Evaluation of hydrocarbons generated from the Permo-Carboniferous source rocks in Huanghua Depression of the Bohai Bay Basin,China

Abstract

The Huanghua Depression is located in the north-centre of Bohai Bay Basin, which is a rift basin developed in the Mesozoic over the basement of the Huabei Platform, China. Permo-Carboniferous source rocks were formed in the Huanghua Depression, which has experienced multiple complicated tectonic alterations with inhomogeneous uplift, deformation, buried depth and magma effect. As a result, the hydrocarbon generation evolution of Permo-Carboniferous source rocks was characterized by discontinuity and grading. On the basis of a detailed study on tectonic-burial history, the paper worked on the burial history, heating history and hydrocarbon generation history of Permo-Carboniferous source rocks in the Huanghua Depression combined with apatite fission track testing and fluid inclusion analyses using the EASY%R_o numerical simulation. The results revealed that their maturity evolved in stages with multiple hydrocarbon generations. In this paper, we clarified the tectonic episode, the strength of hydrocarbon generation and the time–spatial distribution of hydrocarbon regeneration. Finally, an important conclusion was made that the hydrocarbon regeneration of Permo-Carboniferous source rocks occurred in the Late Cenozoic and the subordinate depressions were brought forward as advantage zones for the depth exploration of Permo-Carboniferous oil and gas in the middle-northern part of the Huanghua Depression, Bohai Bay Basin, China.

Keywords

Permo-Carboniferous source rocks burial history thermal history hydrocarbon generation

Introduction

The Bohai Bay Basin is the largest oil and gas production basin among the petroliferous basins in China (Guo et al., 2016; Ma et al., 2016; Zhao et al., 2014). The study area is the Huanghua Depression, located in the north-centre of the Bohai Bay Basin, which overlies the Palaeozoic cratonic basin of North China; it developed as a fault basin in the Mesozoic and then as a rift basin in the Cenozoic. The Huanghua Depression is an oil-rich basin located in North-East China. Whereas petroleum exploration has been an enormous success at the shallow Cenozoic zone in the Huanghua Depression by the Dagang Oilfield since the 1950s, little has been broken through in the deep zone of the three Permo-Carboniferous source rocks (PCSRs). The Permo-Carboniferous is an important formation across the world. Some research focused on Permo-Carboniferous thermal modelling, fluid–rock interactions and geological evolution (Fernandez et al., 2016; Hertle and Littke, 2000; Regenspurg et al., 2016). Recently, exploration prospects and potential problems of Permo-Carboniferous oil and gas were raised with the agenda of the continuous development and reduction of the shallow zone in oil and gas traps. The finding of Palaeozoic primary oil in the Kg3 and Kg4 wells at the deep zone of the Huanghua Depression causes geologic condition problems for the Palaeozoic oil in the deep zone of Bohai Bay Basin (Zhu et al., 2001).

There are three main PCSRs in the Huanghua Depression, i.e. the Benxi Formation, the Taiyuan Formation of the Upper Carboniferous and the Shanxi Formation of the Lower Permian, which are covered by thicker Mesozoic–Cenozoic rocks (Zhu et al., 2010). After formation, PCSRs were tectonically active through the Mesozoic–Cenozoic and underwent a complex history of structural evolution, which can be divided into four structural phases: (1) stable subsidence in the Hercynian-Indo-Chinese Epoch, (2) rift subsidence in the Yanshan stage with a series of northwest-trending fault basins, (3) the Himalayan movement stage I at the end of the Eocene as an exposed rift basin and (4) the thermal depression stage in the Himalayan movement stage II while still in the Quaternary. Therefore, the present tectonic styles are integrating phenomena, which are composed of different episodes, different types and diverse direction tectonics. With the differences in downfaulted tectonic activity, the area can be classified into several tens of subordinate tectonic units in the Huanghua Depression, such as sub-depressions, bulges and sub-bulges, which resulted in respective GH evolutions of Palaeozoic source rocks in each subunit.

