Characteristics and formation mechanism of the fractures in Archaean buried hill: A case study in the BZ19‐6 Block, Bohai Bay Basin, China

Abstract

While Archean buried hill fractured reservoirs have been found for the first time in the BZ19‐6 Block of the Bozhong Sag in the Bohai Bay Basin, the mechanisms driving their creation and evolution are not well known for low degree of exploration. Using core data, image logs, and our knowledge of the regional geology, we characterized the features and explored the formation mechanisms of these fractures. Our core samples captured two types of fractures: high‐angle shear fractures and low‐ to moderate‐angle tensile fractures. Based on the cross‐cutting relationships, four distinct sets of fractures formed at different stages and related to different tectonic events. The first fracture set (Set I) is high‐angle shear fractures having an E‐W orientation, partially filled by calcite, and was formed by transpressive processes in the Early Yanshanian. The second fracture set (Set II) is characterized by moderate angles, partially filled by asphalt and calcite with variable orientations and a more mesh‐like appearance, formed in an extensional environment in the middle Yanshanian. During transpressive activities in the Late Yanshanian, the high‐angle shear fractures of the third fracture set (Set III) formed with NW‐SE orientations and partially filled by asphalt. The fourth fracture set (Set IV), which occurred in the Early Himalayan, involves high‐angle shear fractures that is oriented in the NE‐SW direction and only part of them are filled by clay minerals. Macroscopic fracture features, orientations, and distribution patterns can significantly alter the nature of a reservoir; the fractures in Set I (E‐W‐oriented) and III (NW‐SE‐oriented) are typically the widest (excluded fills), and they tend to have the greatest impact on the overall reservoir permeability.