Abstract:

In the Weixi'nan Sag of the Beibuwan Basin, the high-quality lacustrine source rock in the Liushagang Fm has been poorly understood in terms of its characteristics and its controls on hydrocarbon distribution.To this end, we establish an organic geochemical method to identify quality source rocks, and reveal the source rock distribution pattern and its controls on hydrocarbon distribution, based on the fundamental mechanisms of hydrocarbon generation and expulsion for source rocks.Among others, the major source rocks for the Weixi'nan Sag are the Liushagang Fm shale, which can be divided into three intervals, namely the Liu 3, Liu 2 and Liu 1 members downwardly, with the Liu 2 member as the major zone.Based on the principle of hydrocarbon generation and expulsion for source rocks as well as relations between the TOC content, and the pyrolysis S₁ and chloroform bitumen "A" content of mature source rocks, we identify the lower limit value of TOC content of the high-quality lacustrine source rocks in the Liushagang Fm is about 2%.The black shale at the bottom of the Liu 2 member (El₂) with a TOC content higher than 2%, contains oil-prone organic matters with high hydrocarbon-generating potential, and is high-quality source rocks.Part of the dark mudstone is also high-quality source rocks.The crude oil expelled from the black shale at the bottom of Liu 2 member could migrate in short distance and accumulate under the black shale interval, or could migrate laterally in long distance along the sand bodies in the Liu 3 member and accumulate at higher structures.However, the crude oil expelled from the dark mudstone mainly accumulates in the Liu 1 member and Weizhou Fm, which are vertically above the quality source rocks inside the sag, with faults as the vertical pathway for migration and thus the accumulation is directly characterized by source rock distribution.

Key words: source rock, black shale, dark mudstone, Liushagang Fm, Weixi'nan Sag, Beibuwan Basin