The oil- and gas-bearing basins in the Northern South China Sea(NSCS)are characterized with thick Tertiary sedimentary and good hydrocarbon source rocks(thick Miocene marine source rock and widely distributed Paleogene lacustrine source rock), and by diversified oil and gas geological phenomenon, unique oil and gas accumulation and varied petroleum genetic types(He et al., 2008; Gong et al., 1997). Petroleum exploration in the NSCS has obtained significant breakthrough and progress during the last half century, and a number of large-medium-sized oil and gas fields have been found. The exploration in the slope deep-water area in recent years(Jin, 2006; Zhang et al., 2007)has further proved that the NSCS has a tremendous oil and gas resource potential. However, hydrocarbon prospecting degree in the study area is still quite low, the underst and ing of geology and characteristics of oil-gas accumulation are remained to be further explored and summarized.

Heat flow, which is related to the heat release of the Earth’s interior, provides an important boundary condition for calculating temperatures at depth and for comparing thermal regimes of different tectonic regions. Petroleum hydrocarbon is formed by thermal alteration of organic-rich sediments during burial. Although many factors contribute to organic metamorphism, the process is primarily dependent on the integrated time/temperature history of the buried organic material(Tissot et al., 1974; Tissot et al., 1987). In petroleum exploration, geothermal gradient and heat flow has long been applied to estimate hydrocarbon generation of source rocks, which is closely associated with temperature. It seems that high heat flow regions may easily form larger oil and gas fields with the same other geological conditions(Klemme, 1972; Xie, 1981). In this paper, we sum up and sorted out the thermal conductivity data, geothermal gradient data and heat flow data for the basins in the NSCS, analyze the thermal properties of these basins on basis of geothermal theory, and further discuss the relationship between their geothermal characteristics and hydrocarbon accumulation(distribution).This study provides with new present-day geothermal data for the SCS, which is helpful for the oil and gas resource evaluation and prediction.

The NSCS, surrounded on three sides by the Indian-Australia plate, Pacific plate, and Eurasian plate, is the overlapping area of the Paleo-Tethyan and ancient Pacific characterized by remarkable complex tectonic setting(Briais et al., 1993; Taylor et al., 1983). This study focuses on the four oil-gas-bearing basins in the NSCS, namely the Pearl River Mouth Basin(PRMB), the Qiongdongnan Basin(QDNB), the Beibuwan Basin (BBWB) and the Yinggehai Basin(YGHB), respectively(Fig. 1).

(Fig.1)

Fig.1 Location and major structures of the northern South China Sea YGHB-Yinggehai Basin, BBWB-Beibuwan Basin, QDNB-Qiongdongnan Basin, PRMB-Pearl River Mouth Basin. Circles and numbers along Profile AB show the positions of the expanding spread profile (ESP) midpoints and their serial numbers.

All these basins were developed on an extensional background related with early Tertiary spreading of the SCS, with exception of the YGHB, which was associated with the strike slip along the Red River fault zone. Generally, formation and evolution of these basins experienced a rifting and a post-rifting period, the rifting period consists of multiple episodes of rifting(Ru et al., 1994; Gong, 2004). According to the regional tectonic setting, the evolution of these basins can be subdivided into four stages: pre-spreading initial rifting, syn-spreading intense rifting, post-spreading slow subsidence and post-spreading rapid subsidence(Xie et al., 2011). There are two abrupt environment transformations from the Paleocene to the present, i.e. from terrestrial to paralic, neritic to abyssal environments, which are controlled by tectonic evolutionary stages and related paleogeography. The first face transition occurred at the end of the Oligocene from Paleocene-Oligocene alluvial and lacustrine deposits to Early and middle Miocene near-shore and neritic deposits; and the second transformation of depositional systems occurred at the late Miocene(Lüdmann et al., 1999; Xie et al., 2006) (Fig. 2).

(Fig.2)

Fig.2 Tectonic evolution and sedimentation of the basins in the northern South China Sea (modified from Xie et al.(2011))

Extensive heat flow measurements have been conducted in the South China Sea since the 1970s and provided us a large number of heat flow data(Anderson et al., 1978; Jessop et al., 1976; Nissen et al., 1995;Qian et al.; Ru et al., 1986; Shyu et al., 1998; Taylor et al., 1983; Watanable et al., 1977; Xia et al., 1995; Chen et al., 1991; Rao et al., 1991). He et al.(2001a)colleted the published heat flow data systematically and presented a heat flow map with 589 data. Using a compilation of 592 data, Shi et al.(2003)analyzed the heat flow distribution characteristics of the SCS. In this paper, we report 148 newly determined thermal conductivity, 65 new temperature gradient data calculated from borehole temperature logging, and 65 heat flow data calculated based on these two parameters.

