Identification and hydrocarbon expulsion history simulation of the effective source rocks in the Dongying Formation, Paleogene, Bohai Sea area, eastern China

 

Identificación y simulación de la historia de expulsión de hidrocarburos de la roca fuente efectiva en la Formación Dongying del Paleógeno, región del Mar de Bohai, este de China

 

Fujie Jiang^1,2*, Xiongqi Pang^1,2, Jigang Guo^1,2, Xinhuai Zhou³, Xiaohui Zhou⁴, Dandan Liu³, and Pengwei Wang^1,2

 

¹ State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum, Beijing 102249, China. * jiangfjwl@163.com

² Basin and Reservoir Research Center, China University of Petroleum, Beijing 102249, China.

³ Tianjin Branch of CNOOC China Limited, Tianjin 300452, China.

⁴ Sinopec Geophysical Research Institute, Nanjing in Jiangsu Province 210014, China.

 

Manuscript received: November 27, 2012
Corrected manuscript received: March, 7, 2013
Manuscript accepted: March 13, 2013

 

ABSTRACT

This paper discusses a new method for identification and simulation of the hydrocarbon expulsion history of Effective Source Rocks (ESR) in the Dongying Formation (Ed), Bohai sea area, Bohai Bay basin, eastern China. This new method enables us to reliably identify the distribution and quantitatively determine the hydrocarbon expulsion history of ESR in petroliferous basins. ESR are the material basis for hydrocarbon accumulation, having important implications for oil and gas exploration prospects. The Bohai Bay basin is one of the most petroliferous basins in China, with nearly one third of the total oil production of the country. However, insufficient research on the ESR in the Dongying Formation (Ed), Bohai sea area, Bohai Bay basin obstructs further exploration. In this paper, ESR of Ed are identified with the new "two-stage and three-step" method, and their planar distribution is predicted by combining well data with sedimentary facies, and structure distribution. On that basis, the hydrocarbon expulsion history of ESR in the 2^nd Member (E₃d₂) and 3^rd Member (E₃d₃) of Ed was systematically simulated by using the hydrocarbon generation potential method. The results show that the ESR of E₃d₂ are distributed in four sags of Bohai Sea area, while the ESR of E₃d₃ occur in seven sags. Hydrocarbon expulsion from the ESR of E₃d₂ began during the deposition period of the Minghuazhen Formation Nm) (12 Ma) with a hydrocarbon expulsion quantity of 42 x10⁸ t. The hydrocarbon expulsion centers were located in the Bozhong, Huanghekou, Qikou and Bodong sags. Hydrocarbon expulsion from the ESR of E₃d₃ began around 16.6 Ma ago (hydrocarbon expulsion quantity of 84x10⁸ t), and the hydrocarbon expulsion centers were located in the Bozhong, Qinnan, Huanghekou, Liaozhong, Nanpu and Qikou sags. In conclusion, the hydrocarbon expulsion centers of the ESR of E₃d are mainly located in Bozhong, Huanghekou, Qikou and Bodong sags, with late-stage hydrocarbon expulsion (16.6-2 Ma), which is advantageous to hydrocarbon accumulation and preservation. The uplifts and slope belts around the hydrocarbon expulsion centers would be favorable exploration targets.

Key words: effective source rocks (ESR), hydrocarbon expulsion threshold, Dongying Formation, Paleogene, Bohai Sea, Bohai Bay basin, China.

 

RESUMEN

En este artículo se discute un nuevo método para la identificación y simulación de la historia de expulsión de hidrocarburos de la Roca Madre Efectiva (RME) en la Formación Dongying (E₃d), área del Mar de Bohai, cuenca de la Bahía de Bohai, este de China. Este nuevo método permite identificar de manera confiable la distribución y determinar cuantitativamente la historia de expulsión de hidrocarburos de las RME en cuencas petrolíferas. Las RME son la base material para la acumulación de hidrocarburos y tienen implicaciones importantes para prospectos de exploración de petróleo y gas. La cuenca de la Bahía de Bohai es una de las más ricas en petróleo de China, con cerca de un tercio de la producción total de petróleo en el país. Sin embargo, la investigación de las RME de la Formación Dongying (E3d) en la cuenca de la Bahía de Bohai es aún insuficiente, lo que ha obstruido el avance de la exploración. En este trabajo, se indentifican las RME de E₃d con el nuevo método de "dos etapas y tres pasos", y su distribución planar se predice por medio de la combinación de datos de pozos con facies sedimentarias y distribución de estructuras. Con base en lo anterior se simuló sistemáticamente la historia de expulsión de hidrocarburos de las RME en el 2°Miembro (E₃d₂) y el 3er Miembro (E₃d₃) de E₃dpor medio de la aplicación del método de potencial de generación de hidrocarburos. Los resultados muestran que las RME de E₃d₂ están distribuidas en cuatro depresiones ("sags") del área del Mar de Bohai, mientras que las RME de E3d3 se presentan en siete depresiones. La expulsión de hidrocarburos de las RME de E3d2 inició en el periodo de depósito de la Formación Minghuazhen (N₁m) (12 Ma) con la expulsión de 42x 10⁸ t. Los centros de expulsión se localizan en las depresiones de Bozhong, Huanghekou, Qikou y Bodong. La expulsión de hidrocarburos de las RME de E₃d₃ inició a ~16.6 Ma con la expulsión de 84x10⁸ t, y los centros de expulsión se localizaron en las depresiones Bozhong, Qinnan, Huanghekou, Liaozhong, Nanpu y Qikou. En conclusión, los centros de expulsión de hidrocarburos de las RME de E3d se localizan principalmente en las depresiones Bozhong, Huanghekou, Qikou y Bodong, con expulsión de hidrocarburos tardía (16.6-2 Ma), lo cual es ventajoso para la acumulación y preservación de hidrocarburos. Los levantamientos ("uplifts") y cinturones de talud que rodean a los centros de expulsión de hidrocarburos serían blancos de exploración favorables.

