The hydrocarbon generation potential of the QFY Formation was assessed by analyzing the TOC content of selected shale samples using a LECO CS-230 carbon-sulfur analyzer (He et al., 2022). Powdered samples were treated with hydrochloric acid (HCl) to remove carbonate minerals, and the TOC content was determined by combusting the treated samples at high temperatures in an oxygen-rich environment and measuring the evolved carbon dioxide (CO₂) using an infrared detector.

The mineralogical composition of the core samples was determined through whole-rock X-ray diffraction (XRD) analysis. Representative rock samples were ground to a fine powder using a mortar and pestle and analyzed using a Bruker D8 Advance X-ray diffractometer. The resulting diffraction patterns were analyzed to identify and quantify the relative abundance of mineral phases, including quartz, clay minerals (e.g., illite, smectite, kaolinite, and chlorite), carbonate minerals (e.g., calcite and dolomite), and feldspar (Yang L. R et al., 2022; Nag et al., 2025; Peng et al., 2025). In whole-rock XRD analysis of fine-grained continental sedimentary rocks, acid combinations must be selected for sample digestion according to the mineral composition and target elements. HF + HNO₃ was used in this study, and the digestion process was as follows: (1) Sample preparation: ground to less than 200 mesh to ensure uniformity. (2) Acid mixing: 0.1–0.5 g of the sample was weighed into a polytetrafluoroethylene (PTFE) digestion tank, and 5–10 mL of HNO₃ and 3–5 mL of HF were added. (3) Closed digestion: using a microwave digester or high-pressure digester, the sample was heated to 180°C–200°C for 2–4 h (4) Acid treatment: After digestion, the solution was dried, and HNO₃ was added to redissolve and dry again (to remove residual HF and prevent damage to the ICP-MS atomizer). (5) Volume: Diluted HNO₃ (2%–5%) was added to bring the volume to 25–50 mL. The solution was filtered and tested on the machine.

Porosity and permeability, key parameters for reservoir quality assessment, were measured on cylindrical core plugs extracted from representative samples. Porosity was measured using a helium porosimeter (e.g., Quantachrome UltraPore) which measures the volume of helium gas permeating into the pore spaces of a core plug at a known pressure (Peng et al., 2018). Permeability, a measure of the ability of the rock to transmit fluids, was measured using a nitrogen gas permeameter (e.g., Vinci Technologies Permeameter) by flowing nitrogen gas through the core plug at a controlled pressure gradient and measuring the flow rate and pressure drop (Wen et al., 2024).

Scanning electron microscopy (SEM) was employed to visualize the pore structure and morphology of the QFY Formation. Small rock chips were mounted on SEM stubs, coated with gold to prevent charging during imaging, and imaged using a high-resolution scanning electron microscope (e.g., FEI Quanta). Argon ion polishing was used to remove surface irregularities and expose the internal pore structure for better visualization. Large area backscattering scanning electron microscopy (MAPS) imaging technology, a technique that involves acquiring and stitching multiple high-resolution SEM images, was used to create a mosaic image of the sample surface (Jun and Liang, 2024). This approach allowed for the identification and quantification of different pore types (intergranular pores, interlayer pores in clay minerals, organic matter-hosted pores, and microfractures) based on their size, shape, and connectivity.

Geochemical analyses were performed to determine the concentrations of specific elements and to understand the paleoenvironmental conditions during the deposition of the QFY Formation. Rock samples were digested using a combination of acids, and the concentrations of V, Cr, Ni, Sr, Ba, Al, Fe, Mn, and Mg were measured using inductively coupled plasma mass spectrometry (ICP-MS) or inductively coupled plasma atomic emission spectrometry (ICP-AES). The measured elemental concentrations were used to calculate various paleoenvironmental proxies, including V/Cr and V/(V + Ni) ratios for redox conditions, Sr/Ba ratios for paleosalinity, total rare earth elements (∑REE) and Fe/Mn and Mg/Ca ratios for paleoclimate, and Al content for paleoproductivity (Tang et al., 2023).

Diagenetic analysis was performed to evaluate the impact of post-depositional processes on the reservoir quality of the QFY Formation. Thin sections of selected core samples were prepared and examined under a polarized light microscope to identify diagenetic features, including compaction, cementation, dissolution, and clay mineral transformations. The compaction ratio was estimated based on the intensity of diagenetic alterations observed in the thin sections. The porosity loss due to cementation was evaluated using the Houseknecht method , which involves comparing the measured porosity with a theoretically calculated porosity based on the initial mineralogy of the rock.

