Geochemical Characteristics and Evaluation Criteria of Overmature Source Rock of the Laiyang Formation in Well LK-1, Riqingwei Basin, Eastern China

Abstract

Total organic carbon (TOC) and hydrocarbon generation potential (Pg) are essential parameters for the qualitative evaluation of source rock and the basis for evaluating hydrocarbon resources in petroliferous basins. However, there will be some deviations in evaluating hydrocarbon resources of overmature source rock by using TOC and Pg. The super-thick overmature source rock of the Early Cretaceous Laiyang Formation was found in well LK-1, the Riqingwei Basin. To accurately understand the oil and gas potential of the Riqingwei Basin, this paper conducted a systematic organic geochemical analysis of the overmature source rock of the Early Cretaceous Laiyang Formation found in well LK-1. Combined with the results of previous thermal simulation tests on hydrocarbon generation of low-maturity samples in the Jiaolai Basin, the original total organic carbon (TOC⁰) content of source rock in well LK-1 was recovered and the evaluation criteria of overmature source rock was established. Results: (1) The average TOC content of well LK-1 source-rock samples is 1.25 wt.%, and the average Pg content is 0.11 mg/g. The type of organic matter is mainly type II₂, including a small amount of type II₁ and type III. The average reflectance of vitrinite (R_o) is 4.35%, which belongs to overmature source rock of a poor–fair level and mixed kerogen. (2) After recovery calculation, these samples’ original hydrocarbon generation potential (Pg⁰) contents ranged from 0.63 to 108.1 mg/g, with an average value of 6.76 mg/g. Furthermore, the TOC⁰ contents of the analyzed source-rock samples ranged from 0.62 to 30.6 wt.%, with an average value of 2.01 wt.%. It belongs to fair–good source rock, showing better hydrocarbon generation potential. (3) According to the relationship between the Pg⁰, Pg and TOC content, the evaluation standard for overmature source rock in well LK-1 was established. Under the evaluation standard of overmature source rock, a source rock with a TOC content exceeding 0.6% and a Pg content greater than 0.1 mg/g can be identified as a good source rock. This paper provides a foundation for the fine classification and evaluation of the overmature source rock of the Riqingwei Basin.

1. Introduction

The Riqingwei Basin is a late Mesozoic marine sedimentary rift basin discovered and named by Zhou et al. (2015) [1]. Regarding tectonic division, it belongs to the east Asian continental margin, the junction of the North China Craton and the Yangtze Craton—the eastern section of the Sulu orogenic belt [2,3]. During the late Jurassic and early Cretaceous, the paleo-Pacific plate was subducting to the East Asian continent [4]. The lithosphere of the East Asian continent was passively subducted, stretched and drifted under the drag of the subducting slab [5]. Different degrees of thinning have been observed in the eastern lithosphere of the North China Craton in the setting of strong extension [6], and a series of extensional rift basins (such as the Bohai Bay Basin, Jiaolai Basin, etc.) formed along the main body of the weak lithosphere zone [7]. The origin of the Riqingwei Basin is similar to that of the Jiaolai Basin. Still, the difference is that the regional rifting in the Riqingwei Basin was more intense and formed a narrow rift valley, which evolved into a marine rifted basin and developed a set of early Cretaceous deep water sedimentary system [1,8]. The Lingshan Island Scientific Drilling Project (LK-1) encountered this set of deep-water deposits of the Early Cretaceous Laiyang Formation and found that it has an enormous source-rock thickness (>500 m). Preliminary exploration results in the basin show some prospects for shale gas resources [9]. However, affected by regional thermal uplift in the late Early Cretaceous and ancient deep burial, the source rock of well LK-1 are all in the overmature stage [10]. At present, the degree of oil and gas exploration in the basin is generally low. Thus, there is an urgent need for research into the potential of oil and gas exploration.

