Geology and geochemistry of large gas fields in the deepwater areas, continental margin basins of northern South China Sea

https://doi.org/10.1016/j.marpetgeo.2021.104901Get rights and content

Highlights

  • Deepwater sags of the QDN and PRM basins were formed as a result of strong crust detachment and thinning.

  • Eocene shallow lake and Oligocene transitional–marine gas-prone source rocks are developed in deepwater sags.

  • Gas sources of LW3-1 and LS17-2 gas fields are geologically and geochemically determined.

  • Key geological elements of forming large gas fields in deepwater area were revealed.

Abstract

The Qiongdongnan (QDN) and Pearl River Mouth (PRM) basins are two important Cenozoic petroliferous basins along the northern margin of the South China Sea (SCS). In this study, the geological and geochemical characteristics of the LS17-2 and LW3-1 gas fields, the respective largest gas field in the deepwater areas of the QDN and PRM basins, were investigated. The tectonic evolution of the two basins can be divided into four stages, including rift, rift-depression transition, post-rift thermal subsidence and neotectonic stages. During the early rifting period, strong crustal thinning in the deepwater areas led to the formation of the broad and deep sags where Eocene shallow lake and Oligocene transitional to neritic source rocks were developed whose organic matter was dominated by gas-prone type II2–III kerogens. The gases from the LS17-2 gas field in the QDN Basin are characterized by relatively heavy δ13C1 (−36.8 to −40.1‰) and δ13C2 (−23.5 to −26.2‰) values. The associated condensates contain high abundance of oleanane and low content of bicadinanes, showing close affinity to the Oligocene Yacheng source rocks. The large-scale Huangliu channel sandstones act as reservoirs with a massive mudstone cap of lower Yinggehai Formation. The diapiric faults provide the main pathways for the upward migration of gases into the shallow reservoirs. By contrast, the gases from the LW3-1 gas field in the PRM Basin have δ13C1 values similar to LS17-2 gases, while their δ13C2 values are relatively light (−28.4 to −29.6‰). Like the LS17-2 condensates, LW3-1 condensates also contain abundant oleanane. These features indicate that the gases/condensates could be derived from the lower Oligocene Enping and Eocene Wenchang shallow-lake source rocks. The Zhuhai delta and the Zhujiang submarine fan sandstones and thick marine mudstones of the upper Zhujiang Formation form the favorable “reservoir-cap” assemblage. The effective combination of faults and permeable sandbodies make up the main migration pathway. The anatomy of geological elements for the two large fields suggests that both the source rock facies and reservoirs have controlled the large-scale gas accumulation. This study provides new insights into the hydrocarbon accumulation models in the deepwater areas of two basins, and implies highly promising gas exploration potentials.

Introduction

The deepwater region (water depth >300 m) of the northern margin of South China Sea (SCS) has a relatively large area. The present study covers an area of about 12 × 104 km2 and is focused on the Central Depression and Southern Uplifts in Qiongdongnan (QDN) Basin and the Zhu II Depression in Pearl River Mouth (PRM) Basin (Fig. 1). In recent years, the deepwater regions of the two basins have received considerable attention and have become major hotspots for petroleum exploration because of their huge sedimentary volumes and gas potentials. So far, several commercial gas fields have been discovered, including the LW3-1, LW9-1, LW34-2, PY35-2 gas fields in the PRM Basin (Zhu et al., 2009; Shi et al., 2014, 2016), and the LS17-2, LS25-1, LS25-1 W, LS18-1, LS18-2 gas fields in the QDN Basin. Among them, the LW3-1 gas field is the first large gas field discovered in 2006 in the deepwater area of the northern margin of SCS (Zhu et al., 2009), while the LS17-2 gas field is the largest gas field with a gas-in-place (GIP) reserve of about 110 billion m3 in the deepwater area. Although recent exploration success has greatly enhanced the understanding of the petroleum systems in the deepwater area, some key geological problems are still unresolved. Early studies have demonstrated that the natural gases in the shallow water areas of the PRM and QDN basins are mainly derived from the Oligocene source rocks (Zhu et al., 2009; Huang et al., 2012). However, little work has been done on the formation of the deepwater sags and their source rocks. Some recent studies have been conducted on the origin and charging time of natural gas in the LW 3–1 (Fu et al., 2010, 2019, 2020) and the LS17-2 gas fields (Huang et al., 2016), but the detailed constraints of geological elements on the large-scale gas accumulation are still poorly understood. In this study, we investigated the tectonic evolution of the QDN and PRM basins, analyzed the formation mechanisms of deepwater sags and the hydrocarbon generation potentials of source rocks in them, identified the sources of the two large gas fields and established their gas migration-accumulation models. This work will help to better understand the petroleum systems in deepwater areas and define the favorable exploration targets, and thus reduce the risk of gas exploration in the future.

