1. Introduction
China has made eye-catching breakthroughs in shale oil (or gas) and is keeping pace with the United States, whose target investigations have lasted for over 20 years (e.g., [
1]). The large-scale discoveries of continental facies oil and gas resources represented by the Qingshankou Formation in Songliao Basin and the Yanchang Formation in the Ordos Basin led to triumphant improvements in shale resources [
2,
3]. Meanwhile, the advances targeting the Bohai Bay Basin (BBB) in China are likewise significant in exploration (e.g., the Jiyang Depression, the Dongpu Depression, and the Cangdong Depression). The Shahejie Fm of Qikou Sag has recently been a favorable shale oil area, of which in-depth studies aimed at the primary target strata, the first member (Mbr
1) and the third member (Mbr
3) of the Shahejie Fm, are still needed. Source rocks are vital for the formation of hydrocarbon-rich reservoirs and determining the scale of conventional and unconventional resources (e.g., [
3]). From the perspective of exploration, although there have been substantial breakthroughs found in the wells of Mbr
1 and Mbr
3, and Mbr
3 is superior in terms of single well production and exploitation stability, it cannot be considered as a more desirable layer with greater resource potential. Objective, accurate assessment is needed to qualify hydrocarbon (HC) generation and expulsion capabilities as well as the resource potential of the two target members.
Interest in source rocks has been gradually deepening, from their quality to their quantity, and it has been concluded the capabilities and capacities of HC generation and expulsion, which represent the essential status of source rocks (e.g., [
4,
5]). Current detection methods could be categorized into three mechanisms derived from genetics [
6]: basin simulation [
7], chemical generation dynamics [
8], and material balance dominated by carbon conservation [
9]. The pros are obvious, but the cons of each method are also severe for their unreliable calculations. Former studies have indicated that thermal simulation ignored immature and low-maturity HC when judging the HC generation threshold [
10]. Geologic conditions given by chemical kinetic experiments are far from actual circumstances (e.g., [
11]). Most studies have not had enough samples to represent heterogeneity [
12]. The original HGPM proposed by Pang et al. [
13] overcame these shortcomings from the perspective of statistics and the material balance principle; its main idea is that the generated HC potential (S
1+S
2/TOC) of source rocks is a constant value in any stage of thermal evolution unless HC expulsion occurs. Abundant applications and analysis based on this model in representative petroliferous basins in China (e.g., [
14,
15,
16]) provide practical cases of modification, where the inaccuracy stems from volatile hydrocarbon content (S
1) [
17,
18]. For the revised model, applications demonstrate that the method is not only simple but also accurate and effective.
Moreover, in traditional organic chemistry, a S
2 versus TOC cross-plot, a common plot made for kerogen classification and the evaluation of HC generation capacity, embraces a presupposition that all points on the plot are at the similar maturity stage or are all immature [
19]. A model-assisted method, together with the dimensionless TOC(TOC
DL) versus S
2(S
2DL), was used to determine the derivation of samples (the same/single kerogen population or not) and eliminate maturity overprint in a traditional plot to objectively reflect the generation capacity from Rock-Eval data.
Both of these methods are used on the basis of tremendous Rock-Eval data. The Qikou Sag is an old exploration area. The exploration process in the past 20 years mainly involves conventional resources, which has a supporting pyrolysis data foundation for a clear analysis. It is reasonable to utilize an advanced HGPM and kerogen kinetics to precisely evaluate all kinds of resources and an in-depth comparison between targeted layers. Moreover, applications of the HGPM also proved the possibility of detecting and quantifying shale oil resources, as well as movable resources. The results of the model showing the future resource potential and the comparison between the different layers in specific environments could objectively guide shale resource exploration and effectively guide other areas, such as Qikou.
An updated HGPM and a kerogen kinetics cross-plot are adopted in this study, and Mbr3 source rocks’ generation and expulsion capabilities are confirmed. Conventional and unconventional resource potential was further acquired by the characteristics reflected in the model. Furthermore, the two main shale oil target layers were compared both in generation and expulsion capabilities and processes. The major objectives of the investigation are (1) to systematically state Mbr3 source rocks’ features in geochemistry, (2) to objectively analyze the kerogen population by dimensionless variables TOCDL versus S2DL, (3) to quantify the generated and expelled HC capabilities aimed at Mbr3, and (4) to compare the target layers (Mbr1 and Mbr3) in the generation process, the generated and expelled HC capabilities, and their covariant relationships in the sedimentary environment.