Source rock is the basis for hydrocarbon generation, transport, and reservoir formation. Previous exploration experience in China’s petroleum-bearing basins shows that high-quality mature source rock with high organic-matter abundance and good types play a crucial role in reservoir control [11,12]. Therefore, evaluating source rock is essential in clarifying the formation and distribution of oil and gas reservoirs in the exploration area and proving the oil and gas potential. The qualitative evaluation of source rock is the premise of the quantitative evaluation of source rock and the evaluation of hydrocarbon resources. Currently, evaluating the abundance of organic matter in source rock mainly uses TOC and Pg indicators at domestic and overseas [13,14]. TOC and Pg can accurately reflect immature source rock’s original hydrocarbon generation potential. However, as the degree of thermal evolution increases, the absolute amount of organic matter in the source rock decreases as hydrocarbons are produced and expelled, leading to a gradual decline in TOC [15,16]. Consequently, for source-rock samples in oil-bearing basins that have reached a high-overmature stage and have undergone a large amount of hydrocarbon generation and expulsion, if the TOC content is used to evaluate and judge the oil and gas prospects of an area, it will inevitably cause significant errors [17]. The changes in the hydrocarbon generation potential of source rock, the number of residual hydrocarbons, and the simulation experiment results of hydrocarbon generation and expulsion all indicate the existence of low-abundance effective source rock in the deep part of oil and gas-bearing basins. This view has also been supported by numerous examples of domestic and international oilfield exploration [18,19,20,21]. Therefore, recovering the TOC content in the overmature hydrocarbon source rock of the well LK-1 and re-evaluating it is crucial for accurately assessing the hydrocarbon exploration prospects in the Riqingwei Basin.

The main objectives of this research were to: (1) evaluate the organic-matter abundance, type and maturity of well LK-1 overmature source rock based on the geochemical data from well LK-1 in the Riqingwei Basin; (2) based on previous research findings from thermal simulation experiments of hydrocarbon generation, the relationship between the Pg⁰, TOC and Pg was used to restore the TOC⁰ and Pg⁰ content of well LK-1 overmature source rock; and (3) establish evaluation criteria of high-overmature source rock based on TOC and Pg to provide a basis for the fine classification evaluation of source rock and re-understanding of oil and gas exploration potential in the Riqingwei Basin.

2. Geological Setting

The Riqingwei Basin is located in the coastal area of eastern Shandong, and belongs to the Sulu orogenic belt (Figure 1a,b). It is bounded by the Wulian-Yantai fault in the north, the Jiaolai Basin in the west, the Jiaonan uplift in the west, the Haiyang depression in the east, and the Qianliyan uplift in the south (Figure 1b). Under the background of the Late Jurassic-Early Cretaceous extension, the Riqingwei Basin underwent the initial passive rift basin formation stage during the Laiyang period. By 120 Ma, as a result of crustal uplift and large-scale magmatic activity, the passive rift of the Laiyang period died out prematurely, changing into a volcanic arc basin with active rift properties by the Qingshan period; tectonic inversion from the late Early Cretaceous to the early Late Cretaceous resulted in large scale uplift and denudation in the area. In Late Cretaceous, the stable sedimentary stage was entered, and the Wangshi Formation strata were deposited. At the end of the Paleocene, regional compression was strengthened, and the rift basin stage of development ended, forming the present tectonic pattern [1,22].

Well Lingke 1 (LK-1) (120°09′16.31″ E, 35°45′41.36″ N) is located on Lingshan Island in the Riqingwei Basin (Figure 1c), with a drilling depth of 1352 m and all of the cores have been collected. The stratum encountered is the Laiyang Formation, and the age of the magnetic strata is between 146.5 Ma and 125.8 Ma. The sedimentary age of this set of strata is mainly Early Cretaceous, and the bottom strata may be Late Jurassic [23]. There are many deep-water deposits in the stratigraphic records. Fan delta-subaqueous fan-turbidite fan-delta deposits are developed from bottom to top, forming a complete sequence of rift basin sedimentary infill [8] (Figure 1d). The lithologic combination of the Laiyang Formation is mainly interbedded with mud shales, siltstones and fine sandstones, with a small amount of coarse-grained sediments and several sets of igneous rock interbeds. Among them, the cumulative thickness of dark mudstone is 520 m, and the cumulative thickness of dark argillaceous siltstone and siltstone is 450 m. Overall, the Laiyang Formation in well LK-1 is a potential high-quality source-rock development horizon with dark fine-grained deposits [10] (Figure 1d).

Figure 1. Tectonic map of the Riqingwei Basin and lithological section of the well LK-1: (a) simplified map of China showing major blocks and its boundaries (modified from [24]); (b) geological map of the Riqingwei basin (modified from [25]); (c) geological map of the Lingshan Island, location of well LK–1(modified from [22]); and (d) composite stratigraphic column and sampling locations of the Laiyang Formation in well LK-1(modified from [22]).

