Petrological Characteristics and Hydrocarbon Generation of Carbonate Source Rocks of the Permian Taiyuan Formation in Central and Eastern Ordos Basin, China

Abstract

In order to evaluate the hydrocarbon generation potential and effectiveness of the carbonate source rock from the Lower Permian Taiyuan Formation of the Upper Paleozoic gas reservoirs in the central and eastern Ordos Basin, 87 core samples from the formation were analyzed through the comprehensive application of core observation, thin section analysis, lithofacies division, and organic geochemistry experiments. The results show that the carbonate source rocks of the Taiyuan Formation comprise four lithofacies types with type I–II kerogen: laminar argillaceous micritic limestone, massive micrite, massive bioclastic micritic limestone, and massive algae-clotted limestone. Among them, laminar argillaceous micritic limestone and massive micrite are favorable lithofacies for high-quality source rocks, with a TOC distribution range of 0.99% to 6.07% (average 2.56%) and 0.24% to 8.27% (average 1.77%), respectively. Hydrous gold tube pyrolysis showed that the samples of laminar argillaceous micritic limestone and massive micrite attained a peak yield of nearly 115.0 mL/g TOC (heating rate 2 °C/h) and 101.4 mL/g TOC (heating rate 2 °C/h), respectively, for C_1–5 compounds. Due to the higher maturity of the samples, the hydrocarbon gases were dominated by residual kerogen pyrolysis gases and lacked liquid hydrocarbon cracking gas. Furthermore, the carbonate source rocks had weak methane absorption ability, with a maximum adsorption capacity of only about 0.15 cm³/g. In addition, the hydrocarbon gas generation of carbonate source rocks of the Taiyuan Formation was far greater than 0.2 mL/g rock, which is the lower limit standard for effective gas source rock. Therefore, the carbonate source rocks of the Taiyuan Formation should be regarded as important gas source rocks in subsequent explorations of the central and eastern Ordos Basin.

1. Introduction

At present, oil and gas exploration in carbonate strata has become a research hot spot in China and abroad, and there have been many discoveries, most of which have been made by considering carbonate strata as reservoirs. Large carbonate reservoirs have been discovered in the Sichuan, Ordos, Tarim, and Bohai Bay Basins in China. It is estimated that the amount of petroleum resources in carbonate reservoirs is 340 × 10⁸ t, and the amount of natural gas is 24.3 × 10¹² m³ [1,2,3,4]. However, the study of carbonate source rocks has been quite difficult in the petroliferous basins of China, mainly due to the characteristics of deeply buried resources, old stratigraphic age (mainly Paleozoic), low organic matter content, and high degree of thermal evolution of carbonate rocks [5,6]. Conventional geochemical methods of evaluating low-mature lacustrine source rocks are limited due to these characteristics. However, some researchers directly ignore the contribution of petroleum source rocks that are highly mature and have low organic matter content to hydrocarbon generation and face the problem of mismatch between source rocks and reservoirs. For instance, in the past, resource evaluation in the Tarim Basin usually focused on mudstone with high organic matter content, and the calculated reserves were usually quite different from the actual reserves. Even in some large oil fields of the Tarim Basin, appropriate source rocks in the stage of hydrocarbon generation have not been found [7,8]. Therefore, it is of great strategic significance to consider carbonate rocks as source rocks to evaluate the hydrocarbon accumulation mechanism and exploration prospects of multi-cycle superimposed basins in China.

The Ordos Basin is the second-largest sedimentary basin in China. Research on the existing source rocks has mainly focused on the lacustrine black shale and mudstone in the seventh member of the Yanchang Formation, the Upper Paleozoic coal measure source rocks, and the Lower Paleozoic Ordovician Majiagou Formation. Several rounds of work have been conducted on the seventh member of the Yanchang Formation as the main source rock of oil and gas in the basin. The Upper Paleozoic coal measure source rock, as the main gas source of the large Paleozoic gas reservoir, has always been the focus of theories regarding Upper Paleozoic tight sandstone gas accumulation, such as the Sulige and Yulin gas fields [9,10,11,12]. With the in-depth study of the natural gas theory of the Ordovician karst paleogeomorphology, researchers have found that the natural gas reservoir of the Lower Paleozoic is far from the coal measure source rocks of the Upper Paleozoic, and there are thick and tight salt rocks in the middle. Moreover, the genetic type of natural gas is a mixture of coal-formed and oil-related gas [13,14]. Therefore, the geochemical characteristics and hydrocarbon generation potential of low-abundance marine carbonate source rocks have attracted the attention of researchers. At present, self-generating and self-accumulating gas reservoirs have been discovered in the Ordovician subsalt in the eastern part of the Ordos Basin, and the subsalt carbonate source rocks of the Ordovician Majiagou Formation have been systematically evaluated, and it is considered that they can be treated as effective gas source rocks [3,15,16].

The lower Permian Taiyuan Formation was formed in the transgressive system domain covering the Ordos, Qinshui, South North China, and Bohai Bay Basins. Influenced by the gentle paleotopography in North China, the Late Carboniferous to Early Permian in the Ordos Basin was dominated by land–sea interlacing, with a stable distribution of sedimentary thickness. In the process of frequent transgressions and regressions, large areas of exposed coastal areas were flooded into marshes by encroaching water, and the shallow water sedimentary environment in the stable structure controlled the widespread distribution of coal measure source rocks [17,18]. In the past 20 years of exploration, gas reservoirs of the Taiyuan Formation have been found in Yulin, Shenmu, and other large and medium-sized gas fields [19]. The Taiyuan Formation limestone is widely developed in the central and eastern parts of the Ordos Basin, and the Miaogou, Maoergou, Xiedao, and Dongdayao members developed from the bottom to the top (Figure 1). In the past 2 years, high-production gas reservoirs with great exploration potential have been found in the limestone of the Xiedao and Maoergou members [20]. Whether carbonate rocks other than coal measure source rocks in the Taiyuan Formation can be considered as effective gas source rocks has become an urgent problem to be solved. Due to the W-type burial history in Ordos Basin, the vitrinite reflectance (%Ro) of Carboniferous–Permian coal measure source rocks in most areas of the basin is above 1.2%, and the thermal evolution of organic matter in the Qingyang area in the south of the basin is the highest, with Ro above 2.0% [20]. Aiming at carbonate source rocks with high maturity in the Taiyuan Formation, in this research, the authors designed geochemical experiments based on the analysis of petrological characteristics and optimized evaluation indexes that are less affected by the degree of thermal evolution to describe the characteristics of organic facies of limestone source rocks. Then, the samples can be selected according to the differences in petrological characteristics to carry out a hydrocarbon generation dynamic simulation experiment and rock gas adsorption measurement. Combined with the organic geochemical data of Taiyuan Formation limestone source rocks, the characteristics and effectiveness of hydrocarbon generation and expulsion were determined. The research results can provide a basis for the deployment of limestone natural gas exploration in the Taiyuan Formation, as well as a reference for the study of carbonate source rocks in the Sichuan Basin, Tarim Basin, and other carbonate reservoir exploration hot spots.

