Evaluation of Fault Sealing in the No.2 Fault Zone of the Qiongdongnan Basin

doi:10.19657/j.geoscience.1000-8527.2024.099

Abstract

Abstract:

The No.2 Fault is a key basement fault in the Qiongdongnan Basin, connecting the oil source and trap, and playing a crucial role in transport and distribution of oil and gas. This study aims to clarify the difference in fault sealing across various sections of the No.2 Fault, providing a reliable theoretical basis for subsequent exploration in this area and improving fault sealing evaluation methods through detailed seismic data interpretation. Based on fault activity calculations, the No.2 Fault Zone in the Qiongdongnan Basin can be divided into three sections: eastern, central, and western. Based on this, we quantitatively evaluated the fault sealing ability of different sections of the No.2 Fault Zone using the fuzzy mathematical evaluation method, by calcula-ting normal stress for section (F), mudstone cutting ratio (SGR), and mud filling index (R_m). Additionally, we qualitatively evaluated the sealing properties of fracture-related zones in different sections using fluid inclusions, thin section observation, and scanning electron microscopy, focusing on diagenetic cementation and oil thickening. The results indicate that the fault sealing characteristics of the No.2 Fault Zone exhibit regional variability, with the western section demonstrating superior fracture sealing and serving as an effective barrier for oil and gas migration, followed by the eastern section. The central section has the greatest need for enhanced fracture sealing. The western fracture zone is sealed by mineral cement, whereas the central and eastern sections are filled with thickened crude oil, with sealing occurring relatively later. Overall, the sealing of the No.2 Fault Zone is the result of multi-stage hydrothermal mineral crystallization and oil thickening, occurring on top of compaction and filling processes.

Key words: fuzzy mathematic evaluation, qualitative evaluation, fault closure, No.2 Fault Zone, Qiongdongnan Basin

CLC Number:

TIAN Lanxi, ZHANG Guanjie, LEI Xin. Evaluation of Fault Sealing in the No.2 Fault Zone of the Qiongdongnan Basin[J]. Geoscience, 2024, 38(05): 1235-1247.

Figures/Tables 12

Fig.1 Tectonic zonation and stratigraphic boundary map of the Qiongdongnan Basin (modified from reference [30])

Fig.2 Structural map of the No.2 fault zone in the Qiongdongnan Basin

Fig.3 Activity variation diagram of the No.2 fault zone in the Qiongdongnan Basin (from west to east)

Table 1 Fuzzy evaluation value weight factor and weightcoefficients

断裂评价参数	影响因素个数n	权重因子N	权重系数M
断面正应力(P)	4	0.9375	0.2317
泥质含量(R_m)	3	0.8750	0.2162
泥岩刮削比(SGR)	2	0.7500	0.1853
断层紧闭指数(IFT)	6	0.9844	0.2432
岩性对接	1	0.5000	0.1236

Table 2 Comprehensive evaluation of fault sealing in different sections of the No.2 Fault Zone in the Qiongdongnan Basin

区带	断裂名称	断面正应力	泥质充填指数	断层泥比率	模糊评价值	封闭性定量评价结果
中段	F4	18.78	0.24	0.22	0.37	差
	F16	19.23	0.25	0.35	0.39	差
	F22	26.14	0.19	0.25	0.41	较好
东段	F13	26.85	0.31	0.27	0.40	较好
	F11	27.55	0.37	0.35	0.45	较好
	F2	26.57	0.30	0.37	0.43	较好
西段	F18	25.50	0.32	0.54	0.50	较好
	F20	26.90	0.33	0.58	0.55	好
	F25	22.57	0.40	0.57	0.49	较好

Fig.4 Sealing evaluation results for different sections of the No.2 Fault Zone

Fig.5 St-03 cross-well lithology profile of the eastern line of the No.2 Fault Zone (measurement point locations are shown in Fig.2)

Fig.6 Influencing factors affecting the sealing of the No.2 Fault Zone in the Qiongdongnan Basin

Fig.7 Micrograph of fluid inclusions in the No.2 Fault Zone

Fig.8 Homogenization temperature distribution of fluid inclusions in the No.2 Fault Zone of the Qiongdongnan Basin (a) and burial history of well Y19-1(b)

Fig.9 Cementation types in the fault-induced fracture zones of the No.2 Fault in the Qiongdongnan Basin

Fig.10 Physical characteristics of the No.2 Fault Zone in the Qiongdongnan Basin

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