2. Geological Setting
The BBB, which covers nearly 2 × 10
5 km
2, did not have a fixed shape until the basement of the North China Craton formed [
20] (
Figure 1a). Two essential evolutions occurred 25 Ma: “synrift transformation” and “post-rift conversion”.
The Huanghua Depression, a tectonic unit with fault boundaries in the west and the east, lies in the in the central area of the BBB (
Figure 1b) [
21]. Qikou Sag, the largest half-graben since the Paleogene, is a secondary depression structural unit [
22], whose five secondary sub-depressions compose the area. They are the Banqiao, Beitang, Qibei, Qinan, and Chenghai sub-depressions surrounding the main depression, with an area of 6000 km
2 [
23] (
Figure 1c).
There are four strata in the Cenozoic: the Shahejie, Dongying, Guantao, and Minghuazhen Formations. The Shahejie and Dongying formations were sedimented under fluvial or lacustrine conditions (
Figure 1d). The other formations were more likely sedimented in fluvial conditions. They are more than traditional source rock centers of conventional energy. The Mbr
1 and Mbr
3 strata in Qikou Sag are potential shale resources [
24]. In early resource assessments (before the shale oil breakthrough), Qikou Sag had the largest resource potential, accounting for half of the resources in the entire Dagang exploration area, and the Paleogene Shahejie Fm is the most important resource layer, which account for around 45% of all resources. Mbr
3 is sedimented mudstone, shale, thin layer of sandstone and siltstone which deposited in fan delta, lacustrine and its nearshore, subaqueous sedimentary fan environment. Mbr
2 is in the stable stage of the faulted basin, i.e., in the overall uplift stage, and mainly consists of sandstone, dark mud, and shale. At Mbr
1, gray black mudstone, oil shale, biolimestone, and oolitic limestone, forming in shallow lacustrine and semi-deep lacustrine facies, were deposited.
3. Experiments and Methods
Ninety-one samples were taken from 27 exploratory wells, which are scattered across Sag. Total organic carbon (TOC) in the sample was obtained with a LECO CS-200. Free HCs (S
1), remaining generation potential (S
2), and the temperature at the peak (maximum) generation rate (T
max) were collected using an OGE-II rock pyrolyzer instrument. The hydrogen index (H
I) was the result of a further formula application by Espitalié et al. [
25].
The HGPM was established according to the database mentioned above. Our study combines an advanced model revised by Wang et al. [
26] and Li et al. [
12] with mass balance. The common error derived from hydrocarbon expulsion was eliminated. The improvement of the model improves the HC generation and expulsion process, which can be identified in
Figure 2. Equations and mathematical symbols for the model and an improved establishment of a TOC versus S
2 plot were introduced.
Determining the initial hydrogen index (
, the max HC generation potential of the effective organic carbon) is essential. Tissot et al. (1974) pointed out that it would affect the amount of effective organic carbon and its conversion to HC. The
can be fitted by a statistical model between H
I and T
max, and we use Equation (1) [
12], which breaks through the low maturity condition further compared with the former one [
27].
Here, is as mentioned above, and θ and β are unknown parameters that are originally influenced by kerogen kinetics that can be estimated from data simulations (based on the 91-sample database of Mbr3 in this study). With the extensive database and fitting deduction, optimal parameters are confirmed, and these reflect the HC generation process, including the onset of oil generation and the width of the oil window.
Tr needs to reflect the kerogen decomposition degree. Equation (2) is from Espitalie et al. [
28]:
Equations (1)–(3) were combined to calculate the generated HCs at a specific thermal degree:
where
Qg refers to the generation capability of HC, in mg HC/g TOC.
During the proper processes, kerogen normally transforms beyond adsorption capacity. The HC begins to be expelled abruptly at the hydrocarbon expulsion threshold (HET). The HET [
13] represents the specific geological conditions posited for concluding R
o.
