Controlling Factors of Organic Matter Enrichment in Marine–Continental Transitional Shale: A Case Study of the Upper Permian Longtan Formation, Northern Guizhou, China

Abstract

The marine–continental transitional shale of the Upper Permian Longtan Formation in northern Guizhou is an important source rock in the upper Yangtze region of China, and it holds significant potential for the exploration of shale gas. To investigate the correlation between sedimentary conditions and the accumulation of organic matters in marine–continental transitional shale, this paper performed an extensive analysis using organic geochemical testing, organic petrology examination, a cross-section polisher–scanning electron microscope (CP-SEM), and geochemical analysis. The Jinsha and Dafang drilling cores were selected as the research subjects. The results showed that the TOC of the Longtan Formation in the study area was relatively high, and the TOC content of the tidal flat–lagoon environment (average of 8.37%) was significantly higher than that of the delta samples (average of 2.77%). The high content of Al₂O₃ (average of 17.41% in DC-1, average of 16.53% in JC-1) indicated strong terrigenous detrital input. The proxies indicated that the Longtan Formation shale in northern Guizhou was deposited in a climate that was both warm and humid, with oxic–dysoxic sedimentary water characterized by high biological productivity and a rapid sedimentation rate. The organic-rich shales during the marine and continental transitional phases were affected by various factors, including the paleo-climate, water redox properties, paleo-productivity, sedimentation rate, and other variables, which directly or indirectly impacted the availability, burial, and preservation of organic matter.

1. Introduction

Organic-rich shale plays a dual role as both the source rock for conventional hydrocarbon reservoirs and the primary target for shale oil and gas exploration [1,2]. Additionally, it acts as a repository of invaluable data regarding the geologic climate and paleo-environmental conditions during its deposition; thus, it possesses substantial scientific research significance [3,4,5]. The buildup of organic matter within black shale involves intricate physicochemical processes that are affected by various factors, including the level of paleo-biological productivity, the oxidation–reduction conditions of the underlying water, and the sedimentation rate, as well as post-depositional degradation processes [6]. Therefore, investigating these governing factors along with their associated formation mechanisms is of paramount importance within the realm of unconventional hydrocarbon sedimentology research. The factors that govern the accumulation of organic matter in sediments found in marine environments have been the subject of extensive discussions and debates, and there is a prevailing consensus that the preservation of substantial biomass quantities is closely tied to the presence of abundant organic matter [7,8]. This prerequisite for biomass preservation relies on thriving microbial communities and favorable sediment conditions, such as hypoxia and optimal sedimentation rates. Therefore, the enrichment of organic matter can be attributed to various factors, which can be summarized as primary productivity at the ocean surface and favorable preservation conditions. Different scholars hold different views on whether biological productivity or preservation conditions are more critical for organic matter enrichment, and the enrichment models are called the productivity model and the preservation model [5,9,10]. According to the productivity model, biological productivity at the ocean surface primarily regulates organic matter enrichment, with limited influence from the water redox properties observed in the upwelling current areas along the continental margins [5]. On the other hand, according to the redox model, organic-rich sediments can also form under low marine surface biological productivity in anoxic conditions, particularly in sulfide environments. Notable examples include modern anoxic basins like the Black Sea and the oceanic anoxic events (OAEs) during the Cretaceous period. Some scholars argue that a low sedimentation rate leads to oxidation decomposition and the benthic consumption of organic matter in oxygenated water bodies, while a high sedimentation rate promotes mineral dilution and reduces the total organic carbon (TOC) content in sediments [10].

The current research primarily focuses on the enrichment mechanism of organic matter in the marine shales found in continental shelf basins and lacustrine faulted basins [11]. However, there is a lack of studies on how organic matter becomes enriched in the transitional mudstones that exist between marine and continental environments. Transitional shales are more susceptible to sedimentary conditions compared to marine shales, and they exhibit distinct differences in provenance, sedimentary characteristics, hydrodynamic conditions, and terrigenous input [12]. Therefore, it can be deduced that the mechanism for organic matter enrichment differs between transitional shales and marine shales. The Upper Permian Longtan Formation in northern Guizhou offers an extensive exposure of these transitional mudstones. It contains segments of shale with high levels of organic content, making it a highly prospective region for the exploration of shale oil and gas based on existing drilling results. This region provides an excellent opportunity to investigate how organic matter accumulates within transitional marine and continental shales. Previous studies have mainly focused on analyzing the sedimentary environment, reservoir characteristics, and source rock evaluation and on assessing the potential for shale gas resources within the Longtan Formation in this specific area [13,14,15]. Generally speaking, it is widely acknowledged that the Longtan Formation’s shale deposits possess significant cumulative thicknesses with advanced maturity stages containing high levels of organic carbon content along with an abundant clay mineral content; they also demonstrate excellent gas-bearing properties while holding substantial potential as a source of shale gas resources.

This study aimed to investigate the paleo-climatic conditions, water redox properties, paleo-productivity, and sedimentation rate during the sedimentary period of organic-rich shale in northern Guizhou. To achieve this, samples were collected from the cores of two wells and subjected to organic geochemical tests, organic petrology studies, and CP-SEM, as well as element geochemical analysis. The findings not only contribute to a better understanding of unconventional oil and gas sedimentology but also provide valuable insights into the enrichment model of shale gas in the study area.