The structural types of sedimentary rocks are determined by the arrangement of mineral components. Based on the observations of cores and thin sections, the rock structures of fine-grained sedimentary rocks of the QFY Formation in the study area can be divided into three types: blocky, banded, and lamellar.

The blocky structure is the most common structure in the fine-grained sedimentary rocks of the QFY Formation in the NE Sichuan Basin. It is characterized by no significant changes in mineral composition and particle size, and the rock is homogeneous with a thickness generally greater than 2.5 cm. Different rock structures and lithologic combinations indicate different forming environments. Blocky claystone and blocky silty claystone are fine-grained sedimentary rocks dominated by clay minerals that are transported to deepwater areas far from the provenance, reflecting the semi-deep lake environment with weak hydrodynamics (Figures 2A, B). In contrast, siltstone, muddy siltstone, and shell limestone, which are rapidly deposited under strong hydrodynamics in shallow lakes, are generally blocky in structure and well sorted (Figures 2C, D).

Banded structures are parallel overlaps of alternately dark and bright banded on the core. The majority are horizontal, a few are wavy, and the thickness is greater than 0.5 cm and less than 2.5 cm. The formation of this structure in the study area is mainly related to the sudden increase of hydrodynamics in shallow water areas, such as terrigenous events including storms and turbidity currents (Figure 2E).

The lamellar structure in the study area can be divided into straight laminae and wavy laminae. The horizontal lamellar structure in lamellar silt-bearing claystone, lamellar silty claystone, and laminated claystone is a typical microrock structure of terrigenous hydrostatic sedimentary fine-grained sedimentary rocks (Figures 2F–I). It is formed in a low-energy environment with a warm and humid climate and semi-enclosed or closed water, representing the shallow lake and semi-deep lake environment with weak hydrodynamics. The wavy laminar structure indicates that the sedimentary water is relatively volatile, and it is speculated that the wavy laminae are formed in the shallow lake environment with shallow water and strong hydrodynamics.

Statistically, the TOC content of the QFY Formation in the study area ranges from 0.12% to 3.71%, with an average of 1.85%. In the middle 2nd Member of the QFY Formation, the TOC content is the highest and gradually decreases vertically upward and downward. Compared with marine fine-grained sedimentary rocks, continental fine-grained sedimentary rocks are characterized by rapid sedimentary changes and a complex sedimentary environment. The development of organic-rich fine-grained sedimentary rocks is mainly controlled by the sedimentary environment with deepwater reduction.

Whole-rock XRD analysis shows that the fine-grained sedimentary rocks of the QFY Formation in the study area are mainly composed of quartz, clay minerals, and carbonate minerals, with a small amount of feldspar, pyrite, siderite, and anhydrite. Overall, the content of clay minerals and quartz is high, and the carbonate minerals show local enrichment. It is observed that the quartz content in the analyzed samples varies from 8.7% to 60.9%, with an average value of 38.5%. Similarly, the clay content ranges from 19.4% to 66.6%, with an average of 48.9%, while the carbonate content ranges from 0.2% to 17.8%, with an average value of 2.3%. The clay minerals identified in the QFY Formation of the study area are illite, illite-smectite mixed layer, kaolinite, and chlorite. Notably, the highest content is observed for the illite-smectite mixed layer, which ranges from 28% to 55%, with an average of 42.48%, followed by illite, which ranges from 15% to 43%, with an average of 34%. The kaolinite content ranges from 2% to 35%, with an average of 6.39%, while the chlorite content ranges from 11% to 26%, with an average of 17% (Figure 3; Table 1).

Silty intergranular pores are one of the most common pores in the fine-grained sedimentary rocks of the QFY Formation in the study area. Because the LZ-YB area in the northeastern Sichuan Basin is close to the provenance of the Micangshan-Dabashan in the northern Sichuan Basin, the fine-grained sedimentary rocks are mixed with some silty sediments from the continental crust. These silty particles of terrigenous debris are dispersed in the shale. The intergranular pores are developed at the contact zones of mineral particles, but are affected by the shape of the detrital grains and the surrounding clay minerals. In particular, the silty intergranular pores formed by the dissolution of clay minerals and carbonate minerals are mostly irregular and pore-shaped. Some intergranular pores are distributed at the edges of brittle minerals such as quartz and feldspar grains, and the morphology of brittle minerals can also be seen through the distribution of intergranular pores (Figure 5A). In general, the porosity and permeability of brittle minerals are poor, and the pores are mostly filled with organic matter. Because of the influence of compaction and cementation in the diagenetic process, especially in the early diagenetic stage, a large number of intergranular pores are compressed and filled, resulting in relatively poor connectivity between pore throats.