2. Geological Setting

The Ordos Basin is an asymmetrical rectangular basin that extends in an almost entirely south–north direction. The basin consists of six first-order structural units: Yimeng uplift, Jinxi fault-fold belt, Yishan slope, Weibei uplift, Tianhuan depression, and western thrust belt. The study area is located in the eastern part of the Ordos Basin. The stratum is a west-dipping uniclinal structure with few faults (Figure 1). Within the whole early Permian, this area was composed of carbonate tidal-flat deposits in the south–central area and mixed carbonate–clastic tidal flat deposits in the northern part [21].

In the central and eastern Ordos Basin, the sedimentary sequence of limestone and coal seams in the Taiyuan Formation records multiple processes of seawater encroachment. The thickness and internal fabric of the limestone are variable due to the varying length and scale of transgression in different stages, and there is horizontal contrast in a wide range of the study area. According to the characteristic coal seam and mudstone, the Taiyuan Formation is subdivided into the Miaogou, Maoergou, Xiedao, and Dongdayao members from bottom to top. The Taiyuan Formation limestone is mainly distributed in the Xiedao and Maoergou members and less in the Dongdayao member (Figure 2). The total thickness of strata in the Taiyuan Formation ranges from 30 to 60 m, and the thickness of limestone ranges from 10 to 40 m. The limestone of the Taiyuan Formation is mainly developed in the middle to the southeast of the study area, and the thickness is the largest in the Hengshan area, reaching 35 to 40 m (Figure 1).

3. Samples and Methods

A total of 87 core samples of the Taiyuan Formation were collected from 20 wells (Figure 1). All core samples were measured for their total organic carbon (TOC) content and underwent Rock-Eval pyrolysis, 18 samples underwent X-ray diffraction (XRD) analysis, and kerogen components were identified in 29 samples. Eight samples were used for vitrinite reflectance determination, two for gold tube thermal simulation, and one for an isothermal adsorption experiment. The TOC determination, Rock-Eval pyrolysis, and XRD analysis were conducted at the Yangtze University; kerogen component identification, vitrinite reflectance determination, thermal simulation, and isothermal adsorption experiments were performed at the Guangzhou Institute of Geochemistry, Chinese Academy of Sciences.

All samples were made into powder before conducting TOC determination, Rock-Eval pyrolysis, and XRD analysis. After removing the inorganic carbon with dilute hydrochloric acid (HCl:H₂O = 1:7, V/V), the TOC content in the samples was analyzed using a LECO CS-230 analyzer. The Rock-Eval 6 instrument for pyrolysis analysis was set at 300 °C for 3 min and then heated evenly to 650 °C for 14 min. The XRD analysis was performed on a Bruker D2 PHASER diffractometer system with test conditions of Cu-Kα radiation, scanning speed of 2θ 2°/min, and 2θ scanning area of 5° to 80°.

The organic components were identified using kerogen concentrations under reflected light and fluorescent light (blue light) with a Nikon microscope by the Chinese standard (SY/T 5125-1996) [22]. The selected samples were first leached in HCl for 12 h to remove carbonate minerals and then treated with HF for another 12 h to remove silicate minerals. Distilled water was used to flush the samples several times during this process. These steps were repeated several times to completely remove the mineral constituents in the samples. Finally, the minerals and kerogen could be distinguished by centrifuge separation with heavy liquid (ZnCl₂ + H₂O) with a specific gravity of 2.0–2.1. Maceral abundance refers to the relative area percentage of each maceral. For vitrinite reflectance (%Ro) analysis, core samples were sliced and sintered with water and then polished and placed in a drying vessel for 12 h for reflectivity measurement. The average vitrinite reflectance under oil immersion was measured using a Leica MSP200 microphotometer under green light with magnification ranging from 32 to 125 times. At a wavelength of 546 nm, Ro was obtained according to the percentage of the reflected light intensity of the vitrinite polishing surface to the vertical incident light intensity.

The isothermal adsorption experiment was carried out on a high-pressure adsorption analyzer made in Germany. The samples were cleaned and pulverized to less than 100 mesh and vacuum-dried at 100 °C for 24 h. According to the formation temperature and pressure conditions and thermal evolution history of the Taiyuan Formation, the experimental temperature was set at 70 °C [23]. Methane (purity = 99.99%) was used as the experimental gas, and the experimental pressure range was 0 to 11.0 MPa. The equilibrium time at each pressure point was not less than 12 h.

According to the lithofacies division of the Taiyuan Formation, two samples belonging to the dominant organic facies were selected for gold tube thermal simulation. Before hydrous pyrolysis in sealed gold tubes was performed, the samples were crushed to powder. Kerogen was solvent extracted from the powder of the two samples, which was measured for TOC content. About 15–60 mg of prepared kerogen and deionized water for about 20% of the sample weight were weighed and sealed in the gold tube under argon gas protection according to different final temperatures. Then, the gold tube was put into the autoclave to complete the pyrolysis experiment. The autoclave pressure was 50 MPa with an error of less than ±2 MPa. Each sample was heated in the heating furnace from 200 °C to a final temperature of 20 and 2 °C/h, with an error of less than 0.2 °C, for calculation of kinetic parameters. Then, the autoclave was quenched immediately with cold water to room temperature and was opened to remove the gold tube. The cleaned gold tube was placed in a vacuum system with a fixed volume connected to an Agilent 6890 gas chromatography (GC) device. The gold tube was punctured in vacuum and analyzed online via gas chromatography. Eight columns and a detector were equipped for C₁–C₅ analysis through the GC with nitrogen gas as carrier gases. The initial temperature of GC oven was 50 °C holding for 5 min, then heated up to 180 °C at 4 °C/min and held for 15 min. After gas analysis, the volatile C_6–14 compounds were frozen using liquid nitrogen so that they would gather in a vial, and then 4 mL n-pentane was injected into the vial. Then, the gold tube was cut and put into the cell bottle to merge the liquid hydrocarbon. About 0.025 mg deuterated n-C₂₄ was added to the vial as an internal standard, and the vial was vibrated for 2 min by ultrasonic oscillation. Finally, 1 mL of solution was taken from the vial and used for GC analysis on an Agilent 7890A equipped with a PoraPLOT Q capillary column (30 m × 0.32 mm × 0.25 μm). The remaining solution for GC analysis was recycled to the original vial, and 4 mL dichloromethane solution was added into the vial after the n-pentane was volatilized. Then, the solution was filtered by vacuum filter after ultrasonic vibration for 2 min, and the filtrate containing C₁₄₊ compounds was dried and weighed, obtaining the C₁₄₊ yield.

4. Results and Discussion

4.1. Geochemical Characteristics of Source Rocks

Domestic and foreign scholars have concluded that the lower limit of TOC of carbonate source rocks varies from less than 0.1% to 0.5% based on geochemical data obtained from thermal simulation experiments of hydrocarbon generation and expulsion in different research areas. For instance, Gehman [24] analyzed more than 1400 samples taken from around the world and finally determined TOC = 0.3% as the lower limit for carbonate source rocks. Qin et al. [25] studied the relationship between hydrocarbon expulsion and TOC of carbonate source rocks through simulation experiments and determined that the lower limit of organic carbon of high-mature to over-mature carbonate source rocks was 0.1%–0.25%. Huo et al. [2] studied the relative contribution of low-abundance carbonate source rocks to oil and gas reservoirs in the Tazhong area, Tarim Basin, and set the lower limit of organic carbon of high-mature to over-mature carbonate source rocks at 0.15%–0.20%. Tenger [26] studied the Lower Paleozoic carbonate source rocks in the Ordos Basin by adding trace elements, rare earth elements, and carbon isotopes and determined that TOC = 0.2% was an important limit. Based on the literature, TOC = 0.2% was selected as the lower limit of the abundance of high-mature carbonate source rocks in this study.

