INTRODUCTION

Four dry holes were drilled in Jamaican territory between 1981 and 1983. These results, combined with the seven dry holes recorded between 1955 and 1973, would suggest that conditions did not favour the generation, accumulation and preservation of petroleum in Jamaica. Most of these wells penetrated thick sections of Cretaceous and Early Tertiary sediments but no significant shows were encountered.

The occurrence of hydrocarbons in commercial quantities depends on a combination of several factors essential for petroleum generation and accumulation. Organic-rich beds must be able to generate oil and gas as a result of thermal maturation during a time span in the evolution of the basin when migration of hydrocarbons into traps is feasible without escape to the surface. Permeable conduit beds capable of gathering hydrocarbons generated must be favourably placed in space and time and the migrating hydrocarbons must be contained in porous reservoir beds confined within some trapping configuration by impermeable sealing beds. The tectonic history of the basin subsequent to petroleum accumulation in reservoirs must be favourable to the preservation of these petroleum-filled structures.

An evaluation of the petroleum potential of Jamaica in terms of generation, accumulation and preservation efficiencies is attempted here.

GENERAL GEOLOGY

Tertiary limestones cover more than 70% of the surface geology of Jamaica. Pre-Tertiary rocks are exposed in Cretaceous 'inliers' scattered over the island

dated sedimentary rocks are Albian limestones and a Late Cretaceous to Recent stratigraphic succession is exposed on the island. Metamorphic rocks of greenschist and amphibolite facies derived from greywackes and volcanogenic sediments are probably Cretaceous in age.

Most of the Early Cretaceous (Valanginian to Aptian) comprises rudist-bearing limestones, lava flows and volcaniclastic conglomerates, sandstones and shales. The Campanian to Maastrichtian of western Jamaica is represented by interbedded sandstones and shales more than 2000m thick, with minor conglomerates and thin limestones. Lava flows, volcaniclastics and limestones of equivalent age are present in central and eastern Jamaica.
Tertiary stratigraphy is marked by a gradual upward transition from clastic to carbonate sedimentation. Paleocene to Early Eocene sections over Jamaica are characterised by clastic sedimentation. A reduction in clastic input gave rise to the impure Yellow Limestone Group of Early - Middle Eocene which grades upwards into the pure cherty micrites and sparites of the White Limestone Group (Middle Eocene - Middle Miocene).

PETROLEUM GENERATION

Shales with low organic carbon contents (<0.5% TOC) are found in the Hanover and Sunderland Newman Hall Formations and the Veniella shales of western Jamaica (Fig. 1 and Table I) and in the Richmond and Cross Pass Formations of eastern Jamaica. Marginal organic carbon contents (0.5 - 1.0 % TOC) are indicated for some sections of the Windsor Formation and for parts of the Richmond Formation. Shales with good source rock quality (>1.0% TOC) are confined almost exclusively to the Chapelton Formation cored in the Content well. This Formation is very rich in parts and given the appropriate organic matter types even a thin sequence may be capable of generating significant quantities of hydrocarbons.


Only the Chapelton Formation in the MontpelierNewmarket belt has sufficient concentrations of oil-prone organic matter capable of generating commercial quantities of liquid hydrocarbons. Organic matter types in all other Formations analysed (Table I) are predominantly humic (gas-prone) and reworked (inert) and their low organic matter contents would be incapable of supporting significant gas generation.



Hanover, western Clarendon and Blue Mountain blocks (Fig. I) have been buried to depths greater than 2 km while in the Montpelier-Newmarket and Wagwater belts Tertiary overburden may have been 3-4 km thick. Cretaceous shales generally display vitrinite reflectances greater than 0.6% suggesting that these sequences were buried deep enough for a sufficiently long time for oil generating maturity to be attained. The lower sections of some Cretaceous Formations may have advanced to gas generating maturity.

Tertiary sections analysed (Table 1) have never been buried beyond 2-3 km and are generally immature with respect to petroleum generation. Indeed the only Formation with positive indicators of petroleum source rock quality (Chapelton Formation) has not experienced a thermal history compatible with peak oil generation in the area sampled. Higher maturity levels may be attained where the section is more deeply buried to the south (Fig. 1).

PETROLEUM ACCUMULATION

Most of the sedimentary sections in Jamaica consist of interbedded shales, mudstones and sandstones with some conglomeratic horizons (Plate 1). If hydrocarbons were generated in these shales the most probable reservoirs would be the interbedded sandstones and conglomerates. Detrital grains in many of these sandstones are angular to subangular and sorting is generally poor. Short transport distances and rapid deposition of these sandstones are implied by their poor sorting, angular clastic grains and abundance of unstable lithic fragments and feldspars. Petrographic analyses indicate that the sandstones are litharenites and feldspathic litharenites. Volcanic rock fragments comprise more than 50% of the total detrital grains with feldspars second most common constituent (10-43%).
· Quartz content ranges from 1 to 32% and averages 12%.

Clastic sediments associated with arc-derived detritus are typically low in quartz but high in plagioclase feldspar and volcanic lithic fragments. The mean framework modes calculated for these sandstones plot within or close to the magmatic arc provenance of Dickinson and Suczek (1979) on the Q-F-L diagram.

