J. Cent. South Univ. (2016) 23: 654-668

DOI: 10.1007/s11771-016-3111-3

Oil source and migration process in oblique transfer zone of Fushan Sag, northern South China Sea

WANG Guan-hong(王观宏)^{1, 2}, WANG Hua(王华)^{1, 2}, GAN Hua-jun(甘华军)^{1, 2}, SHI Yang(时阳)³,

ZHAO Ying-dong(赵迎冬)², CHEN Shan-bin(陈善斌)²

1. Key Laboratory of Tectonics and Petroleum Resources of Ministry of Education

(China University of Geosciences), Wuhan 430074, China;

2. Faculty of Earth Resources, China University of Geosciences, Wuhan 430074, China;

3. Exploration and Development Corporation, PetroChina, Guangzhou 510000, China

 Central South University Press and Springer-Verlag Berlin Heidelberg 2016

Abstract:

The oblique transfer zone in the Fushan Sag, a syndepositional dome sandwiched between the Bailian and Huangtong sub-sags, has been the most important exploration target. The major oil observation occurs in the and the . 46 oil and rock samples reveal that the oil in the transfer zone is mostly contributed by the Bailian sub-sag, though the source rock conditions, hydrocarbon generation and expulsion histories of the Bailian and Huangtong sub-sags are similar. The oil, characterized by high maturity, Pr/Ph ratio and oleanane/C₃₀-hopane ratio, shows a close genetic affinity with the source rocks, while the  oil,characterized by lower maturity, Pr/Ph ratio and oleanane/C₃₀-hopane ratio, is suggested to be derived from the  source rocks. The homogenization temperatures of aqueous fluid inclusions, taking the burial history of the reservoirs into account, reflect that the oil charge mainly occurred from mid-Miocene to Pliocene in the oblique transfer zone. The oil transporting passages include connected sand bodies, unconformities and faults in the Fushan Sag. Of these, the faults are the most complicated and significant. The faults differ sharply in the west area, the east area and the oblique transfer zone, resulting in different influence on the oil migration and accumulation. During the main hydrocarbon charge stage, the faults in the west area are characterized by bad vertical sealing and spatially dense distribution. As a result, the oil generated by the Huangtong source rocks is mostly lost along the faults during the vertical migration in the west area. This can be the mechanism proposed to explain the little contribution of the Huangtong source rocks to the oil in the oblique transfer zone. Eventually, an oil migration and accumulation model is built in the oblique transfer zone, which may provide theoretical and practical guides for the oil exploration.

Key words:

oblique transfer zone; oil-source correlation; oil loss; oil migration and accumulation model; Fushan Sag；

1 Introduction

The term “transfer zone” was first defined in thrust belts by DAHLSTROM in 1970 [1]. Since the 1980s, it has been applied to extensional terrane. The transfer zone in an extensional basin can be defined as a coordinated system conserving regional extension [2–7]. The oblique transfer zone is a special transfer zone. It is constructed by two groups of overlapped faults with opposite dips, appearing as a dome or syncline in morphology [3, 8–10]. The transfer zones have become important target areas for petroleum exploration throughout the world [11–14].

In the Fushan Sag, an oblique transfer zone developing in the central region divides the whole sag into two sub-sags (the Huangtong sub-sag in the west and the Bailian sub-sag in the east), resulting in different tectonic styles and sedimentary patterns between the west and east areas [14–15]. The oblique transfer zone accounts for nearly half of the total petroleum reserves. In general, an oblique transfer zone can receive hydrocarbons from the sub-sags on both sides [10, 14]. However, the detailed geochemical investigation based on the Rock-Eval pyrolysis parameters, physical properties, molecular biomarkers reveals that the oil in the oblique transfer zone of the Fushan Sag was mostly contributed by the Bailian sub-sag, though the source rock conditions, hydrocarbon generation and expulsion histories of the Bailian and Huangtong sub-sags are similar. The spatial and temporal fault systems were studied based on 3D seismic data in the Fushan Sag, which suggested that the faults differed sharply in the west area, the east area and the oblique transfer zone, resulting in different influences on the oil migration  and accumulation. Eventually, an oil migration and accumulation model was built in the oblique transfer zone, which may provide theoretical and practical guides for the oil exploration.

