Department of Geology

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Geologic review of hydrocarbons potential of the Rufiji Basin, Tanzania
(Springer, 2022-05-06) Sabuni, Rachel; Mtelela, Cassy; Kagya, Meshacky
The Rufiji Basin is one of the underexplored and least studied basins in the coastal Tanzania, despite the occurrences of oil and gas seeps that indicate the presence of a working petroleum system(s). Consequently, geology and distribution of key petroleum elements and hydrocarbon potentiality of the basin remains poorly understood. This study presents a geological review of the hydrocarbon potential of the Rufiji Basin based on a synthesis of published and unpublished reports of multifaceted studies in the basin, coupled with very limited additional data collected in the course of this study. This review identifies three petroleum plays (play I, play II, and play III) along with associated components, and includes: hydrocarbons play I, which constitutes a Permian–Triassic source rocks that are characterized by kerogen type III with TOC of ~ 6.1 wt% and Tmax values of 465 °C, along with Permian–Triassic fluvial–deltaic sandstone reservoir units, with porosity varying from 7 to 18%; and a Bajocian (restricted marine shales) as a seal. Play II has Bajocian restricted marine shale source rocks that are correlated to kerogen type II/III and III Makarawe shales, which have an average TOC of 1.7 w% and Tmax of 450 ℃, and is marked by Middle Jurassic carbonate reservoirs with an average porosity of 15%, capped with mid-Late Jurassic marine shales. Play III is characterized by Campanian shales as source rocks, Early Cretaceous fluvial–deltaic sandstone reservoir with a porosity of 15–20%, and is capped by Late Cretaceous transgressive marine shales. The analyses indicate that plays I and II are particularly more prospective, as manifested by the gas reserves discovered in offshore Songo Songo Island, making a Rufiji Basin a viable potential basin for hydrocarbon generation and accumulation. The findings of this review study support follow up exploration activities and researches, which can ultimately lead to a commercial discovery oil reserves in the basin.
Acquisition of a Unique Onshore/Offshore Geophysical and Geochemical Dataset in the Northern Malawi (Nyasa) Rift
(GeoScienceWorld, 2016-12) Donna J. Shillington, James B. Gaherty, Cynthia J. Ebinger, Christopher A. Scholz, Kate Selway, Andrew A. Nyblade, Paul A. Bedrosian, Cornelia Class, Scott L. Nooner, Matthew E. Pritchard, Julie Elliott, Patrick R. N. Chindandali, Gaby Mbogoni, Richard Wambura Ferdinand, Nelson Boniface, Shukrani Manya, Godson Kamihanda, Elifuraha Saria, Gabriel Mulibo, Jalf Salima, Abdul Mruma, Leonard Kalindekafe, Natalie J. Accardo, Daud Ntambila, Marsella Kachingwe, Gary T. Mesko, Tannis McCartney, Melania Maquay, J. P. O’Don- nell, Gabrielle Tepp, Khalfan Mtelela, Per Trin- hammer, Douglas Wood, Ernest Aaron, Mark Gibaud, Martin Rapa, Cathy Pfeifer, Felix Mphepo, Duncan Gondwe, Gabriella Arroyo, Celia Eddy, Brian Kamoga, and Mary Moshi
The Study of Extension and maGmatism in Malawi aNd Tanzania (SEGMeNT) project acquired a comprehensive suite of geophysical and geochemical datasets across the northern Malawi (Nyasa) rift in the East Africa rift system. Onshore/offshore active and passive seismic data, long‐period and wideband magnetotelluric data, continuous Global Positioning System data, and geochemical samples were acquired between 2012 and 2016. This combination of data is intended to elucidate the sedimentary, crustal, and upper‐mantle architecture of the rift, patterns of active deformation, and the origin and age of rift‐related magmatism. A unique component of our program was the acquisition of seismic data in Lake Malawi, including seismic reflection, onshore/offshore wide‐angle seismic reflection/refraction, and broadband seismic data from lake‐bottom seismometers, a towed streamer, and a large towed air‐gun source.