The results of Rock-Eval analysis and TOC determination of 87 samples show that the abundance of organic matter in Taiyuan Formation limestone is significantly lower than that in mudstone, and there is a wide variation. The results of the samples from the Xiedao member indicate relatively high organic abundance, as TOC ranged from 0.06% to 8.27% (mean 1.20%), and the hydrocarbon potential index (S₁ + S₂) ranged from 0.04 to 2.54 mg/g (mean 0.51 mg/g). For the Maoergou member, TOC ranged from 0.10% to 3.82% (mean 0.65%), and the hydrocarbon potential index (S₁ + S₂) ranged from 0.04 to 1.13 mg/g (mean 0.27 mg/g). Samples from the Dongdayao member had relatively low organic abundance, TOC ranged from 0.06% to 1.36% (mean 0.42%), and the hydrocarbon potential index (S₁ + S₂) ranged from 0.03 to 0.92 mg/g (mean 0.16 mg/g) (

Table 1. The TOC contents, pyrolysis, and vitrinite reflectance for samples from the Taiyuan Formation in Ordos Basin.

Well	Depth (m)	Member	Sample Type	TOC (%)	S1	S2	S1 + S2	Tmax (°C)	Ro (%)
					(mg/g)
Q71	2849.1	Dongdayao	Core	0.18	0.01	0.02	0.03	479
Q71	2850.3	Dongdayao	Core	0.10	0.01	0.02	0.03	412
Q71	2851.3	Dongdayao	Core	0.09	0.01	0.02	0.03	413
Q71	2852.2	Dongdayao	Core	0.06	0.01	0.02	0.03	420
Q71	2853.3	Dongdayao	Core	0.07	0.01	0.03	0.04	428
Q71	2854.2	Dongdayao	Core	0.12	0.01	0.03	0.04	474
Q71	2854.9	Dongdayao	Core	0.12	0.01	0.03	0.04	359
Q71	2856.1	Xiedao	Core	0.12	0.01	0.03	0.04	475
Q71	2857.0	Xiedao	Core	6.07	0.33	1.81	2.14	445	1.20
Q71	2858.9	Xiedao	Core	2.56	0.07	2.35	2.42	487
Q71	2859.5	Xiedao	Core	1.95	0.24	2.07	2.31	448	1.44
Q71	2860.1	Xiedao	Core	0.13	0.03	0.06	0.09	467
Q71	2861.0	Xiedao	Core	0.33	0.02	0.11	0.13	521
Q71	2861.4	Xiedao	Core	0.50	0.03	0.25	0.28	519
Q71	2861.9	Xiedao	Core	0.88	0.03	0.35	0.38	515
Q71	2862.7	Xiedao	Core	1.15	0.04	0.46	0.50	524
Q71	2862.8	Xiedao	Core	0.31	0.02	0.17	0.19	505
Q71	2863.8	Xiedao	Core	0.65	0.03	0.22	0.25	438
M28	2165.9	Xiedao	Core	0.75	0.03	0.11	0.14	528
M28	2167.7	Xiedao	Core	0.25	0.02	0.12	0.14	313
Y16	2205.0	Xiedao	Core	0.15	0.01	0.05	0.06	470
Y16	2205.9	Xiedao	Core	1.63	0.10	0.77	0.87	317
Y16	2210.8	Xiedao	Core	3.02	0.14	1.26	1.40	361	1.30
Y16	2211.6	Xiedao	Core	4.26	0.19	1.90	2.09	315
Y18	2317.8	Maoergou	Core	0.31	0.02	0.14	0.16	309
Y18	2318.2	Maoergou	Core	0.25	0.01	0.05	0.06	371
Q2	2564.1	Xiedao	Core	2.29	0.05	0.40	0.45	528
Q2	2567.4	Xiedao	Core	0.08	0.01	0.04	0.05	364
Q2	2571.6	Xiedao	Core	0.09	0.01	0.04	0.05	407
Y36	3030.3	Xiedao	Core	0.40	0.02	0.18	0.20	300
Y36	3031.4	Xiedao	Core	0.59	0.02	0.16	0.18	316
Y36	3035.7	Xiedao	Core	0.11	0.01	0.06	0.07	449
M61	2306.6	Dongdayao	Core	0.09	0.01	0.04	0.05	472
M61	2306.8	Dongdayao	Core	1.35	0.03	0.15	0.18	575
S137	2478.5	Maoergou	Core	0.16	0.01	0.07	0.08	481
S137	2479.1	Maoergou	Core	0.13	0.01	0.05	0.06	436
T65	3247.9	Dongdayao	Core	0.37	0.03	0.15	0.18	505
T65	3250.3	Dongdayao	Core	1.25	0.08	0.84	0.92	463
Y93	2466.9	Dongdayao	Core	1.36	0.06	0.12	0.18	583
Y93	2467.4	Dongdayao	Core	0.31	0.01	0.05	0.06	336
Y72	2793.2	Xiedao	Core	0.19	0.01	0.05	0.06	569
Y72	2793.4	Xiedao	Core	1.18	0.02	0.18	0.20	574
Y72	2794.5	Xiedao	Core	2.03	0.02	0.39	0.41	571
Y72	2796.4	Xiedao	Core	1.11	0.02	0.28	0.30	565
J21	3403.5	Xiedao	Core	0.09	0.01	0.04	0.05	473
J21	3404.8	Xiedao	Core	2.88	0.01	0.05	0.06	383
J21	3408.0	Xiedao	Core	0.06	0.01	0.03	0.04	396
J21	3409.8	Xiedao	Core	0.08	0.01	0.03	0.04	481
J21	3410.5	Xiedao	Core	0.14	0.01	0.05	0.06	492
J21	3411.1	Xiedao	Core	1.60	0.05	0.53	0.58	508
J21	3412.6	Xiedao	Core	0.09	0.01	0.04	0.05	371
J21	3412.9	Xiedao	Core	0.10	0.01	0.04	0.05	352
M29	2234.9	Maoergou	Core	1.09	0.04	0.24	0.28	454
M29	2236.2	Maoergou	Core	0.53	0.03	0.23	0.26	414
M29	2236.3	Maoergou	Core	0.94	0.04	0.28	0.32	338
M29	2238.7	Maoergou	Core	0.32	0.02	0.12	0.14	494
M29	2239.7	Maoergou	Core	1.37	0.06	0.28	0.34	524
M29	2240.4	Maoergou	Core	0.18	0.01	0.07	0.08	302
S105	2725.3	Maoergou	Core	0.56	0.04	0.48	0.52	322	1.40
S105	2728.5	Maoergou	Core	0.36	0.02	0.12	0.14	422
S105	2728.6	Maoergou	Core	0.11	0.12	0.19	0.31	428
M154	2451.8	Maoergou	Core	3.82	0.12	0.63	0.75	361
M154	2452.8	Maoergou	Core	0.47	0.02	0.11	0.13	516
M154	2453.7	Maoergou	Core	0.36	0.02	0.11	0.13	497
M154	2454.4	Maoergou	Core	0.13	0.01	0.06	0.07	483
M154	2455.8	Maoergou	Core	0.46	0.02	0.15	0.17	491
M154	2457.7	Miaogou	Core	4.37	0.14	0.90	1.04	492	1.42
Sh217	2830.8	Maoergou	Core	0.60	0.04	0.24	0.28	526
Sh217	2831.4	Maoergou	Core	0.45	0.04	0.12	0.16	492
Sh217	2833.6	Maoergou	Core	0.10	0.01	0.03	0.04	469
Y4	2186.2	Dongdayao	Core	0.18	0.02	0.18	0.20	328	1.45
Y4	2187.5	Dongdayao	Core	0.72	0.02	0.31	0.33	498
Y4	2212.4	Maoergou	Core	0.96	0.02	0.20	0.22	512
Y11	1590.3	Maoergou	Core	0.24	0.02	0.08	0.10	502
Y11	1593.2	Maoergou	Core	1.10	0.08	0.76	0.84	513
Z4	2361.2	Xiedao	Core	0.47	0.05	0.20	0.25	552
Z4	2362.4	Xiedao	Core	8.27	0.24	2.30	2.54	564	1.60
Z4	2366.2	Xiedao	Core	0.06	0.01	0.05	0.06	440
Z4	2368.1	Maoergou	Core	0.22	0.01	0.07	0.08	524
Z4	2070.3	Maoergou	Core	0.16	0.01	0.06	0.07	471
Z4	2372.5	Maoergou	Core	1.28	0.05	0.42	0.47	563
M115	2062.9	Xiedao	Core	0.99	0.07	0.46	0.53	433
M115	2063.9	Xiedao	Core	1.30	0.09	1.06	1.15	439	1.38
M115	2066.4	Xiedao	Core	0.16	0.03	0.18	0.21	434
M115	2070.4	Maoergou	Core	0.54	0.05	0.37	0.42	444
M115	2073.4	Maoergou	Core	0.22	0.02	0.13	0.15	426
M115	2074.7	Maoergou	Core	2.12	0.13	1.00	1.13	429

