Effects of the Sedimentary Environment on Organic-Rich Shale in the Intracratonic Sag of the Sichuan Basin, China

Abstract

The enrichment of organic matter in high-quality marine shale is generally controlled by factors such as the redox conditions of sedimentary environments, productivity levels, terrigenous input, and ancient productivity. However, the controlling effect of the sedimentary environment on organic matter enrichment in intracratonic sag is still unclear. This study takes samples from the Qiongzhusi formation shale in southern Sichuan Basin as the research object, focusing on trace elements as well as rare earth elements in different stratigraphic intervals. The provenance of the Qiongzhusi formation shale is mainly terrigenous, with sediment sources mainly consisting of sedimentary rocks and granites. The primary sedimentary environment transitions from a continental margin setting, influenced by rift-related tectonic activity and sediment influx from adjacent landmasses, to an open oceanic environment characterized by mid-ocean ridge processes and oceanic plate subduction zones. During sedimentation, saline water was present, with predominant sedimentary environments ranging from shallow water to deep water continental shelves. The shale in the study area is characterized by a higher content of silicates and a lower content of carbonate minerals. Its siliceous sources are mainly influenced by biogenic and terrigenous debris, indicating higher ancient primary productivity and representing a favorable target for shale gas exploration.

1. Introduction

Shale gas is an unconventional natural gas resource that, as a clean energy source, is receiving increasing attention from countries around the world [1,2,3,4]. The shale gas from the Wufeng–Longmaxi formation in the Sichuan Basin has already achieved large-scale commercial development [5]. With ongoing breakthroughs in exploration and development, the daily gas production of well Z201, deployed by China National Petroleum Corporation in Neijiang, Sichuan Province, has reached 738,800 cubic meters, indicating that the Qiongzhusi formation in the Sichuan Basin has the potential to become a major layer for shale gas development. The marine shale of the Qiongzhusi formation in the Sichuan Basin, situated within the tectonic background of the Mianyang-Changning intracratonic sag, exhibits high thermal maturation and significant burial depth [6]. The enrichment of organic matter in these shales is typically controlled by factors such as the redox conditions of the sedimentary environment, productivity levels, terrigenous input, and paleoproductivity [7,8]. However, exploration of the Qiongzhusi formation in the Sichuan Basin is still in its early stages, and the role of the sedimentary environment in controlling organic matter enrichment within the intracratonic sag of the Sichuan Basin remains unclear.

To clarify the reservoir characteristics and organic matter enrichment mechanisms of the Qiongzhusi formation shale, this study takes well Z201 in the Zizhong area of the Sichuan Basin as an example. It analyzes the mineralogical characteristics, organic geochemical features, and key trace element characteristics of the Qiongzhusi formation shale. The study reconstructs the sedimentary environment during the deposition of the Qiongzhusi formation shale within the extensional basin, revealing the mechanisms of organic matter enrichment. This provides geological foundations for the next phase of shale gas exploration and development in the Sichuan Basin.

The maturity of organic matter in shale samples is commonly assessed using the vitrinite reflectance (R_o). R_o is a measure of the reflectance of vitrinite, a component of organic matter, under a microscope. It provides an indication of the thermal maturity of the organic matter, which is crucial for evaluating the potential for hydrocarbon generation. In this study, the organic geochemical characteristics of the shale samples are described using several parameters. S₀ refers to the initial organic carbon content of the shale, representing the total amount of organic carbon present in the rock (measured in mg/g). S₁ represents amount of free hydrocarbons released from the sample upon heating, which provides an indication of the volume of hydrocarbons that are readily extractable (measured in mg/g). S₂ denotes the potential hydrocarbon generation capacity of the sample, calculated as the difference between the total hydrocarbons produced upon heating and the free hydrocarbons measured as S₁ (measured in mg/g). S₂ reflects the remaining organic matter’s potential to generate hydrocarbons upon further maturation.

2. Geological Setting

The Sichuan Basin, located in the northwest part of the Yangtze Plate, is a multi-layered, rhomboid superposed structure oil and gas basin [9]. Well Z201 is situated structurally in the low-amplitude fold belt of central Sichuan, with the ancient landform located in the central segment of the Mianyang-Changning intracratonic sag. During the Late Sinian to Middle Cambrian period, the Sichuan Basin was influenced by the Tongwan and Xingkai rift movements, as well as by basement uplift and crustal extension. This led to significant uplift and erosion of the strata, resulting in the formation of the north–south-oriented Mianyang-Changning intracratonic sag [10,11]. The sag can be divided into three distinct areas: the external highlands, the sag margins, and the central sag [12]. During the sedimentation of the Qiongzhusi formation, the sag was in its mature stage of development, with a set of thick black shales of shelf facies deposited in the central sag, reaching thicknesses of 300 to 700 m [13]. The Qiongzhusi formation is referred to by different names in various regions, such as the Niutitang formation, Shuijingtao formation, Guojiahe formation, and Jiulaodong formation [14]. The Mianyang-Changning intracratonic sag primarily features middle to deep water shelf facies deposits, while the sag itself exhibits outer shallow shelf facies. The water body within the sag was notably deeper than that on the flanks (Figure 1). The Qiongzhusi formation in the Sichuan Basin is deposited above the Maidiping or Dengying formations and overlain by the Canglangpu formation. Its burial depth is generally less than 5000 m, becoming shallower towards the basin margins [15].