The P and S wave velocity structure of the mantle beneath eastern Africa and the African superplume anomaly
(Wiley, 2013-09-24) Mulibo G.D., and Nyblade A.A.
P and S relative arrival time residuals from teleseismic earthquakes recorded on over 60 temporary AfricaArray broadband seismic stations deployed in Uganda, Tanzania, and Zambia between 2007 and 2011 have been inverted, together with relative arrival time residuals from earthquakes recorded by previous deployments, for a tomographic image of mantle wave speed variations extending to a depth of 1200 km beneath eastern Africa. The image shows a low-wave speed anomaly (LWA) well developed at shallow depths (100–200 km) beneath the Eastern and Western branches of the Cenozoic East African rift system and northwestern Zambia, and a fast wave speed anomaly at depths 􏰋 350 km beneath the central and northern parts of the East African Plateau and the eastern and central parts of Zambia. At depths 􏰌350 km the LWA is most prominent under the central and southern parts of the East African Plateau and dips to the southwest beneath northern Zambia, extending to a depth of at least 900 km. The amplitude of the LWA is consistent with a 􏰉150–300 K thermal perturbation, and its depth extent indicates that the African superplume, originally identified as a lower mantle anomaly, is likely a whole mantle structure. A superplume extending from the core-mantle boundary to the surface implies an origin for the Cenozoic extension, volcanism, and plateau uplift in eastern Africa rooted in the dynamics of the lower mantle.
Mantle transition zone thinning beneath eastern Africa: Evidence for a whole-mantle superplume structure
(Wiley, 2013-08-22) Mulibo G.D., and Nyblade A.A
[1] P to S conversions from the 410 and 660 km discontinuities observed in receiver function stacks reveal a mantle transition zone that is ~30–40 km thinner than the global average in a region ~200–400 km wide extending in a SW-NE direction from central Zambia, across Tanzania and into Kenya. The thinning of the transition zone indicates a ~190–300K thermal anomaly in the same location where seismic tomography models suggest that the lower mantle African superplume structure connects to thermally perturbed upper mantle beneath eastern Africa. This finding provides compelling evidence for the existence of a continuous thermal structure extending from the core-mantle boundary to the surface associated with the African superplume.
The uppermost mantle shear wave velocity structure of eastern Africa from Rayleigh wave tomography: constraints on rift evolution
(Oxford, 2013-05-01) O’Donnell, J.P., Adams, A., Nyblade, A.A., Mulibo G.D., and Tugume F
An expanded model of the 3-D shear wave velocity structure of the uppermost mantle beneath eastern Africa has been developed using earthquakes recorded by the AfricaArray East African Seismic Experiment in conjunction with data from permanent stations and previously deployed temporary stations. The combined data set comprises 331 earthquakes recorded on a total of 95 seismic stations spanning Kenya, Uganda, Tanzania, Zambia and Malawi. In this study, data from 149 earthquakes were used to determine fundamental-mode Rayleigh wave phase velocities at periods ranging from 20 to 182 s using the two-plane wave method, and then combined with the similarly processed published measurements and inverted for a 3-D shear wave velocity model of the uppermost mantle. New features in the model include (1) a low-velocity region in western Zambia, (2) a high-velocity region in eastern Zambia, (3) a low-velocity region in eastern Tanzania and (4) low-velocity regions beneath the Lake Malawi rift. When considered in conjunction with mapped seismicity, these results support a secondary western rift branch striking southwestwards from Lake Tanganyika, likely exploiting the relatively weak lithosphere of the southern Kibaran Belt between the Bangweulu Block and the Congo Craton. We estimate a lithospheric thickness of ∼150–200 km for the substantial fast shear wave anomaly imaged in eastern Zambia, which may be a southward subsurface extension of the Bangweulu Block. The low-velocity region in eastern Tanzania suggests that the eastern rift branch trends southeastwards offshore eastern Tanzania coincident with the purported location of the northern margin of the proposed Ruvuma microplate. Pronounced velocity lows along the Lake Malawi rift are found beneath the northern and southern ends of the lake, but not beneath the central portion of the lake.
