The characteristics and origin of chlorite coats in the Oligocene gas sandstone reservoirs of the Upper Huagang Formation, Xihu Depression, East China Sea Shelf Basin

Abstract

Clay coats are widespread in the Oligocene braided river channel sandstones in the Xihu Depression on the East China Sea Shelf Basin. High clay coat coverage in braided channel sandstones is typically associated with good reservoir quality. The morphology, origin and distribution of clay coats and their effect on reservoir quality in different braided channel facies are investigated by core description, core analysis and a series of petrographic techniques. The present clay coats consist of two parts, the inner and outer coats. The inner coats are mainly composed of illite and chlorite components. The inner illite coats are transformed from smectitic clay precursors, while the inner chlorite coats may originate from thermally driven recrystallization of Fe-rich clay precursors. The outer clay coats, composed of well-crystallized chlorite, are possibly the result of neoformation directly from Fe- and Mg-rich pore water. The distribution of inner clay coats is controlled by sediment transport and deposition processes. The inner illite coats that are better developed in terms of content and thickness typically form in low energy braided channels due to relatively less transport abrasion of the smectitic clay precursors. The inner chlorite coats are observed exclusively in high energy mud clast-rich braided channels. The abundance of mud clasts and Fe-rich clay precursors are likely derived from the intense erosion of adjacent overbank mudstones, which are enriched in flocculated fluvial Fe oxides and hydroxides. The high coverage outer chlorite coats tend to occur in high energy braided channels with well-developed primary pores, since they provide sufficient growth space for chlorite. High energy braided channels with relatively low ductile component content generally maintain better porosity and permeability, which facilitates the development of the outer chlorite coats, thereby preserving more pore space by inhibiting quartz cementation and promoting subsequent dissolution of unstable silicate minerals, further improving reservoir quality. The results of this study provide new insights into the prediction of chlorite-coated reservoir quality of gas fields in Xihu Depression while also being relevant to braided river sandstones having comparable depositional settings worldwide.