Senegal's large-scale compartments provide new exploration potential

DSDP core holes show interesting anomolies

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Regional map of offshore Senegal showing the major fracture trends, basin compartments, and DSDP core holes.
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A new exploration play is evolving in the deep offshore waters of Senegal where organic-rich marine source rocks appear to be present. New data is revealing a mature and active petroleum system. Large-scale structural and stratigraphic traps have been identified on recently reprocessed 2D seismic data. Between Mauritania and The Gambia, only seven wells have been drilled offshore, all on the continental shelf (<200 meters of water), and none in the deeper waters that would have tested this prospective play.

The Senegal Basin occupies the central portion of the large Northwest African Coastal Basin. Like many other western African offshore basins, it formed as a result of the opening of the North Atlantic Ocean beginning in late Triassic time. It is a typical passive margin basin.

Onshore, the basin is bound on the north by the Regubiat High, on the south by the Guinea Fracture Zone, and on the east by the Pan African/Hercynian overthrusts. The deepwater portion of the basin (>200 meters), is recognized as the Gambia Basin, and lies between the Cape Verde Rise on the north and the Sierra Leone Rise to the south.

The deep offshore basin is floored by syn-rift Upper Jurassic oceanic basalts that transition to pre-rift Paleozoic rocks underlying the onshore portion of the basin. The deepwater portion of the basin covers over 100,000 sq km off Senegal and The Gambia, and contains a wedge of Paleozoic through Cenozoic sediments up to 9.5 km thick. The Gambia/Senegal basin can be further sub-divided into discreet compartments as a result of major fracture zones (FZ) that cut the basin.

The largest of these is known as the Dakar-Banjul compartment, and is bound by the Blake Spur - Cayar FZ on the north, and the Jacksonville FZ to the south. This compartment is characterized by a large number of volcanic intrusions, including the Miocene intrusions on the Dakar peninsula and the fact that no salt, or evidence of diapiric movement, has been found within this compartment. Immediately south is the Casamance-Bissau compartment, which contains numerous salt diapirs in shallow, near-shore waters. One of these domes is the site of the giant (1 billion boe) Dome Flore non-commercial oil field.

DSDP coreholes

In 1975, a series of five core holes were drilled off the northwestern coast of Africa as part of Deep-Sea Drilling Program (DSDP) Leg 41. The objective was to learn more about the formation of the North Atlantic Basin. Three of these DSDP core holes were drilled off the coast of Senegal. Of particular interest is DSDP Corehole 367. Located in 4,748 meters of water, the well penetrated 1,153 meters of sub-subsurface sedimentary section, bottoming in oceanic basalt. A total of 40 individual cores were attempted over a 347-meter section, with 174.3 meters actually recovered.

The basalts are believed to be oceanic basalts emplaced as a result of rifting. They have been age dated as Lower Jurassic. Immediately overlying the basalts is a series of Late Jurassic to Early Cretaceous limestone units. These limestones represent deepwater pelagic deposition, and are draped over the basalt basement. Overlying the limestones are red and green varigated claystones of Barremian to Early Aptian age. A 312-meter section of Late Aptian to Early Turonian black shales lies above the claystones.

These black shales have a very high organic content and represent a change from oxic to anoxic water conditions. Total organic content values range from a low of 8% to a high of 28% for these shales. They have high methane content, and pieces of the shale were reported to "burn when lit." These middle Cretaceous black shales are equivalent in age to other important source rocks found around the north Atlantic Basin. The black shale interval at DSDP 367 is somewhat laminated and calcareous in the lower cores, grading upward into light green and olive-gray clays in the upper cores.

Overlying the black shale interval is Late Cretaceous to Late Paleocene multicolored silts and clays. Diatom rich clays, cherts, and porcelaneous cherts of Eocene age were found above the claystone interval. A thin Miocene to Pleistocene section consisting of clays, marls and thin sands complete the sedimentary section.

There is almost continuous deposition from Upper Jurassic through Holocene. This lithologic column is similar to others seen in core holes drilled at other locations in the northern Atlantic. The discovery of these thick, organically rich, black shales enhance the exploration potential of the deepwater basin offshore Senegal and The Gambia.

Seismic correlation

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The lithologic column encountered at DSDP 367 (Spathopoulus et al. 1998).
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After the cores were recovered, two single-channel seismic profiles were acquired over the site. Four distinct zones of seismic character were observed below the sea floor.

A strong reflector (No. 1) at 0.38 sec TWT (two-way-time) correlates to the high impedance cherts of the Eocene section. Fairly strong reflectors at 0.7 sec TWT and 0.8 sec TWT are a result of calcareous or dolomitic layers within the black shale interval (No. 2).

A very strong reflector at 1.1 sec TWT (No. 3) ties to the top of the limestone section. At 1.2 sec. TWT, a subtle reflector (No. 4) may indicate the top of the oceanic basalt basement. There are no multi-channel lines over the DSDP 368 location. There are no direct seismic ties between the DSDP 367 and the various seismic data sets offshore Senegal.

Exploration history

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A portion of an E-W dip line from the reprocessed and depth-migrated data set showing correlatable units and seismic character.
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Hydrocarbon exploration in Senegal began in 1953, with the first exploration well drilled onshore. During the last 47 years, 144 hydrocarbon exploration wells have been drilled in the Senegal Basin. Of these wells, only 49 have been drilled offshore and only seven of these 49 have been drilled along the 300 km shoreline between Mauritania and The Gambia. None of these wells are in water depths greater than 200 meters. No wells have been drilled in the present day deepwater environment along the entire northwestern Continental Margin.