The Seismotectonics of southeastern Tanzania: Implication for the propagation of the Eastern Branch of the East African Rift
(Wiley, 2016-04-02) Mulibo G.D., and Nyblade A.A
Seismicity patterns and focal mechanisms in southeastern Tanzania, determined from data recorded on temporary and permanent AfricaArray seismic stations, have been used to investigate the propagation direction of the Eastern branch of the East African Rift System southward from the Northern Tanzania Divergence Zone (NTDZ). Within the NTDZ, the rift zone is defined by three segments, the Eyasi segment to the west, the Manyara segment in the middle, and the Pangani segment to the east. Results show that most of the seismicity (~ 75%) extends to the south of the Manyara segment along the eastern margin of the Tanzania Craton, and at ~ 6–7° S latitude trends to the SE along the northern boundary of the Ruvuma microplate, connecting with a N–S zone of seismicity offshore southern Tanzania and Mozambique. A lesser amount of seismicity (~ 25%) is found extending from the SE corner of the Tanzania Craton at ~ 6–7° S latitude southwards towards Lake Nyasa. This finding supports a model of rift propagation via the Manyara segment to the southeast of the Tanzania Craton along the northern boundary of the Ruvuma microplate. However, given the limited duration of the seismic recordings used in this study, the possibility of another zone of extension developing to the south towards Lake Nyasa (Malawi) cannot be ruled out. Focal mechanisms along the boundary between the Victoria and the Ruvuma microplates and offshore southeastern Tanzania show a combination of normal and strike slip faulting indicating mainly extension with some sinistral motion, consistent with the mapped geologic faults and a clockwise rotation of the Ruvuma microplate.
Surface-wave imaging of the weakly-extended Malawi Rift from ambient-noise and teleseismic Rayleigh waves from onshore and lake-bottom seismometers
(Oxford, 2017-04-01) Accardo, N. J., Gaherty, J. B., Shillington, D. J., Ebinger, C. J., Nyblade, A. A., Mbogoni, G. J., Chindandali, P. R. N., Ferdinand, R. W., Mulibo, G. D., Kamihanda, G., Keir, D., Scholz, C., Selway, K., O'Donnell, J.P., Tepp, G., Gallacher, R., Mtelela, K., Salima, J., Mruma A.
Located at the southernmost sector of the Western Branch of the East African Rift System, the Malawi Rift exemplifies an active, magma-poor, weakly extended continental rift. To investigate the controls on rifting, we image crustal and uppermost mantle structure beneath the region using ambient-noise and teleseismic Rayleigh-wave phase velocities between 9 and 100 s period. Our study includes six lake-bottom seismometers located in Lake Malawi (Nyasa), the first time seismometers have been deployed in any of the African rift lakes. Noise levels in the lake are lower than that of shallow oceanic environments and allow successful application of compliance corrections and instrument orientation determination. Resulting phase-velocity maps reveal slow velocities primarily confined to Lake Malawi at short periods (T <= 12 s), indicating thick sediments in the border-fault bounded rift basin. The slowest velocities occur within the Central Basin where Malawi Rift sedimentary strata may overlie older (Permo-Triassic) Karoo group sediments. At longer periods (T > 25 s), a prominent low-velocity anomaly exists beneath the Rungwe Volcanic Province at the northern terminus of the rift basin. Estimates of phase-velocity sensitivity indicates these low velocities occur within the lithospheric mantle and potentially uppermost asthenosphere, suggesting that mantle processes may control the association of volcanic centres and the localization of magmatism. Beneath the main portion of the Malawi Rift, a modest reduction in velocity is also observed at periods sensitive to the crust and upper mantle, but these velocities are much higher than those observed beneath Rungwe.