2 Regional geological setting

The Fushan Sag, located in the southeast of the Beibuwan Basin, was formed by extension during Cenozoic [14, 16–18]. It is bounded to the west by the Lingao Fault, to the south by the Anding Fault, and to the east by the Changliu Fault (Fig. 1). The Cenozoic system comprises, in ascending order, the Paleogene Changliu, Liushagang and Weizhou formations, and the Neogene Xiayang, Jiaowei, Dengloujiao and Wanglougang formations [16–17, 19–20] (Fig. 2). Controlled by the asymmetric stretching setting resulted from the sinistral strike slip of the Red River fault zone and the clockwise rotation of the Hainan Uplift, an oblique transfer zone was developed in the central region and divided the whole sag into two sub-sags (the Huangtong sub-sag in the west and the Bailian sub-sag in the east), resulting in different tectonic styles and sedimentary patterns between the west and east areas [14–15].

Fig. 1 Map showing sedimentary basins in northern margin of South China Sea and location of Fushan Sag

Fig. 2 Generalized stratigraphic column of Fushan Sag

As the primary oil reservoir, the Liushagang Formation (E₂l) is generated by a set of lacustrine systems, and can be subdivided into three members (E₂l₃, E₂l₂ and E₂l₁). During the depositional period of E₂l₂, several hundred meters of mudstones were deposited, forming the most important hydrocarbon source rocks. The Changliu (E₁c) and Weizhou (E₃w) formations, deposited mainly as fluvial facies, have never been recognized as effective source rocks.

The major oil observation in the oblique transfer zone occurs mainly in the middle and lower E₂l₁ and the upper E₂l₃ [17]. The mid-fine sandstones of deltaic front facies form the main reservoir rocks [17]. Regional hydrocarbon seals include the 500–700 m thick lacustrine dark mudstones in the E₂l₂ and the thick mudstones in the upper E₂l₁ [17].

3 Samples and experimental methods

A total of 34 core and cutting samples were collected from the Liushagang Formation in the Bailian sub-sag, the Huangtong sub-sag and the oblique transfer zone, and analyzed using a Rock-Eval/ TOC analyzer. Of these, 19 rock samples were subjected for detailed molecular biomarker analyses. 12 oil samples from the oil pools in the oblique transfer zone were sampled for the oil-source correlation. In addition, 113 vitrinite reflectance values and 13 physical property data were collected from PetroChina.

After asphaltene precipitation, source rock extracts and crude oils were fractionated into saturated and aromatic hydrocarbons, and a polar fraction, by sequential elution with n-hexane, toluene and chloroform. Gas chromatography-mass spectrometry (GC-MS) analyses were carried out on a Thermal Finnigan Trace- DSQ mass spectrometer for saturated hydrocarbons, coupled to an HP 6890 gas chromatograph fitted with an HP-5MS column (30 m×0.25 mm (inner diameter)) with a 0.25 μm coating. For analyzing saturated hydrocarbons, the GC oven was initially set at 50 °C, and programmed to 120 °C at 20 °C/min, to 250 °C at 4 °C/min, and then to 310 °C at 3 °C/min, with a final holding of 30 min. The mass spectrometer was operated in selected ion monitoring. Peak identifications and quantitative methods have been reported by JIANG et al [21].

4 Results

4.1 Geochemical characteristics of source rocks

4.1.1 Petroleum potential of source rocks

According to the measured vitrinite reflectance data, the Ro-depth regression equations have been established in the Huantong sub-sag, the Bailian sub-sag and the oblique transfer zone (Fig. 3), suggesting that the threshold depths of oil generation (Ro=0.5%) are 2737, 2844 and 2970 m, respectively. Based on the available drilling data, the maximum burial depths of the E₂l₁ are 3438 m and 3747 m in the Bailian and Huangtong sub-sags, respectively, indicating that the E₂l₁ mudstones in the two sub-sags have entered the oil generation threshold. The mudstones show lower thermal evolution degree for the limited burial depth in the oblique transfer zone. The maximum burial depths of the E₂l₂ and E₂l₃ are 2912 m and 3310 m, respectively, indicating that only the E₂l₃ mudstones can be identified as thermally mature source rocks in the oblique transfer zone.