Figure 1. Tectonic map of the Riqingwei Basin and lithological section of the well LK-1: (a) simplified map of China showing major blocks and its boundaries (modified from [24]); (b) geological map of the Riqingwei basin (modified from [25]); (c) geological map of the Lingshan Island, location of well LK–1(modified from [22]); and (d) composite stratigraphic column and sampling locations of the Laiyang Formation in well LK-1(modified from [22]).

3. Materials and Methods

3.1. Geochemical Analysis

The dark mudstone core samples at different depths in the Laiyang Formation of well LK-1 in the Riqingwei Basin were selected for organic geochemical analysis (Figure 1d). All experiments were completed in the Oil and Gas Geochemistry Laboratory of Northeast Petroleum University.

Methods for evaluating organic-matter abundance in source-rock samples include TOC content determination, rock pyrolysis analysis and Chloroform bitumen “A” content determination. The TOC value of each sample was determined by using the 774 series Carbon and Sulfur Analyzer (LECO Analytical Instruments, St. Joseph, MO, USA) according to the following steps: Crush the rock sample until the particle size is less than 0.2 mm and prepare a sample of more than 10 g. Weigh about 1 g of the sample into a container and slowly add excess hydrochloric acid solution, and put it in a water bath at a temperature of 60~80 °C for more than 2 h. The acid-treated sample is placed in a porcelain crucible, washed with distilled water until neutral, and then dried at low temperature. Add 1 g of iron filings co-solvent and 1 g of tungsten particle co-solvent to the dried porcelain crucible containing the sample, and then measure it on the machine. The organic carbon at high temperature is converted into CO₂, and the volume data are measured by the thermal conductivity detector, and then converted into the percentage of carbon [26]. Rock pyrolysis analysis and determination uses the Rock-Eval VI rock pyrolysis instrument, according to the following steps: Grind the rock sample until the particle size is less than 0.5 mm, weigh about 100 mg and put it into the instrument for measurement. The rock samples continuously released hydrocarbons under the heating condition of programmed temperature (S₁: 300 °C for 3 min; S₂: 300–500 °C at 25 °C/min and holding at 550 °C for 1 min), and the peak areas of S₁ and S₂ were measured by the instrument and S₂ peak temperature T_max [27]. The Chloroform bitumen “A” content of each sample was determined according to the following steps: Crush the sample to less than 0.18 mm and dry it at 40~45 °C for more than 4 h. Weigh an appropriate amount of sample and put it into the extracted filter paper tube, wrap it and put it in the extraction sample chamber. Add purified chloroform and the copper sheet is heated and the heating temperature is maintained below 85 °C until the fluorescence of the extract dripped from the sample chamber weakens to below the fluorescence level 3, and the extraction is completed [28].

The type of organic matter and organic matter maturity in source rock was determined by using a ZEISS Axio Image Z1 Microscope (Carl-Zeiss AG, Oberkochen, Batenwerburg, Germany). Determination the type of organic matter in source rock according to the following steps: Select the rock sample to prepare a light sheet with a diameter greater than 20 mm to ensure that there are more than 800 effective measuring points evenly covering the entire sheet, and count the effective substances under the microscope line by line according to the predetermined line spacing until the end of the light sheet statistics [29]. Determination of organic matter maturity of hydrocarbon rocks by the vitrinite reflectance (R_o) measurement method according to the following steps: The instrument was calibrated using the double standard method so that the reflectance error was less than 0.02%. Measure the maximum reflectance R_max in oil immersion with a polished light sheet that is free of dirt and scratches, and the number of measuring points shall not be less than 30. When vitrinite particles are very small, the random reflectance R_ran can be measured first, and then the R_max can be obtained by conversion method. According to Formulas (1) and (2):

$$\overline{R}=\frac{{\sum }_{i=1}^n{R}_i}{n}$$

(1)

$$s=\sqrt{\frac{n\sum _{i=1}^n{R}_i^2-{\left({\sum }_{i=1}^n{R}_i\right)}^2}{n(n-1)}}$$

(2)

In the formula: $\overline{R}$ is the average reflectance value, %; R_i is the reflectance value of the ith measuring point, %; n is the number of measuring points; s is the standard deviation. Calculate the average reflectance value and standard deviation according to the formula [30].