Lithospheric low-velocity zones associated with a magmatic segment of the Tanzanian Rift, East Africa
(Oxford, 2017-05-01) Plasman, M., Tiberi,C., Ebinger, Gautier, S., Albaric, J., Peyrat, S., Deverchere, J., Le Gall, B., Tarits, P., Roecker, S., Wambura, F., Muzuka, A., Mulibo, G. D., Mtelela, K., Msabi, M., Kianji, G., Hautot, S., Perrot, R., and Gama, R
Rifting in a cratonic lithosphere is strongly controlled by several interacting processes including crust/mantle rheology, magmatism, inherited structure and stress regime. In order to better understand how these physical parameters interact, a 2 yr long seismological experiment has been carried out in the North Tanzanian Divergence (NTD), at the southern tip of the eastern magmatic branch of the East African rift, where the southward-propagating continental rift is at its earliest stage. We analyse teleseismic data from 38 broad-band stations ca. 25 km spaced and present here results from their receiver function (RF) analysis. The crustal thickness and Vp/Vs ratio are retrieved over a ca. 200 × 200 km2 area encompassing the South Kenya magmatic rift, the NTD and the Ngorongoro-Kilimanjaro transverse volcanic chain. Cratonic nature of the lithosphere is clearly evinced through thick (up to ca. 40 km) homogeneous crust beneath the rift shoulders. Where rifting is present, Moho rises up to 27 km depth and the crust is strongly layered with clear velocity contrasts in the RF signal. The Vp/Vs ratio reaches its highest values (ca. 1.9) beneath volcanic edifices location and thinner crust, advocating for melting within the crust. We also clearly identify two major low-velocity zones (LVZs) within the NTD, one in the lower crust and the second in the upper part of the mantle. The first one starts at 15–18 km depth and correlates well with recent tomographic models. This LVZ does not always coexist with high Vp/Vs ratio, pleading for a supplementary source of velocity decrease, such as temperature or composition. At a greater depth of ca. 60 km, a mid-lithospheric discontinuity roughly mimics the step-like and symmetrically outward-dipping geometry of the Moho but with a more slanting direction (NE–SW) compared to the NS rift. By comparison with synthetic RF, we estimate the associated velocity reduction to be 8–9 per cent. We relate this interface to melt ponding, possibly favouring here deformation process such as grain-boundary sliding (EAGBS) due to lithospheric strain. Its geometry might have been controlled by inherited lithospheric fabrics and heterogeneous upper mantle structure. We evidence that crustal and mantle magmatic processes represent first order mechanisms to ease and locate the deformation during the first stage of a cratonic lithospheric breakup.
Subsurface Images of the Eastern Rift, Africa, from the Joint Inversion of Body Waves, Surface Waves, and Gravity: Investigating the Role of Fluids in Early-Stage Continental Rifting
(Oxford, 2017-05-19) Roecker, S., Ebinger, C., Tiberi, C., Mulibo, G., Ferdinand, R. W., Mtelela, K., Kianji, G., Muzuka, A., Gautier, S., Albaric, J., and Peyrat, S.
The Eastern Rift System (ERS) of northern Tanzania and southern Kenya, where a cratonic lithosphere is in the early stages of rifting, offers an ideal venue for investigating the roles of magma and other fluids in such an environment. To illuminate these roles, we jointly invert arrival times of locally recorded P and S body waves, phase delays of ambient noise generated Rayleigh waves and Bouguer anomalies from gravity observations to generate a 3-D image of P and S wave speeds in the upper 25 km of the crust. While joint inversion of gravity and arrival times requires a relationship between density and wave speeds, the improvement in resolution obtained by the combination of these disparate data sets serves to further constrain models, and reduce uncertainties. The most significant features in the 3-D model are (1) P and S wave speeds that are 10–15 per cent lower beneath the rift zone than in the surrounding regions, (2) a relatively high wave speed tabular feature located along the western edge of the Natron and Manyara rifts, and (3) low (∼1.71) values of Vp/Vs throughout the upper crust, with the lowest ratios along the boundaries of the rift zones. The low P and S wave speeds at mid-crustal levels beneath the rift valley are an expected consequence of active volcanism, and the tabular, high-wave speed feature is interpreted to be an uplifted footwall at the western edge of the rift. Given the high levels of CO2 outgassing observed at the surface along border fault zones, and the sensitivity of Vp/Vs to pore-fluid compressibility, we infer that the low Vp/Vs values in and around the rift zone are caused by the volcanic plumbing in the upper crust being suffused by a gaseous CO2 froth on top of a deeper, crystalline mush. The repository for molten rock is likely located in the lower crust and upper mantle, where the Vp/Vs ratios are significantly higher.
