Controlling effect of paleogeomorphology on quality and distribution of sandstone reservoirs in braided river-delta front: A case study of Triassic Xujiahe formation, SW China

Braided-river delta is an important exploration target for oil and gas basins. The delta front is in a transitional environment with strong heterogeneity of sand bodies. This study uses logging responses, seismic interpretation data, porosity measurements, and grain size analyses to determine the influence of paleogeomorphology on the sand body distribution in the braided-river delta front. The results indicate that the Xujiahe fourth member develops two sedimentary facies types: subaqueous distributary channels (SDCs) and mouth bars (MBs). The SDCs are characterized by chaotic and weak reflection seismic facies, while the MBs are characterized by strong, continuous, and parallel reflection. The sedimentary period is divided into three zones based on paleogeomorphology: depression zone, slope zone, and uplift zone. In the slope area, grooves and protrusions develop alternately. High-quality reservoirs (>6%) are relatively developed in SDCs, with valley paleogeomorphology controlling the distribution of SDCs. The compaction effect in the uplifted area is stronger than in the depressed area. On the one hand, it is due to the lack of green mudstone edge cement in the uplifted area, which resists the compaction effect of reducing pore size. On the other hand, the content of plastic debris is relatively high in the rock debris of the uplift area. This plastic debris deforms and blocks intergranular pores during diagenesis due to strong compaction, resulting in the predominance of intragranular pores in sandstone of the uplift zone. The depressed area not only has a higher quartz content, but also has a rock debris composition that is mainly composed of rigid debris particles (including quartz, feldspar, quartzite debris, flint debris, igneous rock debris, etc.), which allows more intergranular pores to be preserved.