3.2. Original Organic Carbon Content Recovery Methods

TOC⁰ calculated method used in this paper is based on the chemical dynamic method proposed by Lu, et al. (1995) [31], and the TOC⁰ of organic matter is restored step-by-step according to the law of organic matter evolution and hydrocarbon generation and expulsion.

The Pg⁰ should be the sum of the Pg in the source rock and the amount of hydrocarbon expelled [31,32], therefore:

Pg⁰ = Pg + (Pg⁰∙X_o+B₀ − B) + Pg⁰∙X_g

(3)

In the formula: Pg⁰ is the original hydrocarbon generation potential, mg/g; Pg is the hydrocarbon generation potential detected today, mg/g; B₀ is the amount of primary bitumen in the source rock (that is, not caused by thermal degradation of kerogen), which can be statistically obtained from immature samples, mg/g; B is the amount of residual oil in source rock, which can be replaced by pyrolysis data S₁, mg/g; X_o is the conversion rate of kerogen to oil, %; X_g is the conversion rate of kerogen to gas, %.

Correspondingly, the TOC⁰ calculated method of source rock can be expressed as:

TOC⁰= TOC+ (Pg⁰ − Pg)·K·100%

(4)

In the formula: TOC⁰ is the original total organic carbon content, wt.%; TOC is the total organic carbon content detected today, wt.%; Pg⁰ − Pg is the hydrocarbon-expulsion amount of the source rock, mg/g; K is the coefficient of converting the product organic matter to organic carbon (that is, the carbon content rate of oil and gas products, generally 0.083) [33].

4. Results and Discussion

4.1. Geochemical Characteristics Analysis of Hydrocarbon Source Rock

Oil and gas are generated from organic matter. The amount of organic matter in rocks, the ability of organic matter to generate hydrocarbons, and the degree of conversion of organic matter to oil and gas are the key factors determining the quality of source rock. Therefore, the qualitative evaluation of source rock is mainly carried out from three aspects: the amount (abundance) of organic matter, the level of hydrocarbon generation capacity (type) of organic matter, and the degree of conversion of organic matter to oil and gas (maturity) [34,35,36].

4.1.1. Abundance of Organic Matter

The TOC content of source-rock samples ranges from 0.29 to 29.7 wt.%, with an average of 1.25 wt.% (N = 269). The samples with TOC values < 1 wt.%, ranges in 1~2 wt.%, and >2 wt.% accounts for 48%, 43% and 9%, respectively. The content of chloroform bitumen“A” is 0.000024~0.0006 wt.%, with an average of 0.00012 wt.% (N = 27). The Pg values range from 0.01 mg/g to 1.73 mg/g, with an average value of 0.01 mg/g (N = 99). The hydrogen index (HI) values range from 0.04–1.5 mg/g, with an average of 0.1 mg/g (N = 99) (

Table 1. Geochemical data of over-mature source-rock samples in well LK-1.

TOC/wt.%	Chloroform Bitumen “A”/wt.%	Rock Pyrolysis Parameters			Ro/%
$\frac{0.29~29.7}{1.25/269}$	$\frac{0.000024~0.0006}{0.00012/27}$	Pg (S1 + S2)/(mg/g)	HI/(mg/gTOC)	Tmax/°C	$\frac{3.59~4.65}{4.3/18}$
		$\frac{0.01~1.73}{0.11/99}$	$\frac{0.04~1.50}{0.10/99}$	$\frac{341~522}{425.8/99}$

TOC = total organic carbon(wt.%); S₁ + S₂ = potential yield(mg HC/g rock); HI = hydrogen index; T_max = temperature with maximum hydrocarbon generation. The meaning of the fraction in the table is: “$\frac{\mathrm{Minimum} \mathrm{value}~\mathrm{maximum} \mathrm{value}}{\mathrm{Average}/\mathrm{Statistical} \mathrm{number}}$”.

).

The source rock of well LK-1 was evaluated according to the source rock geochemical evaluation method issued by the National Energy Administration of China [37]. The results of cross-projection between TOC and Pg values show that, among the source-rock samples of well LK-1, 47% are fair source rock, 37% are good source rock, and 11% are excellent source rock (Figure 2). According to the three data of chloroform bitumen “A”, Pg and HI contents (Table 1), all source rock is poor source rock. It is considered comprehensively that the source rock of well LK-1 belongs to poor–fair source rock.