2. Geological Setting

Northern Guizhou Province is situated within the middle foreland basin under the upper Yangtze plate and the passive marginal fold thrust belt to the south [16]. It encompasses the Bijie arcuate tectonic area of the quaternary tectonic unit and the northern Zhijin broad fold area, which exhibit well-developed folds and faults. The Longtan Formation in northern Guizhou is found on the coastal carbonate strata of the Maokou Formation, with a sequential development from west to east, including a delta plain, lagoon–tidal flat, and carbonate platform (Figure 1) [16]. This succession reflects sedimentary characteristics transitioning from terrestrial to terrestrial–marine facies. From the Sinian to the Middle Triassic, there was no significant orogeny in this region; instead, there was an uplift movement of seawater resulting in a broad sea platform environment with tidal flats. During the Early Cambrian Longwangmiao period, which was influenced by the Caledonian Movement, the seawater gradually receded from this area, leading to the extensive emergence of the platforms that formed the central Guizhou uplift [16]. As this uplift expanded northward and westward, connecting with the southern Sichuan ancient landmasses, seawater extended over the study area until the Early Permian times. From the Late Ordovician to the Permian periods, the Caledonian Movement caused uplifts in central Guizhou, resulting in long-term weathering and denudation and preventing the Lower Permian series development across most areas. Extensive regressions occurred the during the Late Permian Soochow Movement, affecting northwest Guizhou, where the sediment environment shifted from epigenetic to transitional facies due to the northeastward retreat of seawater; clastic rocks were widely deposited during this stage within the Longtan Formation [14,15]. As it was subsequently affected by the Late Hercynian Movement after deposition, it experienced considerable damage.

The DC-1 well is located in the middle western part of the study area (Figure 1). The Longtan Formation of the DC-1 well is mainly a delta plain deposit with grey, dark grey fine sandstone, and siltstone as its lithology, and black shale and coal are developed locally (Figure 1). The JC-1 well is located in the middle part of the study area. The lithology of the Longtan Formation of JC-1 is mainly black shale, silty shale, gray-green siltstone, fine sandstone, and coal. The sedimentary facies are tidal flat and lagoon (Figure 1).

3. Equipment and Materials

Twenty fresh samples without any surface contamination were selected to carry out various experiments from the core of DC-1 and JC-1.

By observing the core surface with the naked eye, it is obvious that some of the core was more enriched in pyrite. Six samples with a high pyrite content were selected for analysis using a cross-section polisher–scanning electron microscope (CP-SEM). The instrument model was a Hitachi SU8010 SEM (Hitachi High-Tech, Tokyo, Japan). The 6 selected shale samples were first prepared by argon ion polishing. The experimental process was as follows: the samples were pre-polished with sandpaper, which followed the principle of from coarse to fine sandpaper selection. Then, the surface of the shale samples was bombarded with an argon ion beam in the polishing instrument. Then, the surface of the polished samples was plated with gold and then fixed on the sample table of the SEM for observation. After observation and acquisition of the SEM photos, image recognition was carried out by ImageJ (version number 1.8.0), an open-source Java-based software. A high reflectance was used to identify pyrite, and important geometric parameters were calculated. After importing the target image into ImageJ, the measurement scale was first corrected, the statistical area was manually circled (Figure 2a), and then the plug-in Image-Adjust-Threshol was applied to separate the pyrite, and the plug-in Process-Binary-Watershed was used to separate the microcrystals (Figure 2b). After these preliminary steps, the pyrite crystals could be further accurately characterized by setting a minimum particle size and minimum roundness (Figure 2c,d). Finally, the corresponding data were exported and processed using the data software.

Twenty fresh unweathered samples were selected from the available shale sections. The sampling position was evenly distributed longitudinally as far as possible. The 20 samples were ground into powder with a particle size of less than 75 μm; this powder was used for the TOC and the analysis of the major, trace, and rare earth elements, respectively. For the TOC content determination, a LECOCS-400 carbon sulfur analyzer was used (Laboratory Equipment Corporation, St. Joseph, MI, USA). About 10 g of each sample was crushed, cleaned, and dried repeatedly in an ultrasonic oscillator with deionized water and then ground to 100 mesh in an agate mortar. After sifting, 6% hydrochloric acid was added to the sample and boiled to remove the carbon in the carbonate. After the reaction was completed (no more bubbles), the sample was dried on an electric heating plate and then tested in the analyzer. The test accuracy was better than 3%.

The major element content was determined by an X-ray fluorescence spectrometer (XRF). The instrument model was Rigaku 100e (Rigaku Corporation, Tokyo, Japan), and its analysis accuracy was better than 5%. After being subjected to a temperature of 920 °C, the 0.5 g sample was homogeneously blended with eight times its mass of Li₂B₂O₇ and treated with one drop of LiBr–NH₄I cosolvent before being poured into a platinum crucible for melting preparation at 1150 °C and evaluated by the machine.

The content of trace and rare earth elements was determined by an inductively coupled plasma mass spectrometer (ICP-MS). The instrument model was PE Elan6000 (PerkinElmer Inc., Waltham, MA, USA), and the analysis error was less than 5%. First, the sample was combusted at 700 °C to remove organic matter. Then, a 40 mg sample was weighed, and 0.8 mL of HNO₃ and HF was added before the sample was dissolved in a sealed autoclave at 190 °C for 48 h, followed by drying. Subsequently, it was diluted to a ratio of 1:2000 with purified HNO₃, and then an internal standard solution of Rh was added. Finally, the sample was tested by the ICP-MS.

4. Results and Discussion

4.1. TOC Content Characteristics

The results show that the TOC content of the study area ranged from 1.19% to 11.75%, with an average of 5.57% (Figure 3,

Table 1. TOC and major element oxide contents in Longtan Formation shale samples in study area (%).

