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Deltaic to Estuarine Regime Change on Proximal Paleo-Orinoco Shelf: Morne L’Enfer Formation, South Trinidad

Ariana Osman and Ron Steel
The University of Texas at Austin, Austin, TX

The Morne L'Enfer Formation, onshore southern Trinidad, records the regressive-to-transgressive shelf transit of the Late Pliocene Orinoco delta as it built across south Trinidad. Much of our current understanding of paleo-Orinoco sand reservoirs come from the storm-wave dominated outer-shelf and shelf-edge areas in the Columbus Basin. The Morne L'Enfer Formation, however, encapsulates an early regressive deltaic phase (possibly a tide-dominated delta), the sequence boundary and the turnaround to a tide-dominated estuarine phase in the proximal paleo-Orinoco. Integration of outcrop and well data provides a window into proximal reservoirs, illustrating the contrasting style of tide-dominated reservoir facies and architectures.

The basal regressive deltaic deposits show repeated coarsening upward parasequences, up to 25m thick. These sand packages have a very heterolithic character suggesting a fluvial or tidal dominated, rather than wave-dominated regime.

The overlying transgressive deposits are characterized by thick channelized units of blocky coarse-grained sands, ranging from 25 – 50m thick. The tidal character of the sands are evident in outcrop by repeated thick sandprone units with stacks of 2-D dunes, sigmoidally cross stratified sands and rhythmically arranged sand and mud strata. The blocky but partitioned nature of the sand units suggests that this estuarine system provides a more prolific hydrocarbon reservoir than the related regressive deltaic deposits.

These proximal delta-estuary couplets lead to an understanding of potential proximal-distal regime change on the paleo-Orinoco shelf, and of contrasting reservoir geometry between regressive and transgressive phases on the inner shelf.