Key words: roca madre efectiva, umbral de expulsión de hidrocarburos, Formación, Dongying, Paleógeno, Mar de Bohai, cuenca de la Bahía de Bohai, China.

 

INTRODUCTION

The study of effective source rocks (ESR) relates to the accuracy of estimates of hydrocarbon resources and directly affects the exploration prospects in petroliferous basins. Tissot and Welte (1978) defined source rocks as rocks that already generated or have the ability to generate petroleum. Momper (1978) proposed the concept of ESR, defined as the rocks that could generate and expel commercial petroleum. Since then, many definitions of these rocks have been proposed, which can could be divided into two major types: (1) source rocks that have already generated and expelled hydrocarbons (Sheng, 1989; Wang et al., 2003), and (2) source rocks that already expelled hydrocarbons and also have formed commercial hydrocarbon reservoirs via oil-source correlation (Hunt, 1979; Jones, 1981; Li et al., 1999; Jin, 2001; Jiang et al., 2002; Rao et al., 2003). Due to different cognition and research methods (Fu et al., 1983; England, 1987; Lewan and Williams, 1987; Leythaeuser, 1988; Chen et al., 2004; Chen, 2005), certain differences also occur in the definitions of ESR. However, generating and expelling hydrocarbon is the precondition of ESR, and whether they can form commercial hydrocarbon reservoirs needs to be verified by exploration, which is difficult as it is affected by exploration progress and late-stage preservation. In addition, in the situation of the coexistence of multiple source rocks, although the amount of hydrocarbon expelled from some source rocks can be relatively small and is lost during secondary migration, this process can effectively avoid loss of hydrocarbon expelled from other source rocks, so those source rocks also make a contribution to hydrocarbon accumulation and can been considered to be ESR as well.

The Bohai Bay basin is one of the most petroliferous basins in China, with an exploration area of approximately 4.2x10⁴ km² (Li, 1980), accounting for nearly one third of the total oil production of China. Oil and gas exploration in the Bohai sea area began in 1966; seven oil and gas fields have been successively confirmed in this area, including PL19-3, SZ36-1, among others. So far, 48 oil and gas fields and 100 hydrocarbon bearing structures have been discovered in the Bohai Sea area with proven OOIP (original oil in place) of about 24x10⁸ m³ and OGIP (original gas in place) of about 500x10⁸ m³. With its cumulative production exceeding 50x10⁶ t oil equivalent (Zuo et al., 2010), the Bohai Sea area is still the main oil and gas exploration target in eastern China. Currently discovered oil and gas fields show an unbalanced geological distribution, with shallow buried fields being more abundant than deep buried ones, and fields are locally distributed. The main reason is the complex geological conditions and inadequate research on source rocks, especially the research of ESR, which handicaps further exploration. A new method for identification and hydrocarbon expulsion history simulation of ESR in E₃d, Bohai Sea area is discussed in this paper. This new method enables us to reliably recognize the distribution and quantitatively determine the hydrocarbon expulsion history of ESR in petroliferous basins.

 

GEOLOGICAL SETTING

Geographic location and tectonic framework

The Bohai Sea area is located between the Tanlu and Cangdong faults, belongs to the seaward extension part of four primary structural units, including Liaohe and Huanghua depressions, Chengning uplift and Jiyang depression. The Bohai Sea area contains mainly 15 sags and 16 uplifts (Figure 1, Zhang, 2000; Tang et al., 2011).