Well No.	Sample No.	Depth/m	TOC	SiO2	Al2O3	Fe2O3	FeO	MgO	CaO	Na2O	K2O	MnO	TiO2	P2O5	CIA	(P/Al)/10−3
DC-1	DS-1	421.58	3.94	58.38	15.57	3.85	1.24	0.85	4.35	1.25	2.22	0.04	0.28	0.12	67.73	4.28
DC-1	DS-2	426.98	3.27	46.38	16.96	4	0.6	1.4	5.08	0.96	2.26	0.04	0.39	0.09	67.95	4.42
DC-1	DS-3	430.27	1.28	61.25	14.37	3.65	1.2	1.28	3.67	1.03	1.98	0.05	0.51	0.11	69.48	6.31
DC-1	DS-4	439.98	1.86	54.94	19.09	4.1	1.46	1.36	4.36	0.65	3.88	0.12	0.82	0.12	69.22	5.31
DC-1	DS-5	442.37	4.38	53.81	17.36	5.16	0.76	1.29	5.09	0.1	2.04	0.03	0.52	0.09	71.47	4.37
DC-1	DS-6	445.22	1.19	58.25	19.31	4.88	1.21	1.53	3.57	0.6	3.59	0.06	0.87	0.19	73.04	8.07
DC-1	DS-7	448.78	3.57	58.24	18.72	3.25	0.69	0.96	7.09	0.1	1.67	0.03	0.31	0.1	68.71	4.18
DC-1	DS-8	455.12	1.85	58.17	16.26	5.63	1.13	1.5	4.78	0.16	2.79	0.08	0.79	0.17	69.42	8.62
DC-1	DS-9	457.28	3.53	61.28	17.46	2.54	0.66	1.28	8.06	1.59	1.03	0.03	0.27	0.45	65.54	21.2
DC-1	DS-10	460.26	2.78	53.78	18.98	4.77	0.93	1.38	5.43	0.12	1.85	0.03	0.46	0.08	72.68	3.61
JC-1	JS-1	520.25	7.27	50.96	16.73	5.4	0.32	1.69	4.11	0.37	1.18	0.02	0.38	0.03	75.06	1.48
JC-1	JS-2	523.11	9.48	52.97	13.62	6.26	0.2	1.36	4.2	0.54	2.5	0.02	0.16	0.02	65.50	1.21
JC-1	JS-3	526.82	9.54	49.25	14.5	7.44	0.36	1.37	3.16	0.26	2.72	0.02	0.29	0.03	70.59	1.82
JC-1	JS-4	529.58	10.16	42.83	14.21	6.13	0.1	1.33	4.56	0.37	1.67	0.01	0.25	0.03	68.61	1.51
JC-1	JS-5	535.39	11.75	53.89	20.48	4.82	2.02	1.74	3.27	0.55	1.92	0.1	0.41	0.1	79.11	3.91
JC-1	JS-6	539.57	4.37	51.08	18.37	5.64	1.61	2.49	5.54	0.53	2.63	0.22	0.37	0.12	68.88	5.16
JC-1	JS-7	541.82	6.45	48.94	15.98	4.09	1.57	2.63	4.32	0.46	1.76	0.14	0.36	0.15	72.57	7.58
JC-1	JS-8	548.78	7.96	48.12	17.56	7.02	2.2	1.36	5.29	0.74	2.88	0.11	0.7	0.09	67.10	3.33
JC-1	JS-9	551.68	9.48	47.98	16.15	6.21	2.22	1.4	5.1	0.65	2.07	0.02	0.53	0.09	68.23	4.8
JC-1	JS-10	554.72	7.28	40.81	17.72	7.33	3.22	5.54	7.93	0.69	1.25	1.69	0.37	0.16	65.49	9.8
Average			5.57	52.57	16.97	5.11	1.19	1.69	4.95	0.59	2.19	0.14	0.45	0.12	69.82	5.55

). Further analysis shows that the average TOC content of the samples of the tidal flat–lagoon environment (JC-1) was 8.37%, which was significantly higher than that of the delta samples (DC-1) (average of 2.77%). This may be related to the different sedimentary environments of the two wells. No significant vertical changes in the TOC was observed in the two wells separately (Figure 3).

4.2. Element Geochemistry

SiO₂ was the most abundant element in the samples, with the content ranging from 40.81% to 61.28%, and the average content was 52.57%. The SiO₂ content of DC-1 ranged from 46.38% to 61.28%, and the average value was 56.45%. The SiO₂ content of JC-1 ranged from 40.81% to 53.83%, and the average value was 48.68% (Figure 3, Table 1).

The second most abundant element oxide was Al₂O₃, with a content ranging from 13.62%~20.48%; the average was 16.97%. The range of the Al₂O₃ content in the DC-1 well was 14.37%~19.31%, with an average of 17.41%. The range of the Al₂O₃ content in the JC-1 well was 13.62%~20.48%, with an average of 16.53%. The range of other major element oxides, namely, the Fe₂O₃, CaO, K₂O, MgO, and FeO contents, were 2.54%~7.44%, 3.16%~8.06%, 1.03%~3.88%, 0.85%~5.54%, and 0.10%~3.22%. The average values were 5.11%, 4.95%, 2.19%, 1.69%, and 1.19%. The content of the other major element oxides was no more than 1.0% (Table 1). In general, the Al₂O₃ content was high, and the Al₂O₃ content of the shale samples across the various geological settings was basically similar, which may be related to the strong terrigenous detrital input [8].

The enrichment factor (EF) can be used as an indicator of the level of element enrichment in sediment [17]. The DC-1 well had an EF_Mo value of 0.69~12.80, with an average of 4.60. The JC-1 well had an EF_Mo value of 1.56~11.64, with an average of 6.53. The DC-1 well had an EF_U value of 0.94~3.27, with an average of 2.08. The JC-1 well had an EF_U value of 1.15~4.43, with an average of 2.53. The results of the EF_Mo and EF_U show relatively high levels of enrichment (

Table 2. Trace element contents and their related parameters in Longtan Formation shale samples in study area.

