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Pre-evaporitic Messinian sediments: a new challenge for oil exploration in the Mediterranean Basin

Albert Permanyer, Patricia Marin, and Lluis Gibert

South-eastern Spain has a large number of Late Neogene basins with substantial evaporitic deposits that developed under an overall NNW-SSE compressional regime as a result of African-European plate collision. Two main evaporitc units have been described, the older is Late Tortonian (8.5-7.2 Ma) and the younger is Late Messinian (6.0-5.3 Ma). Tortonian evaporites are related with the uplift of the Betic seaways and the resulting restriction or isolation of marginal marine basins. The Messinian evaporitic deposition was related to the isolation of the Mediterranean Sea during the Messinian Salinity Crisis. Associate to both evaporitic units organic rich sediments forming oil shales and sapropels occur. Previous studies in the Lorca Basin showed high oil potential for some layers, the marine origin of the organic matter and a low degree of maturity. In this study we analysed Late Tortonian oil shale samples from the Lorca, Minas de Hellín and Cenajo Basins found in boreholes, mines and outcrops.

This work examines the petrological characterisation of the organic matter using fluorescence microscopy and evaluates the main geochemical features of organic-rich deposits for these basins. In general, the organic matter appears as amorphous matter, showing thin yellowish-red fluorescence laminae denoting an undisturbed sedimentary environment, favouring both accumulation and preservation. The results are: organic carbon content up to 20%; Hydrogen Index values between 400 to 900 mg HC/g of TOC; and oil potential up to 150 mg HC/g of rock. The immaturity of organic deposits is indicated by Tmax values varying from 380 to 420 Celsius degrees.

The deposits in onshore Tortonian and Messinian basins do not reach sufficient burial to produce oil. However, in offshore areas the presence of similar deposits might be expected. These contemporaneous oil shales and sapropels would then be covered by up to ~4,000 m of sediments, including detrital deposits derived from the Messinian subaerial exposure, Messinian evaporites and the post-Zanclean flood sediments (<5.3 Ma). The possibility of finding oil shale deposits below the Messinian Salt associated with conditions of high burial and heat flow provides an oil exploration challenge in the Mediterranean offshore.

AAPG Search and Discovery Article #90161©2013 AAPG European Regional Conference, Barcelona, Spain, 8-10 April 2013