Fault-magma interactions during early continental rifting: Seismicity of the Magadi-Natron-Manyara basins, Africa
(Wiley, 2017-11-16) Weinstein, A., C. J. Ebinger, S. J. Oliva, S. Roecker, C. Tiberi, M. Aman, C. Lambert, E. Witkin, J. Albaric, S. Gautier, A. Muzuka, G. Mulibo, G. Kianji, R. Hadfield, F. Illsley-Kemp, M. Msabi, R. Ferdinand-Wambura, S. Perrot, J. Muirhead, A. Rodzianko, T. Fischer
Although magmatism may occur during the earliest stages of continental rifting, its role in strain accommodation remains weakly constrained by largely 2-D studies. We analyze seismicity data from a 13 month, 39-station broadband seismic array to determine the role of magma intrusion on state-of-stress and strain localization, and their along-strike variations. Precise earthquake locations using cluster analyses and a new 3-D velocity model reveal lower crustal earthquakes beneath the central basins and along projec- tions of steep border faults that degas CO2. Seismicity forms several disks interpreted as sills at 6–10 km below a monogenetic cone field. The sills overlie a lower crustal magma chamber that may feed eruptions at Oldoinyo Lengai volcano. After determining a new ML scaling relation, we determine a b-value of 0.87 6 0.03. Focal mechanisms for 65 earthquakes, and 13 from a catalogue prior to our array reveal an along-axis stress rotation of 􏰉608 in the magmatically active zone. New and prior mechanisms show pre- dominantly normal slip along steep nodal planes, with extension directions 􏰉N908E north and south of an active volcanic chain consistent with geodetic data, and 􏰉N1508E in the volcanic chain. The stress rotation facilitates strain transfer from border fault systems, the locus of early-stage deformation, to the zone of magma intrusion in the central rift. Our seismic, structural, and geochemistry results indicate that frequent lower crustal earthquakes are promoted by elevated pore pressures from volatile degassing along border faults, and hydraulic fracture around the margins of magma bodies. Results indicate that earthquakes are largely driven by stress state around inflating magma bodies.
Seismic anisotropy of the upper mantle below the Western rift, East Africa
(Wiley, 2018-08-23) Tepp, G., Ebinger, C. J., Zal, H., Gallacher, R., Accardo, N., Shillington, D. J., Gaherty, J., Keir, D., Nyblade, A.A., Mbogoni, G.J., Chindandali, P.R.N., Ferdinand-Wambura, R., Mulibo, G.D., Kamihanda, G.
Although the East African rift system formed in cratonic lithosphere above a large-scale mantle upwelling, some sectors have voluminous magmatism, while others have isolated, small-volume eruptive centers. We conduct teleseismic shear wave splitting analyses on data from 5 lake-bottom seismometers and 67 land stations in the Tanganyika-Rukwa-Malawi rift zone, including the Rungwe Volcanic Province (RVP), and from 5 seismometers in the Kivu rift and Virunga Volcanic Province, to evaluate rift-perpendicular strain, rift-parallel melt intrusion, and regional flow models for seismic anisotropy patterns beneath the largely amagmatic Western rift. Observations from 684 SKS and 305 SKKS phases reveal consistent patterns. Within the Malawi rift south of the RVP, fast splitting directions are oriented northeast with average delays of ~1 s. Directions rotate to N-S and NNW north of the volcanic province within the reactivated Mesozoic Rukwa and southern Tanganyika rifts. Delay times are largest (~1.25 s) within the Virunga Volcanic Province. Our work combined with earlier studies shows that SKS-splitting is rift parallel within Western rift magmatic provinces, with a larger percentage of null measurements than in amagmatic areas. The spatial variations in direction and amount of splitting from our results and those of earlier Western rift studies suggest that mantle flow is deflected by the deeply rooted cratons. The resulting flow complexity, and likely stagnation beneath the Rungwe province, may explain the ca. 17 Myr of localized magmatism in the weakly stretched RVP, and it argues against interpretations of a uniform anisotropic layer caused by large-scale asthenospheric flow or passive rifting.
