Geometry of Dolomite Bodies within Deep-Water Resedimented Oolite of the Middle Jurassic Vajont Limestone, Venetian Alps, Italy

Abstract

ABSTRACT 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 Limestone has been partly replaced by massive dolomite that can be mapped at both regional and local scales. Dolomite bodies that are present within or are associated with the Vajont Limestone include: (1) a large-scale wedge, 25 km long, 10-15 km wide, and 400-500 m thick (50-94 km3), 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 crosscuts the stratigraphic section where subvertical to vertical faults penetrate the crest of the anticline; (2) Isolated, rootless plume-shaped bodies, 100-200 m wide and >300 m high (2 10-2 km3), which penetrate a footwall 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 synsedimentary fault breccia. These bodies are found in overlying basinal strata (i.e., the Fonzaso Formation, the Ammonitico Rosso, and the Biancone Formation), but emanate from the underlying dolomitized Vajont; and (4) Small-scale wedge-shaped dolomite bodies on the scale of meters found along small faults and fractures. The connection between these dolomite bodies and Alpine-aged faults and fractures clearly indicates that dolomitization was a late burial process. 1 Present address: Mobil Oil Company, Dallas, Texas, U.S.A. End_Page 127------------------------ 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 paleosynsedimentary 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 the one proposed for dolomitization of the Vajont, appear to be active in modern subaqueous thrust zones of the Caribbean and Northwest Pacific Coast. Potential reservoir attributes of Vajont dolomite bodies include their large size and medium to coarsely crystalline replacement fabric that is characterized by significant amounts of partial moldic, intercrystalline, and vug pore space. Visual estimates of porosity within dolomitized grainstone and packstone range up to 10% to 15%, with inferred permeabilities of 1-100 md. Permeability of Vajont dolomite replacement fabrics is enhanced through recrystallization and the formation of touching-vug networks (inferred permeabilities 100 md). Results of this study indicate that (1) massive replacement dolomitization in thermotectonic (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. Consequently, the geometries of the Vajont dolomite bodies may provide analogs for reservoir characterization and new exploration plays in the subsurface. Exploration methods for analogous dolomite reservoirs in the subsurface may include the mapping of dolomitization fronts using core and log data and seismic reflection identification of crosscutting dolomite bodies. The focus of such efforts should be placed on anticlinal and synclinal structures within buried fold and thrust belts, and along zones of deep-seated tectonic fractures and faults within intracratonic basins.