Geochemical Characteristics and Hydrocarbon Generation Potential of Source Rock in the Baorao Trough, Jiergalangtu Sag, Erlian Basin

Abstract

The Baorao Trough of the Jiergalangtu Sag, located in the central Erlian Basin, is rich in petroleum resources. However, due to a lack of systematic geochemical characterization and comparative studies with other source rocks, the hydrocarbon generation potential of its Jurassic strata remains unclear. In this study, 125 samples from the Baorao Trough were analyzed to evaluate their hydrocarbon generation potential, identify organic matter sources and depositional environments, and characterize hydrocarbon generation and expulsion. Results show that source rocks from the first member of the Tengge’er (K₁bt₁) Formation and the Aershan (K₁ba) Formation have high organic matter content, favorable kerogen types, and have reached low to medium maturity. In contrast, Jurassic source rocks are predominantly Type III kerogen and highly mature. K₁bt₁ was deposited in a weakly oxidizing to reducing, brackish environment, while K₁ba formed under weakly reducing, saline conditions. Jurassic source rocks also developed in weakly reducing, brackish to saline settings. Notably, saline and reducing environments promote the development of high-quality source rocks. The lower total organic carbon (TOC) threshold for effective source rocks in the study area is 0.8%, and the hydrocarbon expulsion threshold for vitrinite reflectance ratio (R_o) is approximately 0.8%. Accordingly, K₁bt₁ and K₁ba have undergone partial hydrocarbon expulsion but remain within the oil-generating window, indicating strong oil-generating potential. Jurassic source rocks likely experienced early thermal cracking of Type III kerogen, with generated oil migrating or escaping during early geological activity. However, some gas-generating potential remains. These findings provide significant evidence for assessing resource potential, predicting the distribution of high-quality source rocks and favorable exploration areas.

1. Introduction

Source rock is the fundamental material for hydrocarbon generation, and favorable source rock conditions are essential for the formation of medium to large oil and gas reservoirs [1,2]. The Jirgalangtu Sag in Erlian Basin is a significant petroliferous area in northern China, where heavy oil reservoirs, conventional oil reservoirs, and shallow gas reservoirs have been successively discovered [3,4]. Previous studies have identified three sets of potential source rocks within the Erlian Basin: the second member of Tengge’er (K₁bt₂) Formation, the first member of Tengge’er Formation (K₁bt₁)—Aershan (K₁ba) Formation, and the Jurassic strata [5,6].

In the eastern trough of the Jirgalangtu Sag, source rocks are shallowly buried and thermally immature, making large-scale hydrocarbon generation and expulsion unlikely. As a result, early drilling efforts yielded no significant discoveries. Current exploration is focused on the central and western troughs, where surrounding oil and gas reservoirs demonstrate the controlling role of source rocks in hydrocarbon distribution [7,8]. The Jurassic strata, deeply buried in the central troughs, have not been previously penetrated by wells. Based on Jurassic source rocks encountered at high structural position on the western trough slope, earlier studies reported low organic matter abundance (TOC = 0.30~0.97%) and poor hydrocarbon generation potential [9]. Similarly, the K₁bt₂ Formation, due to its shallow burial and low thermal maturity, also lacks the capacity for large-scale hydrocarbon generation and expulsion. Consequently, the hydrocarbons discovered in the region are believed to have originated from the K₁bt₁ and K₁ba Formations [6,7,10]. However, spatial variations in sedimentary facies mean that source rock properties in high slope areas may not accurately represent those in the sag center. Jurassic source rocks’ development and hydrocarbon potential in the sag center remain poorly understood, primarily due to the lack of available samples. The newly drilled L29X well in the central trough in 2024 revealed the presence of Jurassic black mudstones for the first time, challenging the prevailing view that the Cretaceous is the sole source rock.

To address the above issues, source rock samples from the Jurassic, K₁ba, and K₁bt₁ Cretaceous Formation in well L29X were systematically collected, and a comprehensive analysis was conducted, including total organic carbon (TOC), Rock-Eval pyrolysis, organic matter extraction and separation, saturated hydrocarbon gas chromatography (GC), and gas chromatography–mass spectrometry (GC–MS). This study aims to clarify the conditions for organic matter enrichment, evaluate the hydrocarbon generation potential, and elucidate the processes of hydrocarbon generation and expulsion. The results will provide critical insights into the origin of hydrocarbons, migration pathways, resource potential, and the distribution of high-quality source rocks, thereby supporting exploration efforts in the region.

2. Geological Setting

The Erlian Basin, located in northern China (Figure 1a), is a Mesozoic–Cenozoic sedimentary basin developed upon a Paleozoic folded basement formed during the Hercynian tectonic movement. Structurally, the basin extends from the Daxing’anling Uplift in the east to the Solunshan Uplift in the west, from the Wenduermiao Uplift in the south to the Bayanbaolige Uplift in the north. It is approximately 1000 km long east–west, 20 to 220 km wide north–south, and covers an area of approximately 100,000 km² [11,12]. It comprises a group of Mesozoic–Cenozoic rifted lake basins, including 54 small sags, each with an independent depositional system (Figure 1b) [13,14].

The Jiergalangtu Sag is located southwest of the Sunite Depression in the central Erlian Basin (Figure 1b). It extends approximately 45 km east–west and 10–15 km north–south, covering an area of about 600 km². The maximum burial depth of the Lower Cretaceous reaches about 3500 m. Structurally, it is a half-graben depression trending NE–SW, characterized by a south-dipping boundary fault and northward onlap (Figure 1c) [6,15,16]. The sag comprises three troughs: eastern, central, and western. The central trough is the main hydrocarbon generation zone, with current exploration efforts focused on the central and western parts.

