Requejo, A.G.¹, Zvi Sofer², David L. Stansbury³, Craig F. Schiefelbein¹, James L. Brooks⁴
(1) Geochemical Solutions International, The Woodlands, TX 
(2) Consultant, Houston, TX 
(3) BaselineDGSI, The Woodlands, TX 
(4) TDI-Brooks International, College Station, TX

ABSTRACT: Origin and Distribution of Hydrocarbon Gases in the Columbus Basin of Trinidad: An Emerging Gas Province

Over the last five years the Columbus Basin of Trinidad has emerged as one of the largest gas provinces in the western hemisphere, as development of several fields discovered during the 1990s has increased Trinidad’s annual gas production nearly threefold. Gas accumulations in the basin occur exclusively in Plio-Pleistocene sediments, which were rapidly deposited in association with the paleo-Orinoco delta system. Total gas production to date is nearly 3 TCF and reserve estimates indicate an additional 20 to 30 TCF in place. 
A regional geochemical study of gases from all major producing intervals indicates variable mixtures of both thermogenic and biogenic origin. Up to two-thirds of the gas reserves in the basin may be biogenic, including large fields in the north (El Diablo, East Manzanilla, Red Snapper) and southeast (Corallita, Dolphin). Elsewhere, the thermogenic component varies from 50% to 75% and is higher in the deeper, older reservoirs to the west, decreasing basin-ward toward the present-day shelf edge. 
2-D basin modeling suggests that these distributions are a result of generation and migration from gas-prone, Upper Cretaceous source rocks within the last 1 - 2 million years, which has not allowed sufficient time for thermogenic hydrocarbons to charge shallower and younger structures to the east. Hydrocarbons have migrated vertically through unconsolidated sediments as well as up normal faults. Lateral gas migration occurs from west to east and continues presently, however, the thermogenic front has not reached the shelf edge. Gas compositions suggest generation at maturities ranging from 0.9% to 1.1% vitrinite reflectance equivalent. 
The large volumes of biogenic gas in this basin are not easily explained by conventional low-temperature processes. It is suggested that these accumulations could arise through the rapid burial and trapping of massive gas hydrate layers, which are known to occur in modern slope and basinal depositional environments similar to those that characterize the Plio-Pleistocene section.

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.