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Seismic characteristics of a reef carbonate reservoir and implications for hydrocarbon exploration in deepwater of the Qiongdongnan Basin, northern South China Sea
Marine and Petroleum Geology 26 (2009) 817–823
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Seismic characteristics of a reef carbonate reservoir and implications for hydrocarbon exploration in deepwater of the Qiongdongnan Basin, northern South China Sea Shiguo Wu a, d, *, Shengqiang Yuan a, b, Gongcheng Zhang c, Yubo Ma a, b, Lijun Mi e, Ning Xu f a
Key Laboratory of Marine Geology and Environment, Institute of Oceanology, Chinese Academy of Sciences, 7 Naihai Road, Qingdao 266071, PR China Graduate University of Chinese Academy of Sciences, Beijing 100049, PR China CNOOC Research Center, Beijing 100027, PR China d China University of Petroleum, Qingdao 266555, PR China e Exploration Department of CNOOC China Ltd, BeQing 100010, China f China National Oil and Gas Exploration and Development Corporation, Beijing 100034, China b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 October 2007 Received in revised form 3 April 2008 Accepted 7 April 2008 Available online 10 May 2008
Our analysis of approximately 40,000 km of multichannel 2-D seismic data, reef oil-field seismic data, and data from several boreholes led to the identification of two areas of reef carbonate reservoirs in deepwater areas (water depth 500 m) of the Qiongdongnan Basin (QDNB), northern South China Sea. High-resolution sequence stratigraphic analysis revealed that the transgressive and highstand system tracts of the mid-Miocene Meishan Formation in the Beijiao and Ledong–Lingshui Depressions developed reef carbonates. The seismic features of the reef carbonates in these two areas include chaotic bedding, intermittent internal reflections, chaotic or blank reflections, mounded reflections, and apparent amplitude anomalies, similar to the seismic characteristics of the LH11-1 reef reservoir in the Dongsha Uplift and Island Reef of the Salawati Basin, Indonesia, which house large oil fields. The impedance values of reefs in the Beijiao and Ledong–Lingshui Depressions are 8000–9000 g/cc m/s. Impedance sections reveal that the impedance of the LH11-1 reef reservoir in the northern South China Sea is 8000– 10000 g/cc m/s, whereas that of pure limestone in BD23-1-1 is >10000 g/cc m/s. The mid-Miocene paleogeography of the Beijiao Depression was dominated by offshore and neritic environments, with only part of the southern Beijiao uplift emergent at that time. The input of terrigenous sediments was relatively minor in this area, meaning that terrigenous source areas were insignificant in terms of the Beijiao Depression; reef carbonates were probably widely distributed throughout the depression, as with the Ledong–Lingshui Depression. The combined geological and geophysical data indicate that shelf margin atolls were well developed in the Beijiao Depression, as in the Ledong–Lingshui Depression where small-scale patch or pinnacle reefs developed. These reef carbonates are promising reservoirs, representing important targets for deepwater hydrocarbon exploration. Ó 2008 Elsevier Ltd. All rights reserved.
Keywords: Qiongdongnan Basin (QDNB) Deepwater hydrocarbons Sequence stratigraphy Reef carbonate Seismic attributes
1. Introduction Marginal reef carbonate facies in offshore areas have been confirmed as important deepwater oil–gas reservoirs (Sarg, 1988 Sattler et al., 2004; Zampetti et al., 2004). Qiongdongnan Basin (QDNB) is a large rift basin developed at the margin of the northern South China Sea (NSCS). Development of the basin was controlled by rifting and sustained seafloor spreading within the South China
* Corresponding author. Key Laboratory of Marine Geology and Environment, Institute of Oceanology, Chinese Academy of Sciences, 7 Naihai Road, Qingdao 266071, PR China. Tel./fax: þ86 532 82 898 544. E-mail address: [email protected] (S. Wu). 0264-8172/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.marpetgeo.2008.04.008
Sea. The first 2-D seismic survey in the QDNB was performed in 1970s, marking the commencement of exploration in this area. The study area covers 20,000 km2, and we used data from nearly 40,000 km of 2-D seismic lines and four wells. This paper documents the geological and geophysical characteristics of reef carbonate facies in deepwater areas of the QDNB. There exist differing opinions regarding the conditions of reef development in the QDNB. Many geologists argue about the existence of carbonate reservoirs in the basin: some believe that largescale reef development took place in the mid-Miocene (the Meishan Formation) (Chen and Hu, 1987; Jin and Pang, 1998; Qiu and Chen, 1999; Qiu and Gong, 1999; Qiu and Wang, 2001; Liu, 2003; Sattler et al., 2004; Wei et al., 2005, 2006). However, drilling has failed to reveal any evidence of a large-scale reef reservoir in shallow-water
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areas of the basin. This may reflect the narrow width of the continental shelf in this area and its proximity to land, making it prone to disturbance and blanketing by terrigenous sediments (Fan, 1996; Qiu and Chen, 1999; Qiu and Gong, 1999). Given this lack of success in finding a reef reservoir, it is important to also consider deepwater areas far away from continental source areas. The approaches of sequence stratigraphy and geophysical inversion have been combined in previous studies that sought to predict the locations of carbonate reservoirs in specific environments (King, 1972; Erlich et al., 1990; Mayall et al., 1997; Kusumastuti et al., 2002; Zampetti et al., 2004). In the present study, we first analyzed the stratigraphy and seismic facies of the basin, and then identified favorable carbonate rock provinces as potential reservoirs. We then undertook a paleogeographic study of the area and an inversion of seismic data. Finally, we identified the reef-development characteristics of the study area, combined the seismic facies with the geophysical attributes of carbonate sequences, and compared our findings with those obtained for typical reefs from other locations.