There is a large gap between the abundance level obtained based on the Pg content and the level evaluated based on the TOC content. The reason for this result may be that a large amount of oil and gas in the overmature source rock has been expelled, the existing free hydrocarbons (S₁) and pyrolyze hydrocarbons (S₂) in the rocks are extremely low, and most of the existing organic carbon is non-pyrolyze hydrocarbons of invalid carbon. Therefore, for the source rock of well LK-1, the evaluation indicators of organic-matter abundance have been distorted. Evaluating of well LK-1 source rock based on TOC and Pg is inaccurate. The Pg⁰ and TOC⁰ should be restored before evaluation.

4.1.2. Type of Organic Matter

The organic matter types of source-rock samples can be effectively discriminated by using rock pyrolysis related parameters HI, oxygen index (OI), T_max, etc., [38,39,40]. However, due to the extremely low HI value of overmature source rock (Table 1), it is difficult to effectively discriminate the type of organic matter in the source-rock samples in this study with the relevant discriminant charts of pyrolysis parameters [41]. After the formation of kerogen, various types of kerogen will evolve as the burial depth increases, and oil and natural gas will be generated through thermal degradation and thermal cracking. The basic law of evolution is the O/C atomic ratio and H/C. The atomic ratio decreases successively, and the carbon is enriched, and finally converges to the carbon pole [42,43,44]. Therefore, in the case of a high degree of evolution, it is no longer possible to use the O/C atomic ratio and the H/C atomic ratio to divide kerogen.

According to the calculation method and evaluation standard of predecessors, the T index (TI) is obtained by weighted calculation using the content of each microscopic component of kerogen to distinguish the type of kerogen, according to the formula:

$$TI=\frac{\mathrm{s}\mathrm{a}\mathrm{p}\mathrm{r}\mathrm{o}\mathrm{p}\mathrm{e}\mathrm{l}\mathrm{i}\mathrm{c}\times 100+\mathrm{e}\mathrm{x}\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{e}\times 50-\mathrm{v}\mathrm{i}\mathrm{t}\mathrm{r}\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{e}\times 75-\mathrm{i}\mathrm{n}\mathrm{e}\mathrm{r}\mathrm{t}\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{e}\times 100}{100}$$

(5)

TI ≥ 80 is type I kerogen, 80 > TI ≥ 40 is type II₁ kerogen, 40 > TI ≥ 0 is type II₂ kerogen, TI < 0 is type III kerogen [45,46].

The average value of sapropelic in source-rock kerogen microcomponents is 118.4, the average value of exinite is 63.4, the average value of vitrinite is 47.8, and the average value of inertinite is 70.4. The kerogen microcomponents of each sample were weighted and calculated according to Formula (3), and the relevant TI values were obtained. The kerogen type index (TI) of well LK-1 source-rock samples in the Riqingwei Basin ranges from −15.2 to 43.8 (N = 18) (

Table 2. Macerals and the kerogen types of overmature source-rock samples from the Laiyang Formation in well LK-1.

Sample Number	Depth (m)	Maceral Content				Index of Type (TI)	Type
		Sapropelic	Exinite	Vitrinite	Inertinite
LK-5	60	130	69	40	61	24.5	II2
LK-26	172.5	118	65	43	74	14.8	II2
LK-33	216.4	107	57	57	79	4.6	II2
LK-73	412.8	122	68	43	67	18.9	II2
LK-102	591.1	97	48	68	87	−5.7	III
LK-124	665.3	94	47	60	99	−8.833	III
LK-137	665.3	104	56	50	90	1.5	II2
LK-146	707.3	105	58	49	88	3.1	II2
LK-152	756.9	89	52	67	92	−9.1	III
LK-162	816.5	103	57	49	91	1.3	II2
LK-166	872.7	148	78	35	39	40.6	II1
LK-170	917	146	76	36	42	38.3	II2
LK-177	1031.5	125	54	50	71	14.5	II2
LK-185	1031.5	152	80	30	38	43.8	II1
LK-210	1076.8	149	77	34	40	40.7	II1
LK-222	1122.6	140	73	40	47	33.2	II2
LK-224	1203	120	76	48	56	22.0	II2
LK-265	1312.9	82	50	62	106	−15.2	III

). For the organic matter type, type II₂, II₁ and III account for 61%, 17% and 22%, respectively. The results of kerogen types reflect that the main source of organic matter in well LK-1 is marine plankton and microorganisms, which is consistent with the results of the study on the depositional environment of well LK-1 [8,10]. It shows that the discrimination method of organic matter type in kerogen microcomponents is still effective for overmature source rock.

