Hydrocarbon Systems in the Onshore and Offshore Sicilian Fold-and-Thrust Belt: New Constraints from Geochemical Data

Caldarelli, Carlo ¹; Smith, David ¹
(1) Northern Petroleum Plc, London, United Kingdom.

Northern Petroleum and its partner Shell Italia E&P are jointly engaged in oil & gas exploration in a large area of relatively deep water offshore western Sicily. The area under study contains the offshore extension of the Atlas fold & thrust belt (OFTB) of North Africa which links to the north-east with the onshore Sicilian thrust belt continuing to the Apennine mountain chain. Only one well, Ermione-1 (Agip/Shell, 1989), has been drilled in the OFTB.

A geochemical fieldtrip was undertaken to Sicily sampling the thrust belt with the aim of determining the hydrocarbon potential of all possible source rocks and trying to tie source rocks to oil & bitumen seeps. In order to determine the hydrocarbon potential in the OFTB, the results of geochemical analyses (pyrolysis, GC and total extract data, biomarkers, isotopes, etc) have been integrated with a regional geological model which also included an inferred stratigraphy and tectonic evolution for the OFTB.

A preliminary analysis of seepage combined with stratigraphical correlations indicates the possible extension of U.Triassic-L.Lias intraplatform basins with hydrocarbon potential (i.e. Marineo Basin) to Western Sicily and to the inverted offshore Tortonian-Messinian foredeep.

The Cenomanian-Turonian black shales outcropping in NE Sicily, regarded as a unit equivalent to the Bahoul Fm. in North Africa, have the highest source rock potential (TOC above 20%, HI up to 640, early mature). Our studies suggest these sequences likely formed within slope facies of the Panormide Platform domain and were emplaced on the Argille Scagliose Fm. by large back thrusts. The possible presence of similar sequences in the OFTB has been inferred from regional paleogeographical reconstructions, indicating the likely presence of nappes of equivalent U. Cretaceous slope domains overlain by Pre-Panormide type sequences. The latter units overlain by syn and post orogenic deposits with thickness of about 4km may have provided sufficient burial for hydrocarbon generation.

Organic rich pebbles in the Argille Scagliose Fm. (L. Tertiary, Sicilide Unit) also showed remarkable geochemical properties with TOC concentrations up to 46%, HI up to 515. However, organic rich sediments make up a very limited fraction of the Argille Scagliose Fm. in the outcrops.

In this paper we will present diagrams showing key geochemical properties and comparative cross sections through the OFTB derived from the recently acquired seismic lines.

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.