致密砂岩气田盖层封闭能力综合评价——以四川盆地广安气田为例

Comprehensive evaluation on the sealing capability of cap rocks in tight sandstone gasfield——a case study from Guang’an gasfield in the Sichuan Basin

致密砂岩气作为一种非常规油气资源,具有良好的勘探开发前景,其成藏保存条件问题越来越受到研究人员的关注。在前人研究的基础上,筛选出与盖层封闭能力密切相关的评价参数,如盖层厚度、排替压力、气藏压力系数、断裂对盖层的破坏程度及垂向封堵性,并结合盖层封闭能力有效性概念,建立了盖层封气能力综合评价方法。对我国致密砂岩大气田储量丰度及盖层属性数据进行了统计,厘定了高储量丰度大气田形成所需的各盖层评价参数下限作为其各自均一化标准值。以四川盆地广安气田须家河组五段区域盖层为例,研究发现盖层厚度为30～200 m,排替压力为5.5～9.5 MPa,气藏压力系数为1.05～1.50,断裂活动性较强,断层垂向封闭能力较弱,但盖层封气有效性好;最后,利用综合评价公式计算后得到须五段盖层能够封盖特低储量丰度的气田,盖层封闭能力综合评价指标为0.04～0.24,在工区内总体表现为西高东低的展布特征。该方法预测保存有利区与目前须四段气藏的勘探现状较吻合。

In this paper,we chose several parameters that are closely related with sealing capacity of cap rock,such as cap rock thickness,displacement pressure,pressure coefficient,damage of fault to cap rock and vertical sealing ability.In combination with the concept of sealing effectiveness of cap rock,we proposed a comprehensive evaluation method of cap rock sealing capability.Based on statistics of reserve abundance and cap rock attributes of large tight sandstone gasfields in China,we took the lower limit of each cap rock evaluation parameter necessary for the formation of large gasfield with high reserve abundance as its homogenization standard value.Case study of the regional cap rock of the 5th Member of Xujiahe Formation in Guang’an gas field shows that the cap rock thickness is 30-200 m,displacement pressure is 5.5-9.5 MPa,the pressure coefficient is 1.05-1.50,the fault activity is strong,vertical sealing capacity of the fault is poor,and the sealing effectiveness of cap rock is good.Calculation with the comprehensive evaluation formula obtains a composite sealing capacity evaluation index in the range of 0.04-0.24,indicating that the cap rock of the 5th Member of Xujiahe Formation is capable of sealing gas in reservoirs with ultra-low reserve abundance.The index is high in the west but low in the east as a whole.The play fairways predicted by this method basically coincide with the present exploration results of the 5 th Member in the Xujiahe Formation.