3. Samples and Experimental Methods

3.1. Samples Collection

Thirty-one samples were collected from the Z201 shale gas well, located at the center of the Mianyang-Changning extensional trough in the Sichuan Basin. Subsequently, optical thin section observation, scanning electron microscopy observation, total organic carbon (TOC) analysis, X-ray diffraction analysis of whole rock mineral content, shale pyrolysis analysis at high temperatures, as well as major, trace, and rare earth element analyses were conducted based on these samples. The experiments were completed in the National Key Laboratory of Oil and Gas Resources and Engineering, China University of Petroleum (Beijing).

3.2. Total Organic Carbon Experiment

The TOC content in shale was measured using a CS230HC carbon (Lico Instruments Hong Kong Limited, Hong Kong, China) and sulfur analyzer. Initially, the crucible was calcined at 1000 °C for 2 h. Shale samples were ground to less than 0.2 mm, placed into the crucible, and treated with 1 mol/L hydrochloric acid to remove organic carbon. Excess hydrochloric acid was then washed off with distilled water. The crucible with the treated samples was dried in an oven at 60–80 °C. The samples were subsequently analyzed using the carbon and sulfur analyzer to determine CO₂ and SO₂ levels based on the gas absorption light intensity. Total organic carbon content was calculated by relating the carbon and sulfur measurements to the sample mass.

3.3. X-ray Diffraction Experiment

A D2PHASER X-ray diffractometer and Jade 6.5 software were used to analyze the mineral content of shale samples. The main specifications of the instrument include a Cu target, ceramic X-ray tube, and Kα radiation. The tube operates at a voltage of 30 kV and current of 10 mA, using a LynxEye™ one-dimensional array detector (Brook AXS GmbH, Munich, Germany). The samples were dried at under 60 °C, ground to a particle size less than 40 μm, and placed into the sample loading paddle, ensuring a rough surface. When setting up the instrument, we first selected the X-ray source and set it to a Cu target, ensuring the tube voltage is 30 kV and the current is 10 mA. Next, we configured the detector with the LynxEye^TM one-dimensional array detector. Then, we chose the scanning mode, setting the 2θ range to 5–70°, the step size to 0.01°, and the scan speed to 0.5°/min. This paddle was then inserted into the X-ray diffractometer, where the sample was irradiated, and diffraction patterns were recorded. The obtained data were compared with standard mineral X-ray diffraction patterns to identify the mineral species.

3.4. Major Elements

The major elements (Al, K, Na, P) in the shale samples were analyzed using an Axiosm AX X-ray fluorescence spectrometer (Spectris Limited, Amsterdam, The Netherlands). The test material was fused with anhydrous lithium tetraborate, ammonium nitrate as the oxidizing agent, and lithium fluoride along with a small amount of lithium bromide, serving as flux and release agents, in a mass ratio of 1:8. The samples were melted into glass at temperatures ranging from 1150 °C to 1250 °C using a sample melting machine. Measurements were then performed using the X-ray fluorescence spectrometer. Compton scattering rays served as an internal standard for nickel, copper, strontium, and zirconium to correct for matrix effects. For other elements, the absorption enhancement effects were adjusted using theoretical α coefficients, and the concentrations of major and minor components were determined from fluorescence intensities. Based on repeated measurements of the same sample, the standard deviation of the experimental results is 0.5%, and the error is 0.5%.

3.5. Trace Elements

Trace elements (Co, Li, Ni, Ba, V, Th, Zr, Cr, U) in shale samples were analyzed using an Agilent 7900 Inductively Coupled Plasma Mass Spectrometer (ICP-MS) (Agilent Technologies, Santa Clara, CA, USA). The samples were initially dissolved with hydrofluoric and nitric acids in a closed dissolver. Hydrofluoric acid was evaporated using an electric hot plate, and the residue was then re-dissolved in nitric acid under sealed conditions. After dilution, the samples were analyzed directly using the ICP-MS with an external standard method. Based on repeated measurements of the same sample, the standard deviation of the experimental results is 1%, and the error is 0.5%.

3.6. Rare Earth Elements

The ICP-MS was used in the experiment for the determination of rare earth elements in shale samples. During the experiment, the samples were decomposed using the sodium peroxide melting method. The measured elements were precipitated along with the matrix elements in an alkaline medium. A large amount of melting agent was separated off by filtration. Then, the precipitates were dissolved using acid and then directly determined using the ICP-MS external standard method. Based on repeated measurements of the same sample, the standard deviation of the experimental results is 1%, and the error is 1%.

