--> ABSTRACT: Geometry of Dolomite Bodies within Deepwater Resedimented Oolite of the Middle Jurassic Vajont Limestone, Italy: Analogs for Hydrocarbon Reservoirs Created Through Fault-Related Burial Dolomitization, by W. G. Zempolich and L. A. Hardie; #91021 (2010)

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Geometry of Dolomite Bodies within Deepwater Resedimented Oolite of the Middle Jurassic Vajont Limestone, Italy: Analogs for Hydrocarbon Reservoirs Created Through Fault-Related Burial Dolomitization

ZEMPOLICH, W. G, and L. A. HARDIE

The Middle Jurassic Vajont Limestone of the Venetian Alps, Italy, is predominantly composed of resedimented ooids that were deposited in slope and basin settings. The Vajont has been partly replaced by massive dolomite which can be mapped at both regional and local scales. Dolomite bodies that are present within or are associated with the Vajont include: (1) A large-scale wedge, approximately 25 kilometers in length, 10 to 13 kilometers in width and > or = 400 to 500 meters thick (50 to 94 km{3}), located on the hanging wall of the Alpine-aged, thrust-based Mt Grappa-Visentin Anticline. This dolomite body is located within the axis of the anticline and cross cuts the stratigraphic section where subvertical to vertical faults penetrate the crest of the anticline; (2) Isolated, rootless lume- shaped bodies, 100-200 meters wide and greater than 300 meters high (greater than or equal to 2 x 1O{-2} km{3}), which penetrate a foot-wall syncline within an Alpine-aged thrust sheet. These dolomite "plumes" possess extensively brecciated cores and exhibit sharp to gradational transitions with surrounding Lower to Middle Jurassic basinal limestone; (3) Isolated dolomite "towers" that have partly replaced Cretaceous-age syn-sedimentary fault breccia. These bodies are found in overlying basinal strata but emanate from the underlying dolomitized Vajont; and (4) Small-scale wedge-shaped dolomite bodies on the scale of meters found along faults and fractures. The connection between these dolomite bodies and Alpine-aged faults and fractures clearly indicates that dolomitization was a late burial process.

It is proposed that during the Alpine deformation event convection-driven fluids derived from Late Tertiary seawater were circulated through subaqueous Alpine-aged faults and fractures and paleo-synsedimentary breccias, thus creating the multitude of dolomite bodies now found in the Vajont and other Mesozoic basinal sediments. Paleogeographic, tectonic and hydrologic systems, similar to that proposed for dolomitization of the Vajont, appear to be active in Modem subaqueous thrust zones.

Results of this study indicate that (1) massive replacement dolomitization in thermo-tectonic (i.e., burial) settings may be much more important than previously thought, and (2) significant reservoirs may be hosted in otherwise tight basinal limestones as the result of late-stage burial dolomitization.

AAPG Search and Discovery Article #91021©1997 AAPG Annual Convention, Dallas, Texas.