THICK SEDIMENTS, HIGH POTENTIAL POSTULATED IN BELIZE LICENSE AREA

An exploration license area awarded last month in northwestern Belize has an expected 15,000-20,000 ft of sedimentary section on its west side and is in the most prospective area of the country, says a report prepared for the concessionnaire. Belize signed the agreement with Pentagon Petroleum Inc., Baton Rouge, La., covering 268,000 acre Block V14 (see map, OGJ, Dec. 17, 1990, p. 29). Seismic work is to start in first half 1991. Pentagon's 4 year exploration license calls for a wildcat to

Jan. 7, 1991

4 min read

Belize signed the agreement with Pentagon Petroleum Inc., Baton Rouge, La., covering 268,000 acre Block V14 (see map, OGJ, Dec. 17, 1990, p. 29).

Seismic work is to start in first half 1991. Pentagon's 4 year exploration license calls for a wildcat to spud by the fourth year.

Only a few miles of seismic have been shot previously in the area, known as the Blue Creek region, wrote Jean Cornec, a Benque Viejo, Belize, consulting geologist, in a November 1990 report.

The license area is part of the Peteh basin, and its Cretaceous carbonates are lateral equivalents to those of the producing fields of Reforma and Chiapas in Mexico and the U.S. Gulf Coast basin, Cornec wrote.

Oil has definitely been generated in the basin, he said.

Six oil seeps are found on the basin flank in the Maya Mountain foothills from Benque Viejo to the Belmopan area, where some free, 3040 gravity oil was recovered in four shallow wells through drill stem or production testing (OGJ, Aug. 20, 1990, p. 97).

Live oil shows have been observed in the Cretaceous section of most wells.

The area also has Cretaceous back reef, carbonate anhydrite alternations and organic black shales, and open marine Jurassic sediments that sourced Mexico's Reforma fields might be present in parts of the license area.

LICENSE AREA GEOLOGY

Cornec said the license area has west-northwest regional dip and is intersected by several faults of the Rio Hondo lineament zone running along the eastern margin of the Yucatan platform.

The faults strike north-northeast-south-southwest to north-south and can clearly be seen on aerial photographs and topographic maps. Most drainage in northern Belize is controlled by that pattern, Cornec said.

The faults are interpreted from satellite imagery, based on the orientation of associated faults, as a series of normal faults downthrown to the east and with probable left-lateral reactivation. Some form well developed and linear escarpments several hundred feet high, suggesting movement in recent times.

The faults upthrown towards the basin should have provided excellent trapping for any oil migrating updip, Cornec said.

The fracture zone might have been originated by the Jurassic rotation of the Yucatan block during the opening of the Gulf of Mexico.

Recent reactivation of those lineaments have created a topography that might have hidden older structural trends from aerial interpretation techniques, as suggested by recent geological field work and a seismic survey in the vicinity.

RESERVOIRS

A process of digenetic destruction of porosity by dolomitization and sealing by anhydrite has taken place especially in the shallower parts of the basin.

That process obliterated parts of the original modic and intercrystalline porosity that was as much as 25%. The destruction appears to have taken place in the shallow environment of sedimentation itself, with also possible fresh and mixing water influences, Cornec said.

Therefore areas with higher subsidence rates, like northwestern Belize where the Pentagon license is located, have much better potential for the preservation of original porosity as well as the algal organic matter needed for hydrocarbon generation.

An analysis published in 1988 of cuttings and core chips from the 1 Hill Bank and 1 Blue Creek wells show that a good amount of porosity has survived.

The former Gulf Oil Corp. drilled 1 Blue Creek, in the northeast corner of Pentagon's tract almost to the Mexican border, in 1958. Gulf's 1 Hill Bank is in an adjoining concession area about 10 miles southeast of 1 Blue Creek.

The 1 Blue Creek analysis shows a great thickening of the Cretaceous section, an index for a higher subsidence rate, and recent seismic work confirms that the basin thickens and deepens rapidly towards the west.

The area west and southwest of 1 Blue Creek should then have better reservoir quality potential as well as thickening of the potential pay zones.

Cornec recommended that a combination of satellite, geomorphology, and geochemical studies and gravity data be used to help locate areas for seismic surveys.