Sample No.	V (10−6)	Cr (10−6)	Co (10−6)	Ni (10−6)	Cu (10−6)	Sr (10−6)	Mo (10−6)	Th	U	U/Th	V/Cr	Ni/Co	Sr/Cu	EFMo	EFU
DS-1	351.00	278.00	13.80	83.7	42.10	82.00	22.70	15.90	13.50	0.68	1.16	7.28	2.18	12.80	3.21
DS-2	185.00	169.00	10.70	78.40	75.60	428.00	11.50	9.50	8.40	0.88	1.17	7.33	5.66	4.42	2.22
DS-3	196.00	184.00	12.50	65.40	52.20	211.00	7.30	10.50	7.50	0.71	1.17	5.23	4.04	3.33	2.33
DS-4	133.00	71.60	15.90	81.30	30.80	100.00	2.00	16.30	4.00	0.24	1.75	5.11	3.25	0.69	0.94
DS-5	185.00	168.90	11.10	57.80	50.50	179.00	9.20	11.50	8.10	0.70	1.17	5.21	3.54	3.45	2.10
DS-6	141.00	77.30	13.20	78.20	34.70	123.00	2.10	18.40	5.30	0.29	1.58	5.92	3.54	0.71	1.23
DS-7	158.00	250.00	12.30	80.10	32.70	146.00	14.30	12.20	9.20	0.75	1.02	6.51	4.46	4.98	2.21
DS-8	207.00	86.20	10.70	60.00	59.80	86.00	10.40	16.30	5.60	0.34	1.05	5.61	1.44	4.17	1.55
DS-9	182.00	273.00	18.70	95.50	99.00	331.00	19.00	15.00	12.70	0.85	0.70	5.12	3.34	7.09	3.27
DS-10	201.00	136.40	13.20	43.20	48.10	203.00	12.80	10.70	7.30	0.68	1.56	3.27	4.22	4.39	1.73
JS-1	488.00	799.00	14.80	27.70	32.10	80.20	18.70	15.30	10.00	0.65	0.62	1.87	2.50	7.27	2.69
JS-2	315.00	893.00	15.40	51.70	16.10	64.50	17.50	12.50	7.50	0.60	0.32	3.36	4.01	8.37	2.47
JS-3	257.00	462.00	16.40	49.60	25.00	86.50	18.70	13.30	8.60	0.65	0.59	3.03	3.46	8.40	2.68
JS-4	527.00	605.00	13.60	30.40	24.60	69.70	25.40	13.30	14.00	1.05	0.86	2.24	2.83	11.64	4.43
JS-5	392.00	247.00	19.00	61.60	77.80	213.00	16.70	20.30	10.00	0.49	1.23	3.24	2.74	5.31	2.20
JS-6	301.00	203.00	20.60	89.40	102.30	294.00	21.80	18.10	8.30	0.46	1.41	4.34	2.87	7.73	2.03
JS-7	315.00	189.00	18.40	76.30	69.50	178.00	12.50	16.40	7.20	0.44	1.45	4.15	2.56	5.09	2.02
JS-8	211.00	91.00	19.30	93.60	85.20	227.00	8.60	25.20	5.80	0.23	1.48	4.85	2.66	2.49	1.15
JS-9	386.00	363.00	15.70	98.10	123.00	318.00	18.50	15.20	15.90	1.05	1.05	6.25	2.59	7.46	4.43
JS-10	238.00	121.00	16.80	114.00	60.40	235.00	3.30	14.00	3.50	0.25	1.88	6.79	3.89	1.56	1.16
Average	268.45	283.37	15.11	70.12	57.08	182.75	13.65	15.00	8.62	0.60	1.16	4.84	3.29	5.57	2.30

), and almost all values were much greater than 1. They were very consistent with the trend of the TOC. This is possibly attributable to the fact that these two elements are likely to be associated with organic matter or clay minerals. The value of the JC-1 well was significantly higher than that of the DC-1 well; the EF value in the tide–lagoon environment is obviously higher than that in the delta environment.

The ΣREE value ranged from 45.66 × 10⁻⁶ to 238.60 × 10⁻⁶, with an average of 155.10 × 10⁻⁶ (

Table 3. Rare earth element contents and their related parameters in Longtan Formation shale samples in study area (10⁻⁶).

Sample No.	La	Ce	Pr	Nd	Sm	Eu	Gd	Tb	Dy	Ho	Er	Tm	Yb	Lu	ΣREE	δCe	(La/Yb)N
DS-1	27.3	52.3	6.21	26.3	5.14	0.91	4.25	0.78	4.58	1.26	3.31	0.48	3.13	0.57	126.38	0.71	0.59
DS-2	27.8	48.8	5.85	22.1	3.96	0.71	3.45	0.58	2.99	0.68	1.97	0.33	2.07	0.29	121.98	0.87	1
DS-3	31	61.4	8.76	37.28	4.78	0.89	5.11	0.88	3.24	0.78	2.62	0.49	3.18	0.47	162.38	0.83	0.75
DS-4	42.4	87.1	10.8	41	7.32	1.33	5.85	0.96	5.3	1.02	3.21	0.56	3.43	0.5	212.98	0.91	0.95
DS-5	34.2	64.7	9.24	34.67	6.36	1.09	5.33	0.87	4.17	1.11	2.58	0.48	2.77	0.48	172.65	0.78	1.03
DS-6	52.9	95.8	11.8	43.2	7.73	1.54	6.68	1.1	5.68	1.29	3.59	0.6	3.63	0.54	234.08	0.89	1.03
DS-7	24.9	37.1	5.21	20.6	3.66	0.66	3.12	0.53	2.73	0.58	1.75	0.28	1.69	0.24	99.64	0.8	0.93
DS-8	41.8	85.8	10.5	39.6	7.23	1.25	6.31	1.08	5.58	1.2	3.43	0.58	3.39	0.47	210.52	0.91	0.95
DS-9	28.3	41.2	5.76	21.8	4.38	1	4.39	0.81	4.53	1.16	3.25	0.55	3.26	0.48	120.75	0.74	0.63
DS-10	32.8	45.4	7.83	28.6	6.92	0.98	5.17	0.75	3.16	0.86	2.87	0.42	2.18	0.35	136.79	0.66	1.05
JS-1	18	32.8	4	14.5	2.67	0.55	2.27	0.35	1.87	0.45	1.35	0.23	1.55	0.24	82.32	0.85	0.92
JS-2	12.2	25	3.47	14.2	2.46	0.44	1.99	0.31	1.56	0.35	1.05	0.16	1.08	0.15	66.42	0.81	0.96
JS-3	9.24	16.9	2.25	8.38	1.49	0.26	1.27	0.26	1.59	0.38	1.22	0.2	1.33	0.2	45.66	0.81	0.55
JS-4	22.7	32.6	4.61	16.9	2.75	0.43	2.14	0.34	2.05	0.49	1.46	0.23	1.66	0.24	86.9	0.76	0.93
JS-5	51.9	75.2	9.8	41.6	6.86	1.27	6.47	1.09	7.02	1.51	3.46	0.61	3.64	0.56	209.19	0.77	1.01
JS-6	47.2	69.4	10.52	40.2	7.53	1.35	7.09	1.23	6.29	1.09	3.41	0.7	3.5	0.58	200.09	0.71	0.99
JS-7	41.6	81.4	9.4	38.1	8.25	1.19	6.17	1.15	8.15	1.25	3.29	0.52	3.42	0.52	205.11	0.93	0.91
JS-8	53.8	95.7	11.7	43.7	8.06	1.41	6.26	1.06	6.19	1.15	3.5	0.6	3.7	0.57	238.6	0.86	1.09
JS-9	37.8	66.1	8.5	32.7	6.69	1.28	5.54	1	5.86	1.15	3.37	0.58	3.48	0.57	172.82	0.86	0.76
JS-10	37.5	76.7	8.97	35.2	8.13	1.42	7.22	1.4	7.9	1.42	3.83	0.61	3.56	0.53	196.79	0.92	0.82
AVERAGE	33.77	59.57	7.76	30.03	5.62	1.00	4.80	0.83	4.52	0.96	2.73	0.46	2.78	0.43	155.10	0.82	0.89
UCG	30	64	7.1	26	4.5	0.88	3.8	0.64	3.5	0.8	2.3	0.33	2.2	0.32	146.37	/	/

