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Mixed Carbonate-Siliciclastic Systems: Dynamics and Controls, Eocene-Oligocene Browse Basin


Current deep-water exploration is targeting potential hydrocarbon reservoirs that were deposited downdip of mixed carbonate-siliciclastic shelves (e.g. Eastern offshore Canada and offshore West Africa). Deep-water reservoir facies for pure carbonate and siliciclastic systems have been extensively studied but the stratigraphic complexity and variability associated with mixed depositional systems are still poorly known. We use extensive 2D (peak frequency 30 Hz) and 3D (peak frequency 25-50 Hz) seismic-reflection data, with well control, over 20,000 square kilometers of the Browse basin, Northwest Shelf of Australia, to interpret the spatial variability of Eocene-Oligocene carbonate and siliciclastic depositional systems from shallow to deep water. Detailed 3D seismic mapping reveals 1) the topset region was dominated by a combination of predominantly heterozoan carbonate shelf sediments, carbonate platforms, and wave-dominated siliciclastic shorelines, 2) two deep-water depozones with different slope relief, and 3) a variety of deep-water depositional geometries, including channel-levee systems and lobes, suggesting active turbidity currents downdip of the mixed carbonate-siliciclastic shallow marine systems. Deep-sea drilling in the outboard Argo abyssal plain sampled Oligocene mixed and pure silliclastic turbidites. A first-order control on along-strike variability in the Browse Basin area is the relative position of the advancing shelf margin relative to underlying northeast-southwest oriented structures. The two distinct deep-water depozones are related to inflections in the slope profile overlying the Paleozoic and Jurassic structures. Factors influencing siliciclastic sediment dispersal and localized deep-water deposition along a complex slope profile includes basement tectonic and antecedent topography, shelf physiography and location of carbonate accumulation. The combination of those factors produced a complex seismic-stratigraphic pattern that might be common in analogous mixed systems with a strong overprint of earlier tectonic basin evolution.