Geochemical Evidence of Organic Matter Enrichment and Depositional Dynamics in the Lower Cambrian Yurtus Formation, NW Tarim Basin: Insights into Hydrothermal Influence and Paleoproductivity Mechanisms

Abstract

The lower Cambrian Yurtus Formation (Є₁y) in the Tarim Basin, characterized by its high organic matter content, serves as a critical source rock for oil and gas exploration in the platform basin. This study presents a high-resolution geochemical analysis of a geological section located near the Aksu Cement Plant in the northwest margin of the Tarim Basin. The focus is on elucidating the sedimentary environment, mechanisms of organic matter enrichment, and the depositional history of the Є₁y source rock. The Є₁y exhibits distinctive geochemical signatures, including elevated concentrations of Mo, Ba, and U, with an average rare earth element (REE) content of 155.75 μg/g. The formation shows significant light REE enrichment (LREE/HREE = 1.74–5.57), a moderate Ce negative anomaly (δCe = 0.4–0.71), and a notable Eu positive anomaly (δEu = 0.94–2.14), indicative of a unique depositional environment influenced by hydrothermal processes. Geochemical evidence suggests that the Є₁y siliceous shales were deposited in a highly reducing, anoxic, and sulfide-rich environment, promoting organic matter preservation and enhancing sedimentary productivity. The presence of hydrothermal trace elements, likely introduced by hydrothermal fluids from volcanic activity along fractures and faults, played a critical role in enriching the sedimentary system, preserving organic matter, and boosting paleoproductivity. The model of organic matter enrichment proposed in this study underscores the dynamic interplay between hydrothermal influences and high primary productivity. These findings provide important insights into the formation of high-quality source rocks and have significant implications for the exploration of deep and ultra-deep oil and gas reserves in the Tarim Basin.

1. Introduction

The transition from the Precambrian to the Early Cambrian is a critical period in Earth’s history, marked by significant tectonic and environmental shifts. This era witnessed extensive marine transgressions, hydrothermal activity, and persistent anoxic conditions, shaping global biogeochemical cycles (Figure 1a) [1,2,3]. The widespread deposition of Lower Cambrian black shales globally serves as a key record of marine evolution during this time, offering insights into paleoenvironmental conditions [4]. In the Tarim Basin, the Early Cambrian Yurtus Formation (Є₁y), predominantly composed of siliceous rocks and siliceous shales (Figure 1b), provides a unique opportunity to explore the paleo-marine environment of this period. Notably, the organic carbon content of the Є₁y represents the highest concentrations of marine source rocks in China [5], making it a significant subject of study, especially with the growing focus on deep and ultra-deep strata exploration in the basin [5,6].

The Є₁y is overlain by the Xiaoerbulake Formation, which features reef-shoal facies dolomites with numerous fractures and cavities, while the Wusonger Formation consists of thick mudstones and paste rocks. These strata form an important source-reservoir-caprock system, critical for oil and gas exploration in the Tarim Basin [7]. However, the complex and heterogeneous nature of the basal black rock series in the Є₁y presents significant challenges in understanding its genesis, depositional processes, and sedimentary background. Despite some progress, research on the mechanisms of organic matter enrichment in this series remains limited and debated [8,9]. The Є₁y outcrop in the northwest of the Tarim Basin is thought to have been deposited in environments ranging from restricted shallow shelves to deep-water basins, with the subsurface Є₁y forming in semi-restricted or coastal environments [10,11]. These findings have implications for the exploration of Lower Paleozoic organic-rich source rocks in the basin.

Figure 1. Global palaeogeography during the Early Cambrian (a) and locations of the sections and samples (b) (Fm., Formation; D., Depth; S1, Sample number; PreCam., Precambrian) [10].

Figure 1. Global palaeogeography during the Early Cambrian (a) and locations of the sections and samples (b) (Fm., Formation; D., Depth; S1, Sample number; PreCam., Precambrian) [10].

Organic matter enrichment in sedimentary rocks is primarily controlled by high primary productivity, which is often enhanced by anoxic conditions that facilitate organic matter preservation [12,13,14]. The abundance of organic matter is largely governed by anoxic or sulfidic reducing environments, where preservation potential is high [15]. However, neither the “productivity model” nor the “preservation model” alone fully explains the mechanisms of organic matter enrichment. It is likely that these factors interact to produce the observed organic richness. Current models explaining the development of high-quality source rocks include: (1) the “hydrothermal activity-upwelling-anoxic event model,” which posits that stratification of the water column allows for oxygenated surface waters with high productivity, while the bottom waters are anoxic and reducing [16]; (2) the combined influence of hydrothermal activity, coastal upwelling, sea-level fluctuations, and organic matter preservation conditions on source rock development [17]; (3) the effect of sea-level changes, topography, hydrothermal events, upwelling, and periodic anoxia on organic matter enrichment [18]; and (4) the interplay between redox conditions and productivity, where the dominant control on organic matter enrichment varies by sedimentary period [19].