Geological setting

With an area of 17,000 km², the depression is bounded by the Cangxian uplift to the west, the Yanshan tectonic belts to the north and the overlap or fault transition of the Chengnin uplift to the southeast (Figure 1). It is shaped as a wedge with a SSE–NNW trend, which is distributed from the west–south divergence to the east–north convergence. This area is roughly considered as an asymmetric fault basin controlled by a fault (Meng et al., 1993; Qi et al., 1995).

Figure 1.

The skeleton map and lithologic column showing the subunits of the Huanghua Depression. 1 – sub-depression; 2 – sub-uplift; 3 – place name; 4 – well name; 5 – fault; 6 – Permo-Carboniferous; 7 – Mesozoic; 8 – Lower Tertiary; 9 – Upper Tertiary and Quaternary; 10 – stratum boundary.

The burial history of PCSR

Since the source rocks were formed in the Palaeozoic, the Huanghua Depression experienced several episodes of structural activities from the Triassic to Tertiary (prior state), which resulted in multiple hydrocarbon generation evolutions of Permo-Carboniferous organic matters. The tectonic-sediment history analysis revealed that the burial history of PCSR in the Huanghua Depression mainly experienced three sediment-uplift cycles of alternate evolution.

Indo-Chinese Epoch: During the Hercynian-Indo-Chinese Epoch, the Huanghua Depression belonged to a family of extensional basins that developed in North China with a stabilized background of regional structure, where deposited strata occurred from the Upper-Carboniferous to Mid-Triassic on the ancient weathering crust of the Ordovician, while PCSR experienced the first buried period.

The deposit thickness of the Upper Carboniferous ranges from 100 to 150 m, and that of the Permian is less than that of the Shanxi Formation of approximately 100 by 1000 m. Subsequently, PCSRs were covered by the Low-Mid Triassic, in which the primary deposit thickness was changed from 900 to 1500 m (Zhu et al., 2001). Therefore, the maximum buried depth of PCSR was 2600 m.

Yanshan period: During the Late Indo-Chinese Epoch to Early Yanshan Epoch, the structural differentiation was aggravated by control on regional tectonic uplift, resulting into the different degradation of the Palaeozoic–Triassic in the Huanghua Depression and the formation of regional unconformity. The degradation thickness ranged from 1000 to 2000 m (Zhu et al., 2001). During the Mid-Late Yanshan period, deposition occurred to Huanghua Depression again in response to crustal extension in this area, which was associated with widespread volcanic eruption in East China (Ren et al., 2002), while the deposit thickness was heavily inhomogeneous in the research region by regional tectonic control. The sedimentary thickness was thicker in the south of the Huanghua Depression, where the burial depth of PCSR was less than 1500 m, compared with that in the north, which was approximately 1000 m.

During the Mid-Late Yanshan period, regional rifting accompanied by a mass of magmatic activity occurred in the Huanghua Depression and East China and formed in the Upper Jurassic–Lower Cretaceous containing a large quantity of volcanic rocks, where the PCSRs were buried again. In the Late Cretaceous, the middle-south area uplifted and experienced degradation. However, sediment still existed in some northern areas. Until the Palaeocene, the whole area had uplifted with degradation, leading to the formation of regional uniformity.

Himalayan period: In the Himalayan period, followed by uplift and denudation, the Early Cenozoic rifting stage started, which was probably triggered by the subduction rollback of the oceanic Pacific Plate from the Asian continent (Allen et al., 1997). The Huanghua Depression was represented as a rift basin, which can be divided into two extension phases with Dongyin tectonic movements, i.e. the Early Himalayan phase (a rift basin) and Late Himalayan phase (a thermal depression).

In the Early Himalayan period, the Huanghua Depression developed into a rift basin, and sediment was obviously controlled by the Cangdong downfaulted in the west (Figure 1). Although the deposition was definitive differentiation, by the end of the Early Himalayan, the burial depths of PCSRs were all over 2000 m, and some already exceeded 3000 m, such as the Nanpi sub-depression and Qiku sub-depression. At the end of the Palaeogene, the Dongyin tectonic movement caused basin basement uplift and degradation, but the denudation rate was relatively lower and rounded approximately hundreds of metres.

