The Mode Of Oil Occurrence in the Miocene of Southern Trinidad, B.W.I

Abstract

Trinidad is closely related geologically to Eastern Venezuela and during Miocene times southern Trinidad formed a part of the East Venezuela geosynclinal basin. In Trinidad, as in Eastern Venezuela, the basin is asymmetric with a steep north flank and a gentle south flank rising towards the Guiana shield. The oil fields are situated mainly on secondary flexures around the rims of this basin, and are of the multiple sand reservoir type.

During Miocene times the sedimentation reflects the gradual infilling of the Tertiary basin, beginning with dominantly argillaceous deposits and passing through a series of clay-silt-sand cycles to dominant sand deposition in the final infilling phase. Three major sedimentary cycles are recognized corresponding to the main stratigraphic units, and known as the Cruse, Forest, and Morne l'Enfer cycles, from oldest to youngest respectively. In each cycle the earlier sand deposition is characteristically lenticular; whereas the top cycle sands are more widespread, and in the Forest and Cruse cycles form the more important oil-producing sands. Dominant sheet sand deposition characterises the Morne l'Enfer cycle. The sand distribution during these cycles is investigated by means of sano/shale ratio maps which show the dominance of a southeastern source of sedimentation with a secondary source from the southwest which became of equal importance to the southeastern source in the Upper Miocene, Morne l'Enfer cycle.

The oil occurrence is clearly related to the sand distribution, modified to a greater or lesser degree by the local folding and faulting. Noteworthy is the influence of the transverse Los Bajos tear fault which cuts across the basin from west-northwest to east-southeast, along which a persistent belt of oil accumulation occurs. Much of the oil is thought to be indigenous to the formation in which it occurs, but some redistribution by fault migration has occurred, and in addition some truly migrated oils are found.