Geological model and construction process of the Mesozoic volcanic edifices in Bozhong Sag, Bohai Sea

Fine depiction of volcanic edifices is of great value to mapping volcanic reservoirs. A variety of volcanic edifices under fierce later modification are developed in the Mesozoic of the Bozhong Sag, Bohai Sea. There is, however, a lack of documents on the types, structure and construction process of the volcanic edifices in the area at present. The data of core observation, logging and seismic interpretation are applied to divide the Mesozoic volcanic edifices in the Bohai Sea into four types, that is, lava dome, stratovolcano, pyroclastic sheet and shield volcano. The study serves to depict the eruptive units, lithology and lithofacies within the four types of volcanic edifices, with a geological model established and based on the morphological characteristics and internal structure, the diagrams of four types are built to show the volcanic construction processes. Among others, the lava dome characterized by a high aspect ratio is composed of thick-bedded acidic lava of massive structure within; the stratovolcano widely seen in the Bohai Bay Basin and large in scale is mound-shaped in terms of external morphology, smaller than the lava domes in terms of aspect ratio, and characterized by complex lithological types, most of which are of alternating lava and pyroclastic rocks; and the shield volcano is of gentle low-mound shape, and mainly composed of basic rocks, featuring low-viscosity magma and large flow range, resulting in large-scale volcanic edifices of sheet-like structures inside to a large extent. The pyroclastic sheet is in the shape of sheet and filling, with very small aspect ratio and layered structure caused by multi-stage eruption and superimposition. In addition, the comparison of the physical properties of different volcanic edifices shows that the reservoirs of lava dome and stratovolcano types in the study area are of the best physical properties, which can be viewed as favorable targets for future oil/gas exploration, followed by the pyroclastic sheet and then the shield volcano coming at the worst.