The Scotland Formation, Barbados: From Outcrop-Based Study of the Hydrocarbon Prospectivity of the Southern Barbados Accretionary Prism

Nysha A. Chaderton¹ and Lesli J. Wood^2
1The Jackson School of Geosciences, The University of Texas at Austin, Austin, TX
²The Bureau of Economic Geology, The Jackson School of Geosciences, The University of Texas at Austin, Austin, TX

The Barbados Accretionary Prism (BAP) is a classic example of an end-member accretionary margin and the only one from which hydrocarbons have been produced. Although unique, the hydrocarbon system elements of the BAP have nevertheless not been well studied. The clastic basin fill; Scotland Formation (Eocene) is the main producing reservoir onshore, but its study is complicated because of limited exposure and dramatic structuring.

Seven outcropping locations of the Scotland were examined to document stacking patterns, key surfaces, depositional element geometries, facies occurrences their vertical and lateral extent, and the unit’s gamma response. Six facies were identified in outcrop: silty muds; laminated, centimeter-scale sandstones interbedded with silts and muds; cross-stratified sandstones; massive, medium to coarse-grained sandstones; very coarse grained sands with gravel or pebbles; and rare conglomerates. These facies combine to form architectural elements—channels, levees, and depositional lobes. The architecture varies in geometries over decimeter distances. The erosiveness of many key surfaces suggests deposition within a confined deepwater setting, in environments ranging from confined canyons to large, leveed channel complexes. Tectonic reconstructions based on 2D seismic interpretation of offshore stratigraphy and structure of the Tobago Basin suggest that this basin was much broader during Eocene times. Observations suggest that the Scotland was deposited with the proto-Tobago Basin as forearc basin sediments and not at the toe of the existing accretionary prism.

Hydrocarbon production in the BAP region is therefore not from true accretionary prism sediments but from more conventional, deepwater, forearc basin sediments that became incorporated into the larger BAP through continued compression since Eocene times.

AAPG International Conference and Exhibition, Cape Town, South Africa 2008 © AAPG Search and Discovery