The quest for hydrocarbons continues

Seismic Provides New Look at Trinidad Geology

Thoughts of Trinidad and Tobago conjure up visions of carnival, frosty rum libations adorned with tiny umbrellas, fun times in general.

Yet this alluring Caribbean region is way more than a lush, fab playground for tourists – and locals as well.

To the oil crowd, anyway, it’s all about big deposits of natural gas and other hydrocarbons.

Geophysical surveys first got under way in the southern Tobago Trough (Basin) offshore Trinidad’s north coast in the late 1960s, continuing into the early 1970s when operators were first granted exploration concessions.

In May 1971, a shallow water new field wildcat well drilled in the southern Tobago Basin reached basement total depth at 2,740 meters. Bright, Mio-Pliocene sandstones that appeared on seismic were the objectives.

The well tested dry gas and was abandoned.

But this belies the fact that it was a winner, in that it had discovered the Patao-KK gas trend in the Tobago Trough.

The play harbored 2P recoverable reserves of 935 MMboe within Trinidad’s North Coast Marine Area and 1,206 MMboe within eastern Venezuela’s Marsical Sucre field complex, according to Rick Roberson, senior geoscientist at PGS, sighting IHS 2011 data.

The Basin’s Geology

The Tobago Trough is a modern marine forearc basin in the southeast Caribbean Sea between the Lesser Antilles Arc and Barbados Accretionary Prism.

“The basin is bound on the north by the St. Lucia Ridge and to the south by the Araya-Tobago metamorphic basement of the Araya Peninsula, Venezuela and the Northern Range, Trinidad,” Roberson noted.

Most hydrocarbon reservoirs offshore Trinidad and Tobago are located in Tertiary shelf and shelf-edge deltaic depositional systems, including the highly productive Patao-KK fields.

PGS has been a presence in the area acquiring seismic surveys, including a multi-client 2-D survey over the Tobago Trough, in conjunction with the Ministry of Energy and Energy Affairs of Trinidad and Tobago. The survey was implemented to provide imaging of the Upper Miocene and Lower Pliocene deltaic sandstones and possible deeper turbidite sandstones.

The Tobago Basin holds up to 14 kilometers of Cenozoic sedimentary fill with thick sequences of Mio-Pliocene sediments. About 2,448 kilometers of modern seismic data have been acquired and processed over the Tobago Trough, according to PGS, who describe the basin’s geologic history this way:

The Tobago Basin formed through an initial phase of collision tectonics between the Caribbean Arc and South American continent and evolution to an oblique transform margin with transpressional tectonics governing the Paleogene and transtensional tectonics dominating the Neogene. The age and origin of the basin’s basement is unknown, but evidence suggests that it is Late Cretaceous and of arc affinity.

Principal structural features in the southern Tobago Basin are the North Coast Fault Zone (NCFZ) and the Hinge Line Fault Zone (HLFZ), principal components of a broad zone of east/northeast-west/northwest trending faults associated with the southern wrench fault boundary of the Caribbean plate and controlling the basin’s structural and sedimentalogical evolution.

The majority of clastic sediment deposited in the basin was sourced from the south as the Caribbean plate moved eastward along South America’s passive margin. Slow slab rollback during the Paleogene created broad flexural subsidence across the forearc and controlled Paleogene deposition in the basin.

Flexural and thermal subsidence concluded in the middle Eocene producing a wide, deep-marine forearc basin with approximately eight kilometers of Paleogene sedimentary fill of predominately deep-marine pelagic shales and siltstones and deepwater turbiditic sandstones (Aitken, 2005).

Steady eastward advancement of the Caribbean plate brought it to oblique convergence with the South American plate during the Late Oligocene to Middle Miocene. Clinoforms in the basin indicate north-northeast progradation of the distal Proto-Orinoco delta in response to forced regression during Miocene uplift of the Northern Range in Trinidad and the Paria Peninsula in Venezuela. Sediment thickness of the early Neogene section below the Middle Miocene unconformity is approximately 3.5 kilometers.

Pliocene sediments deposited along the shelf and slope of the Tobago Basin are predominantly silty claystones with thin interbeds of fine-grained sandstones, conformably overlying Top Miocene and reaching thicknesses about 1,200 meters. Pleistocene sediments are primarily shales with interbeds of deltaic sands and reefal limestone conformably overlying Top Pliocene. Sediment thickness above the Middle Miocene unconformity is approximately 3.5 kilometers.

Support for the Premise

The majority of hydrocarbon reservoirs offshore Trinidad & Tobago found to date are in Tertiary shelf and shelf-edge deltaic depositional systems, according to PGS, and Patao-KK fields are no exception.

Reservoir, trap and seal for the Patao-KK petroleum system reside within the Upper Miocene and Lower Pliocene progradational shelf and shelf edge deltaic depositional systems of the Tres Puntas and overlying Cubagua formations. These sediments were deposited within actively deforming basins in shallow marine settings along South America’s northern margin.

Patao-KK gas is very dry and assumed to be either autochthonously sourced from prodeltaic mudstones coeval with Mio-Pliocene reservoir units (Schenk, 2000) or the product of anaerobic bacterial decomposition of thermogenically derived hydrocarbons sourced from deeper in the Tobago Basin.

Condensate production, higher in Patao-KK fields nearest the shallowing basin axis in Venezuela, confirms a thermogenic system functioning in the deeper Tobago Basin.

Reservoirs are Upper Miocene-Lower Pliocene fine-grained deltaic sandstones and siltstones with porosities ranging from 13-29 percent and permeabilities ranging from 10-1,100 mD. These sands are highly unconsolidated with interstitial clays and interbedded shales. Some reservoirs within Mariscal Sucre are rumored to be turbiditic sandstones. Seals in the play are formed by intraformational argillites and shales.

Petro-Canada executed field discoveries Cassra and Sancoche in 2008, extending the Patao-KK biogenic gas play northeastward on what appear to be gentle combination traps unrelated to traditional structural mechanisms of the HLFZ. Cassra currently is under appraisal.

Comprehensive 2009 2-D seismic coverage of the deeper West Tobago Basin and Tobago Platform suggest Mio-Pliocene reservoirs exist along the Tobago Ridge northeast of the island, and extending northward from Sancoche into the deeper Tobago Basin.

Roberson noted that condensate tests from the giant Hibiscus natural gas field in the Carupano Basin, or sub-Basin and the southernmost part of the Tobago Trough, indicate only Tertiary contribution.

Carupano is the biogenic gas-producing region in the Trough. The largest field there is the Patao field in Venezuelan waters, according to the U.S. Geological Survey’s World Petroleum Assessment in 2000.

“A colleague and I were considering potential for Late Cretaceous sediments in the basin,” he said. “But hopes were dashed when no Cretaceous fingerprints were found in condensate samples reviewed to date, indicating that produciton is purely Tertiary sourced in support of Aitken’s earlier conclusions.

“Back in ’08-’09, we shot half of the Tobago Basin with 2-D, which addressed the Trinidad side of the Tobago Trough,” he noted. “The other is to the west and belongs to (island country and commonwealth) Grenada.”

A bid round was ongoing at press time, and Roberson noted that interest was apparent from the block nominations.

At the end of the day, the quest to establish a commercially viable thermogenic petroleum system in the Tobago Basin continues.

Roberson thinks there is reason to be optimistic given the structural and sedimentary disposition of the deeper basin.

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Emphasis: Geophysical Review