Sequence stratigraphic analysis of superimposed coal measure gas-bearing system in Daning-Jixian block, eastern margin of Ordos Basin, China

Abstract

The identification of superimposed gas-bearing systems in coal measures is the basis for expediting the optimization of coal measure gas co-production. Through the analysis of drill cores and log data of Upper Carboniferous Benxi Formation to the member 8 of Middle Permian Lower Shihezi Formation in Daning-Jixian block, eastern margin of Ordos Basin, four distinct superimposed coal measure gas-bearing systems were identified, and their formation mechanism was discussed from the sequence stratigraphic perspective. Type I system mainly contains multiple coal seams, shales and sandstone layers. Type II system is dominated by multiple coal seams and shales. Type III is characterized by multiple sandstone layers, and type IV system is dominated by limestones and mudstones. In general, the gas-bearing systems deposited in barrier-lagoon are type II, those deposited in carbonate tidal flats are type IV, and those deposited in the delta front are types I and III. The marine mudstone, acting as a key layer near the maximum flooding surface, exhibits very low permeability, which is the main factor contributing to the formation of superimposed gas-bearing systems. The sedimentary environment plays a significant role in controlling the distribution of gas-bearing systems. Notably, the vertical gas-bearing systems in the south-western region, where delta front and lagoon facies overlap, are more complex than those in the north-eastern delta front facies.

Graphical abstract

Keywords

coal measure gas / superimposed gas-bearing system / sequence stratigraphic / key layer

1 Introduction

In recent years, China᾽s energy sector has paid extensive attention to the commercial potential of coal measure gas resource (Zou et al., 2019; Su et al., 2020). However, in coal measures, the reservoirs and cap layers are frequently superimposed, resulting in the coexistence of multiple pressure systems and the compatibility issues for co-production, limiting gas production from individual wells (Li et al., 2023a). Therefore, identifying gas-bearing systems is crucial for efficient development of coal measure gas.

Previous studies on the superpositionality of gas-bearing systems has been carried out from various perspectives, including reservoir pressure (Powley, 1990; Chen et al., 2018a), gas content vertical fluctuations (Lei et al., 2012; Wang et al., 2015), hydrogeological conditions (Su et al., 2005; Yang et al., 2015), production dynamics (Tang et al., 2017; Jia et al., 2021), and sedimentary stratigraphy (Martin et al., 1997; Allen and Fielding, 2007; Wang et al., 2020). The study of sequence stratigraphy under epicontinental-sea environments further reveals the formation mechanism of superimposed gas-bearing systems in coal measures. Depositional systems in such environments differ from river-dominated systems due to frequent influences from waves and tides (Cooper et al., 2018; Nowacki and Ganju, 2018; Raff et al., 2018), resulting in the formation of sequence stratigraphy with certain rhythms in the coal-bearing basins at different evolutionary stages. Differences in the systems tracts affect the environment, characteristics, and intensity of coal accumulation and also make the lithology of associated rocks significantly different (Flores and Sykes, 1996; Diessel et al., 2000; Holz et al., 2002; Wang et al., 2019). These variations affect the gas content and seepage capacity of coal reservoirs (Shen et al., 2016), and the development of water-barrier and gas-barrier layers separates gas-bearing units in coal measures, forming distinct gas-bearing systems (Shen et al., 2017). Due to the complexity of geological conditions, there are significant differences in distribution, overlap frequency, and controlling factors of coal measures gas-bearing systems in different regions. However, there is still a need for systematic and in-depth discussions on superimposed gas-bearing systems of coal measures gas from a sedimentary perspective.

In this study, the vertical distribution characteristics of geological conditions are analyzed in details to identify the superimposed gas-bearing systems in Daing-Jixian block of the Ordos Basin. Combined with sequence stratigraphic framework division and sedimentary facies analysis, the formation mechanism of superimposed gas-bearing systems in coal measures is clarified. The outcome of this study is important for optimization of coal measure gas co-production.