Tre is obtained in representation of T
r with Equations (1) and (2) at the HET.
Qe (mg HC/g TOC) is created to describe the HC expulsion capability by defining the function concerning R
o using Equation (4):
where ‘
x’ stands for a given depth or
Ro at which the kerogen is located, whereas
xHET relates to the expulsion conditions. It is important to judge whether the source rocks’ ‘x’ stage condition(s) accord with HET conditions.
Trx is a specific T
r under a certain condition ‘
x’, calculated by Equation (2).
Though HC generation and expulsion can seldom be processed thoroughly, the residual potential component is theoretically
Qr:
It is worth noting that the relation between
Qr,
Qg, and
Qe is suitable for exactly the realistic geological condition or in the fitting model. However, the data from the Rock-Eval experiment are inaccurate because
Qr is not an easy calculation of (S
1 + S
2)/TOC × 100 (mg HC/g TOC). It is well-known that volatile HC content (S
1) is intensely reduced when samples are obtained underground because of the inappropriate preservation measures in the process of obtaining and handling samples [
17,
26]. Therefore, the exact
Qr will fit the posterior Equation (5) when samples undergo non-quantitative losses due to uncertain causes of HC content.
E was used to describe the HC expulsion degree and efficiency (Equation (6)).
The
TOC from the Rock-Eval analysis, except in the reconstruction of the model, consists of inert (
TOCI) and active organic carbon (
TOCA). TOC
I is not an HC generation potential component.
TOCA can convert to HCs under a suitable thermal process.
TOCA is partly
TOCC in certain mature conditions [
19]. The present
TOC is represented as follows:
where
TOCo can be calculated from the recovery function [
19]:
where
α is a constant term, obtained from
/1200, combining Equations (1) and (2).
At an immature stage, the original HC generation potential (S
2o) can be represented by the current HC generation potential (S
2) with the following equation:
Equations (10) and (11) represent the dimensionless
TOC (
TOCdl) and dimensionless
S2 (
S2dl):
Thus, a more appropriate model was constructed. Parameters and functions reflected practical HC generation and expulsion processes and the characteristics of source rock with the conceptual model (
Figure 2b). A dimensionless variable plot was obtained.
Confirming HC generation and expulsion features is important for judging the intensities of HC generation and expulsion, especially under certain geological periods. The HC generation (I
g), expulsion (I
e), and retention intensities (I
r) are defined in Equations (12)–(14):
where Q
g, Q
e, and Q
r are obtained from Equations (3)–(5) at a specific maturity stage in mg HC/g TOC, respectively; R
oe represents the maturity at the HET, inferring the beginning of expulsion maturity, in %; T is the source rock thickness in m; Q
r is the source rock density in g/cm
3; TOC(R
o) is the actual TOC under certain conversion period in wt %; and I
g, I
e, and I
r are in 10
4/km
2.
Models have considered the shale oil system since the breakthrough in the target area. HCs of shale oil are supposed as movable and immovable oil parts [
29]. The movable oil index (OSI) is commonly used and defined as OSI = S
1/TOC × 100. A target layer’s OSI can reach over 100 mg HC/g [
30]. However, recent experiences aimed at Chinese lacustrine shale exploration have shown that dissention of the threshold could be 70 mg HC/g TOC [
31]. The OSI value used here was obtained mainly based on the rock pyrolysis information of 261 shale samples in different areas of the Ordos Basin, China. Although that study was not carried out in the Bohai Bay Basin, it is statistically significant to some extent because it covers almost the entire Ordos Basin and involves a large number of samples. Moreover, the shale in the Ordos Basin was mainly developed in a lacustrine environment, so it is believed that its OSI value is more representative than that of 100 mg/g TOC in North America [
30]. According to the status quo in China, the relevant formula is defined as follows:
where
is the effective part of retentive HCs in
Qr. Consequently, I
re (movable retentive HC intensity) is acquired:
is a calculation of Equation (16) at a specific maturity stage in mg HC/g TOC.
