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

Petrologic features, diagenetic processes and origins of different pore types of Upper Triassic Chang 7 (7th member of Yanchang Formation) tight sandstones in the Ordos Basin were analyzed based on cast thin sections, scanning electron microscope (SEM), and X-ray diffraction analysis (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 formed 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 charging.