AN OVERVIEW OF THE CRETACEOUS GEOLOGIC HISTORY OF TRINIDAD AND IMPLCATIONS FOR HYDROCARBON EXPLORATION

KRISHNA PERSAD
K. Persad & Associates Ltd. 690 Aileen Ave.,
Palmiste, Trinidad
WINSTON ALl
Exploration Department Trinidad & Tobago Petroleum Co. Ltd. Santa Flora, Trinidad

The Cretaceous Period is represented by a varied suite of rock types including igneous, metamorphic and sedimentary rocks which offer significant clues to elucidating the regional geology and early tectonic history of the southeastern Caribbean.
During the late Jurassic/Early Cretaceous the area was undergoing rifting and active spreading, with early basement related, down to the basin faults, having already formed at possibly the El Pilar, the Central Range and the Southern Range. Because of the attenuation of the crust and the active spreading the Northern Range, Caroni Basin, Central Range Etc. were most likely some 50 kms to the north and west and are underlain by transitional crust.
The subsidence due to the rifting and spreading first allowed a sequence of evaporites, fringed to the north by Carbonate reefs, banks and shoals. It is postulated that these sediments are underlain by volcanics and redbeds of Jurassic age, and possibly in areas, by lacustrine shales and aeolain sandstones, all deposited during the early rifting stage. As the drift phase started, large thicknesses of submarine fan deposits formed up until the middle and upper Cretaceous, particularly during periods of lower stands of sea level, interpolated with periods of deposition of algal rich shales.
During the Upper Cretaceous however, as spreading ceased and as sea level rose to the highest level in geologic history, greater thicknesses of algal rich marine shales were deposited in deep anoxic basins. As sea level rose extensive reefs developed in some areas, particularly in Venezuela.
In general, the area can now be divided into two terrains:
A northerly terrain consisting of the clastic and carbonate sediments (with minor volcanics), which have been metamorhposed by deep burial beneath Lower and Middle Tertiary sediments, and later unroofed by an extensive period of folding, uplift and erosion as the sequential oblique collision of the Caribbean Plate with northern South America reached Trinidad.
A southerly terrain, consisting of unmetamorhposed Mesozoic and younger sediments, which form a sedimentary wedge which becomes progressively shallower-water and thinner towards the south.
Source rocks are known to occur within the Cretaceous, the upper Cretaceous Gautier and Naparima Hill Formations, being thick extensive and oil prone, and vary from marginally mature to over-mature. These are now known to be the source of most of the oils in Trinidad and the Eastern Venezuelan Basin. Other older lacustrine source rocks of Jurassic age may also occur.
Reservoir rocks are known to occur in the limestones and submarine fan deposits of Jurassic to Cretaceous age. Dolomites, fringing the evaporites are also known to occur. Aeolian sandstones reservoirs, good potential reservoirs, may occur in some areas. In places the brittle Naparima Hill Formation may be fractured and hence form potential reservoirs.
Early stratigraphic traps will have formed during rifling. Structural traps, by way of fault propagation and fault bend folds, formed in response to Middle Miocene and later compressional events, are also present.
Adequate seals exist, interbedded with the reservoirs, e.g. Cuche, Gautier, Naparima Hill and Guayaguayare.

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