁰ sub-members may have mixed with the petroliferous gas from the Ordovician Majiagou Formation. Moreover, the light carbon isotope of the methane and ethane of the natural gas indicates that more petroliferous gas mixed with the coaliferous gas in Ma5⁷–Ma5¹⁰ sub-members than in the Ma5⁵–Ma5⁶ sub-members.

Figure 4.

Comparisons of the carbon isotopes of methane and ethane (a) and ethane and propane (b) in the natural gas.

Migration characteristics of natural gas

There are barely any faults in Ma5⁵–Ma5¹⁰ sub-members in Jingxi, and the intercrystalline pores, fractures, and their combination constitute the major migration pathways of natural gas. Therefore, the development scale, porosity, and permeability of reservoirs play an important role in controlling the migration of natural gas.

Owing to the successive action of seepage refluxing dolomitization and epigenic karstification in Jingxi (Liu and Jiang, 2017; Liu et al., 2017), there exists widely developed dolomite with intercrystalline pores and fractures in Ma5⁵–Ma5¹⁰ sub-members. Moreover, from the west to the east in Jingxi, the development degree of dolomite and the development scale of intercrystalline pores and fractures gradually decrease, leading to a gradual decrease in porosity and permeability (Figure 5). In the western and central part of Jingxi, intercrystalline pores and fractures show a certain connectivity, thus forming favorable migration pathways. In the eastern part of Jingxi, limy dolomite or dolomitic limestone with lower porosity and permeability may seal off natural gas, leading to the accumulation of natural gas. The reservoirs of Ma5⁵–Ma5¹⁰ sub-members have greater porosity and permeability and thus have stronger lateral migration ability than do Ma5⁷–Ma5¹⁰ sub-members. In addition, the development of fractures led to the formation of a vertical migration pathway for natural gas in Ma5⁵–Ma5¹⁰ sub-members. Therefore, the natural gas in Ma5⁵–Ma5¹⁰ sub-members also shows the trend of migration along fractures from bottom to top.

Figure 5.

Comparison of porosity and permeability of the dolomite reservoirs of the Majiagou Formation from Well S127 to Well S310 (from the west to the east) in Jingxi.

According to the geochemical parameters of natural gas in Ma5⁵–Ma5¹⁰ sub-members from the west to the east, the methane contents gradually increase and the methane carbon isotopes gradually become light (Figure 6), reflecting the migration trend of natural gas from the west to the east along the migration pathway within reservoirs.

Figure 6.

Carbon isotopes of methane (a) and methane content (b) indicating migration direction of natural gas in Ma5⁵ sub-member in Jingxi.

Natural-gas-charging time

According to the observation of fluid inclusions in Ma5⁵–Ma5¹⁰ sub-members, fluid inclusions mainly comprise brine and gaseous inclusions, with a few gas–liquid hydrocarbon inclusions (Figure 7(a) and (b)). The size dimension of the inclusions ranges from 2 to 12 µm, and the inclusions are elliptical, round, or in the form of strips. Hydrocarbon inclusions in Ma5⁵–Ma5¹⁰ sub-members are mainly distributed in the coarse-grained calcite cement. Most fluid inclusions have no fluoresce when illuminated with fluorescent light, and only a small amount of the fluid inclusions emit faint blue-white light (Figure 7(c) and (d)).

Figure 7.

Features and distribution of hydrocarbon inclusions of the Majiagou Formation reservoirs in Jingxi. (a) Well T17, 3657.12 m, Ma5⁵, fluid inclusions in filling calcite at the edge of dissolved pore, transmission light; (b) Well T13, 3633.52 m, Ma5⁵, gaseous and associated brine inclusions in coarse calcite at the center of the dissolved pore, transmission light; (c) Well T18, 3703.96 m, Ma5⁵, gaseous-liquid inclusions in coarse calcite at the center of the dissolved pore, transmission light; (d) Well T18, 3703.96 m, Ma5⁵, gaseous-liquid inclusions in coarse calcite at the center of the dissolved pore, fluorescence light; (e) Well L30, 4058.62 m, Ma5⁹, gaseous and brine inclusions in coarse calcite at the edge and center of the dissolved pore, transmission light; (f) Well L30, 3965.62 m, Ma5⁵, gaseous and associated brine inclusions at the edge and center of the dissolved pore, transmission light.