Fig. 3 Ro-depth regression equations:

As given in Table 1, the rock samples from the Huangtong sub-sag are characterized by gray, dark gray or silty mudstones, containing carbonaceous fragments of terrestrial plants occasionally. The same goes for the E₂l₃ rock samples from the Bailian sub-sag and the oblique transfer zone. This is consistent with their depositional setting of braided delta fronts (Fig. 4). In contrast, the E₂l₁ and E₂l₂ rock samples from the Bailian sub-sag feature dark gray, black gray or black mudstones deposited in the semi-deep or deep lake (Figs. 4(a) and 4(d)). Higher TOC, average (S₁+S₂) and extractable bitumens are observed in the (E₂l₁ and E₂l₂ in the Bailian sub-sag) rock samples compared with the  (E₂l₃ in the Bailian sub-sag), E₂l^h (E₂l in the Huangtong sub-sag) and  (E₂l₃ in the oblique transfer zone) samples, suggesting that the  source rocks have the best petroleum potential. According to the occupation standard of continental source rock evaluation (SY/T 5735—1995) proposed by PetroChina in 1995, the mudstones are classified as good source rocks and the  E₂l^h and  mudstones are classified as moderate source rocks.

Based on the above, the source rocks can be divided into six groups in the Fushan Sag, which are denoted as , (E₂l₁ in the Huangtong sub-sag), (E₂l₂ in the Huangtong sub-sag), (E₂l₃ in the Huangtong sub-sag) and

4.1.2 Biomarker characteristics of source rock extracts

Nineteen of the thirty-four rock samples screened by Rock-Eval/TOC analyses were selected for solvent extraction and further analyses. The measured values of molecular biomarker parameters of the rock samples are given in Table 2. The m/z 85, m/z 191 and m/z 217 mass fragmentograms of saturated hydrocarbon from the  representative rock samples are shown in Fig. 5, Fig. 6 and Fig. 7, respectively. On the GC-MS fingerprints, the source rocks can be distinguished steadily from the E₂l^h and  source rocks by the difference in pristine/phytane ratios and oleanane/C₃₀-hopane values. The displays a pristane to phytane balance and low oleanane/C₃₀-hopane values (0.10–0.25), while the E₂l^h and show obvious pristane predominance and higher oleanane/C₃₀-hopane values (≥0.35). The difference is also reflected by the relative abundance of the C₂₇ ααα 20R cholestane, C₂₈ ααα 20R ergostane and C₂₉ ααα 20R stigmastane. Most of the E₂l₁₊₂^b rock samples show C₂₇≥C₂₉>C₂₈ regular sterane distributions and cluster in the group defined as mixed or phytoplankton predominant organic facies in the ternary plot showing the relative distribution of C₂₇, C₂₈ and C₂₉ regular steranes (Fig. 8(a)), while most of the  E₂l^h and rock samples show C₂₉>C₂₇≥C₂₈ regular sterane distributions and cluster in the group defined as terrestrial plant predominant or mixed organic facies in the ternary plot (Fig. 8(a)). These data features are well consistent with the terrigenous clastic-starved anoxic to suboxic lacustrine depositional setting for the  and the terrigenous clastic-abundant suboxic to oxic lacustrine setting for the  E₂l^h and  [22–26]. Other distinguishing biomarker features include the lower hopane/sterane and higher gammacerane/C₃₀- hopane ratios in the  mudstones than their  E₂l^h and counterparts. High gammacerane is suggestive of a stratified water column [27–29].

Table 1 Physical properties of  and oil in oblique transfer zone, Fushan Sag

Fig. 4 Sand-thickness maps of Fushan Sag in different stages:

Table 2 Basic geochemical parameters of source rocks in Bailian sub-sag, Huangtong sub-sag and oblique transfer zone, Fushan Sag

4.2 Oil-source correlation

Table 3 illustrates the physical properties of 19 oil samples. The physical properties of the samples include 0.76–0.80 g/cm³ density, 0.51–1.29 mPa·s viscosity, 0.44%–4.28% wax content, 0.03%–0.05% sulfur content and –15–9.5 °C freezing point. The oil is predominately aliphatic as indicated by their saturate/aromatic ratio (5.52–17.22) and saturate fraction abundance (76.32%–92.81%). In contrast, the  oil samples show higher density, viscosity, wax content, sulfur content and freezing point and lower saturate/ atomatic ratio. All these data suggest that the  and the oil belong to two different oil families. Source-related biomarker parameters of the oil samples are given in Table 4. The m/z 85, m/z 191 and m/z 217 mass fragmentograms of saturated hydrocarbon from the representative oil samples are shown in Fig. 5, Fig. 6 and Fig. 7, respectively. Most of the oil displays clear odd over even carbon preference in n-alkanes, with the CPI in the range of 1.04–1.14 and the OEP in the range of 1.06–1.15 (Table 4), suggesting low thermal maturity. In comparison with the oils, the CPI and OEP values of the oil are closer to 1, indicating higher thermal maturity. Molecular mechanics calculations indicate that the compounds of the diahopane series should be more stable than those of the hopane series [17, 30]. Thus, increasing maturity should result in increased ratios of C₃₀ diahopane/hopane. Higher C₃₀ diahopane/hopane observed in the oil than their  counterparts indicates higher thermal maturity for the  oils.