During the Late Himalayan, the entire Bohai Bay Basin (including the Huanghua Depression) was converted into a thermal deposit basin in which broad deposition had taken place, with a thickness of approximately 2500 m in the Later Tertiary–Quaternary, resulting in a greater buried depth of PCSR. The buried depth is approximately 3000 m in the south and up to 5000 m in mid-north of the Huanghua Depression, and in some places reached 6000–7000 m (e.g. the middle of the Qiku sub-depression and middle-northern part of the Huanghua Depression). Therefore, the burial depth of PCSR in the Late Himalayan was not only much greater than that of the Yanshan period but also greater than that of the Yinzhi period in many areas, which created a beneficial condition for the further evolution of PCSR organics.

Materials and methods

The characteristics of PCSR

The primary deposit of the Permo-Carboniferous was approximately 1000 m in thickness, which is divided into six groups from bottom to top, i.e. the Benxi Formation and Taiyuan Formation of the Upper Carboniferous, Shanxi Formation, Lower Shihezi Formation, Upper Shihezi Formation and the Shiqianfeng Formation of the Permian. The coal series stratum is mostly composed of the Benxi Formation, Taiyuan Formation and Shanxi Formation; it is approximately 200–300 m in thickness and is considered a candidate for hydrocarbon source rocks in the Huanghua Depression. The organic micro-component is mainly exposed as vitrinite (Figure 2, Table 1) by organic petrography analyses and primarily contains Type III (gas-prone) humic kerogen, deposited as terrigenous-marine interbedded coal-bearing clastics, which can be further divided into paralic lake-swamp and fluvial facies.

Figure 2.

The chart of organic compositions of the Permo-Carboniferous coal series. (a) – Benxi Formation; (b) – Taiyuan Formation; (c) – Shanxi Formation. E – exinite; I – inertinite; S – sapropel; V – vitrinite.

Table 1.

Maceral contents of Permo-Carboniferous coal.

Formation	Vitrinite (%)	Inertinite (%)	Exinite (%)	Sapropel (%)
Benxi	63.4	32.4	3.7	0.3
Taiyuan	68.2	24.74	3.7	0.3
Shanxi	62.7	33.1	3.3	0.9

A total of 16 samples from the Shanxi Formation have total organic carbon (TOC) values ranging from 0.14 to 4.7%, an average of 2.16%, and average S₂ values of approximately 1.15 mg/g. A total of 23 samples from the Taiyuan Formation have TOC values ranging from 0.18 to 4.98%, an average of 2.5%, and average S₂ values of approximately 2.21 mg/g. A total of 14 samples from the Benxi Formation have TOC values ranging from 0.06 to 4.46%, an average of 1.64%, and average S₂ values of approximately 1.09 mg/g.

The coal series included a coalbed with a thickness of 50–60 m and dark mudstone with a thickness of 100–170 m.

The heating history of PCSR

The generation, migration and accumulation of oil and gas source rocks are closely related to palaeotemperature conditions. The evaluation of thermal histories is of great significance to oil–gas basins. During the control by regional structural and magmatic activity, PCSR experienced different geothermic phases in complicated heating processes since its formation in the Huanghua Depression.

At present, no measurable parameter can be directly converted to palaeotemperature, but we can offer some indirect approaches to reveal the heating history of source rocks, such as apatite fission track (AFT) analysis, fluid inclusion and vitrinite reflectance.