Delineation of shallow stratigraphy and aquifer formation at Kahe Basin, Tanzania: Implication for Potential Aquiferous Formation
(Scietntific Research Publishing, 2018-01-30) Mlangi, M.M and Mulibo G.D
Electrical resistivity method was conducted at Kahe-Mtakuja basin aimed at appraising the potential of the basin as a source of groundwater by establish- ing shallow stratigraphy and delineating aquifer formations. A total of fif- ty-eight vertical electrical sounding data (VES) were acquired using Schlum- berger array and the data were analyzed to obtain apparent resistivity and layer depth. The interpretation of resistivity data revealed three main geoelec- tric layers. The first layer has resistivity values ranging from 40 to 230 Ωm with thickness ranging from 0.4 to 2 m. The second layer has resistivity values in the range of 2 to 10 Ωm and thickness of 2 to 25 m. The third layer has slightly high resistivity values ranging from 10 to 60 Ωm and thickness in the range of 30 to 70 m. This layer is mainly dominated with sand. The resistivity cross-sections constructed from the interpretation of VES data indicate that the Kahe-Mtakuja basin has shallow stratigraphy consisting of 3 layers. The layers are composed mainly of top red soil, clay (sometime alternating with sand) and sand formation holding the groundwater. These findings are con- sistent with lithological logs of the borehole drilled near Kahe-Mtakuja that indicate two to five layers composed of alluvial deposits alternating with dif- ferent lithological thicknesses. The high correlation between the VES results and borehole lithological logs near Kahe-Mtakuja suggests that the area is po- tential for aquiferous formation. Based on constructed stratigraphy, the aqui- fer formation of the basin is found in alluvial deposits composed of mainly sand. The potentiality of this area for aquiferous formation is vital for provid- ing additional baseline data on the aquifer characteristics and will assist in reducing water scarce in the area.
Petrophysical Analysis of Reservoirs Rocks at Mchungwa Well in Block 7 Offshore, Tanzania: Geological Implication on the Reservoir Quality
(Scietntific Research Publishing, 2018-08-03) Mjili, A.S. and Mulibo, G.D
The present work highlights the results of the study conducted to estimate the petrophyiscal properties of the Mchungwa well with the aim of assessing the quality of reservoirs rocks. A set of well logs data from Mchungwa well were used for the analysis that involved identification of lithology, hydrocarbon and non-hydrocarbon zones and determinations of petrophysical parameters such as shale volume, porosity, permeability, fluid saturation and net pay thickness. This study was able to mark six sandstone zones with their tops and bases. Of the six zones hydrocarbon indication was observed on four zones from which estimation of petrophysical parameters was done to assess the re- servoirs quality. The petrophysical parameters across the four reservoirs yield an average shale volume ranging from 0.08 to 0.15 v/v. The porosity ranges from 7% to 23%, indicating a fair to good porosity sandstone, while permea- bility ranges from 0.01 to 6 mD. The porosity and permeability results suggest that the quality of the sandstone reservoirs identified at Mchungwa well is poor. Fluid types defined in the reservoirs on the basis of neutron-density log signatures and resistivity indicate a mixture of water and gas. However, high water saturation (50% - 100%) indicates that the proportion of void spaces occupied by water is high, thus, indicating low hydrocarbon saturation of 2.4%, 17.9%, 19.2% and 39.3%. Generally the results show that hydrocarbon potentiality at Mchungwa well is extremely low because of small net pay thickness and very low hydrocarbon saturation. This could be attributed to the geology of the surrounding area where low hydrocarbon saturation sug- gest the presence of non-commercial volumes of either migrant gas or gas generated from the interbedded claystone sediments, which are dominant in the observed well.