Section snippets

Geological setting

The SCS is tectonically located in the joint area of the Pacific and Tethys tectonic domains and is a relatively large and deep marginal sea in western Pacific Ocean. It was formed as a result of the rifting of paleo-SCS platform triggered by the interaction between the Eurasian, the India-Australia and the Philippine plates during the Cenozoic (Gong et al., 1997; Zhu et al., 2007). The PRM and QDN basins, located along the northern margin of the SCS, are Cenozoic rift basins with pre-Paleogene

Samples

A total of 123 cutting and core samples from 12 wells were collected for TOC and Rock-Eval analysis, and 17 gas samples used for geochemical and isotopic analysis were collected during drill stem tests and module formation tests (Table 1). Selected rock samples were Soxhlet extracted with dichloromethane (72 h). After asphaltene precipitation and fractionation using column chromatography, saturate fractions obtained from both oils and rock extracts were prepared for GC-MS analysis.

TOC and Rock-Eval

The total

Continental margin detachment and deepwater sag formation

Available data show that the tectonic evolution processes and characters exhibit some obvious changes from shallow water shelf to the deepwater slope in the QDN and PRM basins. In the shallow water areas of the two basins, the formation of narrow rift sags were first triggered by stretching of weak crust and were controlled by steep normal faults, and then they underwent weak subsidence in late depression stage, gradually evolving into continental shelves (Fig. 4). In contrast, in the deepwater

Gas origin and accumulation of two deepwater large gas fields

As mentioned above, the largest gas fields discovered so far in the PRM and QDN basins are the LW3-1 and LS17-2 gas fields, respectively. The key geological elements of the two gas fields will be discussed in detail in following sections.

Gas exploration prospectivity in deepwater area

Based on the aforementioned results and discussion, it is believed that the deepwater area is geologically favorable for the formation of large- and medium-sized gas fields, and thus has a promising potential for gas future exploration.

Firstly, the deepwater area encompasses the majority of the Central Depression of the QDN Basin, including the Ledong, Lingshui, Songnan, Baodao and Changchang sags, and the Zhu II Depression of the PRM Basin, including the Baiyun and Liwan sags. Compared with

Conclusions

Based on the systematic study of the geology and geochemistry on the formation of large gas fields in the deepwater area of the QDN and PRM basins, the following conclusions can be drawn.

  • (1)

    The QDN and PRM basins in the northern margin of the SCS have gone through four evolution stages, including the rift, rift-depression transition, post-rift thermal subsidence and the neotectonic stages. In response to the above structural evolution, the dominant environment changed from paleo-lakes in the early

Credit author statement

Weilin Zhu: Data collection, Methodology, Formal analysis, Writing – original draft, Hesheng Shi: Data collection, Writing – original draft, Funding acquisition, Baojia Huang: Conceptualization, Validation, Formal analysis, Writing – original draft, Writing – review & editing, Supervision, Funding acquisition, Kai Zhong: Data collection, Investigation, Formal analysis, Writing – original draft. Yiwen Huang: 1D basin modeling, Investigation, Visualization

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The authors are indebted to China National Offshore Oil Corporation (CNOOC) China Limited for providing us with source rock, gas and condensate samples and making available the geological and seismic data. We are grateful to Associate Editor Barry Katz and three anonymous reviewers for their constructive comments that improved the clarity of this manuscript significantly. We also extend our thanks to Dr. Barry J. Katz who helped to polish the English writing. This work was financially supported

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