Fracture stratigraphy and oil first migration in Triassic shales, Favignana Island, western Sicily, Italy

Abstract

This study aims at evaluating the control exerted by fracture stratigraphy and diagenetic processes on oil first migration through an outcropping, mixed carbonate-siliciclastic succession. The present work included results of sedimentological, paleontological, mineralogical, petrographic, structural, and microstructural analyses carried out on organic-rich shales exposed at the Favignana Island of Sicily, Italy. The analyses focus on Upper Triassic yellowish siltstones and greyish laminated dolomitic limestones, which form a 10’s of m-thick succession exposed along the westernmost portion of the Sicilian fold-and-thrust belt. The studied succession deposited in a coastal lagoon associated to a wide carbonate platform, in which anoxic bottom conditions allowed the preservation of the organic matter forming catagenetic patches and veins/fractures infill. In fact, two orthogonal fracture sets perpendicular to bedding are pervaded by organic matter. They are hence interpreted as structural elements that affected the paleofluid circulation and oil migration within the Triassic source rock. These two orthogonal sets form the background fracture network predating the formation of three other sets infilled mainly with calcite cements. The latest fracture set also includes barite cements implying a hydrothermal origin (<200 ◦C), as supported by the R1 and R3 mixed-layer Illite-Smectite (I–S) stacking. Data are consistent with a burial depth of the sedimentary succession of about 3 ± 0.5 km. In light of these considerations, the study area might represent an outcropping stratigraphic and structural analogue of the source rocks buried in the offshore of western Sicily and northern Tunisia.