The Carboniferous Taiyuan Fm, the Permian Shanxi Fm and Xiashihezi Fm of the upper Paleozoic in the Daniudi gas field are a set of typical marine facies tidal flat–offshore delta–continental braided river sedimentary sequences (Fu et al., 2013; Yang and Liu, 2014), with thick reservoir sands and high lateral continuity (Figure 3). In the sedimentary period of Taiyuan Fm, the area was in an epicontinental sedimentary environment. Large-scale barrier bars formed in the study area, with an extension length of 50 km and a dominant sand thickness varying 6 m–20 m; these barrier bars served as effective pathways for gas migration. In the area with small sand thickness around barrier bars, the reservoir densification is more obvious due to the strong compaction. While the densification of thick sand reservoir is mainly related to the quartz overgrowth. In the period of the Mbr 1 of the Shanxi Fm, the study area was in a coastal plain environment, and shore–delta sedimentary systems were developed. Reservoir sands are mainly distributary channel–underwater distributary channel coarse sandstones, and the sandstone thickness is comparatively large in the western study area. Delta plain sediments were mainly developed in the period of the Mbr 2 of the Shanxi Fm; thick-bedded distributary channel sands are distributed in the centre and north. Similar to Taiyuan Formation, the area with thin sand and high shaliness has strong compaction and strong densification. In the sedimentary period of Xiashihezi Fm, the paleoclimate turned dry and multistage braided sands were superpositioned and mainly developed in the Daniudi gas field (Ren et al., 2007). River channels were thick and migrated frequently, and the reservoir sands were 30–80 m thick. In this period, the thickness of sandstone is large, and the reservoir has strong anti-compaction ability. The reservoir densification of Xiashihezi Fm is mainly caused by multi-stage and multi-type cementation.

The upper Paleozoic reservoir rocks of the Daniudi gas field are comprised by medium–coarse-grained lithic sandstone, lithic quartz sandstone and quartz sandstone. The Taiyuan Fm is comprised by quartz sandstone (Figure 4A), while the Shanxi Fm and Xiashihezi Fm are comprised by lithic sandstone and lithic quartz sandstone, with minor quartz sandstone and feldspar lithic sandstone (Figures 4B, C). Clastic quartz includes monocrystalline quartz and polycrystalline quartz; the lithic fragments are mainly epimetamorphic rocks, such as slate and phyllite, with secondary volcanic clasts. The lithic fragments are mostly plastic and underwent intense compaction and then deformation (Figure 5A). Fillings in the reservoirs are mainly matrix and various authigenic cements, with an average content of 10%–15%. The matrix includes argillaceous components, such as kaolinite and illite, with an average content of 8%; the cements mainly include multistage authigenic quartz, calcite and ferrocalcite (Figures 5B–E), with minor ankerite, siderite, chlorite and kaolinite. Microscopy shows that clastic grains have close contact relations, such as linear or concavo-convex, and moderate sorting, subangular and porous cementation (Figure 5F).

In accordance with the statistics of reservoir physical property data, the upper Paleozoic reservoir in the Daniudi gas field has a porosity of 2%–12% with an average of 6.9% and permeability of 0.01 × 10⁻³–10 × 10⁻³ μm² with an average of 0.76 × 10⁻³ μm² (Figures 4D, E), indicating a low-porosity and ultralow-permeability tight reservoir. In various gas sets, reservoir properties are mostly favourable in the Mbr 2 and Mbr 3 of the Xiashihezi Fm (Table 1). The upper Paleozoic reservoir underwent long-term diagenetic reworking; the reservoir space are predominantly secondary pores, including intergranular dissolved pores, intragranular dissolved pores and matrix dissolved pores, with secondary residual primary intergranular pores (Figures 5G, H). Secondary pores are mainly quartz-dissolved pores and secondarily feldspar and lithic-dissolved pores, thereby indicating the reservoirs of the study area have two diagenetic fluid environments, i.e., acidic and alkaline. In addition to pores, there are also some micro-fractures in the study area. The micro-fractures are mainly the granular boundary fractures, which are often curved shape and partially dissolved (Figure 5I).

Statistics of mercury-injection parameters imply that the sandstones in the study area have an average pore throat radius of 0.33 μm and a sorting coefficient of throats varying from 0.95 to 5.95 with an average of 3.54. This result suggests that most of the sandstones have poorly sorted pore throats and are dominated by combinations of micropores and microthroats (Table 2). The Carboniferous–Permian reservoir has a displacement pressure varying from 0.01 MPa to 40 MPa; the Xiashihezi Fm has minimum average displacement pressure, followed by the Shanxi Fm and then the Taiyuan Fm This variation tendency reflects that reservoir densification intensifies as the burial depth increases.

