EXPLORATION Two basins explored in Dominican Republic

Gerald M. Ellis
Director General of Mines
Santo Domingo

Where Caribean Exploration is Active [55223 bytes]

Exploration companies are exploring two tracts in separate basins of the Dominican Republic.

Drilling is under way or planned in the eastern Cibao basin in the northeastern part of the country, where Petrolera Once Once SA holds a 1,001,287 ha concession, and the Azua-Ban basin in the southwest, where Mobil-Murfin holds a 2,266,197 ha concession.

About 75 wells have been drilled onshore in Dominican Republic, but commercial production has not been established.

Petrolera concession

Petrolera Once Once, a Dominican-American-Spanish group led by the Rannik Shipping firm, signed an exploration/development operation contract with the government in 1990. The contract took effect Sept. 15, 1995 on publication in the Official Gazette.

The surface expression of the Cibao basin, the Cibao Valley, extends from near the Haitian-Dominican border about 200 km eastward to Samana Bay near the northeast corner of Dominican Republic.

Basin width is 25-50 km. In cross-section, its deepest part appears to lie just south of the Septentrional fault.

Along strike, the basin attains its greatest depth, 8 km (estimated from gravity anomalies), in its central part.

From seismic reflection data, the Septentrional fault splays into separate faults; sediments in the zone of splaying are compressed and uplifted into a series of flower structures.

The buried flower structure anticline in the central Cibao Valley has a minimum closure of 16 sq km. Surface geology in the western Cibao Valley suggests that additional flower structures are present there. The San Francisco ridge and possibly other flower structures are topped by reefs.

From data of two wells drilled in 1958, Licey al Medio to 3,668 m and Villa Isabela to 3,317 m, sediments appear to be of Miocene and Oligocene age.

Both wells, drilled on gravity anomalies, had abundant gas shows and several oil shows. Gas and condensate seeps occur in the basin.

Subsequent seismic data and a better understanding of the basin's geology suggest four exploration plays with good reservoir potential.

Petrolera Once Once, after a careful study and review of all geological and seismic data, concluded that the eastern Cibao basin, particularly the San Francisco area, had good exploration potential. The group decided to drill three exploratory wells in the San Francisco Block. They are:

Well No. 1, the San Francisco Reef prospect, drilled in 1995 to 6,500 ft (shot point VP 132 on Seismic Line 5 on the San Francisco Block brochure).
Well No. 2, the Patch Reef prospect, drilled in 1995 to 4,260 ft (shot point VP 148 on Seismic Line 2, SF Block Brochure).
Neither well discovered hydrocarbons.
Location No. 3, Pimen- tel Reef prospect, projected to 4,846 ft (shot point VP 192, Seismic Line 2, SF Block brochure).

A fourth location was planned farther east, closer to Samana Bay, where Petrolera Once Once imported its drilling rig and equipment in July 1995. Noncommercial shows were detected in several zones in both wells.

Field work, geological interpretation, and acquisition of more seismic data are under way in preparation to drill a third well, probably the one in the eastern Cibao basin. The company plans to spud in April or May 1996.

Mobil-Murfin work

Mobil Exploration Domin- icana Inc. signed a contract in 1990 that took effect Aug. 31, 1991.

Mobil geoscientists have undertaken seismic mapping offshore and on land, geochemical analysis, basin modeling, geology, gravity studies, seismic reprocessing, depth migration studies, and seismic facies work following reprocessing of seismic data.

The geological field work in part was conducted to investigate the source rock potential and reservoir properties of the Middle Miocene Sombrerito limestone and the Upper Miocene Trinchera formation.

The seismic program offshore along the southern Dominican coast was completed along 1,500 km lines, with rather detailed coverage offshore in Ocoa Bay. The 1991 seismic data defined a large anticlinal structure that could generate important volumes of hydrocarbons.

