Assessing the Source Rock Quality and Hydrocarbon Generation Potential of the Upper Triassic T₃x⁵ Formation in the Southwestern Sichuan Basin: An Organic Geochemical Study

1. Introduction

The Upper Triassic Xujiahe Formation in the Sichuan Basin is one of the most important coal-measure source rock intervals in central China, hosting significant natural gas accumulations [1,2,3]. In recent years, exploration has gradually shifted from conventional structural traps to subtle and lithological traps, increasing attention has been paid on the hydrocarbon generation potential of the Xujiahe Formation, particularly its fifth member (T₃x⁵) [4]. The southwestern part of the Sichuan Basin (hereafter referred to as the study area) is a key region for deep natural gas exploration due to its thick succession of lacustrine dark shales and coaly mudstones [5,6]. However, previous studies mainly focused on the overall source rock characteristics of the Xujiahe Formation, with limited systematic evaluation of the vertical and lateral variations in organic matter abundance, type, maturity, and hydrocarbon generation potential of the T₃x⁵ member [7,8,9,10,11,12]. Furthermore, the thermal evolution history and gas generation intensity of the T₃x⁵ source rocks in this area remain poorly constrained [11,13].

This study aims to systematically characterize the organic geochemical features of the T₃x⁵ source rocks in southwestern Sichuan Basin, including total organic carbon (TOC) content, organic matter type (maceral composition and kerogen type), thermal maturity (vitrinite reflectance Ro), and source rock thickness distribution. In addition, basin modeling is applied to reconstruct the burial and thermal history and to quantify the gas generation intensity and cumulative gas generation. The results will provide a scientific basis for further exploration and resource assessment of the Xujiahe Formation in the study area.

2. Geological Settings

The Sichuan Basin located in southwestern China, covers an area of approximately 1.8×10⁵ km² [14,15]. Tectonically, it is a multi-cycle superimposed sedimentary basin developed on the Yangtze Plate [16]. Based on structural characteristics, the basin can be subdivided into six tectonic units: the western depression, northern depression, central uplift, eastern high-steep structural zone, southern low-steep structural zone, and southwestern gentle structural zone [17]. From the Sinian to the Middle Triassic, the basin was dominated by subsidence and marine carbonate deposition. Due to tectonic inversion during the Late Triassic to Eocene, the basin experienced uplift and deposited transitional and continental clastic sequences [16,18,19].

The Upper Triassic Xujiahe Formation is a clastic unit consisting of five members (T₃x¹ to T₃x⁵). Among these, T₃x⁵ is the youngest and corresponds to a period of maximum lacustrine transgression, with widespread deposition of lacustrine dark mudstones, carbonaceous mudstones, and thin coal seams [20,21,22]. In the study area (southwestern Sichuan Basin), the T₃x⁵ member is divided into upper and lower submembers based on lithological associations and sedimentary cycles. The sedimentary environments range from shore-shallow lake and delta front in the upper submember to semi-deep lake in the lower submember, with locally developed interdistributary bays and lagoons that favored the accumulation of organic-rich sediments [23,24].

The study area is located in the western depression, the southwestern gentle structural and the central uplift zones, where the T₃x⁵ member has been preserved from intense erosion. The present-day burial depth ranges from 300 to 3500 m [25,26]. The source rocks in this area are thermally mature to over-mature, with Ro values generally between 1.0% and 2.0%. The T₃x⁵ source rocks are considered to be the primary gas-generating interval in the southwestern Sichuan Basin (Figure 1).

3. Materials and Methods

A total of 402 core samples were collected from the southwestern Sichuan Basin. The samples cover both the upper and lower submembers of the T₃x⁵ member, with lithologies including dark lacustrine mudstones, carbonaceous mudstones, and coals. Special care was taken to select fresh, unweathered portions of the cores for analysis. The samples were stored in sealed bags to prevent contamination and oxidation before laboratory processing.

3.1. TOC Analysis

The total organic carbon (TOC) content of the samples was determined using the Leco-CS744 analyzer. The samples were ground to below 200 mesh and washed with 5% dilute hydrochloric acid to remove carbonate. After repeatedly washing with distilled water and drying, the samples were combusted at 1100 °C in the instrument to obtain TOC. To ensure data accuracy, the instrument was calibrated with a standard sample after every 10 samples were tested.

3.2. Organic Petrology Analysis

Thin section microscopy analysis is one of the main methods for source rock evaluation. Fresh parts of core samples were selected and cut perpendicular to the bedding plane. Then, the samples were ground down to below 20 mesh and mixed with epoxy resin. After the samples were polished into thin sections, petrographical organic compositions were identified under transmitted light, reflected light, and fluorescence using a Leica DM4500P microscope equipped with an MPS 200 microphotometer.

