渝东南盆缘转换带常压页岩气储层非均质性特征及主控因素

Heterogeneity characteristics and controlling factors of normal-pressure shale gas reservoirs in the basin-margin transition zone of SE Chongqing

四川盆地东南部及其盆缘转换带(以下简称渝东南盆缘转换带)页岩气资源量较大,但其地质条件复杂、地层呈常压特征,给页岩气的经济开发带来了困难。为了深入认识该区页岩气储层的非均质性,借助X射线衍射、氩离子抛光扫描电镜、低温氮气吸附等手段,从岩石骨架、储集空间的角度研究了该区上奥陶统五峰组—下志留统龙马溪组页岩气储层的非均质性及其主控因素。研究结果表明:(1)该转换带五峰组—龙马溪组页岩气储层非均质性主要表现在骨架非均质性和孔隙非均质性两个方面,优质页岩层段与上覆页岩段在脆性矿物含量、黏土矿物含量、有机质丰度、孔隙度等方面差异较大,具有较强的纵向非均质性;(2)储层发育有机质孔、脆性矿物孔、裂缝等多种储集空间类型,并且孔隙在形态、分布、孔径、结构特征方面的差异明显,具有较强的微观非均质性;(3)页岩岩石物理参数泊松比、杨氏模量也相应表现出较大的差异;(4)页岩气储层非均质性的主控因素是沉积环境差异导致的岩石骨架变化,构造缝及其多向性增强了储层的非均质性,成岩作用进一步改变了骨架矿物和有机质的含量,对孔隙类型及结构产生影响,进一步加剧了页岩储层的非均质性。结论认为,龙一下亚段应为该区下一步页岩气开发的首选目的层系,平面上以远离构造相对活动带和深大断裂为宜,区块上优选南川区块。

In the southeastern(SE)Sichuan Basin and its basin-margin transition zone(hereinafter referred to as the basin-margin transition zone of SE Chongqing),shale gas resource is abundant,but its beneficial development is difficult to achieve due to its geological complexity with the characteristics of normal formation pressures.In view of this,the heterogeneity of shale gas reservoirs of Upper Ordovician Wufeng–Lower Silurian Longmaxi Fms in this area and its main controlling factors were studied from the aspects of rock skeleton and reservoir space by means of X-ray diffraction,argon ion polishing-scanning electron microscopy and cryogenic nitrogen adsorption.And the following research results were obtained.First,the heterogeneity of Wufeng–Lower Silurian Longmaxi shale gas reservoirs in this transition zone is mainly manifested as skeleton heterogeneity and pore heterogeneity.Quality shale intervals are different from overlying shale intervals in terms of brittle mineral content,clay mineral content,organic abundance and porosity,and their vertical heterogeneity are stronger.Second,many types of reservoir spaces are developed,including organic pores,brittle mineral pores and fractures.And pores are obviously different in terms of morphology,distribution,sizes and structures,presenting a stronger microscopic heterogeneity.Third,the petrophysical parameters of shale(Poisson's ratio and Young's modulus)are different.Fourth,the main factor controlling the heterogeneity of shale gas reservoirs is the change of rock skeleton caused by the difference of sedimentary environment.Structural fractures and their pleiotropy enhance the heterogeneity of reservoirs.And diagenesis further changes the content of skeleton minerals and organic matters and influences the type and structure of pores,so as to intensify the heterogeneity of shale reservoirs.In conclusion,it is recommended that the lower submember of the first Member of Longmaxi Fm should be the preferred target layer for shale gas development,tectonic active zones and discordogenic faults should be avoided,and Nanchuan Block should be the preferably selected block.