4.1.3. Organic Matter Maturity

Based on the data of vitrinite reflectance (R_o) and rock-pyrolysis peak temperature (T_max) values, the organic matter maturity of the source rock of the Laiyang Formation in the Riqingwei Basin were studied. Both R_o and T_max values will increase along with the maturity, without reversibility. Therefore, it is an important indicator for judging the maturity of source rock [47,48]. The R_o values of the analyzed samples range from 3.59% to 4.65%, with an average value of 4.30% (Table 1). It can be seen that these R_o values have a good linear increase trend with the depth of well LK-1 (Figure 3a), indicating that buried depth is the main reason for the overmature evolution of hydrocarbon source rocks from the Laiyang Formation. As the degree of thermal evolution of kerogen increases, the degree of carbonization increases and the color becomes darker [49]. Observation under the microscope revealed that the color of the sample kerogen was all black (N = 18), showing overmature characteristics. Based on the R_o values and microscopic examination, it is considered that the present source rock of the Laiyang Formation is in the overmature stage.

The variation trend of the pyrolysis peak temperature of the well LK-1 source rock is shown in Figure 3b. The T_max value ranges from 341 °C to 522 °C (N = 99) (Table 1). T_max is the rock pyrolysis temperature corresponding to the maximum hydrocarbon-generation rate when the rock sample generates cracked hydrocarbons (S₂) during pyrolysis. Studies have shown that when the source-rock samples are too mature and the hydrocarbon generation rate of the S₂ is too low (generally lower than 0.2 mg/g), the T_max is often inaccurate. It reflects the maturity of organic matter and can only be used as a reference index for judging the maturity of organic matter [50]. The S₂ content of source-rock samples of the Laiyang Formation in well LK-1 is shallow, with an average value of 0.09 mg/g. So the T_max cannot accurately reflect the maturity of organic matter and the data in Table 1 is non-representative.

4.2. Re-Evaluation of Organic-MatterBabundance of Source Rock in Overmature Evolution Stage

The TOC content for immature source-rock samples can reflect its original hydrocarbon generation potential. However, as the thermal-evolution degree increases, the effective carbon content in the total organic carbon gradually decreases, making it difficult to honestly and objectively reflect the original hydrocarbon-generation potential of source rock [15]. The essence of source-rock quality lies in the difference in its original hydrocarbon-generation potential. Based on the Pg content, the Pg⁰ and TOC⁰ content were restored, and the well LK-1 source rock was re-evaluated. Based on the relationship between the recovered Pg⁰ and TOC⁰ and the measured Pg and TOC, an evaluation scheme for source rock in the overmature evolution stage based on the current abundance index was determined.

4.2.1. Hydrocarbon Conversion Chart

To restore the Pg⁰ and TOC⁰ of the well LK-1 source-rock samples, it is first necessary to select low-mature source-rock samples for thermal simulation experiments to establish a hydrocarbon-conversion rate chart. The hydrocarbon-conversion rate and related thermal-simulation experiment data used the published research results on the Jiaolai Basin by Li et al. (2022) [9].

Structurally speaking, the Jiaolai Basin belongs to the collateral rift basin of the Riqingwei Basin, and there was water communication between the Riqingwei Basin and the Jiaolai Basin in the Laiyang period. The paleoclimate environment and biological populations of the two basins are similar [1,51]. At the same time, the geochemical analysis results of the source-rock samples show that the HI of this sample is 266 mg/g, and the parent material types are type II₂. The measured vitrinite R_o is 0.51%, at a low-maturity stage (

Table 3. Geochemical analysis results of source-rock samples in Well Laikong 2, Jiaolai Basin [9].

No.	Depth/m	TOC/ wt.%	S1/mg/g	S2/mg/g	HI/mg/g TOC	Tmax/°C	Type	Ro/%
1	88.07	0.67 ± 0.06	0.19 ± 0.02	1.78 ± 0.18	266 ± 3	434 ± 2	II2	0.51

). Therefore, this sample is quite representative and can represent low-maturity type II₂ source rock in the Laiyang Formation in well LK-1.