AFT analysis: AFT analysis has been widely used to study the thermal evolution of sedimentary basins (Corrigan, 1991; Gleadow et al., 1986; Green et al., 1989; Kang and Wang, 1991; Naeser and Faul, 1969; Sweeney and Barnham, 1990). Fission track annealing in apatite is primarily controlled by temperature, the chemical composition of the apatite and the duration of heating. Three sandstone cores from three wells (Gg1-1is Permian; Kg3 and Dg1 are Jurassic) were processed for fission track analysis in the study. Apatite was extracted from the cores by using standard heavy liquid and magnetic separation techniques. Samples were tested to obtain track ages and track lengths by using the external detector method. The fission track ages, track lengths and associated errors of the three samples are briefly summarized in Table 2.

Table 2.

Measured apatite fission track data in the Huanghua Depression.

Well no.	Depth (m)	No. crystals	ρ_s	ρ_I	Fission track ages (Ma)	Test track length (µm)	Emendated track length (µm)
Kg3	3297	13	0.42(36)	18.34(1588)	11.7±4.9	6.8±1.4(26)	6.6±1.3(26)
Dg1	1605	9	11.24(830)	25.17(1855)	123.9±12.9	11.9±2(103)	11.5±2.1(103)
Gg1-1	1778	14	10.90(801)	30.20(2220)	87.9±2.2	11±2.1(108)	10±2.2(108)

ρ_s indicates spontaneous track density; ρ_I indicates induced track density. All track densities are 10⁵ cm⁻². The number of tracks counted or measured is shown in parentheses. Uncertainties are quoted at 1σ. Ages are calculated using a zeta of 322.1 ± 3.6 for dosimeter glass CN5 for apatite and a zeta of 325.6 ± 5.9 for dosimeter glass CN1 for zircon, but not determined. The λ_d is 1.55125 × 10⁻¹⁰ a⁻¹, and g is 0.5 in this measurement. The tests were completed in the Institute of Energy Physics, Chinese Academy of Sciences.

The slope correction lengths and ages of AFT among some well samples in the Huanghua Depression exhibit peak shapes as an entirely mixed distribution with most overlapping (Figure 3). Because the length of AFT is decided only by the types and status of rock heating histories (Kang and Wang, 1991), the shape of many peaks overlapping exposes the heating history characters of samples that have experienced multiple thermal events and apatites produced by multiple anneals. By detailed studies of those peaks, the individual contributions can be derived from apatite annealing in different thermal events and shows the heating history of source rocks.

Figure 3.

The distribution maps of fission tracks, anneal lengths and ages in three wells in the Huanghua Depression. (a) Kg3, (b) Dg1, (c) Gg1-1, (d) Kg3, (e) Dg1, and (f) Gg1-1.

The Huanghua Depression experienced a complicated heating history based on the peak study of AFT, regional burial history and fluid inclusion analyses. It went through at least four tectono-thermal events since formation, and three of them occurred in the Middle-Late Yanshan period at 147–135, 100–90 and 76–65 Ma, respectively, which suggested that it was mainly associated with anomalous palaeo-geothermal fields. The fourth is approximately 11.7 Ma, which shows it occurred in the Late Himalayan and resulted from structural-burial affection (Table 3).

Table 3.

The tectonic heating evolution history with AFT.

Well no.	Depth (m)	TTE(I) (J₃–K₁)					TTE(II) (K₂)					TTE(III) (K₂²)					TTE(IV) (N)
Well no.	Depth (m)	T	L_f	dL	t	dt	T	L_f	dL	T	dt	T	L_f	dL	t	dt	T	L_f	dL	t	dt
Kg3	3297 m											65	4.5	72%	98	6.0	11.7	7	57%	110	3.1
Dg1	1605 m	135	11	29%	85	6.0	90	13.5	17%	80	6.9
Dg1	1605 m	147	6.5	60%	100	7.2	90	13.5	17%	80	6.9
Gg1-1	1778 m						110	6.5	60%	97	4.3	70	11.5	29%	80	6.0

AFT: apatite fission track; TTE: tectono-thermal event.

T indicates the age (Ma); L_f indicates the individual emendated track length (µm); dL indicates the annealing ratio; t indicates the heating temperature (°C); and dt indicates the palaeo-temperature degree (°C/100 m), which can be calculated using the ancient buried depth. When computing the annealing ratio of AFT, the induced track length is suggested as 16.3 µm (Kang and Wang, 1991). The tests were finished in the Institute of Mineral Resources Chinese Academy of Geological Sciences.