The sedimentary strata of Erlian Basin are predominantly Mesozoic, with Triassic deposits largely absent across most areas, including the Jiergalangtu Sag. The stratigraphic sequence consists of Jurassic and Cretaceous formations, with the latter subdivided into Aershan (K₁ba), Tengge’er (K₁bt), and Saihantala (K₁bs) formations (Figure 1d and Figure 2). During the early and Middle Jurassic, the Jiergalangtu Sag developed the Alatanheli Group, composed of a set of fluvial–lacustrine sandstone, conglomerate, and relatively thinner-layered dark mudstone. These sediments were deposited in irregular, low-lying areas in a filling-type pattern. In the Late Jurassic, intense tectonic inversion along basement fault zones triggered large-scale volcanic eruptions. The remaining thickness of the Alatanheli Formation is approximately 0~200 m [17]. Subsequently, the Jiergalangtu Sag developed a thick volcanic sequence of the Xing’anling Group, mainly consisting of andesite. During volcanic hiatuses, these fluvial–lacustrine deposits formed, including gray mudstone thin layers [18]. In the Early Cretaceous (Aershan Formation), the Yanshan Movement caused lava overflow and initiated rifting, leading to the expansion of lake basins.

On the basin margins, coarse-grained, impurity-rich silty mudstone and conglomerate were deposited. However, the limited sediment supply led to only small underwater fans being formed at the basin edges. In the central depression, thick layers of dark mudstone and dolomitic mudstone rich in oil-generating material accumulated. During the middle Cretaceous stage (between the deposition of K₁bt₁ and K₁bt₂), two large-scale lake transgressions occurred. The first member of the K₁bt₁ Formation saw the longest and most extensive transgression, marking the main stage of lake basin development. The sedimentation was dominated by lacustrine facies, producing fine clastic rocks, such as calcareous mudstone, dolomitic mudstone, and dark gray mudstone. In the Baorao Trough, multiple deltas developed, with the sedimentary system characterized by proximity to the sediment source, narrow facies belts, and rapid lateral variations [6]. Throughout the Baorao Trough’s deposition, the initial subsidence phase was substantial. Later regional uplift caused erosion of overlying strata, with maximum denudation occurring after the Saihantala Formation deposition. Before the Late Cretaceous (prior to the middle Saihantala deposition), the Jiergalangtu Sag existed as a unified catchment lake basin. However, intense tectonic reactivation ended the Saihantala sedimentation and divided the basin into three troughs along the Baorao–Baofeng and Hani–Xilin transverse structural belts [19,20].

Figure 2. Comprehensive stratigraphic column of the Jiergalangtu Sag [21,22].

Figure 2. Comprehensive stratigraphic column of the Jiergalangtu Sag [21,22].

The study area, the Baorao Trough, is located in the central Jirgalangtu Sag (Figure 1c) and covers approximately 320 km². It consists of three structural elements: the northern steep-slope Hani Structural Belt, the southern gentle-slope Baorao Structural Belt, and the central depression zone, which forms the sag’s principal hydrocarbon-generating trough [7]. The Baorao Trough has significant oil and gas exploration potential, with an estimated resource of 35 million tons. By the end of 2024, cumulative proven reserves reached 10.25 million tons, leaving substantial resources untapped (data from Huabei Oilfield Company, Renqiu, China).

3. Samples and Methods

3.1. Samples

Samples were collected from the K₁bt₁, K₁ba, and Jurassic formations in the Baorao Trough (Xilinhot, China), which consist mainly of gray and dark gray mudstones, deposited in shallow lacustrine settings. A total of 125 samples were taken from eight wells (J55, L7, J58, J59, L4-37X, L29X, J43, and L15) along the west–east axis of the trough (Figure 1c). Of these, 29 samples came from the Cretaceous K₁bt₁ formation, 70 samples from the Cretaceous K₁ba formation, and 26 samples from the Jurassic strata. Notably, four wells (J58, J59, J43, and L15) are located in the trough slope zone, where drilling depths and formation depth ranges vary, reflecting the oilfield company’s selective coring strategy (non-full-well coring). Accordingly, most samples from the K₁ba Formation, which developed originally below the K₁bt₁ Formation, are currently at shallower actual depths than the K₁bt₁ samples in this study. This phenomenon of stratigraphic depth inversion is noted here to prevent ambiguity in the discussions and figures below (e.g., scatter plots of depth versus geochemical parameters). All samples were immediately packed into wooden boxes for transportation after being collected. They were placed in a sample storage room with a constant setting, with a temperature of 24 °C, a humidity of 45%, and protected from light until tested within 20 days.

To assess the abundance, type, and maturity of organic matter and evaluate the hydrocarbon generation potential of source rocks, all samples were analyzed for total organic carbon (TOC) and rock pyrolysis analysis; 18 samples were analyzed for vitrinite reflectance (R_o), 16 samples were analyzed for organic elemental composition, and 10 samples were analyzed for kerogen carbon isotope. To investigate the origin of organic matter and depositional environments, 19 samples were analyzed using gas chromatography (GC) and gas chromatography–mass spectrometry (GC–MS). All analyses were performed at the Central Laboratory of Geological Sciences, Exploration and Development Research Institute, PetroChina Huabei Oilfield Company.

3.2. Methods

Rock debris samples were carefully selected to remove impurities, including drilling mud particles and barite. Core and cutting samples were cleaned with distilled water, dried in an oven at 45 °C for 12 h, and then pulverized to 100 mesh. Pretreatment was performed to remove inorganic carbon prior to TOC analysis. Finely ground samples were treated with an excess of dilute HCl (H₂O: HCl = 7:1 by volume) and heated in a 60 °C water bath for over 2 h. The samples were rinsed with distilled water to neutral PH and dried at 60 °C. TOC content was measured using a LECO CS-844 carbon–sulfur analyzer, with O₂ as the combustion-supporting gas, adjusted to 0.25 MPa, following the Chinese National Standard GB/T 19145-2022 [23].

The Rock-Eval pyrolysis was conducted using a Rock-Eval 6 analyzer, in accordance with the Chinese National Standard GB/T 18602-2012 [24]. Measured parameters included free and volatile hydrocarbon (S₁) released at 300 °C; remaining hydrocarbon-generating potential (S₂) during progressive heating from 300 °C to 650 °C; and the temperature of maximum pyrolysis yield (Tmax).

