Origin of authigenic kaolinite with implications for Permian tight gas sandstone reservoirs in the northern Ordos Basin, central China

Abstract

This study deciphers the origin of authigenic kaolinite in the lower unit of Permian Shihezi Formation (P2x) tight gas sandstones in the Hangjinqi gas field, northern Ordos Basin, China, to determine its implications for reservoir quality. Authigenic kaolinite is the most abundant diagenetic cement and two occurrences of authigenic kaolinite are identified in the P2x sandstones: 1) in-situ replacive kaolinite; and 2) pore-filling kaolinite. Petrographic and geochemical evidence reveals that authigenic kaolinite is mainly derived from dissolution of K-feldspar via flushing with organic fluids from underlying source rocks. This evidence includes: 1) spatial association of partially dissolved K-feldspar and authigenic kaolinite; 2) the amount of K-feldspar dissolution is mostly equal to that of kaolinite precipitation within sandstones; 3) a homogenization temperature of 110–120 °C for aqueous inclusions in quartz overgrowths; 4) negative stable carbon isotopic compositions of calcite cements (δ13CPDB from −16.05‰ to −9.35‰). Dissolution of K-feldspar, coupled with precipitation of kaolinite and quartz, is interconnected fluid-rock interactions within sandstones that are inferred to simultaneously occur in a relatively closed system. Intra-formational diffusive transport is inferred to be the dominant transport mechanism during these fluid-rock interactions, which may result in precipitation of in-situ replacive kaolinite or pore-filling kaolinite. This transport pattern may have been localized (several centimeters to meters). The fluid-rock interactions lead to redistributional secondary porosity and would not significantly enhance total reservoir porosity. The occurrence of authigenic kaolinite greatly reduced reservoir permeability and gas saturation by occluding the pore-throats in the P2x sandstones.