This study focuses on a geological section near the Aksu area in the northwest Tarim Basin, where detailed geochemical analyses are employed to reconstruct the depositional environment and sedimentary conditions of the Є₁y Formation. The goal is to provide new insights into the mechanisms of organic matter accumulation, preservation, and enrichment, as well as to explore the tectonic evolution of the region.

2. Geological Setting

Geologically, the Tarim Basin is divided into three uplifts, four depressions, and seven first-order structural units (Figure 1b and Figure 2). The Aksu area is located on the northwest margin of the Tarim Basin. The Nanhua strata, comprising gray-green and purplish-red feldspar sandstones and siltstones, are approximately 2000 m thick, exhibiting poor sorting and rounding. The Sinian strata consist mainly of sand-mudstone and carbonate rocks of coastal shallow sea-tidal flat facies.

The Early Cambrian (Є₁y) strata were deposited following post-rift subsidence and a large-scale transgression, resulting in the extensive development of black strata across the northwestern region of the Tarim Basin. Recognized as the largest hydrocarbon-bearing basin in China, these strata are primarily composed of black siliceous shales and other dark-colored shales [20], and are situated in the lower section of the Є₁y. With a thickness ranging from 10 to 15 m and spanning approximately 26,000 km², these strata reflect the unique paleoenvironmental conditions of the Early Cambrian. The extensive development of these black strata across the basin has made them a focal point in petroleum geology, particularly in understanding the genesis and distribution of Lower Paleozoic marine hydrocarbons. Previous studies have highlighted that the organic matter within these shales predominantly consists of Type II kerogen, which is highly effective in generating oil under appropriate thermal maturity conditions. Consequently, these strata are pivotal for current hydrocarbon exploration and production efforts and offer valuable insights into the broader paleoenvironmental and tectonic evolution of the Tarim Basin during the Early Cambrian period.

3. Materials and Methods

This study collected high-resolution samples from Cambrian outcrops in geological profiles near the Aksu Cement Plant, located in the northwest Tarim Basin. Sampling intervals ranged from 0.01 to 0.03 m, yielding a total of 100 samples, labeled Cam_1y-1 through Cam_1y-100 (Figure 1b), which were subsequently analyzed. To meet space constraints, this paper presents the results for every fifth sample and discusses their geochemical significance. 1-alkyl-2,3,6-trimethylbenzene compounds (ATMBs) from aromatic fractions of typical source rock samples are analyzed with m/z 133/134 to reveal their critical role in the relationship between Cam_1y source rocks and major hydrocarbons in the Tarim Basin. Pre-processing and testing procedures for biomarker detection and total organic carbon (TOC) testing can be found in our early work [10].

Major and trace element analyses were performed at the Analytical Laboratory of the Beijing Research Institute of Uranium Geology, China National Nuclear Corporation, following the procedures outlined in previous studies [21]. Major elements were quantified using an AB104L Axioma X-ray fluorescence (XRF) spectrometer, with a relative standard deviation (RSD) of less than 1%. Accuracy was confirmed using shale standard material (GBW 03014) measured alongside the samples [21,22]. Trace elements and REE concentrations were determined following multiple rounds of acid digestion, involving HNO₃, HClO₄, and HF. Samples were diluted with 5% HNO₃ to 50 mL before measurement on an ELEMENT XR inductively coupled plasma mass spectrometer (ICP-MS), with an RSD of less than 1.5% for each element. The calculation methods for δCe and δEu are as follows: δCe = [Ce/(La × Pr)^1/2]_{Post-Archean Australian Shale (PAAS)} and δEu = [Eu/(Sm × Gd)^1/2]_PAAS.

4. Results

4.1. Major Elements

The experimental results (

Table 1. Major elements (%) of geological profile samples near the Aksu area in the Tarim Basin.

