Research on the Cause Analysis and Prediction Method of Abnormal Pressure in Oil Shale Formation of Weixinan Sag

Weixinan Sag in the west of the South China Sea is a proven hydrocarbon generating sag. The LII formation is characterized by abnormally high pressure. Previous studies suggested that the formation of abnormal high pressure was mainly due to the undercompaction of mudstone, but with the expansion of exploration fields, especially after drilling well W1, the first oil shale exploration well in China Offshore, it is found that there is a big error between pre-drilling pressure prediction based on mudstone undercompaction model and measured formation pressure. According to the analysis of pressure while drilling and after drilling of oil shale wells drilled in recent two years, it is considered that the abnormal pressure contributing to hydrocarbon generation and pressurization of oil shale exists in addition to the traditional origin of abnormal pressure of undercompaction of mudstone, this is a major breakthrough in the understanding of the origin of pressure in the Beibuwan Basin. In this paper, firstly, the cause of abnormal pressure is determined by the method of velocity-density crossplot analysis of mudstone and oil shale, secondly, the p-wave velocity is reconstructed by using s-wave velocity to eliminate the effect of high TOC oil shale hydrocarbon generation on the velocity, finally, based on the actual pressure measurement points, the contribution template of hydrocarbon generation and pressurization of oil shale is established. The study on the genetic judgement and prediction method of abnormal pressure in the oil shale formation of Weixinan SAG has been successfully applied to the predrilling pressure prediction and the pressure-while-drilling analysis of the new oil shale drilling in Weixinan Sag, it provides the technical support for the discovery of oil and gas reservoirs in the field of oil shale, and provides the technical support for the reasonable well structure design and the improvement of drilling safety.