Fluid inclusion test: Because of fluid inclusion, especially organic fluid inclusion, the original geochemical record was reserved for the occurrence of oil and gas generation and migration, so fluid inclusion analysis as an effective measure is applied to petroleum research more and more extensively. We can get reliable physical and chemical conditions of oil and gas generation and migration, as well as recover the hydrocarbon generation history of source rocks by fluid inclusion analyses. In this study, eight samples were selected for organic fluid inclusion studies on the cores’ calcite arterite of the Palaeozoic in the Huanghua Depression. Fluid inclusion petrography reveals the occurrence of two types of fluid inclusions, and almost all are in a two-phase state (gas–liquid inclusion) except for one single phase state (liquid inclusion). The tested results of fluid inclusion expose the same heating history of PCSR in the Huanghua Depression (Table 4).

Table 4.

The characters of organic fluid inclusion in calcite arterite in the Huanghua Depression.

Well no. (stratum)	Depth (m)	Inclusion size (µm)	Inclusion shape	Ratio of gas/liquid	Phase state	Homogenization temperature (°C)	Other
Qg1(O)	4012	4–7	Anomaly or ellipse	5–10%	Two	126(10)	–
	4101	5–15	Ellipse or anomaly	10–15%	Two	118(4)	Highest ratio of gas/liquid: 30%
	4195	5–15	Rectangle or square	10–15%	Two	213(6)	More
Kg3(O)	3408	10±	Long ellipse	5–15%	Two	167(4)	Litter, but one group is arrayed in orientation
Kg3(O)	3415	2–5	Ellipse	–	Single	–	Poor
Kg4(C)	3811	5–10	Anomaly	5–15%	Two	186(4)	Litter, highest ratio of gas/liquid: 30%
GG1-1(O)	2220	10–15	Ellipse	10–30%	Two	141(2)	Much more
Bg1(O)	2004	10±	Anomaly	5–10%	Two	128(11)	More

For example, 126 (10): Homogeneous temperature (test inclusion number).

EASY%R_o of vitrinite reflectance: Vitrinite reflectance is the most widely used parameter for quantitatively estimating the thermal maturity of sedimentary rock. It has been already proven that the vitrinite reflectance of deposition organics is functionally related to its heating time and heating temperature. The heating time is confirmed as one of the keys to inferring the heating temperature through the vitrinite reflectance of source rocks. Numerous geologists have done extensive research on using the vitrinite reflectance to recover the palaeogeothermics field (Bostick et al., 1978; Wood, 1988).

There are decades of constructing chemistry kinetic models with the vitrinite reflectance, which retains the characteristics of the palaeogeothermics field and heating history of the organic matter, but only several of the models have been spread and applied extensively (Bostick et al., 1978; Hood et al., 1975; Lopatin, 1971; Waples, 1980). However, we suggested that Burnham and Sweeney (1989) brought up and established an inverse method, called EASY%R_o. The method has quickly gained popularity and application in North America, Europe, China, etc. (Qin et al, 1997; Song and Qin, 1998; Sweeney and Burnham, 1990; Yan et al., 2005) and is considered as a kind of valid method that is better and more applicable than other simulation methods (Morrow and Lessler, 1993).

In this study, we collected or tested a large quantity of organic vitrinite reflectivity data, gave a complete systemic analysis for all data and reasonably eliminated the reflectivity mutation caused by local abnormalities (such as point thermal resource). This was done in order to increase the dependability of the analysis and research, as well as combined with district tectonic-burial history research, AFT analysis and fluid inclusion test using EASY%R_o to infer the heating histories and maturity evolution processes of different periods of source rocks in geologic history.

Results

Maturity evolution of PCSR

The heating process of source rocks affects their hydrocarbon generation history, allowing us to reconstruct the heating history and maturity evolution of the Permo-Carboniferous, which is key to exactly appraise their oil and gas resources in the Huanghua Depression.

