I N T E G R A T E D G E O P H Y S I C S C O R P O R A T I O N

FOOTNOTESIGC FOOTNOTE SERIES October 2008

ON INTERPRETATION

GABON REGIONAL STRUCTURAL FRAMEWORK

Isostatic Residual Gravity with Interpretation

Figure 1

OVERVIEW

GEOPHYSICAL& GEOLOGICALDATA

STRUCTURALINTERPRETATION

This regional interpretation of western Gabon is based onour interpretation of satellite-derived gravity data offshore,digital topography onshore, and a compilation of publisheddata both offshore and onshore. Two maps are shown forcomparison and correlation: Figure 1

displays the horizontal gradient of free-airgravity plus the onshore topography, . Important featuresinclude a series of basins and high blocks trendingsubparallel to and subnormal to the coastline. Numerousfault/fracture zones are seen to extend northeast fromoceanic crust areas of the South Atlantic onto the offshoreshelf and the onshore coastal areas of Gabon. One majorfault/fracture zone, the N'Komi, divides coastal Gabon intotwo main structural provinces: the south and the north.

The coastal plain widens northward from the Gabon-Congoborder to the Gabon-Equatorial Guinea border and isbounded to the east by basement outcrop. It contains fivenamed basins, two of which extend into the offshore. Thecoastal plain is also the site of five named high blocks, one ofwhich extends into the offshore.

Gabon's offshore shelf contains five significant basins andfive major high blocks. Those closest to the coastline arepoorly defined by satellite-derived gravity due a normal lossof resolution close to shore. Northeast-trending gravityanomalies or anomaly offsets confirm and/or further definethe fault/fracture zones noted in much of the literature,although location of some features has been modified fromthe literature on the basis of our gravity interpretation. Amajor hinge zone and the ocean-continent crust boundaryare interpreted from the gravity data. Six high-amplitudegravity maxima in northern Gabon are interpreted asvolcanic plugs, two of which are named.

Primary input data for the interpretation consisted of publicsource satellite-derived free air gravity

and bathymetry-topography grids (Sandwell 10.1).The free air gravity was reduced to Bouguer gravity and wethen computed a Moho-Effect (isostatic compensation)gravity grid from a 3D gravity inversion model using theAiry-Heiskanen computation method. This grid was thensubtracted from the Bouguer grid to produce the isostaticresidual gravity map. By use of both Bouguer and Airycomputations, anomalies due to seafloor topography and tothe crust/mantle interface have been attenuated, and theresulting residual maps have enhanced anomalies ofgeologic interest. A horizontal gradient operator wasapplied to the free-air gravity data to generate a map whichcould emphasize geologic boundaries or major faults.

Several published papers map and describe a variety ofgeologic features from onshore basement outcrop to offshoreoceanic crust. However, the existence, location, and trend ofsome features are inconsistent from paper to paper, and notalways consistent with data from the relatively new andcomplete satellite-derived gravity and its enhancements.These inconsistencies made the geologic compilationprocess more challenging, but in the end the best correlationto the gravity residual data took precedence for ourinterpretation.

Two papers include seismic data which straddles the N'KomiFault Zone. They provide depths from three seismicrefraction profiles and sections from a deep seismic projectand thereby contributed to our regional interpretation.

The interpretation area, shown on the index map, liessoutheast of the Cameroon Volcanic Zone and west of theinterior Congo Craton. Coastal Gabon can be divided intotwo main structural provinces, south and north, separated bya major wrench or transfer system, the N'Komi Fault Zone.Several published studies propose that both provinces aredominated by rift tectonics, with rifting progressivelyyounger from south to north. Our regional geologic featuresinterpretation includes features common to both provinces,as well as local structures found in each.

Oceanic-Continental Crust Boundary

The crust boundary is interpreted to have an arcuate shape,first trending northwest parallel the shoreline and along analignment of residual gravity maxima, from the Gabon-Congo offshore border to the N'Komi Fault Zone. FromCongo to the N'Komi the crust boundary is locally offset bynortheast-southwest shears. It has a major dextral offset(shoreward) at the N'Komi, then trends northerly alonganother alignment of residual maxima as far as the Gabon-Equatorial Guinea border. The boundary in the northernsegment is also locally offset by northeast-southwest shearfaults.