Number	Interval	Horizon	Lithology	SiO2 /%	Al2O3 /%	Fe2O3 /%	MgO /%	CaO /%	MnO /%	TiO2 /%	P2O5 /%	LOI /%	FeO /%	TOC /%
Cam1y-5	0.21	Є1y	Siliceous shale	93.09	0.539	0.146	0.426	1.21	-	0.018	0.451	3.87	0.11	20.18
Cam1y-10	0.33	Є1y	Siliceous shale	95.56	0.387	0.408	0.131	0.177	-	0.016	0.019	3.08	0.35	11.25
Cam1y-15	0.43	Є1y	Siliceous rock	54.72	9.19	7.46	1.37	2.06	0.005	0.538	1.03	15.24	0.72	4.87
Cam1y-20	0.64	Є1y	Siliceous shale	83.61	2.04	0.758	0.358	0.452	-	0.106	0.208	10.71	0.59	5.96
Cam1y-25	0.77	Є1y	Siliceous rock	96.13	0.426	0.618	0.115	0.328	-	0.021	0.173	1.82	0.52	0.2
Cam1y-30	0.895	Є1y	Siliceous shale	88.38	0.449	0.568	0.993	2.01	0.006	0.019	0.068	6.6	0.49	2.54
Cam1y-35	1.125	Є1y	Siliceous rock	92.72	0.399	0.501	0.069	0.752	-	0.021	0.029	4.76	0.43	1.49
Cam1y-40	1.295	Є1y	Siliceous shale	48.41	8.04	6.31	1.6	1.26	-	0.543	1.1	22.25	1.63	4.6
Cam1y-45	1.385	Є1y	Siliceous rock	96.32	0.261	0.412	0.066	0.112	-	0.015	0.065	2.04	0.36	0.58
Cam1y-50	1.47	Є1y	Siliceous shale	37.01	8.67	4.43	1.71	5.6	0.004	0.584	2.09	25.03	0.95	2.93
Cam1y-55	1.575	Є1y	Siliceous rock	96.16	0.462	0.322	0.087	0.126	-	0.023	0.064	2.4	0.28	0.85
Cam1y-60	1.632	Є1y	Siliceous shale	51.95	10.91	3.58	1.6	0.662	-	0.676	1.5	18.85	1.21	6.04
Cam1y-65	1.747	Є1y	Siliceous rock	96.42	0.375	0.365	0.064	0.193	-	0.019	0.066	2.26	0.3	0.64
Cam1y-70	1.867	Є1y	Siliceous shale	47.03	8.07	4.8	1.67	1.68	0.004	0.469	1.94	21.6	0.34	5.92
Cam1y-75	1.947	Є1y	Siliceous rock	87.51	0.505	0.514	0.074	2.88	-	0.019	1.71	5.99	0.42	2.05
Cam1y-80	2.007	Є1y	Siliceous shale	56.55	5.59	2.78	0.961	3.95	-	0.29	2.64	20.79	0.82	7.88
Cam1y-85	2.117	Є1y	Siliceous rock	21.61	2.03	1.58	0.288	31.92	-	0.101	23.44	9.56	1.3	3.6
Cam1y-90	2.217	Є1y	Siliceous shale	46.93	9.17	3.92	1.75	2.99	0.007	0.445	1.79	23.43	0.8	10.88
Cam1y-95	2.307	Є1y	Siliceous rock	95.01	0.438	0.247	0.248	0.379	-	0.028	0.02	3.31	0.21	1.46
Cam1y-100	2.397	Є1y	Siliceous shale	46.04	8.95	3.36	1.9	3.1	0.006	0.41	1.61	26.51	1.58	14.04

Note: “-” indicates that the element content is lower than the detected value; LOI: Loss on ignition; TOC: Total organic carbon.

) show considerable variation in the silica content across the profile, with SiO₂ mass fractions ranging from 21.61% to 96.42%, averaging 71.56%. This distribution generally follows a consistent fluctuation pattern. Notably, in samples Cam_1y-10, Cam_1y-35, Cam_1y-45, and Cam_1y-55, the average SiO₂ content is consistent at 71.56%, while in sample Cam_1y-95, the SiO₂ content is exceptionally high, surpassing 90%. The TOC content ranges from 0.2% to 20.28%, with an average of 5.40%. Interestingly, the average TOC content for these five samples is 3.12%, significantly lower than the overall average of 5.40%. Among the other major elements, Al₂O₃ content varies from 0.26% to 10.91%, with an average of 3.85%. In the five samples mentioned, the average Al₂O₃ content is only 0.38%, much lower than the general average. Fe₂O₃ content ranges from 0.15% to 7.46%, with an average of 2.15%. MgO content ranges from 0.064% to 1.9%, with an average of 0.774%, while CaO content varies significantly from 0.11% to 31.92%, with an average of 3.09%. MnO content is generally low and mostly falls below the detection limit. TiO₂ mass fractions range from 0.02% to 0.68%, with a mean value of 0.22%.

4.2. Trace Elements

Table 2. The characteristics of trace elements of geological profile samples near Aksu Cement Plant.

