Feldspar dissolution with implications for reservoir quality in tight gas sandstones: evidence from the Eocene Es4 interval, Dongying Depression, Bohai Bay Basin, China

Abstract

Feldspar dissolution is a pervasive water-rock interaction in siliciclastic sediments and has critical impact on reservoir quality. The purpose of this paper is to evaluate the influence of feldspar dissolution on reservoir quality in tight gas sandstones from the Eocene Es4 interval, Dongying Depression. Based on petrographic, mineralogical, and geochemical analysis, distribution patterns of authigenic minerals associated with feldspar dissolution are recognized. Dissolution of feldspar in Eocene Es4 interval is related to organic acids and CO2 expelled from adjacent source rocks in deep burial environment. Diffusion is inferred to be the predominant transport mechanism for dissolved solids during feldspar dissolution and result in dissolution by-products (e.g. quartz, kaolinite, illite and albite cements) precipitated in-situ or in adjacent pores. Mass balance calculations also demonstrate most of silica derived from K-feldspar dissolution was precipitated as quartz cements within tight sandstone reservoirs. The net porosities created by feldspar dissolution were insignificant but reservoir permeability was reduced significantly by the precipitation of authigenic quartz, kaolinite, albite, and especially by conversion of kaolinite to hair-like illite at higher temperatures.