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The Late Jurassic Great Barrier Reef of the North American Atlantic Continental Margin: Initial Findings From Basin Analysis-Seismic Stratigraphy

Abstract

This investigation provides initial information from an on-going regional hydrocarbon exploration study of the U.S. Atlantic continental shelf incorporating more than 600 migrated 1978-1984 vintage 2-D seismic reflection lines, petrophysical logs and geochemical data from 15 exploration wells. Compared to the Gulf of Mexico, this region is virtually unexplored but may have significant undiscovered upside owing to numerous large oil and gas discoveries in the adjacent and conjugate Atlantic margins of Canada, Ireland, and the African continent. To enhance the geologic information between the sparsely distributed well control and improve the interpretive potential of the legacy 2D seismic, a novel seismic inversion of 2-D stacking velocities was applied in order to generate 2-D panoramas of velocity, density, porosity, and assist with attribute analyses of the interpreted seismic lithofacies. The workflow entailed the integration of “Vail” and “Galloway” approaches to sequence stratigraphy and culminated with the generation of isochrons and isopachs of the borehole constrained seismic lithofacies which provided critical information for virtual well generation for the subsequent basin modeling. Following well calibration, the inversion results and mapping of seismic lithofacies revealed an extensive 2500 km Late Jurassic carbonate buildup trend which evolved from a carbonate ramp to fringing reef and culminated in a barrier reef steepening at the continental shelf edge with the generation of substantial talus. The Jurassic reef responded dynamically to changes in global sea level with abrupt changes in horizontal progradation and punctuation in vertical growth by reciprocal sedimentation recorded as shelf to basin clastic system tracts culminating in sediment gravity flows encased in rich organic shales. By the Cretaceous, the reef gave way to clastics of multiple fluvial-deltaic systems although the southern portion did persist in its growth into the Early Cretaceous, Basin modeling reveals the thermal maturities to be problematic when calculated from the available borehole temperature data as they are not consistent with the Ro and Tmax data. However, incorporating tectonic pulses and non-steady state regional heat flow data with changes in paleo-latitudinal ocean temperatures may provide insight with respect to the spatial and temporal evolution of petroleum system criticals.