Affected by Caledonian, Indosinian and Yanshanian movements, the Bohai Sea area was uplifted and underwent weathering and denudation over a long geological time. During the Cenozoic sedimentation period, influenced by Himalayan movement, the structural evolution of hydrocarbon-bearing areas in Bohai is characterized by polyphase rifting, multicycle superimposition and polygenous mechanism, and is divided into a Paleogene rifting stage and a Neogene—Quaternary post-rifting thermal subsidence stage (Hou et al., 2000; Gong and Wang, 2001; Gong, 2004; Sun et al., 2008). The rifting stage can be further divided into four evolution phases: I, period from the Kongdian Formation (E₁k) to the 4^th Member of the Shahejie Formation (E₂s₄) (55-42 Ma); II, period of the 3^rd Member of the Shahejie Formation (E₂s₃) (42-38 Ma); III, period of the 1^st and 2^nd Member of the Shahejie Formation (E₂s₁₊₂) (38-32.8 Ma); and IV, period of Dongying Formation (E₃d) (32.8-24.6 Ma). The post-rifting thermal subsidence stage can be further divided into phase I corresponding to the sedimentation period from Guantao Formation (N₁g) to the 2^rd Member of the Minghuazhen Formation (N₁m₂) (16.6-5.1 Ma), and phase II from the 1^st Member of the Minghuazhen Formation (N₂m₁) to the Quaternary (5.1-0 Ma) (Figure 2; Gong, 2004).

Correlated with the tectonic evolution, the sedimentation and subsidence center of the Bohai Bay basin migrated from land to sea, forming a structural framework of land depressions intersecting in the sea area. The thickness of sediment increased gradually from the Paleogene to the Neogene in the sea area (Xu et al., 2008).

Sedimentary strata

The sedimentary formations in the Bohai sea area mainly include Paleogene, Neogene and Quaternary sediments from bottom to top (Figure 2; Gong, 2004). Rift valley sedimentation is preserved in the Paleogene strata, which indicate that lake basins with lacustrine sedimentation were well developed. In the Neogene, lakes basically disappeared, and deposition was characterized by fluvial plain and shore-shallow lake environments. Quaternary strata are mainly marine deposits. According to lithologic features, the whole Cenozoic basin-filling sequence can be divided into six formations from the bottom up (Figure 2, Gong, 2004): Kongdian(E₁k), Shahejie (E₂s) and Dongying formations (E₃d) of Paleogene age, Guantao (N₁g) and Minghuazhen formations (N₁+₂m) of Neogene age, and the Pingyuan Formation (Qp) of Quaternary age (Figure 2, Gong, 2004).

Introduction to oil and gas geology, and characteristics of source rocks

Reservoirs in the Bohai Sea area are mainly distributed around Bozhong, Huanghekou and Liaozhong sags, and the biggest offshore reservoir PL19-3 is situated at the Bonan lower uplift between the Bozhong and Huanghekou sags (Figure 1). Industrial oil flows have been found in Shahejie, Dongying, Guantao and Minghuazhen formations, and they are mainly in the 1^st Member of the Dongying Formation and in the Guantao Formation, where the discovered oil and gas reserve is roughly 50% of total discovered reserve (Zuo et al., 2010). The distribution of main source rocks, reservoirs and caprocks in the Bohai Sea area is shown in Figure 2.

Four sets of source rocks are developed in the Bohai Sea area: the 3^rd Member of the Shahejie Formation (E₂s₃), the 1^st and 2^nd Members of the Shahejie Formation (E₂s₁+₂), the 3^rd Member of Dongying Formation (E₃d₃), and the 2^nd Member of Dongying Formation (E₃d₂) (Figure 2, Gong, 2004). So far, due to its deep burial depth, 58 wells have been drilled in the source rocks of E₂s₃, and the bottom boundary is deepest in the Bozhong sag (8000 m) and it exceeds 3000 m in most of the sea area.

The source rocks of E₂s₁+₂ are extensively distributed, but their thickness is rather low with an average thickness less than 200 m. The lithology of the source rocks of E₂s₁+₂ is mainly dark mudstone deposits, and their maximum burial depth can reach 6000 m in the Bozhong sag.

The dark mudstones of E₃d have a large thickness and a wide distribution range. The sedimentary facies include fan delta, braided river delta, sublacustrine fan, and lake (Xu, 1981). Lacustrine deposit systems are widely distributed and two main subsidence centers formed in the Bozhong and Liaozhong sags (Deng et al., 2011). The lithology is thick dark gray-blackish mudstone interbedded with thin sandstone. Deep and semi-deep lake deposits were developed in the deep depressed part of all sags, and shore-shallow lake deposits were distributed in the slope part of lake basins. The maximum strata thickness is 1080 m, with an average of about 600 m. The maximum burial depth is 5500 m in the Bozhong sag. The sedimentary facies of E₃d₂ correspond to fan delta, braided river delta, meandering river delta, reticular river delta, sublacustrine fan, and lake. Semi-deep to deep lake deposits developed in the deep depressed part of sags. The lithology assemblage contains mainly thick-bedded, grayish-black and black mudstone interbedded with thin sandstone; the subsidence center was located in the Bozhong sag. The maximum strata thickness is 1650 m with an average thickness of about 800 m, and on the whole, the burial depth is around 1000 to 4500 m.

