渤南洼陷超压系统划分及结构特征

摘要

Bonan sag is a typical overpressured sag with abundant hydrocarbon resources,and present-day overpressures commonly occur in the Eocene third and fourth members of Shahejie Formation (Es 3 and Es 4 )at depths between 2 300 m and 4 200 m.By assessing present-day pressure distribution and response from drilling stem test (DST ), wire logging and geological date, the characteristics of overpressure and geological factors affecting abnormal pressure are discussed in detail. Sonic transit times of mudstones deviate from the normal compaction trend line to the overpressured area revealed by drilling,but density has no obvious response.On the whole,three overpressure systems have been identified,including an upper system distribution in Es 1 ,a middle reservoir overpressure with main part in lower Es 3 ,and a lower reservoir overpressure in the upper part of Es 4 .The second member in which sand contents are more than 20% generally shows normal pressure.This study indicates that sourced rocks,low permeability sealing rocks,and fault structure are the three major factors controlled the range of overpressure and its structure.Multiphase fluid from mud-rich sources rock composes material basis of an overpressured system.Compacted mudstone within Es 1 ,and cemented sandstone interbeded with mudstone in Es 3 ,as well as mudstone and gypsum beds distribution in the upper part of Es 4 ,formed respectively barrier of each system,can also control the distribution and construction change of overpressures system.Fault structures,which has great influence on formation pressure and distribution of oil,show a complex and duality fault-fluid flow behavior.The optimized and calibrated seismic intervai-velocity data,in conjunction with measured pressure,have been used with considerable success in overall recognition of large-scale overpressured system in Bonan sag.%沾化凹陷的渤南洼陷为富油洼陷，实测超压出现在沙三、四段，埋深为2300~4200 m。综合大量的地压测试、测井和地质资料，研究了现今超压分布和超压的测井及地震响应，对单井、剖面超压发育特征和影响超压结构的地质要素进行了深入分析。超压带内泥岩声波时差偏离正常趋势线，而泥岩密度没有明显偏低的响应。研究表明：渤南洼陷存在3个超压系统，上超压系统为浅层沙一段发育的弱超压，沙三段中下部的强超压构成中超压系统的主体，下超压系统位于沙四段中；砂质质量分数大于20%的沙二段为常压。超压源层、低渗封隔层和断裂构造带3类地质要素控制着超压幅度和超压结构。泥质源岩产生的多相流体是形成超压系统的物质基础；沙一段压实泥岩、沙三中亚段的互层致密砂岩和泥岩、沙四上亚段的膏盐层作为压力系统封隔层，对超压系统的形成和分布起到控制作用；断裂具有泄压和封堵双重性，对地层压力和油气层横向分布具有重要影响。利用地震层速度计算得到了渤南洼陷现今大规模超压系统分布特征的整体认识。