). However, the limestone of the Miaogou member is barely developed, and only one source rock sample was obtained in this study, which is not representative. The heterogeneity of organic matter abundance distribution should be analyzed in combination with the petrological characteristics of the carbonate source rock samples (Figure 3).

The values of T_max in the Rock-Eval analysis were extremely high and extremely low (Table 1), which indicates that Tmax is not reliable as an indicator of maturity (Figure 3). Therefore, vitrinite reflectance analysis of some samples was added to this study. The results show that the reflectance of most vitrinite samples was higher than 1.2% (Table 1), which exceeds the oil-generation stage, indicating that the high degree of thermal evolution of organic matter is one of the reasons for the abnormal Tmax; another reason is that some samples have extremely low TOC (<0.5%) and S₂ (<0.6 mg/g).

Due to the high degree of thermal evolution of organic matter in the study area, it is impossible to use plots of the hydrogen index (S₂ × 100/TOC) vs. Tmax or hydrogen index vs. oxygen index (S₃ × 100/TOC) to determine the type of organic matter. Therefore, in this study, we mainly used a Nikon microscope to identify the components of kerogen so as to identify the type of organic matter by type index TI (amoprhinite + terrigenous liptinite × 0.5 − vitrinite × 0.75 − sameinertinite) in the Chinese standard (SY/T 5125-1996) [22]. A large amount of amoprhinite can be observed under microscopy in most samples, terrigenous liptinite and inertinite are rare, and a few samples are enriched in vitrinite (Figure 4). Meanwhile, the TI values also reflect that the kerogen type of most samples is I or II₁, while very few samples have type III kerogen (

Table 2. The kerogen components and types in the source rock in Taiyuan Formation. TL = terrigenous liptinite.

Well Name	Depth	Member	Amoprhinite	TL	Vitrinite	Inertinite	TI	Type
	(m)		(%)	(%)	(%)	(%)
Q71	2849.1	Dongdayao	91		9	<1	83.8	I
Q71	2858.9	Xiedao	45		55		4.5	II2
Q71	2861.4	Xiedao	51		49		13.9	II2
Q71	2861.9	Xiedao	45		55	<1	2.8	II2
Y36	3030.3	Xiedao	90		10	<1	81.7	I
Y36	3031.4	Xiedao	90		10		81.6	I
Y36	3035.7	Xiedao	89	<1	10	<1	80.9	I
J21	3410.5	Xiedao	95		5		90.9	I
J21	3411.1	Xiedao	68		31	<1	44.0	II1
T65	3247.9	Dongdayao	68		32	<1	43.0	II1
T65	3250.3	Dongdayao	91		9		83.4	I
Y16	2205.9	Xiedao	66		34		41.0	II1
Y16	2211.6	Xiedao	94		6	<1	88.7	I
M115	2062.9	Xiedao	95		5	<1	90.7	I
M115	2074.7	Maoergou	45		55	<1	2.8	II2
M154	2451.8	Maoergou	90		10		82.3	I
M154	2457.7	Miaogou	89		11	<1	80.6	I
Z4	2361.2	Xiedao	89		11	<1	81.2	I
Z4	2372.5	Maoergou	67		32	<1	42.8	II1
Y18	2317.8	Maoergou	95		5		90.4	I
Y72	2796.4	Xiedao	31		69		−20.4	Ⅲ
Y93	2466.9	Dongdayao	70		30		47.5	II1
M61	2306.8	Dongdayao	90		10	<1	82.2	I
M28	2465.9	Xiedao	90		10	<1	81.6	I
M29	2239.7	Xiedao	90		10	<1	81.5	I
S105	2725.3	Maoergou	97		3	<1	93.9	I
Y4	2212.4	Maoergou	89		10	<1	80.4	I
Y11	1593.2	Maoergou	44		56	<1	2.0	II2
Q2	2564.1	Xiedao	61		39		32.6	II2

). These results are also consistent with the sedimentary setting of sea-level fluctuation [21].

4.2. Lithofacies Characteristics of Source Rocks

The lithofacies descriptions include the mineral composition and microscopic distribution. Based on the typical sedimentary structures, the carbonate rocks of the Taiyuan Formation are divided into four lithofacies: laminar argillaceous micritic limestone, massive micrite, massive bioclastic micritic limestone, and massive algae-clotted limestone (Figure 5).

Laminar argillaceous micritic limestone often occurs in the Xiedao and Maoergou members. Samples of this lithofacies are usually dark gray or gray-black in color, with a high clay content, pelitic texture, and lamellar structure (Figure 5a). The pelitic texture mainly comprises reticular clay minerals, along with fine-grained debris with particle sizes less than 156 μm and kaolinite (Figure 5b). The lamination is generally continuous, with wavy laminae and rippled cross-laminae under microscopic observation. The dolomite and calcite in thin sections are mainly micrite and powder crystal and are dispersed. The organic matter in the samples is developed and distributed in microlaminae following the bedding orientation (Figure 5c).

Massive micrite is the overwhelmingly dominant lithofacies and is distributed in all four members of the Taiyuan Formation. Samples of this lithofacies are generally gray to dark gray in color, and large without apparent sedimentary structure under the microscope (Figure 5d–f). Thin sections display a large amount of micritic calcite, scattered silt-sized quartz, organic matter, and punctate iron (Figure 5e,f).

