AAPG/GSTT HEDBERG CONFERENCE

“Mobile Shale Basins – Genesis, Evolution and Hydrocarbon Systems”

June 4-7, 2006 – Port of Spain, Trinidad & Tobago

Mobile Shale Characteristics and Impact on Structural and Stratigraphic Evolution of the Niger Delta

R.W. Wiener¹, M.G. Mann¹, D.M. Advocate¹, M.T. Angelich¹, S.A. Barboza²

¹ExxonMobil Exploration Co.

²ExxonMobil Upstream Research Co.

Early Tertiary overpressured shale forms the mobile substrate that links deformation in the Niger Delta paired extensional-contractional system. This mobile shale has a major impact on the structural and stratigraphic evolution of the delta, along with sedimentation and gravitational controls. Understanding the properties and history of the mobile shale is important because of its impact on trap geometry, bed and fault seal risking, and reservoir distribution associated with structural development. 

Mobile shale in the Niger Delta is characterized by seismic transparency, low velocity, low density, and ductile deformation style, indicating that it is overpressured and mechanically weak. Early Tertiary deepwater facies in the mobile shale and its undeformed stratigraphic equivalents include mass transport complexes, distal turbidites, and hemipelagic sediments.  The mobile shale was deformed during the Oligocene –Recent forming the major structures of the Niger Delta.

The regional detachment in the mobile shale zone links extension, contraction, and strike-slip in this gravity-driven system.  This detachment can be thought of as the base of a ductile shear zone. The basal detachment is seismically observable and follows stratigraphic surfaces for long distances, separated by stratigraphic and structural frontal and lateral ramps. Locally within the mobile shale, reflective packages and brittle deformation style are observed, but the overall geometry of the structures involving thickened mobile shale indicates macroscopic ductile deformation.  Internally the mobile shale is more complexly deformed than overlying Oligo-Miocene strata, indicating a structural decoupling at the top of the mobile shale. This is locally evident in places where thrusts root in the upper detachment; in other places thrusts and normal faults penetrate the top of the mobile shale. The top of the mobile shale reflects the overall geometry of the major trap-forming structures in the Niger Delta.

Physical properties indicate that the mobile shale is overpressured. A regional 3d velocity model shows that the high relief detachment fold province in the “mobile shale” belt is a regional low velocity zone, and the outer fold-thrust belt is a zone of relatively high velocity. Individual structures show velocity sags under anticlines indicating low velocity overpressured shale. Gravity data show a similar relationship with low density in the inner mobile shale belt and high density in the outer fold-thrust belt. In addition, individual structures in the inner mobile shale belt show a striking correspondence of structural highs and gravity lows.  Gravity modeling suggests an overpressured shale-rich lithology best matches the gravity signal. Basin-wide 3D pressure modeling shows high effective stresses and overpressure within the mobile shale structural belt, and that overpressure initiated in the Oligocene and continued to the present.

Deformation of mobile shale has a major impact on the distribution of trap types in the Niger Delta. On the present day shelf, extensional growth faults overprint older detachment folds forming complex structures. The extensional systems generally developed on the downdip flanks of the older detachment folds. In addition, loading of sediments on the downthrown side of normal faults is accommodated partly by withdrawal of mobile shale substrate.  The downdip contractional part of the Niger Delta is characterized by mobile shale-cored ductile detachment folds and shear fault bend and fault propagation folds with a mobile shale core.  The structural style of contractional structures may be controlled by the thickness of mobile shale where thicker mobile shale results in a more ductile deformation style.  Locally, strike-parallel tear faults are an important component of the linked extensional – contractional system and serve to connect deformation at the margins of the delta lobes from the updip to the downdip. Mobile shale forms an important component of the deformation along tear faults. In areas of contractional overstep structures, there are mobile shale cores to the anticlinal features. In addition, mobile shale is present in the core of oblique contractional anticlines that are developed in areas of high lateral displacement gradients.

Structural features associated with mobile shale deformation controlled distribution of paleogeographic elements. The downdip limit of extension marks the paleo-shelf edge, and the downdip limit of contraction marks a major gradient change to undeformed lower slope / basin floor plain. These paleobathymetric features translated basinwards with time and controlled deposition such that distributive deposits formed in the unstructured basin plain, ponded minibasin and channelized deposits formed on the slope where contractional deformation prevailed, and lowstand deltas formed along the shelf margin.