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Intraslope basin stratigraphy documents the evolution of salt wall growth in the Lower Congo Basin, o�shore Angola Institutt for geovitenskap

Department of Earth Science

I. Early post salt (Albian)• Closely spaced synsedimentary normal faults• Parallel to regional strike• Initial redistribution of salt

Growthpackages

II. Upper Cretaceous• Early low relief salt ridges form at regular spacing• Deposition focusses in the centre of the intraslope basins and in the hanging wall of faults• Some early MTDs cross salt walls• First salt welds occur in the centre of intraslope basins

Saltwelds

IV. Miocene• Depocentres migrate toward the centre of the intraslope basins• Abundant channel lobe complexes con�ned by salt walls and major faults• Some intraslope basins sediment starved from the Early Miocene

V. Plio-Pleistocene• Focus of deposition migrates across to other side of most intraslope basins • Shut o� of dominant coarse clastic input• Some deposition of MTDs continues to be con�ned by salt walls and major growth faults

Sedimentstarved

con�nedturbiditic channel

lobe complexes

III. Paleogene• Salt welds widen• Focus of deposition in rim synclines• Most depositional systems cross salt walls• Some south �owing axial deposits (MTDs) and locally derived slumps

Summary and conclusionsIn this study of the post-salt sequence of the Lower Congo basin we use de-tailed mapping of 3D re�ection seismic data, time thickess and other attrib-ute maps to evalutate the structural evolution of elongate salt walls, from the late Cretaceous to the present.

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I. Albian

II. U. Cretaceous

III. Paleogene

IV. Miocene

V. Plio-Pleistocene

The post salt sequence is di-vided into 5 units based on time thickness variations and the type and dominant orientation of depositional systems within these units.

Salt weldswiden

MTDs sourcedfrom east

Axial MTDs

Locally sourcedslump

ReferencesBroucke, O., Temple, F., Rouby, D., Robin, C. Calassou, S., Nalpasand, T., Guillocheau, F., 2004, The role of deformation processes on the geometry of mud-dominated turbiditic systems, Oligocene and Low-er–Middle Miocene of the Lower Congo basin (West African Margin), Marine and Petroleum Geology, 21, 327-348Fort, X., Brun, J-P, and Chauvel, F., 2004, Salt tectonics on the Angolan margin, synsedimentary deforma-tion processes, AAPG Bull., 88, 1523-1544Oluboyo, A.P., Gawthorpe, R.L., Bakke, K. and Hadler-Jakobsen, F., 2013, Salt tectonizc controls on deep-wa-ter turbidite depositional systems:Miocene, southwestern Lower Congo Basin, o�shore Angola, Basin Re-search, 26, 1-24Valle, P.J., Gjelberg, J.G. and Helland-Hansen, W., 2001, Tectonostratigraphic development in the eastern Lower Congo Basin, o�shore Angola, West Africa, Marine and Petroleum Geology, 18, 909-927

AcknowledgementsAcknowledgements

Intraslope basin stratigraphy documents the evolution of salt wall growth in the Lower Congo Basin, o�shore Angola

Leo Zijerveld[1]*, Rob Gawthorpe[1], Ayo Oluboyo[1,2,3]

1 Department of Earth Science, University of Bergen, Allégaten 41, 5007 Bergen, Norway, *e-mail: leo.zijerveld@uib.no2 Present address: Statoil Research Centre, Statoil, Bergen, Norway3 PGS-Reservoir, Weybridge, UK

Institutt for geovitenskapDepartment of Earth Science

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I. Albian

II. U. Cretaceous

III. Paleogene

IV. Miocene

V. Plio-Pleistocene

The post salt sequence is di-vided into 5 units based on time thickness variations and the type and dominant orientation of depositional systems within these units.On the time tickness maps shown below the main dep-ositional centers in the intra slope basins are indicated by synclines. The most impor-tant active faults are also marked.

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Time thickness

V. Plio-PleistoceneShut o� of dominant coarse clastic input (turbidi-ty currents). Deposition of MTDs continues to be con�ned by salt walls and major growth faults. Focus of deposition shifts to other side of most in-traslope basins.

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IV. MioceneDepocentre axes migrate toward the centre of the intraslope basins. Abundant channel lobe com-plexes occur throughout this unit, sourced from the north and mostly con�ned by salt walls and major growth faults. Some intraslope basins are sediment starved due to con�nement of axially �owing turbidity currents from the Early Miocene onwards.