Thermal conductivity, one of the most important parameters, is essential for the study of the thermal structure of lithosphere and deep thermal state of the Earth. We measured 148 thermal conductivity samples of the YGHB and the BBWB(Table 1), most of which are from sedimentary strata. Combined with previous results(Yuan et al., 2009; Rao et al., 1991), the conductivity for each lithology in the study area shows quite large variation. The values of mudstone, s and stone, limestone and basement rocks (including granite, diorite, quartzite and so on)are 1.06~3.69 W/(m·K), 1.04~3.98 W/(m·K), 0.72~3.5 W/(m·K), 1.42~4.43 W/(m·K), respectively. The thermal conductivity of sedimentary rocks generally increases with depth. It shows that the thermal conductivities of mudstone are highly depth dependent (Fig. 3a)while those of s and stone(Fig. 3b) and limestone (Fig. 3c)are much less depth dependent. However, the basement rocks do not exhibit increasing thermal conductivity with depth(Fig. 3d).

(Table 1)

Table 1 Newly acquired thermal conductivity data of YGHB and BBWB

Table 1 Newly acquired thermal conductivity data of YGHB and BBWB

(Fig.3)

Fig.3 Charts showing characteristics of thermal conductivity for rocks from the four basins (YGHB, BBWB, QDNB and PRMB) in the northern South China Sea Left figures show the relationship between thermal conductivity and depth while right ones illustrate the value distribution of thermal conductivity for different kind of rocks in histogram.

As the core samples of the South China Sea basins are very scarce, we cat not measure thermal conductivity of all boreholes that used to calculate heat flow in this paper. Based on the large number of measured thermal conductivity, average formation thermal conductivity of the BBWB and the YGHB were calculated by the weighted average method according to the formation lithology proportion(Qiu et al., 2004), and the results are shown in Table 2.

(Table 2)

Table 2 Average thermal conductivity of each formation in the BBWB and the YGHB

Table 2 Average thermal conductivity of each formation in the BBWB and the YGHB

To obtain accurate heat flow that could better reflect the regional thermal background, reliable geothermal gradients are also essential in addition to the reliable thermal conductivity data. Based on the linear regression method, new geothermal gradient data (Table 3)were acquired from comparatively reliable borehole temperature data. Together with data from previous publications(Nissen et al., 1995; Yuan et al., 2009), this study provides with an updated contour map of geothermal gradients for the study area(Fig. 4). The gradients in the study area show a zonal distribution and the gradients for the deep water area(southward from the 300 m depth of water)are significantly higher than that of the shallow water area.

(Table 3)

Table 3 Newly acquired geothermal gradient and heat flow data of YGHB and BBWB

Table 3 Newly acquired geothermal gradient and heat flow data of YGHB and BBWB

(Fig.4)

Fig.4 Geothermal gradient map of the basins in the northern South China Sea

Heat flow is the direct manifestation of energy equilibrium for various kinetic processes in the Earth’s interior and can be calculated by the following equation:

From Eq.(1), we know that the heat flow value quality is depended on the quality of thermal conductivity and temperature data. Thermal gradients in this paper are calculated based on MDT temperature and DST temperature, which are taken as more reliable temperature data. Heat flow for each borehole location is equal to the thermal gradient multiplied by thermal conductivity value of the temperature depth range. For those boreholes without thermal conductivity measurement, the average formation thermal conductivity values are used(Table 2).

A heat flow database of the NSCS has been set up using data from previous research, data in literature since 2003(Li et al., 2010; Xu et al., 2006)as well as the newly acquired 65 data(Table 3 and Fig. 5). Based on this database, an updated heat flow map is constructed and the average heat flow data of the four sub-basins are evaluated. The average heat flow is 68.7±11 mW/m² for the PRMB, 71.1±13 mW/m² for the QDNB, 65.7±8.9 mW/m² for the BBWB, and 74.7±10 mW/m² for the YGHB, respectively. The regional pattern of heat flow is similar to the characteristics of the geothermal gradients, which increases gradually from the shelf to the slope(Fig. 6).

(Fig.5)

Fig.5 Heat flow data scattered in the study area Solid circles represent the data from previous research while the triangles represent the data from this paper.

(Fig.6)

Fig.6 Heat flow map for the basins in the northern South China Sea

There are two things to be mentioned. First, the geothermal gradient and heat flow maps are constructed based on different kinds of data including borehole data, aerial survey data and probe measurement data, whereas average values acquired in this paper are only based on relatively reliable borehole data. Second, data from the adjacent areas of the NSCS, such as Guangdong and Hainan provinces, has also been used for heat flow map construction in order to decrease the boundary effect(Hu et al., 2000).

The average geothermal gradient of the NSCS, with a value of 37.1±6.3 °C/km, is not only much more higher than those of the cratonic basins in central and western China such as the Erdos basin(24.4 °C/km) (Yuan et al., 2007), Sichuan basin(22.8 °C/km)(Xu et al., 2011), Tarim basin(20°C/km)(Wang et al., 2003) and Junggar basin(21.2~22.6 °C/km)(Wang et al., 2000; Qiu et al., 2001), but also higher than those of some other basins in offshore China, for example, the East China Sea(32.7 °C/km)(Yang et al., 2004)basin and South Yellow Sea basin(28.6 °C/km)(Yang et al., 2003). The average heat flow of the study area, 72.6 mW/m², is more than higher than that of continental areas of China(63±24.2 mW/m²)by 10 mW/m²(Hu et al., 2000) and also higher than those of the East China Sea basin and South Yellow Sea basin(Yang et al., 2004; Yang et al., 2003). All the geothermal characteristics presented above prove that the basins located in the NSCS are typical “hot basins”(Wang, 1996).