However, in many cases, factors such as the earth crust settlement, the fold or uplift, and erosion effects will cause the burial conditions of source rocks, as well as their heating temperatures, to be altered. The heating temperature difference of organic matter between ancient and present times leads to their maturity evolutionary inconformity, causing the organic hydrocarbon generation evolution to change, thus affecting the evaluation, exploration and prediction for oil and gas resources.

To study the maturity evolution of the entire Huanghua Depression, we have anatomized dozens of wells, which must be drilled to the depth of PCSR in the Huanghua Depression. Based on detailed research of the tectonic-burial history, combined with fission track testing and fluid inclusion analyses, the organic maturity of PCSR in the Huanghua Depression was deduced by using the EASY%R_o numerical simulation. For example, Table 5 is the result of using the EASY%R_o numerical simulation in the Kg4 well (Table 5, Figure 4).

Table 5.

The EASY%R_o numerical simulation result of PCSR in the Kg4 well in the Huanghua Depression (testing at the bottom of the Taiyuan Formation: R_o = 0.92%).

Stage of structural	Heating time (Ma)	Temperature (°C)	%R_o	Hydrocarbon generation
Indo-Chinese Epoch	0.00	15.00	0.22	–
	21.30	20.80	0.28
	51.30	26.80	0.30
	66.30	47.85	0.34
	81.30	86.10	0.51	1st
Yanshan Epoch	103.30	32.30	0.52	–
	154.30	52.20	0.52	–
	214.30	112.60	0.74	2nd
Early Himalayan Epoch	246.30	42.90	0.76	–
	265.80	71.50	0.76
	275.30	87.00	0.76
Late Himalayan Epoch	288.00	81.70	0.76	3rd
	308.46	132.30	0.86
	311.30	136.50	0.92

PCSR: Permo-Carboniferous source rock.

Figure 4.

The burial history curve of the Huanghua Depression.

The results show that the highest heating temperatures of PCSR increased by zigzag motions from the Indo-Chinese Epoch to Himalayan Epoch, leading to the organic maturity being enlarged in a stepwise manner (Figure 5). During the process, three large staged increments of organic maturity existed in the PCSR of the Huanghua Depression (Figure 5).

Figure 5.

The evolutional curves of maturity of the Taiyuan Group in the Huanghua Depression (one curve indicates one well, and the total number is 20).

The first stage occurred in the Indo-Chinese Epoch, and the PCSRs were buried for the first time. At that time, the structural activity was faint in the Huabei Platform, North China, where the crust was mainly represented by up and down movements, and fluid inclusion analyses indicated that the palaeo-geothermal field was normal (approximately 3.0–3.3°C/100 m). The maturities were uninterruptedly increased and were mainly controlled by Early-Middle Triassic deposition. The study shows the thickness of the Triassic is between 900 and 1400 m in the Huanghua Depression, so the greatest buried depth of PCSR did not reach 2500 m. Until the end of the stage, the vitrinite reflectivity of PCSR did not exceed 0.7%R_O, so the organic matter belonged to an early stage of maturity in the Huanghua Depression (Figure 6A).

Figure 6.

The maturity evolution of the PCSR in the Huanghua Depression. A – Indo-Chinese Epoch; B – Yanshan Epoch; C – Himalayan Epoch. 1 – well; 2 – the boundary of the Huanghua Depression; 3 – the vitrinite reflectance of PCSR (%); 4 – place name.