For major elements analysis, approximately 1.0 g of powdered sample was weighed into a porcelain crucible and combusted in a muffle furnace at 1000 °C for 4 h to remove organic matter and carbonate with mass loss recorded. The ashed sample was mixed with 0.3~0.4 g ammonium nitrate (NH₄NO₃) and 6 g Li₂B₄O₇-LiBO₂-LiF flux in a platinum crucible, then fused at 1050 °C for 10 min to produce a glass bead. Major element concentrations were determined using a Rigaku 100e X-ray fluorescence spectrometer (Rigaku, Akishima, Japan), with analytical precision better than ±5% following the Chinese National Standards GBW07107 [25]. The chemical index of alteration (CIA) was calculated as $\mathrm{CIA} = \mathrm{molar} [{\mathrm{Al}}_2{\mathrm{O}}_3/({\mathrm{Al}}_2{\mathrm{O}}_3 + \mathrm{CaO}* + {\mathrm{Na}}_2\mathrm{O} + {\mathrm{K}}_2\mathrm{O}\left)\right] \times 100$, where CaO* represents the concentration of CaO in the silicate fraction, derived from a formula based on P₂O₅ content ($\mathrm{CaO}* = \mathrm{molar} \mathrm{CaO}-10/3 \times \mathrm{molar} {\mathrm{P}}_2{\mathrm{O}}_5$) [26]. If the calculated CaO* is lower than Na₂O, CaO* can be directly adopted to calculate CIA, otherwise, it is replaced with Na₂O [27].

Kerogen preparation followed the Chinese National Standards GB/T 19144-2010 [28] prior to vitrinite reflectance, kerogen elemental, and kerogen carbon isotope analyses. Source rock samples were treated sequentially with 15% HCl, a mixture of 15% HCl and 40% HF (1:3 by volume), and 15% HCl (heated in a water bath at 80 °C) to dissolve minerals, then repeatedly washed with ultrapure water to neutral PH and dried to obtain kerogen. The isolated kerogen powder was placed into fixed molds, impregnated with epoxy resin for cold curing, and after solidification, ground and polished using an automatic grinding polisher with alumina suspension. Polished thin section surfaces were examined under a 20× dry objective to ensure the absence of stains, scratches, and unclear boundaries. After drying, random reflectance measurements were taken using a Leica DM4500P polarizing microscope (Leica Microsystems, Wetzlar, Germany) with a 50× objective and an MPM-80 photometer (PerkinElmer, Waltham, MA, USA). Calibration was conducted using yttrium aluminum garnet (R_o = 0.90%), gadolinium gallium garnet (R_o = 1.72%), and cubic zirconia (R_o = 3.17%) in line with the actual reflectance range of the samples. To account for optical variability among vitrinite particles within the same sample, multiple measurements were taken to ensure the representativeness: at least 30 points when the mean R_o was ≥2.0% and at least 20 points when R_o was between 0.5% and 2.0%. The analysis followed the Chinese Industry Standard SY/T 5124-2012 [29], with a standard deviation below 0.2%.

The organic elemental composition (C, H, N, and O) of kerogen samples was measured using a Thermo FLASH EA1112. Helium was used as the carrier gas at a constant flow rate of 150 mL/min. For carbon, hydrogen, and nitrogen analysis, the column temperature and the left oven were set to 65 °C and 950 °C, respectively. For oxygen analysis, the carrier gas flow rate was 130 mL/min, with the column and right oven temperature set to 75 °C and 1060 °C, respectively. The analytical accuracy was better than ± 0.1% (1 sd, n = 9) using IVA33802180 (C = 0.83%, S = 0.014%, N = 0.07%), IVA33802150 (C = 7.45%, S = 0.62%, N = 0.52%), and B2162 (C = 46.47%, S = 0.78%, N = 10.75%) as reference material for calibration, following the Chinese National Standard GB/T 19143-2017 [30].

Kerogen carbon isotope (δ¹³C) analysis was performed using a Thermo EA IsoLink-MAT 253 Plus IRMS (Thermo Fisher Scientific, Waltham, MA, USA) with pure CO₂ as the standard gas, helium as the carrier gas, and reference gas at a constant rate of 180 mL/min and 70 mL/min, respectively; oxygen was used as the combustion-supporting gas at 250 mL/min. The reaction oven and GC oven temperatures were set to 960 °C and 50 °C. Calibration and isotope calculation were performed against three international reference materials—USGS61 (δ¹³C = −35.05‰), USGS43 (δ¹³C = −21.28‰), and USGS62 (δ¹³C = −14.79‰)—with analytical precision better than ± 0.2‰ (1 sd, n = 9), following the Chinese Industry Standard SY/T 5238-2019 [31].

Organic matter was first extracted to analyze biomarkers. First, the samples were enclosed in filter paper cartridges after being crushed to 100 meshes, placed in the extractor chamber, and heated in a 50~60 °C water bath while being continuously extracted with chloroform to obtain chloroform bitumen “A”. Asphaltenes were removed from the extracts by precipitation with n-hexane. The resulting maltenes were separated on a silica gel/alumina column (activated at 150 °C for 12 h) using sequential elution with n-hexane, n-hexane/dichloromethane (1:1 by volume), and dichloromethane/methanol (95:5 by volume) to obtain saturated, aromatic, and resin fractions, respectively. Quantitative analysis of saturated hydrocarbon was performed using GC and GC–MS, following the Chinese National Standards GB/T 18340.5-2010 [32] and GB/T 18606-20 [33].

Gas chromatography (GC) of saturated hydrocarbons was performed on an Agilent 6890N (Agilent Technologies, Santa Clara, CA, USA) equipped with a 30 m × 0.20 mm × 0.25 μm PONA fused silica capillary column, using N₂ as the carrier gas at a flow rate of 1.0 mL/min. The oven was programmed at 50 °C for 10 min, then ramped to 310 °C at 4 °C/min, and finally held at 310 °C for 40 min. The injection and detector temperatures were set to 310 °C. Gas chromatography–mass spectrometry (GC–MS) of the saturated hydrocarbon fraction was performed on a Thermo 1300-ISQ (Thermo Fisher Scientific, Waltham, MA, USA) with a 30 m × 0.20 mm × 0.25 μm HP-5MS fused silica capillary column, using helium as the carrier gas at a constant flow rate of 1.0 mL/min. The vaporizing chamber and transfer line temperatures were 310 °C and 300 °C, respectively. The oven was held at 50 °C for 1 min, then ramped to 100 °C at 20 °C/min, then to 310 °C at 3 °C/min, and held for 20 min. Analyses were run in electron impact (EI) mode at 70 eV in both full-scan and selected-ion monitoring modes, with a scan range of 50–550 amu.