Considering the homogenization temperature of fluid inclusions and the order of the formation of minerals, two types of homogenization temperature distributions of the hydrocarbon inclusions in the cements were discerned. First, for the same sight under a microscope, the homogenization temperatures of fluid inclusions in the cements at the edge of pores are lower than that of the terminally formed inclusions in the cements at the center of pores, thus indicating that the formation of cements and the charging of the natural gas occurred in the continuous buried stage before Late Cretaceous tectonic uplift. Second, the homogenization temperatures of fluid inclusions in the cements at the edge of pores are higher than that of the terminally formed inclusions in the cements at the center of pores, thus indicating that the formation of the cements at the edge of the pores and the early charging of natural gas occurred at a burial depth before Late Cretaceous tectonic uplift, while the formation of cements at the center of the pores and the later charging of natural gas occurred during the tectonic uplift. Consider the example of Well L30, the gas charging time was analyzed along with the thermal history of the reservoir based on the homogenization temperatures of brine inclusions associated with hydrocarbon inclusions (Figure 8). The results show that the homogenization temperatures of these brine inclusions in the cements at the edge of pores range from 135℃ to 175℃, and the temperatures are mainly distributed in the range 145℃–155℃, corresponding to the continuous buried stage in the Middle Jurassic to the Late Triassic (197–170 Ma). The homogenization temperatures of the associated brine inclusions in the cements at the center of pores range from 135℃ to 145℃, corresponding to gas charging during the late tectonic uplift stage (75–65 Ma).

Figure 8.

Determination of the charging time of the natural gas of the Majiagou Formation reservoir in Jingxi.

During Late Triassic–Middle Jurassic, the Permo-Carboniferous coal-bearing source rocks reached the stage of hydrocarbon generation. However, the structural background (high in the west and low in the east) was not conducive for the charging of natural gas into Ma5⁵–Ma5¹⁰ sub-members. Thus, the early charging quantity of natural gas was limited. The study area experienced a strong hydrothermal movement during the Late Cretaceous–Neogene (Wang et al., 2014), promoting the source rocks to generate hydrocarbons. Besides, Ma5⁵–Ma5¹⁰ sub-members experienced tectonic inversion, from west high–east low to east high–west low, during this period; this favored natural gas migration from the west to the east. The present gas distribution matches the gas migration during the Late Cretaceous–Neogene. Therefore, the Late Cretaceous is considered the major gas charging period in the study area.

Natural gas reservoir formation model

The gas pools are dominated by lithologic gas pools and lithologic-structural gas pools in the dolomite reservoirs of Ma5⁵–Ma5¹⁰ sub-members in Jingxi. The gas pools are characterized by natural gas partially charging in the early stage and natural gas massively charging in the late stage. Moreover, this article puts forward the natural gas reservoir formation model: migration of gas over a short distance and then partial charging into dolomite reservoirs from the Late Triassic to Middle Jurassic, along with migration of gas over a long distance and massive charging into dolomite reservoirs during the Late Cretaceous (Figure 9).

Figure 9.

Natural gas reservoir formation model of Ma5⁵–Ma5¹⁰ sub-members in Jingxi. (a) The period from Late Triassic to Middle Jurassic and (b) the period of Late Cretaceous (modified after Liu et al., 2016).

Late Triassic to Middle Jurassic

The natural gas in the central part of Jingxi was dominantly generated by the Upper Paleozoic coal-bearing source rocks. This gas migrated into reservoirs through the charging window where Ma5⁵–Ma5¹⁰ sub-members were in direct contact with the coal-bearing source rocks. During the Late Triassic–Middle Jurassic, Ma5⁵–Ma5¹⁰ sub-members in the area were under the tectonic background of west high–east low, and the natural gas generated in the Upper Paleozoic source rocks migrated over a short distance downward to the reservoirs and accumulated at the top of the reservoir. Moreover, the source rocks of the Majiagou Formation also generated a certain amount of oil and gas, which mainly migrated and accumulated in the lithologic traps within the reservoir.

