Offshore Somalia: Defining crustal type and its implications for prospectivity

Transcription

1 Offshore Somalia: Defining crustal type and its implications for prospectivity Hannah Kearns 1, Douglas Paton 2, Neil Hodgson¹, Karyna Rodriguez¹, Roxana Stanca², Abdulkadir Abiikar Hussein³ 1 Spectrum Geo, United Kingdom, 2 University of Leeds, United Kingdom, ³Ministry of Petroleum and Mineral Resources (MOPMR), Federal Government of Somalia , hannah.kearns@spectrumgeo.com 1. INTRODUCTION Exploration in Somalia began onshore in the 1950s with the drilling of several onshore wells, but no notable economic discoveries. Tragically, the collapse of the Government in 1991 ushered in a long period of political instability, where Somalia remained inaccessible to exploration companies for 25 years. During this time, the majority of Somalia s legacy geological and geophysical data were lost, or destroyed. Since 2012, Somalia has experienced a period of relative political stability, following the inauguration of the Federal Government of Somalia. Positive efforts by the Government have been made to encourage hydrocarbon exploration activity, and seismic companies have successfully acquired new 2D seismic exploration data. Two 2D seismic acquisition programmes undertaken in 2014 and 2015/16 have facilitated the development of new theories and understanding of the passive margin[1]. In this study, we integrate long offset 2D seismic data with offset well information and potential field data to assess the structural configuration of the margin, map the continental to oceanic crustal transition boundary (COB), and we discuss the potential implications on exploration and prospectivity of the offshore Somali margin. Only gas has been found in commercial volumes offshore East Africa to date, however the potential for a large oil discovery is ever-present and there is encouraging evidence for oil-mature petroleum systems at multiple stratigraphic levels offshore Somalia. This is supported by satellite observations of numerous oil slicks along the coast, thought to be unrelated to human activity. 2. METHODOLOGY Both the 2014 and 2015/16 2D seismic datasets have been used in this study. The 2014 dataset consists of 195 lines with a record length of 9s TWT, with variable line lengths extending as long as 230km offshore, and line spacing of approximately 10km; total line length is 20,500km. The 2016 dataset consists of 76 lines with a record length of 15s TWT, with variable spacing and line lengths extending up to 380km offshore, totalling 20,185km. The Meregh-1 well (shallow offshore, direct tie) and DSDP wells 234 (direct tie) and 241 (indirect tie) were used for age correlation. Meregh-1 TD was in Middle Jurassic deposits, DSDP 241 reached 1174m in Upper Cretaceous sediments, and DSDP 234 TD was in Lower Oligocene sediments. Regional magnetic anomaly data are available as part of the Earth Magnetic Anomaly Grid (EMAG2) and represent a compilation of satellite, ship, airborne and ground surveys. The total magnetic intensity anomaly map has been integrated with seismic data for mapping the extent of the oceanic crust towards the shore. Oceanic crust is characterised on magnetic data by linear anomalies perpendicular to the spreading direction. Gravity data used are residual Bouguer and free air gravity anomaly maps covering the East Africa region. Gravity modelling was undertaken by Leeds University. A gravity response profile was modelled across a seismic line in the north in an area of uncertainty of crustal type, and this was compared with the satellite-derived gravity observations. As gravity modelling provides a non-unique solution, a suite of scenarios was modelled to show two alternative models to illustrate the key areas of uncertainty, located between the stretched continental crust and oceanic crust. Even with high fidelity data in frontier areas, there remains uncertainty in the definition of crustal type.