), and the average values of DC-1 and JC-1 were 159.82 and 150.39, respectively. These are close to the total amount of ΣREE in the upper continental crust (UCC) of 146.37 × 10⁻⁶ [18]. The total amount of ΣREE exhibited a considerable decrease compared to that of North American shale composite (NASC), 173.21 × 10⁻⁶, and post-Archaean Australian shale (PAAS), 183.03 × 10⁻⁶ [18]. The content of rare earth elements in the delta environment (DC-1 well) was obviously lower than that in the tidal flat–lagoon environment (JC-1), which may be related to the different sedimentary environments.

4.3. Geologic Climate

Geologic climate change affects the accumulation and conservation of organic matter by impacting the provision of sediment and the stratification of water, which restricts the concentration of individuals and the composition of organisms in aquatic environments [19]. The chemical index of alteration (CIA) serves a broader purpose beyond evaluating the level of chemical weathering. The CIA is also extensively employed to assess alterations in the geologic climate [20].

The CIA of the Longtan shale samples in the study area ranged from 65.49 to 79.11, with an average of 69.82 (Table 1). The range of the CIA of the DC-1 well was 65.54~73.04, with an average of 69.52. The range of the CIA of the JC-1 well was 65.49~79.11, with an average of 70.02%. And, there was no significant difference in the CIA values between the delta environment and the tidal flat–lagoon environment, indicating that the Longtan Formation shale was in a warm and wet climate during the sedimentary period. Although the overall CIA did not change much, the changes in the CIA can still indicate an obvious correlation with the TOC. For example, sample JS-5 of the JC-1 well showed the highest CIA, and the TOC of JS-5 shows the highest value in JC-1. This may indicate that under strong weathering, a humid and hot climate with a higher temperature is conducive to the enrichment of organic matter.

In addition, Sr/Cu is also an effective indicator in characterizing the geologic climate during shale deposition [12]. Usually, Sr/Cu in the range of 1.3–5.0 represents a warm and wet climate, while Sr/Cu greater than 5.0 represents a warm and dry climate [12,21]. The Sr/Cu ratio of the Longtan Formation shale samples ranged from 1.44 to 5.66, with an average of 3.29 (Table 2), and only one sample had a value greater than 5.0, which also indicates a warm and wet climate. According to the above analysis, both the CIA and Sr/Cu indicate that the continental–marine transitional shale of the Longtan Formation in the study area was in a warm and wet climate during the sediment period. Furthermore, Figure 3 shows that the TOC was not consistent with the vertical variation trend of the geologic climate indicators (CIA and Sr/Cu). This may be due to the whole Longtan Formation in the study area being in a similar warm and wet environment during the depositional period, so the response to the change in the TOC was not obvious.

4.4. Redox Condition

The redox condition of the water in this research was divided into four levels, as recommend by Tyson et al. [10], namely oxic, dysoxic, suboxic, and anoxic. In this study, pyrite morphology and size, the trace element ratio, and the EF_U-EF_Mo model were employed for determining the oxidative–reductive conditions of the aquatic environments during the sediment period of Longtan Formation shale.

The size and morphology of pyrite are commonly employed for assessing the redox condition of water bodies where they have been deposited [22]. Pyrite can be divided into a syngenetic type and a diagenetic type. The syngenetic type pyrite usually has smaller grains and is generally formed in reduced water [21]. Diagenetic pyrite, however, usually has larger grains and is formed in an oxic or dysoxic environment [10]. The observation and statistical findings obtained from the CP-SEM shows that the predominant morphology of the pyrite was formed of framboids, columnar, had partial recrystallization, and had an irregular mass (Figure 3). Columnar pyrite is usually formed from pyrite framboids through recrystallization. Part of recrystallized pyrite includes the development of some flocculants at the edge of the pyrite, which is usually caused by fluid-rich diagenesis or low-degree metamorphism. Irregular lumps of pyrite form the edges of the angular development of pyrite. Compared with marine organic-rich shales, the development degree of pyrite framboids in the Longtan Formation shales was relatively low, and their relative content was 34.1%–52.8% (

Table 4. Pyrite content and statistics of framboid size in Longtan Formation shale samples.

