Integrated diagenetic modeling and reservoir quality assessment and prediction of the Água Grande sandstones, early Cretaceous, Recôncavo Basin, northeast Brazil
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This investigation aimed to (1) reconstruct the diagenetic evolution of sandstones of the Água Grande (AG) Formation throughout their burial history by using an integrated approach designed to promote the linking of petrographic and geochemical data on sandstones, shales, organic matter, and pore fluids; (2) examine the effect of variables inherited from depositional environments, such as initial detrital composition, texture, and early diagenetic constituents, on diagenetic processes and reservoir quality and porosity evolution of the AG sandstones. Particular attention was given to explain anomalous values of porosity and permeability identified at greater depth; (3) consider non-compositional controls (temperature, time, pressure, and hydrocarbon emplacement) on diagenetic processes; and (4) assess and predict porosity and permeability of sandstones as hydrocarbon reservoirs in a prospect area by using a forward numerical model. Early Cretaceous AG sandstones were deposited as the last stage of an intracratonic basin as eolian and fluvial deposits. A rifting phase subsequent to sedimentation was responsible for rapid burial of the Agua Grande. Eolian and fluvial reservoirs evolved differently because of detrital composition and early diagenesis. Compaction and quartz cementation were the major diagenetic processes that affected AG sandstones. Compaction reduced original porosity by about half during the first 2,000 m of burial and evenly affected eolian and fluvial sandstones. Quartz cementation occurred when the temperature reached approximately 80°C and affected mostly fluvial sandstones. Eolian sandstones have smaller amounts of quartz cement because detrital quartz grains are coated by clays and iron oxide that inhibited quartz cementation. Reservoir oil emplacement occurred after quartz cement. Modeling of quartz cement abundance indicates that oil entrapment played strong control upon quartz cementation and prevented further pervasive quartz overgrowth. Numeric modeling of quartz cement abundance and compaction indexes corroborates petrographic data and indicates that an integrated burial, thermal, and diagenetic modeling is crucial to identify maximum burial depth and to point out potential targets for oil and gas in deeply buried intervals.