2 Southern Marine Science and Engineering Guangdong Laboratory(Guangzhou), Guangzhou 511458, China;
3 Guangzhou Marine Geological Survey, China Geological Survey, Guangzhou 510760, China;
4 University of Chinese Academy of Sciences, Beijing 100049, China
Oil resources are rich in the Mesozoic strata (Hanson et al., 2007; Safronov et al., 2014; Yuan et al., 2018; Chen et al., 2019). Mesozoic strata are widespread in offshore China (Lu et al., 2014; Hu et al., 2015; Wang et al., 2016; Yang et al., 2020). The northern part of the South China Sea is a key area for offshore oil-gas exploration in China (Wu et al., 2009; Gao et al., 2018). Research on the Mesozoic strata in the northern South China Sea has become one of the frontier subjects in earth science since the discovery of the Mesozoic layers in the 1980s (Yao et al., 1995; Xia and Huang, 2000; Hao et al., 2001; Xu et al., 2013). Recent exploration studies have confirmed the wide existence of the Mesozoic strata in the northern South China Sea (Hao et al., 2009b, Lu et al., 2014). For instance, in the southwest Taiwan basin, multiple wells have drilled into the lower Cretaceous system in the Gaoxiong depression (Zhou, 2002; He et al., 2006) and some wells revealed the presence of organic-rich mid-upper Jurassic strata in the Chaoshan depression (Shao et al., 2007). Moreover, hydrocarbon was even found in the Mesozoic in the Taixinan Basin. The Mesozoic basins in the northern South China Sea are characterized by multi-stage evolution and are superimposed basins of prototype basins with different evolution characteristics (Zhong et al., 2008, 2011; Zhang, 2010).
The distribution of the Mesozoic strata in the northern South China Sea has been delineated based on the seismic profiles and drilling data (Lu et al., 2014; Hu et al., 2015). The Chaoshan depression, located on the uplift of the Dongsha sea area, has the widest and thickest Mesozoic strata. The LF35-1-1 well on the northern slope of the Chaoshan depression confirmed that the Chaoshan depression is a Mesozoic residual depression (Hao et al., 2009a). The Mesozoic strata of the Chaoshan depression are well-preserved and have a sound index of source rock. Moreover, large tectonics is wide-spread. Therefore, Chaoshan depression has good prospects for oil and gas exploration and is very likely to emerge as a new field of oil-gas exploration in the South China Sea (Zhou et al., 2005; Li et al., 2008; Hao et al., 2009a; Ji et al., 2014). The petrological characteristics, depositional environment, source analysis, basin evolution, and reservoir prediction of the Mesozoic strata in the Chaoshan depression was studied using various methods, on which the prospective areas of oil and gas exploration were demonstrated accordingly (Shao et al., 2007). The seismic data combined with well logging data were used to reconstruct the paleogeomorphology of the sedimentary strata and to determine the distribution and range of the sedimentary system. In addition, the sedimentary features have positive significance for the favorable reservoir prediction of the Mesozoic strata in Chaoshan depression (Qiang et al., 2018).
The Dongsha sea area is not well studied and the survey was still in the early stage. Seismic lines were sparse and drilling data were very limited, which hindered complete and consistent interpretation. Furthermore, the Mesozoic strata are very complex due to multi-stage tectonic movements. Due to the lack of a good understanding of basic geological issues such as the Mesozoic sedimentary model and tectonic evolution, it is very difficult to achieve a breakthrough of oil and gas exploration in the Mesozoic strata in the northern South China Sea.
In this study, we conducted fine data processing targeting the Mesozoic strata to obtain clear and reliable images. Integrated interpretations were conducted for favorable structures and traps to identify oil and gas distribution, to select favorable zones and key targets, and to select drilling sites for exploratory wells.
This paper is organized as follows. The material and method are presented in Section 2. Mainly results are presented in Section 3. Finally, discussion and conclusion are given in Sections 4 and 5, respectively.2 MATERIAL AND METHOD
Guangzhou Marine Geological Survey (GMGS) has conducted several long-offset seismic surveys in the Dongsha sea area since 2016 with the key objective of investigating the oil and gas distribution in the Mesozoic strata and providing high degree seismic data for evaluating the resource potential in the promising zone.
The survey is located in the detailed survey area for the key tectonic structure in the Mesozoic strata of the Chaoshan depression in the Dongsha sea area (Fig. 1). That is the A-1 structural area on the central uplift optimally determined by the previous studies (the blue box area in Fig. 1a). The seismic survey was conducted with a grid of 1 km × 2 km. The total field data was about 4 158.8 km and the three-dimensional coverage area about 383 km2. The seismic data were acquired in 2D with a dense grid using a single source and a single cable. Seismic source was triggered at interval of 25 m. Seismic wave was recorded using a 480-channel streamer at group interval of 12.5 m. The minimum offset was 175 m and the maximum offset was 6 162.5 m. The record length was 8 s and the sampling rate is 2 ms. Turbulence was very strong around the Dongsha Island (Fig. 1b), which posed a threat to vessels and underwater towing devices aside from their impact on data quality. In worse cases, the tail might also be damaged (Fig. 1c). The presence of strong ocean turbulence was the major reason why we acquired the seismic data in 2D using single-source and single-streamer acquisition for safety and quality.