4. Results

4.1. Petrological Characteristics

The lithology of the Qiongzhusi formation in southern Sichuan is primarily composed of black, gray-black, or gray shale; calcareous siltstone; and gray-black sandy shale. The shale typically exhibits a fine grain size, with small and uniform particles mainly composed of clay minerals, quartz, and feldspar and particle sizes generally at the mud grain level. At the base of the Qiong 1-1 submember, the lithology is dominated by black and gray-black mud shale, showing distinct horizontal bedding. Towards the top of the Qiong 1-1 submember, there are more occurrences of siltstone and dark gray shale exhibiting thin laminated interbedding, with relatively intact layers (Figure 2a,c,d). In the Qiong 1-2 submember, the lithology consists mainly of thick sections of black shale with some interbedded gray siltstone (Figure 2b). In the Qiong 2 member, the lithology is characterized by thick sections of black shale interbedded with gray-black sandy shale, with calcite veins visible under a single polarized light microscope (Figure 2e,f).

4.2. Characteristics of Organic Matter

The organic carbon content is the most fundamental and critical parameter in the evaluation of source rocks and shale gas systems [16]. For well Z201, the Qiongzhusi formation samples exhibit a wide range of organic carbon content, ranging from 0.28% to 2.37%, with an average of 1.31% (

Table 1. Organic matter abundance content of shale samples from well Z201 of the Qiongzhusi formation in the southern Sichuan Basin.

Sample Number	Depth/m	Interval	S0/mg·g−1	S1/mg·g−1	S2/mg·g−1	Tmax/°C	TOC/%
Z1	4451.74	Q2	0.016	0.16	0.25	380.38	0.28
Z2	4455.09	Q2	0.01	0.12	0.19	368.63	0.84
Z3	4463.15	Q2	0.017	0.63	0.47	388.88	1.55
Z4	4466.28	Q2	0.018	0.89	0.61	394.63	0.95
Z5	4469.09	Q2	0.018	1.32	0.86	393.25	0.98
Z6	4471.31	Q2	0.004	1.11	0.88	396.50	1.31
Z7	4480.52	Q2	0.008	2.07	1.39	403.13	1.42
Z8	4483.91	Q2	0.009	3.66	2.31	416.00	1.08
Z9	4485.82	Q2	0.007	2.69	1.80	406.50	1.42
Z10	4486.04	Q2	0.007	0.87	0.60	406.00	1.01
Z11	4502.42	Q1 2	0.007	0.75	0.51	385.13	1.18
Z12	4506.37	Q1 2	0.009	0.37	0.36	372.38	1.23
Z13	4548.07	Q1 2	0.006	0.30	0.32	374.13	1.14
Z14	4552.08	Q1 2	0.007	0.27	0.29	368.00	1.49
Z15	4554.15	Q1 2	0.008	0.29	0.35	372.50	1.71
Z16	4555.98	Q1 2	0.005	0.20	0.26	366.00	1.48
Z17	4558.01	Q1 2	0.007	0.18	0.25	370.50	1.52
Z18	4560.21	Q1 2	0.009	0.16	0.22	365.50	1.38
Z19	4572.46	Q1 2	0.007	0.11	0.23	377.13	1.69
Z20	4592.57	Q1 2	0.005	0.13	0.25	371.13	2.30
Z21	4612.22	Q1 2	0.005	0.13	0.24	367.75	2.14
Z22	4749.16	Q1 1	0.006	0.14	0.23	361.63	0.49
Z23	4751.47	Q1 1	0.004	0.28	0.61	397.50	2.25
Z24	4756.96	Q1 1	0.005	0.71	2.18	406.50	2.35
Z25	4758.91	Q1 1	0.004	0.56	1.97	402.50	1.93
Z26	4761.15	Q1 1	0.006	0.11	0.33	392.25	2.22
Z27	4767.74	Q1 1	0.008	0.09	0.26	393.75	2.28
Z28	4769.68	Q1 1	0.003	0.07	0.20	385.00	2.37
Z29	4771.83	Q1 1	0.002	0.07	0.19	384.50	2.02
Z30	4773.93	Q1 1	0.001	0.07	0.19	384.00	2.32
Z31	4777.87	Q1 1	0.001	0.07	0.19	384.38	2.23

Caption: Q₂: Qiong 2 member, Q₁ ²: Qiong 1-2 submember, Q₁ ¹: Qiong 1-1 submember.

). Experimental data indicate that the residual organic matter abundance in the Qiongzhusi formation of this well is moderate and exhibits two relatively high w(TOC) intervals, corresponding to the lower part of the Qiong 1-1 submember and the lower part of the Qiong 1-2 submember, which aligns with the two high gamma-ray segments. Among them, the w(TOC) is highest at the bottom of the Qiong 1 submember, generally exceeding 1.5%, approaching 1% in the middle, and less than 1% in the upper part. Overall, the high w(TOC) interval (1%) can reach 130 m.