Many studies of source rocks of the Shahejie Formation have been conducted (Chi, 2001; Hu et al., 2001; Wang and Zhang, 2003; Jiang et al., 2010a), while systematic and further research on the source rocks of E₃d is rare (Li et al., 2001; Huang and Li, 2002). With recent exploration practices, oil and gas discoveries in E₃d has increased greatly (Wang et al., 2006). Therefore, the comprehensive study of the effectiveness and hydrocarbon supply capacity of the source rocks in E₃d and their variation in different sags is of great geologic significance for furthering the recognition of oil and gas sources and expanding the exploration extent in the Bohai Sea area. Especially, with the development of seismic exploration in recent years, plenty of geologic and geochemical data of source rocks have been obtained, enabling systematic evaluation of source rocks. Aiming at the existing problems in exploration, combining with the new recognition of the regional structural evolution history and sedimentary facies distribution, by systematically analyzing drilling data and geochemical data, in this paper the ESR of E₃d are identified and their hydrocarbon expulsion history is outlined. The study provides reference information on oil and gas resource potential to guide exploration directions in the Bohai Sea area.

 

METHODS

Determination of ESR

Many methods for determining ESR have been proposed, including identification through simulation experiments, identification based on a lower limit of organic carbon content, and identification using analysis of the hydrocarbon content in closely sampled core samples (Jiang et al., 2010b). However, the applications of these methods are limited by core availability, experimental analysis cost, sample amount and experiment conditions, which leads to poor maneuverability and lopsided results. This paper discusses a new method for identification of the ESR in E₃d, named the "two-stage and three-step method" (Jiang et al., 2010b). This new method enables us to reliably determine distribution of ESR in petroliferous basins. The process of this method is as follows:

Stage 1: Geological statistics

Step 1. Collect thickness data for dark mudstones from wells in the study area, set up a database of dark mudstone thickness (Figure 3a), then extrapolate to the areas with less o no wells, on the basis of sedimentary facies, seismic facies data, etc., and finally, obtain the planar distribution of the dark mudstone in the study area (Zhang et al., 2007).

Stage 2: Geological analysis

On the basis of stage 1 results, determine the thickness of ESR in bulk dark mudstone by means of geochemical analysis. Stage 2 can be further divided into two steps: step 2 and step 3.

Step 2. Apply the hydrocarbon expulsion threshold theory (Pang, 1995) to identify the hydrocarbon expulsion threshold of source rocks and eliminate the dark mudstones that have not crossed the hydrocarbon expulsion threshold.

Basic approach. The hydrocarbon expulsion threshold refers to the critical geologic condition at which the amount of hydrocarbon generated satisfies the residual needs for hydrocarbon in various processes, including self-adsorption, interstitial water dissolution, oil dissolution (gas), capillary blockage, etc. Consequently, lots of oil and gas can be expelled and can migrate in separate phases during the burial process (Pang, 1995). It is obvious that only dark mudstones that had crossed the hydrocarbon expulsion threshold could possibly expel hydrocarbons, and the hydrocarbon generation potential method can be used for discrimination (Zhou and Pang, 2002; Jiang et al., 2007). The method is theoretically based on the principle of conservation of matter, namely that the amount of organic matter in the source rocks remains constant before and after hydrocarbon generation and migration. A decrease in the amount of the organic matter can occur as result of substance exchange between the source rocks and the surroundings. In terms of Rock-Eval pyrolysis, S₁ represents free volatile hydrocarbons thermally flushed from a rock sample at 300 °C, S₂ represents products of cracking during standard Rock-Eval pyrolysis temperatures (300-600 °C), and the sum (S₁+S₂) is generally used to signify the content of hydrocarbon in source rocks (Espitalié et al., 1977; Tissot and Welte,1978,1984). Thus, the (S₁+S₂)/total organic content (TOC) ratio could be used to delineate the potential of source rocks to generate hydrocarbons. In other words, it could reflect volumes of hydrocarbons generated by per unit of TOC in the source rocks. Zhou and Pang (2002) called the (S₁+S₂)/TOC ratio as Hydrocarbon Generation Potential Index. Hydrocarbon expulsion begins (Figure 3b) when the index starts to decrease and the critical geologic condition is defined as the hydrocarbon expulsion threshold. Those dark mudstones that have not crossed the hydrocarbon expulsion threshold could be reasonably eliminated after determining the hydrocarbon expulsion threshold.

Different types of organic matter in source rocks will result in differences in hydrocarbon generation potential and hydrocarbon expulsion (Pang, 1995). The organic matter types of source rocks in the study area can be classified into three groups: Type I kerogen is mainly located in the center of the sag, Type III is distributed in the edge of the sag, and type II is located between them (Li et al. ,2001; Chi, 2001; Wang et al. ,2004). Thus, in this paper, three hydrocarbon expulsion geological models are established. The models arebased on the type of kerogen (Figure 4), using a database of 1234 Rock-Eval pyrolysis analysis of samples from the Dongying Formation, Bohai Sea area (Table 1).

