Depositional and diagenetic controls on the evolution of abnormally high porosity in braided river deltaic sandstones (Oligocene), Xihu Depression, East China Sea

Abnormally high porosity (AHP) was observed at the depth interval of similar to 3650-3900 m in the Oligocene Huagang Formation braided river deltaic sandstones from the Xihu Depression, East China Sea Shelf Basin. The AHP is similar to 1.35-1.95 times (median values) higher than the background normal porosity (NP). In this work, 136.8 m consecutive core samples located within the AHP zone were collected. We used sedimentary facies interpretation, helium porosity/permeability, thin section petrography, scanning electron microscopy, X-ray diffraction, laser grain size and fluid inclusions to discuss the potential influences of depositional environment and diagenesis on AHP evolution, which can be simplified into three steps. In step (1) (pre-dissolution), greater depositional porosity, and/or weaker cementation, and/or weaker compaction result in greater porosity preservation before entering the dissolution window. One AHP sample has already exhibited a higher porosity than NP samples at this step. In step (2) (dissolution window), greater porosity before entering the dissolution window dominantly results in stronger grain dissolution. Meanwhile, chlorite coats inhibit the growth of quartz overgrowths and therefore enlarge the net porosity increase through grain dissolution. As a result, the porosity gap between AHP and NP samples is further widened. In step (3) (post-dissolution), post-dissolution cements exert negligible influences on the further growth or decline in the porosity gap. Therefore, we proposed eight possible AHP evolution paths. This work may serve as a reference for discussing the evolution of AHP in analogous geological settings.