2. Geological setting Qiongdongnan Basin lies in the west part of northern South China Sea. The deepwater area of the QDNB is located in the southern part of the basin, where the present water depth is at least 200 m. The basin is bounded to the west by the Red River Fault and the Yinggehai Basin, to the east by the Pearl River Mouth Basin, and to the south by the Xisha Rise. The deepwater area of the basin includes the Ledong–Lingshui, Beijiao, and Songnan–Baodao Depressions, and the Beijiao Uplift (Fig. 1). The features of the deepwater area are typical of passive continental margins, with a two-layered structure consisting of a lower syn-rift sequence and upper post-rift sequence (Taylor and Hayes, 1980). Seismic reflector T60 is a breakup unconformity that divides the upper and lower megasequences. The upper sequence is characteristic of post-rift thermal subsidence, containing few fractures, whereas the lower sequence records syn-rift tectonics, marked by well-developed fault systems and half-graben basins. The syn-rift sequences in the deepwater area of the QDNB include the Lingtou, Yacheng, and Lingshui Formations, consisting of
lacustrine, coal-bearing transitional and epicontinental sedimentary facies (Qiu and Gong, 1999) cut by well-developed fault systems. The main source rocks formed during rifting. Post-rift drift sequences consist of Neogene–Quaternary marine terrigenous detritus and carbonate deposits, including the Miocene Sanya, Meishan, and Huangliu Formations, the Pliocene Yinggehai Formation, and Quaternary Ledong Formation, all devoid of syndepositional fault systems. The key factors that control the growth and development of coral are water temperature, salinity, water depth, water turbidity, concentration of dissolved oxygen, substrate, and water dynamics (waves and streams) (Wei et al., 2006). Suitable conditions for coral growth and reef development are as follows: temperature of 23– 27 C, salinity of 30–40&, water depth of <50 m, clear water, and good light penetration for photosynthesis. It is clear that the QDNB meets most of these criteria in Miocene (Sun and Esteban, 1994). The conditions of the deepwater area of the mid-Miocene QDNB were suitable for reef growth. Modern carbonate deposits are distributed mainly between 30 S and 30 N, under tropical to subtropical conditions (Sun and Esteban, 1994). The deepwater area of the QDNB that was suitable for reef development during the midMiocene was located on a shelf margin platform, with only a minor input of detrital terrigenous material due to its distance from the source area.
3. Cenozoic sequence stratigraphy Eight sequences have been recognized based on drilling results, and eight seismic reflectors (T20, T30, T40, T50, T60, T70, T80, and Tg) have been confirmed from synthetic seismograms (Table 1; Figs. 2 and 3). The reflectors divide the Cenozoic strata into eight formations: the Ledong, Yinggehai, Huangliu, Meishan, Sanya, Lingshui, Yacheng, and Lingtou (Figs. 2 and 3). Reflector T60 is a regional unconformity that divides the entire stratigraphy into syn-rift and post-rift sequences (Wang et al., 1998; Wei et al., 2001). The sequence boundaries in the basin are either unconformities or marked by relatively conformable successions (Vail, 1987). Each sequence is recognized by its external form and internal seismic facies, which are also important indicators in identifying reefs on seismic profiles.
Fig. 1. Structural units of the Qiongdongnan Basin. (1) Songnan–Baodao Depression, (2) Ledong–Lingshui Depression, (3) Beijiao Depression, (4) West Beijiao Rise, (5) East Beijiao Rise, and (6) South Beijiao Rise.
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Table 1 Sequence stratigraphy of the Qiongdongnan Basin (the Lingtou and Yacheng Formations are the major source rocks in the basin). Chronostratigraphy
Seismic reflector
Age (Ma)
System
Series
Formation
Quaternary
Pleistocene
Ledong
T20
1.9
Neogene
Pliocene Miocene
Upper
Yinggehai Huangliu
T30 T40
5.5 10.5
Middle
Meishan
T50
15.5
Lower Oligocene
Sanya Lingshui Yacheng
T60 T70 T80
23.3 29.3 35.4
Ecocene
Lingtou
T100
Paleogene
Reservoir
Structural and fill data
Submarine fan wedge Submarine fan Reef turbidite slope fan Incised valley channel-fill fan Fan delta Fan delta Fan delta
Passive continental margin
Fan delta
Period of post-rifting
Shelf–slope system
Open neritic and bathyal
Intercontinental rift
Period of rifting fill
Intracontinental rift
Offshore and neritic Partition offshore and neritic Rifted lacustrine basin
Pre-Tertiary
The syn-rift period consists of the Lingtou, Yacheng, and Lingshui formations, and the tectonic movement was the dominant control factor on sedimentation. The Lingtou Formation is a nonmarine lacustrine facies, indicating the initial period of the syn-rift period, and is one of the main source rocks within the QDNB (Zhong et al., 2004; He et al., 2006). The Yacheng Formation represents the late periods of syn-rift tectonics, when offshore and neritic sedimentary systems developed, and represent the major part of the basin fill. The Lingshui Formation was deposited at the end of the syn-rift period within a semi-closed marine sedimentary system, and is an important reservoir within the QDNB. Post-rift sequences include the Ledong, Yinggehai, Huangliu, Meishan, and Sanya Formations. The Miocene was an important period for the growth of reefs in the QDNB, especially during transgressive and highstand system tracts. Miocene strata consist of the Sanya Formation (23.3–15.5 Ma), Meishan Formation (15.5– 10.5 Ma), and Huangliu Formation (10.5–5.5 Ma). The continental shelf system of the NSCS developed above reflector T60, and it is evident on seismic profiles across the deepwater region that the thickness of strata increases from the shelf-to-slope (Figs. 2 and 3). The shelf-to-slope system was active from 10.5 Ma (T40), meaning that the Sanya and Meishan Formations formed in open neritic and bathyal environments, whereas the Ledong, Yinggehai, and Huangliu Formations belong to the shelf- to-slope system (Table 1). The Miocene sequence can be divided into system tracts identified from seismic profiles. Transgressive and highstand system tracts in the sequence of Meishan Formation were favorable environments for the development of reef carbonate, as recorded in other sequence stratigraphy studies (Handford and Loucks, 1993).
The Sanya Formation contains neritic clastic facies of submarine fans and reef growth, while the Huangliu Formation is dominated by neritic to abyssal terrigenous facies that serve as a good seal for the potential Miocene reef oil field.
