Caribbean's Saba Bank area might hold pre-Eocene potential

Oct. 2, 2000
Reprocessing of several hundred kilometers of seismic data makes it possible for the first time to map structure within the pre-Eocene at Saba Bank in the Netherlands Antilles, eastern Caribbean Sea.

Reprocessing of several hundred kilometers of seismic data makes it possible for the first time to map structure within the pre-Eocene at Saba Bank in the Netherlands Antilles, eastern Caribbean Sea.

Structure mapping indicates a prospect that has at least 28 sq km of four-way closure and about 175 sq km of combined four-way dip and fault closure.

Seismic and gravity data also indicate the presence of a thick undrilled sedimentary section that is likely to be of Cretaceous age. Analyses of kerogens and of extractable hydrocarbons in the samples of the Saba Bank-1 well revealed migrated and reworked Cretaceous deltaic source rock material.

These hydrocarbons should have been derived from the pre-Eocene section which has been partially eroded in the western portion of Saba Bank, and the implication of the data is that the section should contain both source rocks and reservoir. It is this undrilled section that exhibits the large structural closure and commensurately large reserve potential.

The Saba Bank prospect is shallow enough for drilling by a jackup. The probable Cretaceous sediments are only about 1,000 m from the surface, and the section is at least 6,000 m thick.

Saba Bank location

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Saba Bank is a shallow carbonate platform that covers approximately 2,200 sq km. It is located in the northeast Caribbean Sea 5 km southwest of the northern Lesser Antilles island of Saba and approximately 140 km east-southeast of St. Croix, U.S. Virgin Islands (Fig. 1).

Saba is part of the Netherlands Antilles, a constituent entity of the Kingdom of the Netherlands. Petroleum activities are administered by Saba Bank Resources NV, a company jointly owned by the government of the Netherlands Antilles and the island governments of Saba, Saint Martin, and Saint Eustatius.

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Saba Bank Resources NV sponsored a recent program of geologic review, seismic reprocessing, and new seismic acquisition that found a very large four-way closure (Fig. 2) in an area with prospective and undrilled sedimentary section.

Exploration history

Approximately 4,300 km of seismic data have been acquired over the Saba Bank area since 1971.

Only two wells have been drilled in the area. Marathon operated Saba Bank No. 1, the first exploratory well in the Saba Bank area, in 1977. The well was abandoned as a dry hole with minor gas shows after reaching a TD of 2,974.7 m.

Fina Petroleum operated the exploratory well Saba Bank No. 2, located 15 km east-northeast of Saba Bank No. 1, in 1982. It was abandoned as a dry hole with minor gas shows at a TD of 4,231 m.

Data base

The presently accessible seismic database within the Saba Bank area includes 205 km of seismic acquired in early 2000.

Field tapes and prints are available for the 1,708-km 1980 Fina survey and the 343-km 1988 Aladdin survey, plus 60 km of Fina data and the entire Aladdin survey reprocessed during 1998-2000. Paper prints of some older vintage seismic lines are also available, along with gravity and magnetic data acquired by Fina during 1980.

Well data for Saba Bank-1 and -2 include wireline log suites, mud logs, and operational reports, as well as various in-house and consulting reports that analyze and interpret the biostratigraphy and geochemistry of the sediments encountered and the prospectivity of the block.

Geologic setting

Saba Bank is a backarc basin on the west side of the northern Lesser Antilles, a slowly moving active margin island arc on the eastern edge of the Caribbean plate under which Atlantic oceanic crust is being subducted.

Hydrocarbon source, provenance

Seismic and airborne gravity data indicate the presence of a thick undrilled sedimentary section at Saba Bank.

Analyses of kerogens in Saba Bank-1 by Marathon revealed considerable reworked Cretaceous coaly material in the upper Eocene-middle Miocene volcaniclastic sequence. The character of the material indicates it was sourced from an exposed Cretaceous deltaic source that had undergone considerably less deformation than the section exposed at nearby St. Croix.

Reworked Cretaceous calcareous nannofossils were reported from the equivalent but deeper-water section in Saba Bank-2. The relationship of the two wells suggests the Late Cretaceous sediment source was to the west or northwest of the wells, possibly from the western part of Saba Bank and adjacent areas or perhaps the southern Yucatan block, with which the Greater Antilles arc collided during Late Cretaceous time.

Stratigraphy and dating

The Saba Bank-1 and -2 wells encountered a thick Tertiary sedimentary section composed of carbonate and clastic sequences overlying andesites.

Recent analysis of seismic data indicates that a thick sedimentary section is also present beneath the andesites, where fair to good quality reflections have dips which are unconformable with the overlying sequences.

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An interpretation of airborne magnetic data over the Saba Bank-2 location indicates basement is between 6,100 and 6,700 m below sea bottom. Seismic data indicate these pre-volcanic sediments thicken and become shallower to the west (Fig. 3).

Volcanic unit

Potassium/argon radiometric work for Marathon dated the andesite encountered in Saba Bank No. 1 as 64.5 ma, or earliest Paleocene.

Saba Bank No. 2 also penetrated an andesite; whole rock potassium/argon dating by two laboratories determined the age of the andesite encountered at 3,927 m in Saba Bank-2 to be 38.4 ma and 37.3 ma. These dates are both late Eocene. Igneous rocks on Saint Martin Island have been dated from 28 to 32 ma.

Pre-volcanic sequence

The age of the pre-volcanic sequence is uncertain.