Massive bioclastic micritic limestone occurs in the Xiedao and Maoergou members. Samples of this lithofacies are generally gray to dark gray in color and rich in biological debris (Figure 5g). Quartz grains are silt size and present mostly as rounded–subrounded grains. The clay composition is mainly reticulated clay minerals. Biological detritus is composed of fuzulinid, brachiopods, and foraminifera and is often filled with powdery micritic calcite (Figure 5h,i). This lithofacies comprises mostly sedimentary products of a high-energy environment below the average low-tide surface and above the wave base surface.

Massive algae-clotted limestone occurs in the Dongdayao, Xiedao, and Maoergou members. Samples of this lithofacies are generally gray to light gray, very smooth, and low in clay mineral content (Figure 5j). Under the microscope, it can be observed that microbial algal clumps agglomerate and build irregular lattice trellises, which are often semi-filled with calcite, and residual trellises are common (Figure 5k,l). This kind of lithofacies develops in bioherm, where the sedimentary water body is deep and the energy is weak.

4.3. Hydrous Pyrolysis of Gold Tube

Since no solid bitumen has been detected in microscope work and there is not enough evidence in the pyrolysis results to safely conclude its presence, hydrous pyrolysis of the gold tube can be used to analyze hydrocarbon generation patterns in Taiyuan Formation samples. The two samples used for this experiment were from Wells Q71 and Y16 (Xiedao members; 2857.0 and 2210.8 m, respectively). The sample from Well Q71 belongs to laminar argillaceous micritic limestone lithofacies with kerogen TOC of 55.42% and vitrinite reflectance of 1.20%. The sample from Well Y16 belongs to massive micrite lithofacies with kerogen TOC of 33.50% and vitrinite reflectance of 1.30%. Based on gold tube pyrolysis results at two heating rates and using Lawrence Livermore National Lab’s Kinetics software, the kinetic parameters of total hydrocarbon gas (C_1–5) generation and liquid hydrocarbon (C_6–14), including frequency factor and activation energy, were calculated. The range of activation energy of C_6–14 compounds for the samples from Wells Q71 and Y16 was 39 to 52 kcal/mol with a frequency factor of 1.55 × 10¹¹ s⁻¹ and 43 to 57 kcal/mol with a frequency factor of 3.62 × 10¹² s⁻¹, respectively. The results show similar ranges for C_1–5 compounds from Wells Q71 and Y16: 47 to 61 kcal/mol with a frequency factor of 6.00 × 10¹¹ s⁻¹ and 46 to 62 kcal/mol with a frequency factor of 3.71 × 10¹⁰ s⁻¹, respectively; however, the distributions differed: the main peaks of activation energy of Well Q71 were 47 and 57 kcal/mol (22.11% and 20.84%) (Figure 6a), but the main peak of activation energy of Well Y16 was 52 kcal/mol (35.84%) (Figure 6b). Compared with the kinetic characteristics of C_1–5 generation of low-maturity source rocks from the Shahejie Formation of Bohai Bay Basin and the Shanxi Formation of Ordos Basin, the two samples in this study were found to have a narrow range of activation energy distribution, and part of the low values is missing [27,28]. This is mainly due to the high maturity of the Taiyuan Formation carbonate source rock, and the distribution of activation energy corresponding to the hydrocarbon gas generated in the main oil-generating period has been lost.

Non-hydrocarbon gas products mainly include CO₂, H₂, and H₂S, and the yield of CO₂ far exceeds that of H₂ and H₂S. With increased experimental temperature, the CO₂ yield increased rapidly (Table 3). The yield was higher at low heating rates than at high heating rates, and the gas was still generated at about 600 °C (Figure 7a,e). Due to the maturity of the two carbonate source rock samples, the yield of liquid hydrocarbon (C₆₊) was obviously low. The yield of C₆₊ of the two samples reached a peak at 480 °C at a heating rate of 20 °C/h and 432 °C at a rate of 2 °C/h, with peak yields of 10.51/7.19 and 11.30/7.06 mg/g TOC, respectively, and also showed a trend of first increasing and then decreasing with increasing temperature (Figure 7). The yield of C₁₄₊ compounds displayed a similar trend with C_6–14 compounds because the samples were past the main oil-generating period and lacked long-chain hydrocarbons. The CH₄ content was clearly dominant in C_1–5 compounds (Figure 7b,f). However, before the yield of C₆₊ began to decrease, the C_1–5 compounds gradually became dominant in the pyrolysis products, and their yield exponentially increased, indicating that methane was formed by kerogen cracking. The yield of total hydrocarbon gases of both samples exceeded 100 mL/g TOC under the 2 °C/h heating rate at the end of our pyrolysis experiment (Table 3).

Fractionation of the stable carbon isotope of gas compositions occurred with increasing temperature in our gold tube pyrolysis, and the carbon isotope results of methane, ethane, and propane are shown in Table 3 and Figure 8. Many previous studies have shown that, whether in thermal simulation experiments of low-maturity source rock or crude oil, δ¹³C₁ has an evolutionary trend of first decreasing and then increasing during the generation of gas at relatively low temperatures, and this is caused by the complex precursors in immature or low-mature kerogen [28,29,30]. However, the two samples in this study only showed an increasing trend of δ¹³C₁ at heating rates of 20 and 2 °C/h when less than about 450 °C, which coincides with the high maturity of the samples (Figure 8). The δ¹³C₂ values showed a small fluctuation at lower than about 400 °C and then increased, which was caused by the lack of complex branch chain cracking in highly mature kerogen. The δ¹³C₁ values fluctuated above about 550 °C, for which there are two possible causes: (1) in the over-mature stage, water reacts with the generated CH₄ to produce CO₂ and H₂ with light carbon isotope composition, and the CO₂ and H₂ react to produce hydrocarbon gas with light carbon isotope composition [31,32]; or (2) the removal of methyl groups or side chains of aromatic hydrocarbons is the cause of isotopic anomalies in hydrocarbon gas at the over-mature stage [33]. In general, the fractionation effect of the carbon isotope composition of alkane gas in the slow-heating series (2 °C/h) was obviously stronger than that in the fast-heating series (20 °C/h) (Figure 8).

Table 3. Hydrocarbon compositions and carbon isotopes of hydrocarbon gas from pyrolysis experiments for the carbonate source rock samples from the Taiyuan Formation. Ro values were calculated using the Easy Ro model [29,34].