4. Results

4.1. Bulk Organic Geochemical Characterization

4.1.1. Geochemical Parameters of Whole Rock

The K₁bt₁ source rocks show the highest TOC values, ranging from 0.39% to 4.04%, with 58.6% of samples exceeding 1.00% and a mean of 1.45%. In contrast, the K₁ba source rocks have the lowest TOC, with 30% of samples below 0.6% and a mean of 1.19%. Additionally, the Jurassic source rocks exhibit intermediate TOC values, ranging from 0.71% to 1.76%, with an average of 1.37% (Figure 3).

Rock-Eval pyrolysis results show that the sum of free hydrocarbon values (S₁) and remaining hydrocarbon values S₂, S₁ + S₂ in K₁bt₁ and K₁ba source rocks range from 0.68 mg HC/g rock to 21.83 mg HC/g rock and 0.08 mg HC/g rock to 27.9 mg HC/g rock, with mean values of 4.78 mg HC/g rock and 3.45 mg HC/g rock, respectively. In contrast, the Jurassic source rocks have significantly lower S₁ + S₂ values, ranging from 0.20 to 3.78 mg HC/g rock, with an average of 0.96 mg HC/g rock; 38.5% of samples fall below 0.40 mg/g HC/g rock, and 61.5% lie between 0.40 and 6.00 mg/g HC/g rock (Figure 4).

The hydrogen index (HI = S₂ × 100/TOC) values of the K₁bt₁ and K₁ba source rocks (80~479 mg/g TOC and 40~693 mg/g TOC, with average values of 227 mg HC/g TOC and 191 mg HC/g TOC) are relatively higher than those of the Jurassic source rocks (16~212 mg HC/g TOC, with an average value of 60 mg HC/g TOC) (Figure 5). In contrast, the Jurassic has higher Tmax values (the temperature of the peak rate of hydrocarbon generation) (440 °C~528 °C and a mean of 474 °C) than the K₁bt₁ (437~451 °C, with a mean of 446 °C) and K₁ba source rocks (424~451 °C, with a mean of 442 °C).

Major element analysis was conducted on 13 mudstone samples from the L29x well focusing on CIA and Al₂O₃/Na₂O ratios. The CIA values of the K₁bt₁ and K₁ba source rocks are similar, ranging from 54.1 to 68.3 and 61.2 to 64.8, with averages of 62.0 and 62.8, respectively. In contrast, the Jurassic rocks have lower CIA values, ranging from 55.5 to 57.6, with a mean of 56.5. Additionally, the Al₂O₃/Na₂O ratios for the K₁bt₁ and K₁ba range from 3.22 to 7.37 and 4.88 to 5.82, with averages of 5.12 and 5.29, respectively. The Jurassic rocks show lower ratios, ranging from 3.51 to 4.23, with a mean of 3.87.

4.1.2. Geochemical Parameters of Kerogen

The O/C ratios from the K₁bt₁ and K₁ba kerogen samples range from 0.06 to 0.10 and 0.06 to 0.84. Only one sample was obtained from the Jurassic source rocks, with H/C and O/C ratios of 0.54 and 0.08, both significantly lower than those of the K₁bt₁ and K₁ba source rocks. The carbon isotopic compositions (δ¹³C) of K₁ba kerogen range from −31.1‰ to −24.35‰. In contrast, four representative Jurassic samples exhibit higher δ¹³C values than K₁ba source rocks, with a range of −23.06‰~−22.50‰.

4.2. Vitrinite Reflectance Characterization

The mean vitrinite reflectance ratio (R_o) of kerogen from the K₁bt₁ and K₁ba samples range from 0.63% to 0.82% and 0.52% to 1.15%, with averages of 0.72% and 0.74%, respectively. The Jurassic source rocks have higher R_o values, ranging from 1.84% to 2.35%, with an average of 2.04%. The kerogen organic elemental composition shows H/C ratios of K₁bt₁ and K₁ba range from 0.88 to 1.22 and 0.85 to 2.62, respectively.

4.3. Molecular Geochemical Characteristics

4.3.1. N-Alkanes and Isoprenoids

Gas chromatography (GC) analysis of mudstone samples reveals n-alkanes ranging from nC₁₃ to nC₃₆. The n-alkane distributions for the K₁bt₁, K₁ba, and Jurassic source rocks are predominantly unimodal, peaking at nC₁₅~nC₁₈, nC₁₅~nC₂₃, and nC₁₈~nC₂₃, respectively. Odd–even predominance (OEP) values range from 0.81 to 1.23, indicating a generally weak odd-carbon preference. The pristane (Pr)-to-phytane (Ph) ratios vary among the mudstone samples (Figure 6). K₁bt₁ ranges from 0.98 to 1.07 (mean 1.02). K₁ba ranges from 0.55 to 1.50 (mean 0.96), with most values between 0.75 and 1.10. In comparison, that of the Jurassic samples is relatively lower (0.60~0.89, average of 0.68), showing less distinct odd-carbon predominance (

Table 1. Representative biomarker parameters for source rock samples in the Baorao Trough.

