Figure 1 Extensional and Transform Fault Interaction, Influence on the Upper Cretaceous Hydrocarbon System, Equatorial Margin, West Africa. Presented to the 10th PESGB/HGS Conference on Africa E + P September 7-8, 2011, Queen Elizabeth II Conference Center, London By K A Nibbelink Hyperdynamics Corporation With contributions from Steve Barrett, Edward Shaw, Don Rice, Granville Smith, Bonnie Milne, Brian Payne, Sandra Marek and Hande Adiyaman Hyperdynamics Corporation 1
Figure 2 The interaction between the major transform faults or fracture zones and the extensional or rift related faults produce significant bathymetric relief that influences the Upper Cretaceous hydrocarbon system along the Equatorial Margin of West Africa. A generalized model will be presented that defines the preferred areas for richer source and thicker reservoir deposition. New production in Western Ghana and recent discoveries in Sierra Leone and Liberia demonstrate the hydrocarbon system. 2
Figure 3 Onshore geology and offshore gravity map showing the major transform faults or fracture zones that cross the Atlantic as they tip out into the African continent. The production in Eastern Ghana occurs south of the St Paul Fracture Zone and the recent discoveries in Sierra Leone and Liberia occur south of the Four North Fracture Zone. The areas south of the Sierra Leone Fracture Zone in Southeastern Guinea and Romanche Fracture Zone in Eastern Ghana are the currently being explored. Additional, smaller fracture zone occur between these major fractures zones. The next slide is a generalized model showing how the interaction of these transform faults and the extensional faults influence the Upper Cretaceous hydrocarbon system. 3
Figure 4 Generalized model for the transform/extensional fault intersection influence the Upper Cretaceous hydrocarbon system. The topographic or bathymetric low on the south side of the transform fault and the west side of the extensional faults create an embayed area with the potential to develop richer source rocks during the Upper Cretaceous anoxic events. This low embayed area may also be the focus of the thicker deep water sandstone reservoir sections. The high on the north side of the transform fault is a headland area that may have thinner or lesser quality source and reservoir. The model may be used to predict the most robust areas for the Upper Cretaceous hydrocarbon system along the Equatorial Margin of West Africa. However, additional smaller lateral displacement transform faults occur and may also improve the quality of the hydrocarbon system. 4
Figure 5 Stratigraphic section from the shelf to deep water in southeastern Guinea. The sequences defined by the only offshore well, GU-2B-1 drilled on the shelf in 1977 and the recent 2D and 3D seismic data in deep water as related to the Haaq et al world wide sea level curve. The major sequence boundaries and flooding surfaces that can be mapped around the whole South Atlantic are also defined by this data set. From the top they are: the 30 my Oligocene low stand of sea level sequence boundary, SB in orange, the 67 my flooding surface, FS of the top Cretaceous in green, the 69 my Maastrichtian SB in red, the 85 my Santonian SB in purple, the 90 my Turonian maximum flooding surface, MFS in teal, the 98 my Albian SB in blue and the 110 my Aptian SB in red at the bottom of the section. The major source sections include the 108 my Lower Albian ocean anoxic event and the 90 my Turonian ocean anoxic event. Since significant upwelling of organics occurs on the west coast of continents, additional Cenomanian and Campanian source interval have been defined in the West Africa. The major reservoir sections are shelf, slope and deep water sandstones associated with each of the major sequences boundaries. Above the syn-rift section, the Albian SB represent a significant low stand of sea level that is then on lapped by the back stepping, transgressive Cenomanian sandstones and finally capped by the maximum flooding surface of the Turonian source and seal. Progradation of the Upper Cretaceous section above the Turonian maximum flood is punctuated by several low stand sequence boundaries. During these low stands of sea level sandstones are pumped into the deep water producing excellent reservoirs. 5
Figure 6 Erbacher and others document two major oceanic anoxic events in the South Atlantic, OAE, the 108 my Lower Albian, OAE 1b and the 90 my Turonian, OAE 2 section. The result is deep water organic intervals that deposited across the entire restricted South Atlantic basin. These organic intervals of the South Atlantic can be mapped on seismic data as troughs, from a decrease in acoustic impedance, since the organic intervals have lower density and slower velocities. The location of the GU-2B-1 in Guinea is shown on the index map. 6
Figure 7 The GU-2B-1 well was drilled on the shelf in Guinea in 1977. The five major sequences of the Upper Cretaceous are documented, the green, 67 my Top Cretaceous FS, the red, 69 my Maastrichtian SB, the purple, 85 my Santonian SB, the teal, 90 my Turonian MFS and the blue, 98 my Albian SB. The organic interval in the Turonian contains 2.5% TOC and has a vitrinite reflectance of.65, Ro. The points in the deeper part of the section that have Ro values of 1.95 are interpreted as reworked organic material. This Turonian deep water source in typically contains moderate organic content an is immature on the shelf and becomes much richer and more mature in the deep water of the South Atlantic. 7
Figure 8 Extensive basin modeling of the Turonian and Lower Albian source rock intervals was completed by Sandra Marek, Hyperdynamics. Over 250 points were modeled along 2D seismic profiles and resulted in the Turonian Maturity Map where the green areas are greater than.7 vitrinite reflectance, Ro. The heat flow of 1.55, which correlates to a geothermal gradient of 35.5 degrees C per 100 meters was used in the model. The 2D time seismic was data converted to depth with GU-2B-1 well sediment velocity below mud line, BML for less than 1 sec of water and then 95% of stacking velocity, BML was used for water depths of great than 1 sec. The graphic of depth in meters BML versus Ro is shown for the modeled points along with the measured data at the GU-2B-1 well. Several oil seeps typed as Upper Cretaceous marine oil are located on this block, the closest seep is shown on the northern part of the map. 8