T	HR	Ro	C1	C2	C3	CO2	C6–14	C14+	C6+	δ13C1	δ13C2	δ13C3
°C	°C/h	%	mL/g				mg/g			‰, PDB
Well Q71
336.4	20	0.57	0.06	0.002	0.001	2.72	0.72	1.25	1.97	−38.21	/	/
360	20	0.68	0.07	0.004	0.001	4.44	0.96	1.38	2.34	−37.69	/	/
384	20	0.79	0.15	0.013	0.001	5.08	1.24	1.58	2.82	−36.52	−23.42	/
408	20	0.96	0.40	0.036	0.002	6.98	1.56	1.86	3.42	−36.01	−23.42	−21.47
432	20	1.19	1.49	0.125	0.004	8.10	2.19	2.54	4.73	−35.91	−22.24	−21.28
456	20	1.47	6.01	0.357	0.009	9.16	3.38	2.88	6.26	−35.66	−20.67	/
480	20	1.81	18.24	0.608	0.012	13.66	4.35	2.84	7.19	−32.95	−17.32	/
504	20	2.19	35.38	0.467	0.007	19.59	4.10	1.91	6.01	−29.37	−9.91	/
528	20	2.62	51.98	0.357	0.006	29.39	3.18	0.93	4.11	−25.99	/	/
552	20	3.10	69.92	0.247	0.009	41.06	1.87	0.29	2.16	−24.67	/	/
576	20	3.53	84.79	0.168	0.005	48.16	1.04	0.09	1.13	−25.50	/	/
600	20	3.87	99.46	0.178	0.002	53.08	0.70	0.02	0.72	−26.48	/	/
336	2	0.73	0.09	0.005	0.001	4.91	1.07	1.42	2.49	−38.33	/	/
360.1	2	0.86	0.23	0.019	0.001	7.28	1.32	1.57	2.89	−36.78	−22.57	/
384	2	1.08	1.23	0.098	0.003	9.29	2.11	2.13	4.24	−37.00	−23.37	−19.18
408	2	1.36	5.28	0.301	0.006	10.87	3.16	2.79	5.95	−36.87	−20.16	−19.23
432	2	1.69	17.22	0.566	0.009	12.84	4.14	2.93	7.06	−33.70	−16.50	/
456	2	2.09	35.41	0.462	0.005	17.04	4.29	2.02	6.31	−29.99	−14.30	/
480.1	2	2.52	51.60	0.364	0.005	25.59	3.47	1.06	4.53	−26.24	/	/
503.8	2	2.99	68.30	0.268	0.006	37.16	2.15	0.38	2.52	−25.53	/	/
528.2	2	3.46	84.27	0.164	0.004	50.32	0.97	0.10	1.07	−25.49	/	/
552.2	2	3.89	99.31	0.134	0.001	55.91	0.47	0.08	0.55	−28.09	/	/
576.3	2	4.21	110.81	0.123	0.001	58.83	0.31	0.07	0.38	−26.72	/	/
600	2	4.45	114.91	0.133	0.001	61.49	0.16	/	0.16	−24.63	/	/
Well Y16
336.4	20	0.57	0.45	0.023	0.002	7.93	1.78	0.73	2.52	−40.64	−24.08	/
360	20	0.68	0.54	0.038	0.006	10.89	2.26	1.18	3.44	−39.55	−24.45	−23.73
384	20	0.79	0.82	0.081	0.011	10.31	2.99	1.62	4.62	−38.35	−25.90	−24.66
408	20	0.96	1.53	0.227	0.033	15.14	4.01	2.01	6.03	−38.10	−25.70	−24.75
432	20	1.19	4.38	0.687	0.085	14.32	5.80	2.46	8.25	−37.25	−25.10	−23.90
456	20	1.47	10.64	1.723	0.173	19.79	7.53	2.91	10.45	−37.12	−24.77	−22.72
480	20	1.81	24.08	2.553	0.175	19.16	7.64	2.87	10.51	−36.06	−22.15	−18.29
504	20	2.19	42.01	1.761	0.080	30.08	7.80	2.64	10.44	−33.32	−17.23	/
528	20	2.62	52.28	0.542	0.017	40.77	6.77	1.52	8.29	−29.81	−5.36	/
552	20	3.10	56.91	0.161	0.008	67.00	4.41	1.00	5.41	−26.34	/	/
576	20	3.53	60.54	0.116	0.004	99.66	3.08	0.91	3.99	−23.81	/	/
600	20	3.87	64.23	0.064	0.003	103.52	1.40	0.32	1.73	−29.28	/	/
336	2	0.73	0.64	0.038	0.005	12.16	2.24	1.24	3.48	−39.27	−25.09	−24.37
360.1	2	0.86	1.19	0.144	0.019	13.40	3.48	1.97	5.45	−39.17	−26.45	−24.32
384	2	1.08	3.76	0.538	0.048	20.02	5.23	2.39	7.61	−37.97	−27.01	−24.37
408	2	1.36	10.92	1.536	0.133	22.19	6.90	2.76	9.65	−37.77	−25.40	−22.59
432	2	1.69	23.97	2.481	0.151	23.76	8.33	2.96	11.30	−36.44	−22.01	−18.71
456	2	2.09	42.75	2.102	0.058	32.20	8.30	2.24	10.54	−32.86	−13.17	/
480.1	2	2.52	63.32	0.825	0.023	37.99	7.25	1.11	8.36	−30.18	−8.02	/
503.8	2	2.99	77.63	0.352	0.013	55.14	4.86	0.53	5.39	−28.46	/	/
528.2	2	3.46	83.62	0.160	0.007	77.98	2.96	0.34	3.30	−27.39	/	/
552.2	2	3.89	93.88	0.089	0.002	99.05	1.54	0.33	1.87	−31.47	/	/
576.3	2	4.21	99.59	0.083	/	111.12	1.00	/	1.00	−33.31	/	/
600	2	4.45	101.32	0.077	/	111.07	0.67	/	0.67	−31.20	/	/

Note: mL/g and mg/g represent the component yield per gram of TOC; / represents an inability to detect due to very low or no content. T = temperature; HR = heating rate.

Table 3. Hydrocarbon compositions and carbon isotopes of hydrocarbon gas from pyrolysis experiments for the carbonate source rock samples from the Taiyuan Formation. Ro values were calculated using the Easy Ro model [29,34].