2 Geological setting

The Daning-Jixian block is located in the southeast of Ordos Basin, specifically in the southern part of Jinxi flexure fold belt on the eastern margin of the basin (Zhong et al., 2022; Li et al., 2023b; Fig.1(a)). The strata in Daning-Jixian block are predominantly horizontal orientation, with localized faults development. The study area is about 25 km long from north to south, 22 km wide from east to west, with a total area of about 5.5 × 10² km² (Fig.1(b)).

The coal measures of the Daning-Jixian block consist of the Upper Carboniferous Benxi (C₂b), Taiyuan (C₂t), the Permian Shanxi (P₁s), and the member 8 of Lower Shihezi (P₂x₈) Formations. C₂b was deposited in the early Late Paleozoic, and C₂t is in direct contact with the underlying C₂b. The limestone at the bottom of C₂t is Maoergou limestone, and the limestone at the top is known as Dongdayao limestone (Wang et al., 2022). Peitsakou sandstone is a marker layer at the bottom of P₁s, with gray pebbled sandstone and quartz sandstone. Lotopo sandstone is a marker layer at the base of P₂x₈. A total of 10 coal seams are developed in the area, among which No. 5 coal seam of P₁s and No. 8 coal seam of C₂t are stably distributed, which are the main coal seams in the area. In contrast, the other coal seams have significant structural changes and are varying in thicknesses. The thickness of No. 5 coal seam in P₁s is 1.5−7.2 m, with an average thickness of 3.18 m. The thickness of No. 8 coal seam in C₂t is 2.0−9.8 m, with an average thickness of 4.5 m (Fig.1(c)).

3 Identification of superimposed gas-bearing systems

3.1 Reservoir pressure coefficient

The main characteristic of an independent gas-bearing system is its well-integrated fluid pressure system (Li et al., 2023a). The Eaton method is an empirical method for predicting formation pressure coefficient by using the actual measured formation pressure as the constraint (Eaton,1972). By establishing the normal compaction trend line, the magnitude of formation pore pressure is calculated for mudstone and sandstone formations when the actual logging data deviate from the normal compaction trend line. The formulas are as follows:

(1)P_f=OBG-((OBG-P_n)\times (\mathrm{d}{T}_n/\mathrm{d}{T}_o{)}^c),

(2)c=\frac{\mathrm{l}\mathrm{n}(p_0-p_{\mathrm{p}})/(p_0-p_{\mathrm{a}})}{\mathrm{l}\mathrm{n}(\mathrm{d}{T}_{\mathrm{n}}/\mathrm{d}{T}_{\mathrm{o}})},

where {\rho }_{\mathrm{p}} is the strata pressure equivalent density, g/cm³; {\rho }_0 is the overlying strata pressure equivalent density, g/cm³; {\rho }_{\mathrm{w}} is the density of stratified water, g/cm³; \text{d}{T}_{\mathrm{n}} is the normal compaction acoustic time difference, µs/m; \mathrm{d}{T}_{\mathrm{o}} is the measured acoustic time difference, µs/m; c is the Eaton coefficient, P_0 is the overburden pressure, MPa; P_{\mathrm{P}} is the measured pressure, MPa; and P_{\mathrm{a}} is the hydrostatic pressure, MPa.

The comparison of the calculations with the measured pressure coefficients shows that the error is less than 10% (Fig.2).