In the study area,
Tmax was converted to R
o as a result of the limited R
o data. The fitting equation with measured R
o points aimed at the Mbr
3 of Qikou Sag is shown in Equation (17):
5. Discussion
Source rock properties have remarkable effects on resource potential evaluations. These properties can be used to guide future resource predictions: generated HC, expelled HC, and retentive HC. Moreover, though source rocks belong to the same area, generation and expulsion processes in strata, consequently resulting in resource distinctions caused by the OM accumulation environment, OM derivations, etc., may also completely differ from each other. The following discussions refer to the Mbr3 stratum in resource evaluation and the in-depth analysis between Mbr1 and Mbr3 in the generation and expulsion capabilities reflected by the HGPM, the process indicated from the model, and possible factors contributing to the discrepancies.
5.1. Favorable Exploration Target
The HC generation center is usually a desirable target for exploration because of their material foundation (e.g., Peng et al., 2016). It is not suitable for all kinds of resources and areas, since each target area is dominant in a different major resource. This can be further explained by the conceptual model shown in
Figure 2b. The hydrogen index is constant at H
Io until kerogen begins to convert into HCs. The sharply decreasing point of H
I is the hydrocarbon generation threshold (HGT) as a result of the conversion from OM to hydrocarbons. The dramatically expelled point of HCs is the hydrocarbon expulsion threshold (HET). The resources we focus on here are thus described by the HGPM. In detail, the light brown area represents conventional and tight HC resources. The light gray area represents inert carbon, and the vacant part represents the residual HC generation potential (equal to the shale resources). That is to say, when we are concerned about conventional and tight oil petroleum systems, only the expelled HCs, rather than the generated HCs, determine the resource potential. The remaining effective HCs also play a role in shale resources.
Mbr
3 source rocks are of high quality for the following reasons: (1) the thickness in the main depression is over 1200 m; (2) the TOC is comparatively high, with an average value of 1.67%; (3) the source rock over the study area is generally in a mature stage. Various types of HC resources can be directly judged using the HGPM. Among the three kinds of resources, shale resources manifest a higher potential based on the I
r and I
re contour maps (
Figure 10c,d). These resources are radially distributed near the sedimentary center of the Qibei sub-sag and the Qikou main depression. The I
e map indicates the area favorable for conventional and tight resources and shows relatively poor potential compared with shale resources. Targets aimed at conventional or tight resources are generally distributed, but intensities are low (
Figure 10b). By contrast, areas with high I
r and I
re values show a limited distribution.
The advanced HGPM not only reveals generated, expelled, and retentive HC characteristics and demonstrates HC generation, HC expulsion, and HC retention intensities, but also helps in obtaining the resource scale of an area. As source rocks continuously evolve from the HGT point (Ro = 0.48) to the HET point (Ro = 0.78), resources of three types accumulate to maximum values: retention shale oil or gas is equal to 13.3 × 108 t oil, movable shale oil or gas is equal to 8.0 × 108 t oil, and conventional resources with tight resources is equal to 4.7 × 108 t oil equivalent. An overall evaluation indicates that shale oil resources (movable shale oil) preponderate compared with conventional and tight resources, showing a larger exploration potential as exploitation technology allows.
5.2. Differences between Mbr1 and Mbr3 Based on the HGPMs
Mbr1 of the Qikou Sag, another important stratum in the study area in the Bohai Bay, was chosen for further research in order to thoroughly investigate the effects of HC generation and expulsion features on resource evaluation and to identify the causes of such distinctions.
Table 1 lists geological and geochemical characteristics and HGPM parameters in the two strata. The information (cited from [
36]) indicating OM content, maturity, the simulation line, and the established HGPM is shown in
Figure 4,
Figure 6,
Figure 8 and
Figure 9.
5.2.1. Geological and Geochemical Features
In terms of geological and geochemistry, Mbr
1 source rock actually surpasses Mbr
3 in quality (
Figure 4 and
Figure 6), showing more TOC, a superior kerogen type, and a higher maturity. The thicknesses of the source rocks in these two layers are almost equal. More specifically in terms of parameters, the distance between the average TOC content is remarkably large, which causes large differences in resource amounts (
Table 1). Moreover, the far-away distinctions in OM type may cause the maturity and generation potential to change from the kerogen kinetics.