3.3. Thermal Evolution Analysis of Source Rocks

Thermal evolution history modeling of source rocks is based on burial history reconstruction and includes thermal and maturity history. By simulating the thermal evolution history, a dynamic understanding of the thermal background, organic matter maturity, and hydrocarbon generation process in the basin can be obtained, providing a basis for analyzing the spatiotemporal configuration of petroleum accumulation conditions and for evaluating petroleum resources. The basin modeling software used in this study is PetroMod (IES GmbH, Germany), which includes the EASY% Ro theoretical model established by Sweeney and Burnham [28]. The EASY% Ro model is currently the most commonly used chemical kinetic predictive model for thermal evolution analysis of source rocks.

4. Results and Discussion

4.1. Organic Geochemical Characteristics of Source Rocks

4.1.1. Organic Matter Abundance

The TOC content of the T₃x⁵ source rocks in the southwestern Sichuan Basin mainly ranges from 0.5% to 4%, with the maximum reaching 71.73% and the average being 2.09%. More than 71.7% of the samples have TOC greater than 1.0%. The main lithologies of the hydrocarbon source rocks are lacustrine mudstone, carbonaceous mudstone and coal. Among them, lacustrine mudstone is the main lithology of the hydrocarbon source rocks, accounting for 89.3%, while the proportions of carbonaceous mudstone and coal are 9.5% and 1.2% respectively.

Vertically, the T₃x⁵ member has the highest organic matter abundance among all members of the Xujiahe Formation, and the abundance increases from bottom to top. In the upper submember, samples with TOC > 1.0% account for 73.3%, while in the lower submember, this proportion is 71.8%. Overall, the organic matter abundance of the upper and lower submembers is comparable, with the upper submember being slightly higher (Figure 2).

Laterally, multiple high-TOC areas occur in the upper submember, mainly in the central-eastern part, which is related to coal development during that period. In the western lacustrine area, TOC is generally 1.5%–2.5%. In the southern area, organic matter abundance is relatively low, with TOC generally 0.5%–1.0%. For the lower submember, high-abundance source rocks are widely developed in the southwestern part, with TOC generally above 2.5%. Influenced by the sedimentary environment, coal and carbonaceous mudstones developed along the lake shoreline, forming independent high-abundance centers distributed along the shoreline. Overall, the distribution of high-abundance source rocks in the lower submember is broader than that in the upper submember. The planar distribution of organic matter abundance differs significantly due to variations in sedimentary water environments.

4.1.2. Organic Matter Type

Organic petrology analysis was performed in different areas of southwestern Sichuan. Results show that the depression area is dominated by semi-deep to deep lacustrine facies, with quiet water and well-developed lower aquatic organisms, favoring the enrichment of sapropelic organic matter. In well Fushun-1, the content of sapropelic macerals such as alginite and amorphous organic matter reaches 64%, indicating Type II kerogen. In well Qiulin-2, the T₃x⁵ member also contains abundant amorphous algae (sapropelic group accounting for 51%), with framboidal pyrite (grain size <0.5 μm) reflecting a deep-water environment, and the organic matter is generally Type II (Figure 3).

In the lower slope area, which is mainly a semi-deep to shallow lacustrine environment, the input of higher plants increases, and the organic matter type is mainly mixed (humic-sapropelic). The content of amorphous algae (sapropelic) increases to about 45%, while vitrinite (humic) is still present in certain amounts. For example, in well Jiantan-1, the T₃x⁵ member is dominated by laminated alginite and sapropelic amorphous organic matter (45%), with vitrinite content of 42% and solid bitumen content of 13%. In well Yongqian-7, detrital vitrinite and sapropelic amorphous organic matter are developed, with sapropelic group accounting for about 35%, vitrinite 42%, and solid bitumen 23% (Figure 4).

In the upper slope, uplift areas, and basin margins, fluvial-delta sedimentary systems dominate, and the organic matter is mainly derived from terrigenous higher plants, such as vitrinite. Relatively enclosed environments like interdistributary bays can also contain certain amounts of sapropelic organic matter. For example, in well Yin-36, coal fragments and amorphous sapropel are common, with vitrinite accounting for 55%, sapropelic group 25%, and solid bitumen 20%. In the Dujiangyan section, coal seams are developed, and the organic matter consists of 75% vitrinite, 18% sapropelic group, and 7% solid bitumen, indicating Type III kerogen (Figure 5).

