Quantitative Evaluation of Sealing Capacity of High Temperature and Pressure Caprocks in Yinggehai Basin

Yinggehai Basin is the important Cenozoic petroliferous basin in China South Sea. With the gradual improvement of the petroleum exploration and development in shallow layers,middle and deep overpressured layers become the main target of the natural gas exploration. Whether the caprocks can effectively seal the oil and gas turns to be a crucial questions for the research. In recent years,different scholars have conducted a lot of researches on the caprocks of Yinggehai Baisn,but there are still some questions in the sealing mechanism,failure conditions and quantitative evaluation of the caprocks in high temperature and pressure. Based on the systematic analysis of the high temperature and pressure caprocks in Yinggehai Basin,it is clear that the sealing mechanism of caprocks is capillary sealing and hydraulic sealing. Using the relationship among displacement pressure,acoustic time and porosity of mudstone to predict the capillary sealing ability of regional caprocks,the results show that the caprocks in Yinggehai Basin have strong capillary sealing ability. Therefore,hydraulic fractures induced by overpressure are the fundamental cause of oil and gas multilayer accumulation,and then propose the cap hydraulic fracturing pressure coefficient to quantitative evaluate the risk of hydraulic fracturing in caprocks. The evaluation results show that there is a significant correlation between the occurrence of hydraulic fracturing in the caprock and the diapir activity,the risk of hydraulic fracturing in caprocks gradually decreases from the diapiric center to the periphery. LD-B block located in the slope near the depression in Yinggehai Basin which is adjacent to the Ledong diapirs,is a favorable area for oil and gas accumulation.