The magmatic are provenance suggested for these sandstones is compatible with Cretaceous and early Tertiary paleogeography in Jamaica. Cretaceous volcanic centers existed on the Clarendon block in central Jamaica and on the Blue Mountain block to the east. These volcanic piles, situated topographically as paleohighs, would have supplied source material to gravity flows which fed peripheral sedimentary basins with detrital sediments. High relief of the source areas associated with differential tectonic activity resulted in rapid erosion. Modification of the unstable mineralogical suite (consisting predominantly of feldspars and volcanic lithic fragments) was not possible over the short transport distances involved and mineralogically immature, poorly sorted sandstones were deposited. Thus at the site of deposition porosity and permeability were already low due to poor sorting. Alteration of unstable mineralogy with depth and associated diagenesis resulted in a further reduction of the effective porosity and permeability in these sandstones by producing a recurrent sequence of authigenic cements.

Carbonates appear to have some potential as reservoir rocks in Jamaica. Good porosity and permeability were reported in the Guinea Corn Limestone found in the Cockpit well (Clarendon block). A dolomitised rudist limestone in the Negril Spots well (Negril Savanna La Mar belt) had porosity values ranging up to 20%. Four carbonate units encountered in the Negril Spots well all showed porosity from dolomitisation and the development of caverns and vugs with interconnecting fractures.
Structural traps may be present over Jamaica and several large anticlines have been mapped at the surface. Indeed most of the prospects mapped to date have been anticlines, defined either from surface mapping or from subsurface seismic profiles. Anticlinal structures in Jamaica were formed either as a result of tectonic stresses or reflect compaction on basement rocks or paleohighs. Stratigraphic traps may be created where Upper Cretaceous and Early Tertiary sediments wedge out against these structures. However lack of adequate subsurface control at present makes it difficult to locate these stratigraphic traps at depth.

Structural relationships between the Cretaceous and Tertiary are generally obscure and the structure of the Yellow Limestone Group and younger rocks is often discordant upon the Cretaceous structure. Problems arise in exploring for structural traps as the presence of an anticline in Tertiary rocks at the surface does not necessarily reflect a similar structure at depth.

Impermeable shale, mudstone and siltstone beds which may act as adequate cap rocks for sealing traps are present in Cretaceous and Early Tertiary sections of Jamaica. These beds are fairly extensive horizontally, often occurring interbedded with sandstones, and may serve as both source and seal. However most of these impermeable beds appear too indurated and competent and may fracture rather than fold if subjected to tectonic stress.

PETROLEUM PRESERVATION

Jamaica is located in a zone of intense tectonic activity and has had an active tectonic history, particularly since the Middle Miocene. Major faults have prominent surface expressions (Fig. 1) and would have provided avenues of escape for petroleum that may have been trapped at depth. In addition, some prospective Cretaceous sections which have been uplifted are now exposed at the surface and large sections have been destroyed at the present erosion level. Whatever hydrocarbons that may have been generated and stored in these Formations would have been lost to the surface.

Granodiorite intrusions, the large number of faults and the present deep erosion level would have combined to destroy any hydrocarbons that accumulated in the

High geothermal gradients prevailing in the active marginal basin of the Wagwater belt in the early Tertiary would have also destroyed any petroleum generated in the area, except in the shallow parts, most of which have been removed by erosion. Thus large sections of eastern Jamaica are non-prospective.

The Tertiary carbonate cover has been extensively fractured and faulted and these faults probably extend downward to the Cretaceous. Seals would have been breached, thus permitting escape of hydrocarbons. One exception is the Montpelier-Newmarket belt where the relatively simple tectonic history and structural pattern would have favoured preservation of any accumulated hydrocarbons.


The existence of oil and gas seeps in an area is a sure indication that petroleum has been generated. The Windsor gas seep at St. Ann's Bay in the North Coast belt is the only significant surface indication of hydrocarbons identified in Jamaica and would rank as a relatively small seep by international standards. The gas appears to be biogenic in origin. Although the absence of hydrocarbon seeps does not preclude the existence of adequate source and reservoir rocks in the subsurface the lack of surface indications of oil and gas in Jamaica is certainly a negative factor, considering the numerous faults extending at depth and the fact that many Cretaceous sections have been uplifted and their sedimentary cover removed.



CONCLUSIONS

A review of the petroleum geology of Jamaica would suggest that conditions for the generation, accumulation and preservation of commercial deposits of petroleum were not ideal. Perhaps the most obvious negative factors are the apparent lack of adequate reservoirs and the relatively active tectonic history, particularly since the Middle Miocene.
The Chapelton Formation in the Content well has the best potential as source rocks for petroleum generation. Reservoir quality in the area is unknown and sandstones of the Masemure Formation underlying the Chapelton Formation in the cored section of the Content well were generally tight and well cemented. The availability of seals and structures in the southern Montpelier-Newmarket belt remains uncertain. Additional exploration may be justified subject to the identification of adequate reservoirs and structures which are in reasonably close proximity to potential source rocks.

REFERENCES CITED

Dickinson, W.R. and Suczek, C. A., 1979, Plate tectonics and sandstone compositions : Amer, Assoc. Petrol. Geol. Bull., v.63, p.2164 -2182.

Eva, A. N., 1980, Petroleum potential of Jamaica A case study of part of an ancient island arc UN ESCAP CCOP/SOPAC Tech. Bull., v.3, p.143-151.

Table 1:
Formations analysed, shown in relation to age and geologic province.

Figure 1 Structural suhdivision of Jamaica into morpho tectonic units (modified after Eva, 1980). Major faults with surface expression indicated.

PIase 1: Interbedded shales, siltstones and sandstones in the Richmond Fomation of eastern Jamaica. Front Cover)