Traditionally, the Upper Triassic Xujiahe Formation in the Sichuan Basin is considered to be coal-measure source rock mainly derived from higher plants. However, the organic petrology analysis indicates that the organic matter characteristics of the T₃x⁵ source rocks vary considerably with basin structure and sedimentary water environment. Near the basin margin toward the provenance, higher plant input is dominant with Type III kerogen. In the lake center and interdistributary bays, the water is quiet, lower aquatic organisms are well developed, and sapropelic organic matter such as algal amorphous material is abundant, forming Type II kerogen-dominated source rocks.

4.1.3. Thermal Maturity of Source Rocks

The thermal maturity of source rocks generally decreases from west to east (Figure 6). The western Sichuan area experienced large subsidence, leading to high thermal maturity, reaching the high-mature stage and locally the over-mature stage. The thermal maturity decreases towards the east. The lower slope area is generally in the mature stage, locally high-mature. In the central Sichuan and Shunan areas, subsidence was small, and the maturity remains in the early mature stage. The Weiyuan and southern areas are in the low-mature stage. The thermal maturity of the lower submember is slightly higher than that of the upper submember, with a larger area of high maturity. Overall, the western Sichuan area has high thermal maturity and great hydrocarbon generation potential, making it the most favorable exploration target for the T₃x⁵ member.

4.1.4. Thickness Distribution of the Source Rocks

The T₃x⁵ member is dominated by shore-shallow lake and delta front deposits. During this period, the lake basin reached its maximum extent, and the distribution of dark mudstones was also largest. Sand bodies and delta lobes from different provenances exhibit elongate shapes, intersecting to form multiple semi-enclosed bays or lagoons with quiet water, which were very favorable for the deposition of high-abundance source rocks. Due to uplift and erosion, the T₃x⁵ member is absent in the northwestern part of the basin. The upper submember dark mudstone thickness center is widely distributed in the southwestern, central, and west-central parts of western Sichuan, with thicknesses of 120–280 m and a maximum exceeding 320 m. Eastward, the thickness rapidly decreases to 40 m in the Leshan-Zizhong-Hechuan-Guang’an-Sanhui-Yilong area, and to less than 20 m in the Yibin-Chongqing-Liangping area (Figure 7).

The lower submember dark mudstone in the central-southern part of western Sichuan is generally thinner than the upper submember, typically 80–180 m. The thickness center is located in the Chengdu-Mianzhu area, with a central thickness of 160–220 m and a maximum exceeding 240 m. Eastward, the thickness rapidly decreases to 40 m in the Leshan-Zizhong-Hechuan-Guang’an-Sanhui-Yilong area, and to less than 20 m in the Yibin-Chongqing-Liangping area (Figure 8).

Laterally, the T₃x⁵ source rock thickness gradually decreases from southwest to northeast, which is related to the foreland basin structure and provenance supply. Provenance was supplied from the north, east, and south, resulting in coarse clastic deposits and relatively poorly developed source rocks. In the southwest, entering the foreland depression, the mud content increases, and source rocks are thick. Near the provenance, local interdistributary bays can host thick coal seams, providing important hydrocarbon supply for local gas accumulations. Therefore, even though the T₃x⁵ source rocks are relatively thin in the eastern, northern, and southern Sichuan areas, the development of coal and carbonaceous mudstones with high organic matter abundance can still result in large-scale hydrocarbon supply and significant exploration potential. Overall, the T₃x⁵ source rocks are thick and widely distributed, favorable for large-scale hydrocarbon generation.

4.2. Evaluation of Hydrocarbon Generation Potential

The hydrocarbon generation potential of source rocks is closely related to organic matter type, abundance, and thermal maturity. Vitrinite reflectance is the most common and effective indicator for determining source rock maturity. The Xujiahe Formation source rocks are mainly coal-measure source rocks, and vitrinite is widely developed.

In this study, thermal simulation experiments were used to determine the vitrinite reflectance of each source rock interval. After burial history reconstruction and calibration, the thermal evolution history of the Xujiahe Formation was reconstructed. Then, using the basin modeling software PetroMod, the hydrocarbon generation amount of each member of the Xujiahe Formation was simulated and calculated to comprehensively evaluate its hydrocarbon generation potential.

4.2.1. Hydrocarbon Generation History

The thermal maturity of source rocks changes with external conditions, mainly temperature and time. Therefore, the basin thermal history (paleogeothermal gradient or paleo-heat flow evolution) is the main factor influencing the hydrocarbon generation history of source rocks. Referring to previous heat flow values for the northwestern Sichuan Basin (Huang Shaopeng et al., 1990; Yang Huai et al., 2004) and validation by single-well data, the thermal evolution trend is generally consistent with measured Ro (Figure 9).