4. Characteristics of reef carbonate 4.1. Seismic reflection characteristics Drilling reveals that carbonate rocks are abundant within the QDNB deepwater area, where geological conditions favored the development of carbonates during the Miocene in the NSCS. The LH11-1 reservoir beneath the Dongsha Rise (Pearl River Mouth Basin) is a 60-m thick reef carbonate reservoir with reserves of 164 million tons, making it the largest offshore oil field in China (Sattler et al., 2004). There exist 200-m thick limestones and dolomites, along with interbedded mudstone, sandstone, and phosphorous, in the early Miocene Zhujiang Formation and mid-Miocene Hanjiang Formation, as found in well QH36-2-1 drilled into the Shenhu Rise, Pearl River Basin. In addition, more than 60 m of biolithite within the Zhujiang Formation was encountered in well BD23-1-1 upon the southwest margin of the Shenhu Rise, Pearl River Basin (Liu and Feng, 2001). Well Ya 21-1-4 in the eastern neritic part of the basin revealed >60-m thick limestones of the Sanya Formation, as well as discontinuous limestone of the Meishan Formation. Several hundred meters of reef layers have also been reported from boreholes Xiyong-1 and Xiyong-2 upon the Xisha Rise (Fig. 1; Zhang et al., 1989; Xu and Wang, 1999).
Fig. 2. Seismic profile across the Ledong–Lingshui Depression (line A–A0 in Fig. 1). The seismic reflectors are labeled T20, T30, T40, T50, T60, T70, T80, and Tg. The stratigraphic framework is described in Table 1.
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Fig. 3. Seismic profile across the Songnan–Baodao Depression (line B–B0 in Fig. 1). The seismic reflectors are labeled (from top to bottom) T20, T30, T40, T50, T60, T70, T80, and Tg. The stratigraphic framework is described in Table 1.
The reef carbonate is a carbonate platform where growing reef surround or cover it. It is characterized on seismic profiles by chaotic bedding, intermittent internal reflections, chaotic or blank reflections, mounded reflections, and apparent amplitude anomalies (Sattler et al., 2004; Yu et al., 2005). Seismic profiles across the LH11-1 reef oil field reveal that the main reservoir is located within the early Miocene Zhujiang Formation reef reservoir of the Pearl River Basin, which exhibits high amplitude and discontinuous layers; marine shale of the mid-Miocene Zhuhai Formation served as the main seal to the reservoir (Fig. 4a; Yue et al., 2005). The Miocene Meishan and Sanya Formations are regarded to represent ideal conditions for the development of large-scale reef complexes (Chen and Hu, 1987; Wei et al., 2005). The seismic reflection features of these formations are similar to those of the reef reservoir within the LH11-1 oil field (Sattler et al., 2004). To study reef development and distribution within the two target formations, we divided them into a lowstand system tract (LST), transgressive system tract (TST), and highstand system tract (HST), and
analyzed the seismic characteristics of the three types of tracts. The Beijiao and Ledong–Lingshui Depressions are characterized by high amplitude, lack of continuity and low frequency, and chaotic or mounded seismic facies (Fig. 4b). A >60-m thick pure biolithite was drilled in the Zhujiang Formation at well BD23-1-1 on the southern margin of the Shenhu Rise (equivalent to the Meishan Formation within the Qiongdongnan Basin). This unit shows strong reflections and amplitude, and high continuity in seismography (Fig. 4c). The core drilled at this site revealed a compact, barren reef (Liu and Feng, 2001), different in character from the reservoir reef rocks. The seismic profiles across the Ledong–Lingshui Depression contain features similar to those of typical patch reefs within the Meishan Formation (Fig. 4d). Based on this finding, and in combination with the geologic background of this area, we deduce the existence of patch reefs or pinnacle reefs in the Ledong–Lingshui Depression. The deeper zones within the depression contain smaller deepwater carbonate buildups with mound configurations and high amplitudes (Erlich et al., 1990).
Fig. 4. Seismic profiles of representative reef carbonates in different areas. (a) Profile across the LH11-1 field; (b) profile across the Beijiao Depression; (c) profile across well BD23-1-1; and (d) profile across the Ledong–Lingshui Depression.
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4.2. Sedimentary facies High-resolution sequence stratigraphic analysis reveals that bioherms occurred in TST and HST of the Meishan Formation. Fig. 5 provides an HST sedimentary facies map of the Meishan Formation, which also shows the reef and carbonate platform distribution of HST Meishan Formation. The depocenter of the Meishan Formation was located in the northeast of its distribution. The southern Beijiao Uplift was part of an area of denudation (Fig. 5). The sedimentary facies of HST reveal that the southern part of the uplift was not strongly disturbed by terrigenous material because of the northern deepwater, the depth of the depression, and the presence of the Xisha Rise to the south of the uplift. The Xisha Rise has been the site of large-scale reef development from the Miocene until the present day, meaning that the environment surrounding the Beijiao Uplift experienced favorable conditions for the growth of large bioherms during Miocene period.
4.3. Seismic attributes The seismic attributes and acoustic impedance of the reef carbonates within the Beijiao and Ledong–Lingshui Depressions are highly similar to those of the proven oil- and gas-bearing bioherm within the LH11-1 oil field (Fig. 6a). Seismic inversion is an important method in determining rock properties such as impedance and porosity (Huuse and Feary, 2005), and has been successfully applied to sandstone reservoirs; however, erroneous results have been obtained when applied to carbonate reservoirs (Cerepi et al., 2003). Therefore, in the present study we employed the constrained sparse spike inversion method (Mu, 2005) in evaluating rock properties, subsequently comparing them with the properties of rocks in other carbonate oil fields, such as the LH11-1 field. The seismic profile across the LH11-1 oil field shows clear high-amplitude anomalies (Fig. 6a) and discontinuous and chaotic reflections within the bioherm, with relatively continuous highamplitude anomalies above and below, indicative of dense rock strata.
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The acoustic amplitude values of the reef carbonate in the Beijiao and Ledong–Lingshui Depressions indicate a stronger anomaly than that expected for a layer of pure limestone, which can be recognized based on high amplitude and good continuity; there exists only one obvious high-amplitude layer within well BD23-1-1 (Fig. 6b). This type of limestone is generally characterized by low porosity, resulting in poor reservoir potential (Liu and Feng, 2001). In contrast, the profile across the Beijiao Depression (Fig. 6c) reveals similar seismic attributes to those of the LH11-1 bioherm: a lack of continuity, high amplitude, and chaotic reflections. The seismic attributes of typical bioherms in the Ledong–Lingshui Depression are different from those of the LH11-1 oil field, which has a smaller area of scattered high-amplitude anomalies in the lower uplift zone, perhaps indicating a patch reef or pinnacle reef (Fig. 6c, d). In terms of the absolute acoustic impedance of these three areas, the dense limestone in well BD23-1-1 recorded values of >10000 g/cc m/s, higher than the values of 8000–10000 g/cc m/s in the LH11-1 oil field and 8000–9000 g/cc m/s in the Beijiao Depression.