The second stage occurred in the Yanshan Epoch, where the organic matter of the coal series showed a second maturity evolution. The tectonic control on extension was probably the subduction rollback of the Pacific Plate relative to the eastern margin of Asia, while the style of Late Mesozoic tectonics along the eastern Asian margin was related to the velocity of the Pacific Plate relative to the Asian Plate (Watson et al., 1987). As a result, magmatic activity occurred extensively in the research region, the palaeo-geothermal field was an obvious abnormity, the geothermal gradient was as high as 6–7°C/100 m (Sun et al., 2006) and the maturities of PCSR were advanced. In spite of the relatively high temperature regime, the buried depth of PCSR was comparatively shallower, and the maturities had been finitely enhanced, while the maximum was not greater than 1.1%R_O. Because of the very inhomogeneous buried depths, the maturities were exposed in different stages, and the whole tendency shows a higher maturity in the southern than the northern Huanghua Depression. The maturity maximum was greater than 1.1%R_o at the Nanpi sub-depression (Figure 6B).

The third stage occurred in the Himalayan Epoch, where the Huanghua Depression experienced Early Tertiary rifting and a Late Tertiary–Quaternary thermal sedimentation period. During the rifting period, PCSRs were again buried by the lacustrine deposition of the Upper Tertiary within some basalt sedimentary eruption in the Huanghua Depression, so that the palaeo-geothermal gradient was slightly higher at approximately 3.5°C/100 m (Qi et al., 1995). After shortage uplift, there were deposits of the Late Tertiary–Quaternary, and PCSRs were greatly increasingly buried, particularly in the northern part of the Huanghua Depression, and the organic matter maturity obviously increased (e.g. the area with a maturity over 1.5%R_o at the middle area in Qiku sub-depression). The maturity distribution was higher at the northern and lower at the southern Huanghua Depression (Figure 6C).

The hydrocarbon generation evolution of PCSR

By the control of tectonism, the twice or triple hydrocarbon generations of PCSR were worked out in the Huanghua Depression (Table 5), which was proven with fluid inclusion analysis (Table 6).

Table 6.

The temperature measurement results of fluid inclusion in the calcite arterite of two wells.

Well no. (depth, m)	Homogenization temperature (°C)	Freezing point (°C)	Period	Fluid system of diagenism	Salinity (wt%, NaCl eq)
Kg33405–3571	67–75(4)138–148.5(7)	−3.0 to −3.9(2)−0.2 to −0.9(4)	1st2nd	CaCl₂–H₂ONaCl–KCl–CaCl₂–H₂O	7.2–12.90.4–3.5
Qg14102	95–103(8)138–143(5)	−3.8 to −4.2(6)−2.9 to −3.2(5)	1st2nd	CaCl₂–H₂ONaCl–CaCl₂–H₂O	5.0–6.72.7–6.7

wt% indicates the salinity content in percentage; eq indicates the salts other than sodium chloride (NaCl). The periods are identified from the relation analysis for cutting with two calcite arterites in hand specimens.

To calculate hydrocarbon, the generation quantity of Type III (gas-prone) humic kerogen, we conducted systematic experiments for the hydrocarbon generation evolution of organic macerals in humic coal and prepared the measuring map of the hydrocarbon regenerated amount and maturation of Type III kerogen in organic matter (Figure 7).

Figure 7.

The measuring map of the hydrocarbon regenerated amount and maturation of Type III kerogen in organic matter (mg/g is the relative quantity of hydrocarbon regeneration/organic matter of PCSR).

The first hydrocarbon generation of PCSR occurred in the Indo-Chinese Epoch, where organic matter maturity was not greater than 0.65%R_o, while the hydrocarbon generation was minimal and no more than 16 mg/g (Figure 8A).

Figure 8.

The relative quantity of hydrocarbon regeneration of PCSR in the Huanghua Depression. A – Indo-Chinese Epoch; B – Yanshan Epoch; C – Himalayan Epoch. 1 – well; 2 – the boundary of the Huanghua Depression; 3 – the relative quantity of hydrocarbon regeneration/organic matter of PCSR (mg/g); 4 – place name.

After the hydrocarbon generation of PCSR, the Huanghua Depression underwent complex tectonic movements, so that the generated oil and gas was unable to remain but is insignificant for exploration today. However, the maturity step of PCSR will have more intensive influence following hydrocarbon regeneration.

