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Pimenteiras Shale: Characterization of an Atypical Unconventional Petroleum System, Parnaiba Basin, Brazil.

Abstract

The Devonian Pimenteiras Shale is the source rock for the conventional gas discoveries in the Parnaiba Basin. In this study it’s evaluated as a potential shale gas play. Average depths are 2000m (6,562ft) with up to 40m (131ft) of organic-rich mudstones. Based on forty eight sidewall core plugs from a wildcat well the Pimenteiras Shale was divided into eight main microfacies: muddy-sandstone, muddy-sandstone couplets, sandy-mudstone couplets, bioturbated sandy-mudstone couplets, laminated claystone; organic-rich mudstone; chaotic muddy-sandstone and siliciclastic hornfels. The trace fossils assemblage suggests a Distal Cruziana ichnofacies. The environment of deposition is interpreted as storm-dominated shallow marine system. The total organic carbon (TOC) varies from 0.38% to 4.77% with types II and III kerogen, overprinted by the thermal effect of the Triassic and Cretaceous sills. The strong correlation between pyrite content and TOC suggests that periods of anoxia were responsible for the accumulation and preservation of the organic matter. Estimated vitrinite reflectance maturity is highly variable due to the thermal effect of the sills within the fine-grained interval and it’s closely related to the sill’s thicknesses. SEM analysis showed a wide variety of porosity systems with better development of organic porosity in samples closer to the sills therefore with higher thermal maturity. Besides the usually high clay content the interbedded nature of organic-rich facies and quartz-rich facies might be a positive aspect for hydraulic fracturing. Log interpretation suggests an overpressured compartment with open horizontal fractures. Adsorption and geomechanical data still lack for a complete characterization.