Well No.	Sample No.	Depth/m	Pyrite Framboids/%	Average Size/μm	Maximum Size/μm	Minimum Size/μm	Standard Deviation /μm
DC-1	DS-4	439.98	34.1	8.3	14.72	4.34	2.85
DC-1	DS-6	445.22	52.8	7.74	16.7	3.27	2.62
DC-1	DS-7	448.78	39.1	7.04	15.41	2.19	2.31
JC-1	JS-2	523.11	45.7	7.47	12.19	2.09	2.00
JC-1	JS-5	535.39	36.4	7.24	12.57	2.14	2.35
JC-1	JS-7	541.82	42.38	7.68	15.35	3.578	2.65
	Average		41.75	7.58	14.49	2.93	2.46

). It is speculated that the existence of oxygen may limit their development.

The statistics of the relevant parameters of the pyrite in the shale (Table 4) show that the average particle size of the pyrite framboids in the shale was 7.04–8.30 μm, which is higher than the average size of pyrite framboids in an anoxic marine environment (5.0 ± 1.7 μm, Wilkin et al. [23]). The particle size of pyrite framboids in modern oxygen-poor marine environments (7.7 ± 4.1 μm, Wilkin et al. [23]) is basically the same as that of pyrite framboids in modern oxygen-poor marine environments, and there were a large number of irregular massive pyrite samples and some recrystallized pyrite in the samples in this study (Figure 4a,b), indicating that the Longtan shale was formed in an oxygen-poor environment.

In addition, cross-plots between the average particle size and the standard deviation of the particle size of pyrite framboids were employed to identify the redox properties of the water bodies during the sedimentary period of the continental–marine transitional shale of the Longtan Formation. It is evident from the figure that five out of the six shale samples were situated within the region characterized by oxic–dysoxic behavior (Figure 5), which further indicates that the Longtan shale was mainly formed in oxic–dysoxic surroundings. The water body in the delta environment (DC-1 well) is oxic–dysoxic. The water body in the tidal flat–lagoon environment (JC-1 well) is dysoxic–suboxic (Figure 5).

The ratio of elements such as U/Th, V/Cr, and Ni/Co is also widely used to indicate the redox state of water bodies. Although some scholars believe that these ratios may be affected by diagenesis, there are certain deviations in the indicative results, and the general trend is recognized, that is, the oxidation degree of water bodies increases as the ratios become smaller. The strength of the reduction degree in the water body intensifies with an increase in the ratio [24]. The U/Th, V/Cr, and Ni/Co ratios of the rock samples in this study ranged from 0.23 to 1.05, 0.32 to 1.88, and 1.87 to 7.33 (with mean values of 0.60, 1.17, and 4.84) (Table 2). The range of U/Th of the DC-1 well was 0.24~0.88, with an average of 0.61; the range of U/Th of the JC-1 well was 0.23~1.05, with an average of 0.59. The range of V/Cr of the DC-1 well was 0.70~1.75, with an average of 1.23; the vertical variation in the U/Th ratio in the DC-1 well was greater. This indicated that the redox condition of the delta may have changed more frequently. The variation range of JC-1 was much greater than that of DC-1, indicating that the redox conditions of JC-1 varied greater than those of DC-1.

The range of V/Cr of the JC-1 well was 0.32~1.88, with an average of 1.16. The V/Cr ratio of the two wells varied sightly, and all the V/Cr ratios of the two wells were less than 2. The range of Ni/Co of the DC-1 well was 3.27~7.33, with an average of 5.66; the range of Ni/Co of the JC-1 well was 1.87~6.79, with an average of 4.01. These results indicate that in the study area, the water body of the organic-rich shale was in an oxic or dysoxic state during the sedimentary period.

The redox characteristics of water bodies can be effectively differentiated by using the EF_U-EF_Mo covariant model. Figure 6 shows that all the data points fell in the dysoxic region. This phenomenon is consistent with the conclusion obtained from the analysis of pyrite and trace elements. Further analysis shows that compared with the samples from the DC-1 well, the sample points of the JC-1 well were more concentrated and closer to the anoxic end, which reflects that the oxygen enrichment degree of the tide–lagoon was lower than that of the delta environment.

4.5. Paleo-Productivity

The nutrition level in a water body is directly correlated with the extent of paleo-productivity. The more abundant the supply of nutrients, the stronger the vitality of the biological life processes, which in turn enhance the ability to capture carbon through photosynthesis and ultimately lead to a rise in biological productivity. The direct measurement of paleo-productivity poses considerable challenges. In this study, qualitative discussions were conducted using the trace element Mo along with the major element P [26].

In the past few years, an increasing number of studies have validated the existence of a clear and positive association between the TOC and the Mo content in sediments that are rich in organic matter [26]. The Mo concentration in the black shale within the study area was (2.0~25.4) × 10⁻⁶, with an average value of 13.65 × 10⁻⁶. The range of the Mo content in the DC-1 well was (2.00~22.70) × 10⁻⁶, with an average of 11.13 × 10⁻⁶; the range of the Mo content in the JC-1 well was (3.30~25.40) × 10⁻⁶, with an average of 16.36 × 10⁻⁶. These are significantly higher than the value in PAAS (1.0 × 10⁻⁶). This demonstrates that the study area had high paleo-productivity during the sedimentary period. The content of Mo showed an obvious positive correlation with the vertical trend of the TOC. Figure 7c demonstrates a positive correlation between the TOC and the Mo content, and the R² values of the correlation between the Mo content and the TOC in DC-1 and JC-1 were 0.68 and 0.52, respectively, suggesting that the marine–continental transitional shale relied greatly on paleo-productivity for the accumulation of organic matter.

