南堡凹陷馆陶组火山岩盖层封闭能力定量评价

为探讨南堡凹陷馆陶组三段火山岩盖层之下天然气的分布规律,在掌握了馆陶组三段火山岩发育及分布特征的基础上,利用地震数据及油气井实测资料,开展了针对其封气机理、封闭能力和对天然气聚集与分布控制作用的研究,结果表明:南堡凹陷馆陶组三段火山岩整体破碎程度较高,对油气运聚未能起到有效遮挡作用,但火山喷发期次多、强度大,且具轻微含泥蚀变作用,导致垂向上玄武岩与泥岩及玄武质泥岩频繁互层,层段内的泥岩及泥质岩含量较高,泥质岩含量与单井天然气日产量呈良好的线性正相关关系,说明真正对下伏天然气起到封闭作用的是馆陶组三段火山岩盖层内的泥质岩夹层,且封闭能力与泥地比呈正相关关系。通过井震结合的方法,预测得到研究区内馆陶组三段盖层泥地比由凹陷西南端向东北缘逐渐减小,表明其封闭能力也由凹陷西南端向凹陷东北边部逐渐减小。进一步对凹陷内天然气垂向分布特征、盖层厚度及泥岩含量的拟合和分析可确定,泥质体积分数30%是馆陶组三段盖层封闭住下伏天然气的下限值,另外考虑到断裂对盖层的破坏作用,划定断接厚度大于180 m的区域为馆陶组三段有效封盖区,结合泥质含量下限值与断接厚度下限值,可给出馆陶组三段盖层厚度与泥地比互补关系的定量公式。根据上述参数与指标,刻画出南堡凹陷内馆陶组三段火山岩盖层下的天然气成藏分布有利区。

Quantitative evaluation of the volcanic caprock sealing capacity for Guantao Formation in Nanpu Sag

In order to explore the distribution law of the natural gas under the volcanic caprock of Member Ng3 in Nanpu Sag, based on the grasped development and distribution characteristics of the volcanic rock in the member, and moreover with the help of the seismic and logging data of the hydrocarbon wells, the sealing mechanism, sealing capacity and controlling actions on the natural gas accumulation and distribution were searched, the results show that the volcanic rock in Member Ng3 was highly broken in general, which makes it hard to preventing the hydrocarbon migration and accumulation;however, this member of the rock is characterized by many volcanic eruption period-times, strong intensity and slight mud-alteration action, thus results in the vertical frequent interbeds between the basalt and the mudstone and basaltic mudstone, and furthermore the contents of the mudstone and argillaceous rock in Member Ng3 are pretty higher, the argillaceous rock content has much better linear positive correlation with the daily gas production of the single well, which indicates that the argillaceous interbed in the volcanic caprock is the real one who can seal the natural gas beneath Member Ng3, and the sealing capacity is associated with the mudstone-formation ration. By means of the well-seismic combined method, it is predicted that the mudstone-formation ratio in Member Ng3 of the interesting area will gradually decrease from the southwestern edge to the northeastern margin of the sag, so as the sealing capacity. The further match and analyses on the vertical distribution characteristics of the natural gas, cap thickness and the mudstone mass fraction in the sag determine that 30% content is the lower limit value for the caprock to seal the underlying natural gas in the member. In addition, considering the damage action of the fault to the caprock, the determined region where the faulted connected thickness greater than 180 m is the effective sealed area in Member Ng3, by integrating the lower limit values of both the mud content and connected thickness, the quantitative formula showing the complementary relationship between the thickness and mudstone-formation ratio of Member Ng3 caprock can be obtained. With the help of the parameters and indexes stated above, the favorable plays of the natural gas accumulation and distribution can be characterized under Member Ng3 volcanic caprock in Nanpu Sag.