Discussion on the development mechanism of lacustrine high-quality source rocks in oil-rich sags in offshore China

Oil-rich sag is the resource base for the formation of large oil and gas fields, which controls or affects the formation, enrichment and distribution of large and medium-sized oil and gas fields (reservoirs), and high-quality source rock is the material base for the formation of oil-rich sag. It is of great significance to explore the development mechanism and genesis of high-quality source rocks in oil-rich sags in offshore China for the selection of potential oil-rich sags. The development and distribution of high-quality lacustrine source rocks in proven oil-rich sags in offshore China are controlled by four major factors: strong fault depression, moderate barrier, eutrophication and thermal events. The strong fault depression can form deep sags, providing sufficient accommodation space for the development of source rocks, and forming a semi deep-deep lake reducing environment, which is conducive to the enrichment and preservation of organic matters. During the depositional stage of source rocks, the grit chamber on the provenance migration path moderately blocks the megaclast, which can control the input of megaclast into sags, so that the sag area, especially the deep depression area, mainly develops fine-grained sediments to form argillaceous source rocks. The deep hydrothermal activities, volcanic ash and rivers flowing through the granite provenance area in the sag and its surrounding areas input rich nutrients into the sag, leading to water body eutrophication in the lake basin, which can promote the bloom of algae and provide a large amount of sedimentary organic matters, thus forming an oxygen-deficient environment. This is conducive to the development of high-quality source rocks. Late thermal events such as faulting and magmatism have accelerated the thermal evolution of source rocks, and expanded the distribution range of effective source rocks, which is conducive to the large-scale oil generation of source rocks.