Abstract
Devonian sandstones of the Uerê Formation are important oil exploration targets in the Solimões Basin, western Brazilian Amazonia. The basin fill comprises Ordovician to Permian sedimentary successions, Triassic diabase dykes and sills, and Cretaceous to Recent continental deposits. This study deals with the Upper Devonian interval, which consists of sharp-based, progradational sandstones, attributed to a storm-dominated shelf complex formed during an overall transgressive system tract, overlain by Frasnian-Famennian black shales. In spite of their large lateral extent, the exploration of these sandstones is complicated by intense diagenesis, which strongly affected reservoir quality. The main processes of porosity reduction are mechanical and chemical compaction and cementation by quartz overgrowths, carbonates (siderite and dolomite) and fibrous illite. Porosity (up to 28%) was preserved by the inhibition of quartz overgrowth cementation and pressure dissolution by grain-rimming, eogenetic, microcrystalline quartz and associated chalcedony, or smectite. Early diagenetic silica precipitation is related to the dissolution of sponge spicules, which were concentrated in storm-reworked hybrid arenites and in interbedded spiculite deposits. Locally, massive quartz cementation and recrystallisation occurred as a consequence of hot fluids circulation related to Triassic magmatism.
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