Geological conditions and the main controlling factors for the tight oil accumulation in Liushagang Formation of Fushan Depression, Beibuwan Basin

Unravelling the accumulation conditions and their controlling factors for the tight oil in the Liushagang Formation, Fushan Depression, Beibuwan Basin, is critical for the further evaluation of the tight oil distribution and the favorable exploration area.Based on comprehensive analysis of seismic, well logging, core and lab tests in this study, the effective source rocks, favorable reservoir, and the source-reservoir matching relations were investigated.It can be proposed that the high-quality source rocks were formed in Huangtong, Bailian two sub-sags during the maximum lake flooding periods in Liushagang depositional stage and were characterized as high organic content and moderate degree of thermal evolution, and thus they provide the material basis for the formation of tight oil; due to the influence of multi-source supply, tight sandstones penetrated into these two sub-sags and they were widely distributed with superimposed source-reservoir assemblages. The development of dissolution pores and structural fractures was favorable for the migration and enrichment of tight oil; the tectonic conditions of the sag were relatively stable and no major fault system was active when the main sources entered peak oil window, and thus the relatively stable tectonic environment enabled the formation and preservation of tight oil accumulations.Therefore, the Liushagang Formation of the Fushan Depression have favorable geological conditions for the formation of tight oil, which mainly distribute in the lower L1 submember and L2 member vertically and Huangtong and Bailian sub-sags horizontally.The distribution of high-quality source rocks, the distribution of "sweet point" reservoirs being rich in dissolution pores and favorable superimposed source-reservoir assemblages are the three main controlling factors for the formation and distribution of tight oil in Liushagang Formation of Fushan Depression, Beibuwan Basin.