Favorable conditions of inter-salt shale oil formation and key parameters for geological sweet spots evaluation: a case study of Eq34-10 rhythm of Qianjiang Formation in Qianjiang Sag, Jianghan Basin

The inter-salt fine-grained sedimentary rocks of the Eq34-10 rhythm of the Qianjiang Formation in the Qianjiang Sag, Jianghan Basin, are favorable for shale oil exploration and development. They are excellent source rocks according to the analysis of cored intervals from specialized/concurrent shale oil exploration wells and the archival data. The organic matter is composed of the amorphous components of the sapropelic group, mainly of types Ⅱ1 and I, and is currently in the peak period of oil generation and expulsion. The reservoir properties are good especially for dolomites and argillaceous dolomites, and are equivalent to the reservoir properties of main shale oil production layers at home and abroad. Thick layers of salt distributed continuously above and below the inter-salt fine-grained sedimentary rocks constitute high-quality top and bottom seals of the inter-salt shale oil. The inter-salt fine-grained sedimentary rocks of the Eq34-10 rhythm developed two types of shale oil, including the lateral migration and enrichment type in the sedimentary rocks in the Wangchang anticline area, and the in situ and near-source enrichment type in the Benghu Deep Sag and the southern slope area. Some key parameters and limits for geological sweet spot evaluation for the two enrichment types were put forward. The results provided the evidence and reference for shale oil exploration and development evaluation of other inter-salt fine-grained sedimentary rocks in the Qianjiang Sag, Jianghan Basin.