碱性环境成岩作用及对储集层孔隙的影响——以鄂尔多斯盆地长7段致密砂岩为例

Alkaline diagenesis and its effects on reservoir porosity: A case study of Upper Triassic Chang 7 tight sandstones in Ordos Basin, NW China

根据大量岩石薄片观察、扫描电镜和X衍射分析,详细研究鄂尔多斯盆地三叠系延长组长7段致密砂岩的岩石学特征、成岩作用及孔隙成因等,探讨长7致密砂岩碱性环境成岩作用及发育的各种伴生孔隙,并分析孔隙的演化过程及与其油气成藏的关系。长7段致密砂岩广泛发育碱性环境成岩作用,包括硅质的溶解及被交代、碳酸盐矿物胶结交代、钠长石化作用及大量伊利石、绿泥石沉淀。碱性环境下发育了多种伴生孔隙,如石英溶孔、黏土矿物晶间微孔、云母层间孔和钠长石化伴生孔隙。埋藏过程中,碱性环境成岩作用出现于早成岩B阶段和中成岩A2阶段,其中早成岩B期是主要碱性环境伴生孔隙发育阶段,大量黏土晶间微孔的出现严重损害了储集层的渗透能力,同时也导致后期酸性溶蚀强度降低,因此推测在大量烃类充注之前,长7段砂岩已经严重致密化,为典型的先致密后成藏。

Petrologic features, diagenetic processes and origins of different pore types of Upper Triassic Chang 7 tight sandstones in Ordos Basin were analyzed based on cast thin sections, scanning electron microscope （SEM）, and X-ray diffraction （XRD）. The diagenetic processes caused by alkaline pore fluid and associated pores were studied in detail and the relationship between porosity evolution and hydrocarbon accumulation was analyzed. The Chang 7 tight sandstone commonly experienced alkaline environment and associated diagenesis, including quartz dissolution or replaced by mica, clay and carbonate minerals; carbonate cementation; abundant authigenic illite and chlorite with minor kaolinte and smectite; and albitization. Several types of associated pores were created in the alkaline environment, including quartz dissolution pore, intercrystal pore of clay minerals, interlamination pore of hydrated mica and pores associated with albitization. The alkaline diagenesis mainly occurred in two periods during the burial process： B stage of early diagenesis and A2 stage of mesogenetic diagenesis. The abundant intercrystal micropores of clay minerals, which mainly occurred during the B stage of early diagenesis, seriously reduced the permeability of sandstone. As a result, the acid dissolution in the later stage was not intense enough due to the low permeability, so it is inferred that the sandstone had already been compacted very tight before the major hydrocarbon accumulation period.