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III. PaleogeneSalt welds widen, focus of deposition in rim syn-clines. Abundant evidence for depositional sys-tems (dominated by mass transport deposits MTD) crossing salt walls from east to west. Some south �owing axial deposits (MTDs) and locally derived slumps.

Time thickness

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II. Upper CretaceousFocus of sedimentation in the centre of the in-traslope basins and in the hanging wall of a limit-ed number of faults in the north. Early low relief salt ridges form at regular spacing. Some evi-dence suggests early depositional systems or-thogonal to salt walls �owing towards W. By the end of this stage, the �rst salt welds occur in the centre of most intraslope basins.

Time thickness

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I. Early post salt (Albian)Closely spaced synsedimentary normal faults with associated growth sequences. These are oriented parallel to regional strike and associated with ini-tial redistribution of salt.

Time thickness

Faults

Flow obstucted

Sediment input

RGB blend of high de�nition frequency decompos-tion (GeoTeric) on a surface in the lower part of Unit III (Paleogene). Mottled appearance corresponds to chaotic seismic facies and is characteristic of mass transport deposits (MTD). Direction of transport is from the east. Al-though obstructed by the salt walls, in many cases these deposits also cross the salt walls.

Sediment

input

Sediment

input

RGB blend high de�nition frequency decompostion (Geoteric) on a surface in the upper part of Unit IV (Miocene). Channel lobe complexes con�ned by salt walls and major growth faults. Intraslope basins in the east do not have active sedimentation.

Sediment starved

Institutt for geovitenskapDepartment of Earth Science

Intraslope basin stratigraphy documents the evolution of salt wall growth in the Lower Congo Basin, o�shore Angola

INTRODUCTIONPost-salt stratigrapy in the Lower Congo Basin provides a detailed insight into the evolution of elongate salt walls from the late Cretaceous to the present. A large number of horizons were generated from 3D re�ection seismic sur-veys using a commercial, semi-automatic, geo-model based interpretation technique. Visualising these horizons and associated time thickness, variance and amplitude maps allows us to analyze structural, thickness and depositional trends in the intraslope basins and their implications for the evolution of the salt walls through time.

GEOLOGICAL FRAMEWORKThe evaporites of the Aptian-Barremian Loeme Formation in the Lower Congo Basin are overlain by carbonates, marls and clays of the Albian to Late Cretaceous age followed by a thick sequence of predominantly clastic sedi-ments (Late Cretaceous to present) (Broucke et al. 2004). The structural style in the post salt section in this area is typ-ical for gravitational gliding with extensional and compressional structures in the upslope and downslope domaines respectively. This deformation started during the Albian and continues to the present day (Valle et al. 2001, Fort et al. 2004, Oluboyo et al. 2013) as evidenced by the presence of active structures on the present day seabed.

In an earlier study, Oluboyo et al. (2015) described the interaction of submarine gravity �ows and the evolving salt-related topography in the study area during the Miocene. We extend this work to the entire post-salt sequence and adopt the same no-menclature for the main structural elements.

METHODOLOGY

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NW SEIntraslope basin 4 Intraslope basin 1Intraslope basin 3Intraslope basin 3 Intraslope basin 0

SubsaltIII. PaleogeneAptian saltI. Early post salt (Albian)

II. Upper Cretaceous

IV. MioceneV. Plio-Pleistocene

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Study area

AngolaN

Saltwall domainIsolated diapirs and faults

Intraslope basin 2

Intraslope basin 3

Intraslope basin 4

Intraslope basin 1

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Main structural elements at top MioceneFrom Oluboyo et al. 2013

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Saltw

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Isolated diapirs and faults

Geomodel based interpretationAttribute analysis

Frequency decompositionand RGB blending

Horizon stackCorrelate patches

Model grid

A detailed interpretation is performed using Paleoscan. After calculating an initial model grid based on the location of peaks and troughs, grid nodes are connected auto-matically and the resulting patches can be further correlated manually. The end result is a horizon stack that can contain any number of surfaces (200 in this study).

Horizons are imported into Petrel for detailed analysis of time thickness

and seismic attributes including RMS amplitude and variance.

Frequency decomposition and RGB blending in GeoTeric allows for more detailed analysis of sedimentological features.

Locally sourcedslump

Leo Zijerveld[1]*, Rob Gawthorpe[1], Ayo Oluboyo[1,2,3]

1 Department of Earth Science, University of Bergen, Allégaten 41, 5007 Bergen, Norway, *e-mail: leo.zijerveld@uib.no2 Present address: Statoil Research Centre, Statoil, Bergen, Norway3 PGS-Reservoir, Weybridge, UK