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Mechanical stratigraphy control on oil first migration: the Triassic shale
of Favignana Island (Sicily)

Parrino, N.

Dipartimento di Scienze della Terra e del Mare, Via Archirafi 22, Università di Palermo, Italy

Corresponding author email: nicolo.parrino@community.unipa.it

Keywords: mechanical stratigraphy, first migration, fracture.

Rock fracturing is strictly controlled by variations of mechanical stratigraphy of different layers. This pro-
cess increases secondary permeability and, as consequence, allows high production rates of oil and gas from
rocks characterized by low primary porosity/permeability (e.g. shale). In light of this, the analysis of the inter-
play between rock fracturing and their properties, such as lithology, texture, facies and diagenetic evolution,
may improve the understanding of processes responsible for migration of fluids. In light of this, the integrated
analysis of the fracture network and host rock properties (e.g. lithology, texture, facies and diagenesis), may
improve the understanding of processes responsible for migration of fluids.
The aim of this study is to improve knowledge on the role of the mechanical stratigraphy on the first migra-
tion of oil and gas. To achieve this goal, integrated sedimentological, structural and microstructural analysis
has been performed in the Upper Triassic shale of Favignana Island (Aegadian Archipelago). These rocks
belong to a succession that crops out in the locality of Punta Faraglione (NW sector of Favignana Island). It
consists of meters to centimeters thick beds of yellowish clayey marls and marls that alternate to thick beds of
grayish to blackish stromatolitic and loferitic limestones. A pervasive dolomitization affects these rocks that
were detected in a limb of a Miocene syncline, with an axis roughly oriented N48E. Dolomitic and stromato-
litic well bedded sediments are typical of epeiric sea lagoons characterized by euxinic environments capable
of preserving its organic matter. This outcrop allows direct mesoscopic observations on different patterns of
fractures and veins that occur in lithologically different layers. An analogue to this succession is exposed in the
Marettimo Island (Punta Bassano section), close to the Favignana Island.
Three high-angles and one low-angle (to bedding) opening mode fracture sets were detected in the afore-
mentioned outcrop, in which both joint and calcite filled veins were included; these sets strike roughly N10E,
N100E, N50E and N130E. Only a small number of joints (oriented to N100E and N50E) cross the marls
layer, since the greatest part of them are strata bounded into the dolomitic/stromatolitic layers. Differently,
the greatest part of the veins crosscut multiple layers; furthermore, veins pertinent to the set N10E frequently
show macroscopic oil evidence at the vein edges. Analysis of thin section allows recognizing several bed par-
allel catagenetic micro-fractures. Their genesis was interpreted to be related at the increase of the oil volume,
which occurred during organic matter maturation. The syntaxial nature of the calcite crystals inside the veins
indicates several opening phases, in which the opening starts at the center of the vein and propagates towards
the edges. These recurrent phases are also testified by oil in the central part of a vein related to the set N100E.
These suggest that the two sets of fractures were formed during the migration of oil, and were probably reac-
tivated during the folding phase, in which the last two sets were formed.
The new dataset provides the first evidence of fracture sets related at the increase of oil volume, which oc-
curred during organic matter maturation, in the Upper Triassic shale of Favignana Island. Thus, the mechanical
stratigraphy of the study sedimentary succession played a key role in the oil first migration in the Aegadian
Islands.

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