Sedimentary characteristics and exploration significance of sub-lacustrine fan of highstand system tract in the first member of Liushagang Formation in the Weixinan sag

doi:10.3969/j.issn.1672-7703.2020.05.004

Abstract

Abstract: Sub-lacustrine fans in the highstand system tract in the first member of the Liushagang Formation (the Liu-1 member) in B sub sag of the Weixinan sag are an important field for lithologic reservoir exploration. However, the characteristics, classification, distribution, genetic mechanisms, and development patterns of sub-lacustrine fans are not well understood, which constrains the exploration process and achievements. In this study, drilling, core, well logging, laboratory analysis and 3D seismic data are used to systematically describe the sub-lacustrine fans in the highstand system tract in the Liu-1 member in B sub-sag. The results show that: (1) The sub-lacustrine fans in the highstand system tract in the Liu-1 member in B sub-sag can be divided into two major types and four sub-types: mass-flow sub-lacustrine fans in the early highstand stage (which can be further divided into incised-channel center type and incised-channel flank type), and debris-flow sub-lacustrine fans in the late highstand stage (which can also be further divided into non-channelized sand-rich type and non-channelized mud-rich type). (2) Different types of sub-lacustrine fans all present different characteristics in lithology and in well logging and seismic data. (3) There are two development models, which are jointly controlled by lake level changes, sediments source systems, and transport routes. The first depositional model is meandering river delta - sedimentary flexure slope break - mud-rich sub-lacustrine fan with the sediments source co-axial with the sub-sag to the west. The second depositional model is fan delta - fault transition slope break - sand-rich sub-lacustrine fan with the sediments source to the northwest of the sub-sag. (4) Sub-lacustrine fan sand bodies of the incised- channel center mass-flow type and the non-channelized sand-rich debris-flow type, supplied by fan delta sediments from the northwest source, have shallow burial depths and good reservoir properties. The sand bodies form a self-generating, self-storage, and self-sealing hydrocarbon accumulation assemblage with the lacustrine mudstones in the Liu-1 member—an assemblage which has great exploration prospect.

CLC Number:

Hu Desheng, Fan Caiwei, Zhu Hongtao, Liu Sheng, Gong Liyuan. Sedimentary characteristics and exploration significance of sub-lacustrine fan of highstand system tract in the first member of Liushagang Formation in the Weixinan sag[J]. China Petroleum Exploration, 2020, 25(5): 23-31.

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