Phosphorus plays pivotal in the metabolic processes of living organisms while also being an essential component of the skeletal frameworks found in various species. Upon their demise, these structures may become buried within sediments and subsequently utilized to characterize biological productivity. In the Longtan Formation, the content of P was generally higher, reflecting higher biological productivity. However, due to the influence of terrigenous detritus input, the absolute content of P is uncertain and cannot be used to directly measure the level of paleo-productivity. To exclude the influence of terrigenous detritus, the values of P/Al or P/Ti can more accurately reflect the level of biological productivity. The P/Al (×10⁻³) values ranged from 1.21 to 21.20, with an average of 5.55 (Table 1). The range of P/Al of the DC-1 well was (3.61~21.20) × 10⁻³, with an average of 7.04 × 10⁻³; the range of P/Al of the JC-1 well was (1.21~9.80) × 10⁻³, with an average of 4.06 × 10⁻³. Comparing the different sedimentary environments, it was found that the P/Al value of the shale samples formed in the tidal flat–lagoon environment (JC-1) was significantly lower than that in the delta environment (DC-1); this is the opposite of the result indicated by the Mo content. Although the P/Al value of the shale samples from the JC-1 well was lower than that of the DC-1 well, the TOC content was higher in JC-1. It is speculated that this may be related to the redox state of the water. Previous studies have shown that the redox state of sedimentary water has an important impact on the enrichment of P [27]. Generally, in a partial reduction environment, P will dissolve into the water from the sediment, while in a partial oxidation environment, P is easily adsorbed in the oxides of iron and manganese. Therefore, the low P/Al value of the JC-1 well does not mean that the paleo-productivity of the sample in the geological history is low.

4.6. Sedimentation Rate

The rare earth element partitioning model and (La/Yb)_N provide a qualitative assessment of the rate at which sedimentation occurs. Previous studies have proven that REE exists in water by combining with debris or suspended matter, and varying the duration of the stay in the water will inevitably cause differences in the REE differentiation degree [28,29]. When the rate of sedimentation is elevated, the contact time between REE and clay minerals is short, leading to a lack of distinctiveness. On the contrary, when the rate of sedimentation is reduced, there is an increased opportunity for REE to interact with clay minerals, leading to a significant level of differentiation. Accordingly, the sedimentation rate of sediments can be reversely deduced according to the REE differentiation degree. The (La/Yb)_N (UCC-normalized) ratio is a reliable indicator of the REE differentiation degree [29,30]. When the (La/Yb)_N ratio is close to 1.0, REE is basically non-differentiated or weakly differentiated, corresponding to a high sedimentation rate. When the (La/Yb)_N ratio is significantly higher or lower than 1.0, it reflects strong REE differentiation and corresponds to a low sedimentation rate. The (La/Yb)_N ratio of the Longtan Formation shale samples in the study area ranged from 0.55 to 1.09, with an average of 0.89 (Figure 3, Table 3). The range of the (La/Yb)_N ratio of the DC-1 well was 0.55–1.09, with an average of 0.89; the range of the (La/Yb)_N ratio of the JC-1 well was 0.59–1.05, with an average of 0.89.

As is shown in Figure 2, only four samples had a (La/Yb)_N ratio of less than 0.8. Almost all the (La/Yb)_N ratios were close to 1.0. This result indicates that the Longtan Formation shale exhibits an elevated sedimentation rate. By comparing the (La/Yb)_N ratio in the different sedimentary environments, it was found that the (La/Yb)_N ratio in the delta environment (DC-1 well) was closer to 1.0 than that of the shale samples in the tidal flat–lagoon environment (JC-1 well). The delta environment has a higher sedimentation rate than the tidal flat–lagoon environment.

4.7. Enrichment Factors of Organic Matter

The study area’s Longtan Formation underwent climatic conditions that were warm and characterized by high humidity, resulting in oxic–dysoxic sedimentary water and leading to high biological productivity and sedimentation rates. The key factors that influence the accumulation of organic matter in marine sediments are the paleo-productivity levels and the conditions that promote its preservation through burial. Compared to marine shale, the transitional shale found in both marine and continental environments is more susceptible to variations in sedimentary conditions. To identify the primary factors governing the increase in the concentration of organic matter within these transitional shales, the TOC was used as a proxy for assessing the degree of enrichment.

The correlation analysis revealed that there were weak negative correlations between the TOC and the values of V/Cr and Ni/Co (R² = 0.13 and 0.05, p value = 7.04 × 10⁻⁴ and 3.04 × 10⁻⁵, respectively) in the DC-1 well, but the TOC of the JC-1 well exhibited a positive correlation with the values of V/Cr and Ni/Co (R² = 0.44 and 0.52, p value = 3.86 × 10⁻⁹ and 6.39 × 10⁻⁵, respectively) (Figure 7a,b). Although the R² value was not high, it is clear that the TOC increased with the increase in the two redox proxies in the JC-1 well. The correlation indicates that under the tidal flat–lagoon environment (JC-1 well), the degree of redox has an obvious controlling effect on the enrichment of organic matter, while under the delta environment (DC-1 well), the degree of redox has little influence on the enrichment of the TOC.

The figure shows a positive correlation between the TOC and Mo content (Figure 7c) under both the delta environment (DC-1 well) and the tidal flat–lagoon environment (JC-1 well) (R² = 0.68 and 0.52, p value = 9.67 × 10⁻⁴ and 1.90 × 10⁻³, respectively). These results show that paleo-productivity is one of the key factors in organic matter enrichment, and the positive effect of paleo-productivity on organic matter enrichment under the delta environment is greater than that under the tidal flat–lagoon environment.

No significant correlation was observed between the TOC and CIA or (La/Yb)_N (Figure 7d,e). This may be due to the fact that both the delta environment and the tidal flat–lagoon environment in the study are under a warm and wet environment and maintained a high sedimentation rate. It can be observed that the variation range of the proxies was small, and small changes in the paleo-climate and sedimentation rate in the study area may not have been enough to have a significant impact on the variation in organic matter enrichment.

These findings imply that organic-rich shale can be influenced by various elements, including the paleo-climate, the redox condition, paleo-productivity, and the sedimentation rate. The allocation and interaction of these factors have a significant impact on the determination of the presence or burial/preservation of organic matter in transitional-phase environments between marine and continental settings. Among these factors, paleo-productivity and the water redox properties were identified as the key factors in the enhancement of organic matter of the shale in study area.

