DISCOVER OF A PULL-APART BASIN MAY SIGNAL OIL POTENTIAL IN BELIZE

Susan Morrice
S. Morrice & Associates Ltd.
Denver

Belize excites oil and gas explorationists because of its placement on the rim of the prolific Southern Gulf of Mexico basin, containing most of the major Mexican oil production (Fig. 1). Thus Belize offers access to the same stratigraphic sequences of source, reservoir, and seal facies.

All of the Mexican fields together produce as much oil as the entire U.S. Gulf Coast-about 2.5 million b/ d, noted John A. Masters, formerly with Canadian Hunter Exploration Ltd., in a recent article.

Belize Natural Resources Ltd., Belize City, initiated a multidisciplinary study to identify why commercial reserves have not been discovered off Belize. As a result of this in-depth study BNR has developed a logical explanation for the past lack of success.

The initial study was so encouraging that BNR acquired the Glover's Reef license block and commenced an exploration program building on the 5,000 km of seismic data shot across the area through 1987 BNR Pecten International Co., Houston (Fig. 2).

The geology is complicated as Belize sits close to the junction of the North American and Caribbean tectonic plates; indeed, a tightly flown aeromagnetic survey across the study area revealed two very distinct tectonic environments located in the western and eastern portions of the area (Fig. 3).

The western style (including the Camels Hump area) stood out as being extremely complex and exhibited many "broken up magnetic anomalies," but the eastern area showed up as magnetically very simple with distinct deep seated anomalies. BNR believes it has been able to define a new geological setting called the Gladden pull-apart basin within this restively quiet eastern area. None of the offshore wells drilled to date has penetrated this newly defined basin.

ACCUMULATION MODEL

BNR personnel integrated the Pecten seismic data, regional well data, gravity, magnetics, hi-h resolution seismic, and seep geochemical data to construct a meaningful hydrocarbon accumulation model within the Gladden pull-apart basin. The key interpretations resulting from the study include:

1. The Gladden pull-apart basin is a newly defined, undiscovered pull-apart basin (Fig. 7).

2. The pull-apart basin has remained tectonically quiet since the Early Tertiary, thus enabling the generated hydrocarbons to stay in place (Fig. 7).

3. The presence of a hydrocarbon charge and migration timing are favorable for hydrocarbon accumulation (Fig. 5).

4. Ten sizable structures have been delineated, five of which could be part of one large carbonate bank. Recoveries from such a bank could be comparable to the Campeche multibillion barrel finds (Fig. 2).

5. Well data imply a stratigraphic column similar to many of the Mexican fields (Fig. 4) with similar reservoir, source, and seal rocks.

RESERVOIR

Drilling of the Camels Hump wells confirms that both Upper and Lower Cobana Cretaceous carbonates are the primary reservoir objectives with expected porosities ranging to more than 30%.

In addition to the matrix porosity, cavernous porosity can be expected as evidenced by drill bit drops and thick sections of no returns. The reservoir potential has been further enhanced (secondary porosity) by typically highly fractured carbonates and subaerial exposure of the sections.

SOURCE POCK, MIGRATION TIMING

Oil shows reported over thick sections of several wells and evidence of surface seeps from Barringer Patents Inc. airborne seep-trace technology confirm that source rocks are present and a hydrocarbon charge has occurred in the basin.

Some 600 ft of dark, finely laminated Upper Jurassic limestones in the 1 Seal Caye well to the south are comparable in both ace and appearance to the Tithonian shales and argillaceous limestones that source Mexico's Reforma and Campeche oil fields.

Vitrinite reflectance measurements from various wells and thermal modeling studies indicate that the oil window occurs between 6,000 and 10,000 ft below the sediment-water interface. Within the new Gladden basin the Upper Jurassic/Lower Cretaceous source rocks would have entered the oil window during the Tertiary and migration of hydrocarbons would have occurred after the Early Tertiary Toledo seal was in place (Pig. 5).

Additional source rock potential occurs in the organic rich horizons within the Cretaceous carbonates as well as in the hemipelagic lime mudstones and shale sequence of the overlying Toledo shales (buried below 10,000 ft in the central Gladden pull-apart basin).

SEAL

The shale lithologies of the Upper Cretaceous-Lower Tertiary Toledo formation are not permeable and form an excellent primary seal.

A strong seismic reflector from the Toledo verifies that the seal has remained intact within the Gladden pull-apart basin. Outside the basin the seal has been breached by recent fracturing.

STRUCTURE

The BNR study focused on 5,000 km of marine seismic data, mostly 1987 vintage, that revealed major Cretaceous structural features and an undisturbed Tertiary sequence.

Seismic data accurately tied with the 1 Coco Plum well, which reached total depth of 9,860 ft still within the Cretaceous.

An important gravity profile verified the presence of a thick sedimentary sequence within the Glaaden pull-apart basin (depth to basement of 20,000 ft).

In addition, BNR carried out a detailed "high sensitivity-high resolution" aeromagnetic survey on a 2 km grid that supported the gravity observations and tied nicely with the seismic interpretations of the structural setting.

PLAY TYPES

The primary objective (play type) is the Lower Cretaceous Coban carbonate bank with fracturing and erosion enhancing permeability (Fig. 6, points 1, 2, 3).

The overlying Toledo shale is an effective impermeable seal. Source rocks are found in the Upper Jurassic, Lower Cretaceous, and within the Toledo shales (Fig. 6, point 5).

Hydrocarbon seeps in the area indicate maturity of source and an active charge. Tertiary patch reefs encased in shales, localized about the high points on the carbonate bank (Fig. 6, point 4) provide a shallow secondary target when drilling.

The extensive talus slope development is a third important target in this basin (Fig. 6, point 6). It is important to note that all previous drilling has been associated with recent wrench tectonics which has breached the original traps.

CONCLUSIONS

The study has identified large structural features comparable to many of the Mexican fields within the prolific Southern Gulf of Mexico basin.

The discovery of a "quiet" previously undrilled pull-apart basin has enabled this proven sequence of source, reservoir, and seal to remain intact. This is the first time that unbreached Cretaceous carbonate structures are clearly recognized in Belize.

The sediments within the Gladden pull-apart basin have not been penetrated to date but volumetric studies suggest a potential yield of 4 billion bbl of recoverable oil.

BIBLIOGRAPHY

Taylor, Gary, Mexico has recipe for 'hot play,' AAPG Explorer, July 1993, p. 8.

Bishop, W.F., The petroleum geology of northern Central America Journal of Petroleum Geology, vol. 3, 1980, pp. 3-59.

Petroleum projects and potential of the Belize basin, Ministry of Natural Resources, Geology: and Petroleum Office, Belmopan, Belize, 1986, p. 66.

Glover, reef license technical report, Pecten International Co., 1988.

Sanchez-Barreda, Luis A., Why wells have failed in southern Belize area, OGJ, Aug. 20, 1990, p. 97.

Airborne magnetic survey, Belize Project 90071, BGM Inc., 1990.

Gay, Parker, interpretation report, aeromagnetic survey, Belize offshore concession, Applied Geophysics Inc., 1990.

Timbell, Ned., Hydrocarbon basin model study of the Gladden pull-apart basin, Belize, GES International Inc., 1992.

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