Origin and significance of secondary porosity: A case study of upper Triassic tight sandstones of Yanchang Formation in Ordos basin, China

The Chang7 and Chang6 sandstones in the southwestern Ordos basin are the typical tight oil sandstones in China. Pore textures and their genetic classes of secondary porosity were studied in detail using a combination of optical petrography, SEM and BSE. In order to understand generation process of secondary porosity better, the diagenetic history was reconstructed by a variety of methods, including thin section observation, SEM, BSE and fluid inclusion. Three main diagenetic events is considered to be most likely related to the generation of secondary porosity, these are: 1) meteoric water flushing; 2) formation of authigenic clay minerals; 3) leaching of feldspars, rock fragments and carbonate cements in deep burial sandstones. Particular attention was paid to the generation of secondary porosity and mass transformation in these diagenetic events. This study suggests only meteoric water flushing, which occurs at shallow depths, may result in a net increase of porosity because dissolved minerals can be removed out of sandstones in such an open system. However, below the zone of freshwater flow, dissolved minerals would precipitate as new diagenetic minerals at nearby pore spaces in sandstones, suggesting secondary porosity formed by dissolution only represents a local redistribution of the solids rather than a net increase of porosity.