The structures do not control the distribution of the Yuanba Gas Field, but it is done by organic reef and bank facies at the platform margin (Figure 3). It is a structural and lithologic gas field with reservoirs in both the Changxing and Feixianguan formations. The central pay zone of the Yuanba Gas Field is the Changxing Formation gas reservoir, with a burial depth of 6,682.9 m in the middle of the gas reservoir, which is an ultra-deep, pore-type, and fracture-pore-type gas reservoir (Guo et al., 2014 and Guo et al., 2018; Yuan et al., 2019). The Feixianguan Formation was composed of low-yield gas reservoirs only found in the northwest of Yuanba and had a burial depth of 6,317 m in the middle of the typical gas reservoir (Guo et al., 2014). The mechanism of hydrocarbon accumulation has been debated since the discovery of the Yuanba Gas Field (Fan, 2012; Shao, 2012; Fan, 2014; Guo et al., 2014; Guo et al., 2018). However, the accumulation mechanism is still unknown, such as the gas-bearing difference between the Changxing and Feixianguan formations and the stratum of the caprock.

The Wujiaping Formation of the Upper Permian is the source rock of the Yuanba Gas Field, with bioclastic limestone and mudstone (Figure 4). The thickness of the source rock in the Yuanba area is 60,100 m. The organic matter in the source rock is primarily sapropel, with a significant part of terrestrial organic matter (Li, 2009).

The primary reservoir rock in the Yuanba Gas Field is the Upper Permian Changxing Formation, which has a burial depth of about 6,680 m, an ultra-deep pore-type fracture-pore-type gas reservoir (Guo et al., 2014; Guo et al., 2018; Yuan et al., 2019). Dolomite and biolimestone are the reservoir rocks (Figures 2, 4). The development and distribution of the reef flat influenced the distribution of the gas reservoir (Figure 3). The Changxing Formation has two reservoir types: porosity-type and fracture-porosity-type. The dominant lithology is crystalline dolomite, bioclastic dolomite, and clastic dolomite, with a trace of clastic limestone and bioclastic limestone (Figures 2, 4). The reservoir rock of the Changxing Formation is mainly found at the platform margins in reefs, retro-reef shoals, and reef flat complexes (Figure 3). The Changxing reservoir rock covers an area of 350,450 km². The reservoir rock thicknesses range from 30 to 150 m, with an average of 70 m. The reservoir rock in the platform-margin reef flats is thick, whereas those in the retro-reef shoal and intra-platform flat are thin.

For the Yuanba Gas Field, the regional caprock is gypsum, polyhalite, salt rock, and gypsum dolomite of the Lower-Middle Triassic (Figures 2, 4). However, the main gas pay zone of the Yuanba Gas Field is the Changxing Formation of the Upper Permian, and the Feixianguan Formation directly above it only produces gas locally. The Changxing Formation has a local caprock, the marlites, in the Lower Triassic Feixianguan Formation base.

The Changxing gas reservoir was formed through a complex process, and it is a pyrolysis gas reservoir. The Wujiaping Formation reached its peak oil generation during the Late Triassic to the Early Jurassic, and the paleo-oil reservoir of the Changxing Formation was formed. The oil was generated from the Wujiaping Formation and injected into the Changxing Formation reservoir (Zhang, 2010; Guo et al., 2014; Li et al., 2016). The paleo-oil reservoir was pyrolyzed to form a paleo-gas field at the end of the Middle Jurassic (Zhang, 2010; Guo et al., 2014; Li et al., 2016). The paleo-gas reservoir was adjusted to its present status during the late Yanshanian movements and Cenozoic Himalayan movements.

It was found that tectonic deformation in the Yuanba Gas Field was weak, with low-magnitude anticlines and no large-scale faults (Figure 1). Different strata have been subjected to various tectonic movements, resulting in multiple fractures. In terms of attitudes, imaging logging data show that the fractures in the Changxing Formation are mostly steep (Figure 5 and Figure 6A) and strike NW-SE (Guo et al., 2010), with 8% at 0–30° dip angles, 35% at 30–60°, and 57% at 60–90°. The fractures in the Feixianguan Formation have multiple strikes and are mostly low-angle fractures and stylolites (Figures 5, 6B), with dip angles of 0–30° accounting for 19%, 3,060° accounting for 52%, and 60–90° accounting for 29%.