2 3. REGIONAL GEOLOGY Several regional tectonic events have shaped the Somali margin, associated with rifting of the Gondwana supercontinent from the Carboniferous to Early Jurassic[2], and the opening of the Indian Ocean in the Cretaceous. These tectonic events have divided the offshore area into three basins, each with their own gravity signature, and 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 Early to Middle Jurassic syn-rift sediments. Jurassic rifting and continental breakup coincided with a marine transgression and the deposition of organic-rich marine sediments in a restricted embayment, where northerly transform faults created partial barriers to oceanic circulation. This was 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 Figure 1. a) Free air gravity map. Spectrum s seismic datasets shown in black. margin can be attributed to differences b) Residual Bouguer gravity anomaly map offshore Somalia showing interpreted in sediment supply, depositional style, lineaments accommodation space, sub-basin tectonics, and post-depositional processes. The Obbia Basin in the north is characterised by continentally-derived Permo-Triassic Karoo pre-rift sediments, rotated by large Jurassic faults [Figure 2]. The offset of some of these Jurassic faults is considerably larger than is observed to the south. This may be a result of multiple phases of extension along this part of the margin, linked to the breakup of the Gondwanan supercontinent from the Carboniferous period. The growth strata in the accommodation space created by rifted half grabens are thought to consist of a mixture of shales deposited in an anoxic, restricted marine environment, interbedded with ponded coarse-grained sediments. These may create alternating source/reservoir packages, and organic-rich syn-rift shales may act as sealing lithologies for potential accumulations within pre-rift sediments. Reservoir-quality sediments may be encountered in pre-rift Permo-Triassic Karoo and Early Jurassic Adigrat Sandstone sediments. On localised highs created by the uplifted flanks of rotated fault blocks, carbonate build-ups are observed which may also act as suitable reservoirs for hydrocarbon accumulations, sealed by fine-grained post-rift deepwater sediments [Figure 3]. The Mesozoic post-rift consists primarily of reef build-ups, carbonate platforms and reworked carbonate sediments, as described by shallow offshore well Meregh-1. Reservoir-quality sediments might be encountered in shelf sandstones and

3 coarse-grained basin floor fans and turbidite deposits. Numerous small scale faults intersect the Cretaceous section in parts of the Obbia Basin, and these may compromise the sealing efficiency of the Cretaceous sequence in places, as well as act as potential conduits for charging shallow traps from a deeper Jurassic or Permo-Triassic source. Most post-rift faulting is high angle and planar, reflecting the brittle nature of the calcareous post-rift sediments. The Cenozoic section in the Obbia Basin is relatively thin (<1.5s TWT) compared to the Coriole and Juba-Lamu basins to the south. This is probably due to the absence of a major delta system in the north, creating a sediment-starved basin during the Cenozoic. The seabed on the shelf here is rugose, with deep canyons and channels carved by fluvial incision. Strong ocean-bottom currents formed contourite features within Cretaceous and Cenozoic sediments. Many large-scale contouriteshaped features are seen in Cretaceous and Cenozoic sediments [Figure 2]. Figure 2. Seismic dip line through Obbia Basin showing tilted fault blocks and crustal architecture Figure 3. Seismic strike line through Obbia Basin showing interpreted carbonate build-ups (blue) on local highs The Coriole and Juba-Lamu Basins contain a thick, highly deformed, post-rift deltaic sequence. Across much of the basin, the rift architecture is buried very deeply (>8s TWT), and it is difficult to image on seismic data due to this deformation and the presence of interpreted mud diapirs. Some authors describe these diapirs as comprising Early Jurassic salt [5] (salt was drilled in an isolated basin along the coast of Tanzania, and salt features have been interpreted in the Majunga Basin offshore NW Madagascar). However, recent interpretations describe shale-cored anticlines, as Jurassic sections encountered by Kenyan wells consist mainly of siliciclastic deposits and carbonates [6]. Jurassic salt deposits may indeed be present in the Lamu Basin further south, however offshore Somalia, modern data show coherent low frequency

4 Cretaceous reflectivity beneath the diapirs with no associated velocity pull-up, as well as a reverse phase source layer feeding these structures [Figure 4], indicating that they are likely to be disequilibrium compaction-related shale diapirs rather than Jurassic salt. Figure 4. Seismic dip line through the Juba-Lamu Basin showing interpreted shale diapirs The Cretaceous and Cenozoic sections comprise a thick siliciclastic sequence sourced by the Shabeelle/ Juba/ Tana River delta system. This significant post-rift pro-deltaic sequence hosts a number of surfaces which have formed décollements leading to slope failure events across multiple stratigraphic levels, with some active through to the present day. Gravity sliding occured in discrete episodes during the Cretaceous and Cenozoic. The décollement horizons may comprise organicrich shales, which decreased in viscosity and increased in pore pressure as they matured, creating the mechanism for slope instability, and facilitating the low angle displacement of a relatively unconsolidated overburden. They may therefore act as suitable source rock intervals for hydrocarbon generation. 4. CRUSTAL STRUCTURE & COB Offshore Somalia is a non-volcanic rifted margin; it displays limited rift-related volcanism and no seaward dipping reflectors. The low level of rift-related magmatism may be indicative 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, the continental crust exhibits a 200km wide necking zone, where large rifted continental blocks detach along major listric basement faults and can be seen soling out onto the lower crust/mantle on an intracrustal R Reflector [Figure 5].