The Carboniferous–Permian reservoir of the Daniudi gas field has a median capillary pressure of 1.04–112.1 MPa. This capillary pressure averages 32.84 MPa in the Xiashihezi Fm, 36.8 MPa in the Shanxi Fm and 19.67 MPa in the Taiyuan Fm, generally displaying opposite characteristics to displacement pressure and decreasing as the burial depth increases. The median pore throat radius averages from 0.007 μm to 0.705 μm, which is 0.12 μm in the Xiashihezi Fm, 0.075 μm in the Shanxi Fm and 0.21 μm in the Taiyuan Fm. The maximum average pore throat radius is found in the Taiyuan Fm, and the minimum average pore throat radius is observed in the Shanxi Fm (Table 2). These findings suggest that the maximum pore throat radius has great influences on displacement pressure, and the median pore throat radius has great influences on reservoir properties.

The upper Paleozoic source rocks of the Daniudi gas field are mainly marine–continental transitional facies coal-measure source rocks, followed by dark mudstones. The organic matter types are mainly III–II type kerogens (Li et al., 2014; Liu et al., 2015; Wu et al., 2017). In accordance with various indexes (organic abundance) of coal rocks and dark mudstones (Figures 6, 7), the coal rocks have an average w (TOC) of 60.55%, average w (chloroform bitumen “A”) of 0.8225%, average total hydrocarbon (HC) of 3,275 μg/g and potential of generating hydrocarbon S1 + S2 of 88.5 mg/g on average. Eighty percent of the samples have the potential to generate hydrocarbon of >50 mg/g; the indexes indicate that the coal rocks are of high quality as source rocks. Organic carbon is widely distributed in dark mudstones; about 75% of the samples have w (TOC)>0.5%, with an average of 1.77%, and about 30% of the samples have W (chloroform bitumen “A”) > 0.05%, with an average of 0.0447%. About 27% of the samples have total hydrocarbon >200 μg/g, with an average of 170 μg/g; about 55% of the samples have the potential to generate hydrocarbon S1 + S2>0.5 mg/g, with an average of 1.5 mg/g. These indexes suggest that dark mudstones contribute to hydrocarbon generation and are of moderately–highly favourable source rocks.

The upper Paleozoic source rocks are unevenly distributed in the Daniudi gas field; thick-bedded source rocks are chiefly distributed in the southwest (Lu and Miao, 2018). Coal-measure source rocks are 18–30 m thick in the area, and dark mudstones are 35–55 m thick and are a main natural gas generating zone. A comparison of the organic abundance parameters of various Fms and Members of the upper Paleozoic in the Daniudi gas field (Table 3) indicates that the organic abundance indexes of the First Member coal rocks and dark mudstones of the Taiyuan Fm suggest moderately–highly favourable source rocks. The dark mudstones and coals in the Second Member of the Taiyuan Fm have organic abundance indexes higher than those of the First Member and are also a better type. However, because coal rocks are distributed in a small area, their potential to generate hydrocarbon is lower than that of the First Member. The organic abundance of the dark mudstones in the Xiashihezi Fm is slightly higher than that of the Taiyuan Fm, while the organic abundance of coal in the Xiashihezi Fm is slightly lower than that of the Taiyuan Fm. In the plan view, the dark mudstones in the Shanxi Fm have a TOC that is generally higher in the west and lower in the east, and the coals have a TOC that is lower in the centre and higher in the periphery. The dark mudstones in the Shanxi Fm have slightly lower organic abundance indexes, and the strata almost no coal. The Xiashihezi Fm dark mudstones have a TOC that is mostly lower than 0.5% and its organic abundance obviously lower than those of the Taiyuan Fm and the Shanxi Fm, thereby reflecting that the main hydrocarbon-generating horizons of the upper Paleozoic in the Daniudi gas field are Taiyuan and Shanxi Fm source rocks.

The upper Paleozoic tight sandstone gas in the Daniudi gas field is composed of alkane and minor non-hydrocarbon gases (Table 4). The alkane content ranges from 82.5% to 95.8%, the C_2-5 content ranges from 2.2% to 15.3%, and the non-hydrocarbon gases mainly include CO₂ and H₂, with a content of <5%. The methane content has a positive correlation to aridity coefficient, and the aridity coefficient (C1/C1-4) ranges from 0.845 to 0.977. Natural gas in the Xiashihezi Fm is dry gas, with an aridity coefficient of >0.95, while natural gas in the Taiyuan and Shanxi Fms are comparatively humid, with aridity coefficients of <0.95. The strata near source rocks in Shanxi Fm exhibit greatly increased wet content, thereby reflecting favourable fractionation effects and a high content of near-source heavy hydrocarbon gas. The C isotopic value of methane in the Upper Paleozoic tight sandstone gas varies mainly from −37‰ to −34‰, the C isotopic value of ethane ranges from −26.5‰ to −23.5‰ and the Ro value ranges from 0.5% to 1.5%. Compared with the tight gas in the Sulige and Jingbian gas fields, tight gas in the Daniudi gas field has an obviously lower aridity coefficient, and natural gas is mainly controlled by thermal maturity and not mixed with biogas (Zhao et al., 2014; Yang et al., 2016).

