Exploration methods including high-resolution aeromagnetics, organic geochemistry, and seismic provide exciting perspective on the Dominican Republic's giant offshore prospect.
Murfin Dominicana Inc. (MDI) is exploring for oil and gas on a 2.8 million acre concession in the southern Dominican Republic. The Dominican Republic occupies the eastern two-thirds of the island of Hispaniola, and the Republic of Haiti is located on the western third.
The concession encompasses two basins, the San Pedro basin in the east and the potentially productive Azua basin (Fig. 1) in the west. Although there have been numerous reports of the occurrence of oil on Hispaniola, the Azua basin is the only area on the island that has produced oil.
The Azua basin is located in the southern Dominican Republic onshore and extends into Ocoa Bay. MDI is concentrating its exploration on the eastern Azua basin comprising the Maleno and Higuerito subcommercial oil fields onshore, and the Ocoa Bay anticline offshore.
Exploration history
The American consul to the administration of Abraham Lincoln in 1862 first reported occurrences of oil in the Dominican Republic.
These were oil seeps located near Higuerito1 in the southern part of the country. The American oil company of Lancaster & Kreider drilled its first well, L&K No. 1, at the seeps in 1904. The well was drilled to 277 m and reportedly blew out at 400 bo/d for about a week.
Sustained production was not attained, and other companies later focused exploration on the Maleno anticline 11 km west of Higuerito. The Interocean Maleno-3 was drilled near the faulted crest of the anticline in 1923. The well had encouraging oil shows from 425 m to TD 895 m.
In 1939 the Dominican Seaboard Maleno-1 discovered oil at 122 m. Maleno-1 and Maleno-1A have produced over 23,000 bbl of oil. Again, sustained production proved elusive. The Azua oils were found to be 19.85° -22.34° gravity with sulfur content averaging 3.5%.
Dominican Seaboard drilled seven wells between 1939 and 1947 on the Maleno anticline, all very closely spaced and shallow with most TDs less than 244 m.
Petrolera Azuana/Dominicana drilled at least four more wells in late 1959 and the early 1960s. All of these were close to the original Seaboard wells. All had oil and gas shows.
The deepest wells in the basin are the Petrolera DT-1 drilled to 3,026 m on the western flank of the structure in 1960, and the Petrolera Azuana KM19/2 wildcat drilled to 3,049 m on the adjacent KM19/2 anticline in 1958. Both encountered numerous gas shows. All 44 wells in the Azua basin were drilled without the benefit of seismic.
In the 1960s-70s International Resources Ltd., Gas y Petroleo Dominicanos, and SA Petrolera Las Mercedes explored onshore. In the Azua basin, the work consisted of primarily gravity and poor quality seismic surveys. Several wells were drilled in the Azua concession, but little is known about the boreholes.
In 1969 Tenneco acquired 169 miles of seismic data in Ocoa Bay. The 12 fold data defined a large anticline. It was reported that the company planned to drill an offshore wildcat,2 but it later relinquished the concession.
Upon being awarded a concession in 1991, Mobil Oil Corp. acquired 1,556 km of seismic data in the southern Dominican Republic. The data included 510 km of a 11/2 km by 2 km grid in Ocoa Bay consisting of 60 fold data. The rest of the seismic data was a regional survey conducted throughout the offshore concession.
The Mobil 1991 seismic survey of Ocoa Bay confirmed the structural integrity of what is now informally known as the Ocoa Bay anticline and provided details, which the earlier data lacked.
Upon drilling the Pta. Salinas-1, a dry hole on the flank of the Azua basin in 1995-96, Mobil assigned the concession to MDI in 1996.
MDI acquired a high-resolution aeromagnetic survey of 11,300 km in 1996. The flight pattern was a north-south grid at 1 km spacing and an east-west grid at 2 km spacing.
The aircraft flew at an altitude of 152 m above ground level over the coastal plain and 333 m over the hills. The lower altitude ensured more precise data and the higher altitude was necessary for the safety of the aircraft.
The contractor was Airmag Surveys Inc., and the airborne and ground instruments were cesium optical precision magnetometers.
Pearson Technologies reduced and interpreted the data. In addition MDI acquired onshore seismic data, performed field work, geochemical surveys, and compiled and evaluated all earlier data.
