Sources of authigenic quartz in the Permian tight sandstones close to Gaoqing Fault, Dongying Sag, Bohai Bay Basin, China

Abstract

Abstract Authigenic quartz is a significant cause of reduced porosity and permeability in the cement of the Permian Upper Shihezi Formation (P2s) sandstones in the Gaoqing area, Dongying Sag, Bohai Bay Basin, China. The sandstones are mainly quartzarenite with minor amounts of clay minerals. Authigenic quartz is generally the most commonly occurring cement and occurs as quartz overgrowth, microfracture-healing quartz, pore-filling quartz, and microquartz in the sandstones. Three distinct, separate phases of authigenic quartz (I, II, and III) were distinguished by using cathodoluminescence microscopy (CL). In the scanned CL images, phase I quartz generally appears with light brown luminescence in the 450–670 nm range, phase II quartz is non-luminescent in the 420–480 nm range, and phase III quartz appears with dark brown luminescence in the 470–650 nm range. Phase I quartz, originating mainly from pressure solution, has a composition similar to that of detrital quartz grains and was formed at approximately 79.3–104 °C. In the phase II quartz, Li+, Na+, Mg2+, Al3+, and Ba2+ are enriched, and the homogenisation temperature of some aqueous inclusions is significantly higher than its maximum burial temperature of 130 °C. The δEu of phase II quartz is up to 1.42. The silica sources for phase II quartz are dominated by hydrothermal fluids; pressure solution, feldspar dissolution, and illitisation of smectite also provided a small amount of silica. In the phase III quartz, all of the aqueous inclusions are almost higher than its maximum burial temperature. Al3+, Mg2+, and Rb+ are enriched in most of this quartz, which was sourced mainly from hydrothermal fluids; pressure solution also provided a very small amount of silica. The hydrothermal fluids providing the silica source for phases II and III quartz migrated into the P2s sandstones along the adjacent Gaoqing Fault.