5. Discussion 5.1. Paleogeography and bioherm sedimentary model The central South China Sea basin opened during the period 32– 17 Ma (Taylor and Hayes, 1980). The spreading of South China Sea have been ceased and the QDNB entered post-rift thermal subsidence in Meishan period (15.5–10.5 Ma). The scale and thickness of deposition decreased during this period. The northern part of the Beijiao Depression was a deepwater environment at this time, barely reached by terrigenous material. Therefore, the clastic material in the basin was probably derived from the Indo-China Peninsular or the Xisha Rise. The amount of debris was minor, with most being bioclastic. During the Meishan period, the ancient Hainan Island could have provided small amounts of terrigenous material to neritic parts of the northern QDNB. In other words, continental material is unlikely to have bypassed the Ledong–Lingshui Depression and
Fig. 5. Sedimentary facies of the highstand system tract for the Meishan Formation in the deepwater area of the Qiongdongnan Basin.
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Fig. 6. Attribute profiles of representative reef carbonates from different areas. (a) Profile across the LH11-1 oil field; (b) profile across Well BD23-1-1; (c) profile across the Beijiao Depression; and (d) profile across the Ledong–Lingshui Depression.
affected bioherm development in the Beijiao Depression (Fig. 5). The Indo-China Peninsular could not have supplied large amounts of debris material to the Beijiao Depression during the Meishan period: such continental material would have first filled the Ledong–Lingshui Depression, which was the center of subsidence at that time, making it difficult to reach the Beijiao Depression. In contrast, material would have been derived from the Xisha Rise, as large-scale reefs existed at this site until the Quaternary (Zhang et al., 1989; Xu and Wang, 1999). Based on our analysis of the seismic facies, sedimentary facies, provenance, and geophysical features of the study area, we infer that the Miocene sedimentary environment of the Beijiao and Ledong–Lingshui Depressions favored bioherm development. The bioherm in the Ledong–Lingshui Depression was a small patch reef
or pinnacle reef, while that in the Beijiao Depression developed mainly on the slope and even the central area of the depression, but not the area of uplift. In terms of sedimentary facies (Fig. 5), the depression consisted of neritic facies during the Meishan period. The south uplift part of Beijiao Depression formed before deposition of the Meishan Formation and was partly denuded during deposition of the lowstand, transgressive, and highstand system tracts. The bioherm around the uplift was a shelf margin atoll. 5.2. Schematic model of the reef-type reservoir The reef carbonate reservoir could become a focus of future oil exploration in the deepwater area of the QDNB. The two development periods of source rock in the QDNB are represented by
Fig. 7. Model of hydrocarbon accumulation and transfer within a reef reservoir in the deepwater area of the Qiongdongnan Basin. The Lingtou Formation to Lingshui Formation (T60–T100) represents the source rock (red area), and the reef developed in the Meishan Formation (T50–T40) could be the reservoir, and Huangliu Formation as the seal. These yellow arrows are the hydrocarbon migration pathways.
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the Lingtou and Yacheng Formations (Wang and He, 2003). The main reservoirs within the study region are located in the Lingshui, Sanya, and Meishan Formations; the Huangliu Formation acts as the regional seal formation (Fig. 7). Faults, diapers, and unconformity surfaces were the major migration pathways for oil and gas. Unconformity surfaces and diapers were especially important during the Meishan period when faults were largely inactive. Only a small number of traps have been located in the layers between the S40 and S60 reflectors (Fig. 7). Given that few clastic reservoirs are well developed in the Lingshui and Meishan Formations, bioherm carbonate reservoirs have become the main target for deepwater hydrocarbon exploration in this basin. In addition, the bioherm carbonates in the Beijiao and Ledong–Lingshui Depressions are large, with excellent reservoir capability and great potential for the occurrence of accumulated oil and gas. A model of reef carbonate reservoirs in the deepwater area of the QDNB is presented in Fig. 7. In the model, the source rock came from the Yacheng and Lingshui Formations, the oil and gas migrated through the fault to the reef reservoir, which sealed by the upper deepwater mud of Huangliu Formation. If had the proper space–time allocation relationship, there could be a big reef oil (gas) filed in QDNB. 6. Conclusions (1) The seismic facies of reef carbonates in the deepwater area of the QDNB are characterized by chaotic bedding, intermittent internal reflections, chaotic, blank, or mounded reflections, and apparent high-amplitude anomalies on seismic profiles. (2) Two mid-Miocene bioherms in the Beijiao and Ledong– Lingshui Depressions are recognized based on seismic and drillhole data. Shelf margin atolls are well developed in the Beijiao Depression; the reefs in the Ledong–Lingshui Depression are small patch reefs or pinnacle reefs. (3) The geology and seismic characteristics of the bioherms in the Beijiao and Ledong–Lingshui Depressions are similar to those in proven classic areas of reef carbonate oil reservoirs. These bioherms might therefore become important targets for hydrocarbon exploration. Acknowledgements This work was supported by the CAS Knowledge Innovation Program (KZCX2-YW-203), the National Basic Research Program of China (2007CB411703), the National MLR National Petroleum Resource Strategic Target Survey and Evaluation Program, and the Taishan Scholarship Program of Shandong Province. We are grateful to Wu Jingfu, Lv Ming, Xu Qiang, Liang Jianshe, Liu Zhibin, He Shibin, and Huang Ruojun of the CNOOC Beijing Research Center for their constructive suggestions and great assistance. Special thanks are due to Luiz Gamboa of Petrobras Company for his thoughtful modifications to the manuscript. References Cerepi, A., Barde, J.-P., Labat, N., 2003. High-resolution characterization and integrated study of a reservoir formation: the Danian carbonate platform in the Aquitaine Basin (France). Marine and Petroleum Geology 60, 1161–1183. Chen, S.Z., Hu, Z.P., 1987. The bioherm of Tertiary in the Pearl River Basin and its application in exploration for oil. China Offshore Oil and Gas 1 (1), 3–10. Erlich, R.N., Barrett, S.F., Guo, B., 1990. Seismic and geological characteristics of drowning events on carbonate platforms. AAPG Bulletin 74, 1523–1537. Fan, J.S., 1996. Reefs and Oil–Gas in China. Science Publishing House, Beijing.