According to the XRD analysis of clay minerals of rock samples in the QFY Formation, the content of illite–montmorillonite mixed layer is the highest in clay minerals of fine-grained sedimentary rocks of the QFY Formation in the study area, followed by illite, a small amount of kaolinite and chlorite, and montmorillonite is not developed. The illite–smectite mixed layer is the transitional mineral from montmorillonite to illite, which is honeycomb-shaped and flocculent. The illite is scaly and feathery, scattered on the surface of other mineral particles and filled between particles in the form of clay bridges. The morphology of these two clay minerals is mainly schistose. It gives the interlayer pores in the clay minerals in the study area a generally strong directionality and makes them show a parallel distribution and lamellar distribution with the cleavage plane of the clay minerals (Figure 5B). The interlayer pores can be filled by pyrite and organic matter (Chen et al., 2013). The grain boundary microfractures developed along the surface of clay minerals are mostly long banding. The long axis of the pores is generally between 10 nm and 50 nm, and the short axis is usually less than 1 μm. Due to the poor compressive resistance of clay minerals, the interlayer pores in clay minerals are sometimes curved under the influence of compaction in the early diagenetic stage. Because of the high content of clay minerals in fine-grained sedimentary rocks, the interlayer pore in clay minerals is the main pore type of the QFY Formation continental fine-grained sedimentary rocks in the study area, and its proportion is higher than that of other pores.

Pyrite is formed in an anoxic reducing environment, and it is nodular-shaped and banded bedding or in a dispersed distribution in claystone and silty claystone. Pyrite is mainly developed in the form of framboidal aggregates in the fine-grained sedimentary rocks of the QFY Formation in the LZ-YB area (Figure 5C). Because the crystals are compacted, the spherical framboidal aggregates are formed. The aggregate diameters range from several microns to tens of microns. The pore size of the intragranular pores in the aggregates is large, approximately 10–80 nm. There are usually clay minerals or organic matter among framboidal pyrite, and the intragranular pores in pyrite are easily blocked by surrounding organic matter or authigenic clay minerals.

Organic matter-hosted pores are usually formed in the process of hydrocarbon generation and exist within organic particles (Borjigin et al., 2021; Li H et al., 2022). The pore sizes of organic matter-hosted pores in the fine-grained sedimentary rocks of the QFY Formation in the study area are mainly nanoscale, with a diameter of approximately 30–50 nm, and the overall connectivity is poor. Two types of organic matter-hosted pores can be seen using SEM. One is in dispersed distribution with the organic matter filling in the silty intergranular pores as the main carrier (Figure 5D). Another is the honeycomb-shaped irregular oval or round hole (Figure 5E), with asphalt filled in microfractures as the main carrier, which is the main place for shale oil and gas preservation. The organic matter-hosted pores in fine-grained sedimentary rocks in the study area, observed by argon-ion-polishing SEM, are developed in the structural vitrinite (Figure 5F), and the pores are not developed in the vitrodetrinite and inertrodetrinite. With increasing burial depth and the influence of compaction, the scale of organic matter-hosted pores increases from microns to nanometers. In addition, organic matter-hosted pores are produced with the generation of organic hydrocarbons. Therefore, its pore characteristics are related to the type of organic matter, TOC, and maturity.

Macroscopically, shale bedding fractures and bedding slip scratches can be seen through the core (Li et al., 2021; Li H et al., 2025). Small-scale inclined joints and high-angle fractures are locally developed, with a length of 3–30 cm, and some fractures are filled with oil bloom and asphalt (Figure 5G). However, irregular inclined joints develop in the siltstone, while vertical fractures develop relatively, with a length of 3–24 cm and a width of 0.2–2 mm. Argon ion polishing SEM reveals that microfractures are mainly developed between clay mineral grains, the margins of organic matter, and the margins of the shell (Figure 5H).

According to the MAPS, the content of silty intergranular pores, interlayer pores in clay minerals, and intragranular pores in pyrite is the highest among all types of pores in continental fine-grained sedimentary rocks in the study area, followed by microfractures, and the content of organic matter-hosted pores is the lowest at less than 10%. (Figure 6).