In this study, the upper Paleozoic pressure in the basin was classified into four types: abnormally low pressure (pressure coefficient < 0.9), low pressure (pressure coefficient between 0.9 and 0.96), normal pressure (pressure coefficient between 0.96 and 1.06), and high pressure (pressure coefficient between 1.06 and 1.2). Calculations show that the pressure coefficient of C₂b to P₂x₈ in the study area ranges from 0.36 to 1.10, with a concentration of between 0.75 and 0.98, and with low pressure and normal pressure predominant. Pressure coefficients stays constant from the top of C₂b to the top of the member 2² of Shanxi Formation (P₁s_2-2) and sharply decrease to a low value from the top of P₁s_2-2 to the bottom of the member 2¹ of Shanxi Formation (P₁s_2-1). Pressure coefficients stays constant from the bottom of P₁s_2-1 to the top of the member 1 of Shanxi Formation (P₁s₁) and sharply decline from the top of P₁s₁ to the bottom of P₂x₈ (Fig.3). Therefore, the pressure system from C₂b to P₂x₈ in the study area can be divided into three separate gas-bearing systems: 1) from the top of C₂b to the top of P₁s_2-2, 2) from the bottom of P₁s_2-1 to the top of P₁s₁, and 3) from the bottom of P₂x₈ to the top of P₂x₈.

3.2 Gas content

The vertical variation of gas content serves as a crucial indicator to classify the vertical gas system (Zhang et al., 2015; Chen et al., 2018b). Based on the statistics of gas measurement data, the distribution of total hydrocarbon is depicted to characterize the vertical variation of gas-bearing property. According to the relationship between total hydrocarbon value and burial depth in A4 wells, it can be seen that the total hydrocarbon value can be divided into four stages: 1) from the top of C₂b to the top of the member 2 of Taiyuan Formation (C₂t₂), 2) from the bottom of the member 1 of Taiyuan Formation (C₂t₁) to the top of P₁s_2-2, 3) from P₁s_2-1 to the top of P₁s₁, and 4) from the bottom of P₂x₈ to the middle of P₂x₈. The total hydrocarbon values increase and then decrease with increasing burial depth, with noticeable drop-off sections between each section. Moreover, the magnitude of vertical fluctuation of gas content varies, indicating that the independent gas-bearing systems are developed in different layers (Fig.4).

3.3 Fluid geochemical characteristics

The fluid geochemical characteristics could be used to distinguish gas-bearing systems (Guo et al., 2020; Zhang et al., 2022). A comparison of natural gas carbon isotope test results from 10 wells in the study area shows that the average δ¹³C₁ value of C₂b in Daning-Jixian block is −31.7‰. The average δ¹³C₁ value of P₁s_2-3 is −27.27‰. The average δ¹³C₁ value of P₁s₁ is −26.03‰ and the average δ¹³C₁ value of P₂x₈ is −24.6‰ (Tab.1).

The carbon isotopes of methane in the study area progressive increase in weight from C₂b to P₂x₈ (Fig.5). It is evident from the figure that there is a pronounced differentiation in carbon isotope composition among the natural gas samples in the Daning-Jixian block. The carbon isotopes of natural gas are significantly heavier in P₁s_2-3 than in C₂b, suggesting that the fluid in P₁s_2-3 may not have communicated with the fluid in C₂b. The carbon isotopes of natural gas are significantly heavier in P₁s₁ than in P₁s_2-3, and they are heavier in P₂x₈ than in P₁s₁. As a result, between P₁s₁ and P₁s_2-3, and between P₂x₈ and P₁s₁, low permeable layers could hinder the fluid communication between the overlying and underlying strata.

3.4 Gas-bearing system identification

Ensuring the accuracy of gas-bearing system classification in the study area requires a comprehensive assessment of multiple factors, including the consistency of reservoir pressure coefficients and natural gas geochemical characteristics. Although this consistency is necessary, it is not sufficient on its own. Therefore, it is important to consider the vertical differentiation of pressure coefficients in coal measure formations, gas content in reservoirs, and natural gas geochemical characteristics.

According to the variation of pressure coefficient of coal measures, gas content and vertical divergence of natural gas geochemical characteristics in Daning-Jixian block, the coal measures from C₂b to P₂x₈ can be classified into four independent gas-bearing systems. They are from the top of C₂b to the top of C₂t₂, from the bottom of C₂t₁ to the top of P₁s_2-2, from the bottom of P₁s_2-1 to the top of P₁s₁, and from the bottom of P₂x₈ to the middle of P₂x₈ (Fig.6). Each of these gas-bearing systems is separated by confining layers, indicating isolation from one another. Additionally, the geochemical characteristics of natural gas differ significantly among these systems, while the pressure coefficients remain relatively consistent within each individual gas-bearing system.

