18	The Naparima Hill - Cruse/Forest/Gros Morne Petroleum System of Trinidad: A Quantitative Evaluation of Petroleum Generated. * By Dr. KIRTON RODRIGUES

Trinidad is situated at the eastern end of the prolific Eastern Venezuelan Basin (Fig. 1), where more than 9.5 billion barrels of oil have been produced over the last 80 years, with recoverable reserves estimated at 40 billion barrels (James, 1990). The Orinoco Heavy Oil Belt, the world's largest known hydrocarbon accumulation, is located in the Eastern Venezuelan Basin.
Following Magoon's (1988) definition and nomenclature of petroleum systems, the Naparima Hill - Cruse/Forest/Gros Morne (NH - C/F/GM) petroleum system is proposed for the Trinidad area. This system covers an area of 14000 km² and has produced 2.5 billion barrels of oil from more than 25 oil fields, including 7 giants (>100 million barrels).
More than 6 km of sediments, primarily of continental origin and ranging in age from the Lower Cretaceous Cuche shales to the Pleistocene Cedros sands and clays, have been deposited within the NH - C/F/GM system (Fig. 2 and 3). These sediments include the source beds of the Upper Cretaceous Naparima Hill mudstones and the principal reservoir rocks of the Miocene to Pliocene Cruse, Forest and Gros Morne deltaic sandstones.
Traps associated with petroleum occurrences in Trinidad are mainly anticlinal and fault (both thrust and wrench) -related structures, and these began to form since the Miocene. Seals are provided by sand-shale interfaces and faults. Thermal history reconstructions indicate that oil generation and expulsion from the Naparima Hill source beds occurred over the last 8 million years (since the late Miocene) in most parts of the NH - C/F/GM petroleum system. The entire process, from source rock deposition to accumulation within the NH - C/F/ GM system, extended from the Upper Cretaceous to the present, a duration of approximately 100 million years (Fig. 4).
Based on an average organic carbon content of 3.0 % by weight and hydrogen index of 500 milligrams HC/grams TOC, the Naparima Hill source beds (type II kerogen) have a generative capacity of 76 to 127 x 106 barrels/km3. Assuming 25% of the 500 meters average thickness of the Naparima Hill Formation is net source rock, and that 67% of the present day area (14000 km²) of the NH - C/F/GM petroleum system is effective source rock kitchen area, the Naparima Hill source beds are capable of yielding between 88 and 148 x l0⁹ barrels of oil. This represents more than 4 to 7 times the volume of reservoired oil (producible and non producible) calculated for the Trinidad area.


References
James, K.H, 1990, The Venezuelan hydrocarbon habitat, In: Classic Petroleum Provinces (Edited by J. Brooks), Geological Society Special Publication No.50, pages 9-36.
Kugler, H.G., 1959, Geological map and sections through Trinidad: The Petroleum Association of Trinidad, 1961.
Magoon, L.B., 1988, The Petroleum System: A Classification Scheme for Research1 Exploration and Resource Assessment. In: Petroleum Systems of the United States (Edited by L.B. Magoon), U.S. Geological Survey Bulletin 1870, pages 2-15.


*Extended Abstract of a paper accepted for presentation at the 1993 AAPG Annual Convention
in New Orleans, April 25-28, 1993.

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