四川盆地东南部下志留统龙马溪组一段页岩微—纳米观地质特征

下志留统龙马溪组一段是四川盆地东南部页岩气勘探开发的目标层系之一,以往对其地质特征的研究多停留在单方面、定性层面。为此,在钻井岩心观察与描述的基础上,应用微—纳米分析技术和模拟技术,对该层系的微—纳米观地质特征进行了分析。结果显示:(1)页岩矿物组成横向发育稳定,但纵向变化明显,龙一1亚段以灰黑色富有机质硅质页岩为主,有机质含量高,石英含量高且主要为有机成因石英;(2)龙一2亚段以灰黑色—黑色含粉砂质页岩及粉砂质页岩为主,较之于龙一1亚段,其砂质含量明显增多,TOC和石英含量则相对较低;(3)龙一3亚段主要为黑灰色黏土质页岩,硅质矿物含量较低,有机质常富集在黏土矿物间呈断续顺层分布;(4)该区页岩主要发育有机质孔隙、黏土矿物孔隙、脆性矿物孔隙和裂缝(含页理缝)等4种储集空间类型,其中有机质孔隙十分发育,多数为纳米级,是页岩气赋存的主要储集空间;(5)龙一_1亚段属于IUPC分类方案中的中孔,孔隙度和渗透率均较高,连通性中—差,喉道直径变小,向上到龙一_2、龙一_3亚段则以IUPC分类方案的大孔为主,但孔隙度降低、连通性变差;(6)该区页岩孔缝细小,孔隙表面积大,对页岩气具有很强的吸附能力,吸附气量随压力增加而增加,页岩气在低速渗流时呈现滑脱效应,渗透率和孔隙压力越小,滑脱效应越明显。

Micro- and nano-scale geological characteristics of the shale in the first Member of Lower Silurian Longmaxi Fm in SE Sichuan Basin

The first Member of Longmaxi Fm, Lower Silurian is one of the target strata for shale gas exploration and development in the southeastern Sichuan Basin. The previous studies on its geologic characteristics are mainly unidirectional and qualitative. In this paper, drilling cores were observed and described. And then, their micro- and nano-scale geological characteristics it were analyzed by means of micro-nano analysis and simulation. It is shown that mineral compositions of the shale are developed stably in the lateral direction, but vary greatly in the vertical direction. The Long 11 Sub-Member is mainly composed of grayish black organic rich siliceous shale with high organic content and quartz content, and its quartz is mainly of organic origin. The Long 12 Sub-Member is mainly composed of grayish black—black siltstone bearing shale and silty shale. And compared with the Long 11 Sub-Member, its sandy content is much higher and its TOC and quartz contents are lower. The Long 13 Sub-Member is mainly composed of blackish gray clay shale with fewer siliceous minerals, and the organic matter is mainly distributed discontinuously between clay minerals along the layer. Besides, in this area, four types of shale reservoir space are mainly developed, i.e., organic pores, clay mineral pores, brittle mineral pores and fractures (including lamellation cracks). And among them, organic pores are quite developed as the main reservoir space of shale gas, and most of them are in nanometer scale. In addition, the Long 11 Sub-Member is characterized by moderate pores in the IUPC classiffication scheme, higher values of porosity and permeability, moderate to poor connectivity and small throat diameters. In Long 12 and Long 13 Sub-Members, large pores are dominantin the IUPC classiffication scheme, but porosity and connectivity are lower. Finally, in this area, shale pores and fractures are thin and large in areas, so their adsorption capacity on shale gas is very high. And the adsorbed gas increases with the rising of pressure. The slippage effect occurs when shale gas flows at a low speed. The lower the permeability and pore pressure, the more obvious the slippage effect.