Hydrocarbon accumulation characteristics of beach-bar sandstones in the southern slope of the Dongying Sag, Jiyang Depression, Bohai Bay Basin, China

Abstract

A number of beach-bar sandstone reservoir beds are developed in the upper fourth member of the Eocene Shahejie Formation (Es4s) on the southern slope of the Dongying Sag. Based on the analysis of seismic and logging data, with characterization and petrographic studies of core and cutting samples, this paper analyzes the hydrocarbon accumulation characteristics in two typical blocks of the Boxing and Wangjiagang oilfields, especially reservoir bed heterogeneity and migration conditions that influence oil and gas distribution, calculates the index of reservoir bed quality (IRQ) with a mathematical method, and discusses the relationship between driving force and resistance of hydrocarbon accumulation. Taking into account the characteristics of thin interbeds in beach-bar sandstones, an experimental model simulated the characteristics of hydrocarbon migration and accumulation in thin interbedded sandstones with reservoir bed heterogeneity. The results showed that hydrocarbon distribution and properties were extremely non-uniform. Reservoir bed and migration conditions controlled hydrocarbon accumulation in beach-bar sandstones. IRQ is above 0.4 in the main hydrocarbon region. Sand body distribution, structural configuration and fault systems controlled the direction of regional migration and location of hydrocarbon accumulation. Simulation experiments indicated that the change of driving force for hydrocarbon migration affected selective accumulation mechanisms. Hydrocarbon moved vertically along fault zones to the reservoir and resulted in the distribution of hydrocarbon in the reservoir. Two kinds of hydrocarbon accumulation models exist in the study area. One is a hydrocarbon accumulation model controlled by reservoir bed heterogeneity and the second is a hydrocarbon accumulation model controlled by a complex migration system with faults connecting sandbodies. Finally, different exploration strategies should be adopted for the detailed exploration for beach-bar sandstone reservoirs according to different geological backgrounds.