Number	Mo /ppm	Ba /ppm	U /ppm	Ba/Sr	V/(V + Ni)	Th/U	U/Th	Zn/Ti	Ni/Co	Rb/K	Th/Sc	Y/Ho	Cd/Mo
Cam1y-5	2.29	300	10.1	3.75	0.88	0.06	15.68	0.55	5.67	30.80	3.32	46.70	1.19
Cam1y-10	7	216	4.11	4.33	0.85	0.05	18.77	0.71	5.96	35.26	0.98	42.60	0.26
Cam1y-15	283	730	113	2.87	0.83	0.06	16.89	0.53	17.39	11.97	0.64	45.97	0.08
Cam1y-20	9.92	362	23	3.38	0.98	0.06	16.43	0.15	25.70	16.76	0.74	43.77	0.37
Cam1y-25	7.98	93.8	6.75	3.57	0.98	0.03	33.92	0.34	13.90	19.93	0.78	43.81	0.11
Cam1y-30	15.9	377	12.8	6.78	0.78	0.01	72.73	1.82	11.55	13.56	0.83	42.50	0.36
Cam1y-35	15.4	441	14.5	7.16	0.93	0.01	73.98	1.55	12.79	14.67	0.51	41.89	0.21
Cam1y-40	176	619	187	2.36	1.00	0.04	25.34	0.07	21.23	20.39	0.74	41.02	0.07
Cam1y-45	14.9	102	9.42	6.46	0.98	0.03	31.82	0.42	11.20	28.35	0.81	41.30	0.10
Cam1y-50	144	308	121	1.49	1.00	0.07	13.55	0.09	17.81	29.48	0.55	41.20	0.13
Cam1y-55	7	74.3	11.2	3.83	0.99	0.03	35.11	0.24	25.32	23.98	0.66	42.29	0.30
Cam1y-60	25.6	519	173	2.39	1.00	0.07	14.42	0.06	21.31	27.22	0.75	45.82	0.51
Cam1y-65	11	233	6.77	7.24	0.97	0.05	21.16	0.78	30.73	36.08	0.62	47.39	0.16
Cam1y-70	54.6	376	80.2	1.57	1.00	0.09	11.44	0.19	32.92	25.71	0.66	51.49	0.82
Cam1y-75	7.37	138	14.6	2.11	0.97	0.03	32.52	0.9	28.27	25.61	0.72	50.50	1.51
Cam1y-80	17.1	306	45.5	1.74	0.99	0.09	11.46	0.27	38.94	22.49	0.61	54.15	3.27
Cam1y-85	8.02	3734	101	4.24	0.92	0.03	38.40	1.7	26.88	19.18	0.71	54.18	7.63
Cam1y-90	50.5	594	64.5	2.93	0.96	0.09	11.50	0.94	31.61	23.77	0.48	39.72	3.29
Cam1y-95	4.57	550	2.47	18.15	0.92	0.07	14.03	1.53	12.76	30.75	0.48	41.40	4.18
Cam1y-100	14.8	421	45.8	1.52	0.97	0.13	7.58	0.62	73.11	22.20	0.64	42.45	7.64
Average	43.85	524.71	52.34	4.39	0.94	0.06	25.84	0.67	23.25	23.91	0.81	45.01	1.61

presents the trace element compositions of the samples. Notably, the samples show elevated concentrations of Mo, Ba, and U. Mo levels range from 2.29 to 283 μg/g, with an average concentration of 43.85 μg/g. Ba concentrations range from 743 to 3734 μg/g, with an average of 524.71 μg/g, while U content varies from 2.47 to 187 μg/g, with an average of 52.34 μg/g. Moreover, V/(V + Ni) is generally greater than 0.85, indicating a hypoxic sulfurization condition. These elevated trace element levels are indicative of specific geochemical processes influencing the sedimentary environment, and further analysis may reveal insights into the hydrothermal contributions and redox conditions during deposition.

4.3. REEs

REE concentrations, normalized to PAAS standards [23], are shown in

Table 3. The characteristics of REE of geological profile samples near Aksu Cement Plant.

