Discovery of “detachment-core complex type” basins offshore the northern South China Sea and their oil and gas geological conditions:A case study of the Kaiping sag in the northern South China Sea

doi:10.13745/j.esf.sf.2024.7.1

Abstract

Abstract:

Kaiping sag in the Pearl River Mouth Basin is located in the crustal thinning zone offshore the northern South China Sea. It is a typical “detachment-core complex type” basin (sag) newly discovered around the world. It has a unique lithospheric extension-thinning-fracture process. It is of great significance to study its structural type, genetic mechanism and oil and gas geological conditions.This article is based on the detailed interpretation of three-dimensional seismic data and comprehensive geological research, discovering a new type of basin offshore the northern South China Sea called “detachment-core complex”, revealing its basin formation and hydrocarbon generation process, promoting new understanding of depression structure, hydrocarbon source distribution, and reservoir formation mode, and discovering a large oil field for the first time in this type of sag.The research results indicate that: (1) Kaiping sag is a typical “detachment-core complex type” sag developed under the strong extensional continental margin background offshore the northern South China Sea, and its formation process is influenced by horizontal extensional superimposed metamorphic core complex vertical uplift; The sedimentation period of the Wenchang Formation went through two major stages of basin formation and evolution: early faulting, late detachment, and uplift and transformation of core complex rocks, forming a depression structure of “north faulting and south overtaking”; (2) The source rocks of the Wenchang Formation have a coupled hydrocarbon generation mechanism of “fault detachment controlling basin-early depression late slope-high temperature heating”. The high-quality source rocks of the semi-deep lake and deep lake facies in the fourth section of the Wenchang Formation have undergone deformation and displacement to the southern slope zone under the combined action of detachment and core complex uplift. At the same time, they are affected by the high temperature field and magma heating, showing high heat and fast maturation characteristics, with a shallower hydrocarbon expulsion threshold, significantly enhancing the resource potential of the Kaiping sag; (3) The southern slope zone of Kaiping sag has a reservoir formation model of “basin controlled by fault detachment, combined control of hydrocarbon transport by fault ridges, multi-stage episodic filling, and nose shaped structure controlled accumulation”.The discovery of Kaiping South Paleogene oil field has opened up a new area of deep oil and gas exploration in the the Pearl River Mouth Basin, becoming the first successful case of commercial discovery of continental margin “detachment-core complex type” sag in the world, which is expected to provide important reference for oil and gas exploration of similar sag structures at home and abroad.

Key words: “detachment-core complex type” basin (sag), dynamic mechanism of basin formation, mechanism of hydrocarbon generation, sedimentary evolution, accumulation mode, enrichment pattern

CLC Number:

XU Changgui, GAO Yangdong, LIU Jun, PENG Guangrong, CHEN Zhaoming, LI Hongbo, CAI Junjie, MA Qingyou. Discovery of “detachment-core complex type” basins offshore the northern South China Sea and their oil and gas geological conditions:A case study of the Kaiping sag in the northern South China Sea[J]. Earth Science Frontiers, 2024, 31(6): 381-404.

Figures/Tables 17

Fig.1 Structure of core complex (a) and basin structure of “detachment core complex type” (b) pattern diagram. Modified after [7].

Fig.2 Classification of extensional basin types based on differences in extensional strain. Modified after [18].

Fig.3 Structural outline and stratigraphic evolution of Kaiping sag in the Pearl River Mouth Basin

Fig.4 Deep crustal structure of the Pearl River Mouth Basin

Fig.5 3D of the main detachment fault section in Kaiping Depression (a) and the free air gravity anomaly map in the surrounding area (b)

Fig.6 Structural characteristics of the “detachment core complex type” depression in Kaiping (see Figure 3 for the cross-sectional position)

Fig.7 Classification model of passive continental marginal structural zones and basin types. Modified after [54].

Fig.8 Structural evolution profile of Wenchang Period in Kaiping Depression

Fig.9 Migration characteristics and interpretation model of layered reflection surfaces within the crust of Kaiping Depression

Table 1 Comparison and analysis of typical drilling source rock characteristics in Kaiping Depression

井名	钻遇层位	厚度/m	干酪根类型	TOC含量/%	氢指数HI/(mg·g^-1)	沉积相	综合评价
A7	文三段	91	Ⅱ₁—Ⅱ₂型	0.6~4.2(2.6)	162~378(264)	浅湖—半深湖	好
A7	文四段	145	Ⅱ₁—Ⅱ₂型	1.3~9.6(5.4)	134~377(266)	半深湖—深湖	好—很好
A9	文三段	13	Ⅱ₂型	0.3~0.76(0.5)	134~253(207)	辫状河三角洲前缘	较差
A9	文四段	182	Ⅱ₁型	2.1~4.8(3.4)	347~589(437)	半深湖—深湖	好—很好
B4	文三段
B4	文四段	84	Ⅱ₁型	0.9~7.0(2.5)	146~506(305)	半深湖—深湖	好—很好

Fig.10 Analysis of geochemical indicators of hydrocarbon source rocks in Wenchang Formation of Kaiping Depression

Fig.11 Distribution of source rocks and fault trap overlay in Wenchang Formation of Kaiping Depression

Fig.12 Comparison of measured geothermal gradients during drilling in Kaiping Depression

Fig.13 Sedimentary facies map of the main strata of the Paleogene in the Kaiping Depression (a, lower section of the Enping Formation; b, upper section of the Wensi Formation)

Fig.14 Sedimentary evolution model of the Paleogene in Kaiping Depression

Fig.15 Intersection of permeability and depth in Kaiping Depression

Fig.16 Oil and gas accumulation model diagram of the “detachment core complex type” depression in Kaiping

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