Li et al. (2022) established the evolution characteristics of the conversion rate of II₂ organic matter kerogen oil generation, kerogen gas and oil cracking gas with R_o. They calculated the evolution characteristics of net oil and total gas with R_o. The gas and oil conversion rates of II₂ organic matter kerogen were obtained, and the hydrocarbon-conversion rates charts of II₂ organic matter were established (Figure 4) [9].

4.2.2. Evaluation of Organic-Matter Abundance in Source Rock at Overmature Stage

According to the results of hydrocarbon generation conversion of type II₂ kerogen, when R_o was 1.16%, the kerogen–oil conversion rate (X_o) reaches the maximum value (0.494). When R_o value is 3.02%, the kerogen–gas conversion rate (X_g) reaches the maximum value (0.487). The minimum R_o value in the study area is 3.59%, and X_o, X_g is currently in the maximum range. B is the amount of residual oil in source rock, which can be replaced by pyrolysis data. S_1. B₀ is the amount of primary bitumen in the source rock (not caused by thermal degradation of kerogen), which can be statistically obtained from immature samples. Immature samples were not found in the study area, so the content of raw asphalt in immature samples was set as 0.16 mg/g by referring to previous reports.

Based on the TOC recovery Formulas (3) and (4), combined with the data of TOC, Pg, B, B₀ and the hydrocarbon-formation conversion rate, the TOC⁰ and Pg⁰ of well LK-1 source-rock samples were recovered. The recovery results are shown in Figure 5a.

The TOC⁰ content values ranged from 0.62 to 30.6 wt.%, with an average of 2.01 wt.% (N = 99). The Pg⁰ content values ranged from 0.63 to 108.1 mg/g, with an average of 6.76 mg/g (N = 99) (Figure 5a). The TOC⁰ recovery coefficient ranged from 1.03 to 9.16, with an average value of 1.59, which is in good agreement with the TOC recovery coefficient (1.57–2.08) of type II high-overmature source rock calculated by Pang et al. (2014) [17]. At the same time, it is consistent with the recovery coefficients obtained by other restoration methods (ineffective carbon conservation method 1.10–1.93, organic matter conservation method 1.25–2.44, chemical reaction conservation method 1.44–3.21) [17]. The comparison shows that the recovery results of TOC⁰ and Pg⁰ can reflect the original organic-matter abundance of the source rocks of the Laiyang Formation in the Riqingwei Basin.

The organic-matter abundance of the restored source rock in well LK-1 was evaluated according to the geochemical evaluation method for source rock issued by the National Energy Administration of China [37]. According to the results of TOC⁰ and Pg⁰, organic-matter abundance of source rock of the Early Cretaceous Laiyang Formation was found in well LK-1 belongs to fair, good, and excellent source rocks (Figure 5b). Compared with the TOC and TOC⁰, the proportion of good and excellent source rock has increased by 17% and 14%, respectively (Figure 5c). Compared with the Pg, the organic-matter abundance level of the Pg⁰ has been greatly improved, with 85% belonging to fair, good, and excellent source rock. The proportion of fair, good, and excellent source rock has increased by 60%, 19%, and 5%, respectively (Figure 5d). In general, the organic-matter abundance of the restored source rock belongs to fair–good source rock. Compared with the evaluation based on TOC and Pg, source-rock samples in well LK-1 show better organic-matter abundance and hydrocarbon generation potential.

Although the restoration of TOC⁰ and Pg⁰ can be used to effectively classify and evaluate the quality of high-overmature source rock according to national standards, the operation could be more convenient. Subsequent new samples still need to be recovered before they can be evaluated, which is not conducive to quickly and accurately understanding the quality of source rock. Therefore, it is necessary to establish a set of evaluation criteria for source rock in the high-overmature stage.