T	HR	Ro	C1	C2	C3	CO2	C6–14	C14+	C6+	δ13C1	δ13C2	δ13C3
°C	°C/h	%	mL/g				mg/g			‰, PDB
Well Q71
336.4	20	0.57	0.06	0.002	0.001	2.72	0.72	1.25	1.97	−38.21	/	/
360	20	0.68	0.07	0.004	0.001	4.44	0.96	1.38	2.34	−37.69	/	/
384	20	0.79	0.15	0.013	0.001	5.08	1.24	1.58	2.82	−36.52	−23.42	/
408	20	0.96	0.40	0.036	0.002	6.98	1.56	1.86	3.42	−36.01	−23.42	−21.47
432	20	1.19	1.49	0.125	0.004	8.10	2.19	2.54	4.73	−35.91	−22.24	−21.28
456	20	1.47	6.01	0.357	0.009	9.16	3.38	2.88	6.26	−35.66	−20.67	/
480	20	1.81	18.24	0.608	0.012	13.66	4.35	2.84	7.19	−32.95	−17.32	/
504	20	2.19	35.38	0.467	0.007	19.59	4.10	1.91	6.01	−29.37	−9.91	/
528	20	2.62	51.98	0.357	0.006	29.39	3.18	0.93	4.11	−25.99	/	/
552	20	3.10	69.92	0.247	0.009	41.06	1.87	0.29	2.16	−24.67	/	/
576	20	3.53	84.79	0.168	0.005	48.16	1.04	0.09	1.13	−25.50	/	/
600	20	3.87	99.46	0.178	0.002	53.08	0.70	0.02	0.72	−26.48	/	/
336	2	0.73	0.09	0.005	0.001	4.91	1.07	1.42	2.49	−38.33	/	/
360.1	2	0.86	0.23	0.019	0.001	7.28	1.32	1.57	2.89	−36.78	−22.57	/
384	2	1.08	1.23	0.098	0.003	9.29	2.11	2.13	4.24	−37.00	−23.37	−19.18
408	2	1.36	5.28	0.301	0.006	10.87	3.16	2.79	5.95	−36.87	−20.16	−19.23
432	2	1.69	17.22	0.566	0.009	12.84	4.14	2.93	7.06	−33.70	−16.50	/
456	2	2.09	35.41	0.462	0.005	17.04	4.29	2.02	6.31	−29.99	−14.30	/
480.1	2	2.52	51.60	0.364	0.005	25.59	3.47	1.06	4.53	−26.24	/	/
503.8	2	2.99	68.30	0.268	0.006	37.16	2.15	0.38	2.52	−25.53	/	/
528.2	2	3.46	84.27	0.164	0.004	50.32	0.97	0.10	1.07	−25.49	/	/
552.2	2	3.89	99.31	0.134	0.001	55.91	0.47	0.08	0.55	−28.09	/	/
576.3	2	4.21	110.81	0.123	0.001	58.83	0.31	0.07	0.38	−26.72	/	/
600	2	4.45	114.91	0.133	0.001	61.49	0.16	/	0.16	−24.63	/	/
Well Y16
336.4	20	0.57	0.45	0.023	0.002	7.93	1.78	0.73	2.52	−40.64	−24.08	/
360	20	0.68	0.54	0.038	0.006	10.89	2.26	1.18	3.44	−39.55	−24.45	−23.73
384	20	0.79	0.82	0.081	0.011	10.31	2.99	1.62	4.62	−38.35	−25.90	−24.66
408	20	0.96	1.53	0.227	0.033	15.14	4.01	2.01	6.03	−38.10	−25.70	−24.75
432	20	1.19	4.38	0.687	0.085	14.32	5.80	2.46	8.25	−37.25	−25.10	−23.90
456	20	1.47	10.64	1.723	0.173	19.79	7.53	2.91	10.45	−37.12	−24.77	−22.72
480	20	1.81	24.08	2.553	0.175	19.16	7.64	2.87	10.51	−36.06	−22.15	−18.29
504	20	2.19	42.01	1.761	0.080	30.08	7.80	2.64	10.44	−33.32	−17.23	/
528	20	2.62	52.28	0.542	0.017	40.77	6.77	1.52	8.29	−29.81	−5.36	/
552	20	3.10	56.91	0.161	0.008	67.00	4.41	1.00	5.41	−26.34	/	/
576	20	3.53	60.54	0.116	0.004	99.66	3.08	0.91	3.99	−23.81	/	/
600	20	3.87	64.23	0.064	0.003	103.52	1.40	0.32	1.73	−29.28	/	/
336	2	0.73	0.64	0.038	0.005	12.16	2.24	1.24	3.48	−39.27	−25.09	−24.37
360.1	2	0.86	1.19	0.144	0.019	13.40	3.48	1.97	5.45	−39.17	−26.45	−24.32
384	2	1.08	3.76	0.538	0.048	20.02	5.23	2.39	7.61	−37.97	−27.01	−24.37
408	2	1.36	10.92	1.536	0.133	22.19	6.90	2.76	9.65	−37.77	−25.40	−22.59
432	2	1.69	23.97	2.481	0.151	23.76	8.33	2.96	11.30	−36.44	−22.01	−18.71
456	2	2.09	42.75	2.102	0.058	32.20	8.30	2.24	10.54	−32.86	−13.17	/
480.1	2	2.52	63.32	0.825	0.023	37.99	7.25	1.11	8.36	−30.18	−8.02	/
503.8	2	2.99	77.63	0.352	0.013	55.14	4.86	0.53	5.39	−28.46	/	/
528.2	2	3.46	83.62	0.160	0.007	77.98	2.96	0.34	3.30	−27.39	/	/
552.2	2	3.89	93.88	0.089	0.002	99.05	1.54	0.33	1.87	−31.47	/	/
576.3	2	4.21	99.59	0.083	/	111.12	1.00	/	1.00	−33.31	/	/
600	2	4.45	101.32	0.077	/	111.07	0.67	/	0.67	−31.20	/	/

Note: mL/g and mg/g represent the component yield per gram of TOC; / represents an inability to detect due to very low or no content. T = temperature; HR = heating rate.

4.4. Lithofacies Types and Organic Matter Enrichment

The abundance of organic matter in the samples is closely related to the type of lithofacies. Re-evaluation of the relationship between TOC and S₁+S₂ based on lithofacies classification shows that the laminar argillaceous micritic limestone had the highest abundance of organic matter and clay minerals, with a TOC distribution range of 0.99% to 6.07% (average 2.56%), which belongs to high-quality source rocks (Figure 9a). The TOC of massive micrite ranges from 0.24% to 8.27% (average 1.77%), second only to laminar argillaceous micritic limestone. Algal-clotted limestone has the lowest abundance of organic matter and clay minerals, with TOC ranging from 0.06% to 0.47% (average 0.14%), and most of the samples did not belong to valid source rocks (Figure 9b). These kinds of lithofacies features have often been mentioned in previous studies. For example, laminar argillaceous micritic limestone in the South Oman Salt Basin contributes 34.0% of the oil and gas in the carbonate subsystem of the Birba and Harweel platforms [35]; laminar micrite deposited in the shallow subtidal zone of the lower Smackover Formation along the Gulf Coast of the United States has become the most important source rock of the granular carbonate reservoir in the upper Smackover Formation [36]; and the source rocks of Zechstein in the Southern Permian Basin of Europe are mainly laminar micrite and calcarine mudstone, which developed in the toe-of-slope apron, slope, and marginal platform facies [37].

Oehler [38] concluded that carbonate source rocks generally have four main characteristics: (1) high salinity and low dissolved oxygen conditions; (2) located in a hydrostatic bottom environment caused by terrain obstacles or stratification of the overlying water mass; (3) high abundance of algae and bacteria as bio-precursors; and (4) relatively low abundance of terrestrial clasts and organic matter. Subsequently, Cordell [39] determined the main lithological characteristics of argillaceous limestone and micrites as source rocks: (1) generally dark gray or black; (2) fine-grained or microcrystalline, bedded or micro-bedded, with TOC that increases with decreasing particle size; (3) the presence of numerous algal and bacterial components; and (4) varying amounts of clays or other impurities. Laminar argillaceous micritic limestone and massive micrite, the favorable facies for the development of carbonate source rocks in this study, are characterized by dark color, fine grain, type I–II kerogen, and certain clay mineral content (Figure 4 and Figure 5). Fine grain and microcrystallinity indicate a sedimentary environment and diagenesis favorable to the preservation of organic matter, while laminarity often reflects the absence of bioturbation and an anoxic environment. Moreover, carbonate rocks usually lack terrigenous organic matter and are dominated by algae and bacteria due to the special environment in which they are formed. The relatively high content of clay minerals in these two types of lithofacies can absorb organic matter, leading to these impure units containing higher amounts of organic matter than purer carbonates [40]. Due to the strong hydrodynamics of the forming environment, the organic matter is not easily preserved in the lithofacies of massive algae-clotted limestone [20]. Under the microscope, microbial lattices formed by the growth of algae can be observed, and most of the biological cavities and algal lattices are cemented by bright crystal calcite (Figure 5j–l). Thus, massive algae-clotted limestone facies are not favorable to the development of high-quality carbonate source rocks. This method of classifying carbonate source rock lithofacies was used to analyze the key wells in the study area. Taking Well Q71 as an example, although there are thick carbonate rocks in the Dongdayao member, these carbonate rocks belong to massive algae-clotted limestone facies with low organic matter content, which means they cannot be considered valid source rocks. However, the limestone in the Xiedao member is mainly micrite, and the organic matter content is significantly higher, even reaching the standard of high-quality source rock (Figure 10). In summary, the distribution of carbonate source rocks is highly heterogeneous in the vertical plane, and an analysis method combining lithofacies types and geochemical parameters could provide reference data for the prediction of carbonate source rocks in this study area.

