Sedimentary characteristics and hydrocarbon accumulation of glutenite in the fourth member of Eogene Shahejie Formation in Shengtuo area of Bohai Bay Basin, East China

Abstract

The genetic type and hydrocarbon accumulation conditions of glutenite in Shengtuo Area have been studied based on cores, well-logs, and formation test data. During deposition of the fourth member of Eogene Shahejie Formation (Es4), Shengtuo Area was predominated by nearshore subaqueous fan and sublacustrine fan, controlled by the Chennan Fault and the Tuo-Sheng-Yong Fault. Nearshore subaqueous fan and sublacustrine fan were respectively developed on the downthrown of the Chennan Fault and the Tuo-Sheng-Yong Fault. Both of nearshore subaqueous fan and sublacustrine fan can be subdivided into three subfacies, including inner fan, middle fan and outer fan, from proximal to distal provenance. The braided channel within the middle fan, characterized by conglomeratic sandstones and sandstones with high secondary porosity and overpressured microfissures resulted from the effect of organic acid and reservoir overpressure, is the best reservoir among these three subfacies. Inner fan subfacies, consisted of thick-bedded, poorly-sorted and matrix-supported conglomerates, has low primary and secondary porosity, is the best sealing among this depositional system. Outer fan sediments, comprised of thick dark mudstones interbedded with thin sandstones in which overpressured microfissures developed very well, accounts for the migration pathway for oil and gas to enter braided channel reservoir from source rocks. In addition, the glutenite fans were developed near the Lijin Sag and the Minfeng Sag, which deposited two sets of high quality source rocks (Es4s (upper section of Es4) and Es4x (lower section of Es4) source rock). With regional cap rock and appropriate preservation conditions, the different subfacies in glutentite fans worked together and formed a "self-generation and self-storage" lithostratigraphic trap characterized by "migrating through outer fan, accumulating in middle fan and sealing by inner fan".