RESERVOIR CHARACTERISTICS AND FORMING MECHANISM IN BURIED-HILLS OF GRANITE:A CASE STUDY OF B SAG IN THE PEARL RIVER MOUTH BASIN,THE SOUTH CHINA SEA

Granite reservoirs are special reservoirs that are quite different from traditional sedimentary rock reservoirs and contain rich hydrocarbon reserves under the certain geology condition. As the largest hydrocarbon generation sag on the northern continental shelf of the South China Sea, B Sag, the Pearl River Mouth Basin has a good exploration prospect, granite of Yanshanian is widely distributed in the basement. However, the previous researches mainly focus on the Cenozoic sedimentary strata and tectonic evolution, the exploration field of buried-hill oil and gas is almost blank, which is mainly limited by the weak petrographic and reservoir property studies. Based on lastest sidewall core, cuttings samples, thin sections, whole-rock geochemical analysis, drilling data and seismic data, we evaluated the petrological characteristics and explored the main factors controlling granite reservoir formation, and distribution laws of the reservoirs were investigated in the B Sag. In this area, three types of lithofacies are identified: Medium acid intrusive rock, eruptive rock, and metamorphic rock; among these rocks, granite of Yanshannian is widely distributed. Brittle minerals such as quartz and feldspar are the material basis for the formation of granite reservoirs. The reservoir spaces of granite are divided into fractures and pores produced via weathering and structural movements. Fractures can be subdivided into structural fractures and dissolution fractures, and pores include dissolved intergranular pores, dissolved intragranular pores and vugs. Numerous structural fractures and dissolution fractures provide the main reservoir spaces for hydrocarbons. The reservoir quality of granite is primarily controlled by faults, fractures, lithofacies, weathering and paleogeomorphology. Lithofacies and minerals are the internal factors. While paleogeomorphology, faults and weathering are the external factors. Medium acid intrusive rocks are high-quality reservoir lithofacies, and high paleogeomorphology areas and fault zones are the optimal locations for fractures and pores. This study has scientific significance for investigating the reservoir-forming mechanisms and practical applications for exploring and developing oil and gas fields of granite buried hill in coastal China.