--> Sedimentological Characterization of the Highest Permeability Deepwater Wilcox Sandstones — Interaction Between Gravity Flows and Ocean Bottom Currents

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Sedimentological Characterization of the Highest Permeability Deepwater Wilcox Sandstones — Interaction Between Gravity Flows and Ocean Bottom Currents

Abstract

Over the past decade numerous hydrocarbon discoveries have been made in the subsalt Palaeogene Wilcox play (Gulf of Mexico). Despite large in-place volumes their extraction poses significant challenges; seismic imaging is limited, well data sparse and the reservoirs are heterogeneous. The distal deep-water Wilcox represents channelized lobe complexes separated by laterally extensive intraformational mudstones. Sediments were transported ~600 km from the contemporaneous shoreline before being deposited on a relatively flat basin floor. The unconfined setting allowed the flows to gradually spread out without encountering any major topography. Classical turbidites dominate the succession but a large proportion of the sandstone beds have a much higher content of dispersed clay than expected from beds deposited from turbidity currents, and have been attributed to sediment gravity flows transitional between laminar and turbulent. The conceptual geological models are further complicated by the occurrence of ripple-laminated, fine-grained, and well sorted sandstones interbedded within hemipelagite-dominated stratigraphic intervals interpreted to represent periods of low sediment flux. These ripple-laminated beds also occur at the very top of lobes and can account for up to six percentage of the total reservoir volume. We interpret these “out of place” sediments to have been reworked from sediment gravity flow deposits, transported and winnowed by bottom currents and deposited away from their gravity flow feeding system, forming ripple trains and sheets which could be detached from their genetically linked lobe/channel source. Ocean bottom currents were probably active throughout deposition of the entire Wilcox Formation, but evidence of their activity is generally restricted to times of gravity flow quiescence. Analysis of ~2000 core plugs, ~200 thin sections, and >11 500 event beds from the deep-water Wilcox Formation show that deposits interpreted as bottom-current reworked occupy a separate permeability class, primarily due to their better sorting, higher porosity and lower clay content compared to depositional products from turbidity currents and transitional flows. Relatively closely spaced wells and extensive core coverage give some on constrains on the 3-dimenstional distribution, and even though the bottom-current reworked beds are volumetrically less significant than gravity flow sandstones, their contribution to hydrocarbon recovery can be substantial.