Geochemistry and Sedimentation of Organic Matter in the Triassic-Liassic Carbonate Laminated Source Rocks of the Ragusa Basin (Italy)

E. Brosse, J. P. Loreau, A. Frixa

The Noto and Streppenosa formations of the Ragusa basin (southeastern Sicily) are considered the main source rocks for oil in this area. They display various styles of sedimentation in a generally carbonate context.

The organic matter is basically of marine planktonic origin but with some variations, especially in terms of O/C ration and kinetic behavior. The oxygen-rich kerogens present a broader distribution of activation energies than the oxygen-poor ones, which means they will release their cracking products less suddenly upon thermal evolution.

Three main styles of sedimentation occurred within these formations: (1) laminates in a dominantly carbonate rock with thin recurrent interlayers of black shales; (2) alternating layers of marls and limestones, both containing interlayers of black shales and with occasional laminations in the limestones; and (3) silty shales, more or less rich in carbonates (30-70%). Laminites are themselves of several types: algal-sedimentary laminites deposited in intertidal environment, and yearly or seasonal varves and resedimented laminated limestones both deposited in shallow environments.

The highest petroleum potentials are neither strictly associated with the algal-sedimentary laminites nor with the basinal silty facies but with the black shales interbedded in the different facies or abruptly overlying limestones. In these black shales, oxygen-poor kerogens are dominant.

Limestones of the alternated layers are generally organic lean (TOC < 1%), and oxygen-rich kerogens are dominant. The transition from one type of kerogen to the other occurs in the marly layers of the sequence.

A tentative integration of both sedimentological and geochemical results is proposed, at the scale of the core, to interpret the respective influence of the depositional pattern and the diagenetic conditions on the content and nature of the kerogen in the source rocks.

AAPG Search and Discovery Article #91032©1988 Mediterranean Basins Conference and Exhibition, Nice, France, 25-28 September 1988.