Origin and control of grain-coating clays on the development of quartz overgrowths: example from the lower Paleozoic Barik Formation sandstones, Huqf region, Oman

Abstract

Understanding the origin of grain-coating clays and its effects on the inhibition of cementation by quartz overgrowths plays a crucial role in porosity preservation versus destruction in deeply buried sandstones. This study aims to decipher the origin of grain-coating clays and their effect on the quartz overgrowth cementation of the mouth-bar/shoreface and tidal channel sandstones of the lower Paleozoic Barik Formation in the Huqf region of Central Oman. The study is based on petrographical and mineralogical investigations using the conventional optical microscope, scanning electron microscope (SEM) equipped with a dispersed energy spectrometer (EDS), and X-ray diffraction (XRD). The results reveal that the grain-coating clays are predominantly mesogenetic-transformed illite and, to some extent, chlorite. The illitic clays occur as 2–5-μm-thick rims covering the entire detrital grain surface, as well as at grain-to-grain contacts. The illitic clays display honeycomb and cornflake textures with filamentous termination. The chloritic clays occur as 1–3-μm-thick rims, apparently covering the entire detrital grains and at grain-to-grain contact; they display honeycomb-like and platelet morphologies. Such textural morphologies demonstrate the development of illite and chlorite through a transformation process from smectitic clay precursors. The smectitic clay precursors are typically near-surface clay fractions that infiltrated mechanically into the mouth-bar/shoreface and tidal channel facies association sandstones through the tidal hydrodynamic pumps. The predominant occurrence of illite over chlorite is attributed to the availability of potassium ions that are sourced through the extensive dissolution of detrital potassium feldspar grains. The illitic and chloritic clay rims covering the entire detrital grains effectively inhibited the growth of quartz cement by limiting the nucleation sites on the detrital grains. This study may also serve as an analogue model for a similar depositional environment to better understand grain-coating clays’ origin and its role in controlling cementation by quartz overgrowths in deeply buried sandstone reservoirs globally.