Facies Architecture Model of the Shimentan Formation Pyroclastic Rocks in the Block‐T Units,Xihu Sag,East China Sea Basin,and its Exploration Significance

A gas field consisting of volcanic reservoir rocks was discovered in the block-T units of the Xihu Sag, East China Sea Basin. The lithology of the volcanic rocks is dominated by tuff and reworked tuff. The lithofacies are dominated by base surge deposits of explosive facies. As the architecture model of volcanic facies is still uncertain, it has restricted the exploration and development of mineral resources in this area. Using core and cuttings data, the lithology, lithofacies, geochemistry as well as grain size characteristics of volcanic rocks were analyzed. Based on these analyses, the volcanic rocks in the well section are divided into three eruptive stages. The transport direction of each volcanic eruption is analyzed using crystal fragment size analysis. The facies architecture of the block-T units was established based on the reconstruction results of paleo-geomorphology. The results show that the drilling reveals proximal facies (PF) and distal facies (DF) of the volcanic edifices. However, the crater-near crater facies (CNCF) are not revealed. Compared with the reservoirs of the Songliao Basin, it is shown that the volcanic rocks in the Xihu Sag have good exploration potential; a favorable target area is the CNCF near the contemporaneous fault.