碳酸盐岩盖层封闭性讨论--以塔里木盆地塔中北斜坡奥陶系为例

摘要

Based on well log and logging data, using experimental test method, the sealing condition and mecha-nism of three sets of Ordovician cap rocks in the northern slope of the central Tarim Basin were studied. Thick mudstones of the Upper Ordovician Sangtamu Formation cover the whole study area and provide good regional sealing for oil and gas. The difference of shale content of carbonate cap rocks in the Lower Ordovician Lianglitage 3rd-5th members is obvious, but the distribution of oil and gas is different, illustrating that shale content is an important factor affecting the sealing capacity of cap rocks in the Lianglitage 3rd-5th members. When GR value reaches 20 API, the Lianglitage 3rd-5th members work as effective cap rocks, sealing oil and gas in the under-lying Yingshan Formation. Due to the influence of filling, cementation and selective corrosion of atmospheric pre-cipitation, dense and high-resistivity layers were formed in the Yingshan Formation, blocking oil and gas mainly by the mechanism of displacement pressure difference. When the displacement pressure difference between high-resistance layer and underlying reservoir bed is above 1.5 MPa, the high-resistivity layer is considered as a local cap rock, providing certain sealing for oil and gas inside the Yingshan Formation.%在统计分析录井及测井资料、运用实验测试手段的基础上，对塔中北斜坡奥陶系3套盖层的封闭条件或封闭机理进行研究发现，上奥陶统桑塔木组泥岩覆盖整个塔中北斜坡，厚度大，具有很好的封盖作用。下奥陶统良里塔格组良3-5段碳酸盐岩盖层泥质含量差别明显，油气分布层位也不同，说明泥质含量是影响良3-5段盖层封闭性的重要因素，且泥质含量对应的自然伽马测井响应值达到20 API时，良3-5段泥灰岩段可作为有效盖层。由于受到充填作用、胶结作用和大气降水选择性溶蚀作用的影响，鹰山组内部形成了致密高阻层，主要靠压力差封闭机理来遮挡油气，即高阻层与下伏储层的排替压力差在1．5 MPa以上时，高阻层可作为局部盖层，对鹰山组内部油气具有一定的封闭作用。