Data were processed with multiple seismic data processing systems to fully exploit the advantages of each system. The main processing workflow included noise attenuation, multiple attenuation, regularization, and pre-stack time migration. A complete workflow is displayed in Fig. 2. First, noise attenuation was applied in the way that amplitude is preserved. To attenuate the abundant multiples on the seismic data, we jointly used multiple methods including the classic surfacerelated multiple elimination (SRME) method, radon transform based method, and frequency-dependent method. In addition to the conventional semblance analysis method, anisotropic bi-spectral high-density velocity analysis method was applied to obtain a better velocity model. In this way, the characteristics of seismic wave train were largely enhanced. Regularization plays a very important role in compensating the uneven distribution of energy, can greatly improve the lateral continuity, and is particularly import for 3D processing of the closelyspaced 2D seismic. Hence, before migration, a careful regularization was applied. Finally, a pre-stack time migration method was applied to obtain the final result. Our 3D processing strategy differs from previous 2D processing in many ways. In 2D processing, the 2D SRME and 2D time migration are applied to each line. The 3D SRME and 3D time migration were applied to the whole date set including all the lines. Moreover, during data processing, the characteristics of the large-offset data were acquired using single-source and single-streamer and the sedimentary features of the Mesozoic strata were taken into account. The data resolution of the middledeep target stratum was effectively improved, which ensured the image quality of the 3D pre-stack migration volume (Xu et al., 2009; Xing et al., 2015; Deng et al., 2019).3 RESULT 3.1 Fine seismic processing
As shown in Fig. 3, after the refined processing, fault surface waves and diffraction waves were converged, the fault was clearer, the inclined reflectors were clearly imaged, and the relationship of the strata was more reliable. The signal-to-noise ratio of the data has been greatly improved. Therefore, the structure of a specific stratum could be further analyzed and interpreted based on the time slice shown in Fig. 4. From the profile imaging, the location of the target strata was accurately tracked, and the geological structure characteristics of the study area were more intuitively reflected, so as to serve the interpretation work.3.2 Structural feature analysis and comprehensive interpretation
Chaoshan depression is a Mesozoic residual depression, which has experienced the subsidence in the Middle-late Jurassic, the uplift in the late Jurassic and the early Cretaceous, the burial in the early Cretaceous and late Cretaceous and the uplift and denudation in the late Cretaceous, forming structural traps with diverse shapes, such as compression anticline, broken noses, and fault blocks. The residual Mesozoic strata are 2 000–6 000-m thick and are rich in Jurassic marine hydrocarbon source rocks. The coastal sandstone, delta sandstone, submarine fan sandstone, turbidite fan sandstone, and fluvial sandstone in the middle and upper Jurassic are favorable reservoirs (Xia et al., 2004; Chen, 2007; Yao et al., 2009). Although primary oil reservoir might have been destroyed due to large-scale uplift and erosion, large-scale oil and gas reservoir is still probable, considering the widespread presence of the Mesozoic strata and the transformation of two stages due to tectonic overturn.
The Chaoshan depression can be divided into six sub-units, namely, the eastern sag, western sag, central high, northern slope, western slope, and southern slope. The A-1 structure in the central high was selected as the favorable exploration target.3.2.1 Favorable structure analysis
The A-1 structure is a structure in the shape of a broken nose or broken block. It is a typical uplift within a depression and its associated traps have been well determined. The Cenozoic strata in the area of A-1 are thicker than that in other areas, and the lower source rocks may be in the mature stage. The thickness of the residual Mesozoic strata is generally above 1 600 m. The Mesozoic structure is more stable and favors a better hydrocarbon source condition (Ji et al., 2014). Moreover, in the Jurassic, carbonate rocks of platform facies and submarine fan sandstone developed, resulting in good reservoirs.
By analyzing the oil and gas accumulation conditions in the area, we believed that the uplift and erosion between the Cretaceous and the early Eocene might destroy or transform previous oil and gas reservoirs, and thus inferred that the secondary generation of hydrocarbon of the Jurassic source rock is crucial for the latter oil and gas accumulations (Yao et al., 2009). Moreover, the thickness of the Cenozoic sediments is larger than that of erosion, which is good for the secondary generation of hydrocarbon in the source rock. Therefore, it is very likely that the secondary generation of hydrocarbons occurred in the Jurassic source rock.