Well	Strata	Depth (m)	Pr/Ph	Pr/nC17	Ph/nC18	OEP	C27 Ster. (%)	C29 Ster. (%)	C19+20 TT (%)	C21 TT (%)	C23 TT (%)	Ga/C30H	Ga/C31H	S/H	C35H/C31+32H
L4-37X	K1bt1	1860~1870	0.98	0.19	0.19	1.13	25.67	60.49	0.44	0.30	0.26	0.07	0.14	0.14	0.08
L4-37X	K1bt1	1950~1960	1.07	0.22	0.23	1.08	27.66	56.43	0.45	0.29	0.27	0.10	0.21	0.21	0.19
L4-37X	K1bt1	2040~2050	1.02	0.15	0.14	1.00	27.86	54.83	0.44	0.31	0.25	0.11	0.21	0.27	0.19
L4-37X	K1bt1	2140~2150	1.00	0.12	0.13	1.03	27.24	57.25	0.48	0.37	0.15	0.13	0.27	0.37	0.24
J43	K1ba	1390.58	0.64	0.52	0.89	1.20	14.96	71.68	0.43	0.27	0.30	0.44	2.28	0.28	0.22
J43	K1ba	1394.4	0.55	0.63	1.22	1.21	18.76	67.45	0.40	0.26	0.35	0.10	0.42	0.36	0.24
J43	K1ba	1400.2	0.57	0.51	0.98	1.19	15.31	72.64	0.38	0.28	0.34	0.11	0.50	0.28	0.18
J55	K1ba	1306.8	1.40	0.37	0.26	1.02	25.17	57.48	0.46	0.28	0.26	0.17	0.39	0.46	0.49
J55	K1ba	1315.34	1.45	0.46	0.31	1.01	26.63	55.94	0.40	0.27	0.33	0.12	0.33	0.39	0.33
J58	K1ba	1520.75	1.08	0.28	0.21	1.02	22.20	63.10	0.42	0.26	0.31	0.15	0.30	0.32	0.30
J58	K1ba	1523.55	0.88	0.28	0.28	1.08	28.81	55.26	0.40	0.32	0.28	0.20	0.61	0.16	0.42
J58	K1ba	1527.6	0.99	0.28	0.30	1.01	21.70	62.98	0.46	0.31	0.22	0.21	0.50	0.44	0.48
J58	K1ba	1548.38	1.12	0.40	0.32	1.13	30.16	57.28	0.44	0.29	0.26	0.23	0.64	0.19	0.41
J58	K1ba	1602.26	0.79	0.22	0.27	1.03	17.03	66.52	0.41	0.33	0.26	0.28	0.89	0.20	0.30
J58	K1ba	1603.76	0.75	0.22	0.29	1.06	18.86	65.62	0.41	0.34	0.25	0.20	0.65	0.22	0.27
J59	K1ba	1166.01	0.59	0.64	1.19	1.24	28.88	53.87	0.40	0.35	0.26	0.25	1.53	0.33	0.29
J7	K1ba	1673.06	1.08	0.17	0.18	1.17	26.71	58.27	0.34	0.29	0.38	0.12	0.44	0.18	0.34
J15	K1ba	1388.25	1.50	0.64	0.43	1.23	52.98	38.53	0.50	0.22	0.27	0.11	0.28	0.26	0.29
J29X	J	2820~2824	0.89	0.23	0.22	0.96	29.85	52.72	0.39	0.28	0.33	0.10	0.24	0.35	0.24
J29X	J	2857.1	0.60	0.33	0.55	0.91	29.60	53.97	0.40	0.28	0.32	0.16	0.48	0.35	0.22
J29X	J	2858.95	0.61	0.18	0.29	0.98	32.98	48.93	0.35	0.26	0.39	0.15	0.37	0.37	0.26
J29X	J	2860.6	0.62	0.14	0.19	0.82	26.46	57.20	0.37	0.27	0.37	0.18	0.49	0.36	0.24

Note: Pr/Ph = pristane/phytane; Pr/nC₁₇ = pristane/nC₁₇; Ph/nC₁₈ = phytane/nC₁₇; OEP (odd–even predominance) = ${\left[{(c}_i+{6c}_{i+2}+c_{i+4})/\left({4c}_{i+1}+{4c}_{i+3}\right)\right]}^{{(-1)}^i+1}$, in which i stands for the carbon number of the main peak carbon; C₂₇Ster. (%) = C₂₉Sterane/(C₂₇Sterane + C₂₈Sterane + C₂₉Sterane); C₂₉Ster. (%) = C₂₉Sterane/(C₂₇Sterane + C₂₈Sterane + C₂₉Sterane); Ga/C₃₀H = gammacerane/C₃₀hopane; Ga/C₃₁H = gammacerane/C₃₁hopane; S/H = steranes/hopanes; C₃₅H/C₃₁₊₃₂H = C₃₅hopane/(C₃₁hopane + C₃₂hopane.

).

4.3.2. Terpenoids

Abundant terpenoid compounds were detected in source rocks from the Baorao Trough, including minor tricyclic terpenes, trace tetracyclic terpenes, and predominant pentacyclic triterpenoids (hopane series). Terpenes mass chromatograms (m/z 191) of all source rock extracts show high intensities of C₃₀ hopane (C₃₀H) as the dominant peak, with abundance decreasing from C₃₁ hopane to C₃₅ (Figure 7). The C₃₅H index, expressed as the C₃₅H/C₃₄H ratio, is lower in K₁bt₁ than in K₁ba and Jurassic source rocks, ranging from 0.53 to 0.65, 0.37 to 1.06, and 0.63 to 0.67, with averages of 0.58, 0.64, and 0.65, respectively. Gammacerane (Ga) abundance varies (Figure 7), with the Ga index (Ga/C₃₀H) between 0.18 and 1.72 overall. Compared with the K₁bt₁ and Jurassic source rocks, the K₁ba shows marginally higher Ga indexes, ranging from 0.24 to 1.72, with a mean of 0.52. In comparison, the ratios of 0.18~0.32 for K₁bt₁ show the lowest values, with an average of 0.26. In contrast, the ratios of the Jurassic range between the two sets of source rocks (0.21~0.40), with an average of 0.33. The C₂₁TT, C₁₉₊₂₀TT, and C₂₃TT percentages for the K₁bt₁ source rocks range from 28.7% to 36.7%, 43.5% to 47.9%, and 15.4% to 26.5%, with means of 31.6%, 45.0%, and 23.4%, respectively. Samples from the K₁ba source rocks exhibit more heterogeneous percentages and similar averages to K₁bt₁, ranging from 22.4% to 34.5%, 33.6% to 50.2%, and 22.2% to 37.8%, with averages of 29.1%, 41.8%, and 29.1%, respectively. In contrast, Jurassic source rocks exhibit relatively low C₂₁TT (25.8%~28.1%) and C₁₉₊₂₀TT (35.4%~39.8%) but higher C₂₃TT (32.3%~38.8%), with means of 27.1%, 37.8%, and 35.1%, respectively (Table 1).