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Marine and Petroleum Geology journal homepage: www.elsevier.com/locate/marpetgeo
Seismic characteristics of a reef carbonate reservoir and implications for hydrocarbon exploration in deepwater of the Qiongdongnan Basin, northern South China Sea Shiguo Wu a, d, *, Shengqiang Yuan a, b, Gongcheng Zhang c, Yubo Ma a, b, Lijun Mi e, Ning Xu f a
Key Laboratory of Marine Geology and Environment, Institute of Oceanology, Chinese Academy of Sciences, 7 Naihai Road, Qingdao 266071, PR China Graduate University of Chinese Academy of Sciences, Beijing 100049, PR China CNOOC Research Center, Beijing 100027, PR China d China University of Petroleum, Qingdao 266555, PR China e Exploration Department of CNOOC China Ltd, BeQing 100010, China f China National Oil and Gas Exploration and Development Corporation, Beijing 100034, China b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 October 2007 Received in revised form 3 April 2008 Accepted 7 April 2008 Available online 10 May 2008
Our analysis of approximately 40,000 km of multichannel 2-D seismic data, reef oil-field seismic data, and data from several boreholes led to the identification of two areas of reef carbonate reservoirs in deepwater areas (water depth 500 m) of the Qiongdongnan Basin (QDNB), northern South China Sea. High-resolution sequence stratigraphic analysis revealed that the transgressive and highstand system tracts of the mid-Miocene Meishan Formation in the Beijiao and Ledong–Lingshui Depressions developed reef carbonates. The seismic features of the reef carbonates in these two areas include chaotic bedding, intermittent internal reflections, chaotic or blank reflections, mounded reflections, and apparent amplitude anomalies, similar to the seismic characteristics of the LH11-1 reef reservoir in the Dongsha Uplift and Island Reef of the Salawati Basin, Indonesia, which house large oil fields. The impedance values of reefs in the Beijiao and Ledong–Lingshui Depressions are 8000–9000 g/cc m/s. Impedance sections reveal that the impedance of the LH11-1 reef reservoir in the northern South China Sea is 8000– 10000 g/cc m/s, whereas that of pure limestone in BD23-1-1 is >10000 g/cc m/s. The mid-Miocene paleogeography of the Beijiao Depression was dominated by offshore and neritic environments, with only part of the southern Beijiao uplift emergent at that time. The input of terrigenous sediments was relatively minor in this area, meaning that terrigenous source areas were insignificant in terms of the Beijiao Depression; reef carbonates were probably widely distributed throughout the depression, as with the Ledong–Lingshui Depression. The combined geological and geophysical data indicate that shelf margin atolls were well developed in the Beijiao Depression, as in the Ledong–Lingshui Depression where small-scale patch or pinnacle reefs developed. These reef carbonates are promising reservoirs, representing important targets for deepwater hydrocarbon exploration. Ó 2008 Elsevier Ltd. All rights reserved.
Keywords: Qiongdongnan Basin (QDNB) Deepwater hydrocarbons Sequence stratigraphy Reef carbonate Seismic attributes
1. Introduction Marginal reef carbonate facies in offshore areas have been confirmed as important deepwater oil–gas reservoirs (Sarg, 1988 Sattler et al., 2004; Zampetti et al., 2004). Qiongdongnan Basin (QDNB) is a large rift basin developed at the margin of the northern South China Sea (NSCS). Development of the basin was controlled by rifting and sustained seafloor spreading within the South China
* Corresponding author. Key Laboratory of Marine Geology and Environment, Institute of Oceanology, Chinese Academy of Sciences, 7 Naihai Road, Qingdao 266071, PR China. Tel./fax: þ86 532 82 898 544. E-mail address: [email protected] (S. Wu). 0264-8172/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.marpetgeo.2008.04.008
Sea. The first 2-D seismic survey in the QDNB was performed in 1970s, marking the commencement of exploration in this area. The study area covers 20,000 km2, and we used data from nearly 40,000 km of 2-D seismic lines and four wells. This paper documents the geological and geophysical characteristics of reef carbonate facies in deepwater areas of the QDNB. There exist differing opinions regarding the conditions of reef development in the QDNB. Many geologists argue about the existence of carbonate reservoirs in the basin: some believe that largescale reef development took place in the mid-Miocene (the Meishan Formation) (Chen and Hu, 1987; Jin and Pang, 1998; Qiu and Chen, 1999; Qiu and Gong, 1999; Qiu and Wang, 2001; Liu, 2003; Sattler et al., 2004; Wei et al., 2005, 2006). However, drilling has failed to reveal any evidence of a large-scale reef reservoir in shallow-water
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areas of the basin. This may reflect the narrow width of the continental shelf in this area and its proximity to land, making it prone to disturbance and blanketing by terrigenous sediments (Fan, 1996; Qiu and Chen, 1999; Qiu and Gong, 1999). Given this lack of success in finding a reef reservoir, it is important to also consider deepwater areas far away from continental source areas. The approaches of sequence stratigraphy and geophysical inversion have been combined in previous studies that sought to predict the locations of carbonate reservoirs in specific environments (King, 1972; Erlich et al., 1990; Mayall et al., 1997; Kusumastuti et al., 2002; Zampetti et al., 2004). In the present study, we first analyzed the stratigraphy and seismic facies of the basin, and then identified favorable carbonate rock provinces as potential reservoirs. We then undertook a paleogeographic study of the area and an inversion of seismic data. Finally, we identified the reef-development characteristics of the study area, combined the seismic facies with the geophysical attributes of carbonate sequences, and compared our findings with those obtained for typical reefs from other locations.
