PETROLEUM POTENTIAL OF THE MESOZOIC OF TRINIDAD & TOBAGO
By Krishna Persad, Ph.D.
Manager - Exploration and Production, Premier Consolidated Oilfields PLC
ABSTRACT
To the present time no commercial discovery has yet been made in the Mesozoic Rocks in Trinidad. This is after
the drilling of some thirty-seven (37) wells over the last four (4) decades.
While the Upper Cretaceous has been found to contain major thicknesses of mature oil source rocks, potentially
productive reservoirs have been more elusive, and where found, have tested water or heavy oil. Likewise Lower Cretaceous
and Jurassic Rocks have found no reservoirs.
Recent work on stratigraphic and structural re-interpretations suggests the probability of major potential reserves
of oil from Upper Cretaceous Rocks, as well as the possibility of finding more elusive Lower Cretaceous and older
reservoirs.
Paleogeographic studies indicate the widespread presence, in Southern Trinidad, of sandy facies equivalents of
the Gautier Formation, which is largely shaly in much of Trinidad.
Well data from this area indicate sand percentages of 15-25% and two (2) wells did test oil, with one (1) structure
being potentially commercial.
A recent structural re-interpretation based on modern geological concepts, including plate tectonics, indicates
that most of the Cretaceous wells in South Trinidad either did not penetrate the prospective Gautier interval or
were offstructure and reveals the existence of several potential structures, within the prospective sandy zone,
with an upside potential of over one (1) billion barrels of oil.
In addition, two (2) potential Jurassic plays are believed to exist with as yet unknown, but possibly large potential.
The first is a potential dolomitized edge of the Jurassic Carbonate Trend where it abuts against the
Evaporite Basin in the Gulf of Paria.
The second is a potential Jurassic Reef Trend in unmetamorphosed rocks off the Manzanilla Coast in the deep offshore.
The total potential of the Mesozoic is exciting, and the first discovery could lead to the opening up of an entirely
new province, and may lead to a new high in Trinidad's oil production.
INTRODUCTION
In order to place things in perspective, I should mention:-
· Over 90% of total petroleum production to date comes from Pliocene and younger rocks, the remainder from
mid-Miocene. That is to say no commercial production has been established from the Mesozoic.
· The Upper Cretaceous has been established, however, as a major mature oil source rock, indicating that
to anticipate oil in the Mesozoic would not be unrealistic.
· To date, however, the vast majority of wells drilled to the Cretaceous have not penetrated the horizons
which contain reservoirs. The remainder have been, by and large, off structure. (Figures 1a & 1b).
· In fact, possibly the only well which penetrated the Cretaceous in a structurally favourable position,
and in an area where reservoir conditions should have been favourable resulted in an oil discovery, which may yet
be commercial.
Hence my initial optimism which I may mention has been further enhanced by recent geological work on the Mesozoic.
PREREQUISTIES FOR OIL ANO GAS
The prerequisites for oil (and gas) accumulation are:-
1. Suitable reservoir rocks.
2. Potential for trapping (either structural or stratigraphic).
3. Formation of trap early enough to allow migration to occur.
4. Seal.
5. Mature source rock
6. Migration pathway from the source to the reservoir.
RESERVOIR ROCKS
To the present time, the major reason for the failure to find hydrocarbons in the Mesozoic is lack of reservoir.
In order to better predict potential reservoir trends one must look at the paleogeographic history Of the Mesozoic,
specifically from the Jurassic (which contains the oldest known rocks in Trinidad) through to the Upper Cretaceous
(Fig 2 & 3)
PALEOGEOGRAPHY LATE JURASSIC - BASALLOWER CRETACEOUS
(Fig. 4)
During this period the arid conditions were widespread and in the Trinidad-Eastern Venezuelan Area a major reef
trend existed. To the south in the Gulf of Paria there was a persistent evaporite basin, south of which is an arid
foreland with possible aeolan sandstones.
The potential reservoir rocks are:-
porous carbonates within the reef trend.
The dotomitised edge of the reef trend where it abuts against the evaporites formed by a chemical process of molecule-by-molecule
replacement of Calcium Ions in limestone by smaller Magnesium Ions).
Aeolian sandstones to the south.