Number	La /ppm	Ce /ppm	Pr /ppm	Nd /ppm	Sm /ppm	Eu /ppm	Gd /ppm	Tb /ppm	Dy /ppm	Ho /ppm	Er /ppm	Tm /ppm	Yb /ppm	Lu /ppm	Y/Ho
Cam1y-5	2.86	3.62	0.50	1.94	0.37	0.10	0.40	0.08	0.46	0.10	0.30	0.05	0.29	0.04	46.70
Cam1y-10	1.41	1.88	0.26	1.03	0.20	0.06	0.19	0.04	0.22	0.05	0.16	0.03	0.16	0.02	42.60
Cam1y-15	73	85.5	15.4	68.9	14.1	3.04	15.7	3.07	19	4.22	11.7	1.84	10.2	1.39	45.97
Cam1y-20	11.1	10.9	2.26	9.97	1.96	0.45	2.13	0.43	2.87	0.71	2.19	0.40	2.42	0.36	43.77
Cam1y-25	1.86	1.82	0.38	1.8	0.38	0.09	0.43	0.08	0.60	0.14	0.46	0.09	0.50	0.08	43.81
Cam1y-30	2.66	3.09	0.57	2.48	0.51	0.13	0.55	0.10	0.66	0.16	0.44	0.08	0.42	0.06	42.50
Cam1y-35	2.29	2.44	0.58	2.62	0.46	0.14	0.54	0.10	0.65	0.16	0.50	0.09	0.52	0.08	41.89
Cam1y-40	92.1	68.8	17.1	74.7	14.8	3.27	16	2.99	19.4	4.51	13.6	2.38	13.6	1.88	41.02
Cam1y-45	2.43	2.48	0.59	2.77	0.59	0.14	0.68	0.15	0.98	0.25	0.77	0.13	0.79	0.12	41.30
Cam1y-50	90.5	80.2	22.8	114	26.4	6.09	31.9	6.29	40.2	9.03	25	3.83	20	2.69	41.20
Cam1y-55	3.81	3.53	0.99	4.7	0.97	0.23	1.11	0.28	1.43	0.35	1.1	0.19	1.09	0.15	42.29
Cam1y-60	94.1	81.5	19.2	84.5	17.1	3.90	20.8	4.37	30	7.66	24.2	4.28	25	3.72	45.82
Cam1y-65	3.53	3.5	1.00	5.02	1.16	0.29	1.54	0.31	2.19	0.54	1.66	0.30	1.56	0.22	47.39
Cam1y-70	51.8	46.2	9.51	38.6	7.10	1.52	8.07	1.65	11.8	3.03	10.1	1.86	11	1.66	51.49
Cam1y-75	20.7	21.4	5.82	27.3	5.88	1.30	6.89	1.33	8.38	1.81	5.15	0.74	3.66	0.47	50.50
Cam1y-80	37.6	36.7	7.65	34.4	6.63	1.48	7.98	1.56	10.4	2.53	7.55	1.25	6.9	1.01	54.15
Cam1y-85	121	104	24.8	114	23.5	5.51	29.3	5.6	36.6	8.49	25.7	4.02	20.6	2.89	54.18
Cam1y-90	42.4	44.1	9.16	40.7	8.34	1.9	9.54	1.85	12	2.82	8.48	1.5	8.93	1.36	39.72
Cam1y-95	0.76	0.78	0.15	0.64	0.13	0.07	0.16	0.03	0.21	0.05	0.17	0.03	0.25	0.04	41.40
Cam1y-100	43.9	46.9	9.23	40.1	8.12	1.75	8.81	1.68	10.8	2.45	7.14	1.21	7.1	1.04	42.45

, with the distribution pattern illustrated in Figure 3. The average ΣREE value is 155.75 μg/g, with the LREE/HREE ratio averaging 3.24, ranging from 1.74 to 5.57. The Y/Ho ratio averages 45.01, and the mean δCe and δEu values are 0.5 (ranging from 0.4 to 0.71) and 1.10 (ranging from 0.94 to 2.14), respectively. These REE characteristics are consistent with a geochemical signature influenced by hydrothermal fluids [24], suggesting that the trace element and REE distributions reflect both the depositional environment and the hydrothermal processes that affected the region during the Early Cambrian.

4.4. Biomarkers

ATMB represents an extremely unique compound of aromatic biomarkers that are highly resistant to secondary alterations. These biomarkers are particularly significant in geochemical studies because they indicate a distinct source, namely green sulfur bacteria, and are associated with photic zone euxinia (PZE) environments. Such environments are known to enhance the enrichment and preservation of organic matter [10,18]. In this study, these compounds were identified through gas chromatography-mass spectrometry (GC-MS) analysis, where they exhibited characteristic mass spectral patterns (Figure 4). The structural confirmation of these compounds was achieved by comparing them with synthetic standards of 1-alkyl-2,3,6-trimethylbenzenes (C₁₃–C₂₂), as established by pioneering researchers in the field [18,24]. This analysis provides valuable insights into the sedimentary environment and the organic matter preservation processes, reinforcing the significance of hydrothermal and anoxic conditions in the Early Cambrian.

5. Discussion

5.1. Role of Terrigenous Influx

To explore factors such as paleoproductivity and redox conditions, it is necessary to assess whether the samples were influenced by terrigenous clastic components. In this study, thorium (Th) was employed as an index to measure terrigenous clastic input [25]. Additionally, Th, Al, Y/Ho, and Ti were used to assess the contribution of terrigenous detritus to diagenesis [16]. The results reveal a strong correlation between Th and Al, as well as Th and Ti (Figure 5, R² = 0.92 and 0.97, respectively), but no correlation with Y/Ho (Figure 5, R² = 0.001), indicating that the chemical composition of the samples was influenced by terrigenous detrital input [26]. The correlation between Al and ΣREE (Figure 6, R² = 0.84) further corroborates the presence of terrigenous material input [27].

