Tectonic geomorphology of the eastern Trinidad shelf : implications for influence of structure on reservoir distribution and nature in older basin fill

Abstract

Few deltaic systems in tectonically active areas of the world exhibit more data for detailed study than the Orinoco Delta extending along the hydrocarbon-rich, narrow, eastern shelf of the island of Trinidad. The entire region has undergone tectonic extension, compression and transpression during the late Tertiary and into the Quaternary. Paralic and shelf reservoir sand distribution and geometry have been significantly influenced by both structuring and strong offshore current activity, as well as large forced and unforced sealevel regressions. These sedimentary deposits hold significant shallow gas resources across the region, but the complexity of their distribution and architecture is poorly understood. A large merged 3D seismic survey (~9,000 sq km) was integrated with well penetrations across the modern shelf to examine the influence of structuring on near-modern basin fill depositional morphology and architecture. Key reflecting horizons were mapped across the area and these document the structural opening of the basin by extension superimposed over ongoing uplift of compressional anticlines. Seismic attributes image reservoir elements such as channels and channel belts, tidally inundated interfluves and interdistributary areas which facilitate analysis of the scale and form of these features as well as enable evaluation of the influence of structure on deposition. Both larger channels (1-2 km wide) and smaller channels (less than 100 meters) show patterns of avulsion and lateral migration and appear to be ubiquitous in some areas of the shelf during lowstand times. The largest major sediment fairway (valley) is long-lived (~1.0 million years) and structurally confined by east-west trending anticlinal uplifts, funneling sediments down the axis of the basin. Alternations between surfaces with well defined depositional elements interpreted to be indicative of subaerial conditions, and surfaces devoid of such features which have been interpreted to be submarine in nature suggest large scale fluctuations in the depositional environment over time under the influence of changes in sea level. Extensional faults, which show a complex pattern of displacement, both spatially and temporally; appear to remain active up to near present day time. Coupled with the apparent long-lived nature of the tectonic uplifts in this region, observations suggest that the structure is controlling sedimentation, as opposed to sedimentation driving deformation. Structure is playing a significant role in accommodation creation and therefore the location of the axis of sediment transport and accumulation throughout the Pleistocene history of the basin. This conclusion would be in keeping with the regional structural history of the basin showing transpression between the east-southeastward moving Caribbean Plate and stationary South American plate, initiated in the early Tertiary and continuing to the present day. Quantitative data on systems tract architectural elements, including spatial orientation and distribution should significantly improve 3D modeling of these reservoirs and improve understanding of sand distribution and the processes of sediment transfer from proximal sources to shelf staging areas