Diagenesis of braided fluvial sandstones and its implications for prediction of reservoir quality: a case study on the Neogene Shawan Formation, Junggar Basin, NW China

This study states the spatial and temporal distribution of diagenetic alterations and their influence on reservoir quality in the lower member of Shawan sandstones of the Chepaizi Uplift in the Junggar Basin, NW China. Sandstones in this unit, which contains heavy oil, are composed of mostly lithic arkose deposited in a braided fluvial environment. The diagenetic processes that have affected the sandstones include mechanical compaction, cementation by carbonates, quartz, and clay minerals, and dissolution of framework grains and cements. Relatively good reservoir quality of the studied arkosic heavy oil reservoir rocks deposited in the braided fluvial environment occurs when feldspar, lithic fragments, and calcite cement experience dissolution in channel lag sandstones. However, the pervasive distribution of early carbonate cements mainly concentrated in braid bar deposits has resulted in a great reduction of reservoir quality. This study reveals that understanding of diagenetic evolution of braided fluvial sediments is helpful for providing a better elucidation and prediction of reservoir quality, and the results of this study can be applied to guide the direction for exploration and production of heavy oil.