4.3.3. Steroids

Mass chromatograms for steroids (m/z 217) exhibit a predominance of C₂₉ steranes over C₂₇ and C₂₈ steranes in all three source rock groups, with slight differences in ratio distributions (Figure 8). K₁bt₁ source rocks display narrow ranges (13.8%~17.3% of C₂₇ steranes, 25.7%~27.9% of C₂₈ steranes, and 54.8%~60.5% of C₂₉ steranes). K₁ba samples are more variable, ranging from 8.5% to 17.4%, 15.3% to 30.2%, and 53.9% to 72.6% of C₂₇, C₂₈, and C₂₉ steranes, respectively. Jurassic samples are relatively stable, ranging from 16.3% to 18.1%, 26.5% to 33.0%, and 48.9% to 57.2%, respectively. The steranes-to-hopanes (S/H) ratios are similar across groups, with a mean of 0.36 for Jurassic, 0.25 for K₁bt₁, and 0.29 for K₁ba (Table 1).

5. Discussion

5.1. Hydrocarbon Generation Potential

5.1.1. Organic Matter Abundance

The abundance of organic matter reflects its relative content in source rocks and is the primary factor determining their hydrocarbon generation potential. Common indicators include total organic carbon (TOC) content, chloroform bitumen “A” content, and hydrocarbon generation potential S₁ + S₂ from rock pyrolysis [34,35,36].

Statistical analysis shows that source rocks in the study area have relatively high TOC contents but low S₁ + S₂ values and bitumen “A” contents (0.0984~0.7376% in K₁bt₁, 0.0051~0.2762% in K₁ba, and 0.0077~0.0312% in Jurassic), particularly pronounced in the Jurassic, consistent with high maturity and significant hydrocarbon generation and expulsion. Comprehensive evaluation indicates that K₁bt₁ and K₁ba are fair-quality source rocks. In contrast, Jurassic source rocks are relatively poor (Figure 9a). Additionally, the Jurassic samples exhibit stable TOC values, with 96.2% falling between 1% and 2%. In comparison to the Jurassic source rocks, the K₁bt₁ and K₁ba show greater variability, ranging from 0.39% to 4.04% and 0.08% to 4.81%, respectively. Furthermore, 89.6% of K₁bt₁ samples have TOC values greater than 0.6%, while only 70.0% of K₁ba source rock samples present TOC content above this threshold (Figure 3). The pronounced TOC heterogeneity in K₁bt₁ and K₁ba reflects unstable depositional environments and variable organic matter sources during that time, whereas Jurassic source rocks were deposited in a more stable setting with consistent organic matter supply.

5.1.2. Organic Matter Type

The type of organic matter is another critical parameter in source rock evaluation, as it determines the products generated during the maturation [35,37]. Type I kerogen is oil-prone, whereas Type III is gas-prone [34,35]. Both the K₁bt₁ and K₁ba source rocks are dominated by Type II₁-II₂ organic matter. However, the K₁ba contains a higher proportion of Type I and Type III kerogen than K₁bt₁, indicating greater heterogeneity. This suggests that K₁ba was deposited under more variable conditions, including different organic matter sources and depositional environments. They are mainly composed of types II₂–III organic matter, implying limited oil-generation capacity but some potential for gas generation (Figure 9b). In addition to the hydrogen index (HI), organic element ratios and carbon isotopes are important indicators of organic matter type.

Kerogen is primarily composed of carbon (C), hydrogen (H), oxygen (O), sulfur (S), and nitrogen (N), and its elemental composition varies by origin. Aquatic-derived kerogen is hydrogen-rich, oxygen-poor, and enriched in liptinite macerals, allowing type determination through kerogen elemental analysis. Organic matter typing is conventionally performed by plotting hydrogen/carbon (H/C) and oxygen/carbon (O/C) atomic ratios on a “Van Krevelen” diagram (Figure 9c) [35,38]. Furthermore, the carbon isotopic composition (δ¹³C) of kerogen in sedimentary rocks is largely determined by the isotopic signature of the precursor organic matter, with a slight enrichment in δ¹³C (generally <2‰) observed with increasing thermal maturity [39]. Type III kerogen generally has δ¹³C values around −25.5‰, while Type II falls between −28‰ and −25.5‰ [40]. δ¹³C analysis reveals that K₁ba source rocks predominantly contain Type II kerogen, whereas Jurassic source rocks show characteristics of Type III. These findings are consistent with the results of the HI and kerogen elemental analysis.

5.1.3. Thermal Maturity

Vitrinite reflectance (R_o), a measure of the optical properties of vitrinite in kerogen, is widely used to evaluate the thermal evolution of organic matter in source rocks [35,36]. In the study area, R_o ranges from 0.52% to 2.35%, placing most samples just before the peak oil-generation window. Jurassic source rock samples have higher R_o values, indicating high thermal maturity and entry into the high-temperature gas-generation stage. In contrast, the K₁bt₁ and K₁ba source rocks have reached the oil-generation stage.

Based on the R_o-depth trend, the oil-generation threshold (R_o = 0.5%) occurs at depths of less than 1200 m (Figure 10a). Additionally, due to the limited number of measured R_o samples, restricted by kerogen preparation difficulties, these data are insufficient for a comprehensive maturity assessment. Therefore, the Tmax parameter is used as supplementary evidence. Equivalent vitrinite reflectance values calculated from T_max (R_Tmax = 0.018 × T_max − 7.16) [41] show good agreement with measured R_o values from corresponding samples (Figure 10b).

5.2. Geochemical Characteristics

5.2.1. Organic Matter Sources

Organisms exhibit distinct n-alkane patterns: phytoplankton typically dominate in low-molecular-weight n-alkanes, while terrestrial higher plants are characterized by high-molecular-weight n-alkanes [42]. Therefore, n-alkane profiles can indicate the sources of organic matter. The K₁bt₁ and K₁ba exhibit a predominance of short-chain n-alkanes (Figure 6), suggesting a greater contribution from lower organisms such as algae. In contrast, the Jurassic source rocks peak at nC₂₁ to nC₂₃, indicating a higher input from terrestrial higher plants. Thermal maturity affects these patterns, as increasing maturity promotes the cracking of long-chain to short-chain n-alkanes [43]. The low maturity of K₁bt₁ and K₁ba means their n-alkane distributions reliably reflect original organic matter input. In contrast, the Jurassic source rocks, with maturity up to 2.0% R_o, have experienced significant alteration. Therefore, it can be inferred that in their early thermal history, the Jurassic source rocks likely peaked at higher n-alkanes, reflecting a greater terrestrial plant contribution.

