鄂尔多斯盆地三叠系延长组富有机质页岩孔隙特征及发育机制

页岩储层孔隙发育特征是页岩储层评价的重要内容，也是开展页岩油气赋存机理研究的基础。提出了通过“基质类型—孔隙产状—孔隙成因”划分孔隙类型的方案。利用氩离子抛光和场发射扫描电镜技术观察孔隙发育情况，根据基质类型和孔隙产状，识别了4类孔隙：粒间孔、粒内孔、有机质孔和微裂缝，并根据孔隙成因将粒间孔分为碎屑颗粒间原生孔、黏土矿物片体间孔和颗粒间溶蚀孔，将粒内孔分为长石颗粒内溶蚀孔、黏土矿物片体内孔和黄铁矿晶体间孔。有机质孔主要为有机质颗粒内的微裂缝和有机质内孤立分布的孔径较小的孔隙。微裂缝主要表现为纹层缝或页理缝。综合孔隙图像分析、低温液氮吸附实验结果、孔隙结构参数与矿物组成、有机碳含量和有机质成熟度等参数相关性分析，认为沉积条件、成岩作用和有机质热演化控制了孔隙的形成和保存。半深湖—深湖沉积环境下，富有机质页岩中发育重力流成因的薄层砂质纹层，纹层段碎屑颗粒含量高，有利于形成碎屑颗粒粒间孔、碎屑颗粒粒内孔和顺层微裂缝。早期成岩作用阶段形成黄铁矿，有利于形成黄铁矿晶间孔，但黄铁矿也是压实作用的主要参与者；压实作用造成粒间孔和粒内孔的孔径变小和孔体积降低，碎屑颗粒和黄铁矿与有机质颗粒间呈凹凸接触，有机质孔在压实作用下闭合导致有机质孔不发育；溶蚀作用促进长石粒间溶孔和长石粒内溶孔的形成，一定程度改善储层质量。在成熟度达到一定阶段(R_O≈0.75%)后，开始出现有机质孔。有机质孔发育程度差，一方面受成熟度的影响，另一方面可能是压实作用造成的。此外，富有机质泥页岩渗透性较差，烃类被吸附在有机质表面或溶于干酪根内部，造成干酪根体积膨胀也可能是有机质孔不发育的一种原因。

Characteristics and occurrence mechanism of organic-rich shale in the Triassic Yanchang Formation,Ordos Basin,China

Pore characteristics of shale reservoir are significant content for shale reservoir appraisal,and also the basic for the occurrence mechanism study of shale oil and gas.One scheme for pore types division was recommended in this paper.The scheme is based on mineral matrix types,attitudes and origins of pores.Ar-iron milling and field emission scanning electronic microscope were used to observe pore characteristics.Based on mineral matrix types and attitudes of pores,four kinds of pores were determined,including intergranular pore,intragranular pore,organic matter pore and micro-fracture.Then,according to the origins of pores,sub-types of pores were distinguished.Intergranular pore was divided into primary pores among clastic grains,pores among clay minerals aggregation,and dissolved pores among clastic grains.Intragranular pore include dissolved pores in feldspar,pores among (between) plates of clay minerals,and pores among pyrite crystal.Organic matter pores mainly incorporate micro-fracture in organic matter grain and pores with micro diameter discretely distributed in organic matter grain.Micro-fracture mainly includes fracture between lamina and lamellation.Sedimentary environment,diagenesis and thermal evolution of organic matter co-controlled the origin and preservation of pores in shale.Semi-deep to deep lacustrine environment was conducive to forming organic-rich shale accompanied by thin sandy lamina with origin of gravity flow.Shale deposited with these features was favor of primary pores among clastic grains,pores in calstic grains,and micro-fracture along lamina.Pyrites that formed during eodiagenesis were favor of pores among pyrites,but pyrite also participated in compaction.Compaction caused pore diameter becoming narrower and pore volume becoming smaller,particularly for intragranular pore and intergranular pore.Contact relation between clastic grains and organic matter grains was concave-convex,also between pyrites and organic matter grains.Organic matter pores closed under compaction resulting in organic matter pore not well developed in Chang-7 Member.Dissolution was helpful to form dissolved pores among or in feldspar,as a result,reservoir quality may be improved.There may be no organic matter pore until maturity reached some level (maybeR_O≈0.75%).One of the reason why organic matter pore not well developed is the low maturity,another reason is compaction during burial.Moreover,as permeability of organic rich shale is very low and hydrocarbon may be adsorbed on or dissolved to kerogen,the volume of kerogen may swell resulting in organic matter pore not well developed.