4 Sequence stratigraphic analysis of superimposed gas-bearing systems

A complete gas-bearing system comprises three key elements: source rocks, reservoir, and seal rocks (Peters and Cassa, 1994; Ayers, 2002; Law, 2002). The effective source rocks in coal measure mainly include coal and organic-rich mudstone, and the reservoirs mainly include coal seams, coal seams-mudstone interbedding, mudstone-sandstone interbedding, and coal seams-mudstone-sandstone interbedding.

4.1 Sequence stratigraphic framework characteristics and sedimentary facies

According to base level recognition standard, the sequence boundaries in the study area are as follow.

1) Regional unconformity surface. The regional unconformities formed by the paleotectonic movement is an isochronous surface, mainly marked by the weathered crustal bauxite on the top surface of the Middle Ordovician limestone.

2) Incised valley (Shanley and McCabe, 1994). Some of the large-scale distributed sandstone bodies in the area are generally undercut valley-fill deposits of low stage, and their base is an erosional unconformity with graded bedding of medium to fine sandstones (Hou et al., 2023a). The study area is dominated by the Jinci Sandstone (K1) at the base of C₂t, the Beichakou Sandstone (K7) at the base of P₁s, and the Lotopo Sandstone at the base of P₂x₈.

3) Maximum flooding surfaces. The coal seams and limestone in C₂t in the study area are important markers.

Because the depositional characteristics of different sedimentary cycles show large differences, which have a great impact on the delineation of the superimposed gas-bearing systems in Daning-Jixian block, it is important to clarify the control of sedimentary cycles on the coal-measure gas-bearing system. Based on the identification of the base level from C₂b to P₂x₈ in Daning-Jixian block and combined with the sequence stratigraphy at the eastern margin of the Ordos Basin (Yang et al., 2005; Yang et al., 2010; Fu et al., 2021), 9 long-term cycles and 14 short-term cycles were delineated (Fig.7), among which 5 long-term ascending semi-cycles and 4 long-term descending semi-cycles were developed.

The sedimentary environment controls coal accumulation, lithology and diagenetic facies of coal-bearing strata (Lu et al., 2017). Therefore, the identification of sedimentary facies types is the basis for ascertaining the significance of the sedimentary environment on the gas-bearing system of coal measure. By analyzing the data collected from over 50 wells and sedimentary facies markers, two sedimentary facies, four subfacies and seven microfacies were identified from C₂b to P₂x₈. The types of sedimentary facies are barrier-coast facies and shallow-water delta facies. The barrier-coast facies mainly develop tidal flats, lagoons and barrier islands. The tidal flat facies can be subdivided into mud flat, sand flat, mixed flat, carbonate tidal flat and peat swamp. The delta front subfacies can be divided into underwater distributary channel, distributary bay, mouth bar and peat swamp (Tab.2).

During the SQ2 period, the whole area was dominated by the barrier-lagoon facies, and seawater invaded the study area from the east. Localized carbonate tidal flat facies were developed in the middle of the study area, and barrier sand bars were developed in specific areas (Fig.8(a)). In the SQ3 period, seawater continued to intrude from the east and south, resulting in the development of barrier-lagoon phase. Two barrier sand bars were developed in the south-east and north-east corners (Fig.8(b)). During the period of SQ6, seawater retreated. The southern and northern provenance areas were uplifted, and a wide range of delta facies was developed (Fig.8(c)). In the SQ8 period, the sedimentary environment was similar to that in the SQ6 period, and the delta front facies was developed under the influence of the south-north provenance. The mouth bar is more developed, forming a high-quality sand zone, and the catchment is enriched with far sand bar (Fig.8(d)).