Fig. 5 m/z 85 mass fragmentograms of saturate fractions from representative oil and rock samples:

Fig. 6 m/z 191 mass fragmentograms of saturate fractions from representative oil and rock samples:

The Pr/Ph ratios and oleanane/C₃₀-hopane values of the oil are in the range of 0.75–1.76 and 0.14–0.28, respectively, showing good correlations with the source rocks. The gammacerane/C₃₀-hopane of the oil is in the same range as the counterparts of the source rocks but different from the counterparts of the E₂l^h and source rocks, suggesting that the oil has a closer genetic relationship with the mudstones. Supporting evidences also come from the ternary plots showing the relative distribution of C₂₇, C₂₈ and C₂₉ regular steranes based on ααα 20R isomers (Fig. 8(a)) and the cross plot of gammacerane/C₃₀-hopane versus Pr/Ph (Fig. 8(b)). The oils cluster in the source rocks group, suggesting a close genetic affinity.

All biomarker parameters relative to maturity indicate that the oil has much higher thermal maturity than the in situ source rocks, suggesting that the  mudstones contribute little to the oil. As there is little difference in the source-related biomarker parameters between the oils and the E₂l^h source rock extracts, source-related biomarkers cannot be used here as source differentiators. However, it is suggested that all molecular parameters relative to migration fractionation indicate that the oils are mainly derived from the source rocks in the Bailian sub-sag [17, 31]. The isomerization ratio at the C-20 position (20S/20S+20R) in the C₂₉ ααα steranes, which is theoretically zero at the depositional surface, increases with thermal maturity, reaching the endpoint value of about 0.55 at peak oil generation. The αββ-iso steranes relative to ααα-normal steranes ratios increase with maturity, reaching an endpoint about 0.75 [32–33]. In the cross plots of T_s/(T_s+T_m) versus C₂₉T_s/(C₂₉T_s+C₃₀-hopane) (Fig. 8(c)) and C₂₉ ααα sterane 20S/(20S+20R) versus C₂₉ sterane αββ/(ααα+αββ) (Fig. 8(d)), theoil samples show much higher thermal maturity than the rock samples, suggesting the little contribution from thesource rocks to theoil. The  oil samples also show higher thermal maturity than the source rocks. However, because no wells penetrate the in the depocenter to date, the realistic maximum maturity of the  source rocks should be much higher. It is suggested that the oil is contributed by the  source rocks, and a large proportion may originate from the underlying  mudstones in the depocenter.

Fig. 7 m/z 217 mass fragmentograms of saturate fractions from representative oil and rock samples:

Fig. 8 Ternary plot showing relative distribution of C₂₇, C₂₈ and C₂₉ regular steranes based on ααα 20R isomers (a), cross plot of gammacerane/C₃₀-hopane versus Pr/Ph (b), cross plot of T_s/(T_s+T_m) versus C₂₉T_s/(C₂₉T_s+ C₃₀-hopane) (c) and cross plot of C₂₉ sterane αββ/(ααα+αββ) versus C₂₉ sterane ααα 20S/(20S+20R) (d) for oil and rock samples

Table 3 Basic geochemical parameters of  and  oil in oblique transfer zone, Fushan Sag

Table 4 Physical properties of and oil in oblique transfer zone, Fushan Sag

According to the above discussion, it is reasonable to conclude that the oil is derived from the source rocks, and the oil is derived from the  source rocks. In comparison with the Bailian source rocks, the contribution of the Huangtong source rocks to the oil in the oblique transfer zone can be neglected.

5 Discussion

5.1 Timing of oil accumulation

5.1.1 Oil generation and expulsion history

In the Fushan Sag, the source rocks of the Bailian and Huangtong sub-sags are characterized by similar oil generation and expulsion history. The source rocks entered into the hydrocarbon generation threshold in the late Eocene (38 Ma), and went into the hydrocarbon generation and expulsion peak in the mid-Miocene to Pliocene [34].