4.3. Characteristics of Mineral Composition

Based on core description, optical and scanning electron microscopy observations, and XRD whole-rock mineral analysis of shale samples from well Z201, the mineral composition characteristics of different intervals of the Qiongzhusi formation are identified. Experimental results indicate that the mineral composition of the Qiongzhusi formation shale is relatively complex, including minerals such as quartz, feldspar, calcite, dolomite, clay minerals, and pyrite. Quartz and feldspar are the predominant minerals, accounting for more than 50% of the material with quartz being predominant in the Qiong 1-1 submember, exceeding 50%. The content of quartz gradually decreases with decreasing depth. Carbonate minerals mainly comprise calcite and dolomite, with the content in the Qiong 1 member not exceeding 10% and increasing to 10–20% in the Qiong 2 member, with overall low carbonate mineral content in the formation. The clay mineral content ranges from 10% to 30%, showing significant differences between different intervals, with generally higher clay mineral content in the Qiong 2 member compared to the Qiong 1 member. Pyrite is present throughout the formation, with a content ranging from 3% to 6%. Phosphorite is developed in a very few intervals (

Table 2. Mineral composition content of Qiongzhusi formation shale in the southern Sichuan Basin.

Sample Number	Depth/m	Interval	Mineral Content/%
			Quartz	Feldspar (Albite)	Calcite	Dolomite	Clay Mineral (Kaolinite, Illite, Illite Mixed Layer and Chlorite)	Pyrite
Z1	4451.74	Q2	36	19	4	7	31	3
Z2	4455.09	Q2	36	19	4	7	31	3
Z3	4463.15	Q2	38	17	6	4	32	6
Z4	4466.28	Q2	39	13	5	5	34	4
Z5	4469.09	Q2	34	12	9	8	32	4
Z6	4471.31	Q2	35	14	5	4	37	5
Z7	4480.52	Q2	38	15	6	6	31	4
Z8	4483.91	Q2	40	13	7	6	30	5
Z9	4485.82	Q2	38	13	8	5	33	5
Z10	4486.04	Q2	36	15	8	5	3	4
Z11	4502.42	Q1 2	38	18	8	4	29	4
Z12	4506.37	Q1 2	43	33	3	2	17	3
Z13	4548.07	Q1 2	41	35	2	2	18	3
Z14	4552.08	Q1 2	41	31	2	2	18	6
Z15	4554.15	Q1 2	44	28	2	3	21	3
Z16	4555.98	Q1 2	38	34	4	3	19	3
Z17	4558.1	Q1 2	39	32	1	3	2	3
Z18	4560.21	Q1 2	41	33	1.	2	18	4
Z19	4572.46	Q1 2	41	37	2	2	16	3
Z20	4592.57	Q1 2	42	36	2	2	17	2
Z21	4612.22	Q1 2	43	30	2	2	18	5
Z22	4749.16	Q1 1	40	37	3	2	16	3
Z23	4751.47	Q1 1	45	34	3	3	10	5
Z24	4757.96	Q1 1	25	19	13	28	11	3
Z25	4758.91	Q1 1	40	40	4	3	10	4
Z26	4761.15	Q1 1	40	37	5	4	11	4
Z27	4767.74	Q1 1	48	31	2	6	9	4
Z28	4769.68	Q1 1	49	11	2	14	10	4
Z29	4771.83	Q1 1	68	17	2	1	9	3
Z30	4773.93	Q1 1	80	5	4	1	9	2
Z31	4777.87	Q1 1	77	10	7	0	5	1

Caption: Q₂: Qiong 2 member, Q₁ ²: Qiong 1-2 submember, Q₁ ¹: Qiong 1-1 submember.

).

The primary diagenetic processes in the Qiongzhusi formation shale of the Sichuan Basin include compaction, dissolution, replacement, and cementation. With burial depths generally below 5000 m, compaction is intense, often resulting in the fragmentation of brittle minerals into fine components and mineral fractures. The Qiongzhusi formation shale frequently exhibits argillaceous, siliceous, and calcareous cementation, with observable replacement of potassium feldspar by quartz. In terms of dissolution, feldspar dissolution in the Qiongzhusi formation shale is highly developed, featuring irregularly dissolved particle edges and intragranular dissolution pores.

4.4. Major and Trace Elements

Experimental analysis of major and trace elements indicates that the Qiongzhusi shale in well Z201 shows increased concentrations of Al and Ba compared to other elements in the region (

Table 3. Major and trace element contents in shale samples from the Qiongzhusi formation in the southern Sichuan Basin.

