NAPARIMA HILL FORMATION

Originally defined at its type locality of San Fernando Hill in the western Central Range, this formation has been found to extend in the subsurface to the south, west and east. Compositionally it is an organic rich siliceous mudstone, and thus represents a significant departure from the pre-existing shale dominated formations.Of all the Cretaceous and Paleogene formations this has probably received the most study because if its economic importance as the main source rock for the majority of the hydrocarbons produced in the region (Talukdar et al 1991).
Fine grained siliceous mudstones of this form and extent are likely to be pelagic deposits. As mentioned earlier this unit is generally massive and though this is most probably a result of continuous deposition the abundant bioturbation will have had some homogenising effect. The burrows usually appear as dark wisps, but lines between the walls of larger burrows reveal their true nature. The most common trace fossil is Zoophycos, a form indicative of relatively deep quiet water depositional environments.
Higher silica percentages in the Naparima Hill of the eastern Central Range (Chert Hill Member) might be indicative of a greater depth of deposition (>200m).
The Naparima Hill is absent to the north of the Central Range.
Wells penetrating the Naparima Hill include: Emerald 1, Morne Diablo 34, Marabella 1, Antilles Trinity 35, Moruga East 15, Marac 1, Lizard 1RD, Guayaguayare 646, Guayaguayare 603, Rocky Palace 1St., Ste. Croix 1, Iguana River 1, ABM 102, ABM 54, ABM 1, FW 214

All Photographs by C. Archie

SUMMARY OF UPPER CRETACEOUS NAPARIMA HILL FORMATION

· Rock Types / Lithologies Sandstones Siltstones Clay/Shales Biogenic Lime Mudstones Biogenic Siliceous Mudstones Dolomite (MD 34 @ 13,786 feet)

· Sedimentary Structures Graded Bedding Laminations Burrows Slump Folding

· Deposition
Quiet-water sedimentation in a slope to basmal setting
Interrupted by influx of coarse clastic sand deposition by turbidity currents

· Porosity
Two (2) types of porosity are present in the Naparima Hill Formation:
Intergranular porosity is preserved in turbidite sandstones that have not undergone calcite cementation
Fracture porosity is preserved in non-fissile biogemc mudstone rocks that have not undergone calcite or silica cementation

REGIONAL GEOLOGIC CONTROLS ON RESERVOIR QUALITY
· Middle to Upper Cretaceous biogenic sediments are common across the Maracaibo/Eastern Venezuelan Basins in Trinidad. This belt is represented by carbonates, cherts and siliceous shales of the La Luna Formation, the Guyatura Group and the Naparima Hill Formation. These are brittle rocks, they do produce oil from natural fractures m the Maracaibo and eastern Venezuelan Basins and it is likely that they may also produce oil in commercial quantities in Trinidad.

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