Tectonics, structure
Hispaniola is located in the east-west trending, left lateral strike slip fault system that consists of the northern boundary separating the North American and Caribbean plates.3
The Azua basin is relatively modest in size, covering a combined 400,000 acres on and offshore (Fig. 2). The central basin is a flat coastal plain bordered on the east by the Cordillera Central, the highest mountains in the Caribbean.
The major Los Pozoz thrust fault separates mountainous Eocene and older island arc terrain from Caribbean oceanic basalt terrain3 in the Azua basin. To the northwest the basin is separated from the adjoining San Juan basin by Pliocene volcanics.
Mann and others3 have interpreted the Azua basin's complex faulting and folding as having resulted from indentation of a Late Pliocene to Recent aseismic ridge on the Caribbean seafloor by strike-slip motion. The surface folds of the Maleno and Higuerito subcommercial oil fields and the Ocoa Bay anticline are a result of this motion.
The aeromagnetic survey was intended to aid in defining the structural grain of the Azua basin. Interpretation of the total intensity magnetic profiles4 produced a magnetic basement map, and the total intensity magnetic profiles allowed the differentiation of magnetic basement, surface, and subsurface volcanics.
The on- and offshore magnetic basement profiles are illustrated in Fig. 3a and 3b, and the Las Yayas volcanic field is shown in Fig. 3c. Additional subsurface anticlines were located beneath the Azua Plain by the magnetic data (Fig. 2).
Azua stratigraphy
Paleontology studies by Bermudez in 1949 and most recently by Mann and others3 have provided the present stratigraphic framework for the basin.
Since there has been no offshore drilling in Hispaniola, the stratigraphy within the offshore Ocoa Bay (Fig. 4) is inferred from the adjacent onshore, the 1991 seismic survey, and the high-resolution aeromagnetic survey. However, the Mobil 1991 seismic survey data acquired near shore on its north-south lines enhances confidence in the stratigraphy. Along with velocity analyses the outcropping Sombrerito formation on the coast structurally correlates to the strong reflector in Ocoa Bay seismic data.
Although they have not been observed within the basin, the oldest sediments are thought to be marine Eocene limestones of the Neiba formation. The oldest known sediments that have been observed within the basin are Middle Miocene reefal to deep marine limestones of the Sombrerito formation. These sediments are overlain by marine turbidite shales and sandstones of the Middle to Late Miocene Trinchera formation.
Lying above the Trinchera formation are marine shales and siltstones of the early marine Pliocene Quita Coraza formation. Directly above are regressive marine to nonmarine Pliocene to Pleistocene conglomerates, sandstones, and shales that comprise the Arroyo Blanco formation.
The youngest formation in the bay is a recent reefal limestone here informally called the Ocoa Bay limestone that forms the seafloor and the surface reflector (Fig. 5). Plio-Pleistocene volcanic fields are located within the northeast margin of the basin.
The total stratigraphic thickness within the offshore basin may be approximately 5.2 km to 6.1 km in the center of the basin as suggested by the aeromagnetic survey. Basement is thought to be Late Cretaceous Caribbean basalts.
Hydrocarbons
Onshore oil samples from the seeps at Higuerito, Maleno, and tar from near Playa de Monte Rio on the beach at Ocoa Bay suggests two sources for hydrocarbons in the basin.
Onshore oils are sulfur rich, have a low pristane/phytane ratio, and the aliphatic and aromatic biomarker distributions suggest the source rock may be a Miocene carbonate. The oils have an abnormally high amount of carbon 13 isotope (C13) of -17%.5
There also may be a possible evaporitic input.6 These authors suggest a possible shallow anoxic depositional environment. Although much energy has been expended collecting and analyzing potential source rocks, none have been found and they remain an enigma.
