Characteristics and vertical zonation of large-scale granitic reservoirs, a case study fromPenglaioil field in the Bohai Bay Basin, North China

Primary pores are undeveloped in the granite, but multiple types of reservoir spaces can form during the processes of weathering and tectonic movement. Taking Penglai (PL) oil field as an example, the vertical zonal characteristic of the buried granite is defined and the relationship between different zones and reservoir is discussed. The granite can be divided into weathered zone (WZ) and bedrock zone (BZ) vertically according to weathered or not, and the WZ can be divided into weathered clay zone (WCZ), weathered glutenite zone (WGZ), and weathered fracture zone (WFZ) further. The lithology, mineral composition, and weathering indices (geochemistry) can distinguish different zones. Both WZ and BZ can form good reservoirs, among them the reservoir of WGZ is dominated by pores, while that of WFZ and the BZ are mainly composed of fractures, and they constitute the high-quality reservoir together. There is an obvious positive correlation between the thickness of fracture zone in BZ and the thickness of WZ, and both WZ and BZ are influenced by tectonic movement. Moreover, WZ superimposed with meteoric water dissolution makes it the best reservoir in the granitic buried hill, and the development of fractures in BZ extends the thickness of reservoir greatly.