Authigenic berthierine and incipient chloritization in shallowly buried sandstone reservoirs: Key role of the source-to-sink context

Abstract

Abstract Chlorite is recognized worldwide as a key mineral that inhibits the development of quartz cement in deeply buried sandstone reservoirs. Iron-rich chlorite is mainly formed by the transformation of a precursor clay mineral; however, few studies have focused on the early stages before the crystallization of chlorite. This study analyzed shallowly buried (400–1000 m) coastal sandstones from within the Wealden Group of the Paris Basin. Berthierine, a 7 Å trioctahedral clay mineral belonging to the serpentine group, approximatively with same chemistry as chlorite but a different crystal structure, has been identified in a 900-m-deep well but not in a 400–600-m-deep well. Berthierine has mainly been observed as clay coatings around detrital grains with a typical honeycomb texture. Nanopetrographic observations suggest that the honeycomb textural organization of the clay particles found in deeper buried sandstone reservoirs (>1500 m) is acquired from a berthierine precursor at shallow depths. However, small amounts of quartz overgrowths are observed on the surface of detrital grains at shallow depths and low temperature (below 40 °C), and it is believed that precursor berthierine coatings are primarily responsible for the inhibition of quartz overgrowths before Fe-rich chlorite is formed. This suggests that the key mineral primarily controlling the reservoir quality of deeply buried sandstone reservoirs is berthierine rather than iron-rich chlorite, which challenges the commonly accepted assertion that chlorite coating is the main process that inhibits quartz overgrowths. The source-to-sink context of the Paris Basin during the Early Cretaceous was decisive with respect to the supply of sands and berthierine clay precursors (in particular kaolinite and iron-rich, hydroxy-interlayered clay minerals) to the center of the basin.