5.1.2 Accumulation timing

The homogenization temperatures of aqueous fluid inclusions, taking burial history of the reservoirs into account, can reflect the hydrocarbon accumulation time [35]. The homogenization temperatures of aqueous fluid inclusions in the oil-bearing sandstones of the oblique transfer zone range from 100 to 110 °C in the E₂l₁ interval and from 105 to 115 °C in the E₂l₃ interval (Fig. 9(a)). Combined with the burial history modeling, it is believed that the hydrocarbon charge mainly occurred around the end of mid-Miocene to Pliocene in the oblique transfer zone (Fig. 9(b)), corresponding to the main stage of the Dongsha Movement.

5.2 Transport system

In the Fushan Sag, oil transporting passages include three basic types: connected sand bodies, unconformities and faults.

5.2.1 Connected sand bodies

The thick-layer medium-fine sandstones of braided delta fronts with high connectivity formed the important lateral migration channel. The prodeltas were characterized by thin-layer fine sandstones with single layer thickness below 0.6 m. The thin sand layers were in direct contact with source rocks, serving as a bridge with the unconformity or faults.

5.2.2 Unconformities

The most important unconformity was between the E₂l₁ and E₂l₂ units in the Fushan Sag [14]. It contacted with the source rocks directly or through the thin-layer sandstones of prodeltas, and thus formed the important lateral migration channel.

5.2.3 Faults

Faults are the most complicated and significant transporting passage in the Fushan Sag. Spatial and temporal fault systems were researched based on the 3-D seismic data. The 3-D seismic data comprise a region of approximately 2000 km² and cover the major hydrocarbon exploration area. It is suggested that the faults differ sharply in the west area, the east area, and the oblique transfer zone [18–19].

As shown in Fig 10(a), the fault system in the west area was characterized by numerous NEE-extending consequent faults, cutting through the E₂l₃ section and terminating in the Neogene Formation. They intersected the Huangtong sub-sag along strike direction. It has been suggested that these faults were initiated during the Eocene Zhuqiong Movement^1st, and remained active until the Neogene Dongsha Movement [20, 36]. They were characterized by high activity and large displacement, for example, the maximum activity rate of the Meitai Fault could be up to 240 m/Ma (Fig. 11(b)) and the maximum displacement could be more than 1000 m.

As shown in Fig. 10(b), the fault system in the east area comprised a set of EW-trending antithetic faults, which intersected the Bailian sub-sag along strike direction. They were basement faults cutting through the E₂l₃ section and terminating in the E₂l₂ thick-layer mudstones. In contrast to the faults in the west area, those in the east area were characterized by lower activity rate, smaller displacement and shorter activity duration [20, 36]. They were active mainly during the Eocene Zhuqiong Movement^1st.

Fig. 9 Histograms of homogenization temperatures of aqueous fluid inclusions from E₂l₁ strata of Well H7-2x (Sample depth: 2613.0–2623.0 m) (a) and E₂l₃ strata of Well H2-1 (Sample depth: 2833.0–2848.0 m) (b), and Buried and thermal history model of the Well H7-2x (c) and Well H2-1 (d) (1–Range of homogenization temperature; 2–Buried depth of fluid inclusion samples)

Fig. 10 Structural profiles in Fushan Sag:

The oblique transfer zone in the central sag was manifested as a syndepositional dome in morphology. The formation thickness on the flanks of the oblique transfer zone is obviously thickening toward the Huangtong and Bailian sub-sags (Fig. 10(d)). As shown in Fig. 10(c), a special “double layer structure” occurred in the oblique transfer zone, including a deep antithetic fault system and a shallow consequent fault system. The deep antithetic fault system comprised basement faults cutting through the E₂l₃ section and terminating in the E₂l₂ thick-layer mudstones. The shallow consequent faults occurred in the E₂w and E₂l₁ strata, and did not cut through the E₂l₂ section.

Fig. 11 Plane graph showing fault system in west area of Fushan Sag (a), histogram showing vertical activity rate of Meitai Fault (b), and observation points (c)

5.3 Fault controls on oil migration and accumulation

The distinctive petroleum entrapment in the oblique transfer zone is that the oil was mostly derived from the Bailian sub-sag, though the source rock conditions, hydrocarbon generation and expulsion histories in the Bailian and Huangtong sub-sags are similar. The faults are the most complicated and significant transporting passages in the Fushan Sag, and may take the responsibility for the distinctive petroleum entrapment.

