Offshore Somalia – Defining crustal type and its implications for prospectivity
September 11, 2017

Spectrum’s Hannah Kearns, Neil Hodgson & Karyna Rodriguez alongside Douglas Paton (University of Leeds) and Abdulkadir Abiikar Hussein (Ministry of Petroleum and Mineral Resources (MOPMR), Federal Government of Somalia) have developed this integrated study, using gravity modelling, to reveal a new understanding of the crustal structure offshore Somalia.

Prior to 2014, the geology of offshore Somalia was largely unknown. Two 2D seismic acquisition programmes undertaken in 2014 and 2016 have facilitated the development of new theories and understanding of the passive margin. We integrate long offset 2D seismic with wells and potential field data to assess the structural configuration of the margin, map the continental to oceanic crust transitional boundary (COB), and discuss the implications on exploration potential.

fig1

Fig 1 – Free air gravity map.

Regional Geology
Several regional tectonic events have shaped the Somali margin, associated with rifting of the Gondwana supercontinent from the Carboniferous to Early Jurassic and opening of the Indian Ocean in the Cretaceous. These events have divided the offshore area into three basins, each with their own distinct structural and sedimentological regimes: from north to south, Obbia, Coriole and the Juba-Lamu Basins [Figure 1]. Multiple plate reorganisations have led to rift-related normal faulting creating the basin geometry and accommodation space for deposition of Jurassic restricted marine sediments, overprinted by strike-slip movement from the oblique drift of Madagascar, and Cretaceous to Palaeogene compression and inversion. A variation in the nature and distribution of sediments along the margin can be attributed to differences in sediment supply, depositional style, accommodation space, sub-basin tectonics, and post-depositional processes.

The Obbia Basin is characterised by continentally-derived Karoo pre-rift sediments, rotated by large Jurassic faults. Rifted half-grabens are filled with syn-rift deposits. The Mesozoic post-rift consists primarily of reef build-ups, carbonate platforms and reworked carbonate sediments.

The Coriole and Juba-Lamu Basins are overlain by a thick post-rift deltaic sequence with multiple décollement levels creating toe thrust and gravity slide domains throughout several levels. Gravity sliding occured in several discrete episodes during the Cretaceous and Cenozoic. Large flower structures occur towards the Auxilliary Rescue and Salvage (ARS) Fracture Zone to the east. Increasing numbers of volcanic sills and dykes are associated with this fracture zone. Scarps can be seen in the oceanic crust outboard where transform fault movement has juxtaposed oceanic crusts of different ages.

CRUSTAL STRUCTURE & COB
Offshore Somalia is a non-volcanic rifted margin as it displays limited rift-related volcanism, and no seaward dipping reflectors (SDRs) have been identified. The low level of rift-related magmatism may be an indication of a slow spreading rate, supported by the appearance of a highly faulted oceanic crust on seismic data. A marked difference in the crustal architecture is observed between the Obbia and Juba-Lamu Basins, and we relate this to variations in structural deformation styles generated by the Gondwanan rift and oceanic transforms.

In the Obbia Basin, seismic data indicate the continental crust exhibits a wide 200km necking zone [Fig 2]. The gravity signature is consistent with hyper-extension of continental crust, transitioning through to exhumed mantle and oceanic crust. Hyperextended crust is weak and deformation-prone due to crustal thinning and serpentinisation. A relatively high geothermal gradient is anticipated due to high crustal attenuation and shallow Moho.

In the Juba-Lamu Basin, the necking zone is narrower and oceanic crust and the COB are interpreted to lie close to the base of the continental slope.

Fig2

Hydrocarbon Potential
Possible regional source rocks are present in Permo-Triassic Karoo sediments, synonymous with the deposition of a world class source rock observed from Yemen to South Africa. Potential may also exist in syn-rift sediments deposited in Early Jurassic rifted half grabens, and in Cretaceous sediments in restricted marine embayments where transform movement created barriers to oceanic circulation.

In the Coriole and Juba-Lamu basins, shale décollements may comprise candidate hydrocarbon source rocks. Burial causes maturation of organic-rich layers, reducing the viscosity and increasing pore pressure sufficiently to facilitate low angle displacement. Multiple stacked source rocks may therefore be present throughout the Cretaceous and Cenozoic sediments.

To understand regional heat flow and source rock maturity, model gravity response profiles have been generated for the Obbia and Juba Lamu Basins. The thermal maturity window is predicted to shallow towards the north, corresponding with higher geothermal gradients observed in onshore wells. The hyper-extended continental crust model is associated with high heat flow, placing shallow Early Jurassic syn-rift and Permo-Triassic Karoo sources in the peak oil-generating window. The Juba-Lamu Basin is interpreted to be underlain by Jurassic oceanic crust with a lower heat flow. This puts the more deeply buried Cenozoic, Late Cretaceous and syn-rift Jurassic source rocks in the oil-generating window.

Conclusion
An integrated study supported by gravity modelling reveals a new understanding of the crustal structure offshore Somalia. Hyper-extended continental crust in the northern Obbia Basin is associated with high heat flow and shallow syn-rift source in the peak oil-generating window. Oceanic crust underlies a thick clastic sequence in the Juba-Lamu Basin in the south, where low heat flow puts the more deeply buried post-rift and Jurassic syn-rift source rocks in the oil window. The Somalian margin is sure to yield numerous exciting oil plays at multiple stratigraphic levels.

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Offshore Somalia – Defining crustal type and its implications for prospectivity