TOC and Pg are the two most commonly used parameters to evaluate the quality of source rock; however, due to the different types of kerogen, the effective carbon content in the same TOC will vary. Therefore, for marine source rock and coal-measure source rock, the TOC division boundaries of source rock of the same quality are different. However, the key to the quality of source rock is how many hydrocarbons can be generated per unit mass of source rock; that is, the hydrocarbon generation potential of source rock. The study found that whether it is the national standard for marine mudstone [37], or the evaluation standard for coal-measure mudstone [52], the division of Pg is the same. That is, Pg < 2 mg/g is poor source rock, 2.0 mg/g < Pg < 6.0 mg/g is fair source rock, 6.0 mg/g < Pg < 20.0 mg/g is good source rock, and Pg > 20.0 mg/g g is excellent source rock. Therefore, based on this standard, we can establish a classification and evaluation standard for high-overmature source rock based on TOC and Pg, combined with the relationship between TOC and Pg of source rock in the Laiyang Formation before and after recovery (

Table 4. Evaluation criteria for abundance of organic matter in mudstone of different maturity.

Thermal Evolution Stages	Geochemical Indexes	Poor	Fair	Good	Excellent
Low-mature stage [37]	TOC (wt.%)	<0.5	0.5–1	1–2	≥2
	Pg (mg/g)	<2	2–6	6–20	≥20
High-overmature stages	TOC (wt.%)	<0.1	0.1–0.6	0.6–1.5	≥1.5
	Pg (mg/g)	<0.03	0.03–0.1	0.1–0.3	≥0.3

). According to the new evaluation standard, TOC < 0.1 wt.% is poor source rock, 0.1 wt.% < TOC < 0.6 wt.% is fair source rock, and 0.6 wt.% < TOC < 1.5 wt.% is good source rock, TOC ≥ 1.5 wt.% is excellent source rock. Pg < 0.03 mg/g is poor source rock, 0.03 mg/g < Pg < 0.1 mg/g is fair source rock, 0.1 mg/g < Pg < 0.3 mg/g is good source rock, and Pg ≥ 0.3 mg/g is excellent source rock.

5. Conclusions and Outlook

This paper evaluated the overmature hydrocarbon source rock in well LK-1 in the Riqingwei Basin with regard to organic-matter abundance, type, and maturity, based on a large amount of geochemical data. The TOC content of the Laiyang Formation overmature source rock in well LK-1 in the Riqingwei Basin was 0.29–29.7 wt.%, with an average of 1.25 wt.%, and the Pg was 0.01–1.73 mg/g, with an average of 0.1 mg/g, and belongs to poor–fair source rock as a whole. The kerogen microcomponent index (TI) indicated that the type of organic matter is mainly type II₂. The vitrinite reflectivity (R_o) was 3.59–4.65%, with an average of 4.3%. The organic matter is generally in the overmature evolutionary stage, and the main oil and gas production is dry gas.

Combined with the hydrocarbon conversion rate of low-maturity type II₂ kerogen obtained from the thermal simulation experiment of hydrocarbon generation, the TOC⁰ and Pg⁰ contents were restored and evaluated. The TOC⁰ content was 0.62–30.6 wt.%, with an average of 2.01 wt.%, and the Pg⁰ content was 0.63–108.1 mg/g, with an average of 6.76 mg/g. The original organic-matter abundance of the source rock belongs to fair–good source rock, which has been upgraded by one level compared with the previous evaluation. According to the relationship between the Pg⁰, Pg and TOC, the high overmature source-rock evaluation standard in well LK-1 based on TOC and Pg was re-established. Under the high-overmature source-rock evaluation standard in well LK-1, a source rock with TOC content exceeding 0.6% and Pg greater than 0.1 mg/g can be identified as good source rock.

In conclusion, the large set of overmature source rocks found in well LK-1 has completed the hydrocarbon generation and expulsion process, and it has low potential to produce hydrocarbons at the present time. However, its TOC⁰ and Pg⁰ indicate that this set of source rocks has a high original organic-matter abundance. Restoring the TOC⁰ and Pg⁰ of overmature source rock and re-establishing the evaluation criteria for organic-matter abundance in overmature source rock is of great theoretical significance and practical value for fine evaluation of overmature source rock and the accurate evaluation of hydrocarbon resource potential. The hydrocarbon production and discharge process of hydrocarbon source rock of the Laiyang Formation was finished long ago in the area of Lingshan Island under the influence of tectonic processes, ancient burial depths, and magmatic activity. The results suggested that hydrocarbon geological evaluation studies should be conducted on the contemporary strata that are not affected by thermal action in the sea area of the Riqingwei Basin in the future.