Offshore, rich source rocks were discovered in cores from the Deep Sea Drilling Project sites 146-149, 150, 151, and 153 in the Venezuelan basin beneath the Caribbean Sea. These drill holes (Fig. 5) recorded a rich source rock horizon in Late Cretaceous Coniacian-Santonian radiolarian limestones.7
The samples obtained from the Lamont-Doherty Geological Observatory were analyzed for total organic carbon (TOC), hydrogen index (HI), vitrinite reflectance (Ro), and carbon 13 (C13). The results were encouraging with TOCs varying from 1.06 to 4.4%, and HIs from all but one sample of 114 to 535. The C13values averaged -28%. The Ro indicated immature source rocks (0.55 to 0.60) and was expected because the samples were only buried to 367-750 m beneath the Caribbean seafloor.8
Natural tar samples collected from the beaches of Ocoa Bay near Playa de Monte Rio and Loma de La Vigia were collected for source rock analyses. These samples were severely biodegraded. Because the origin of the samples was unknown, analyses were performed to see if the tar was natural or simply bunker fuel washed up on the beach.
The significantly high presence of aliphatic biomarkers and terpanes, which are most resistant to biodegradation but are commonly found in crude oils, strongly suggests that the tar samples are natural.
The absence of pyrolyzed aromatic compounds, such as benzo(A)pyrene, benzo(E)pyrene, and benzo(B+K)fluoranthane, that are present in heavy, high-temperature refined products such as Bunker C oil corroborates the natural origin of the tars.9 The low amount of -26.86% of C13and other indicators suggest that the Higuerito oils have a different origin. Indeed, the Ocoa Bay tars are much more indicative of a normal marine origin in the Late Cretaceous, similar to the source rocks in most of Latin America and the Venezuelan basin.
Petroleum geology
While the Maleno and Higuerito structures are fairly moderate in area at 3,292 acres and 1,500 acres, respectively, the Ocoa Bay anticline has approximately 18,000 acres of areal closure in three culminations. It may be the single largest drillable prospect in the Dominican Republic.
Structure
The Ocoa Bay anticline trends east-northeast to west-southwest.
A left-lateral (?) strike slip fault strikes parallel to the anticline on its northern flank. The fault separates a northern culmination from the other two that are on the south side of the fault. The eastern part of the fault displays bifurcation (Fig. 2).
The crests of the structure are 1,000 m below the seafloor. The north-south rollover is shown in 91 MDRH-23 (Fig. 5) of the western culmination.
The strong reflectors at the top of the seismic line show the present day reef. The present day reef front has remained in approximately the same position since the Middle Miocene (Fig. 5). Recent Ocoa Bay fault movement can be easily seen disrupting the stratigraphic section.
A smaller anticline, the Punta Serrano structure, trends in the same direction as the larger Ocoa Bay anticline. It is located to the north of the larger structure in 23 m of water just seaward of the tar sample locations.
Reservoir
The primary reservoir in Ocoa Bay is the Sombrerito limestone of Middle Miocene age. Although a generally deep marine carbonate, where reefal it is potentially prolific.
The Seaboard Maleno-7, drilled in 1947, encountered the Sombrerito at 877 m, and the well is thought to have blown out within the interval 925-1,732 m (TD) at an estimated 50,000 b/d of hot sulfur water for a week.10
Porosity observations of the well suggest biomoldic, intrafossil, fracture, and microvugular porosity.11
The Seaboard Maleno-1A encountered the Sombrerito at a depth of 901 m (TD 924 m). The Seaboard Maleno-1A found corals throughout the limestone, and oil stains were observed in all cored limestone intervals. The well produced a limited 4,553 bbl of oil from the overlying Trinchera formation.12 The reefal nature of the Sombrerito can be observed in outcrop at Loma de la Vigia and Cerro El Peñon on the rim of Ocoa Bay.
Secondary reservoirs consist of two Middle to Late Miocene sandstones within the Trinchera formation and are located at the Maleno anticline. These sandstones are known informally as the A and B sands and are comprised of fine to medium grain glauconitic sands with occasional conglomerates and occasional fossil leaves. The sands are bound by varying hard to soft gray calcareous marine shales and sandy shales of the Trinchera formation. The depositional environment is unknown, but the present data suggest debris flows and-or turbidity currents originating from the Cordillera Central to the east.
At present the sands are known to occur in all of the wells on the Maleno anticline. The reservoir in the B sand is capable of producing 300 bo/d.1 The A sand varies from 5 m to 28 m in gross thickness, while the B sand varies from 6 m to 61 m in thickness.
Hydrocarbons
An unexpected result of the aeromagnetic survey was the observation of "intra-sedimentary magnetic anomalies"13 of moderate amplitude and low frequency magnetic profiles.
