Abstract: Platform aggradation and basin starvation - the key to carbonate buildup petroleum systems (Canadian Upper Devonian and Italian Middle Triassic)

VAN BUCHEM, FRANS S.P., MARCO STEFANI, ALAIN Y. HUC

The analysis of the geometrical, stratigraphic and geochemical (TOC; carbonate distribution) aspects of two carbonate buildup systems and their adjacent basinal sediments demonstrates a strong similarity in the distribution of the reservoir facies and source rocks. The formation of both reservoirs and source rocks is genetically related and occurs in two 3rd order cycles of the general, 2nd order sea level rise.

The Frasnian carbonate system in the Canadian Rocky Mountains, and the Anisian/Ladinian carbonate system in the Italian Dolomites were both deposited during a 2nd order, transgressive-regressive cycle (approx. 10 m.y.), which can be subdivided in six 3rd order sequences. In both cases best reservoirs were formed during fast sea-level rise in sequences 3 and 4, when platforms and buildups aggraded and locally retrograded, forming thick, stacked packages of carbonates. Reservoir qualities were enhanced in the Canadian example by early diagenesis and, in the case of the Italian example, by late diagenesis. In the adjacent, starved basins large amounts of marine organic matter accumulated (Canada up to 12%; Italy up to 42%) due to the low sedimentation rates and the enhanced preservation conditions below the stratified water column. In the Canadian example, a distinct dilution relationship between TOC and carbonate could be demonstrated. During the 2nd order regression, this first petroleum system is sealed by the basin infill of clays in Canada, and by volcanic deposits and clays in the Italian example. A second, smaller petroleum play developed during the slowing down of the sea-level rise, when the carbonate system prograded out again. In Canada these carbonates are underlain by a thin marine source-rock interval, and in the Italian example a level enriched in terrestrial organic matter is found at this stratigraphic position.

Despite the difference in total thickness of the successions (Canada up to 450m, Italy up to 1300m), due to the different subsidence history and the occurrence of volcanism in Italy, the main geometrical relationships and the distribution pattern of reservoir and source rocks are strikingly similar. This model may thus have a strong predictive potential for similar carbonate buildup petroleum systems.

AAPG Search and Discovery Article #90942©1997 AAPG International Conference and Exhibition, Vienna, Austria