Sample Number	Depth/m	Interval	Major Elements/%			Trace Elements/ppm
			Al	Na	K	Li	Co	Ni	Ba	V	Th	Zr	P	Cr	U
Z1	4451.74	Q2	7	1	3	49	16	53	1375	204	10	161	930	130	4
Z2	4455.09	Q2	8	1	3	43	19	49	1545	230	12	158	760	140	9
Z3	4463.15	Q2	7	2	3	37	14	50	1270	158	13	273	1030	140	13
Z4	4466.28	Q2	7	2	3	35	13	59	1300	151	12	282	1050	130	12
Z5	4469.09	Q2	8	2	3	40	14	50	1405	157	13	280	880	140	11
Z6	4471.31	Q2	7	2	3	37	14	38	1380	152	12	265	860	130	9
Z7	4480.52	Q2	7	2	3	38	14	47	1340	137	12	267	960	150	11
Z8	4483.91	Q2	7	2	3	38	13	61	1280	150	11	282	1040	140	9
Z9	4485.82	Q2	7	2	3	36	15	64	1190	138	11	248	930	140	12
Z10	4486.04	Q2	7	2	2	35	12	54	1350	156	11	280	1140	120	11
Z11	4502.42	Q1 2	7	2	3	37	13	62	1280	152	13	307	980	140	13
Z12	4506.37	Q1 2	8	2	3	40	15	52	1260	153	12	222	1000	140	11
Z13	4548.07	Q1 2	7	2	3	44	16	82	1570	227	12	194	900	130	11
Z14	4552.08	Q1 2	7	2	3	45	15	54	1525	168	12	189	880	140	14
Z15	4554.15	Q1 2	7	2	3	46	16	80	1450	175	11	196	890	140	14
Z16	4555.98	Q1 2	8	2	3	45	16	78	1470	185	12	185	860	140	12
Z17	4558.01	Q1 2	8	1	3	44	17	54	1515	176	12	173	780	140	11
Z18	4560.21	Q1 2	2	0	1	12	4	12	1595	54	3	46	460	40	3
Z19	4572.46	Q1 2	8	2	3	48	18	90	1495	179	12	194	880	120	13
Z20	4592.57	Q1 2	7	2	2	33	15	142	1430	536	12	306	1140	130	25
Z21	4612.22	Q1 2	7	2	3	41	19	54	1335	145	12	264	990	120	10
Z22	4749.16	Q1 1	7	2	3	28	18	90	1365	1435	13	233	1150	180	14
Z23	4751.47	Q1 1	6	1	2	25	13	102	1405	1300	9	187	880	210	14
Z24	4756.96	Q1 1	6	1	2	30	15	100	1345	1315	10	193	1120	200	15
Z25	4758.91	Q1 1	6	2	2	29	16	72	1395	169	9	174	830	140	13
Z26	4761.15	Q1 1	6	2	2	25	16	85	1430	152	10	184	850	140	12
Z27	4767.74	Q1 1	7	2	2	25	18	74	1730	160	10	189	990	130	30
Z28	4769.68	Q1 1	6	2	2	27	17	114	1395	966	10	181	1040	160	18
Z29	4771.83	Q1 1	7	2	2	30	19	137	1570	1795	11	196	1320	180	17
Z30	4773.93	Q1 1	6	2	2	26	15	106	1470	1570	11	217	1050	200	15
Z31	4777.87	Q1 1	5	2	2	25	15	123	1295	1590	8	164	890	160	16
Element detection limit			0.01%	0.01%	0.01%	0.1 ppm	0.1 ppm	0.01 ppm	1 ppm	1 ppm	0.01 ppm	1 ppm	1 ppm	1 ppm	0.1 ppm

Caption: Q₂: Qiong 2 member, Q₁ ²: Qiong 1-2 submember, Q₁ ¹: Qiong 1-1 submember.

). The Al content ranges from 2% to 8%, with an average of 6.77%; Ba content varies between 1190 and 1730 ppm, averaging 1472.26 ppm; V content ranges from 54 to 1795 ppm, averaging 486.90 ppm; Ni content is comparatively low, ranging from 12 to 142 ppm, with an average of 73.7 ppm. Compared to the post-Archean average shale (PAAS), the Qiongzhusi shale exhibits significant enrichment in V, while Ni, Cr, Zr, Sr, and Co show depletion. The remaining elements show comparable concentrations to those in the PAAS.

4.5. Characterization of Rare Earth Elements

The rare earth element (REE) analysis experiments showed that the ∑REE of 31 shale samples from the Qiongzhusi Formation in the southern Sichuan area ranged from 64.09 to 223.96 ppm, with an average of 181.35 ppm, which is similar to the total amount of rare earth elements in Australian shales (184.77 ppm). The content of LREE ranged from 43.90 to 168.20 ppm with an average of 134.70 ppm. The HREE content ranged from 20.44 to 61.55 ppm with an average of 46.68 ppm. The La/Yb ratios ranged from 9.61 to 14.45 with an average of 12.33, which is lower than that of Australian shale (13.54). The rare earth elements of the shale samples, normalized to PAAS (Australian Shale), show a deficit of light rare earth elements (LREE), specifically La, Nd, Pr, and Ce. The elements Lu also exhibit a notable deficiency, while other LREEs do not display significant deficits. In contrast, heavy rare earth elements (HREE) are relatively horizontal and relatively enriched (Figure 3).

