Origin of the reservoir quality difference between Chang 8 and Chang 9 Member sandstones in the Honghe Oil Field of the Southern Ordos Basin, China

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Abstract Understanding the reservoir quality and heterogeneity of tight oil sandstone reservoirs is essential for their exploration and production. An integration of lithological, diagenetic and hydrocarbon emplacement factors was analyzed to understand the reservoir quality difference in Chang 8 and Chang 9 sandstones of Yanchang Formation, Honghe Oil Field, Ordos Basin, using core, well logs, casting thin sections, SEM, cathode luminescence images and x-ray diffraction data. The results show that Chang 8 and Chang 9 sandstones are mainly fine to medium-grained feldspathic litharenites and lithic arkose with moderately well to well sorting, from braided-delta front subaqueous distributary channel facies. The average porosity and permeability of Chang 8 and Chang 9 sandstones are 8.52%, 0.27mD and 12.96%, 2.86mD, respectively. The main diagenesis types are mechanical compaction, authigenic chlorite, calcite cement, and dissolution. The Chang 8 and Chang 9 sandstone reservoir quality difference is mainly a result of the oil charging and diagenetic environment change. The Chang 8 sandstones that close to the source rock have a high content of feldspar and lithic dissolution and grain collapse from the acidic fluids injection, which dramatically decrease porosity and permeability. Whereas, the Chang 9 sandstones far from the NW-SE direction faults have a small amount of oil charging and acidic fluid injection, which result in high content of chlorite that narrow the throats and reduce permeability, but slightly reduce porosity. Only the sandstones at the bottom of Chang 8 or close to the NW-SE direction faults in Chang 9 Member have a small amount of acidic dissolution, medium content of chlorite, which lead to the relatively high porosity and permeability. High calcite cement contents are mainly observed at the top or bottom of the sandbody, which is not the main factor that leads to the Chang 8 and Chang 9 sandstone reservoir quality difference.