Characteristics, controlling factors and petroleum geologic significance of fractures in the Archean crystalline basement rocks: A case study of the South Jinzhou oilfield in Liaodong Bay depression, North China

Fractures play an important role in hydrocarbon storage and migration in basement buried hills. Taking the South Jinzhou oilfield (JZS oilfield) as an example, based on the data from cores, thin sections, seismic, logging, and experimental analysis, we clarified the genetic types and characteristics of fractures in the JZS crystalline basement oilfield and discussed controlling factors and their significance to petroleum geology. Tectonic and non-tectonic fractures are present in the study area, in which tectonic fractures are predominant. Tectonic fractures show three dominant directions with E-W, NE-SW, and NW-SE trends; among them, fractures with E-W trends are best developed. Most of the fractures are oblique, with a dip angle between 45° and 60°. The shear-compression zone trigged by strike-slip faults in-plane is favorable for fracture development. The granitic rocks with low compressive strength are more prone to fracture than diabase. Meteoric water and organic acids promote dissolution along fractures, expanding the tectonic fractures further. The fractures can increase the permeability of crystalline basement rocks by 103 times compared with the matrix. The dissolution along fractures can effectively improve the storage capacity of the crystalline basement rocks. Furthermore, there are a large number of fractures developed below the oil-water contact, which are important lateral migration channels for hydrocarbons.