Genesis of Grain-Coating Clays and Their Effect on the Development of Feldspar Overgrowths: A Case Study of the Late Cambrian to Early Ordovician Barik Formation Sandstone Reservoir, Haushi-Huqf Region, Central Oman

Abstract

The Barik Formation sandstones accumulated under a deltaic system and constituted important deep and tight gas reservoirs in the Interior Oman Salt Basin. This study aims to understand the genesis of grain-coating clays and their role in controlling the development of feldspar overgrowths. The study is based on integrated different analytical techniques, including thin-section petrography, scanning electron microscopes coupled with dispersed energy spectrometers, and X-ray diffraction analyses. These are achieved to determine the mineralogical and chemical composition, texture and mode of occurrence of the grain-coating clays. These analyses revealed that the grain-coating clays are typically illite and, to a smaller extent, chlorite. The observed illite forms ~ 5 μm thick envelop rimming partially to entirely the detrital grain surfaces and also along grain-to-grain contacts. The illite shows honeycomb-like texture and cornflake textures with filamentous terminations. The chlorite forms ~ 3 μm thick envelop rimming, seemingly the entire detrital grains and at grain-to-grain contacts. The chlorite also shows a honeycomb-like texture and curved platelet morphologies. The described illite and chlorite textural morphologies and mode of occurrence strongly support their growth through a transformation process from smectite precursors. The transformation of smectite to illite and chlorite were taken place during the mesodiagenesis. The smectite clay precursors are naturally near-surface clay fractions that are ultimately introduced mechanically into sandstones by hydrodynamic pumps during high tides. The larger extent of illite compared to chlorite can be related to the enrichment of the system by potassium ions, which are sourced internally via the widespread dissolution of detrital potassium feldspar grains. It is noticed that when the illite and chlorite, partially to entirely, envelop the detrital feldspar grains, they effectively hinder and inhibit the precipitation of authigenic feldspar overgrowth by limiting the nucleation sites on the detrital grains. This study may be regarded as an analogue for similar sandstone reservoirs to understand better the genesis and role of grain-coating clays on reservoir quality.