Figure 9 The regional stratigraphic for the South Atlantic is well defined by the GU-2B-1 well and the 2D and 3D seismic data. The following 5 sequences are the similar to the stratigraphic section present in the recent discoveries in Sierra Leone and Liberia and productive area in Western Ghana. They are the top Cretaceous 67my MFS in green, Maastrichtian 69my SB in red, Santonian 85my SB in purple, Turonian 90my MFS in teal and Albian 98my SB in blue. The Albian low stand of sea level represents a topographic maximum formed by rift structures and erosion. The transgressive Cenomanian reservoir sequence on laps and is then capped by the maximum flooding surface of the Turonian source and seal. Progradation above the Turonian MFS includes the major Santonian and Maastrichtian low stands of sea level which produce excellent deep water reservoirs. 9
Figure 10 Generalized Upper Cretaceous shelf to deep water system in southeastern Guinea. This map shows the large east-west transform fault and the interaction with the northwest to southeast oriented extensional faults. A lateral ramp and a large graben occur where the main rift faults are offset. This creates a focus area for deposition of the Upper Cretaceous sediments. A generalized outline of the Upper and Lower Cenomanian sandstones and some of the deep water fans are shown in yellow and orange. The Upper Cretaceous shelf sandstones in the GU- 2B-1 well averages over 30% porosity. 10
Figure 11 Isochron of the Upper Cretaceous from the Turonian MFS to the Top Cretaceous. The Upper Cretaceous deep water sandstones are interpreted south of the major transform fault in the thick, blue areas. The black lines show bathymetry in kilometers. Just north of the A-A line, a thin, green area occurs because the Upper Cretaceous section is eroded by later canyons. The next slides are a dip seismic section A-A and a strike seismic line drawing B-B. 11
Figure 12 Dip seismic section in time showing the shelf to deep water section in southeastern Guinea. The five major surfaces defining the Upper Cretaceous section are: Green 67my MFS Top Cretaceous, Red 69my SB basal Maastrichtian Unconformity, Purple 85my SB Santonian Unconformity, Teal 90my MFS Turonian and the Blue 98my SB Albian Unconformity. The Albian low stand of sea level represents a topographic maximum formed by rift structures and erosion. The transgressive Cenomanian reservoir sequence on laps and is then capped by the maximum flooding surface of the Turonian source and seal. Progradation above the Turonian MFS includes the major Santonian and Maastrichtian low stands of sea level which produce excellent deep water reservoirs. An inset of a PSDM Kirchhoff Far offset depth section is shows the dramatic improvement from 2D seismic to PSDM 3D seismic data. This depth section also shows the Upper and Lower Cenomanian on lapping sandstones above the Albian and then sealed by the Turonian flood. 12
Figure 13 Strike seismic section line drawing in time showing the deep water section in southeastern Guinea. The five major surfaces defining the Upper Cretaceous section are: Green 67my MFS Top Cretaceous, Red 69my SB basal Maastrichtian Unconformity, Purple 85my SB Santonian Unconformity, Teal 90my MFS Turonian and the Blue 98my SB Albian Unconformity. The deep water sandstone reservoirs are shown in yellow. 13
Figure 14 Recent Upper Cretaceous discoveries in Sierra Leone and Liberia. The Geologic map shows the general location of the transform and rift faults and provides a regional context for the exploration block map and cross section from the Tullow, 2011 half-yearly results presentation on Sierra Leone and Liberia. The Tullow map shows the Venus and Mercury discoveries and the additional deep water fan exploration targets. The five major surfaces defining the Upper Cretaceous section are added to the Tullow cross section, Green; Top Cretaceous, Red; Maastrichtian, Purple; Santonian, Teal; Turonian, Blue; Albian. The deep water sandstone reservoirs are shown in orange. 14
Figure 15 New oil production and recent Upper Cretaceous discoveries in Western Ghana. The Geologic map shows the general location of the transform and rift faults and provides a regional context for the exploration block map from Anadarko s presentation to the Enercom conference, 8/17/2011 and cross section from the Tullow, 2009 half-yearly results presentation on Western Ghana. The Anadarko map shows the Jubilee production area and numerous nearby discoveries and additional deep water fan exploration targets. Five major surfaces defining the Upper Cretaceous section are added to the Tullow cross section, Green; Top Cretaceous, Red; Maastrichtian, Purple; Santonian, Teal; Turonian, Blue; Albian. The deep water sandstone reservoirs are shown in orange. 15
Figure 16 The Upper Cretaceous hydrocarbon system along the Equatorial Margin of West Africa has become the focus of significant exploration following the success in Western Ghana. The discoveries in Sierra Leone and Liberia demonstrate success can be repeated along trend. The interaction of the major transform and rift faults are interpreted to have an influence on the hydrocarbon system by providing bathymetric lows or embayed areas with richer source and better deep water reservoirs on the south side of the transform faults where they tip out into the rift related extensional fault systems. From the displays just presented and using this model of transform/rift fault interaction the next logical places to explore this hydrocarbon system include the where the Romanche Fracture Zone tips out in Eastern Ghana and where the Sierra Leone Fracture Zone tips out in Southeastern Guinea. 16