Structural diagenesis of bed-parallel and bed-normal fractures, Cretaceous Crato Formation carbonate rocks, NE Brazil

McKinnon, Elizabeth Anne
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Here I show that structures exposed in the limestone quarries of the Araripe Basin provide information on the fracture porosity evolution in laminated members of early Cretaceous Brazilian interior basins and provide insight in to the Crato Formation as an analog for offshore reservoirs and its role as a semi-permeable aquitard in the Araripe Basin. Permeable pathways in the microbial laminites of the Crato Formation were enhanced first by fracturing caused by elevated fluid pressures, next by fracturing caused by increasing overburden related to burial, next by jointing caused by decreasing overburden related to uplift and finally by dissolution along joint walls. Porosity was reduced by cementation of fractures, both during and after opening, and later enhanced by dissolution of cements and included host rock within veins. Bed-parallel fractures generated by fluid overpressure may be present in well-layered regions of pre-salt source rocks assuming petroleum generation is significantly high or fluid overpressure occurred sufficiently early in the burial history of pre-salt source rocks. Bed-parallel fractures may serve as storage sites or assist in the lateral transport of fluids to vertical escape pathways. The orientation of bed-normal structures in the Crato Formation is likely different from the orientation of bed-normal structures in pre-salt source rocks due to differences in the basement framework between the Araripe Basin and offshore basins such as the Campos Basin. Open joints are unlikely in subsurface reservoirs due to their relationship with exhumation. According to speleothem growth, open and partially occluded joints in the Crato Formation likely provide permeable pathways for water between the superior and middle aquifers of the Araripe Basin. This fluid movement may be enhanced along bedding planes and/or semi-permeable, bed-parallel gypsum veins with high porosity.