Late Cretaceous

During the Late Cretaceous, the study area was tectonically inversed. Good reservoirs were formed by the effect of tectonization and dissolution, and intercrystalline pores and fractures became major migration pathways for the natural gas. The natural gas generated in the Upper Paleozoic source rocks migrated over a long distance from the west to the east and accumulated in the favorable traps, which play an important role in the present gas distribution. In addition, multiple fractures were present in Ma5⁵–Ma5¹⁰ sub-members, and the natural gas generated from the Majiagou Formation migrated and mixed with the natural gas from the Upper Paleozoic source rocks.

Favorable natural gas exploration targets

The natural gas reservoir formation model suggests two types of favorable exploration targets in Ma5⁵–Ma5¹⁰ sub-members in Jingxi, including the dolomite reservoir in the hydrocarbon-charging window and that to the east of the hydrocarbon-charging window.

The natural gas in Ma5⁵–Ma5¹⁰ sub-members was mainly generated by the Upper Paleozoic source rocks, and thus, the charging ability of natural gas is the key to control gas enrichment. S345 and S217 gas-bearing areas in the central part of the study area are favorable natural-gas-charging windows, followed by T18 and Z18 gas-bearing areas in the north; the southern ZT1 gas-bearing area is relatively poor. The dolomite reservoirs at the charging window were favorable sites for the migration and accumulation of natural gas in the Late Triassic–Middle Jurassic, while the dolomite reservoirs to the east of the charging window were the favorable sites for the migration and accumulation of natural gas in the Late Cretaceous.

Owing to the influence of the contact relationships between source rocks and reservoirs, the natural-gas-charging ability varies among different sub-members. The amount of natural gas from the Upper Paleozoic that charged into Ma5⁵–Ma5⁶ sub-members exceeds the amount that charged into Ma5⁷–Ma5¹⁰ sub-members. Moreover, part of the natural gas that charged into Ma5⁷–Ma5¹⁰ sub-members may have been vertically transported into Ma5⁵–Ma5⁶ sub-members along fractures, leading to larger gas accumulation in Ma5⁵–Ma5⁶ sub-members than in Ma5⁷–Ma5¹⁰ sub-members. Thus, Ma5⁵–Ma5⁶ sub-members are the key intervals for further exploration in Jingxi. In addition, Ma5⁷–Ma5¹⁰ sub-members contain more natural gas from the Ordovician source rocks. Thus, the related intervals are the key exploration targets for petroliferous gas pools as well.

Conclusions

The gas in Ma5⁵–Ma5¹⁰ sub-members in Jingxi is mainly generated by the Upper Paleozoic coaliferous gas, which is mixed with the petroliferous gas from the Ordovician. The ratio of coaliferous gas to petroliferous gas in Ma5⁵–Ma5⁶ sub-members is higher than that of the two gases in Ma5⁷–Ma5¹⁰ sub-members.

Natural gas in the dolomite reservoirs migrated from the west to the east, and then accumulated when the physical properties of the dolomite reservoirs worsened.

Gas charging occurred in two stages, from the Late Triassic to Middle Jurassic and in the Late Cretaceous, in Ma5⁵–Ma5¹⁰ sub-members in Jingxi. The natural gas reservoir formation model was summarized as the migration of gas over a short distance and partial charging into the dolomite reservoirs from the Late Triassic to Middle Jurassic, and the migration of gas over a long distance and massive charging into the dolomite reservoirs during the Late Cretaceous.

Based on a comprehensive analysis, S345 and S217 gas-bearing areas in the central part of the study area and the dolomite reservoir in the east too are favorable exploration targets. Ma5⁵–Ma5⁶ sub-members are predicted as favorable exploration intervals, and the mostly petroliferous gas pools in Ma5⁷–Ma5¹⁰ sub-members also deserve attention.

Footnotes

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 is granted by the Foundation of State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum, Beijing (No. PRP/open-1604).

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