In terms of fillings and filling degrees, we counted the fillings of the fractures on the core and identified the fillings. Fully filled fractures in the Changxing Formation account for 57%, semi-filled fractures account for 19%, and non-filled fractures account for 24% (Figure 7A). Based on the types and distribution of fillings, there are mainly five types of filling materials: dolomite, calcite, quartz, muddy, and asphalt. The fractures filled with dolomite, calcite, muddy, and asphalt account for 2.7, 15, 8, and 73%, respectively; others were filled with pyrite (Figure 7B). The fractures in the Feixianguan Formation have a filling degree of about 54%, followed by non-filled and semi-filled fractures, accounting for about 23% (Figure 7A). The fillings are mostly calcite materials (Figure 7B), accounting for about 65%, followed by mud accounting for about 16%, and the remaining part is asphalt.

Delaminated fracturing refers to various fracture densities and patterns in different layers. It is determined by the magnitude of tectonic stresses, the mechanical properties, and the thicknesses of the rocks. The fractures are required to improve the reservoir properties, but extensive deformation will result in fault formation, which will destroy the gas reservoirs. Because of differences in the inner structures, mineral composition, and grain size, the rocks in the Earth’s crust have varying mechanical properties. Different rocks have different fracture conditions, implying that the rock will fracture when the stress reaches the rock strength. In general, the compressive strengths (which also reflect shear and tensile strengths) of rocks increase in strength from dolomite to dense limestone, dense sandstone, anhydrite, mudstone, shale, and gypsum (Ferrill et al., 2017). In terms of strengths, given a rock section composed of shale (mudstone), dolomite, tight limestone, and marlite, the sequence of forming fractures should be shale (mudstone), marlstone, dense limestone, and dolomite. However, there are strains that different rocks can withstand before fractures can vary. As a result, some rocks with higher strengths will fracture earlier than rocks with lower strengths under identical tectonic movements (Figure 8; Ferrill et al., 2017).

For rocks with identical mechanical properties, the density and attribute of fractures formed in one-time deformation will vary due to various thicknesses. Researchers show that joint space in layered sedimentary rocks is proportional to the thickness of the fractured layer (Bai and Pollard, 2000). The delaminated fractures often are observed in outcrops around the Sichuan basin (Figure 9). The inter-bed cohesion will affect fracture development (Hooker et al., 2013; Surpless and Wigginton, 2020).

Medium deformation occurred in the Yuanba Gas Field, and the Feixianguan Formation and Changxing Formation are with fractures but without faults (Figures 1, 3, 10). Fractures increase the porosity and particularly the permeability of the dolomite reservoirs in the Changxing formation (Guo, 2011; Fan, 2012). Because the Upper Permian and Lower Triassic contacted conformally, the Changxing and Feixianguan formations experienced the same tectonic action processes (Figure 10). The Early Indosinian movements in the late Middle Triassic were responsible for forming the NW-SE fractures in the Changxing Formation (Figure 11). The density, dip, strike, and interstitial materials of fractures in the Feixianguan Formation differed from those in the Changxing Formation, indicating delaminated fracturing.

The Mohr envelope curves for dolomite and limestone were obtained by measuring the strength of rock samples collected from well cores in the Yuanba Gas Field and field crops (Table 1 and Figure 12).

The Mohr envelope curves of dolomite are as follows: σ = 0.0034 τ 2 − 4.7685 , R 2 = 0.9991.

The Mohr envelope curves of limestone are as follows: σ = 0.0052 τ 2 − 13.5530 , R 2 = 0.9889.

The burial depth of the Changxing and Feixianguan formations in the Late Triassic was 2,200 and 2,000 m, respectively (Figure 13). The density of the overburdened rocks was 2.63 g/cm³. The Poisson’s ratio of rocks was 0.35–0.448. The Poisson stress in the horizontal direction was 19.8 and 18 MPa for the Changxing and Feixianguan formations, respectively. The maximum principal stress measured by the acoustic emission in Indosinian movements was 152.1 MPa (Zhang, 2010). Due to zero porosity in the tight limestone, the paleo-pore fluid pressure was 28.6 MPa for dolomite in the Changxing Formation and 0 for limestone in the Feixianguan Formation (Zhang, 2010). Considering all the above, we got the maximum and minimum principal stresses of 123.5 and −8.8 MPa in Changxing Formation dolomite, respectively. The maximum and minimum principal stresses were 152.1 and 18 MPa in Feixianguan Formation limestone, respectively. Mohr stress circles were constructed for the dolomite and limestone according to the maximum and minimum principal stresses. The figure showed that fractures occurred in the dolomite of Changxing Formation but not in the limestone of the Feixianguan Formation (Figure 14).