C₂₇ steranes are typically derived from plankton, metazoans, and red algae; C₂₈ steranes from chlorophyll-c-containing phytoplankton (e.g., diatoms, green and golden algae); and C₂₉ steranes predominantly from terrestrial higher plants [44,45]. GC–MS analysis of saturated hydrocarbons shows enrichment in C₂₉ steranes across all source rocks (Figure 11a), indicating a dominant terrestrial organic matter input [46,47]. Additionally, the sterane/hopane (S/H) ratio reflects the relative contributions of eukaryotic (mainly algae and higher plants) versus prokaryotic (bacterial) organisms [47,48], while S/H values >1 suggests algal-derived input. The study area shows values 0.14~0.49, indicating a predominance of terrestrial or microbially altered organic matter, consistent with interpretation from the sterane ternary diagram.

Organic-matter typing reveals a discrepancy. The K₁bt₁ and K₁ba source rocks are dominated by Type II kerogen, whereas the Jurassic source rocks are dominated by Type III kerogen and should contain a greater contribution from terrestrial higher plants in theory. However, the Jurassic shows lower C₂₉ regular sterane content than the other two intervals (Figure 11a). Previous studies indicate that C₂₉ steranes can also derive from diatoms, cyanobacteria, green algae, and brown algae [47,49], reducing their reliability as indicators of terrestrial input. In addition to multiple biological sources, thermal maturation can also affect C₂₉ sterane abundance. In crude oil maturity assessment, the C₂₉αααS/(S + R) and C₂₉αββ/(ααα + αββ) ratios are robust maturity indicators, but during source rock thermal evolution, relative C₂₉ sterane abundance may shift. This study shows weak positive correlations between these ratios and thermal maturity (R² = 0.27 and 0.24) (Figure 12). Once equilibrium values are reached, sterane degradation occurs, further reducing their reliability as biological indicators. Thermal degradation likely explains the lower C₂₉ sterane content in the Jurassic compared with K₁bt₁ and K₁ba.

5.2.2. Sedimentary Environment

Pristane and phytane are commonly used to assess sedimentary environments. Pr/Ph ratios of <0.5, 0.5~1.0, 1.0~2.0, and >2.0 generally indicate strongly reducing, reducing, weakly reducing to weakly oxidizing, and strongly oxidizing conditions, respectively [47,50]. Based on Pr/Ph ratios and Pr/nC₁₇ vs. Ph/nC₁₈ diagrams (Figure 13a,b), the K₁bt₁ source rocks were deposited in a weakly oxidizing to reducing environment, while the K₁ba and Jurassic source rocks formed under weakly reducing conditions.

Tricyclic terpenes (TTs) are widely distributed in petroleum and sedimentary organic matter and are thought to originate from both vascular plants and lower organisms. For example, C₁₉TT and C₂₀TT are mainly derived from diterpenoids of vascular plants and serve as typical markers of terrestrial higher plant input. Source rocks from marine or saline lacustrine facies typically exhibit C₂₃TT dominance, whereas freshwater lacustrine facies are generally characterized by C₂₁TT predominance [51,52]. Therefore, the tricyclic terpenes distribution patterns are closely linked to depositional environments [53,54]. All samples in this study show relatively high C₂₁TT and C₁₉₊₂₀TT percentages, indicating a predominantly freshwater depositional environment for the three sets of source rocks (Figure 11b). However, subtle salinity variations can significantly affect organic matter enrichment even within freshwater systems. Therefore, additional parameters are needed to identify the salinity range among the three source rock groups more precisely.

Among pentacyclic triterpenoids, the homohopane index (C₃₅/C₃₁₊₃₂) measures paleosalinity, with elevated values typically reflecting marine and saline lacustrine environments [55,56]. Gammacerane abundance also reflects stratified water columns caused by vertical salinity gradients, with high values often observed in hypersaline lacustrine and marine sediments. Elevated water salinity during deposition generally increases both gammacerane indices (Ga/C₃₀H; Ga/C₃₁H) and homohopane indices (C₃₅/C₃₁₊₃₂) [57,58]. In this study, the K₁bt₁ source rocks developed in a brackish environment. The K₁ba shows relatively higher and more variable salinity, suggesting a brackish to saltwater depositional setting. In comparison, the Jurassic source rocks exhibit intermediate salinity but also formed in brackish water (Figure 13b,c).

Comprehensive analysis shows that the K₁bt₁ and K₁ba formations share similar source rock characteristics, as both developed in comparable environments, continuous humid to semi-humid climates with shallow to semi-deep lacustrine deposits (Figure 2) [21], resulting in their overall similarity (Figure 14). The chemical index of alteration (CIA) is widely used to evaluate the degree of chemical weathering in sediment provenance areas [26,59]. CIA value is closely linked to weathering intensity and climate: higher CIA values indicate stronger chemical weathering and warmer, more humid climates. The Al₂O₃/Na₂O ratio has a similar indicative effect [60,61]. In the Baorao Trough, the CIA values and Al₂O₃/Na₂O ratios from well L29X show consistent trends across the sedimentary stages. Both indices are lowest during the Jurassic, increase during the K₁ba period, then decline before rising again through the K₁bt₁ period. This pattern reflects a cold, dry climate with weak weathering in the Early Jurassic, transitioning to a warm, humid climate with moderate weathering in the Early Cretaceous (K₁ba), followed by a slightly cooler, drier phase in early K₁bt₁, and finally a warm, humid climate in the middle to late K₁b_t1. This climatic evolution aligns with previous findings [21].

The K₁ba source rocks display greater heterogeneity, reflected in their wider TOC range (Figure 9), variable sterane and tricyclic terpene distributions (Figure 11), and broader gammacerane and homohopane indices (Figure 13). This variability may result from their widespread distribution, long sedimentary period, and extended transport distance of terrigenous organic matter. In contrast, Jurassic source rocks show markedly different characteristics. Lower Jurassic deposition occurred under semi-arid conditions with minimal environmental fluctuations, leading to relatively stable organic inputs and depositional settings (Figure 2). Dominated by terrestrial higher plants and deposited rapidly in weakly reducing, brackish to saline waters, these rocks are relatively thin and uniform with moderate quality and Types II₂~III kerogen, suggesting limited gas-generation potential (Figure 14).

