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Abstract:
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The study area is situated along the obliquely converging boundary of the Caribbean and South American plates offshore eastern offshore Trinidad . Major structural elements in the shelf break and deep -water slope regions include normal and counter -normal faults to the south and large transpressional fault zones to the north .
Well logs and biostratigraphic information were analyzed for twenty -four wells in the study area to refine previous depositional environment interpretations .
For purposes of this net sand distribution analysis it was decided to consider the deltaic portion of the shelf transit cycle , against the marine portion of the shelf transit cycle and were named T and R cycles , respectively .
T and R cycles were interpreted based on well log patterns and depositional facies shifts . Six T /R cycles were interpreted within the Pliocene to recent stratigraphic succession and shelf edge trajectories were also mapped for each of these cycles based on 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 based on their proximity to the corresponding shelf edge .
Mapping of the shelf edge trajectories (SET ) revealed that (1 ) SET migrate northeasterly across the Columbus Basin through time and (2 ) shelf edge orientations are parallel to the strike of growth faults in the south but deflect to the northeast near the Darien Ridge indicating a strong underlying structural control . The NTG plots and maps also revealed that (1 ) For T cycles , NTG values never exceed 60 % and are inversely proportional to total thickness , (2 ) For R cycles , NTG values are highly variably ranging from 35 % to 90 % , (3 ) NTG values increase as the shelf break is approached and (4 ) The distribution of NTG ratios is also controlled by accommodation space created by local structures .
The Guiana current is believed to play an important role in the redistribution and reworking of sand in the Columbus Basin .
Aggradation and progradation distances were computed for each interval and the results suggest that the younger Sequences C2 (T -R cycle E ) and C3 (T -R cycle F ) show a stronger progradational trend than the older C4 , C5 and C6 . This strong progradational trend might indicate delivery of sand basinwards , while for the older intervals ; the aggradational trend suggests an increase in sediment storage .
In long -term scale (1 -2 m .y . ) the Orinoco Delta seems to behave as an aggradational delta that increases sediment storage due to growth fault and high subsidence rates . However , in the short -term scale , the Orinoco delta seems to behave as a rapid progradational delta , for the younger sequences C2 and C3 , where sediment bypass is more likely to occur ; and as a rapid aggradational (slow prograding ) margin for the older intervals C4 , C5 and C6 . |