The Ti content in sediments is used to calculate the amount of terrigenous detritus with the formula: terrigenous (T, %) = (Ti_sample/Ti_shale) × 100, where Ti_shale = 5995 ppm (PAAS) [28]. The T values range widely, from a minimum of 1.5% to a maximum near 70%, with an average of 21.80%. This indicates that terrigenous input in the target section should be considered, as it may exert a dilution effect on organic matter enrichment, leading to a decrease in TOC.

5.2. Paleoproductivity

Paleoproductivity refers to the amount of organic matter produced per unit area and time during a specific geological period. Geochemical indices serve as reliable tools for quantitatively evaluating paleoproductivity [29], with barium (Ba) being a particularly representative element [7,30]. Ba in marine sediments can originate from terrigenous detrital, biogenic, and hydrothermal sources. However, due to the influence of terrigenous detrital components, Ba cannot directly indicate productivity. Thus, it is necessary to remove the terrigenous Ba component. Typically, titanium (Ti) or aluminum (Al), which have a singular terrigenous origin, are used to eliminate the influence of terrigenous detritus [30]. The formula used to calculate the non-terrestrial Ba content is as follows:

w(Ba_{non-terrestrial}) = w(Ba_total) − w(Ti_total) × (w(Ba_total)/w(Ti))_PAAS

where w(Ba_{non-terrestrial}) represents the content of Ba from non-terrestrial sources, w(Ba_total) is the total Ba content in sediments, w(Ti_total) is the total Ti content, and (w(Ba_total)/w(Ti))_PAAS is the mid-continental Ba/Ti ratio of the PAAS, which is 0.11. The results indicate that the non-terrestrial Ba content in samples Cam_1y-5 to Cam_1y-45 ranges from 21.7 to 417.9 μg/g, indicating a higher supply of nutrients.

Moreover, the ratio of Zn/Ti is increasingly recognized as a reliable proxy for paleo-productivity, reflecting nutrient availability and primary productivity in ancient oceans [31]. A higher Zn/Ti ratio is often indicative of nutrient-rich environments conducive to enhanced biological activity. For instance, when the Zn/Ti ratio exceeds 0.2, it typically signals abundant nutrients, favoring biological productivity and organic matter accumulation. This is supported by studies suggesting that elevated Zn concentrations, linked with high productivity zones, correlate well with the growth of primary producers in nutrient-rich marine environments. The results indicate that the Zn/Ti ratio of the tested samples is generally greater than 0.2, with an average value of 0.67 and a maximum value of 1.82, which also reflects that the relevant source rocks are developed in water bodies with sufficient nutrient supply.

5.3. Sedimentary Environment

The sedimentary environment plays a critical role in the enrichment of organic matter. Various geochemical indicators have been proposed to identify the sedimentary environment of organic matter (

Table 4. The trace element ratio criteria of sedimentary environment.

Ratios	Oxidation	Sub-Oxidation	Anoxic
U/Th	<0.75	0.75–1.25	>1.25
V/Cr	<2	2–4.25	>4.25
Ni/Co	<5	5–7	>7
V/(V + Ni)	<0.46	0.46–0.6	0.6–0.85 (>0.85: sulfidic)

) [26,32,33]. In this study, the U/Th ratios range from 7.58 to 73.98 (average 25.84), V/Cr ratios from 1.07 to 11.31 (average 4.73), Ni/Co ratios from 5.67 to 76.11 (average 23.25), and V/(V + Ni) ratios from 0.78 to 1.00 (average 0.94), along with high Mo content and a Ce negative anomaly (δCe < 1). These characteristics suggest that the deposition environment of the samples was an anoxic sulfide-rich environment. The Rb/K ratio, indicative of sedimentary water depth [32], ranges from 11.97 to 36.08, with an average of 23.91. These values are lower than those of the source rocks from the Heituao Formation in semi-deep sea facies deposits on the northeastern margin of Tarim, where the Rb/K ratio ranges from 37.28 to 50.48 (average 44.98). This suggests that the black rock series of the Є₁y was deposited in shallower waters than those of the Heituao Formation.