2. Geological setting Qiongdongnan Basin lies in the west part of northern South China Sea. The deepwater area of the QDNB is located in the southern part of the basin, where the present water depth is at least 200 m. The basin is bounded to the west by the Red River Fault and the Yinggehai Basin, to the east by the Pearl River Mouth Basin, and to the south by the Xisha Rise. The deepwater area of the basin includes the Ledong–Lingshui, Beijiao, and Songnan–Baodao Depressions, and the Beijiao Uplift (Fig. 1). The features of the deepwater area are typical of passive continental margins, with a two-layered structure consisting of a lower syn-rift sequence and upper post-rift sequence (Taylor and Hayes, 1980). Seismic reflector T60 is a breakup unconformity that divides the upper and lower megasequences. The upper sequence is characteristic of post-rift thermal subsidence, containing few fractures, whereas the lower sequence records syn-rift tectonics, marked by well-developed fault systems and half-graben basins. The syn-rift sequences in the deepwater area of the QDNB include the Lingtou, Yacheng, and Lingshui Formations, consisting of
lacustrine, coal-bearing transitional and epicontinental sedimentary facies (Qiu and Gong, 1999) cut by well-developed fault systems. The main source rocks formed during rifting. Post-rift drift sequences consist of Neogene–Quaternary marine terrigenous detritus and carbonate deposits, including the Miocene Sanya, Meishan, and Huangliu Formations, the Pliocene Yinggehai Formation, and Quaternary Ledong Formation, all devoid of syndepositional fault systems. The key factors that control the growth and development of coral are water temperature, salinity, water depth, water turbidity, concentration of dissolved oxygen, substrate, and water dynamics (waves and streams) (Wei et al., 2006). Suitable conditions for coral growth and reef development are as follows: temperature of 23– 27 C, salinity of 30–40&, water depth of <50 m, clear water, and good light penetration for photosynthesis. It is clear that the QDNB meets most of these criteria in Miocene (Sun and Esteban, 1994). The conditions of the deepwater area of the mid-Miocene QDNB were suitable for reef growth. Modern carbonate deposits are distributed mainly between 30 S and 30 N, under tropical to subtropical conditions (Sun and Esteban, 1994). The deepwater area of the QDNB that was suitable for reef development during the midMiocene was located on a shelf margin platform, with only a minor input of detrital terrigenous material due to its distance from the source area.
3. Cenozoic sequence stratigraphy Eight sequences have been recognized based on drilling results, and eight seismic reflectors (T20, T30, T40, T50, T60, T70, T80, and Tg) have been confirmed from synthetic seismograms (Table 1; Figs. 2 and 3). The reflectors divide the Cenozoic strata into eight formations: the Ledong, Yinggehai, Huangliu, Meishan, Sanya, Lingshui, Yacheng, and Lingtou (Figs. 2 and 3). Reflector T60 is a regional unconformity that divides the entire stratigraphy into syn-rift and post-rift sequences (Wang et al., 1998; Wei et al., 2001). The sequence boundaries in the basin are either unconformities or marked by relatively conformable successions (Vail, 1987). Each sequence is recognized by its external form and internal seismic facies, which are also important indicators in identifying reefs on seismic profiles.
Fig. 1. Structural units of the Qiongdongnan Basin. (1) Songnan–Baodao Depression, (2) Ledong–Lingshui Depression, (3) Beijiao Depression, (4) West Beijiao Rise, (5) East Beijiao Rise, and (6) South Beijiao Rise.
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Table 1 Sequence stratigraphy of the Qiongdongnan Basin (the Lingtou and Yacheng Formations are the major source rocks in the basin). Chronostratigraphy
Seismic reflector
Age (Ma)
System
Series
Formation
Quaternary
Pleistocene
Ledong
T20
1.9
Neogene
Pliocene Miocene
Upper
Yinggehai Huangliu
T30 T40
5.5 10.5
Middle
Meishan
T50
15.5
Lower Oligocene
Sanya Lingshui Yacheng
T60 T70 T80
23.3 29.3 35.4
Ecocene
Lingtou
T100
Paleogene
Reservoir
Structural and fill data
Submarine fan wedge Submarine fan Reef turbidite slope fan Incised valley channel-fill fan Fan delta Fan delta Fan delta
Passive continental margin
Fan delta
Period of post-rifting
Shelf–slope system
Open neritic and bathyal
Intercontinental rift
Period of rifting fill
Intracontinental rift
Offshore and neritic Partition offshore and neritic Rifted lacustrine basin
Pre-Tertiary
The syn-rift period consists of the Lingtou, Yacheng, and Lingshui formations, and the tectonic movement was the dominant control factor on sedimentation. The Lingtou Formation is a nonmarine lacustrine facies, indicating the initial period of the syn-rift period, and is one of the main source rocks within the QDNB (Zhong et al., 2004; He et al., 2006). The Yacheng Formation represents the late periods of syn-rift tectonics, when offshore and neritic sedimentary systems developed, and represent the major part of the basin fill. The Lingshui Formation was deposited at the end of the syn-rift period within a semi-closed marine sedimentary system, and is an important reservoir within the QDNB. Post-rift sequences include the Ledong, Yinggehai, Huangliu, Meishan, and Sanya Formations. The Miocene was an important period for the growth of reefs in the QDNB, especially during transgressive and highstand system tracts. Miocene strata consist of the Sanya Formation (23.3–15.5 Ma), Meishan Formation (15.5– 10.5 Ma), and Huangliu Formation (10.5–5.5 Ma). The continental shelf system of the NSCS developed above reflector T60, and it is evident on seismic profiles across the deepwater region that the thickness of strata increases from the shelf-to-slope (Figs. 2 and 3). The shelf-to-slope system was active from 10.5 Ma (T40), meaning that the Sanya and Meishan Formations formed in open neritic and bathyal environments, whereas the Ledong, Yinggehai, and Huangliu Formations belong to the shelf- to-slope system (Table 1). The Miocene sequence can be divided into system tracts identified from seismic profiles. Transgressive and highstand system tracts in the sequence of Meishan Formation were favorable environments for the development of reef carbonate, as recorded in other sequence stratigraphy studies (Handford and Loucks, 1993).
The Sanya Formation contains neritic clastic facies of submarine fans and reef growth, while the Huangliu Formation is dominated by neritic to abyssal terrigenous facies that serve as a good seal for the potential Miocene reef oil field.
4. Characteristics of reef carbonate 4.1. Seismic reflection characteristics Drilling reveals that carbonate rocks are abundant within the QDNB deepwater area, where geological conditions favored the development of carbonates during the Miocene in the NSCS. The LH11-1 reservoir beneath the Dongsha Rise (Pearl River Mouth Basin) is a 60-m thick reef carbonate reservoir with reserves of 164 million tons, making it the largest offshore oil field in China (Sattler et al., 2004). There exist 200-m thick limestones and dolomites, along with interbedded mudstone, sandstone, and phosphorous, in the early Miocene Zhujiang Formation and mid-Miocene Hanjiang Formation, as found in well QH36-2-1 drilled into the Shenhu Rise, Pearl River Basin. In addition, more than 60 m of biolithite within the Zhujiang Formation was encountered in well BD23-1-1 upon the southwest margin of the Shenhu Rise, Pearl River Basin (Liu and Feng, 2001). Well Ya 21-1-4 in the eastern neritic part of the basin revealed >60-m thick limestones of the Sanya Formation, as well as discontinuous limestone of the Meishan Formation. Several hundred meters of reef layers have also been reported from boreholes Xiyong-1 and Xiyong-2 upon the Xisha Rise (Fig. 1; Zhang et al., 1989; Xu and Wang, 1999).