The importance of primary productivity and conducive preservation conditions in the process of enhancing the concentration of organic matter in marine surfaces has been emphasized by previous researchers [31,32]. It is important to note that the physicochemical enhancement of organic matter is a highly intricate process that can be affected by multiple factors, including the paleo-climate, paleo-productivity, the water redox condition, and the sedimentation rate. Therefore, it can be concluded from the above analysis that a single explanation like a “productivity model” or a “preservation model” cannot solely account for the organic matter enrichment of marine–continental environments within the Longtan Formation in northern Guizhou. Instead, multiple interacting factors need to be considered.

4.8. Relationship between Sedimentary Environment and Organic Matter Enrichment

The high paleo-productivity observed during the Late Permian sedimentation in northern Guizhou provided a substantial foundation for the organic-rich shales. Following the production of organic matter, it undergoes further burial and preservation. Organic matter enrichment can only occur when the accumulation rate surpasses the sedimentation rate [33]. In the study area, the transitional shales of the Longtan Formation were deposited in tidal flat–lagoon and delta environments, which facilitated the continuous accumulation of abundant plant-derived detritus organic matter and acted as a good location to enhance the accumulation of organic matter. Previous studies have also shown that the predominant type of kerogen in northern and western Guizhou is type III, followed by type II₂, and the kerogen is of a humic type [34,35,36,37]. The rich humus components indicate that the Longtan Formation mainly received its input of organic matter from higher plants, and the presence of sapropelic components also indicates the input of lower aquatic organisms. While the organic-rich shale was being deposited in the Longtan Formation, a warm and wet paleo-climate prevailed. This climate not only favored the growth of terrigenous higher plants but also promoted biogeochemical processes that enhanced the weathering of the parent rock and facilitated the reproduction of lower aquatic organisms [38]. Higher plant detritus and lower aquatic organisms provide abundant organic matter sources for organic-rich shale deposits.

However, the oxic–dysoxic water did not promote the preservation of the organic material, as its rapid sedimentation rate reduced the duration of exposure for the organic matter in the oxic–dysoxic water. Therefore, the organic matter could not be oxidized or decomposed in large quantities. Based on factors such as paleo-climate, the water redox properties, paleo-productivity, and the sedimentation rate, organic matter enrichment models for the shale in the delta and tidal flat–lagoon environments were established, respectively (Figure 8).

The warm and wet climate, high paleo-productivity, and high sedimentation rate were the main controlling factors affecting the accumulation of organic matter under the delta environment in the study area. The river carried terrigenous debris and terrestrial plant debris into the basin at a high sedimentation rate. Although it was in an oxic–dysoxic environment, a large amount of organic matter was buried before being decomposed, resulting in the enrichment of the organic matter (Figure 8a).

The sediments of the tidal flat–lagoon in the study area were also in a warm and wet paleo-climate with a high sedimentation rate. This also provided favorable conditions for the rapid burial and preservation of organic matter. Although the tidal flat–lagoon in the study area was also mainly under oxic–dysoxic conditions, the analysis of pyrite shows that the tidal flat–lagoon environment was more reductive. Under such conditions, the change in the water redox properties were enough to affect the preservation of the organic matter. Therefore, the redox conditions were also an important factor affecting the organic matter enrichment in the tidal flat–lagoon environment. The accumulation of organic matter in the tidal flat–lagoon was controlled by the warm and wet paleo-environment, high paleo-productivity, high sedimentation rate, and redox conditions. The relatively low oxygen content in the water body also made the organic matter in the tidal flat–lagoon more abundant than in the delta (Figure 8b).

Based on the above analysis, although oxygen-rich water is not favorable for the conservation of organic matter, it can also cause organic matter enrichment under specific geological conditions such as high biological productivity and a high sedimentation rate. The controlling factors of organic matter enrichment in marine–continental transitional shale can enrich and improve upon unconventional oil and gas sediment-related theories.

5. Conclusions

(1) The Longtan Formation shale in northern Guizhou was formed in a delta and a tidal flat–lagoon with intricate hydrodynamic conditions. The lithology is predominantly composed of black shale and silty mudstone, with localized occurrences of coal seams, gray-black siltstone, fine sandstone, and limestone. The TOC is primarily distributed within the range of from 1.19% to 11.75%, averaging at 5.57%. The TOC content of the samples from the tidal flat–lagoon environment (JC-1) (average of 8.37%) is significantly higher than that of the delta samples (DC-1) (average of 2.77%). The organic matter in the Chaoping lagoon is more abundant than that in the delta.

(2) The Longtan Formation marine–continental transitional shale was formed during a period with a warm and wet climate, as indicated by the geochemical indices and geometric characteristics of the pyrite. Moreover, the sedimentary water body of the delta and tidal flat–lagoon exhibited oxic–dysoxic characteristics, with a high level of paleo-productivity and a high sedimentation rate. Although oxic–dysoxic water is not conducive to organic matter preservation, organic matter enrichment can also occur under specific geological conditions such as high paleo-productivity and a high sedimentation rate, such as that in the study area.

(3) The presence of shale with a high organic matter content in the Longtan Formation indicates that it was impacted by a combination of various factors, including the paleo-climate, water redox condition, sedimentation rate, and paleo-productivity. These factors influenced the presence and preservation of organic matter, directly or indirectly, in the transitional environment. Notably, within this set of factors, the paleo-productivity and the redox proxies had a higher correlation with the TOC.

(4) During the Late Permian sedimentary period, the high paleo-productivity provided a robust material foundation for the organic-rich shales in the transitional facies between the marine and continental environments. While the presence of oxic–dysoxic water may not be favorable for organic matter preservation, its rapid sedimentation rate can effectively reduce the duration of organic matter exposure in such conditions; therefore, the organic matter cannot be oxidized or decomposed in large quantities. These factors together resulted in the enrichment of organic matter in the Longtan Formation in the study area.