PALEOGEOGRAPHY LOWER LOWER CRETACEOUS
(Fig. 5)
The reef trend persists from the Jurassic but the evaporite sequence is now replaced by a lagoon in which alternating
sands and shales were deposited with sand percentage increasing southwards.
The potential reservoir rocks are:-
porous carbonates within the reef trend.
. Sandstone~ in the lagoonal area to the south.
PALEOGRAPHY UPPER LOWER CRETACEOUS
(Fig. 6)
The reef trend continues to persist but the area to the south becomes more shaly in the Trinidad Area.
The only potential reservoir rocks are:-
Porous carbonates in the reef trend.
PALEOGRAPHY LOWER UPPER CRETACEOUS
(Fig. 6)
The formerly persistent reef trend has now disppeared and been, replaced by-. outersheIf dark shales which, are
replaced southward by gradually thickening shelf sands of the Gautier Formation.
These sands have good porosity, are thick, extensive and form the single best known prospective reservoir in the
Mesozoic, but are present only in the extreme southern parts of Trinidad.
PALEOGEOGRAPHY - UPPER CRETACEOUS
(Figs. 8,9 & 10)
Conditions similar to those in Gautier times persist, except however, that the sandy facies has retreated further
southward and no reservoirs are expected.(Fig. 8)
(The shales, however, of the Naparima Formation, are known to be excellent oil source rocks).
(Fig. 9)
The Uppermost Cretaceous is also similar with deep water marls present in m6st areas.
(Fig. 10)
In summary then, only two potential reservoir rocks are known, first carbonates to the north and east and Gautier
sands to the south.
While the carbonates remain as good future prospects, especially to the East, they are over-mature in most other
areas and hence non-prospective. In the East their existence has yet to be proven, and they will remain for the
present a future prospect.
Let us look, therefore at the Gautier Sands, and at the other factors which may contribute to oil accumulation
therein.
TRAP
(Fig. 11)
In the South Trinidad Area, a major wrench system, which appears to have persisted for much of the Tertiary, has
resulted in large scale wrench". tectonics with a prominent (though young) primary shear fault, as well as
other older wrench faults, both P-Shear and antithetic left lateral.
The wrenching movement which caused these faults also caused the formation of a series of basement related anticlinal
features of the type. generally associated with wrench tectonics.
What is significant, though is:-
1. that in several of these structures the top of the Cretaceous is very accessible depthwise (less than 1500 meters).
2. that most of these lie within the gautier Sand Trend.
This fulfills two of the major prerequisites for oil entrapment i.e. Reservoir Rocks and structure. Let us examine
the others.
TIMING
The formation of the structures began during the Tertiary and continued throughout, and was early enough to allow
entrapment of oil in the Moruga East Discovery, and is hence considered not to be a problem.
SEAL
The interbedded shales within the Gautier as well as the overlying shales of the Naparima Hill Formation form adequate
seals.
SOURCE AND MIGRATION PATHWAY
The underlying shales of the Cuche Formation as well as the overlying shales of the Naparima Hill Formation have
been found to be good mature oil source rocks, which are thick and regionally extensive, and are in direct contact
with the Reservoir beds. The overlying shales, of course, can form an adequate seal.
OIL POTENTIAL
We find therefore that all the prerequisites for oil entrapment in the Mesozoic are satisfied in the southeast
Trinidad area in several structures which are considered highly prospective for Gautier Sands.
In these structures alone, if one assumes an average of 400 feet (120m) pay and recovery of 250 barrels per acre
foot, (326 cubic meters per hectare metre), the upside potential is 1200 million barrels, (190.8 million cubic
meters) of recoverable oil. It should be mentioned that one structure in Moruga East is a discovery, albeit fairly
heavy oil, which is potentially economic.
I will be remiss, however, if I did not add a note of caution to counter my reputation of optimism.
In these times of over abundance of crude, and soft prices, the high Supplemental Petroleum Tax imposed by the
Government, militates strongly amongst other things, against exploration in high risk provinces like the Mesozoic,
and there exists a clear case, in my personal opinion (shared I am sure by the many others present here today),
for not only a reduction in the Supplemental Petroleum Tax but also the provision of additional incentives for
"frontier" exploration (for example in the Mesozoic).
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