Step 3. Determine the ESR that have crossed the hydrocarbon expulsion threshold using the hydrocarbon expulsion threshold chart.

Basic approach. The abundance of organic matter in source rocks is strongly heterogeneous. Therefore, the bulk dark mudstone which has crossed the hydrocarbon expulsion threshold may still contain some parts that cannot expel hydrocarbons due to poor quality. These kinds of dark mudstones can be calibrated using a hydrocarbon expulsion threshold identification chart. The effective thickness of source rocks can be determined by using the correlation chart of organic matter abundance, kerogen type and thermal evolution degree, which can get rid of the dark mudstone that cannot expel hydrocarbon due to their poor quality (Figure 3c). Data calibration and mapping are performed to get the planar thickness distribution of ESR by using the ratio of the obtained ESR to the formation thickness in wells. Hydrocarbon Source Rock Index (SRI) charts are used in this paper (Pang, 2003). The SRI represents the percentage of the actually expelled hydrocarbon compared to the amount of hydrocarbon that would be expelled from optimal source rocks. Source rocks of the semi lacustrine to deep lacustrine facies of the Qing Shankou Formation, deposited in Songliao Basin, are considered to be the best ones in the China eastern continental sedimentary basin because they have the highest organic matter abundance and high to very high organic matter maturity. Therefore, source rocks of Qing Shankou Formation were utilized as a standard in the evaluation of source rocks, being termed as the optimal source rocks. The equation to calculate the SRI is as follows:

Where Q_p and Q_rm represent the hydrocarbon generation amount of source rocks and the critical saturation amount of residual hydrocarbons, respectively; Q_em represents the amount of hydrocarbon expulsion from optimal source rocks.

Values of SRI<0 mean that the amount of hydrocarbons generated in rocks is less than the critical saturation amount of residual hydrocarbons, and thus the rocks are non-source rocks; on the contrary, rocks are considered source rocks when SRI >0. The larger the SRI, the better the quality of source rocks (Figure 3c). A statistical analysis of numerous geologic data undertaken by Pang (1995) shows that the hydrocarbon generation, retention and expulsion amount is determined by TOC, vitrinite reflectance (Ro) and source rock type. Based on this recognition, Pang (1995) established the relation chart of SRI versus the above three parameters to distinguish source rocks from non-source rocks.

Method for hydrocarbon expulsion history simulation

The hydrocarbon generation potential method (Zhou and Pang, 2002; Pang et al., 2005) is used for the reconstruction of hydrocarbon generation and expulsion processes. When no hydrocarbon has been expelled, the hydrocarbon potential of source rocks is known as initial hydrocarbon generation potential index (HCI_o); when hydrocarbon expulsion begin, the hydrocarbon potential decreases gradually and the hydrocarbon potential can be known as residual hydrocarbon generation potential index (HCI_p). The differentials between the maximum hydrocarbon generation potential index and the residual hydrocarbon generation potential index is the hydrocarbon expulsion ratio q_e, which is the hydrocarbon amount expelled per unit of organic carbon, since source rocks crossed the hydrocarbon expulsion threshold, in mg/g. Q e represents the cumulative hydrocarbon expulsion amount of source rocks in geological history, Q r represents the residual hydrocarbon amount in source rocks (unit: g), and Q_p is the hydrocarbon generation amount of source rocks (unit: g) (Pang et al., 2005, Figure 5).

The hydrocarbon generation potential index represents the hydrocarbon expulsion ratio of source rocks, and the profile area represents the cumulative hydrocarbon expulsion amount of source rocks in geological history. Based on this principle, the equatuions for calculating the hydrocarbon expulsion intensity and hydrocarbon expulsion amount are as follows:

Where E_hc represents the hydrocarbon expulsion intensity in t/km²; q_e(z) represents the hydrocarbon expulsion ratio per unit of organic carbon in mg/g; Q_e represents the hydrocarbon expulsion amount in t; Z represents the burial depth in m; Z₀ represents the hydrocarbon expulsion threshold; p(z) represents the density of source rocks in g/ cm³; TOC represents the organic carbon mass fraction in wt%; H represents the thickness of source rocks in m; S(n) represents the area of source rocks in m².

The hydrocarbon potential evolution of source rocks in each geological epoch can be simulated by combining the hydrocarbon generation potential index of source rocks with the burial history results. Combined with organic matter abundance, organic matter type, as well as evolution degree, the hydrocarbon expulsion process of source rocks in each geological epoch can be simulated.