5 Figure 5. Seismic dip line section through Obbia Basin showing interpreted R Reflector Gravity modelling showed that the gravity signature is consistent with hyper-extension of continental crust in all scenarios modelled, with a transition through to a possible exhumed mantle domain, and oceanic crust [Figure 6]. Hyperextended crust is weak and deformation-prone due to crustal thinning and serpentinisation[3]. A relatively high geothermal gradient is anticipated due to high crustal attenuation and shallow Moho. It implies a crustal thinning from 8.5 s TWT in the west to <1 s TWT outboard (with β potentially increasing to infinity into the possible exhumed mantle domain).. The nature of the uplifted body to the east has several possible interpretations, including a fragment of continental crust (i.e. a micro-plate, similar to the Seychelles), a tectonically uplifted basement block uplifted across a transform fault, or a magmatic edifice. Figure 6.Modelled gravity profiles for area of uncertainty of the nature of the crust beneath the Obbia Basin a) Exhumed mantle scenario between continental and oceanic crust (density 3.2 g/cm²); b) Exhumed mantle outboard of the stretched continental crust but edifice of magmatic origin (density 2.9 g/cm²). In the southern Coriole and Juba-Lamu Basins, the necking zone is much narrower (around 80km) and the oceanic crust and COB are interpreted to lie close to the base of the continental slope, based on the acoustic appearance and thickness

6 of the crust. The oceanic crust is highly faulted, consistent with a slow spreading rate. A thick Cretaceous and Cenozoic clastic sequence overlies the rift structure, deformed by several episodes of gravity sliding and shelf collapse. Isolated magnetic anomalies along the margin may be associated with the presence of basic intrusions and shallow basement. Increasing numbers of volcanic sills and dykes are associated with the Auxiliary Rescue and Salvage (ARS) Fracture Zone to the east. Scarps can be seen in the oceanic crust outboard where transform fault movement has juxtaposed oceanic crust of different ages. In the outboard, very large flower structures and transpressional anticlines are present, linked to transform movement along the ARS Fracture Zone in the east. In the southern Coriole and Juba-Lamu Basins, this transform movement is interpreted to have been active throughout the Cretaceous and Palaeogene periods, linked to the northward movement of the India block. Increasing numbers of volcanic sills and dykes are also associated with this fracture zone [Figure 5]. 5. 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. Source potential may also exist in syn-rift sediments deposited in Early Jurassic rifted half grabens, and in Cretaceous sediments deposited in restricted marine embayments where transform movement has created barriers to oceanic circulation. In the Coriole and Juba-Lamu basins, shale décollements may comprise hydrocarbon source rocks. Onshore and shallow nearshore well data show increasing geothermal gradients towards the north, and thus a shallowing of the oil window is expectedin the Obbia Basin. 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 in the north [Figure 7]. The Juba-Lamu Basin is interpreted to be underlain by Jurassic oceanic crust with a lower heat flow, however it has the thickest post-rift stratigraphy over the shelf and slope, up to 12 km, and puts the more deeply buried Cenozoic, Late Cretaceous and syn-rift Jurassic source rocks in the oil-generating window. Figure 7. PSDM examples from the Obbia andjuba-lamu basins showing predicted thermal maturity windows

7 6. 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. 7. REFERENCES [1] Kearns, H., Berryman, J., Hodgson, N. and Rodriguez., Somalia s Exploration Journey, GEO ExPro, GeoPublishing Ltd, London, 13 (2), pp , 2016 [2] Macgregor, D., History of the development of the East African Rift System: A series of interpreted maps through time, Journal of African Earth Sciences, 101, pp , 2015 [3] Doré, T. and Lundin, E., Hyperextended continental margins knowns and unknowns, Geology, 43 (1), pp , 2015 [4] Stanca, R., Kearns, H., Paton, D., Hodgson, N., Rodriguez, K. and Hussein, A., Offshore Somalia: crustal structure and implications on thermal maturity, First Break, EAGE, 34, pp , December 2016 [5] Rabinowitz et al. (1982) Rabinowitz, P. D., Coffin, M. F. and Falvey, D., Salt diapirs bordering the continental margin of Northern Kenya and Southern Somalia, Science, 215 (4533), pp , 1982 [6] Cruciani, F., and Barchi, M.R., The Lamu Basin deepwater fold-and-thrust belt: An example of a margin-scale, gravitydriven thrust belt along the continental passive margin of East Africa, Tectonics, 35, pp , 2016