We observed that from the seismic profile passing through the A-1 structure, with denser seismic lines, the seismic profile adequately reflects the structural morphology of the Mesozoic strata and accurately depicts the imaging of the complex geological structure of the target strata. The seismic profile shows the thickness of the Jurassic Tj in the A-1 structure is relatively stable while that of the Cretaceous Tk has a large variation due to erosion. The exploratory well A-1-1 is initially established based on comprehensive interpretation of the data.3.2.2 Traps and reservoir prediction
First, based on several seismic profiles near the exploratory well (Fig. 5), it can be observed that the A-1 is a compressed fold structure. A large fault exists in the south side and some small fractures develop on the top. The plane time-slice (Fig. 6) shows that the northwestern body of the A-1 structure is a complete NE-trending anticline structure and the secondhighest point to its southeast is cut by fractures and forms a structure in the shape of a broken nose. From the profile and time slice, the A-1 structure is a fold with a clear and complete shape. Reliable traps with good correlations occur in all the strata of Tj0, Tj2, Tj1, Tk0, and Tg.
Secondly, the sandstone reservoir of the A-1 structure may be mainly concentrated in the lower Jurassic and Cretaceous, and its scale is large. The Cretaceous and Jurassic sand bodies all tend thinning from northwest to southeast. The thickness of the Cretaceous sandstone is unstable and the Cretaceous sandstone even misses in the A-1 area, which may be related to the erosion of the late Cretaceous. Multiple sand bodies occur in the upper Jurassic but a single sand body is thin and has a large lateral variation in thickness. However, in the middle Jurassic, a single sand body is relatively thick and has a relatively small lateral variation. Sand bodies in the lower Jurassic are similar to that in the upper Jurassic, but are more homogeneous (Yi et al., 2012).
Thirdly, the data acquisition involved some seabed geochemical sampling in the research area. By sample testing and microbial anomaly background analysis, it was found that the Mesozoic in the Chaoshan depression has multiple sets of source rocks in different maturity levels. Some are at a high-maturity level and in the stage of dry gas generation, while some are at a mature level and in the stage of oil generation (He et al., 2008). A combined analysis of LF35-1-1 drilling and seismic profiles indicates that the western part of Chaoshan depression represents the dominant hydrocarbon-generating site. Multiple sets of source rocks were developed in the middle and upper Jurassic (Yang et al., 2008). The A-1 structure is located on the eastern edge of the western depression, which facilitates hydrocarbon filling.
Finally, the oil and gas reservoirs in the A-1 structure formed in the Cretaceous, and mainly occur in the middle and upper Jurassic strata. The oil and gas reservoirs may undergo multiple-stage of adjustments or refills because of the tectonic movement (He et al., 2010). Among them, the uplift and erosion in the late Cretaceous-Paleogene greatly influenced the Mesozoic oil and gas reservoirs. In the Neogene, the A-1 structural area is in the stage of stable subsidence, which provided regional cap rock for the underlying Mesozoic structure and enhanced the overburden pressure. Then, the faults opened due to the uplift in the late Cretaceous were closed again, facilitating the re-discharge hydrocarbons in the deep Jurassic source rocks and the re-filling of the existed structure. Therefore, the A-1 structure has a sound space-time configuration for the source-reservoir-cap relationship and good storage conditions, which allows it to form medium-large oil reservoirs.3.2.3 Preliminary implementation of well location for exploratory well drilling
Based on the analysis of the trap, the correlation of structures, and the prediction of reservoir, an exploration well A-1-1 was proposed at the summit of the structure A-1. As this well is close to the summit of all the strata, it can reveal the lower strata in a relatively complete manner.
