Development characteristics of the unconformity on the bottom Paleogene in the northern Bozhong Sag and its relationship with hydrocarbon accumulation

Unconformity is an important part of hydrocarbon migration system and a favorable place for hydrocarbon accumulation. Taking the northern Bozhong Sag in Bohai Bay Basin as an example, we analyzed the characteristics of unconformity development on bottom Paleogene and its influence on hydrocarbon accumulation comprehensively by using the data from 3D seismic, drilling and logging, and core thin-section. Results show that there are five contact types and seven structural types of the Paleogene bottom unconformity in the northern Bozhong Sag. Unconformity channel types can be divided into double migration channel type Ⅰ, single migration channel type Ⅱ₁ and Ⅱ₂ and plugging type Ⅲ. The single migration channel type Ⅱ₁ is the main type of unconformity hydrocarbon accumulation in the study area. The migration and accumulation types include lateral type, trap and lateral type, vertical and lateral type, and plugging type. The hydrocarbon migration capacity of sandstone above the unconformity is related to the microscopic characteristics of sandstone and the dip angle of fracture. The hydrocarbon migration and storage capacity of semi-weathered rock strata are controlled by the volcanic rocks with good original porosity and permeability and difficult to form weathered clay layer.