WIDESPREAD 3D SEISMIC SURVEY COVERS MATURE FIELD IN GABON

D. Riley, M. Fleming
Western Geophysical
Houston

J. Delvaux
Elf Gabon
Port Gentil

The exploration potential of the Port Gentil region, characterized by some of the earliest petroleum discoveries in Gabon, continues to be of important interest today.

Existing infrastructure and close proximity of oil production and handling facilities can make even small discoveries profitable.

Available seismic data are of an older vintage (1974-82), recorded with low common mid-point (CMP) fold, They are critically void of coverage through the transition zone.

The geology is highly complex characterized by salt structures and strong tectonic activity. An intensive joint exploration and reservoir definition campaign is crucial to full evaluation of this area.

For these reasons, Elf Gabon, a subsidiary of Elf Aquitaine Production, decided to acquire 3D seismic over its Ile Mandji prospect and its environs.

This article describes the 3D survey conducted during 1992 and early 1993 over a mature oil field in and around Port Gentil and incorporating elements of land, transition zone, and shallow marine data acquisition-the 3D Mandji program.

THE AREA

The economic capital of Gabon's petroleum-based economy, Port Gentil and its 80,000 inhabitants are located on l'Ile Mandji Peninsula on the extreme west coast of Gabon, just south of the equator.

The area is bordered on the west by the Atlantic Ocean, where heavy swells are created by permanent longshore currents.

Port Gentil sits on the sheltered east side of the peninsula, overlooking the shallow water of the Cap Lopez Bay.

The land area is divided evenly between savanna, with occasional strips of forest, and mangrove swamp to the north near Cap Lopez. The area contains an international airport, oil refinery, commercial seaport, golf course, and many pipelines and gathering centers.

The Mandji 3D program covered a surface area of about 250 sq km.

It included marine acquisition in water depths of more than 8 m on the Atlantic side, a steep sandy beach transition zone, land acquisition over the peninsula, and shallow-water recording on the east side.

SURVEY PLANNING

The difficult operating environment necessitated careful presurvey planning to guarantee the successful acquisition and subsequent merging of data from a variety of seismic sources, detectors, and acquisition systems. Analysis of aerial photographs, large scale maps, and older seismic data was combined with a thorough scouting of the area.

This included bathymetric measurements to determine accurate water depths at proposed receiver locations and detailed observation of the most congested areas of Port Gentil to determine the feasibility of deploying seismic cables.

Fig. 1 shows the survey area, Fig. 2 the pattern of source and survey lines around Port Gentil.

Close liaison with the civil authorities and a public information campaign in the local media by Elf, combined with a thoroughly planned and executed permitting effort, ensured safe, trouble free operations in Port Gentil.

DATA ACQUISITION

The acquisition cycle incorporated state of the art Differential Global Positioning System (DGPS) technology.

Static DGPS provided first order location for the line opening exercises. Real time DGPS was used for primary vessel navigation and positioning for the transition zone and shallow marine operations.

Additionally, a man-portable backpack DGPS system was employed for shot and receiver positioning in urban areas.

Prior to data acquisition, a variety of parameters were tested to maximize data quality, minimize cultural interference, and ensure safe and practical working conditions.

Seven different explosive shotpoint patterns were tested, with results offering different solutions for each of the terrain types.

A 2D vibrator test line was recorded four times with different linear and nonlinear sweep parameters. Additionally, a particle motion velocity test was conducted to ensure the explosives and vibrator parameters chosen would not adversely affect the structures and citizenry of Port Gentil.

The transition zone data were acquired during the summer with an RF digital telemetry system and a 2,000 cu in. sleevegun array. Data were recorded with a nominal 30 fold CMP multiplicity by hydrophones offshore and geophones onshore.

Land data acquisition took place in a complex multiple-source, multiple-receiver environment. Data were recorded with a twisted pair cable digital telemetry system.

Explosives, vibrators, and sleeve-guns provided source energy, as dictated by the constantly changing terrain. IN 576 channel recording spread, configured as six receiver lines, acquired nominal 24 fold CMP coverage, using a mix of hydrophones, land geophones, and marsh case geophones.

Safety concerns dictated that explosives operations be limited to daylight hours. All shot hole patterns drilled to a depth of 7 m or more were preloaded, and explosives were fitted with charge anchors to prevent tampering. All shallow pattern areas were predrilled but were not loaded with explosives until the day they were detonated.

Vibrator operations occurred primarily from 10 p.m. through 4 a.m. to take advantage of the diminished cultural noise in Port Gentil. The vibrators also encountered fewer obstructions and were more mobile because of the reduced traffic late at night.

Airgun operations were conducted whenever operationally convenient. Deeper water and a lack of obstructions allowed night operations along the coastline.

Operations in the bay were tidally dependent and restricted to daylight hours.

RECEIVERS OFFSET

Due to the multitude of existing wellheads, flowstations, and pipelines, and with close proximity to a major urban area, a majority of the source and receiver locations were offset from their theoretical preplot positions.

Prior to the start of drilling operations on each swath, a preplot chart was generated to ensure proper CMP fold coverage and source receiver offset and azimuthal distributions. Deviations from the planned operations were given to the quality control geophysicists for confirmation and approval.

Observers' reports and camera monitors were scrutinized each day for proper completion of recording operations for each swath.

Postplot coverage charts were generated to reflect the actual data acquisition.

Additionally, quality control stack sections from each swath were generated in the field to verify data content and quality. Whenever possible, these brute stack lines were compared with previously acquired 2D data sections.

Frequent communication between the field analysts and processing center personnel assisted in the design of field processing flows and routines.

Ancillary data in an industry standard format were transmitted by model to the processing center to facilitate the assignment of the correct geometry to the seismic data.

CONCLUSION

Fig. 3 is a multiplicity plot showing final 3D coverage. The seismic section in Fig. 4 shows the seamless coverage from a blue-water marine environment across the transition zone and mangrove swamp, through the town of Port Gentil, and into the waters of Cap Lopez Bay.

Careful planning and quality control throughout the acquisition cycle combined with good communications among the participants effected a successful 3D survey in a complex environment.