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Bureau Seminar, Nov 19, 2010

Sand Distribution along Shelf-Edge Deltaic Systems:
A Case Study from Eastern Offshore Trinidad.

Link to streaming video TBA: available 11.19.2010 at 8:55am

Anmar Davila-Chacon, Lorena Moscardelli, and Lesli Wood
Bureau of Economic Geology

The study area is situated along the obliquely converging boundary of the Caribbean and South American plates in eastern offshore Trinidad. Major structural elements in the shelf break and deep-water slope regions include growth and counterregional faults to the south and large transpressional fault zones to the north.

Well logs and biostratigraphic data were analyzed for 24 wells in the study area to refine previous depositional environment interpretations. Transgressive and regressive cycles were interpreted on the basis of well log patterns and depositional facies shifts. Six T/R cycles were interpreted within the Pliocene to Recent stratigraphic succession. Shelf-edge trajectories were also mapped for each of the six T/R cycles on the basis of earlier stratigraphic correlations. Net-to-gross (NTG) ratios were calculated for each component of the T/R cycles and plotted against total thicknesses and net-sand values. In addition, NTG trends were mapped for each interval and analyzed on the basis of their proximity to the corresponding shelf edge.

Mapping of shelf-edge trajectories (SETs) revealed that (1) SETs migrate northeasterly across the Columbus Basin through time and (2) shelf-edge orientations parallel the strike of growth faults in the south but deflect to the northeast near the Darien Ridge, indicating strong, underlying structural control. NTG plots and maps also revealed that (1) for transgressive units, NTG values never exceed 60%, and they are inversely proportional to total thickness; (2) for regressive cycles, NTG values are highly variable, ranging from 35% to 90%; (3) NTG values increase as the shelf break is approached; and (4) distribution of NTG ratios is also controlled by accommodation space created by local structures.



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