According to the location of the A-1-1 well in the inline and crossline profile shown in Fig. 7, the drilling was predicted to encounter the Neogene, Paleogene, lower Cretaceous, upper Jurassic, middle Jurassic, and lower Jurassic from top to bottom. The major targets are sandstone reservoirs between 1 400 and 1 650 m of the upper Jurassic, between 700 and 825 m of the lower Cretaceous, and between 2 556 and 2 840 m of the middle Jurassic. All of the targets are shaded in yellow in Fig. 8. The proposed well will be completed at the upper part of the lower Jurassic with the drilling depth reaching 3 710 m under the sea. The ultimate goal of the drilling was to reveal the oil and gas conditions of the Chaoshan depression and to obtain breakthroughs in oil and gas exploration in the Mesozoic strata in the northern South China Sea. Meanwhile, the structural characteristics of the Chaoshan depression are expected to be further analyzed and the Mesozoic stratigraphy in the northern South China Sea will be further established.4 DISCUSSION
Many previous wells in the northern part of the South China Sea have revealed the presence of Mesozoic strata. However, in the study area, only one well LF35-1-1 was drilled, which is located in the northern slope. This well shows that the upper part of the middle-upper Jurassic is gray-black lamellar mudstone and argillaceous siltstone intercalated with siliceous rock, containing a small amount of micrite limestone; and the lower part is gray-black laminar mudstone and argillaceous siltstone intercalated with sandstone and limestone. Rocks and mudstones are rich in organic debris. The Cretaceous is generally characterized by river-lacustrine deposits; the upper part is a combination of purple-red mudstone, siltstone, and sandstone with a small amount of marlstone, and the lower part is a combination of gray laminar mudstone, siltstone, and sandstone, containing some organic debris (Hao et al., 2009a). Geochemical analysis results show that there are two sets of hydrocarbon source rock. Both are located in the middle-upper Jurassic with one distributed at 1 800–2 000-m depth and the other at 2 100–2 400 m. The cumulative thickness of the source rock distributed at the 1 800–2 000-m interval is 82.28 m, and the average organic carbon content is 0.67%. This corresponds to source rock of low quality. The cumulative thickness of the source distributed at the 2 100–2 400-m interval is 46.16 m, and the average organic carbon content is 1.32%. This corresponds to source rock of medium quality (Yang et al., 2008). Both of the two sets of source rock are widely distributed in the middle-upper Jurassic marine strata.
Although well LF35-1-1 was failed to detect oil and gas, it encountered radiolarian-rich marine Mesozoic and Jurassic source rocks in high organic matter abundance, which fully shows that the area has good source rock conditions for oil and gas accumulation. The source rock drilled in well LF35-1-1 also encountered multiple sets of mudstone in the southeast direction, showing that the transgression comes from the southeast part of the structure. The southeast part has a larger accommodating space, which is more suitable for the formation of hydrocarbon source rock and the preservation of organic carbon. Therefore, it is possible that source rocks in the southeast of the LF35-1-1 structure are richer and the hydrocarbon generation potential may be larger.
Based on data processing and profile interpretation, we focused on the A-1 structure located on the central uplift of the Chaoshan depression.
The structure is located in the west of the low uplift in the middle of Chaoshan depression, adjacent to the western depression and the western depression is rich in hydrocarbon source rock. The mud hydrocarbon source rock is thick and is in the mature to over-mature stage. The upper Jurassic and middle Jurassic, the target strata of the structure, are adjacent to the source rock in good conditions of hydrocarbon source. Moreover, the formation time of trap matches well with the peak period of the generation and expulsion of hydrocarbon. The strata are characterized by submarine fan sandstone and biogenic reel limestone. This type of sandstone has good performance in accumulating oil and gas. Moreover, sandstone and carbonate reservoir are developed in the main target section of the Jurassic from the amplitude anomaly of root mean square, waveform analysis, and density inversion result. The structure has good indicators of oil gas and good cap conditions. Anomaly associated with oil and gas was detected in the target strata and flat point was observed on the waveform section, indicating the presence of oil and gas in this area. The Cenozoic semi-pelagic sediments covered the Jurassic and Cretaceous strata and form good caps. Therefore, the overlying mudstone is stable, and a good group of accumulation and cap formed, which is beneficial to oil and accumulation and preservation.
A-1 structure is characterized by stable Mesozoic strata and favorable hydrocarbon source conditions. According to the integrated analysis on the structure shape, the traps, the cap and the migration conditions, it can be concluded that the A-1 structure favors the formation of medium-to-large-scale oil reservoirs. Based on all the research and analysis results, the exploratory well A-1-1 located in the key area of the A-1 structure was finally selected.5 CONCLUSION
In this work, long offset seismic data were acquired and processed in 3D way. Benefiting from the new images we can draw the following conclusions.
Close-spaced 2D long offset seismic data could provide good images with a high signal-to-noise ratio, clear characteristics of target strata, and reliable geological structures. This technique could be used in other settings where conventional industrial 3D seismic survey is hard to conduct.
By conducting integrated interpretations of drilling data and new images, five boundaries in the Mesozoic strata were determined, including Tj0, Tj2, Tj1, Tk0, and Tg. Good traps for oil-gas accumulation were confirmed in Mesozoic strata. An exploratory well A-1-1 was consequently proposed with the aim of guiding future drilling project in the Chaoshan depression.
The A-1-1 well is expected to reveal the complete sequence of the strata. In this way, the Mesozoic stratigraphy of the South China Sea may further be established. Hence, our work may provide a reliable basis for achieving breakthroughs in oil and gas exploration in the Mesozoic of the Dongsha sea area.6 DATA AVAILABILITY STATEMENT
The data including raw data and processed data are not publicly available due to the confidentiality issues. The data are available from the Guangzhou Marine Geological Survey upon reasonable request.7 ACKNOWLEDGMENT
We thank all colleagues who participated in seismic acquisition. Thank Bin LIU from Guangzhou Marine Geological Survey for improving the manuscript.
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