5.3.1 Oil losses along faults in west area

During the main hydrocarbon charge stage, the faults in the west area still had relatively strong activity, for example, the activity rate of the Meitai Fault could still be up to 80 m/Ma (Fig. 11(b)). In addition, the Huangtong sub-sag was blanketed by braided river deltas, alluvial fans and braided river deposits (Fig. 4), leading to the strata with high sand content and brittlement. Therefore, the faults were characterized by good oil transporting capacity, and thus acted as favorable pathways of oil migration in the west area [20]. The supporting evidence comes from the occurrence of the deep sourced oil in the shallow E_3w reservoirs in the Bohou and Yongan oilfields (Fig. 12(c)). Because the Weizhou Formation has never been recognized as effective source rocks, the E₃w oil is suggested to be derived from the E₂l source rocks along the faults in the west area after a long-distance vertical migration. However, because of the limited storage space of the E₃w traps, the oil may have been mostly lost during the vertical migration.

Another feature of the faults in the west area was the spatially dense distribution (Figs. 11(a)). Along the north-south direction, the density of the NEE-extending faults was 0.5–0.9 strips per kilometer. With the increase in the number of large-scale commercial oil pools discovered around the Bailian sub-sag, few breakthroughs have been made around the Huangtong sub-sag (Fig. 12). The statistics show that only 6% of the proven oil reserves are derived from the Huangtong sub-sag (Fig. 12(a)), and 90% from the Bailian sub-sag. There appears to be no other mechanism to explain this phenomenon, except that the oil generated by the Huangtong source rocks was mostly lost along the faults in the west area during the vertical migration. This can be also the mechanism proposed to explain the little contribution of the Huangtong source rocks to the oil reserves in the oblique transfer zone.

5.3.2 Oil literal migration along faults in east area

The faults in the east area kept active mainly during the Eocene Zhuqiong Movement^1st. Therefore, they were characterized by good sealing during the main hydrocarbon charge period. The supporting evidence comes from the oil accumulation in the upper E₂l₃ reservoirs around the Bailian sub-sag (Fig. 12(b)).

It has been suggested that the literal migration along fault planes is also a significant hydrocarbon migration pathway [37]. A structural high position lying in the direction of fault strike and good vertical sealing are necessary conditions for long-distance literal migration along fault planes. In the Fushan Sag, the oblique transfer zone formed a structural high position and the E₂l₂ mudstones provided good vertical sealing. The oil generated by the Bailian source rocks migrated literally along the fault planes into the reservoirs in the oblique transfer zone.

Fig. 12 Statistical graphs showing relative reserves of each oilfield in Fushan Sag (a) and relative oil reserves of E₂l₁, E₂l₂ and E₂l₃ intervals around Bailian sub-sag (b), and distribution of oilfield locations (c)

5.3.3 Two different oil families in oblique transfer zone

The deep antithetic faults terminating in the E₂l₂ thick-bedded mudstones were characterized by good vertical sealing. The E₂l₂ mudstones made the oil derived from the  source rocks accumulate in the  reservoirs on one hand and prevented them migrating upwards into shallower reservoirs on the other hand. Therefore, theand oil belong to two different oil families in the oblique transfer zone. It was also proved by the geochemical evidence.

5.4 Migration and accumulation model of oil pools in oblique transfer zone

An appropriate model of oil migration and accumulation may determine the appropriate exploration strategies. Based on the above discussion, an oil migration and accumulation model is illustrated in Fig. 13, to account for the oil pools in the oblique transfer zone. The details are as follows:

(1) The oil in the transfer zone occurs mainly in theand intervals. The mid-fine sandstones of deltaic front facies form the main reservoir rocks [17]. Regional cap rocks include the 500–700m thick lacustrine dark mudstones in the E₂l₂ and the thick mudstones in the [17]. Mudstones in the E₂l^b, E₂l^h and intervals can be recognized as effective source rocks.

(2) During the main hydrocarbon charge stage, the faults in the are area are characterized by bad vertical sealing and spatially dense distribution. As a result, the oil generated by the Huangtong source rocks is mostly lost along the faults in the west area during the vertical migration (Fig. 13(a)).

