Carbonaceous Shales in the Araripe Basin, NE Brazil: A Potential Shale Gas Reservoir

Neto, João Adauto S.; Mort, Haydon P.; Pereira, Ricardo; Barbosa, José A.; Neumann, Virginio H.; Vortisch, Walter; Filho, Osvaldo J.C.; Brandão, Paulo de A.L.S.; Pacheco, José G.A.

Recent interest in potential Brazilian shale-gas reservoirs has opened up exploration in a variety of sedimentary basins in the country. One basin, the Araripe Basin (8000 km2, Pernambuco, Northeast Brazil), contains a series of Aptian-Albian deposits. We undertake a multi-disciplinary investigation on a series of intercalated carbonaceous pyrobitumen shales underling a sequence of Albian evaporites, in order to characterize their paleodepositional environment and assess the region's potential as a shale-gas reservoir. Sampling was undertaken at several gypsum mines in the area, which contain the deposits. Coring reveals that the shales, which possess a pungent hydrocarbon aroma, vary from 10-15 m in thickness. Our data is derived from the top 0.5-2 m of this shale bed, which outcrop within the mines. High organic matter content (up to 29 %), total phosphorus and barium values (up to 2%) indicate that the shales were deposited under a high productivity regime. This remarkable enrichment in organic matter is confirmed by thermogravimetric analysis, which shows a significant drop in sample weight percentage between 200-500°C. Meanwhile, significant pyrite content, elevated vanadium concentrations (up to 817 ppm) and low organic phosphorus content (despite high organic matter content) suggest regionally intense sulfate reduction and hypoxia, which possibly led to phosphorus recycling. X-ray diffraction analysis reveal expansive and interlayered clays with illite/smectite (I/S) values ranging from between 10-30% of illite layers. This is consistent with the moderately deep alteration of smectite at temperatures in the order of 100°C (dry-gas to upper oil window). Preliminary GC-MS analysis reveals demethylated 17α(H)-hopanes and an absence of n-alkanes and isoprenoids, suggestive of biodegradation of organic matter, at least within the shales outcropping in the mine. Given the high paleoproductivity, preservation and apparent thermal maturity of the organic matter, we conclude that the shales of the Araripe Basin likely represents a large shale-gas reservoir, worthy of further investigation.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013