Since the Mesozoic, the Ordos Basin has tectonically undergone three periods, i.e., development of gentle, flat structures in the middle Triassic (Indosinian movement period), development of paleo-uplifts in the late Triassic–early Cretaceous (Yanshan movement period) and formation of present-day slopes (Xishan movement period) (Yang et al., 2005; Liu et al., 2016; He et al., 2019). In the late Triassic, the basin quickly subsided, which greatly increased the paleo-geothermal gradient to 33°C–48°C/km. In the late Early Cretaceous, after the strata reached maximum burial depth, they were uplifted, leading to decreased geothermal gradient and formation temperature.

Stratigraphic and tectonic evolutions are comparatively consistent within the Daniudi gas field. According to the thermal maturity history analysis of typical wells (Figure 8), the upper Paleozoic source rocks within the gas field mainly underwent three stages of thermal evolution history, including late Triassic, late Jurassic and Cretaceous–late Paleocene. At the end of the Triassic, the bottom of the Taiyuan Fm had a burial depth of up to 2,500 m, and the source rocks in the bottom entered the maturity threshold, started hydrocarbon generating evolution and subsequently suffered from tectonic uplifting; however, the uplifted denudation volume was small. In Jurassic, source rocks entered the second stage of hydrocarbon generation evolution, evolving into a mature stage with the Ro value reaching up to 1.0%, and then starting to generate a massive amount of hydrocarbon. As a result of the continuous subsidence and sedimentation in the early Cretaceous, source rocks continued their hydrocarbon generation evolution and their Ro values evolved to 1.3% in Carboniferous. Under the fast subsidence and sedimentation during the Late Cretaceous–Paleocene, the source rocks evolved quickly, generated hydrocarbon and entered the third hydrocarbon generation evolution stage, having Ro values exceeding 1.5% and reaching a highly mature stage. Eocene tectonic uplifting terminates the source rock evolution even up to the present. The maximum burial depth of the reservoir was 3,800–4,000 m, and the formation temperature decreased from 170°C to 190°C to the present 80°C–90°C.

The upper Paleozoic coal-measure source rocks of the Daniudi gas field have high thermal maturities with Ro mainly varying from 1.2% to 1.5% and Tmax ranging from 460°C to 480°C. The primary pores in the reservoir are hardly exist, the reservoir spaces are minor secondary dissolution pores and microfractures, and the porosities are less than 10%. Clay minerals of the reservoir are dominated by illite/smectite formation, with secondary kaolinite and chlorite; the degree of order of illite/smectite formation varies from 15% to 30%. According to China’s criteria for classification of diagenetic stages of clastic rocks (SY/T 5477-2003), the reservoir is mainly in the middle diagenetic Stage B. The upper Paleozoic sandstone reservoir in the Daniudi gas field underwent complicated diagenetic evolution; the main causes of reservoir densification include long-term mechanical compaction and multistage of siliceous and calcite cementation.

The upper Paleozoic sandstone reservoir has a maximum burial depth of up to 4,000 m and underwent long-term intense compaction. This condition is mainly expressed by the elongated and bent epimetamorphic lithic fragments, the clastic grains orientated in linear or concavo-convex contacts and observable cracks on the surface of some rigid grains. Calculation indicates that the reservoir compaction rate varies from 22.2% to 97.5% and the loss ratio of pores varies from 7.56% to 33.15% with an average of 25.3%. The reservoir has multistage siliceous and calcareous cements as well as minor clay mineral cements, which further compact the reservoir. Microscopic observation indicates that quartz has typical three stages of overgrowth. Stage I quartz overgrowth occurs in one side of the quartz grains (Figure 9A) with small widths (<0.02 mm) and minor quantity; inclusions inside the overgrowth have a homogenisation temperature of 78°C. Stage II quartz overgrowth surrounds the quartz grains (Figure 9B), with large widths and automorphic crystal faces; inclusions inside the overgrowth have a homogenisation temperature of 91.6°C. Stage III quartz overgrowth is mostly in zigzag or sutured linear contacts, with two stages of dust line (Figure 9C); the overgrowth occupies almost all residual intergranular pores, with widths of >0.05 mm and basically no automorphic crystal faces; inclusions have homogenisation temperatures of 112.4°C. The calcareous cement types include early-stage carbonate cements, such as calcite and dolomite, and late-stage carbonate cements, such as ferrocalcite and ferrodolomite (Figure 9D). Microscopic observation indicates that the reservoir has two obvious stages of calcareous cement: Stage I cements are mainly calcite in intergranular pores, which usually occur as thin films along pores and appear yellow in cathode luminescence; Stage II cements are mainly calcite and ferrocalcite in intergranular pores; as a result of the high Fe ion content, the cements show a bright colour (generally bright yellow) in CL, which are greatly different from early-stage cements (Figure 9E). In addition, intergranular pores are filled with argillaceous material and autogenic clay minerals, which is also an important factor for reservoir densification (Figure 9F).

