海相页岩裂缝孔隙发育机制及地质意义

天然裂缝形成机制研究是开展页岩气富集机理研究和“甜点区”优选的基础性工作。以国内外6个大型页岩气田为主要研究对象，通过开展黑色页岩天然裂缝发育特征及其形成机制研究发现，海相页岩天然裂缝的形成至少存在前陆盆地冲断褶皱与页岩层滑脱变形、晚期构造反转与页岩层滑脱变形、走滑断层周期性活动等3种机制。前陆盆地冲断褶皱与页岩层滑脱变形是北美地台Appalachian-Ouachita褶皱带的诸多前陆盆地普遍存在的页岩地层造缝机制，一般具有水平剪切作用时间长、页岩层发生水平位移距离较远、在页岩层中产生的共轭裂缝和微裂隙十分发育等特点。晚期构造反转与页岩层滑脱变形是裂谷盆地和叠合盆地中重要的造缝机制，页岩地层天然裂缝发育程度主要与晚期构造反转的强度和规模有关。走滑断层周期性活动主要存在于少数盆地的局部构造带，造缝区域相对局限。通过开展海相页岩裂缝孔隙形成机制探讨，认为有2点地质意义值得关注：一是基于富有机质页岩层滑脱作用形成的裂缝型气层是页岩气开发的重要“甜点”；二是中国南方海相页岩分布区具有形成裂缝型页岩气藏的有利构造条件，其中四川盆地龙马溪组发育裂缝型页岩气“甜点”，勘探前景值得期待。

Development mechanism of fracture pores in marine shale and its geological significance

Formation mechanism of natural fracture is fundamental to the research of shale gas concentration mechanism and "sweet spot" selection.This paper studied the characteristics and formation mechanism of natural fracture in black shale based on six large domestic and foreign gas fields,and the results showed that there are at least three kinds of mechanisms for the natural fracture in marine shale:Foreland thrust-fold and detachment-deformation of shale bed,late-stage tectonic inversion and detachment-deformation of shale bed,as well as periodic movement of strike-slip fault.Foreland thrust-fold and decollement-deformation of shale bed,which is generally characterized by long time of horizontal shearing,large horizontal displacement and well-developed conjugated and micro-fracture,are very common in many forelands in Appalachian-Ouachita fold belt of North America platform.Late-stage tectonic inversion and decollement-deformation of shale bed are important fracture-mechanism in rift basins and superimposed basins,with the development degree of natural fracture depending on the intensity and scale of tectonic inversion.Periodic movement of strike-slip fault mainly exists in partial tectonic belt of a few basins with relatively limited fracture area.According to the formation mechanism of natural fracture in marine shale,two aspects of geological significance are emphasized:Fractured-reservoir forming from the decollement of shale bed is important “sweet spot” in shale gas development;tectonic conditions of marine shale in southern China are favorable for the formation of fractured shale gas reservoir,especially the Longmaxi Formation in Sichuan Basin with fractured-sweet spot and good exploration prospect.