Two primary factors govern the development of high-quality source rocks: the depositional environment, which controls organic matter preservation, and organic matter input, which influences paleoproductivity and thus source rock quality. Organic matter enrichment can be evaluated through a correlation analysis between TOC content and representative parameters. In this study, TOC content shows clear correlations with the gammacerane index and Pr/Ph ratio. Higher gammacerane values correspond to higher TOC (R² = 0.44), while lower Pr/Ph ratios are associated with increased TOC (R² = 0.22). These relationships indicate that saline water and reducing environments favor the development of high-quality source rocks (Figure 15). By contrast, organic matter indicators, such as S/H (reflecting algae, higher plants and bacteria) and C₂₉ sterane (reflecting terrestrial higher plants) show no correlation with TOC (Figure 16), suggesting that organic matter preservation is the dominant control on source rock quality in the study area, while the biological source type plays only a minor role.

Terrestrial lacustrine basins exhibit two primary source rock development models: the high-productivity model and the organic matter preservation model, or a combination of both. High primary productivity increases the organic carbon content of source rocks and promotes bottom-water anoxia by oxidizing excess organic matter, thereby enhancing preservation [62]. For example, the Bohai Bay Basin, a rift basin of China’s eastern coast, experienced tectonic subsidence and climatic changes during the syn-rift evolution of the Bozhong Sub-basin. These changes altered lake hydrology and water chemistry, influencing phytoplankton communities and primary productivity, and resulting in source rock assemblages with varying hydrocarbon potential and biomarker profiles [63].

In contrast, the preservation model emphasizes sustained anoxic bottom-water conditions as critical for forming organic-rich sediments [64]. For example, during deposition of the Third Member of the Shahejie Formation in the northern Dongpu Sag (southwestern Bohai Bay Basin), the environment was predominantly brackish to saline with deep, stratified water. This created an anaerobic–anoxic bottom-water setting, where there is limited input of terrestrial organic matter. High-quality source rocks formed primarily through preservation under saline, anoxic conditions [65].

The development model in the Jirgalangtu Sag aligns more closely with this preservation-dominated model. Due to the structural segmentation typical of rift basins, the sag comprises several troughs without clear barriers. During lake salinization, deeper sub-sags exhibited higher salinity and stronger anoxic conditions, promoting the formation of high-quality source rocks.

5.3. Hydrocarbon Generation and Expulsion Characteristics

Source rocks in the study area exhibit favorable hydrocarbon generation potential, with effectiveness depending on successful hydrocarbons expulsion. A key requirement is that generated hydrocarbons must first reach saturation within the rock matrix before migrating into carrier beds and reservoirs [66,67].

For source rocks with similar organic matter types and maturity, hydrocarbon generation potential positively correlates with TOC content [68]. If generation does not exceed rocks’ retention capacity, low TOC prevents effective expulsion. Expulsion occurs only when generation surpasses retention, enabling the rock to act as an effective hydrocarbon source. Thus, higher TOC enhances both generation and expulsion capacities. The minimum TOC content required for expulsion is known as the expulsion threshold.

The pyrolysis parameter S₁ and bitumen content represent residual hydrocarbons. In source rocks without hydrocarbon expulsion, they reflect the amount of hydrocarbons already generated [69]. Therefore, the S₁/(S₁ + S₂) and bitumen/TOC ratios indicate residual hydrocarbons per unit of organic matter. For source rocks with similar organic matter types and maturity, these ratios generally increase with TOC. In the study area, both ratios rise with TOC up to 0.8%, then decline, marking the onset of expulsion (Figure 17). Therefore, the expulsion threshold corresponds to a TOC content of 0.8%.

Regarding the thermal maturity threshold, the envelope curves of the soluble organic matter transformation ratio [S₁/(S₁ + S₂)] and hydrocarbon index (HCI = S₁/TOC, mg HC/g TOC) increase with burial depth, peak at around 1910 m, corresponding to the hydrocarbon expulsion threshold, and then decline. Some elevated K₁ba values may reflect the influence of migrated hydrocarbons (Figure 18).

Although the R_o range is narrow, the transformation ratio of soluble organic matter, bitumen/TOC, [S₁/(S₁ + S₂)], and HCI all show an initial increase followed by a gradual decline with depth. The R_o and R_Tmax values at the peak of their envelope curves are around 0.8%, corresponding to the hydrocarbon expulsion threshold (Figure 19).

In summary, compared to the mature-stage K₁bt₁ and K₁ba formations, the Jurassic source rocks are over-mature, with hydrocarbon generation and expulsion largely complete, as indicated by the extremely low transformation ratios of soluble organic matter. However, some gas-generating potential still remains.

6. Conclusions

The K₁bt₁, K₁ba, and Jurassic source rocks in the Baorao Trough of the Jirgalangtu Sag all meet or exceed the medium-quality standard for source rocks (TOC > 0.6%). K₁bt₁ and K₁ba source rocks contain abundant organic matter, relatively favorable kerogen types (Type II₁~II₂), and have reached the low to medium maturity, making them favorable oil-generating formations. In contrast, Jurassic source rocks are dominated by Type III kerogen, have reached high maturity, and are gas-prone.

K₁bt₁ and K₁ba source rocks share similar geochemical characteristics, indicating organic matter input from both aquatic algae and terrestrial plants, whereas Jurassic source rocks are mainly derived from terrestrial higher plants. The depositional environment of K₁bt₁ was weakly oxidizing to reducing and brackish, while K₁ba was deposited under weakly reducing, relatively high saline conditions. Meanwhile, Jurassic source rocks formed in weakly reducing, brackish to saline settings. Additionally, saline, reducing environments favor the development of high-quality source rocks overall.

The effective TOC threshold for source rocks in the study area is 0.8%. Hydrocarbon expulsion begins when TOC exceeds these values in K₁bt₁ and K₁ba The organic matter conversion rate peaks at around 1900 m, with a corresponding R_o value of approximately 0.8%, marking the thermal maturity threshold for expulsion. Analytical results show that K₁bt₁ and K₁ba have undergone partial hydrocarbon expulsion but remain within the oil-generating window, retaining strong oil-generating potential. By contrast, Jurassic source rocks experienced early thermal cracking of Type III kerogen, with generated oil largely expelled or lost during early geological processes, though some gas-generating potential remains today.