These anomalies suggest changes in the magnetic component of the sedimentary section overlying the basement. The anomalies could be diagenetic in origin, change in lithofacies, or hydrocarbon biodegradation through bacterial action.14
These anomalous magnetic signatures were observed prominently in two areas. Onshore, one was observed in an east-west band from the Higuerito oil seeps to the subcommercial Maleno oilfield (Fig. 3e), a distance of some 11 km. The second was observed to form a narrow band over the Ocoa Bay anticline along the Ocoa Bay fault (Fig. 3d). Smaller more localized anomalies are also located on the anticline (Fig. 2).
The narrow band of the magnetic anomalies along the Ocoa Bay fault, suggests a hydrogeological environment that is conducive to bacterial action capable of hydrocarbon biodegradation along the active Ocoa Bay fault.
MDI work plan
The Ocoa Bay anticline is 16 km offshore in 43 m of water.
Speculative reserves of the 18,000-acre structure are large (300 million bbl of oil), and a discovery could significantly provide a positive change in the country's economy and stimulate further exploration on Hispaniola.
At present, the Dominican Republic depends totally on imported oil. The oil refinery at the Port of Haina has a capacity of 80,000 b/d and is within 40 miles of Ocoa Bay. With nearby ports such as Caldera, Haina, and at Barahona in adjacent Neiba Bay, importing drilling equipment would be simplified.
At present MDI is committed to begin testing the Maleno-1 and Maleno-1A wells and drill three wildcats on the unexplored eastern flank of the Maleno anticline. When the evaluation of these activities is completed, MDI will then turn to the "giant" structure in Ocoa Bay.
References
- Nelson, P., "A Report on Oil Prospects in the Dominican Republic," Western Natural Gas Co., 1954.
- Allen, C.C., "Final Report Dominican Republic Petroleum Exploration Venture," International Resources Ltd., unpublished report, 1970.
- Mann, P., Draper, G., and Lewis, J., eds, "Geologic and Tectonic Development of the North America-Caribbean Plate Boundary in Hispaniola," GSA Special Paper 262, 1991.
- Pearson, W.C, "Status Report on Dominican Republic Aeromagnetic Survey and Initial View of Data," Pearson Technologies, 1996.
- Walters, C.C., and Rooney, M.A., "Organic Geochemistry Oils form the Azua Basin Dominican Republic," unpublished DRL Mobil Report, 1990s.
- Alimi, H., and Kaplan, I., "Summary of Evaluation of Geochemical Reports Prepared by Mobil on Two Oils and One Seep from Azua Basin, Dominican Republic," Global Geochemistry Corp., unpublished report, 1995.
- Initial reports of the Deep Sea Drilling Project, U.S. Government Printing Office, Washington, DC, Vol. 15, 1971.
- Mobil Research & Development Corp., "Technical Service Job No. 506-7996, Geochemical Analyses of DSDP Cores, Leg 15 Caribbean Sea, and Outcrop Samples," Dominican Republic, 1989.
- Alimi, H., "Hydrocarbon Characterization of a Tar Sample from the Dominican Republic," Global Geochemistry Corp., 1996.
- Boardman, R., "Geological Report on Mella Well No. 1, Enriquillo Basin-Domn. Rep.," Unpublished Dominican Seaboard Report, 1947 (Author's note: The report is mislabeled and describes the Melano-7 well drilled in the Azua basin).
- Cooley, T.W., "Work Request No.: CD20549, Biostratigraphic Paleo-Environmental Analyses of Selected Core Samples from the Maleno-7 and Ditch Samples from the Anschutz Candelon #1 Wells, Dominican Republic," MDRC, interoffice correspondence, 1988.
- Seaboard, "Dominican Seaboard Oil Co. Higuerito Concession," unpublished summary sheet, 1939.
- Pearson, W.C., personal communication, 1997.
- Machel, H.G., "Magnetic Contrasts as a Result of Hydrocarbon Seepage and Migration," AAPG Memoir 66, 1996.
The author
Stephen Pierce ([email protected]) is a geologist with Murfin Dominicana Inc., Dallas, an international affiliate of Murfin Drilling Co., Wichita, Kan. He has more than 20 years' experience exploring the Dominican Republic. He has a BS degree in geology from Long Beach State University and an MS degree in geology from San Diego State University.