The upper Paleozoic tight sandstone reservoir in the Daniudi gas field underwent complicated diagenetic evolution (Figure 10), including fast burial compaction, diagenetic evolution in acid–alkaline fluid environments and strongly cemented densification stages. Compaction in the early diagenetic B stage decreased the reservoir porosity by 15%–20%. Quartz overgrowth formed from the early diagenetic B stage to the middle diagenetic A₂ stage under formation temperatures of 80°C–130°C and an acid fluid environment, decreasing the porosity by 5%–10%. Carbonate cements usually occurred when the fluid environment turned alkaline after quartz enlargement rims were formed, which decreased the porosity by 3%–5%. The finalisation of reservoir densification is ∼140 Ma BP (early Cretaceous), the formation temperature was ∼130°C and Ro was 0.8%. Under intense diagenetic reworking, reservoir properties showed very strong anisotropism in space, and natural gas in the reservoir hardly migrated because of buoyancy.

Natural gas that has been extracted from the upper Paleozoic in the Daniudi gas field is mostly high-maturity coal-generated gas and mainly composed of CH₄ (Yang et al., 2010; Li et al., 2016). According to the C isotope assay results of CH₄, the two major gas pay zones in the Daniudi gas field have greatly different CH₄ δ¹³C values (Figure 10; Table 4). In the gas reservoirs of the Xiashihezi Fm in the southwest, methane has δ¹³C ranging from −36.15‰ to −33.28‰, representing high-maturity natural gas; while in the gas reservoir of the Taiyuan Fm in the northeast, methane gas has low δ¹³C, ranging from −39.02‰ to −36.58‰, representing low-maturity natural gas. This phenomenon indicates that the two major gas pay zones in the Daniudi gas field have different natural gas accumulation processes. According to the natural gas generation–migration history of the upper Paleozoic in the study area, gas accumulation can be classified into two types, i.e., self-sourced and near-source accumulations.

Self-sourced accumulation mainly occurs in the Taiyuan and Shanxi Fms; in the sedimentary evolution system from barrier coast–tidal flat to delta, the reservoirs mainly include barrier bar sands and distributary channel sands (Liu et al., 2019), and the cap rock is the mudstone in the coal-measure strata, forming a self-generation and self-accumulation system with coal rocks within the interval and dark mudstones. Natural gas that was generated from gas generation centres in the Shanxi and Taiyuan Fm entered the reservoirs quickly. Barrier bar reservoirs have favourable physical properties; thus, continuous natural gas plumes can be formed, providing sufficient buoyancy for natural gas lateral migration. Natural gas migrated towards the northeastern Daniudi gas field and accumulated in favourable lithologic traps. Natural gas accumulation in this stage occurs earlier than the formation of tight reservoirs. Near-source accumulations Near-source accumulation mainly occurs in the Xiashihezi Fm. The source rocks are the underlying Taiyuan Fm and Shanxi Fm coal-measure source rocks; the reservoirs are mainly fluvial facies sands, and the overlying Shangshihezi Fm hugely thick mudstone and the mudstone adjoining reservoirs are the cap rock, forming a lower-generation, upper-accumulation combination. After tight sandstone reservoirs were formed, massive high-maturity gases were generated. However, natural gas migration was difficult because of poor reservoir physical properties and high capillary force. Fractures that formed in the late Cretaceous stratigraphic uplifting provided access for natural gas migration. Natural gas migrated vertically through these fracture beds and accumulated in sandstones with developed fractures. In accordance with the geologic conditions, transportation system, natural gas accumulation tracer, migration force and generation–accumulation combination, the accumulation model of the upper Paleozoic tight sandstone gas reservoir was proposed (Figure 11). Generally, in the period of the Shanxi Fm and Taiyuan Fm, favourable lithologic traps near gas generation centres are the areas with the highest exploration potential.