If the age dating done for Fina is correct, the pre-volcanic section could consist of Paleocene to middle Eocene age sediments overlying Upper Cretaceous and older sediments, or perhaps of solely Cretaceous sediments.

Dredge hauls over the Aves Swell and outcrops on Puerto Rico and St. Croix Island indicate widespread presence of Upper Cretaceous rocks in the region.

Petroleum geology Well results

The Saba Bank No. 1 well was drilled on the southeast part of the Saba Bank to test the hydrocarbon potential of a mid-Tertiary reef. The 934-m thick reef contained several intervals of porous and permeable carbonates, but the only indications of hydrocarbons were minor gas shows.

The Fina group drilled the Saba Bank No. 2 well on a seismic anomaly located on a perceived basement high deeper in the basin. It was expected that the well would encounter a carbonate buildup with associated reservoir facies. However, the equivalent to the reef facies penetrated by the previous well was a series of deepwater clastic sediments.

No significant hydrocarbon shows were encountered while drilling, although minor gas shows were encountered and log interpretation indicated the presence of possible gas-bearing reservoirs. No liquid hydrocarbons were recovered from testing, but a small amount of C1-C5+ gas was recovered from the drill collars.

Source rock potential and geochemistry

A reinterpretation of samples from the Saba Bank-1 well data is that most or all of the extractable hydrocarbons in the samples represent migrated hydrocarbons rather than indigenous material.

Kerogen maturation studies by Marathon indicate the thermal maturity of the penetrated section is low; the samples near the bottom of the well are approaching the oil generation zone but are not thermally mature enough for generation to have taken place. Therefore the migrated hydrocarbons should have been derived from the deeper, pre-Eocene section.

Marathon noted older reworked organic material in several samples. A sidewall core contained much fungal material and many pieces of light brown cuticle with a color indicating thermal maturity, as well as fusinite and wood fragments, all believed to originate from a Cretaceous coal seam. Similar but less abundant material was noted in another sidewall core.

Several ditch samples also contained plentiful fungal material, wood, and fusinite along with a suite of medium brown Mesozoic spores (Ro=1.0%) probably associated with the fungal material. The investigator again suggested that a Cretaceous coal or lignite may have been one of the sources for some of the organic material in this sample.

Geochemical studies on cuttings and sidewall and conventional cores from the Saba Bank No. 2 well were carried out by several parties. Vitrinite reflectance values indicate that the well drilled through the top of the oil window between 2,760 m and 3,100 m and reached the mature zone for oil generation.

Regional geochemical considerations

A review of Caribbean hydrocarbon source rock data strongly suggests that early Late Cretaceous rocks are the most likely source for hydrocarbons on Saba Bank. These highly prolific sediments deposited along the rifted margins of North and South America are well documented, while source rocks of the allochthonous suite of the Caribbean Plate are less well known, and their hydrocarbon potential has yet to be realized. Nearby supporting evidence for source potential from Puerto Rico, approximately 200 km west of Saba Bank, is found in outcrop samples and known oil occurrences.

Reservoir geology

The most promising prospects are Cretaceous reservoirs in the pre-andesite sequence. The reworked coaly material encountered in both wells suggests the presence of marginal marine deltaic sediments, which would be expected to have significant sand bodies of good reservoir quality.

Seismic

Selected Petrofina seismic lines (60 km) and all of the Aladdin lines (343 km) were reprocessed and reinterpreted to determine the potential of the untested pre-Eocene sediments.

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The major concern was elimination of the multiples which obscure the deeper data. While the old sections showed little more than flat reflectors and seemed to indicate shallow basement and little petroleum potential, the results from two different processors show geology that is more consistent with the expectations for the area (Fig. 4).

Western Geophysical tested a new post-stack multiple attenuation technique on a key line which ties the eastern area of Saba Bank to the western area. The two wells were drilled in the eastern area and the newest data were shot in the western area, where the pre-Eocene reflections are shallower and the data quality is much higher. This key seismic line crosses the poorly understood boundary between these areas, which had never been satisfactorily imaged or interpreted. The reprocessing cleared the picture sufficiently to allow the new interpretation that the boundary zone is a wrench fault (Fig. 3).

Prospect evaluation, reserve potential

The character of the reworked material found in the two Saba Bank wells suggests that it was sourced from an exposed Cretaceous deltaic sequence.

The reprocessed seismic indicates that during the pre-Eocene, the southwestern corner of what is now Saba Bank tilted up and was peneplaned, apparently serving as part of the provenance of these reworked sediments.

New seismic infill data confirm a large structure in that area and suggest a drilling location. Potential reserves are very high; a 100-ft net oil pay section could result in a 500 million bbl recoverable reserve.

The western half of Saba Bank is shallow enough for drilling by a jackup rig, and the objective section is much shallower than in the eastern portion. The probable Cretaceous sediments are only about 1,000 m from the surface, and the section is at least 6,000 m thick. In much of the eastern half of the bank, the depth to base Eocene is greater than 4,000 m.

The authors

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Kevin R. Allison, president of Pangaea Energy Corp., has worked in 40 countries as a geoscientist for major and independent companies. He is a member of the AAPG committee on continuing education. He has a BS degree in geology from Louisiana State University. E-mail: [email protected]

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Richard Church, a Denver consultant, has worked for major and independent oil companies and as an exploration advisor in the international oil industry for more than 40 years. He was formerly vice president-exploration for Apache International. He has a geological engineer's degree from Colorado School of Mines and an MS degree in geology from the University of Alaska.