Dickite in shallow oil reservoirs from Reconcavo Basin, Brazil; diagenetic implications for basin evolution

Fluvial and aeolian sandstones of the Sergi Formation are the most important reservoirs of the Reconcavo Basin, Brazil. Optical and scanning electron microscopy, X-ray diffraction and infrared spectroscopy revealed the occurrence of dickite, a clay mineral indicative of deep burial conditions (T>100 degrees C), in the shallow Buracica (630-870 m) and Agua Grande (1300-1530 m) oilfields. Vermicular dickite replaces K-feldspar and plagioclase grains, and fills intra- and intergranular pores. Its vermicular habit is a product of pseudomorphic kaolinite transformation during burial. The presence of dickite is in accordance with the intensity of compaction, post-compactional quartz cementation and delta (super 18) O values of calcite cements (T up to 109 degrees C). These petrological features of deep burial, as well as apatite fission-track analyses, indicate that uplift and erosion of at least 1 km, and probably >1500 m, affected the central part of the Reconcavo Basin and possibly the entire region. This uplift has not been detected previously by conventional structural and stratigraphic models.