辽河坳陷东部凹陷沙三段根缘气成藏条件与分布预测

Accumulation condition analysis and distribution prediction of source-contacting gas in third member of Shahejie Formation,eastern sag of Liaohe depression

对辽河东部凹陷储层和烃源岩等成藏条件进行系统研究,通过对东部凹陷多口钻井沙三段致密砂岩底部含气层段进行厚度统计,并结合成藏条件分析,对根缘气藏发育有利区进行了预测。研究结果表明:沙三段具有根缘气藏发育的基本地质条件:沙三段晚期沉积环境,以及后期成岩演化作用,使沙三段45.1%的砂岩储层孔隙度≤13%,25.5%的渗透率≤1.5×10-3μm2,形成较大规模的致密砂岩发育带,成为根缘气储层发育的主体;沙三段烃源岩干酪根类型以利于生气的ⅡB型和Ⅲ型为主,有机质丰度高,且已进入大规模生气阶段,为根缘气藏的形成提供了有力气源保障。由于运聚机理与常规储层气不同,根缘气具有致密砂岩底部含气特点,在气测曲线形态上形成下部异常的特征。凹陷北部茨榆坨-牛居地区和南部黄金带-大平房地区为根缘气藏发育最有利地区。

Comprehensive analyses of accumulation conditions including reservoir and source rock indicate that the third member of Shahejie Formation of eastern sag Liaohe depression possesses the basic geological conditions for forming source-contacting gas accumulations.The distribution thicknesses of gas bearing tight sandstones of the third member of Shahejie Formation of lots of drilling wells were analyzed using this method,and the favorable areas for source-contacting gas accumulations are predicted combined with the accumulation condition analyses.The results show that there are 45.1% of porosity and 25.5% of permeability of sandstone reservoirs in the third member of Shahejie Formation below 13% and 1.5×10-3 μm2,respectively.The third member of Shahejie Formation is the main body for accumulating source-contacting gas due to the sedimentary environment and the diagenetic evolution.The source rocks in the third member of Shahejie Formation are the mainly types of ⅡB and Ⅲ which are favour of generating natural gas.The organic matter abundances are high and the source rocks are in the stage of large scale of generating natural gas, which supplies enough gas for the forming of source-contacting gas accumulation.Source-contacting gas has the characteristic of bearing gas in the bottom of tight sandstone reservoir due to the migration and accumulation mechanism which is different from conventional gases.It has the characteristic of showing abnormal at the bottom of gas surveying curve.Ciyutuo-Niuju area in the north and Huangjindai-Dapingfang area in the south are the most favorable areas in eastern sag of Liaohe depression for source-contacting gas accumulations.