Log Facies Analysis and Reservoir Properties of Basement Granitic Rocks in the Pearl River Mouth Basin, South China Sea

Abstract

Basement granitic rocks represent potential hydrocarbon reservoirs; however, our understanding of their reservoir properties remains limited. In the Pearl River Mouth Basin (PRMB) of the northern South China Sea continental margin, granitic rocks account for ~57% of the basement in terms of lithology, which provides an excellent opportunity to investigate the characteristics and distribution of basement granitic reservoirs. Here we report results from a log facies analysis of the basement granitic rocks in the PRMB based on conventional well-logging data of 13 exploration wells. In the analysis, methods of change-point analysis for segmentation and fuzzy c-means clustering for log facies classification were integrated. As a result, five log facies (LF), named LF 1 to 5, were identified. These log facies were interpreted by integrating the analysis of the lithologic significance of different well logs, the analogy with similar cases confirmed in the literature, and the calibration from the limited cutting and sidewall coring data. Reservoir properties of the basement granitic rocks in the PRMB are highly varied, depending on the degree of the development of secondary porosities and fractures. It is suggested that the granitic rocks with developed secondary porosities and fractures, as represented by the low density (DEN), high acoustic transit time (AT), and relatively high neutron porosity (NP) log facies (LF 3), have the best reservoir potential, followed by the high gamma ray (GR), relatively high AT, and low NP (LF 2), and the low NP, moderate DEN, and AT (LF 4) log facies with variable amounts of fractures, respectively. Granitic rocks represented by the remaining LF 1 and 5 are overall tight and of poor reservoir properties. Favorable reservoir log facies, consisting of LF 2 through 4, are heterogeneously distributed. These findings will be beneficial to deepening our understanding of the reservoir properties of basement granitic rocks.