Although there is a small amount of gas in the Feixianguan Formation, it is mainly caprock. The fractures may cut the marlite at normal temperature and pressure. Due to high confining pressure and temperature in the deep crust, ductility or plasticity increases, leading to good caprocks like shale. (Figure 8; Donath, 1970; Ferrill and Morris, 2003 and Ferrill and Morris, 2008; Ferrill et al., 2012 and Ferrill et al., 2017). The strength of the marlite decreases like salt (Weijermars et al., 1993; Jackson and Vendeville, 1994).

The deformation behavior of the marlite in limestone samples from well cores was similar to that of the Solnhofen limestone, according to the strength measurements of the marlite of the limestone samples (Figure 15 and Table 2). The limestone will undergo semi-brittle deformation, with a confining pressure of 53 MPa. The limestone will enter the plastic deformation stage when the confining pressure reaches 140 MPa.

When gas production peaked at the end of the Middle Jurassic, the burial depth of the Yuanba Gas Field was 5 km. The Poisson horizontal stress was 70.80 MPa, and the vertical stress was 131.5 MPa. Considering that the Sichuan basin was a foreland basin during the middle Jurassic period, the horizontal stress would have been greater than 70.80 MPa. The acoustic emission measurement on the J1z sandstone in well YB102 at a 3,921 m depth revealed two horizontal stresses of 101.1 and 96.1 MPa, with a vertical stress of 100.4 MPa. These stresses were referred to as Yanshanian movement stress strength. The marlite has progressed to the semi-brittle stage. Consequently, the marlite of the Feixianguan Formation was continuously buried before being uplifted by the late Yanshanian movements. Now, it has a 6,000 m burial depth and is denudated with thicknesses of 1,000–3,400 m (Qiu et al., 2008). The present maximum principal stress is in the NW-SE direction, according to borehole caving and fracturing data. The vertical stress in well YB1 is 186.0 MPa, and the horizontal stresses are 243.8 and 154.8 MPa at a depth of 7,081–7,150 m. The vertical stress in well YB2 is 173.0 MPa, and the horizontal stresses are 235.9 and 140.8 MPa at a depth of 6,677–6,700 m. As a result, the confining pressures in geological compression settings were greater than 53 MPa or 96.1 MPa. Marlite has been in a high plastic state to be a good caprock, similar to shale.

Furthermore, the well cores revealed stylolites formed in the Feixianguan Formation, with most of them filled with mud (Figure 5D). They were sealing materials for oil and gas (Heap et al., 2018; Toussaint et al., 2018; Bruna et al., 2019). Many factors, including delaminated fracturing, plastic deformation due to high confining pressures, and mud-filled stylolites, determined the Feixianguan Formation marlite to be a good cap (Figure 16). Since the late Jurassic, it has prevented the pyrolysis gas from leaking out of the Changxing Formation dolomite reservoir.

The major fractures in the Changxing Formation were formed during the initial formation of oil reservoirs in the Yuanba Gas Field from the Late Triassic to the Early Jurassic by integrating pore fluid pressure with the Indosinian movements in the Late Triassic. The fractures increased the porosity and permeability of the dolomite from the Changxing Formation. Due to the medium extent of deformation, no faults cutting through the stratum section from Permian to Jurassic systems were formed. The oil migrated through the fracture network from the underlying source rock, the Wujiaping formation, to charge the overlying Changxing Formation dolomite reservoirs (Figure 17). Later, in the Late Jurassic, the oil was pyrolyzed, resulting in the formation of a paleo-gas field. The subsequent tectonic movements had a minor impact on the paleo-gas field, and finally, the Yuanba Gas Field was formed.

A few vertical fractures were found in the overlying Feixianguan Formation marlite, and they were not connected to the fractures in the Changxing Formation dolomite. Because of the plasticity caused by the deep burial, high confining pressure, and mud-filled stylolites, the marlite served as a good caprock to keep the gas in the underlying Changxing Formation.