Geometry and continuity of fine-grained reservoir sandstones deformed within an accretionary prism - Basal Unit, West Woodbourne



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Texas A&M University


The Basal Unit of West Woodbourne Field in Barbados is a 250 m thick succession of finely-interbedded sandstones and mudstones deposited by Paleogene, fine-grained, deep-water systems off the northern South American margin and deformed as sediments were translated to the subduction zone of the Caribbean and Atlantic plates. Closely spaced gamma ray, neutron, density, spontaneous potential, formation microimager and dip meter logs, limited core, and published reports of local outcrops, were used to define three scales of vertical stratigraphic variation within this 1.5 km2 field: (1) decimeters to meters thick log facies; (2) meters to tens of meters thick log successions; and (3) tens to hundred meter thick intervals that are continuous laterally across the field. These variations record changes in sediment supply and depositional energy during progradation and abandonment events varying in scale from local shifts in distributary channels to regional changes in sediment transport along the basin. Well log correlations suggest the Basal Unit comprises a turbidite fan system (250 m thick) trending north to northeast, composed of six, vertically-stacked, distributary channel complexes. Three architectural elements are identified within each distributary channel complex: (1) Major amalgamated channels (30-40 m thick, 150-200 m wide and at least 900 m long) pass down depositional dip into proximal second-order channels that bifurcate basinward (15-20 m thick, symmetric successions); (2) Lobe deposits (20-50 m thick, 400 m wide, and at least 400 m long) are composed of upward-coarsening successions that contain distal second-order channels (1-10 m thick); and (3) Laterally extensive overbank deposits (5-10 m thick), which vertically separate distributary channel-lobe complexes. Reservoir heterogeneities within the Basal Unit are defined by the lateral extent and facies variations across a hierarchy of strata within channel-lobe complexes. Although laterally extensive muddy overbank deposits generally inhibit vertical communication between stacked channel-lobe complexes, in places where high-energy first-order channel sandstones incise underlying muddy overbank deposits, sandstones in subsequent intervals are partially connected. The Basal Unit is bounded on the southwest by a northwest-southeast trending fault that rises 30 degrees towards the northwest to define a structural trap on the northeast side of the field.