(3) The oil-source correlation results combined with the analysis of reservoirs, seals and transporting passages suggest that two petroleum systems can be distinguished in the oblique transfer zone. The lower petroleum system consists of the reservoirs in and the source rocks in The EW-extending faults (Fig. 13(b)) and connected sand bodies (Fig. 13(a)) form oil lateral transporting passages. The oil generated by the E₂l₃^b source rocks can migrate laterally into the fault-related traps in the oblique transfer zone and accumulate in the E₂l₃^U interval. The upper petroleum system consists of the reservoirs in and the source rocks in The unconformity between E₂l₁ and E₂l₂ and connected sand bodies near the oblique transfer zone form migration passages. In addition, thin-layer sand bodies of prodeltas are in direct contact with the source rocks, potentially serving as a bridge with the unconformity or connected sand bodies near the transferzone. The oil generated by the source rocks can migrate laterally into reservoirs in the oblique transfer zone, and form lithologic updip pinch-out oil reservoirs and fault-related oil reservoirs.

Fig. 13 Conceptual migration and accumulation model of oil pools in oblique transfer zone (based on lateral section in Fig. 10(d)) (a) and conceptual model of oil literal migration along strike direction of faults in east area (b)

6 Conclusions

1) The oblique transfer zone in the Fushan Sag, a syndepositional dome sandwiched between the Bailian and Huangtong sub-sags, has been the most important exploration target. The major oil observation occurs in the and the  The geochemical investigation suggests that the oil in the oblique transfer zone is mostly contributed by the Bailian sub-sag, though the source rock conditions, hydrocarbon generation and expulsion histories in the Bailian and Huangtong sub-sags are similar.

2) The oil transporting passages include connected sand bodies, unconformities and faults in the Fushan Sag. Faults are the most complicated and significant. The fault systems differ sharply in the west area, the east area and the oblique transfer zone, resulting in different influence on the oil migration and accumulation. During the main hydrocarbon charge period, the faults in the west area are characterized by bad vertical sealing and spatially dense distribution. As a result, most of the oil generated by the Huangtong source rocks is lost along the faults in the west area during the vertical migration. This can be the mechanism proposed to explain the little contribution of the Huangtong source rocks to the oil in the oblique transfer zone.

3) An oil migration and accumulation model is built to account for the oil pools in the oblique transfer zone. This model may provide theoretical and practical guides for oil exploration.

Acknowledgements

The authors are grateful to South Oil Exploration and Development Company of PetroChina for assistance in data and sample collection. Thanks also go to ZHANG Xi-man and MA Qing-lin for their excellent technical assistance.

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(Edited by YANG Bing)

Foundation item: Project(41272122) supported by the National Natural Science Foundation of China

Received date: 2014-09-12; Accepted date: 2015-09-23

Corresponding author: WANG Guan-hong, PhD Candidate; Tel: +86–1587170983; E-mail: wangguanhong2015@163.com

Abstract: The oblique transfer zone in the Fushan Sag, a syndepositional dome sandwiched between the Bailian and Huangtong sub-sags, has been the most important exploration target. The major oil observation occurs in the and the . 46 oil and rock samples reveal that the oil in the transfer zone is mostly contributed by the Bailian sub-sag, though the source rock conditions, hydrocarbon generation and expulsion histories of the Bailian and Huangtong sub-sags are similar. The oil, characterized by high maturity, Pr/Ph ratio and oleanane/C₃₀-hopane ratio, shows a close genetic affinity with the source rocks, while the  oil,characterized by lower maturity, Pr/Ph ratio and oleanane/C₃₀-hopane ratio, is suggested to be derived from the  source rocks. The homogenization temperatures of aqueous fluid inclusions, taking the burial history of the reservoirs into account, reflect that the oil charge mainly occurred from mid-Miocene to Pliocene in the oblique transfer zone. The oil transporting passages include connected sand bodies, unconformities and faults in the Fushan Sag. Of these, the faults are the most complicated and significant. The faults differ sharply in the west area, the east area and the oblique transfer zone, resulting in different influence on the oil migration and accumulation. During the main hydrocarbon charge stage, the faults in the west area are characterized by bad vertical sealing and spatially dense distribution. As a result, the oil generated by the Huangtong source rocks is mostly lost along the faults during the vertical migration in the west area. This can be the mechanism proposed to explain the little contribution of the Huangtong source rocks to the oil in the oblique transfer zone. Eventually, an oil migration and accumulation model is built in the oblique transfer zone, which may provide theoretical and practical guides for the oil exploration.