 

RESULTS

Distribution of ESR in the E₃d in the Bohai Sea area

Distribution of dark mudstone and source rocks in E₃d

So far, 165 wells have been drilled in the E₃d₂ and 36 wells have been drilled in the E₃d₃ of the Bohai Sea area. The results show that the dark mudstone thickness of E₃d₂ ranges from 2 to 814 m, averaging 180 m, and that the thickness of E₃d₃ ranges from 3.5 to 496 m with a mean thickness of 143 m. Considering that these unevenly drilled wells are mainly located on uplifts, the relationship between the dark mudstone thickness and sedimentary facies is obtained by evaluating the ratio of the dark mudstone thickness to the formation thickness in wells and sedimentary facies (Figure 6). For the area of the dark mudstone with less wells, thickness can be predicted from sedimentary facies and strata thickness. Finally, the map view distribution of the dark mudstone of E₃d can be derived (Figure 7).

The results suggest that the dark mudstone in E₃d₂ is widely distributed throughout the area. The maximum thickness is 700 m in Bozhong sag, much thicker than that of 500 m in Liaozhong sag, 400 m in Qikou sag and about 400 m in Huanghekou sag (Figure 7a). Compared with that in E₃d₂, the distribution of dark mudstone in E₃d₃ is relatively limited in some regions. It is distributed mainly in Bozhong and Liaozhong sags, with maximum thicknesses of 700 m and 600 m respectively. The thickness of E₃d₃ dark mudstone is relatively small or only appears in deep depressions of other sags (Figure 7b). As a whole, although the dark mudstone thickness in E₃d is large, the percentage of ESR, which is of great importance for the further exploration, remains unclear.

Determination whether the dark mudstone has crossed the hydrocarbon expulsion threshold. According to the hydrocarbon expulsion threshold depth (Figure 4), the dark mudstone that has not crossed the hydrocarbon expulsion threshold can be directly eliminated, thus the dark mudstone thickness which has crossed the hydrocarbon expulsion threshold can be determined.

Determination of the dark mudstone that cannot expel hydrocarbon using the hydrocarbon expulsion threshold chart. Typical wells with sufficient geochemical data and lithologic profiles were selected for statistical analysis, and actual hydrocarbon expulsion determination charts of typical wells in different strata were established (Figure 8) to discriminate whether dark mudstones have crossed the hydrocarbon expulsion threshold. In addition, combining the results of the dark mudstone that have not crossed the hydrocarbon expulsion threshold as described above, a comprehensive data table (Table 2) of the percentage of ESR in the bulk dark mudstones in each stratum can be obtained. Based on the distribution of dark mudstone, the distribution characteristics of the ESR in E₃d can be determined in map view (Figure 9).

The ESR in E₃d₂ is distributed in Bozhong, Huanghekou and Qikou sags (Figure 9a). In other sags, there is no ESR in E₃d₂ due to the relatively shallow burial depth in most cases. In addition, although have crossed the hydrocarbon expulsion threshold, a part of source rocks still cannot expel hydrocarbon due to the relatively low organic matter abundance and low evolution degree, which limits the distribution of ESR. The ESR thickness in E₃d₂ has a maximum of 500 m in Bozhong sag and 300 m in Huanghekou sag. Although the maximum ESR thickness in Qikou sag is 400 m, it has a relatively limited distribution. The distribution of the ESR in E₃d₃ is relatively wider in Bozhong, Huanghekou, Liaozhong and Qikou sags. The maximum thickness is 600 m in Bozhong sag and 400 m in the north of Liaozhong sag.

The maximum thickness is 300 m in both Huanghekou and Qikou sags (Figure 9b).

Hydrocarbon expulsion history of source rocks in E₃d

After recognizing the hydrocarbon expulsion threshold and the relationship between hydrocarbon expulsion ratio and depth in different organic matter types, based on the data of source rock thickness, current buried depth, organic matter abundance and organic types, etc., the hydrocarbon expulsion intensity and hydrocarbon expulsion amount of source rocks can be calculated by using the hydrocarbon generation potential method (Equations 2 and 3). In addition, the hydrocarbon generation potential curve of source rocks, reflecting the hydrocarbon expulsion evolution at different burial depths, is a function of the effects of various geologic factors. Therefore, on the basis of ascertaining the burial history of source rocks, defining the extent of ESR by hydrocarbon expulsion threshold, determining the evolution of the hydrocarbon generation potential curve, combined with data on effective source rock thickness, organic matter abundance and organic matter type, etc., the hydrocarbon expulsion history of source rocks and the hydrocarbon expulsion amount during each period can be derived.