4.2 Sedimentary environment of gas-bearing systems

The coal measures were formed in the sedimentary environment of the terrestrial-marine transitional environment and experienced multiple water transgressive and regressive cycles (Petersen and Andsbjerg, 1996). As a result, the storage capacity of coal and sandstone reservoirs significantly varies, giving rise to distinct assemblages of source, reservoir, and seal rocks. This diversity of rock types ultimately determines the presence of different gas-bearing systems within the area.

According to the assemblage of coal seams, shales, and nearby sandstones, four types of gas-bearing systems are identified.

Type I: System developing multiple coal seams, shales, and sandstone layers. Frequent overlapping of thin source rocks (coal or shale) with adjacent reservoirs increases the contact area, facilitating gas expulsion and conversion into free gas. In addition, the thin interbedded reservoirs with different lithologies are favorable to the development of natural fractures and the formation of high-permeability superposed reservoirs (Zou et al., 2022). In this case, multiple coal seams and sandstones are superimposed, reflecting the cyclicity of sedimentary environment evolution. The complex changes in the pore structure of coal significantly influence the physical properties of coal reservoirs (Hou et al., 2023b). Coal seams and nearby sandstones act as reservoirs, accumulating hydrocarbon gases generated from coal seams, which are coordinated with the appropriate regional cap rock to accumulate the coal-measure gas (Fig.9(a)).

Type II: System only developing multiple coal seams and shales. In general, mudstones and siltstones are superior to sandstones, sand-mud interbeds, and carbonates for coalbed methane preservation (Hou et al., 2019). In this case, the sandstone layers are underdeveloped or not developed, which reflects that the sedimentary environment has been in the marsh environment for a long time and the supply of terrigenous detrital is not sufficient. The well-developed coal seams and shales provide ample gas sources, while the thick mudstones offer suitable sealing conditions for coal measure gas preservation (Fig.9(b)).

Type III: System develops multiple sandstone layers. The scarcity of coal seams and the prevalence of sandstone layers above and below them signify an abundant supply of terrigenous detrital. Hydrocarbon gases from coal are stored directly in nearby sandstones (Fig.9(c)).

Type IV: System develops multiple limestones or mudstones. The combination is generally formed in the sedimentary environment of carbonate platform with favorable sealing conditions. Limestones are widely developed, while sandstones are less developed. Single coal or shale gas reservoir could form in this type of environment (Fig.9(d)).

The sediments in the barrier-lagoon facies generally consist of fine clastic rocks and mudstones. The sandstones are mostly found in barrier bars and sand flats, which are generally located in a few locations. The gas-bearing systems are more likely to form type II assemblages. The mudstone on the roof and floor of coal seams is developed, and the capping ability is strong.

In carbonate tidal flat facies, the coal seam is generally thin and the sandstone is not developed. The gas-bearing systems are more likely to form type IV assemblages. The roof of coal seams is mostly carbonate rock.

In the delta front phases, the underwater distributary bay is a favorable environment for coal accumulation, and the continuity of coal seams is good. At the same time, distributary channel sandbody and mouth bar sandbody are widely developed. The gas-bearing systems are mostly composed of type I and III.

4.3 Impact of sequence stratigraphy on distribution of key layers

The degree of superpositionality of gas-bearing systems in coal measures is controlled by the sequence stratigraphic framework (Shen et al., 2016). Through data surveys and the analysis of gas-bearing systems in individual wells within the study area, it has been observed that the strata contain low-permeable mudstone and siderite, known as key layers. These key layers in the gas-bearing systems served as barriers during the formation of independent gas-bearing systems (Shen et al., 2019). The key layers are generally found in close proximity to the maximum flooding surface. This occurrence can be attributed to the weak reducing environment near the maximum flooding surface, characterized by iron-rich, oxygen-poor conditions, limited hydrodynamic activity, and an ample supply of CO₂. Such conditions favor the formation of low-permeability mudstone containing siderite. Consequently, it is feasible for key layers to exhibit a continuous distribution within the ascending cycle.