Jones and Manning (1994) drew the cross plot of the V/Cr and the degree of pyrite (DOP) in the Upper Jurassic mudstone in the Norwegian North Sea (Jones and Manning, 1994). Since then, other scholars have improved the accuracy of the V/Cr data points based on the analysis of the cross plot (Zhang et al., 2018). They believed that when V/Cr was less than 1.2, this was an indication of an oxidizing environment, while when V/Cr was more than 1.2, this indicated an anoxic–dysoxic environment. Combined with the theory by Hatch (Hatch and Leventhal, 1992), Zhang et al. (2018) studied the shale of the Wufeng–Longmaxi Formation in southeastern Chongqing, China, and found that when V/(V + Ni) was less than 0.46, this indicated an oxidizing environment; when the value ranged from 0.46 to 0.57, it indicated a weak oxidizing environment; if the value was observed to be between 0.57 and 0.83, it indicated an anoxic environment; and values between 0.83 and 1.00 indicated a euxinic environment. The value of V/Cr in the study area ranged from 1.22 to 1.54 with an average of 1.39, while the V/(V + Ni) ranged from 0.64 to 0.74 with an average of 0.71. These values reflect that in the sedimentary period, the QFY Formation in the study area was a sedimentary environment with an anoxic reducing condition. According to the scatter plot, V/Cr and V/(V + Ni) are positively correlated with specific surface area (S_bet) and total pore volume (V_bjh) (Figures 10A–D). The specific surface area of pores and pore volumes were observed to be larger in anoxic reducing conditions; in contrast, the two will shrink in oxygen-bearing oxidizing conditions. This shows that redox conditions can control the pore structures of the reservoir.

The value of Sr/Ba was used to determine the paleosalinity of sedimentary water in the QFY Formation, northeastern Sichuan Basin (Fu et al., 2023). The content of Sris relatively small in the freshwater zone, and the Sr/Ba is usually less than 1 in freshwater sediments, while the content of Sr increases correspondingly in saline-water sediments, and the Sr/Ba is greater than 1. The value of Sr/Ba in the study area ranged from 0.19 to 0.35, with an average of 0.28, indicating that in the sedimentary period, the QFY Formation in the LZ-YB area was a freshwater lake sedimentary environment. The value of Sr/Ba is negatively correlated with specific surface area (S_bet) and total pore volume (V_bjh) (Figures 10E, F). This indicates that the higher the salinity of the lake water, the smaller the reservoir space. Lake water with high salinity provides the material environment for the formation of shell limestone because the environment is suitable for the growth of lamellibranchiate organisms. With increasing carbonate content, it is easy for cementation to occur in the process of diagenesis blocking and filling pores and damaging the reservoir space.

According to the statistics, the total content of rare earth elements (∑REE) in the core samples of YL4 well in the study area was found to be approximately 237.68 μg/g, which is relatively high. This demonstrates the large terrestrial input and warm and humid climate in the sedimentary period. The ratios of two elements (Fe/Mn and Mg/Ca), which indicate paleoclimate, averaged 85.57 and 5.01, respectively, in the QFY Formation in the study area. Both high Fe/Mn and low Mg/Ca values reflect the warm and humid climate of the QFY Formation in the study area. The values of ∑REE and Fe/Mn are positively correlated with specific surface area (S_bet) and total pore volume (V_bjh), while the value of Mg/Ca is negatively correlated with them (Figures 10G–L). This means that the reservoir space in fine-grained sedimentary reservoirs develops more easily in warm and moist climates. A wetter climate will bring more precipitation, which will make the water level rise in the lake basin and the deepwater area enlarge (Falahatkhah et al., 2025). All these conditions facilitate the buildup of clay minerals and organic matter. The higher the content of clay minerals and organic matter in fine-grained sedimentary rocks, the more interlayer pores in clay minerals and organic matter-hosted pores, which makes it easier to form a better reservoir space.

The content of the Al element in the study area was found to range from 5.82% to 9.15%, with an average of 7.05%, which indicates high paleoproductivity. By comparing the scatter plots, it can be seen that the content of the Al element is positively correlated with the specific surface area (S_bet) and total pore volume (V_bjh) of the reservoir (Figures 10M, N). Because terrigenous debris can provide nutrients for organic matter production, the more terrigenous inputs into the lake basin, the more favorable it is for the formation of organic matter-hosted pores (Tan et al., 2024). The QFY Formation in the study area is in a warm climate during the sedimentary period, which is conducive to plant growth. The large runoff into the lake brought abundant terrestrial plants and nutrients to the lake basin; as a result, the aquatic plankton thrived, leading to an increase in the productivity of organic matter. With the rising organic matter content, organic matter-hosted pores also increased.

The reservoir space was found to be well developed in fine-grained sedimentary rocks formed in the sedimentary environment with anoxic reducing conditions, fresh water, wettability, and sufficient terrigenous debris. Therefore, the sedimentary environment of the QFY Formation in the study area was found to be one of the controlling factors of the reservoir space in continental fine-grained sedimentary rocks.