The burial and thermal history of the Xujiahe Formation source rocks in the southwestern Sichuan Basin are shown in Figure 10 and Figure 11. From the end of the Late Triassic to the end of the Early Jurassic, the Upper Triassic source rocks in the study area reached the hydrocarbon generation threshold (early stage), with Ro ranging from 0.5% to 0.7%, generating a small amount of early kerogen cracking gas. The threshold depth is approximately 2100 to 2400 m.

By the end of the Late Cretaceous, the T₃x⁵ source rocks reached the mature stage, with Ro ranging from 0.9% to 1.3%, generating large amounts of kerogen cracking gas, initiating primary migration, and being preserved in adjacent reservoirs. By the end of the Paleogene, the organic matter of the T₃x⁵ member reached the high-mature stage, with Ro > 1.3%, entering the late gas generation stage, which is the key period for natural gas generation. Subsequently, the Himalayan movement caused uplift and erosion, reducing the burial depth of the Xujiahe Formation source rocks. In the central and southern Sichuan areas, the organic matter did not evolve to higher thermal maturity stages and essentially maintained the maturity at the end of the Paleogene. Only in the main body of the western Sichuan depression did the organic matter continue to evolve to higher stages, reaching the over-mature stage.

In the lower slope area, the T₃x⁵ source rocks entered the mature stage in the Late Cretaceous (Figure 1-26), then slowly increased in burial depth. By the end of the Late Cretaceous, uplift occurred, thermal evolution ceased, and the source rocks essentially remained in the mature stage.

4.2.2. Natural Gas Generation Intensity and Volume

For total geological resource assessment, basin modeling is the most widely used and generally considered to be a reliable method. Basin modeling fully utilizes existing geological and geochemical data to dynamically simulate the hydrocarbon generation and expulsion processes of each source rock interval based on reconstructed burial and thermal history, ultimately obtaining the hydrocarbon generation and expulsion amounts at different periods. The main parameters for calculating gas generation volume include source rock volume, organic matter abundance, type, thermal maturity, and gas generation rate. After determining the relevant parameters, this study used the PetroMod software to calculate the gas generation intensity and volume for each member of the Xujiahe Formation in the northwestern Sichuan Basin.

Overall, the T₃x⁵ gas generation intensity is highest in the central-southern section of the western Sichuan depression, with a maximum exceeding 70×10⁸ m³/km². The gas generation intensity in the Qiulin, Jinhua, Jianyang, and southwestern Sichuan areas is generally greater than 20×10⁸ m³/km². The area with gas generation intensity >20×10⁸ m³/km² is about 2.3×10⁴ km², indicating significant resource potential. In the central Sichuan area, the intensity is generally 10-12×10⁸ m³/km², and in the area around well Mo-208 to well Tongnan-105, due to the development of coal seams of certain scale, the gas generation intensity is higher, reaching 15-20×10⁸ m³/km² (Figure 12). According to the simulated calculation, the total hydrocarbon generated from the T₃x⁵ member is approximately 19.2 trillion cubic meters.

Considering that the T₃x⁵ member is characterized to be near-source accumulation [3],the areas with low gas generation intensity still have considerable exploration potential. In summary, the southwestern Sichuan Basin has high gas generation intensity and gas source conditions favorable for forming large gas fields, making it the preferred area for further exploration.

5. Conclusions

(1) The T₃x⁵ source rocks in the southwestern Sichuan Basin have high organic matter abundance, with an average TOC of 2.09%. The upper submember has a slightly higher abundance than the lower submember. High-abundance source rocks are widely distributed in the western lake area and along the paleo-shoreline.

(2) Organic matter types vary systematically: Type II kerogen dominates in the deep lake center and interdistributary bays, Type II–III mixed types occur in the lower slope area, and Type III kerogen dominates near the basin margin and provenance areas.

(3) Thermal maturity is high in the west (high-mature to over-mature) and decreases eastward (mature to low-mature). The western Sichuan depression has the highest maturity and the greatest hydrocarbon generation potential.

(4) The T₃x⁵ source rocks are thick (up to 320 m) and widely distributed, with thickness decreasing from southwest to northeast.

(5) Basin modeling results show total gas generation of approximately 192 tcm, with high gas generation intensity (>20×10⁸ m³/km²) covering an area of 2.3×10⁴ km², indicating that the southwestern Sichuan Basin has excellent gas source conditions and is a priority exploration target.