Controls on abnormally high porosity in a deep burial reservoir of the Lingshui Formation in the Qiongdongnan Basin, South China Sea

Based on information including porosity and permeability, petrography, the stable isotopic composition of carbonate cements, and homogenization temperatures of aqueous fluid inclusions, we have studied the main factors for the development of abnormally high porosity in the Lingshui Formation reservoir of the Yacheng area. We found the sandstones were mainly subarkose, arkose, and lithic arkose and were texturally and compositionally immature. The research suggested that the sandstones existing close beneath the regional unconformity were formed during the Late Oligocene. Early diagenetic calcite cements leached to form intergranular secondary pores without the precipitation of late-diagenetic calcite cements in most sandstones. The isotopic composition of carbonate cements suggested a significant incursion of meteoric freshwater in the sandstones. Early diagenetic meteoric freshwater leaching reactions provided favorable conduits for the penetration of organic acids during the later period. Thermal fluid activities allowed source rocks to mature rapidly; therefore, the organic acid generation period was extended and feldspars were corroded to form abundant intragranular secondary pores. The abundant corroded minerals and the small amounts of associated authigenic minerals suggested that the dissolution of minerals most likely occurred in an open geochemical system. The dissolution of feldspars and calcite minerals generated an enhanced secondary porosity of approximately 9%-13% in thin sections of these sandstones.