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Block M - Equatorial Guinea

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2004 Seismic Interpretation and Prospects

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Regional Geological Setting

The Rio Muni Basin forms part of the West Africa margin basin system formed during continental separation and the creation of the South Atlantic Ocean which occurred through the Cretaceous and Tertiary periods. The basin contains a thick section of mid Cretaceous and younger sediments lying on an early Cretaceous rifted section that transitions to oceanic crust.

 

Block M lies in the distal part of the Rio Muni basin which is totally unexplored by drilling. The proximal part of the basin by contrast has seen extensive recent drilling which has defined the geological sequence and petroleum systems. The syn-rift section consists of Late Barremian to Mid-Aptian terrestrial clastics and lacustrine shales with extensive rollover structuring to large listric faults updip. This is overlain by a transitional sequence of locally developed salt and good quality marine oil prone source rocks of Aptian age. An Albian (Madiela) carbonate platform is followed by a Cenomanian-Turonian sand shale sequence which contains a major source rock interval. This post-salt sequence commonly forms extensional rafts detaching on the Albian-Aptian shale or salt sequence. An extensive Senonian section onlaps the earlier rafted topography and contains channel and turbidite sands that form reservoirs to petroleum deposits. The latest drift sequence is dominated by a thick wedge of Miocene to Recent clastics. Commercial oil accumulations have been typed to both the Aptian and Cenomanian-Turonian sources. Productive reservoirs are proven in Santonian and Campanian clastic reservoirs with other possible reservoirs throughout the section.

In the distal part of the basin, the pre-drift section is unknown but could logically be oceanic crust.  It is likely that this will be overlain by Aptian and younger section with both source and reservoir potential. Regional seismic profiles and outcrop geology show a thick (up to 10kms) sedimentary sequence extending all the way to and beyond the islands of Sao Tome and Principe. Outcrops on these islands have good quality quartz matrix reservoirs of early Tertiary age (?) with oil seeps that have been typed to Cretaceous age source rocks.  

Block M is located approximately 100 kms from the Rio Muni coast, midway to Principe island and in water depths ranging 1800 2300m.

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2004 Seismic Interpretation and Prospects

In 2003, 2000km of 2D seismic was acquired over block M. The new 2D seismic has been interpreted and tied into the existing data base and several prospects and leads have been identified (Figure 1&2)

Prospect 1, is a giant basement high with four way dip closure, mapped on the southern side of the significant regional feature known as the Kribi Fracture zone which trends north east south west across Block M and beyond. This basement high covers a very large area with separate culminations. Several possible prospective horizons are draped above the basement high.

This prospect 1 has been mapped at the Cretaceous 1 level, which is postulated to be Campanian age. This is age-equivalent to the main reservoir at the Ceiba oil field, which is located approximately 75 km to the east. The Cretaceous 1 level has four-way dip closure with 60ms of relief and it covers approximately 67 sq. km..

The interval between Basement and the Cretaceous 1 level is characterised over the area of closure by high amplitude events which may indicate the existence of hydrocarbons in this interval. These events occur over a vertical interval of at least 300 milliseconds (500 meters) and may represent stacked reservoirs. The water depths over the Prospect 1 area range from 2100m to 2200m.

This prospect could easily have been sourced by hydrocarbons migrating from source rocks adjacent and north of the basement high. These attain depths of 5,000m or more below seabed and have sufficient Miocene and younger burial to be oil generative with reasonable heat-flow assumptions. 

Several other significant prospects have been identified. To the east and north of Prospect 1, three compacted channel features have been identified in the Tertiary section.

 

 

 

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