As the geoscientists indicated, the studies showed that the Azua basin was not deep enough to generate significant volumes of hydrocarbons, but it appeared that hydrocarbons could be generated under an east-dipping subduction zone that runs north-south across the basin's eastern side. Within the downthrown plate, source rocks generated oil that migrated to the surface along the thrust front. About 30,000 bbl of oil has been produced at shallow depths from sands in Maleno and Higuerito oil fields located along the thrust front.

This sets up a play trend that extends for 70 km onshore and 30 km offshore, including the Punta Salinas prospect. The most likely location for a hydrocarbon discovery would be in a prospect located directly along this trend, Mobil concluded.

The reservoir objective is reefal Middle Miocene Sombrerito limestone that is folded into large anticlinal traps of the thrust front. Several prospects may exist along this trend, but the best defined is the Punta Salinas prospect. This prospect has potential in excess of 100 million bbl of oil in place and could be tested with an 1,800 m well onshore near Punta Salinas that would evaluate the play concept for the entire trend. Mobil identified another prospect in Ocoa Bay.

Mobil-Murfin spudded the 1 Punta Salinas well in late December 1995, and completed drilling earlier this year at 5,150 ft. It did not encounter hydrocarbons.

Mobil-Murfin is conducting field work, geologic interpretation, an aeromagnetic survey, and acquiring seismic data. It plans to start in May to locate a new drill site.

The Punta Salinas site is onshore southeast of Maleno and Higuerito oil fields. Decades ago these fields yielded limited production from shallow wells. The areas contain seeps and have served as outstanding sites of geologic interest.

The Maleno oil samples were collected from the Maleno 1 well. Oil is actively leaking from this open wellbore because the valve was stolen from the wellhead years ago. The cellar is filled with oil, and a stream of oil flows downhill about 30 m from the cellar. Gas slowly breaks out as the oil rises to the surface in the well casing.

A gas seep was found in an old wooden well cellar about 500 m north of the Maleno 1 well. The gas had a petroliferous odor and a hydrogen sulfide smell, and a black scum resembling an asphalt stain lined the edge of the water table in the cellar.

The Higuerito seep is an impressive area. Eight gas vents occur within a 200 sq m area, and an active oil seep occurs at the crest of the hill. Two gas samples were collected from the west (oily) side of the seep, and two samples were collected from the east side.

Analyses showed that all the gas samples were identical within analytical error. The gas vents are active hot water springs. A caliche is formed in the soil down hill from these hot springs, suggesting that the water has a high calcium carbonate content. The gas has a petroliferous odor and a slight hydrogen sulfide smell.

Murfin Drilling Co., a 70 year old Wichita, Kan., drilling contractor and exploration company, took a farmout from Mobil and became operator with 75% interest last year in its first overseas venture, Murfin Dominicana Inc.

Correlation of aeromagnetic data with currently available reprocessed gravity data, subsurface geology, and surface geochemistry will allow for the generation of prospects that can be further highgraded by a minimal seismic program and drilled quickly.

A 1992 vintage high resolution gravity study produced data that revealed potential structures not seen before in the Azua basin. The high resolution airborne magnetometer study, therefore, is needed to confirm the 1992 results. The 9,000 line km, flown at 1/2 km spacing, will produce more detail from the seismic work.

The Punta Salinas well is the first location drilled in the concession.

Additional smaller potentially commercial prospects include the re-evaluation of the oil seeping structures at Maleno and Higuerito.

Century Offshore and Maleno Exploration are now present in the Azua basin.

Other opportunities

Dominican Republic has large areas of sedimentary basins but a sporadic history of exploration since 1904. The areas are:

Western Cibao basin, 3,250 sq km in the northwestern part of the country.

Eastern Cibao basin, 3,000 sq km in the northeast.

Eastern Plains basin, 4,800 sq km in the southeast.

Enriquillo basin, 2,000 sq km, west and southern part.

San Juan basin, 2,000 sq km in central and western areas.

Azua-Ban basin, 1,500 sq km in the south and southwestern part.

Bahoruco basin, 1,200 sq km in the southwestern part.

The director general of mines plans to hold a one week meeting in Santo Domingo during 1996 to brief oil and gas and mining firms on the country's potential and investment opportunities.