5. Discussion

5.1. Total Organic Carbon and Organic Matter

Existing research indicates that, on a horizontal plane, organic matter is more concentrated within the Mianyang-Changning extensional trough compared to outside the trough [17]. Vertically, the Qiong-1 submember shows the highest organic matter content, with a trend of decreasing TOC from the base to the top of the formation. Organic matter in the Qiong 1-1 submember can be observed using scanning electron microscopy (SEM) (Figure 4). Sea level fluctuations further influence the enrichment and preservation of organic matter. Therefore, the Qiongzhusi formation shale is characterized by high organic matter abundance, moderate maturity, and excellent hydrocarbon source rock potential.

Currently, relevant studies have shown significant variations in the maturity of the Qiongzhusi formation’s organic matter. Studies suggest that the overall equivalent R_o of the Qiongzhusi formation is generally greater than 2.0%, corresponding to an increasing maturation trend from the central to northern parts of the Sichuan Basin [18]. Asphalt reflectance R_o on the northern margin of the Sichuan Basin can locally reach 3.0% to 3.5%. In this study, the maturity of source rocks in well Z201 of the Qiongzhusi formation was determined using thermal alteration T_max data from 31 rock samples. The maturity stages of black shale are classified as follows: R_o < 0.5% and T_max < 435 °C indicate an immature stage; 0.5% < R_o < 1.3% and 435 °C < T_max < 475 °C indicate a mature stage; 1.3% < R_o < 2% and 475 °C < T_max < 500 °C indicate a high maturity stage; T_max > 500 °C indicates an overmature stage, with R_o ranging from 2% to 3% indicating early over-maturity and from 3% to 4% indicating late over-maturity [19]. For highly evolved source rocks of the Qiongzhusi formation, the overall T_max measured by rock pyrolysis is less than 435 °C. However, this does not necessarily indicate that the rock is in an immature stage. The reason is that the source rocks of the Lower Cambrian typically have a conversion rate reaching up to 90%, resulting in lower S₂ values in pyrolysis. This makes the samples susceptible to “pseudo-peaks” caused by contaminants, which can obscure the true S₂ peak, leading to an inaccurately low T_max value. Therefore, this study shows that T_max fluctuates between 361.63 °C and 416.00 °C. S₁ ranges from 0.07 mg·g⁻¹ to 3.66 mg·g⁻¹, and S₂ ranges from 0.19 mg·g⁻¹ to 2.31 mg·g⁻¹. The relatively high values of S₁ and S₂ indicate that there is a significant amount of residual hydrocarbons in well Z201 of the Qiongzhusi formation, indicating good hydrocarbon generation potential and suggesting that the organic matter evolution of source rocks has entered a high maturity stage.

5.2. Sediment Source

In different sedimentary environments, the contents and ratios of major and trace elements in shale vary [20]. Therefore, certain specific content and ratios of major and trace elements can serve as important indicators for identifying sediment sources and discerning the tectonic background of source areas. For instance, the value of n(Al₂O₃)/n(Al₂O₃ + Fe₂O₃) can indicate the tectonic background of sediments, while the value of n(SiO₂)/n(Al₂O₃) can indicate the provenance of sediments.

Previous studies suggest that when the n(SiO₂)/n(Al₂O₃) ratio in rock samples approaches 3.6, it indicates a predominantly terrestrial source for the rock sediment [21]. In the study area, the n(SiO₂)/n(Al₂O₃) ratio ranges from 3.52 to 6.33, with an average of 4.81, and shows minimal variation longitudinally (Figure 5), indicating a predominantly terrestrial source. The tectonic background of sediment is typically determined using the n(Al₂O₃)/n(Al₂O₃ + Fe₂O₃) ratio. Ratios between 0.1 and 0.4 suggest an oceanic ridge and seamount sedimentary environment, while ratios between 0.6 and 0.9 indicate a continental margin sedimentary environment. Ratios between 0.4 and 0.7 suggest a deep-sea oceanic sedimentary environment [22,23]. In the study area, the n(Al₂O₃)/n(Al₂O₃ + Fe₂O₃) ratio ranges from 0.31 to 0.74, as shown in the Fe₂O₃/TiO₂-Al₂O₃/(Al₂O₃ + Fe₂O₃) ternary diagram (Figure 6), indicating a transitional tectonic environment from continental margin sedimentation to deep-sea oceanic sedimentation. The intersection of the La/Yb and ΣREE diagrams (Figure 7) indicates that the sediments primarily originate from deep-sea sediments, granite, and alkaline basalt. Considering the provenance tracing of Qiongzhusi formation sediment and the Paleoproterozoic sedimentary configuration of the ancient continent, it is likely that the Lumian ancient continent provided deep-sea sedimentary mudstone, submarine volcanic basalt, and mafic rocks for the Qiongzhusi formation in the rift zone to the east. Based on geochemical analysis, during the Qiongzhusi period, the Lumian ancient continent served as a source area, with the Qiongzhusi shale exhibiting particles of fine sand and mud grades. This is likely the result of long-distance transport and is closely related to the fine particle size and low quartz content of the source area sediment. The sedimentary materials of the Qiongzhusi formation are primarily derived from sedimentary rocks and granite.