Fig. 2. Seismic profile across the Ledong–Lingshui Depression (line A–A0 in Fig. 1). The seismic reflectors are labeled T20, T30, T40, T50, T60, T70, T80, and Tg. The stratigraphic framework is described in Table 1.
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Fig. 3. Seismic profile across the Songnan–Baodao Depression (line B–B0 in Fig. 1). The seismic reflectors are labeled (from top to bottom) T20, T30, T40, T50, T60, T70, T80, and Tg. The stratigraphic framework is described in Table 1.
The reef carbonate is a carbonate platform where growing reef surround or cover it. It is characterized on seismic profiles by chaotic bedding, intermittent internal reflections, chaotic or blank reflections, mounded reflections, and apparent amplitude anomalies (Sattler et al., 2004; Yu et al., 2005). Seismic profiles across the LH11-1 reef oil field reveal that the main reservoir is located within the early Miocene Zhujiang Formation reef reservoir of the Pearl River Basin, which exhibits high amplitude and discontinuous layers; marine shale of the mid-Miocene Zhuhai Formation served as the main seal to the reservoir (Fig. 4a; Yue et al., 2005). The Miocene Meishan and Sanya Formations are regarded to represent ideal conditions for the development of large-scale reef complexes (Chen and Hu, 1987; Wei et al., 2005). The seismic reflection features of these formations are similar to those of the reef reservoir within the LH11-1 oil field (Sattler et al., 2004). To study reef development and distribution within the two target formations, we divided them into a lowstand system tract (LST), transgressive system tract (TST), and highstand system tract (HST), and
analyzed the seismic characteristics of the three types of tracts. The Beijiao and Ledong–Lingshui Depressions are characterized by high amplitude, lack of continuity and low frequency, and chaotic or mounded seismic facies (Fig. 4b). A >60-m thick pure biolithite was drilled in the Zhujiang Formation at well BD23-1-1 on the southern margin of the Shenhu Rise (equivalent to the Meishan Formation within the Qiongdongnan Basin). This unit shows strong reflections and amplitude, and high continuity in seismography (Fig. 4c). The core drilled at this site revealed a compact, barren reef (Liu and Feng, 2001), different in character from the reservoir reef rocks. The seismic profiles across the Ledong–Lingshui Depression contain features similar to those of typical patch reefs within the Meishan Formation (Fig. 4d). Based on this finding, and in combination with the geologic background of this area, we deduce the existence of patch reefs or pinnacle reefs in the Ledong–Lingshui Depression. The deeper zones within the depression contain smaller deepwater carbonate buildups with mound configurations and high amplitudes (Erlich et al., 1990).
Fig. 4. Seismic profiles of representative reef carbonates in different areas. (a) Profile across the LH11-1 field; (b) profile across the Beijiao Depression; (c) profile across well BD23-1-1; and (d) profile across the Ledong–Lingshui Depression.
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4.2. Sedimentary facies High-resolution sequence stratigraphic analysis reveals that bioherms occurred in TST and HST of the Meishan Formation. Fig. 5 provides an HST sedimentary facies map of the Meishan Formation, which also shows the reef and carbonate platform distribution of HST Meishan Formation. The depocenter of the Meishan Formation was located in the northeast of its distribution. The southern Beijiao Uplift was part of an area of denudation (Fig. 5). The sedimentary facies of HST reveal that the southern part of the uplift was not strongly disturbed by terrigenous material because of the northern deepwater, the depth of the depression, and the presence of the Xisha Rise to the south of the uplift. The Xisha Rise has been the site of large-scale reef development from the Miocene until the present day, meaning that the environment surrounding the Beijiao Uplift experienced favorable conditions for the growth of large bioherms during Miocene period.
4.3. Seismic attributes The seismic attributes and acoustic impedance of the reef carbonates within the Beijiao and Ledong–Lingshui Depressions are highly similar to those of the proven oil- and gas-bearing bioherm within the LH11-1 oil field (Fig. 6a). Seismic inversion is an important method in determining rock properties such as impedance and porosity (Huuse and Feary, 2005), and has been successfully applied to sandstone reservoirs; however, erroneous results have been obtained when applied to carbonate reservoirs (Cerepi et al., 2003). Therefore, in the present study we employed the constrained sparse spike inversion method (Mu, 2005) in evaluating rock properties, subsequently comparing them with the properties of rocks in other carbonate oil fields, such as the LH11-1 field. The seismic profile across the LH11-1 oil field shows clear high-amplitude anomalies (Fig. 6a) and discontinuous and chaotic reflections within the bioherm, with relatively continuous highamplitude anomalies above and below, indicative of dense rock strata.
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The acoustic amplitude values of the reef carbonate in the Beijiao and Ledong–Lingshui Depressions indicate a stronger anomaly than that expected for a layer of pure limestone, which can be recognized based on high amplitude and good continuity; there exists only one obvious high-amplitude layer within well BD23-1-1 (Fig. 6b). This type of limestone is generally characterized by low porosity, resulting in poor reservoir potential (Liu and Feng, 2001). In contrast, the profile across the Beijiao Depression (Fig. 6c) reveals similar seismic attributes to those of the LH11-1 bioherm: a lack of continuity, high amplitude, and chaotic reflections. The seismic attributes of typical bioherms in the Ledong–Lingshui Depression are different from those of the LH11-1 oil field, which has a smaller area of scattered high-amplitude anomalies in the lower uplift zone, perhaps indicating a patch reef or pinnacle reef (Fig. 6c, d). In terms of the absolute acoustic impedance of these three areas, the dense limestone in well BD23-1-1 recorded values of >10000 g/cc m/s, higher than the values of 8000–10000 g/cc m/s in the LH11-1 oil field and 8000–9000 g/cc m/s in the Beijiao Depression.