Evolution of hydrocarbon generation kitchens

Hydrocarbon expulsion history of source rocks in E₃d₂. Source rocks of E₃d₂ began to expel hydrocarbons at around the end of Minghuazhen Formation sedimentary period (12-2 Ma), with corresponding burial depth of2800 m, and were mainly restricted to Bozhong and Qikou sags. In that period, source rocks in Qikou sag just crossed the hydrocarbon expulsion threshold and had a limited hydrocarbon expulsion extent and low hydrocarbon expulsion intensity of less than 40x10⁴ t/km² (Figure 10a). Source rocks of E₃d₂ in Bozhong sag reached the maximum hydrocarbon expulsion intensity of 80x10⁴ t/km² at the burial depth of 3400 m (Figure 10a). At present, the maximum expulsion rate of source rocks is 550 mg/g (type I), and its expulsion extent has been gradually increased. The maximum hydrocarbon expulsion intensity has reached 40x10⁴ t/km² and 160x10⁴ t/km² in Qikou and Bozhong sags, respectively (Figure 10b).

Hydrocarbon expulsion history of source rocks in E₃d₃. Source rocks of E₃d₃ began to expel hydrocarbons around the end of Guantao Formation sedimentary period (12 Ma), with corresponding burial depth of2800 m, and were mainly restricted to Bozhong and Qinnan sags. During this period, the maximum hydrocarbon expulsion intensity was 160x10⁴ t/km² in the Bozhong sag and 40x10⁴ t/km² in the Qinnan sag (Figure 11a). By the end of Minghuazhen Formation sedimentary period, when the average burial depth was around 3400 m, source rocks reached their maximum hydrocarbon expulsion rate and their extent was increasing. There was hydrocarbon expulsion in the Bozhong, Qinnan, Qikou, and Huanghekou sags and in the northern part of the Liaozhong sag, and the maximum hydrocarbon expulsion intensity in Bozhong sag was 200x10⁴ t/km². A small difference of the maximum hydrocarbon expulsion intensity existed in other sags with a maximum hydrocarbon expulsion intensity range of 40x10⁴ - 80x10⁴ t/km², whereas their hydrocarbon expulsion extents were slightly different (Figure 11b). The source rocks of E₃d₃ have expelled hydrocarbons in large areas, and Bozhong, Bodong and Qinnan sags are characterized by continuous hydrocarbon expulsion, with a hydrocarbon expulsion rate of 550 mg/g (type I). The accumulative maximum hydrocarbon expulsion intensity was 280x10⁴ t/km² in Bozhong sag and 80x10⁴ t/km² in Bodong and Qinnan sags. Hydrocarbon expulsion in Qikou, Liaozhong and Huanghekou sags also occurs over wide areas, but their maximum hydrocarbon expulsion intensity varies little and mainly ranges from 40x10⁴ to 80x10⁴ t/km² (Figure 11c).

Evolution of hydrocarbon expulsion amount variation

Hydrocarbon expulsion intensity reflects the migration evolution of hydrocarbon expulsion centers of source rocks in each period, and the expelled amount is the best indicator of the amount of hydrocarbons supplied from source rocks. Through volume integration of hydrocarbon expulsion intensity of source rocks in each period, total amount of hydrocarbons expelled from these two sets of source rocks in E₃d was obtained. Accumulative average thickness, accumulative areas of hydrocarbon expulsion, and expelled volume of hydrocarbons for source rocks of E₃d in different geological times are reported in Table 3.

Evolution of hydrocarbon expulsion amount of source rocks in E₃d. The hydrocarbon expulsion amount of source rocks of E₃d₂ was 27x10⁸ t during about 12-2 Ma, with 25x10⁸ t hydrocarbon derived from the Bozhong sag, making up 90% of the total expulsion amount. The amounts of hydrocarbon expulsion in Bodong and Qikou sags were only 0.65x10⁸ t and 1.0x10⁸ t, respectively, indicating that the expulsion center of the source rocks in E₃d₂ was Bozhong sag during this period. Influenced by sedimentary environment and burial conditions, the source rocks of E₃d₂ in Bozhong sag had a relatively high evolution degree and a large expulsion extent. During the sedimentation period of 2.0-0 Ma, the total hydrocarbon expulsion amount was 15x10⁸ t, including 8x10⁸ t in Bozhong sag, 2.5x10⁸ t in Huanghekou sag, 4x10⁸ t in Qikou sag, and 0.6x10⁸ t in Bodong sag. These results suggest that the hydrocarbon expulsion of the source rocks in E₃d₂ was relatively scattered in this period, and the expulsion amount remarkably increased in Qikou and Huanghekou sags (Figure 12). The evolution of the expulsion amount indicate that the source rocks of E₃d₂ began to expel hydrocarbons in the Minghuazhen Formation sedimentary period, and that the hydrocarbon expulsion center was mainly located in Bozhong sag. Due to long term sedimentation and thick sediments, the source rocks reached hydrocarbon expulsion peak rapidly. From 2 Ma to present, the hydrocarbon expulsion amount was reduced significantly, but the expulsion extent increased slightly. For other sags, because of the relatively shallow burial depth, source rocks have not expelled hydrocarbons until now.