During the Middle-Late Yanshan period, controlled by a tectono-thermal abnormity in the region, the heating temperature of PCSR was heightened, and the organic matter maturity was advanced, leading to hydrocarbon regeneration. However, equilibrium was not reached. The hydrocarbon generation mainly occurred in the southern part of the Huanghua Depression, e.g. that of the Nanpi sub-depression could arrive at 40 mg/g (Figure 8B). The formed oil and gas came through the Himalayan movement and a trap was impossibly formed.

In the Cenozoic, the Huanghua Depression experienced Early Tertiary rifting and a Late Tertiary–Quaternary thermal sedimentation period. During the rifting period, the Huanghua Depression was represented as a contiguous half graben, controlled by the Cangdong Fault and further subdivided into many sub-depressions and bulges by basement faults. As a result, the buried depths of PCSR were very different in space. In spite of the slightly higher thermal gradient, the heating temperatures of PCSR were too low to form hydrocarbon regeneration in most areas.

Entering the Late Tertiary, the rifting in the Huanghua Depression gradually died out and turned into regional thermal sedimentation. The PCSRs were further buried, and heating temperatures increased so much that hydrocarbon regeneration occurred in an extensive region, particularly at the middle-northern part of the Huanghua Depression. For example, the quantity of hydrocarbon generation was more than 100 mg/g in the Qiku sub-depression (Figure 6C).

Both AFT analysis and fluid inclusion testing show that the hydrocarbon generation occurred in the Late Himalayan Epoch (Figure 5, Table 6). AFT analysis of Kg3 indicated it occurred 11.7 Ma, and the age testing for authigenic illite shows it at approximately 24.3 Ma in oil, and its origin is suggested to come from the Palaeozoic.

Because the large quantity hydrocarbon generation was formed later, and most maturity evolved in the oil window, we will suggest exploration in the Permo-Carboniferous of the Huanghua Depression with enhanced investigation of the petrolatum geology of PCSR.

Conclusions

Based on the detailed analyses of the burial history, heating history and hydrocarbon generation history of PCSR in the Huanghua Depression, the following conclusions have been achieved in this study:

Investigation shows that the coal series of the Permo-Carboniferous in the Huanghua Depression underwent a complex tectonic-burial history. Three buried processes are distinguished, i.e. the Hercynian-Indo-Chinese Epoch, Yanshan Epoch and Late Himalayan Epoch. The maximum burial depth of PCSR occurred in the Late Himalayan Epoch, which was of significance to the hydrocarbon generation of PCSR.

Combining AFT, fluid inclusion and EASY%R_o numerical simulation, the heating temperatures of PCSR remained highly disconnected, leading to the maturity increasing in steps. The maturity evolutions of PCSR have been divided into three phases, belonging to the Indo-Chinese Epoch, Yanshan Epoch and Himalayan Epoch, respectively, and the Himalayan Epoch was subdivided into the early period and late period. The maximum heating temperature of PCSR was determined to be in the Late Himalayan Epoch at most areas, particularly at the middle-northern part of the Huanghua Depression.

By control of tectonism, double or triple hydrocarbon generations of the PCSR were worked out in the Huanghua Depression, occurring in the Indo-Chinese Epoch, Yanshan Epoch and Late Himalayan Epoch. The most important hydrocarbon generation of PCSR occurred in the Late Himalayan Epoch. The largest quantity of hydrocarbon generation occurred later, and most maturity evolved in the oil window. This paper recommends increased exploration in the Permo-Carboniferous of the Huanghua Depression with enhanced investigation of the petrolatum geology of PCSR, particularly at the middle-northern part of the Huanghua Depression, such as in the Qiku sub-depression.

Footnotes

Acknowledgements

Grateful acknowledgement is made to the Dagang Oil Company and the individuals who contributed cores and data for this study. The authors wish to thank W. S. Chen for helping with the fluid inclusion studies and S. C. Wang for helping with the fission track studies.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This paper is supported by the ‘Fundamental Research Funds for the Central Universities’ programme (No. 2017CXNL03).

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