5.3. Siliceous Source

Typically, the Si/(Si + Fe + Al) ratio reflects the source of silicates, with higher values, usually greater than 0.85, indicating a predominantly biogenic origin of silica in the study area [24]. However, test results indicate an average Si/(Si + Fe + Al) ratio of 0.78 in the study area (Figure 5), suggesting a combined influence of biogenic and terrigenous on the siliceous origin of the Qiongzhusi formation in the southern Sichuan Basin. Additionally, there is an influence from the ancient Kangdian landmass input. During the sedimentation period, prolific development of biota such as sponge spicules, radiolarians, and algae in deep water environments provided conditions for the formation of silicates [25].

5.4. Paleowater Depth

A considerable amount of research indicates that variations in the content of Sr and Ba in sedimentary environments can serve as indicators of ancient salinity fluctuations [26]. The Sr/Ba ratio to some extent reflects the depth of the sedimentary water body. Generally, the Sr/Ba ratio decreases from the coast to the deep sea. Additionally, Zr in sedimentary environments is a typical lithophilic element primarily transported mechanically and deposited in offshore areas. However, the distribution of Zr in sedimentary rocks is influenced by Al elements. Thus, the Zr/Al ratio can represent the proximal terrestrial components and variations in water depth transported over short distances [23,27]. Generally, a smaller Zr/Al ratio indicates a greater distance from the shore and deeper waters. By selecting Sr/Ba and Zr/Al values sensitive to ancient salinity changes for spot discoveries (Figure 8), it was found that the depth of the sedimentary water body during the deposition period of the Qiongzhusi formation corresponds closely to ancient salinity fluctuations. During the deposition period of the Qiongzhusi formation, significant marine transgression occurred, which led to the alleviation of the previously restrictive sedimentary conditions in southwestern Sichuan, resulting in a more favorable environment for the deposition of organic-rich shales. This led to deepening water and decreasing salinity, gradually forming a sedimentary environment dominated by shallow to deep water continental shelves. Overall, the shale of the Qiongzhusi formation exhibits a trend of decreasing water depth from bottom to top.

5.5. Paleo-Redox Conditions

The oxidation-reduction sensitive trace elements are those trace elements whose solubility is significantly controlled by the redox state of the sedimentary environment, leading to their migration and self-enrichment in reducing water bodies and sediments [28]. The enrichment and accumulation of elements such as Ni, U, Th, and Cr in sediments can reflect the redox environment at that time [29]. Therefore, this study employs several trace element ratios including U/Th, V/Cr, and V/(V + Ni) to determine the paleo-redox conditions.

Typically, a U/Th ratio <0.75 indicates oxidizing conditions, a ratio ranging from 0.75 to 0.15 indicates oxygen-poor conditions, and a ratio >1.25 indicates anoxic-sulfidic conditions [30,31]. The ratios of V/Cr and V/(V + Ni) can also serve as indicators of paleo-redox conditions, where a V/Cr ratio <2 indicates oxidizing conditions, a ratio ranging from 2 to 4.25 suggests oxygen-poor conditions, and a ratio >4.25 indicates weakly oxidizing-reducing conditions. When V/(V + Ni) ranges from 0.46 to 0.6, it indicates oxygen-poor conditions, a ratio ranging from 0.6 to 0.84 suggests oxygen-depleted conditions, and a ratio >0.84 indicates sulfidic conditions.

According to the geochemical indicator diagrams of oxidation-reduction in the Qiongzhusi formation in the southern Sichuan Basin (Figure 9 and Figure 10), during the deposition of the Qiongzhusi 1-1 submember, the water body was relatively deep, primarily situated in an anoxic and sulfidic environment within the deep water continental shelf. As the depth of the sedimentary water body decreased, the overlying Qiongzhusi 1-2 submember and Qiongzhusi 2 member gradually transitioned from anoxic and sulfidic conditions to anoxic and weakly oxidizing conditions. A comprehensive analysis of the stratigraphic distribution trends of elements and various indicators indicates an early sulfidation enhancement in the Qiongzhusi 1 member, followed by a general weakening of sulfidic conditions until suboxic and sulfidic conditions prevailed. In general, the Qiongzhusi 1-2 submember of the lower part of the Qiongzhusi formation in the southern Sichuan region was under an anoxic and sulfidic environment, with the water body gradually oxidizing upwards, and by the Qiongzhusi 2 Member, it was in a suboxic and weakly oxidizing environment.