Several geophysical surveys have been conducted in the offshore waters of Senegal. One of the most complete regional grids of seismic data was acquired by Shell between 1971 and 1975. This survey consists of over 5,000 km of multi-channel 2D lines covering the shelf, slope and portions of the deep water. The National Oil Company of Senegal, Petrosen, has recently authorized the reprocessing and post-stack depth migration of this dataset. First Exchange Corporation of Houston, Texas has been retained by the government to underwrite this reprocessing. Veritas Geophysical Company reprocessed the data in their Houston, Texas Processing Center. Final products include both DMO migrated time sections and poststack depth-migrated sections.

Seismic character

Reprocessed lines show significant improvement in data quality, multiple suppression, and retention of frequency content over the original processing. Distinct coherent reflectors are observed and can be character-tied to seismic profiles shot over DSDP 367, located 200 km to the southwest. These character ties can be mapped regionally over the offshore area.

Examination of the reprocessed data (see figure) shows a strong impedance event (blue horizon) between 7,500 - 9,000 meters below sea level. The event can be reasonably correlated to the Upper Jurassic - Lower Cretaceous limestones (reflector No. 3) seen at DSDP 367. This event shows parallel and continuous reflectors that drape over what appears to be faulted basement blocks.

Basement blocks are either pre-rift continental material, or syn-rift volcanics that have been faulted into a series of alternating high and low blocks due to rifting. This block-faulted geometry is seen on other lines in the reprocessed dataset. Similar block faulting or horst and graben complexes are seen in other West Africa locations, such as offshore Angola, where major hydrocarbon accumulations are found on the structurally high blocks (Pawlowski, 1999).

Organically rich black shales at DSDP 367 were bracketed by claystones having a seismically transparent character. Immediately above the blue horizon is an area of seismic opacity, which is interpreted as the lower claystone interval. The top of this zone has been mapped as the brown horizon. The seismic character begins to exhibit some parallel, but discontinuous reflectors between the brown and the red horizons. These reflectors may be the calcareous lower zones of the black shales as described in the cores from DSDP 367 (reflector No. 2). The parallel and discontinuous reflectors stop at the red horizon, and become indistinct along some portions of the line.

The red horizon is interpreted to be the top of the black shale interval. The upper claystone overlies the black shale interval, and is characterized by another zone of seismic opacity. The black shale interval between the brown and red horizons can be mapped throughout the data set, and ranges in thickness from 300 meters over the basement high blocks, to over 1,000 meters in the deepest portions of the basin.

A thick zone of discontinuous, parallel, low frequency events is above the upper claystone interval. This interval is interpreted as the Upper Cretaceous. The Upper Cretaceous section is greatly expanded compared to the correlative interval at DSDP 367. Strong reflectors are discontinuous but prevalent, suggesting the deposition of alternating lithologies of contrasting impedance. Amplitude anomalies are present in the upper-most Cretaceous and Tertiary sedimentary sections. Some of the amplitudes are quite strong and areally extensive.

A prominent unconformity surface separates predominantly low-frequency data from the overlying high-frequency content data. This unconformity is mapped as the yellow horizon and is thought to be the base Tertiary. Above the base Tertiary unconformity, the seismic geometry displays characteristics of a prograding sedimentary wedge. Significant thickening into the deep basin is observed above the Jurassic limestones. As much as 8,000 meters of overburden have been observed above the black shale horizon on some lines.

Several major drainage systems along the western continent, such as the Gambia and Senegal Rivers, move sediments to the offshore. Deeply incised offshore canyons, such as the Cayar Canyon (Area A), moved sediment into the deeper waters. Evidence of ancestral channels is present on many lines. One such channel (Area B) is seen at a depth of about 3,500 meters.

Presence of faults

Numerous faults can be interpreted. Many of these faults are deep seated, cutting from basement all the way through the sedimentary sections. These deep-seated faults provide excellent migration pathways. One or more of these deep faults cuts the ancestral channel at 3,500 meters. The floor of the ancestral channel contains

an amplitude anomaly. Many lines in the re processed data set contain similar amplitude anomalies. Other interesting features such as gas chimneys and mud volcanoes at fault intersections with the sea floor have been observed.

Recent basin modeling studies show that the middle Cretaceous black shales are mature and prolific. They have been generating hydrocarbons since the Middle Miocene in some parts of the offshore basin. It would appear that significant volumes of hydrocarbons have been generated and are moving through the deepwater petroleum system.

In addition to stratigraphic traps associated with ancestral depositional channels, several large structures have been observed on the seismic. Some of these structures are on the order of 25 km long and 10-12 km wide. The structures formed as a result of deeper igneous intrusion.

Government offer

The Republic of Senegal is currently soliciting applications for concessions on all available shallow and deepwater blocks. The government revised its country's Petroleum Code in 1998 to enact industry friendly measures to stimulate exploration by foreign companies. In October of 1999, the government of Senegal announced it has granted a concession covering much of the deepwater area to Vanco of Houston.

The Senegal deepwater basin is under-explored and contains all the elements needed for a significant exploration play. Recently reprocessed seismic data shows a deep basin containing areally extensive and organically rich source rock potential.

Basin modeling shows sufficient overburden has been deposited to mature potential source rocks. Contained within the overburden are inferred clastic reservoirs that show both stratigraphic and structural trap geometries. Deep-seated faulting provides the necessary migration pathway between potential source rocks and reservoirs. Amplitude anomalies, especially in Tertiary sediments, may be direct hydrocarbon indicators. The tectono-stratigraphic history of the basin is similar to that of other productive basins along the western margin of Africa. A progressive government has amended its Petroleum Code to make exploration and development attractive to the International Oil Industry.

References

A full reference list is too long to present here. Contact John Dombrowski in Houston, Texas for a complete listing at Tel: (281) 556-5656 or E-mail: dombrj@bellsouth.net

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