The present-day geothermal regime could be influenced by many factors, such as latest tectonic movement, extensional degree of lithosphere, crustal thickness, and thermal conductivity structure(Yuan et al., 2006). It is widely accepted that basins in the northern continental margin of the South China Sea have experienced multi-stage episodic rifting and subsequent thermal subsidence since the late Cretaceous(He et al., 2001b; Ru et al., 1986). The multi-stage rifting caused crust thinning as well as substantial tectonic subsidence. Gravity and deep seismic data show that the crust of the northern margin is obviously thinning southeastwards, the crust thickness changed from 30 km on the shelf to 22 km on the slope, and further to 14 km at the foot of the lower slope, just 15 km in the Xisha Trough and only 12 km in the central ocean basin(Nissen et al., 1995;Qiu et al., 2001; Qiu et al., 2003)(Fig. 7). The study area has also experienced several episodes of magmatic activities, at least there episodes of magmatism in the PRMB over the Paleocene-Eocene, Oligocene-mid Miocene and late Miocene-Quaternary periods. The magma changed from predominantly acidic in the late Mesozoic, to neutral-acidic in the Palaeogene period, and to neutral-basic in the Neogene(Yan et al., 2006; Li et al., 1998; Zou et al., 1995). These magma activities could have produced some local high heat flow anomalies such as high values in the Shunde depression of the PRMB(Rao et al., 1991). In addition to magma activity, fault activity could be another reason for the high anomalies in the study area. For example, the anomaly in Yangjiang depression is likely to be the result of northern fault activity of the PRMB while the high heat flows in the southwest YGHB and western QDNB are attributed to the activity of the No.1 fault.

(Fig.7)

Fig.7 Crust structure of the northern South China Sea (modified from Yao et al., 1998) Location of the section AB is shown in Fig. 1.

There are two approximately east-west trending basin(depression)zones in the NSCS, namely the northern basin(depression)zone and central-southern basin(depression)zone. The former consists of BBWB and the northern depression belt of the PRMB while the latter is composed of YGHB, QDNB and the ZHU II depression as well as the Chaoshan depression of the PRMB(Zhang et al., 2010)(Fig. 6).

It seems that oil fields of the NSCS prefer to be located in the northern basin(depression)zone, where the crust is thicker and heat flow(average is 65.7±9.4 mW/m²)is lower compared to the central-southern basin(depression)zone. While the gas fields are more likely to centralize in the central-southern zone(average heat flow is 87.1±16 mW/m²). After years of exploration, several oil field clusters have been found in the northern zone such as the Lufeng oil field cluster, Huizhou oil field cluster, Xijiang oil field cluster, Panyu oil field cluster, Wenchang oil field cluster and Weizhou oil field cluster while some gas fields have been discovered in the central-south zone such as the YA13-1 gas field, Dongfang1-1 gas field and Ledong22-1 gas field(Fig. 6) (Zhang et al., 2010).

Geothermal is one of the most control conditions for hydrocarbon formation and evolution. However, the statement that high heat flow values are favorable for oil and gas generation is not accurate and rigorous. Generally, when the source rock burial depth(present or paleo)is large, the high heat flow prefers to over-mature source rock, resulting in abundant gas but much less oil. On the contrary, the source rock may be un-mature and thus poor in both gas and oil. Under other conditions, shallow source rock burial depth for example, relatively high heat flow is favorable for hydrocarbon accumulation. Therefore, knowledge of heat flow is of great significance in oil and gas evaluation and exploration(Wang, 1992). We have analyzed the geothermal characteristics as well as the oil&gas accumulation in the northern continental margin basins of the South China Sea, and found that there is a good correlation between them. This kind of correlation, which currently stays in the qualitative phase, may have a guiding effect on the oil&gas prediction and exploration prospects.

The basins in the NSCS, with average geothermal gradient of 37.1±6.3 °C/km and average heat flow of 72.6 ±15.6 mW/m², have a present geothermal field similar to the typical “hot basin”.

Heat flow distribution of the NSCS is primarily controlled by the regional tectonic setting and locally influenced by hydrothermal activities associated with magma and fault activities. In response to the thinning of lithosphere and crust, the heat flow increase from the northern basin zone to the central-southern basin zone.

Results of exploration indicate that there is a close correlation between the hydrocarbon accumulation and thermal regime in the NSCS. The distribution of hydrocarbon fields in the study area shows a coupling relationship with the approximately zonal distributed heat flow, i.e., oil fields centralized in the northern basin zone while the gas field mainly located in the central-southern basin(depression)zone.

The temperature data used in this paper were provided by the CNOOC Research Institute and we are grateful to Zhanjiang Branch, CNOOC(China)Co., Ltd. for the help in thermal conductivity measurement. This work was supported by the National Major Special Project of Science and Technology “Key Techniques for Hydrocarbon Exploration in Deep Water Offshore”(2011ZX05025-006-05).