Characteristics and Genetic Mechanism of Granite Weathering Crust of Songnan Low Uplift, Qiongdongnan Basin, South China Sea

Abstract

Recently, a large-scale gas reservoir was discovered in granitic buried hills of the Songnan Low Uplift in the Qiongdongnan Basin. However, the strong heterogeneity of granite reservoirs limits further exploration and evaluation. Based on observations of sixty core samples and sixty thin sections, mineral composition, zircon dating, apatite fission tracks, physical properties, image logs, outcrop surveys and seismic interpretations, the characteristics of granite weathering crust of the Songnan Low Uplift are analyzed, and its controlling factors and evolution process are evaluated. The results show that weathered granite in the study area can be divided into several zones, from top to bottom: eluvium–slope zone, sandy zone, weathered fracture zone and horizontal undercurrent vuggy zone. The reservoirs in the eluvium–slope zone are dominated by microfissures and intergranular dissolution pores and have an average porosity of 4.68% and permeability of 2.34 md; the reservoirs in the sandy zone are composed of intergranular and intragranular dissolution pores and have an average porosity of 11.46% and permeability of 4.99 md; the reservoirs in the weathered fracture zone consist of various fractures and have an average porosity of 3.91% and permeability of 2.5 md; the reservoirs in the horizontal undercurrent vuggy zone are subhorizontal fractures and vugs and have an average porosity of 2.7% and permeability of 0.23 md. The development of granite reservoirs is jointly influenced by petrology and minerals, long-term exposure in a warm humid paleoclimate, faults, diverse topographies and shallow buried depth. Based on the above, our study establishes a development model of weathering crust and suggests that only the gentle slope and platform remain strongly weathered zones. After undergoing a complex evolution process of formation–destruction/denudation–regeneration–preservation, the current weathering crust of the Songnan Low Uplift is finally established. The results of this study have important theoretical and application value for the hydrocarbon exploration of buried hills in the Qiongdongnan Basin and provide a reference example for other granite reservoirs worldwide.