5. Discussion 5.1. Paleogeography and bioherm sedimentary model The central South China Sea basin opened during the period 32– 17 Ma (Taylor and Hayes, 1980). The spreading of South China Sea have been ceased and the QDNB entered post-rift thermal subsidence in Meishan period (15.5–10.5 Ma). The scale and thickness of deposition decreased during this period. The northern part of the Beijiao Depression was a deepwater environment at this time, barely reached by terrigenous material. Therefore, the clastic material in the basin was probably derived from the Indo-China Peninsular or the Xisha Rise. The amount of debris was minor, with most being bioclastic. During the Meishan period, the ancient Hainan Island could have provided small amounts of terrigenous material to neritic parts of the northern QDNB. In other words, continental material is unlikely to have bypassed the Ledong–Lingshui Depression and
Fig. 5. Sedimentary facies of the highstand system tract for the Meishan Formation in the deepwater area of the Qiongdongnan Basin.
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Fig. 6. Attribute profiles of representative reef carbonates from different areas. (a) Profile across the LH11-1 oil field; (b) profile across Well BD23-1-1; (c) profile across the Beijiao Depression; and (d) profile across the Ledong–Lingshui Depression.
affected bioherm development in the Beijiao Depression (Fig. 5). The Indo-China Peninsular could not have supplied large amounts of debris material to the Beijiao Depression during the Meishan period: such continental material would have first filled the Ledong–Lingshui Depression, which was the center of subsidence at that time, making it difficult to reach the Beijiao Depression. In contrast, material would have been derived from the Xisha Rise, as large-scale reefs existed at this site until the Quaternary (Zhang et al., 1989; Xu and Wang, 1999). Based on our analysis of the seismic facies, sedimentary facies, provenance, and geophysical features of the study area, we infer that the Miocene sedimentary environment of the Beijiao and Ledong–Lingshui Depressions favored bioherm development. The bioherm in the Ledong–Lingshui Depression was a small patch reef
or pinnacle reef, while that in the Beijiao Depression developed mainly on the slope and even the central area of the depression, but not the area of uplift. In terms of sedimentary facies (Fig. 5), the depression consisted of neritic facies during the Meishan period. The south uplift part of Beijiao Depression formed before deposition of the Meishan Formation and was partly denuded during deposition of the lowstand, transgressive, and highstand system tracts. The bioherm around the uplift was a shelf margin atoll. 5.2. Schematic model of the reef-type reservoir The reef carbonate reservoir could become a focus of future oil exploration in the deepwater area of the QDNB. The two development periods of source rock in the QDNB are represented by
Fig. 7. Model of hydrocarbon accumulation and transfer within a reef reservoir in the deepwater area of the Qiongdongnan Basin. The Lingtou Formation to Lingshui Formation (T60–T100) represents the source rock (red area), and the reef developed in the Meishan Formation (T50–T40) could be the reservoir, and Huangliu Formation as the seal. These yellow arrows are the hydrocarbon migration pathways.
S. Wu et al. / Marine and Petroleum Geology 26 (2009) 817–823
the Lingtou and Yacheng Formations (Wang and He, 2003). The main reservoirs within the study region are located in the Lingshui, Sanya, and Meishan Formations; the Huangliu Formation acts as the regional seal formation (Fig. 7). Faults, diapers, and unconformity surfaces were the major migration pathways for oil and gas. Unconformity surfaces and diapers were especially important during the Meishan period when faults were largely inactive. Only a small number of traps have been located in the layers between the S40 and S60 reflectors (Fig. 7). Given that few clastic reservoirs are well developed in the Lingshui and Meishan Formations, bioherm carbonate reservoirs have become the main target for deepwater hydrocarbon exploration in this basin. In addition, the bioherm carbonates in the Beijiao and Ledong–Lingshui Depressions are large, with excellent reservoir capability and great potential for the occurrence of accumulated oil and gas. A model of reef carbonate reservoirs in the deepwater area of the QDNB is presented in Fig. 7. In the model, the source rock came from the Yacheng and Lingshui Formations, the oil and gas migrated through the fault to the reef reservoir, which sealed by the upper deepwater mud of Huangliu Formation. If had the proper space–time allocation relationship, there could be a big reef oil (gas) filed in QDNB. 6. Conclusions (1) The seismic facies of reef carbonates in the deepwater area of the QDNB are characterized by chaotic bedding, intermittent internal reflections, chaotic, blank, or mounded reflections, and apparent high-amplitude anomalies on seismic profiles. (2) Two mid-Miocene bioherms in the Beijiao and Ledong– Lingshui Depressions are recognized based on seismic and drillhole data. Shelf margin atolls are well developed in the Beijiao Depression; the reefs in the Ledong–Lingshui Depression are small patch reefs or pinnacle reefs. (3) The geology and seismic characteristics of the bioherms in the Beijiao and Ledong–Lingshui Depressions are similar to those in proven classic areas of reef carbonate oil reservoirs. These bioherms might therefore become important targets for hydrocarbon exploration. Acknowledgements This work was supported by the CAS Knowledge Innovation Program (KZCX2-YW-203), the National Basic Research Program of China (2007CB411703), the National MLR National Petroleum Resource Strategic Target Survey and Evaluation Program, and the Taishan Scholarship Program of Shandong Province. We are grateful to Wu Jingfu, Lv Ming, Xu Qiang, Liang Jianshe, Liu Zhibin, He Shibin, and Huang Ruojun of the CNOOC Beijing Research Center for their constructive suggestions and great assistance. Special thanks are due to Luiz Gamboa of Petrobras Company for his thoughtful modifications to the manuscript. References Cerepi, A., Barde, J.-P., Labat, N., 2003. High-resolution characterization and integrated study of a reservoir formation: the Danian carbonate platform in the Aquitaine Basin (France). Marine and Petroleum Geology 60, 1161–1183. Chen, S.Z., Hu, Z.P., 1987. The bioherm of Tertiary in the Pearl River Basin and its application in exploration for oil. China Offshore Oil and Gas 1 (1), 3–10. Erlich, R.N., Barrett, S.F., Guo, B., 1990. Seismic and geological characteristics of drowning events on carbonate platforms. AAPG Bulletin 74, 1523–1537. Fan, J.S., 1996. Reefs and Oil–Gas in China. Science Publishing House, Beijing.
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