Furthermore, evidence for anoxic, sulfide-rich conditions is supported by our earlier organic geochemistry studies, which identified ATMBs indicative of PZE [24,34,35,36,37,38]. ATMBs are derived from isorenieratene, a pigment in green sulfur bacteria (Chlorobiaceae), and are well-preserved in sediments [38]. These compounds serve as reliable biomarkers for PZE in geological records, as green sulfur bacteria perform anoxygenic photosynthesis in environments where light and hydrogen sulfide (H₂S) coexist. The presence of ATMBs indicates H₂S was present in the water column during sediment deposition, with light reaching anoxic layers, thereby confirming PZE conditions. This biomarker distinguishes the source rocks of the Є₁y from other formations [39] and is prevalent in deep to ultra-deep crude oil deposits in the central, northern, and southwestern Tarim Basin (Figure 7). This suggests that the Є₁y source rocks containing these compounds significantly contribute to the deep to ultra-deep oil and gas reserves in the Tarim Basin [10,39,40,41]. Additionally, it indirectly confirms that the Є₁y source rocks, although relatively thin (10–15 m), have exceptionally high TOC content (exceeding 20% in this study and reaching 29.8% in the well LT1 [42]), which developed under PZE conditions favorable for organic matter enrichment.

5.4. Hydrothermal Activity

Hydrothermal activity plays a crucial role in the enrichment of organic matter, with REE partition patterns and parameters serving as powerful tools for its investigation. A positive Eu anomaly is widely recognized as a significant indicator of hydrothermal fluid activity, though it is essential to account for false positives caused by Ba [23]. Correlation analysis between δEu and Ba or Al reveals no significant correlation, indicating that the positive Eu anomaly in these samples is not due to terrigenous detrital influence.

Additionally, samples influenced by hydrothermal solutions typically exhibit Fe/Ti ratios greater than 15 [10], and Y/Ho ratios close to modern seawater values (~45). In contrast, samples affected by terrigenous debris show lower Y/Ho ratios (Y/Ho = 28). The Fe/Ti ratio of the studied samples ranges from 8.50 to 68.25 (average 32.14), and Y/Ho from 39.7 to 54.2 (average 45.0), indicating the influence of hydrothermal activity. Analysis of samples Cam_1y-5 to Cam_1y-45 shows that non-continental Ba content varies from 21.7 to 417.9 μg/g, with a significant correlation with δEu (Figure 8a, R² = 0.53). Using Al as a control index for terrigenous detritus yields similar results (Figure 8b, R² = 0.53). These findings suggest that hydrothermal activity enhanced paleoproductivity. Additionally, correlations between δEu and V/Cr or Ni/Co (Figure 9, R² = 0.49 and 0.48, respectively) indicate that hydrothermal fluids influenced the preservation environment of organic matter, though their contribution to paleoproductivity was more significant. This is consistent with the conclusions drawn from previous mercury isotope analyses [43].

5.5. Organic Matter Enrichment Mechanism

Two primary models for source rock development include the preservation model, which emphasizes an anoxic environment, and the productivity model, which highlights high paleoproductivity. The prevailing model is still debated [9]. A pioneering study [44] analyzed and summarized the Cd/Mo ratio across various marine basins with different retention degrees, establishing 0.1 as the threshold between the two models. When Cd/Mo > 0.1, organic matter enrichment is dominated by high productivity; when Cd/Mo < 0.1, an anoxic preservation environment prevails. The Cd/Mo ratio in most samples from this study exceeds 0.1, with only two samples falling below this threshold, indicating that paleoproductivity primarily governs organic matter enrichment. Furthermore, analysis of the relationship between TOC and paleoproductivity indicators Ba and Cu, as well as preservation environment indicators V/Cr and Ni/V, shows a strong correlation with paleoproductivity indicators Ba and Cu (R² = 0.50 and 0.98, respectively) (Figure 10) and a weak correlation with preservation environment indicators V/Cr and Ni/V (R² = 0.01 and 0.03, respectively). This further supports the conclusion that paleoproductivity, with hydrothermal activity playing a significant role, is the dominant mechanism for organic matter enrichment in the Є₁y in the Keping area.

The presented composite bar chart (Figure 11) summarizes the high-resolution geochemical data from 100 densely sampled points across a 2.4 m thick siliceous rock layer, which includes both siliceous rocks and siliceous shale interlayers. The chart highlights key geochemical trends that reveal the mechanisms behind organic matter enrichment in the study area. From the bottom to the top of the section, Total Organic Carbon (TOC) displays two prominent features. First, the siliceous rocks are characterized by low organic matter content, with TOC generally less than 2%. In contrast, the siliceous shale interlayers, located in the middle of the siliceous rocks, exhibit much higher TOC values, with some reaching over 20%. The siliceous shale interlayers show two distinct high TOC intervals: one in the lower section (0 to 1.4 m) and another in the upper section (1.4 to 2.4 m). TOC increases at the bottom, decreases in the middle, and then increases again towards the top, which corresponds well with ancient environmental conditions and hydrothermal events.

