Impact of detrital composition and diagenesis on the heterogeneity and quality of low-permeability to tight sandstone reservoirs: An example of the Upper Triassic Yanchang Formation in Southeastern Ordos Basin

Reservoir quality is key to hydrocarbon exploration and often poses challenges for the production of low-permeability to tight sandstone reservoirs. In this paper the Yanchang Formation in the southern Ordos basin was characterized by different reservoir types and heterogeneity through comprehensive analysis of petrological features, petrophysical properties, diagenetic sequences and hydrocarbon charging. Eventually we identified the high-quality reservoirs, more importantly, the controlling factors on their petrophysical properties. This study reveals that low-permeability to tight sandstone reservoirs in the study area exhibit strong heterogeneity. Four types of sandstones were identified and classified as calcareous, dark lamiae-rich, oil-bearing and water-bearing ones. Prior to oil charging, dark lamiae-rich sandstones and calcareous sandstones experienced strong compaction and cementation, respectively. Therefore, they are typical of poor petrophysical properties and usually exist as interlayers. On the contrary, although the oil-bearing and water-bearing sandstones underwent complex diagenetic processes, they maintained good petrophysical properties and became high-quality reservoirs. Now that the sediments have different provenances, the high-quality reservoirs in the southern provenance have more ductile grains, typical of compactional porosity loss. As a result, their reservoir properties are relatively inferior. In comparison, as the high-quality reservoirs in the northeast suffered less from compactional porosity loss, they were developed with superior porosity and permeability. But on the downside, the presence of large amounts of authigenic chlorite cements caused some damage to the reservoir quality.