4.5. Hydrocarbon Generation Potential of the Taiyuan Formation

Hydrous pyrolysis analysis of the two samples representing laminar argillaceous micritic limestone and massive micrite facies can reveal the hydrocarbon generation of the carbonate source rocks. As shown in Figure 7, the relationship between hydrocarbon yield and temperature was determined by gold tube hydrous pyrolysis. In order to further understand the hydrocarbon generation process of the carbonate source rocks in the Taiyuan Formation, it is very important to establish the relationship between organic matter maturity and hydrocarbon yield. Due to the limitations of the experimental conditions, the Ro values were calculated using the Easy Ro model, and the changes in hydrocarbon yield with increasing maturity are shown in Figure 11. In many previous conventional pyrolysis experiments, low-maturity samples attained peak values of oil fraction (C₆₊) yield at about 0.9% for Ro, and compounds C_6–14 and C₁₄₊ had different peak values [28,41]. However, the cumulative yield of C_6–14 compounds displayed the same trend as C₆₊ and C₁₄₊ compounds in the two samples in this study (Figure 7 and Figure 11), and the C_2–5 yield was negligible relative to C₁ yield (Figure 7b,f). This occurred because the maturity of the two samples is high and the process of generating liquid hydrocarbons is basically over. In the process of generating hydrocarbons from organic matter, the liquid hydrocarbon may preferentially crack to heavy hydrocarbon gas (C_2–5); then, C_2–5 compounds are degraded to methane with increased maturity [42,43]. That is to say, the samples in the study area have lost the important source of gas from liquid hydrocarbon cracking and rely only on the thermal degradation of high-mature residual kerogen to produce gas. Therefore, as shown in Figure 11, the C_1–5 yield showed a continuous and stable increasing trend and was not affected by the rise and fall of the C₆₊ yield. The sample from Well Q71, representing laminar argillaceous micritic limestone facies, displayed obviously higher gas-generation capacity than the sample from Well Y16, representing massive micrite facies. Due to the stability of methane and weak liquid hydrocarbon generation of the residual kerogen, the yield of C_1–5 compounds in the two samples showed very low values of less than 10 mL/g TOC when Ro was less than about 1.4%. However, when Ro was higher than 2.5%, the yield of C_1–5 compounds from the Well Y16 sample showed an obvious slow growth trend, with a maximum of 101.4 mL/g TOC (heating rate 2 °C/h), while that of the Well Q71 sample still showed a continuously increasing trend, with a maximum of more than 115.0 mL/g TOC (heating rate of 2 °C/h) (Figure 11).

In order to verify the hydrocarbon generation potential of carbonate source rocks in the Taiyuan Formation against the existing maturity background, we used the low-mature shale of the Shanxi Formation, which is also in the Lower Permian, for comparison. The peak yield of liquid hydrocarbon in the shale of Shanxi Formation, an important gas source rock formation of the Upper Paleozoic natural gas in the Ordos Basin, was less than 60 mg/g TOC during the experiment (Figure 11), which may be caused by the type II₂ kerogen of the sample [44]. When Ro is greater than 3.5%, the C_1–5 yield of the Shanxi Formation shale can reach up to 120.35 mL/g TOC, which is roughly consistent with the peak C_1–5 yield of the Taiyuan Formation samples in this study. However, when Ro is less than 2.5%, the C_1–5 yield curve of the Shanxi Formation shale is obviously higher than that of the Taiyuan Formation carbonate source rocks, mainly due to the cracking of liquid hydrocarbon and heavy hydrocarbon gas generated by the low-mature Shanxi Formation shale. In summary, the hydrocarbon gas generation potential of highly mature samples with type I-II₁ kerogen in the Taiyuan Formation is not negligible.

Moreover, as the effective gas source rock, the premise is to overcome the adsorption effect of rock on gas. In this study, a sample from Well Q71, representing lithofacies with high clay mineral content, was selected, and a CH₄ adsorption experiment was carried out. The results demonstrate that the sample of carbonate source rock gradually reached the equilibrium state of adsorption after the pressure was greater than 4 MPa; compared with the shale of the Upper Triassic Yanchang Formation in Ordos Basin [45], the adsorption capacity of the sample from the Taiyuan Formation is significantly lower, with a maximum of only about 0.15 cm³/g (Figure 12). Based on the most samples representing laminar argillaceous micritic limestone lithofacies and massive micrite meeting the standard organic matter content of effective source rocks, when the TOC of the samples of these two types of lithofacies reached 0.6%, the maximum C_1–5 yield exceeded 0.6 mL/g rock, which is far higher than the evaluation standard of effective source rocks (C_1–5 > 0.2 mL/g rock) [28]. In conclusion, based on the lithofacies types, the hydrocarbon-generation capacity of residual kerogen, and the methane-adsorption capacity, we can conclude that the carbonate source rocks of the Taiyuan Formation are able to supply hydrocarbons to the medium–large natural gas reservoirs in the study area. The possibility of hydrocarbon being contributed by limestone of the Taiyuan Formation must be considered in the subsequent exploration of natural gas reservoirs.

5. Conclusions

(1)

Based on the core characteristics, mineral components, and primary sedimentary structure, the carbonate rocks of the Taiyuan Formation in the central and eastern Ordos Basin can be divided into four lithofacies types: laminar argillaceous micritic limestone, massive micrite, massive bioclastic micritic limestone, and massive algae-clotted limestone.

(2)

Samples of these four lithofacies types displayed characteristics of higher maturity and kerogen composition dominated by amoprhinite. Laminar argillaceous micritic limestone with stripes of enriched organic matter and massive micrite with dispersed organic matter are favorable lithofacies for source rock development. The organic matter content of massive algae-clotted limestone lithofacies is the lowest, and most of the samples do not meet the standard of effective carbonate source rocks.

(3)

Based on our gold tube pyrolysis, the carbonate source rocks of the Taiyuan Formation displayed low liquid hydrocarbon (C₆₊) yield, and maturity corresponding to peak C₆₊ yield lagged significantly, which was caused by the high maturity of the samples. However, the peak C_1–5 yield reached 101.4 mL/g TOC (heating rate of 2 °C/h) for the massive micrite lithofacies sample and reached 115.0 mL/g TOC (heating rate of 2 °C/h) for the laminar argillaceous micritic limestone lithofacies sample. The carbonate source rocks of the Taiyuan Formation have significant hydrocarbon gas-generation ability from residual kerogen without the support of liquid hydrocarbon cracking gas. Consequently, the limestone of the Taiyuan Formation in the central and eastern Ordos Basin could form economic natural gas fields if gas reservoir formation and preservation conditions permit an effective gas source rock of 0.2 mL/g rock and low methane-adsorption capacity.