Former studies confirmed that the OM source of Mbr
1 is highly dependent on the outbreak of algae in that period (e.g., [
37]). The fitted curve shown in
Figure 4b indicates a high ratio of active carbon, at 75%, and a large potential that is similar to that of the simulation of H
Io in
Figure 8. The Es
3 source rock samples exhibited a larger proportion of Type II
2–Ⅲ OM, showing 69% under the microscope, and humic OM is significantly larger in scale, which indicates the source of biological and higher plant residues [
38]. The TOC
DL-S
2DL cross-plot shown in
Figure 4b provides support for an obscure linear relation and the possibility of more than two thermal pathways. The possible simulated lines demonstrate active carbon proportions of 30–55%, indicating a lower potential of HC generation from the source foundation.
The distinctions in influences shown by the HGPM is discussed in the following section.
5.2.2. Generation and Expulsion Features from the HGPMs
The improved parameters mean that the target embraces a larger Q
g as a material foundation. Based on the HGPMs, the curves derived from the statistical numerical simulations of the two layers show distinct H
Io values of 746.99 (Mbr
1) and 605.49 mg HC/g TOC (Mbr
3), respectively. However, this reflects the OM type differences from kerogen kinetics. Kerogen Ⅰ (considering Mbr
1 as representative) and kerogen II–Ⅲ (considering Mbr
3 as a typical layer) underwent distinct processes, as shown in the patterns in
Figure 9.
The source rocks of the Mbr
1 layer show an overwhelming advantage over those of Mbr
3 in Q
g in every stage of degradation. The Mbr
1 source rocks almost reached the HET when Mbr
3 began to generate HCs. In the same thermal stage (R
o = 0.6), Mbr
1 showed an obvious advantage in Q
g, at 590.29 mg HC/g TOC. This resulted from an earlier generation and a much faster course of maturity (
Figure 9b). An earlier HET does not necessarily indicate an obvious advantage in expulsion capability for Es
1, since Q
e has an inconspicuous distance from the maximum values of 168.77 and 231.57 mg HC/g TOC (difference = 62.8 mg HC/g TOC in max). Based on the final values of expulsion efficiency (E) (
Figure 9f), the two layers nearby both reached 30%, and none of these layers had a leading position. Similarly, Q
r exhibits an acceptable difference from the maximum values of 436.24 and 515.42 mg HC/g TOC (difference = 79.18 mg HC/g TOC in max).
Discrepancies in geological and geochemistry characteristics gave rise to an immeasurably vast difference in the generation and expulsion processes of the two studied layers. The most dramatic discrepancy found was between Mbr
1 and Mbr
3; the former was in a much lower maturity grade (in terms of HGT and HET conditions). The maturity differences between the HGT R
o and the HET R
o was not as dramatic, with differences of 0.1% and 0.28%, respectively (
Figure 9a,b). Former studies commonly showed that, during HC generation, the earlier the source rocks reached the HET, the more the HCs were expelled. Moreover, this had a positive influence on conventional and tight resource accumulation. A later HET leads to more retained HCs. This is in favor of accumulating shale resources. However, these two layers provide examples where early and later expulsion both benefit retention resource accumulation. Though source rocks in Mbr
1 reached the HGT and HET at low maturity, a single Type I kerogen population converted to HCs within a very narrow temperature window (
Figure 9b). An HC expulsion capability restriction is likely to limit conventional and tight resource potential [
36], since such potential is far from that of the retention shale oil. In comparison, a later HET and a comprehensive quality of Mbr
3 source rocks showed that they advanced to a mature stage with a constantly rising temperature and pressure, and expulsion proceeded as the temperature changed (
Figure 9a). A longer interval of expulsion indeed helped the Mbr
3 source rocks to achieve an identical degree of expulsion, which contributes to the conventional and tight resource accumulation. However, the predominated positions of Q
e in these two layers both contribute to the shale resource potential. As for movable shale resources, Mbr
1 has 12.21 × 10
8 t oil equivalent, while Mbr
3 has 8.0 × 10
8 t oil equivalent, and the proportions accounting for total retentive resource are 53% and 60%, respectively. The former layer actually shows a greater potential, whereas the latter has a desirable movable proportion together with a fair amount of resources.