As TOC increases, several other key geochemical parameters show parallel trends: (1) nutrient elements (Zn/Ti and w(Ba_{non-terrestrial})): these indicators also exhibit an increasing trend, suggesting that the rise in TOC is significantly influenced by paleoproductivity; terrigenous detrital input; conversely, parameters related to terrigenous input (T) show a decreasing trend, implying that the organic matter enrichment might have been subject to dilution effects from terrigenous sources; (2) V/(V + Ni) ratio: this ratio is consistently greater than 0.85, indicating a sulfur-deficient anoxic environment, which is highly favorable for the preservation of organic matter; (3) hydrothermal activity intensity (Fe/Ti): the Fe/Ti ratio also increases, indicating stronger hydrothermal activity, which further suggests that hydrothermal processes played a significant role in the organic matter enrichment of the siliceous shale.

Thus, the organic matter enrichment mechanism in the siliceous shale can be inferred as follows: Hydrothermal activity induced a sulfur-deficient anoxic environment, which facilitated the preservation of organic matter. The nutrients brought by hydrothermal fluids enhanced ancient productivity, while the increased input of hydrothermal substances relative to terrigenous material led to a reduction in the terrigenous dilution effect. As a result, hydrothermal activity created an optimal environment for organic matter accumulation by promoting a coupling of high ancient productivity, sulfur-deficient anoxia, and low terrigenous dilution. This triple coupling mechanism ultimately contributed to the high organic matter content observed in the siliceous shale, with TOC values exceeding 20%.

5.6. Depositional Model

The Th/U ratio (<0.1) of the studied samples is significantly lower than that observed in previous studies, indicating the incorporation of deep-sea materials during the deposition of siliceous rocks [10]. The Cambrian sedimentary environment in Wells Yingdong 2, Tadong 1, and Tadong 2 was characterized by anoxic conditions and high gamma-ray anomalies, with barite nodules observed in the Keping fault uplift [20], suggesting the presence of upwelling currents. Siliceous rocks of hydrothermal origin were deposited not only in the northwest platform, which was characterized by shallow water and gentle slopes, but also in the northeast basin in a deep-water sedimentary environment. This suggests that the Tarim block experienced extensive fracturing during the Early Cambrian, allowing hydrothermal fluids to erupt along these fractures and contemporaneous faults [43]. These hydrothermal fluids introduced reducing gases (H₂, CH₄, H₂S, etc.) into the water, creating anoxic stratification that facilitated the preservation of organic matter. They also transported large quantities of metals and essential elements for life, promoting the growth of thermophilic communities. However, excessive hydrothermal H₂S could lead to the poisoning of deep-water fauna and flora, accelerating the death of marine organisms and enhancing paleoproductivity. The trace elements introduced by these fluids also increased the rate and extent of organic matter decomposition during subsequent evolutionary processes, catalyzing hydrocarbon generation in source rocks [45]. Hydrothermal activity played a significant catalytic role in enhancing paleoproductivity, preserving and generating organic matter, and developing source rocks in the Early Cambrian Tarim block. Thus, combined with the exploration of the organic matter enrichment mechanism in Section 5.5. Organic matter enrichment mechanism, a developmental model of the Early Cambrian source rock was established (Figure 12).

6. Conclusions

(1)

The Є₁y Formation exhibits notable geochemical signatures, including elevated concentrations of Mo, Ba, and U, and a ΣREE of 155.75 μg/g, with a strong enrichment of light rare earth elements (LREE/HREE = 1.74–5.57), moderate Ce negative anomaly (δCe = 0.4–0.71), and a significant Eu positive anomaly (δEu = 0.94–2.14), indicating a unique depositional environment influenced by hydrothermal processes;

(2)

Geochemical parameters, including trace element and rare earth element distributions, point to the deposition of the Є₁y black shales in a highly reducing, anoxic, and sulfide-rich environment, which was conducive to organic matter preservation and enhanced sedimentary productivity;

(3)

The presence of hydrothermal trace elements, introduced via hydrothermal fluids, suggests a critical role in enriching the sedimentary system, preserving organic matter, and boosting the overall paleoproductivity in the shallow marine environment of the Tarim Basin during the Early Cambrian;

(4)

Hydrothermal fluids, likely emanating from volcanic activities along fractures and faults during the Early Cambrian, were instrumental in enhancing marine primary productivity, fostering the accumulation of organic matter, and contributing to the formation of high-quality source rocks in the Є₁y Formation. The organic matter enrichment model has been established;

(5)

The integrated geochemical findings suggest that the Early Cambrian period in the Tarim Basin experienced a dynamic interplay between hydrothermal influences and organic productivity, laying the foundation for significant oil and gas reserves in the deep and ultra-deep strata of the basin.