Organic-rich sediments in the Late Tortonian to Early Messinian Betic Basins of SE Spain: potential source analogs for the Western Mediterranean pre-salt play

Abstract

Recent hydrocarbon discoveries in the Eastern Mediterranean basin have generated an increased interest on organic rich sediments pre-dating the extensive evaporitic deposits originated during the Messinian Salinity Crisis (MSC). In South-eastern Spain a large number of Late Neogene basins with substantial evaporitic deposits developed under an overall NNW-SSE compressional regime related to the collision between the African and European plates. These basins are in the Alpine Betic range, and can be considered as marginal basins that became gradually isolated from the Mediterranean Sea. The Messinian evaporitic sedimentation was related first to a restriction in the communication with the Atlantic Ocean during the period from ∼ 6 to 5.5 Mas, followed by isolation of the Mediterranean Sea between ∼5.5 and 5.3 Ma. Various evaporitic units accumulated in these marginal basins before and during the restriction stage of the MSC. Evaporitic conditions followed periods of organic matter accumulation.

A previous study of the pre-evaporitic deposits from the Lorca Basin described a high oil potential, organic matter of marine origin and a low degree of maturity. In this work we extend the area of interest and regionally characterize these organic–rich sediments by presenting results from boreholes, mines and outcrops samples analysis in the Lorca Basin as well as in other recently dated Betic basins such as Minas de Hellín and Cenajo (Late Tortonian and Early Messinian in age).

The petrological characteristics of the organic matter has been established by using fluorescence and scanning electron microscopy, and the main geochemical features determined by using Rock Eval pyrolysis, Gas Chromatography and Gas Chromatography-Mass Spectrometry. The analytical results show an organic carbon content up to 20%; Hydrogen Index values between 400 and 900 mg HC/g of TOC; and oil potential reaching 150 to 183 mg HC/g of rock. Immaturity of organic matter is indicated by Tmax values varying from 380 to 420°C.

The organic rich deposits from the marginal Mediterranean basins (presently onshore) show very high oil potential; however they did not reach sufficient maturity to produce oil due to insufficient burial. In an offshore situation the occurrence of similar deposits (sapropels) might be expected, buried under up to -4,000 m of sediments accumulated during and after the Messinian subaerial exposure (Messinian evaporites and Plio-Quaternary detrital sediments). These conditions might have favoured maturity and therefore hydrocarbon generation. A model presented in this work synthesizes this hypothesis.

AAPG Datapages/Search and Discovery Article #90226 © 2015 European Regional Conference and Exhibition, Lisbon, Portugal, May 18-19, 2015