The formation environment and development modes of Cenozoic source rocks in the Qiongdongnan Basin

Abstract

According to the drilling results, there are three sets of source rocks of the Cenozoic Group in the Qiongdongnan Basin, including the Yacheng Formation transitional facies mudstone, the Oligocene Lingshui Formation semi-enclosed marine facies mudstone, and the Miocene neritic-bathyal facies mudstone. The hydrocarbon generating conditions of the source rocks in the study area are mainly controlled by sedimentary environment and organic matter input. The abundance of organic matter in the Oligocene source rocks which were formed in a low-energy back shore environment is confined by organic matter input and hydrodynamic condition. The Miocene source rocks formed in the open marine environment lack terrestrial organic matter input, whose organic matter abundance is constrained by the primary production of the ocean and the preservation condition of organic matter. In addition, the redox conditions are also significant to the formation of source rocks in the study area. According to the differences in key factors controlling the development of source rocks in different time periods and structural belts revealed by drilling practice, three development patterns of source rocks have been put forward. The first one is the transitional facies model featuring a great deal of terrestrial organic matter and weakly hydrodynamic conditions which are apt to develop high effective source rocks in the Yacheng Formation. The second one is the semi-enclosed marine facies featuring abundant terrestrial organic matter, weakly hydrodynamic condition and fine preservation condition in the central depression zone, which are apt to form fine-quality source rocks in the Lingshui Formation. The third one is the marine facies model featuring strong hydrodynamics in the shore-shallow area of northern continental shelf and the lack of benthic algae which are against the development of source rocks. It can be predicted that the continental slope is the perfect place to form source rocks where adequate amounts of light favor the photosynthesis of plants, besides the fine preservation and weakly hydrodynamic conditions. In the deep sea area, although accompanied with weakly hydrodynamic conditions and a reduction environment, it is not suitable for the development of source rocks on account of less aquatic organisms causing low primary production.