Abstract

The reservoir quality of fluvial sandstones of the Upper Jurassic Boipeba Member, Recôncavo basin, northeastern Brazil, is highly heterogeneous and controlled by eodiagenesis under semiarid climate, mesodiagenesis during burial to a depth of 3500 m, and telodiagenesis due to local uplift. Eodiagenesis resulted in mechanical compaction, calcite cementation, clay infiltration, and limited grain dissolution, whereas mesodiagenesis resulted in the precipitation of calcite cement and quartz over growths, intergranular quartz-grain dissolution, chloritization and illitization of smectite, and albitization of feldspars. Sandstones continuously buried at maximum burial depths of about 1600 m (T = 65°C) since 125 Ma display a relatively greater degree of mesogenetic modifications and, on average, poorer reservoir quality than sandstones that were buried deeper (2100 m, T = 75°C) prior to uplift, but only since 13 Ma. Uplift, which affected the sequence along the western border of the basin, has resulted in telogenetic dissolution of framework silicates and formation of kaolinite. Relatively good reservoir quality in the deeply buried sandstones occurs when (1) the grains are coated with a thin layer of chloritized infiltrated smectite, (2) there is little or no pseudomatrix, and (3) there are widely scattered patches of eogenetic calcite cement that supported the framework of sandstones against compaction.

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