The distribution of the sedimentary system under the restriction of the sequence framework controls the distribution of the independent gas-bearing system. In the lagoon-tidal flat facies, the number of sedimentary cycles is high and the ratio of sand to mud is low. The sediments are dominated by fine-grained clastic rocks, which reflects low depositional environmental energy. Additionally, limestone is more developed and coarse-grained clastic rocks are rare under the influence of seawater. The low permeability mudstone and frequent depositional cycles in this phase impede fluid flow between gas-bearing assemblages and the independent gas-bearing system is well developed. Compared with the lagoon-tidal flat facies area, the delta front facies area has a coarser sediment size and higher sand-mud ratio. The number of sedimentary cycles is low and the thickness of individual cycles is large. the surrounding rock of coal seams acts as a barrier, preventing coalbed methane leakage. These sedimentary characteristics facilitate fluid connections between gas-bearing assemblages and contribute to relatively unified fluid pressure systems.

Five first-order key layers and two locally developed second-order key layers were identified based on the stratigraphic sequence. The first-level key layers are located near the maximum flooding surface of SQ2, SQ3, SQ6, SQ7, and SQ8, respectively, named Ky1, Ky2, Ky3, Ky4, and Ky5 from bottom to top (Fig.10). Ky1 is located at the bottom of C₂t, with a thickness of 2−11 m, and consists of mudstone and silty mudstone. Ky2 occurs at the top of C₂t, with a thickness of 2−12 m, and is composed of mudstone, silty mudstone and limestone. Ky3 is located at the top of P₁s₂, with a thickness of 3−15 m, and is composed of mudstone and silty mudstone. Ky4 is located at the top of P₁s₁, with a thickness of 3−14 m and is composed of mudstone and silty mudstone. Ky5 is located in the central part of P₂x₈ and is composed of mudstone and silty mudstone. The secondary key layer is located in the south-west of the study area. The two key layers are located in the period of SQ7 and SQ4, respectively, with a thickness of 2−8 m, and consist of mudstone and silty mudstone. Changes in the pressure coefficient, gas content, and fluid geochemical characteristics of the coal measures are in good accordance with the distribution characteristics of the key layers.

The north-east part of the study area is dominated by delta front sedimentary facies with weak sealing properties and relatively simple gas-bearing systems. In the south-west, the delta front and lagoon-tidal flat overlapped, forming more complex gas-bearing systems (Fig.10).

5 Conclusions

Based on variations of the reservoir pressure coefficient, gas content, and natural gas carbon isotope, the coal measures from C₂b to P₂x₈ in Daing-Jixian block can be classified into four independent gas-bearing systems. These systems are identified as 1) from the top of C₂b to the top of C₂t₂, 2) from the bottom of C₂t₁ to the top of P₁s_2-2, 3) from the bottom of P₁s_2-1 to the top of P₁s₁, and 4) from the bottom of P₂x₈ to the middle of P₂x₈.

The four gas-bearing systems exhibit different formation combinations and sedimentary environments. Type I system develops multiple coal seams, shales, and sandstone layers. Type II is characterized by multiple coal seams and shales. Type III is dominated by multiple sandstone layers, while type IV is primarily composed of limestones or mudstones. Gas-bearing systems deposited in barrier-lagoon are mainly composed of type II, those deposited in carbonate tidal flats are mostly composed of type IV, and those in the delta front are mostly composed of types I and III.

Sequence stratigraphic framework controls the distribution of key layers, which are generally found in close proximity to the maximum flooding surface. The sedimentary environment contributes significantly to the distribution of gas-bearing systems. Compared with the delta interdistributary bay, the tidal flat-lagoon phase has greater water depth and a stronger reducing property. leading to the formation of several key layers. Consequently, the vertical gas-bearing systems located in the overlapped environment of delta front and lagoon in the south-west of the study area are more complex than those located in delta front facies in the north-east of the study area.

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