The expulsion of hydrocarbon from source rocks of E₃d₃ began at about 16.6-12.0 Ma with hydrocarbon expulsion of 20x10⁸ t, including 18.4x10⁸ t in the Bozhong sag, and only 1.6x10⁸ t in the Qinnan sag. From 12.0 Ma to 2.0 Ma, the hydrocarbon expulsion amount increased rapidly to about 38x10⁸ t. The source rocks in Bodong, Huanghekou, Liaozhong, Liaodong and Qikou sags also crossed the hydrocarbon expulsion threshold and began to expel. The largest hydrocarbon expulsion amount of 26x10⁸ t was in the Bozhong sag, followed by Qikou sag with 4x10⁸ t, and Qinnan sag with 2x10⁸ t. However, for the other sags, the hydrocarbon expulsion amounts are mostly less than 2.0x10⁸ t, for instance, Huanghekou sag had the lowest hydrocarbon expulsion amount of about 0.85x10⁸ t. From 2.0 Ma to the present, Nanpu sag expelled 2.5x10⁸ t of hydrocarbon. The hydrocarbon expulsion amount in Bozhong sag diminished sharply to only 8.5x10⁸ t, indicating the source rocks had passed their hydrocarbon expulsion peak. The hydrocarbon expulsion amount in Qikou sag increased to 4.5x10⁸ t, which was equivalent to that expelled during 16.6-12.0 Ma. The hydrocarbon expulsion amounts were 2.4x10⁸ t in the Bodong sag, 3x10⁸ t in Qinnan sag, 2.2x10⁸ t in Liaozhong sag, 1.6x10⁸ t in Liaodong sag, and 1x10⁸ t in Huanghekou sag. Hydrocarbon expulsion amounts in these sags increased slightly compared with previous expulsion periods (Figure 12).

The results of hydrocarbon expulsion amount evolution suggest that the source rocks in E₃d₃ began to expel hydrocarbons at about 16.6-12Ma ago, and the hydrocarbon expulsion center was located in the Bozhong sag. From 12 Ma to 2 Ma, source rocks in the Bozhong sag reached the hydrocarbon expulsion peak, and the hydrocarbon expulsion extent increased to seven hydrocarbon expulsion sags. From 2 Ma to present, although the amount of hydrocarbon expulsion diminished sharply in the Bozhong sag, it increased slightly in other sags, indicating that the source rocks in medium to small sags just entered their hydrocarbon expulsion peak during this period.

 

CONCLUSIONS

Dark mudstones of E₃d in Bohai Sea area are widely distributed with large thickness, being the thickness of E₃d₂ dark mudstone larger than that of E₃d₃. However, the effective source rocks of E₃d have a limited distribution. ESR of E₃d₂ are mainly located in Bozhong, Huanghekou, Qikou and Bodong sags, with an average thickness of 200 m; while ESR distribution of E₃d₂ is relatively broad, mainly in Bozhong, Bodong, Huanghekou, Liaozhong, Qinnan, Qikou and Nanpu sags, with an average thickness of 230 m.

The hydrocarbon expulsion history simulation results indicate that the source rocks of E₃d₂ began to expel hydrocarbons at around 12-2 Ma (Minghuazheng Formation sedimentary period). So far, the cumulative hydrocarbon expulsion amount is 41.95><10⁸ t, with the hydrocarbon expulsion center mainly located in the Bozhong sag, from where the hydrocarbon expulsion accounts for over 90%, whereas the hydrocarbon expulsion amount is very small in Bodong, Huanghekou and Qikou sags. Source rocks of E₃d₃ began to expel hydrocarbons at about 16.6-12 Ma (Guantao Formation sedimentary period). So far, the cumulative hydrocarbon expulsion amount has been 83.75x10⁸ t. In addition to the hydrocarbon expulsion center of Bozhong sag, hydrocarbons are also expelled from source rocks in Liaozhong, Qinnan, Qikou and Huanghekou sags. Source rocks of E₃d₃ make greater contribution to hydrocarbon accumulation than those of E₃d₂ in Bohai Sea area.

Comprehensive analysis reveals that the Minghuazhen Formation sedimentary period was the main hydrocarbon accumulation stage. Source rocks are still expelling hydrocarbon, which is very significant to hydrocarbon accumulation, advantageous for oil and gas gathering, and favorable for the preservation of oil and gas.

 

ACKNOWLEDGEMENTS

This work was supported by the China National Natural Science Foundation Project (Grant No. 41102085), National Key Basic Research and Development 973 Program Project (Grant No. 2011CB201100), National Science and Technology Major Project (Grant No. 2011ZX05006-006), Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20110007120001), and Science Foundation of China University of Petroleum, Beijing (Grant No. KYJJ2012-01-08). We appreciate the Tianjin Branch of CNOOC China Limited for providing background geologic data and permission to publish the results. We also thank anonymous reviewers, who helped to improve the manuscript.

 

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