High potential in the offshore Nigeria transform margin

The African Transform Margin is a new major petroleum province, with offshore basins still relatively poorly explored, but the new Nigeria Transform Margin survey is revealing major clues to exploration success.

The Nigeria Transform Margin (NTM) GeoStreamer® MC3D survey, shot in association with the Department of Petroleum Resources, is situated in the west of Nigeria and positioned to cover the eastern extension of the post-transform turbidite fan play in open blocks OPL 312, 313 and 314. Petroleum systems are proved to be working through the presence of the Aje Field to the north, and the Hihon and Fifa discoveries to the west, in Benin.

Seeps and giants

Evidence of oil seeps and tar sands have long been recognized along the outcrop of Late Cretaceous sediments of the African Equatorial transform margin. The thickness of the sedimentary cover in the onshore and shelfal areas is insufficient for hydrocarbon maturity so these strata were followed offshore onto the shelf. The earliest fields were discovered in the late Sixties and early Seventies, such as the Seme Field (1968), the Lome Field (1970) and the Saltpond Field (1970). These fields were small, had limited upside potential, and did not produce the volumes that had been hoped for. Smaller independent operators continued to invest in the region, and further discoveries were made, with a number of fields being productive.

Things changed dramatically when the Jubilee Field, Ghana, was discovered in June 2007 by Tullow and partners, in a post-transform Turonian-age turbidite fan. The quality of both the oil and the reservoir was apparent very early, and the Jubilee Field was soon termed a world class field with exceptional upside potential. Further discoveries were made in the immediate vicinity in a series of turbidite fans and channels, such as Odum, which extended in the play stratigraphically up into the Campanian, then Tweneboa, which continued the Turonian play fairway to the west. The more recent discovery of the Venus Field, Sierra Leone, in 2009 by Anadarko confirmed the extension of the post-transform turbidite fan play fairway 1,100 km to the west.

Innovative acquisition and processing

The dual-sensor streamer acquisition used to acquire the survey offers improved pportunities, demonstrated fully in this area. The combination of pressure sensor and vertical particle velocity sensor allows wavefield separation, which is not possible in conventional data. By separating up-going pressure, down-going pressure and the velocity wavefield, it is possible to completely remove the receiver ghost whilst towing the streamers deeply enough to reduce noise and increase signal penetration. This acquisition method directly produces a significantly broader, un com promised seismic bandwidth and better signal to noise ratio over the entire time window. These advantages mean that interpretation of dual-sensor seismic data is easier, more reliable and seismic inversion techniques produce better results (Ozdemir, 2009 First Break; Reiser et al., 2010 EAGE, Dhelie and Reiser, 2011 AAPG). For this survey, relative inversion volumes of acoustic impedance, shear impedance and Vp/Vs ratio were produced using PGS’ Prospect Scanner AVO/AVA workflow.

The data was processed with a state of the art anisotropic pre-stack time processing workflow comprising wavefield separation, pre-stack time migration, automatic dense velocity picking, wavelet destretch, spectral offset balancing and trim-statics. Close collaboration between the geophysicists and geoscientists ensured preservation of the signal and frequency bandwidth at maximum offset and the flatness of the gathers. The resulting data is therefore optimized for reliable interpretation, AVO analysis and seismic inversion workflows.

Geological features clearly

Imaged The seismic stack clearly images the geological features which are characteristic of a transform margin. The high frequency content allows accurate imaging with high resolution, and the low frequency content ensures good penetration down to the Cretaceous targets and better reflection continuity.

Within the intervals of reservoir interest, the Turonian and Albian sandstones, there are many interesting fan and channel anomalies. Interpretation and auto-tracking of these anomalies is reliable, as reflectors are continuous and well resolved throughout the section.

One of the interpreted features is a large fan with a very strong amplitude response which dominates the 2,845 km2 survey. The most appropriate analogue to this is the Jubilee Field further west on the transform margin. The feature is also in the same Turonian hydrocarbon system as Jubilee, functioning locally up-dip at the Aje Field.

Another lead shows the combined use of full stack, relative inversion and angle stack information in order to better understand an anomaly. This was identified as a channel feature which includes a four way dip closure. The closure occurs on two levels, but when viewed with inversion attributes or an AVO screening attribute it is clear that the lower structure is higher risk, which is not so evident on the full stack image. Even working in an exploration context, using pre-stack information and attributes can provide a useful perspective and improve interpretation of the area. This area has many interesting anomalies, so use of pre-stack attributes can significantly help characterization and understanding.

Expectation of success

The African Transform Margin is a new major petroleum province, with offshore basins still relatively poorly explored. The majority of wells have historically been drilled on the shallow platform areas with the deeper slope and basin areas still frontier areas.

There is great expectation of exploration success in this extension to the prolific post-transform turbidite play fairway. The interpretation of this dual sensor seismic survey has reinforced this view. The proven reservoir interval has high potential with many anomalies, and the combination of high quality seismic data and attributes (AVO and pre-stack inversion) allows reliable interpretation and better understanding of these features.

ACKNOWLEDGMENTS: The authors would like to thank PGS and DPR for permission to show the data examples and the many colleagues at PGS Reservoir and PGS Data Processing who have contributed to this project.