惠民凹陷临南洼陷古近系沙河街组超压成因机制及分布预测

渤海湾盆地惠民凹陷临南洼陷古近系沙河街组油气勘探过程中钻遇不同规模的异常高压，超压成因的不确定性限制了压力预测的可靠性。利用临南洼陷丰富的钻杆测试（DST）压力数据、泥浆密度和测井资料，细致分析了渗透性砂岩流体压力发育特征和超压段泥岩测井响应，依据常压和超压段声波速度、密度测井数据建立判识超压成因的有效应力-测井响应图版，综合讨论了沙河街组超压的成因机制，并预测了异常高压的空间分布。研究发现，临南洼陷沙河街组砂岩储层超压主要发育在3 000 m以深的沙三段（Es³）和沙四段（Es⁴），最高过剩压力分别为23.82 MPa和14.04 MPa；超压段泥岩测井响应表现为偏离正常压实趋势的异常高声波时差、低密度和高中子孔隙度，具有典型的欠压实特征。沙河街组储层超压最主要的成因是相邻泥岩机械压实不平衡作用形成超压的传递，表现为大多数的超压数据均符合加载曲线趋势，只有深洼陷区（埋深为4 000~4 300 m）的超压呈现出卸载特征，可能存在由于富有机质泥岩深埋达到较高成熟度（R_o为0.90%~1.05%）引起的生烃增压贡献。利用平衡深度法计算的流体压力与DST压力数据吻合度高，印证了沙河街组超压主要来自泥岩不均衡压实作用的认识，沙三段过剩压力呈现围绕着洼陷中心呈环状分布，向周围的斜坡区和隆起带逐渐减小为常压。研究成果可以为临南洼陷钻前压力评估提供有价值的指导和借鉴。

Mechanism and distribution prediction of abnormal high pressure of the Paleocene Shahejie Formation in Linnan Sag,Huimin Depression

In the process of oil and gas exploration,various abnormal high pressures were tested in the Paleogene Shahejie Formation in Linnan Sag,Huimin Depression.The uncertainty of overpressure origin would reduce the reliability of pressure prediction.This study elaborated both the characteristics of fluid pressure in porous sandstones and the well log responses of overpressured mudstones through such data as drill stem tests (DSTs),mud density and logging. Besides,based on acoustic and density logs in normal and abnormal high pressured intervals,we established an effective stress-log response plot to deduce the origin of overpressure in the Shahejie Formation,and predicted the distribution of abnormal pressures in the study area.The results show that overpressures in the sandstone reservoirs of the Shahejie Formation mainly occur in the 3^rd (Es³) and 4^th (Es⁴)members of the formation with a depth over 3 000 m,and their maximum excessive pressures are 23.82 MPa and 14.04 MPa,respectively.The logging responses of overpressured mudstones typify undercompaction,usually exhibiting abnormally high interval transit time,abnormally low density,and high neutron poro-sity.Overpressures in the formation mainly originate from overpressure transmission among neighboring mudstones induced by disequilibrium compaction.Most overpressure data points follow the trend of loading curves,and only the overpressure data points in local deep depression zones(with burial depth being more than 4 000-4 300 m)follow unloading curves, possibly due to the pressure increase with hydrocarbon generation resulting from a higher maturity (R_o is within 0.9%-1.05%)of the deeply buried organic-rich mudstones.Besides,fluid pressures calculated with equilibrium depth method match well with the pressure data obtained from DSTs. This verifies the conclusion that the abnormal high pressures in the formation are fundamentally caused by disequilibrium compaction. The excessive pressures of the Es³ show a ring-like distribution surrounding the core of the sag and progressively lower towards and become normal in its neighboring slopes and uplifts. These research results will be of valuable guidance for pre-drilling pressure assessment in Linnan Sag.