5.6. Paleoproductivity

The level of paleoproductivity directly determines the material basis for the formation of organic matter, and high paleoproductivity is conducive to the formation of shale with high TOC content [32]. Ancient climate has a significant impact on the level of paleoproductivity. In arid to semi-humid climates, an increase in climate humidity can improve productivity levels, while excessively humid and hot climates are not conducive to the survival of organisms and can reduce paleoproductivity levels.

Previous studies have shown that Ba and P are indicators used to infer paleoproductivity [33]. In practical applications, the priority for inferring paleoproductivity using Ba is higher than that of P. Among them, the content of biogenic barium (Ba_bio) is an important indicator of primary paleoproductivity [34]. Typically, the content of barium associated with biological activities is calculated, excluding the input of terrigenous barium, to indicate paleoproductivity. The biogenic barium content is calculated using Formula (1) [35]:

Ba_bio = Ba_sample − Al_sample × (Ba/Al)_PAAS

(1)

Here, Ba_bio is the content of biogenic barium, in μg/g; Ba_sample indicates the content of Ba measured in shale samples, in μg/g; Al_sample is the content of Al measured in shale samples, in μg/g; (Ba/Al)_PAAS is the standard value of Australian shale.

By plotting the barium content of shale in the Qiongzhusi formation, it is clear that the paleoproductivity of the Qiongzhusi formation in well Z201 is extremely high, with an average Ba_bio value of 1411.56 μg/g. Within the Qiongzhusi formation, the paleoproductivity of the Qiong 1 member is slightly higher than that of the Qiong 2 member (Figure 11), indicating that the bottom shale of the Qiongzhusi formation in well Z201 has a higher paleoproductivity level than the top shale. This inference is highly consistent with the TOC content of the intervals, with the TOC content of the Qiong 1 member higher than that of the top Qiong 2 member. Additionally, the calculated Ba_bio content is close to the measured barium content of the shale samples, suggesting minimal terrigenous input of barium. The barium in the shale of this interval is primarily of biogenic origin. Overall, well Z201 in the rift basin within the deep water continental shelf area exhibits a very high paleoproductivity level.

5.7. Organic Matter Enrichment Model

The enrichment of organic matter in shale is not controlled by a single factor but by multiple elements such as paleo-water depth, paleo-climate, and paleo-redox conditions. Higher paleoproductivity can provide abundant sources of organic matter, while anoxic depositional environments can offer favorable preservation conditions. During the deposition of the Qiongzhusi 1 submember, the southern Sichuan Basin was characterized by predominantly clayey mud shale and black shale. This period coincided with a rapid rise in sea levels, resulting in relatively deep depositional water bodies with low terrigenous input, tranquil water conditions, weak hydrodynamic action, and abundant nutrients. The values of biogenic barium and TOC were relatively high, favoring high primary paleoproductivity levels. Concurrently, the paleo-redox conditions were anoxic and sulfidic, gradually creating favorable conditions for organic matter preservation, marking a phase of increasing organic matter enrichment. During the deposition of the Qiongzhusi 1-2 submember, the southern Sichuan Basin was characterized by the predominant deposition of black shale, siltstone, and sandy shale in both deep and shallow water continental shelf environments. This period coincided with the early stage of sea-level regression, resulting in shallower depositional water bodies compared to the deposition of the Qiongzhusi 1-1 submember. Terrigenous input was lower with finer sediment grains relative to the Qiongzhusi 1-1 submember, yet paleoproductivity levels remained high. The depositional environment was predominantly a continental margin, representing a key stage of organic matter enrichment. During the deposition of the Qiong 2 member, as sea levels gradually decreased, the evolution shifted from deep to shallow water continental shelf environments, characterized mainly by the deposition of shallow water siltstone and sandy shale. In combination with oxidizing conditions, paleoproductivity levels were relatively lower compared to the Qiongzhusi 1 submember, resulting in a decrease in the degree of organic matter enrichment. This could be attributed to slow oxidation of organic matter during sedimentation or an increase in water column circulation and terrigenous sediment supply, diluting the enrichment of organic matter in both seawater and sediments (Figure 12).

6. Conclusions

(1)

The sedimentary source of the Qiongzhusi formation shale is terrigenous, with the sediment mainly derived from sedimentary rocks and granite. Its sedimentary environment transitions from a continental margin to a deep-sea environment. The shale is characterized by abundant silicates and traces of carbonate minerals, and the siliceous components are primarily of biogenic origin. During the deposition of the Qiongzhusi formation, a sedimentary environment dominated by shallow to deep water continental shelves gradually formed. The Qiongzhusi formation shale exhibits a trend of decreasing water depth from bottom to top.

(2)

The lower section of the Qiongzhusi formation was deposited in an anoxic and sulfidic environment, gradually transitioning upwards to a hypoxic and weakly oxidizing environment. Within the rift basin located in the deep water continental shelf area, well Z201 exhibits a high overall paleoproductivity level, providing a favorable basis for the development and enrichment of shale gas.