Atlantic Hinge Zone

The Atlantic Hinge Zone, or shelf-slope break, south ofN'Komi Fault Zone can be mapped along the seaward edge ofa northwest-southeast residual gravity maximum trend. It canalso be traced along a high-amplitude trend of the horizontalgradient of free air gravity. Residual and gravity gradientanomalies offshore suggest that a hinge zone could continuenorth of the N'Komi before either turning northeast betweenLoiret and Fang Fault Zone or trending north to theEquatorial Guinea border.

displays regionalgeologic features over-posted on an isostatic residual gravitymap and Figure 2

(Sandwell version16.1)

Figure 2

Free-Air Gravity Horizontal GradientShaded Relief Topography

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Basement Outcrop and Subcrop

South Gabon

North Gabon

CONCLUSIONS

SELECTED REFERENCES

Basement outcrops on the eastern edge of the coastal plainfrom the Gabon-Congo border to the Gabon-EquatorialGuinea border. The outcrop areas include major provincessuch as the Congo Precambrian Craton, NyangaPrecambrian Basin, Franceville Precambrian Basin,Mayumba Range Fold Belt, and Eburnean Fold Belt.Although the province boundaries generally conform tofeatures in the digital topography, further study might showthat a closer fit is possible.

Our depth to basement contours increase seaward from 1km (subsea) near outcrop to 8 km in both the northern andsouthern provinces. Much of the contouring is based onpublished maps, with subsequent local modifications wheredictated by gravity interpretation. Ten discrete basementdepth values, ranging from 6 to 12 km, were obtained frompublished refraction seismic data and provide additionaldepth control offshore.

The regional geologic interpretation highlights 13 majorstructures and features in South Gabon. They are the SouthSubbasin, Kaba High and M'Bya Terrace, Banio Graben,Lucina Uplift, Mayumba Spur, Vera Graben, Gamba-Bigorneau High, Sette Cama High, Dentale Trough, SouthGabon Basin, Rembo Kotto-Ilongo High, and N'Komi FaultZone

The regional geologic interpretation highlights 14 majorstructures and features in the northern Gabon area. Theyinclude the Dianongo Trough, North Basin, Atlantic Basin,Lambarene Horst, Interior Basin, GLB Fault, Kobe Fault,Ekouata Fault, Loiret Plug, Gombe Plug, North Subbasin,Fang Fault Zone, Ascension Fault Zone, and Port GentilFault Zone. The locations of four apparent, but unnamed,igneous plugs are shown as questioned.

An important conclusion is that if appropriately compiled,processed and interpreted, public source data can be used toprovide an early-stage and cost-effective aid to regionalgeologic understanding of a large area. Details can beverified, added, or reinterpreted as data from new and moreintensive geologic/geophysical programs becomeavailable.

A second conclusion based on our interpretation is that in theoffshore and coastal provinces of Gabon, as in many basinalareas, basement deformation has strongly influenced basinsedimentary structure and stratigraphy. Major geologicfeatures, such as fault zones, structural highs, and depocenters---critical to forming a favorable environment for oil and gasaccumulation---are seen as basement-related and are presentboth onshore and offshore Gabon. Our interpretation of theisostatic gravity residual and horizontal gradient of free-airgravity gradient and other data enhancement maps, whenintegrated with published maps and data, leads not only to aviable offshore extension of three distinctive onshore basementprovinces, but also provides regional guidance for planning offuture hydrocarbon exploration plays.

Bessoles, B., 1977. Geologie de l'Afriquele craton ouestAfricaine, BRGM Memoir No. 88

Rabinowitz, P. D., 1982. Gravity measurements borderingpassive continental margins, in Dynamics of Passive Margins,Geodynamics Series, vol. 6,AGU

Simpson, R. W. et al, 1983.AIRYROOT: a FORTRAN programfor calculating the gravitational attraction of an Airy isostaticroot out to 166.7 km, USGS Open-file Report 83-883

Teisserenc, P. and J. Villemin, 1989. Sedimentary basin ofGabongeology and oil systems,AAPG Memoir 48

Vidal, J. et al, 1975. L'exploration petroliere au Gabon et auCongo, in Transactions, Ninth World Petroleum Congress

Wannesson, J. et al, 1991. Structure and evolution of adjoiningsegments of the west African margin determined from deepseismic profiling, in Continental Lithosphere: Deep SeismicReflections, Geodynamics Series, vol. 22,AGU.