THE FURRIAL OIL DISCOVERY AND ITS SIGNIFICANCE TO TRINIDAD
K.M. Persad
Petroleum Consultants Trinidad Limited
El Furrial is a large new oilfield recently discovered in the Maturin sub-Basin in Eastern Venezuela. The initial
wells tested at high rates from the Oligocene Naricual Formation, e.g. 1 Furrial tested at 7,331 bod of 26.50 oil
from perforations between 13,700'-13,900' , 2 Furrial tested 5,780 bod and 4 Furrial tested 4,300 bod. Pay zones
are thick, e.g. 4 Furrial found 1,400' net oil sand, 2 Furrial found 900' and 2 Furrial found 950'
The Furrial structure itself is large, and is an elongate thrusted asymmetric anticline covering 17,000 acres.
Continuing drilling along the structure has been very encouraging, El Carito 1 tested 6,040 bod of 32.90 oil, Furrial
6 tested 8,017 bod of 24.70 oil and 1 X Musipan
found 1,800' of oil pay.
Wells are all deep and T.D. around 17,000 feet. Pay zones are 13,000 to 16,000. (Another field, El Orocual Field
to the NW, has also been discovered to be productive in Naricual turbidites, as well as, upper Cretaceous sandstone
reservoirs of the San Juan Formation). The Furrial structure is one of at least 12 that have been identified by
PDVSA in a 7,000 sq. km. area stretching from Furrial in a broad belt trending ENE as far as the Gulf of Paria.
In fact the trend continues into Trinidad and forms part of the thrust fold belt, which is now recognised as the
structural and stratigraphic response to the oblique collision of the Caribbean Plate with the South American Plate.
This oblique collision resulted in crustal downbuckling, and initially, turbidite deposition in the deep ocean
basin so formed, followed by thrusting and subsequently, molasse sedimentation. The effects of this collision in
Trinidad were:
1. Emplacement of a series of ENE trending nappes in a fold-thrust belt which extends from the Southern Range in
the south, to the El Pilar Fault in the north. In addition the tectonic ridges of the Columbus Basin are considered
as very young blind thrusts.
2. Downward loading of the crust SSE of the fold thrust belt to form a deep remnant ocean basin, roughly triangular
in shape, the southern boundary paralleling the former shelf edge, i.e. EW, the northern boundary being the ENE
trending edge of the Southern Antilles Arc pushed by the Caribbean Plate - this basin was entirely filled during
the Tertiary.
3. The development of a thick sequence of turbiditic flysch and occasionally, wildflysch sediments in the
deep basin just in front of the fold-thrust belt, which forms the major source of the sediments, also trending,
therefore ENE. A number of these turbidite fairways are known in Trinidad, the Herrera being the best developed
of those known. The Naricual Fm. of eastern Venezuela, is another such deposit.
4. Overthrusting of the previously deposited flysch sequence by a series of thrusts which change westward into
oblique wrenches as the direction changes from ENE to WNW.
What are the implications for Trinidad? These features run from Furrial in Venezuela to offshore Eastern Trinidad.
In the Trinidad area, older discoveries like the large Herrera, Brighton and Oropouche Fields are well known. These
well known turbidite trends, Nariva, Retrench and Herrera in fact form part of entire thrusted turbidite fairway
complex, and the Naricual is seemingly an older part of the same trend which should be present NNW of the Nariva
trend.
Within the fold-thrust belt, exploration is sparse both onshore and offshore, and the potential for large oil discoveries
is extremely good. Particularly attractive are several of the blocks to be offered by the Government in the Second
1988 Bidding Round. Structures are expected to be elongate asymmetric over-thrust anticlines typical of fold-thrust
belts of this type. The major reservoirs will be the turbidite sands of Oligo-Miocene age found productive in Furrial
and in Trinidad. The source is the upper Cretaceous Naparima Hill Fm. which became mature with the downbuckling,
and migration probably occurred in the over-thrusting phase that followed.
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