In sum, the Mbr1 layer shows a superior HC generation and expulsion capacity. When a specific thermal stage is given that is identical to that of the Mbr1 layer, all resources are more considerable.
5.3. Possible Controlling Factors and Interpretations of Different HGPMs
The capabilities of generation and expulsion are commonly considered to be influenced by the kerogen type, maturity, and TOC (e.g., [
39,
40,
41,
42]). There is no doubt that this assumption is true. When we attempted a deeper exploration, the controlling factors were derived from the sedimentary environment in which the OM accumulated. Though most applications have an evolution style similar to that of Mbr
3, the extreme HGPM pattern of Mbr
1 was not an exception. The Eocene Wenchang Formation source rock of China and the Eocene Green River oil shale in the United States are analogous examples in kerogen kinetics [
19,
43]. The kerogen of these typical sets of source rocks is nearly a simplex molecular structure deposited in a lacustrine background [
19] or restricted environment. A lacustrine or evaporative environment is more suitable for the accumulation of kerogen with the same origin, while marine environments, along with fluvial, deltaic, and other unrestricted environments, are more likely to embrace various OM, leading to mixed kerogen as a consequence of significant terrestrial input [
27].
Former studies have confirmed that the environment of Mbr
1 was a brackish lacustrine one with a mild climate [
43], during which algae blooming occurred [
37]. Gammacerane, as a main biomarker of a salt lake, showed a range of 0.14–1.84 in the Mbr
1 layer (average = 0.63), reflecting the salty condition [
44]. Such a setting not only provides a single source of kerogen, but also creates a stratified water mass where OM can preferentially accumulate and be conserved. This clearly explains why outstanding geochemical features exist in Mbr
1 compared with Mbr
3.
In comparison, during the Mbr3 sedimentary interval, although the Qikou main depression area was still in a widespread semi-deep to deep lake setting, submerged fans of different sizes developed in the north, south, and west (including the Banqiao Sub-sag) sides of the main depression. This indicates that there was a strong input of external sources in that period, which had a negative impact on the stable sedimentation and accumulation of OM and further caused a diversity of OM sources and OM types under certain circumstances. Meanwhile, there is no evidence of salinization in the lake water mass, implying a wetter climate and stronger communication with external water bodies, such that no isolated lacustrine environment was formed.
It is reasonable to conclude that the Mbr3 source rock was not superior to that of Mbr1 in terms of HC generation potential and resources for the non-isolated environment in which it is located. The diverse sources of OM led to complicated thermal processes in its evolution. It is possible that the salinity was also a significant controlling factor that limited OM conservation.
Based on this analysis, considering a typical layer in the study area (the Mbr1 Member) for comparison, the detailed characteristics of geochemistry, HC generation and expulsion, thermal processes, and further resource potential were quantified. The probable in-depth controlling factors were explored. The HGPM was shown to be effective and important for exploration and quantification in unconventional resource evaluations.
6. Conclusions
A series of assessments aimed at Mbr3 of Qikou Sag was carried out for source rock quality evaluation and quantification evolution features established by an advanced HGPM.
The Mbr3 source rocks were widespread and at a desirable thickness, and their TOC content was 1.66% on average and dominated by Type Ⅱ and Ⅲ kerogens. They were in a mature thermal stage, where source rocks are considered to be of high quality. Establishment of the HGPM provided more detailed information of the source rocks’ evolution features. The model revealed that HC was expelled when Ro = 0.78%. The Qg, Qe, and Qr are, at maximum, 605.89, 169.65, and 436.24 mg HC/g TOC, respectively. The maximum intensities of the generated, expelled, retentive, and effective retentive HCs were 250 × 104, 65 × 104, 170 × 104, and 110 × 104 t/km2, correspondingly located in the Qikou main depression and the Qibei Sub-sag. The predicted resource potentials are as follows: the conventional and tight oil and gas resources: 4.7 × 108 t; remaining shale and movable shale oil and gas: 13.3 × 108 t and 8.0 × 108 t oil equivalent, respectively.
Mbr3 source rocks exhibit large disparities compared with Mbr1 in basic geochemical parameters, including thermal process and generation, expulsion features derived from the kerogen type, and non-isolated deposit environments.