Petrophysical Analysis of the Mpapai Well Logs in the East Pande Exploration Block, Southern Coast of Tanzania: Geological Implication on the Hydrocarbon Potential.
(Scietntific Research Publishing, 2018-08-03) Lyaka, A.L. and Mulibo, G.D.
This study presents results of log analysis from Mpapai well, which is located in the East Pande Block, southern coast of Tanzania. The study aimed at as- sessing the hydrocarbon potential of lithological units encountered during drilling of Mpapai well. To achieve the general objective, suites of wire-line logs from Mpapai well were used for the analysis. Based on wire-line logs, three types of lithology were identified which include sandstone, shale and li- mestone. Seven sandstone bodies marked as MpapaiA, B, C, D, E, F and G were identified with their tops and bases at the depth interval from 3004 m to 4008 m. Four zones among seven sandstones bodies marked as MpapaiB, E, F and MpapaiG were identified as reservoir zones. Computed petrophysical pa- rameters for the four reservoir zones gave an average total porosity ranging from 14% to 21% with low permeability in the range of 3.92 mD to 13.67 mD. The low permeability indicates that the reservoir sand bodies are impermea- ble, that might have been affected by the geology of the area where high con- tent of clay minerals reduces permeability due to filling in open spaces. The fluid type defined in the reservoir zones is basically water with high saturation greater than 75%, which indicates that the proportion of void space occupied by water is high, consequently low hydrocarbon saturation and production. Despite of fair to good porosity, the low permeability and high-water satura- tion indicate that the quality of Mpapai prospect is poor.
Petrophysical Analysis of Mpera Well in the Exploration Block 7, Offshore Tanzania: Implication on Hydrocarbon Reservoir Rock Potential
(Scietntific Research Publishing, 2018-08-03) Mheluka, J.M. and Mulib,o G.D.
The present study provides evaluation and estimation of petrophysical para- meters and assessment of lithology and their thicknesses in order to charac- terize present reservoir rocks at Mpera well located in Exploration Block 7, deep offshore Tanzania. To achieve the objectives the wire-line logs, Techlog program was used for assessment, analysis, computation and interpretations of petrophysical parameters and results were integrated through interpreta- tion of well logs. The results from wire-line logs reveal three (3) non hydro- carbon-bearing reservoir rocks i.e., Mpera splay (sandstone), Mpera deep sand 1 (sandstone and limestone) and Mpera deep sand 2 (sandstone and li- mestone) with gross thickness of 94.335 m, 28.905 m and 12.967 m respec- tively. The average permeability values of the reservoir rocks were 9.47 mD, 6.45 mD and 4.67 mD, while average porosity values were 14.57%, 17.4% and 16.75%, with average volume of shale 25.7%, 23.5% and 9.7% at Mpera splay, Mpera deep sand 1 and Mpera deep sand 2 respectively. These results signify poor permeability; good porosity and good quality reservoir in terms of vo- lume of shale. Fluid type defined in the reservoirs was basically water. High water saturation (90.6% - 97.7%) in the reservoir zones of the Mpera well in- dicates that the proportion of void spaces occupied by water is high, thus, in- dicating less than 10% hydrocarbon saturation. The findings indicate that Mpera well reservoir rocks are of low quality with non-hydrocarbon bearing such that it is not potential for hydrocarbon production.
Tectono-Sedimentary Evolution of the Offshore Hydrocarbon Exploration Block 5, East Africa: Implication for Hydrocarbon Generation and Migration
(Scietntific Research Publishing, 2018-08-03) Seni, E.J and Mulibo, G.D.
Sedimentary deposits in Block 5, offshore Tanzania basin have been imaged using two-dimensional (2D) seismic data. The seismic data and well data re- veal four tectonic units representing different tectonic events in relation to structural styles, sedimentation and hydrocarbon potential evolved in Block 5. Results show that during Early to Late Jurassic, Block 5 was affected by the break-up of Gondwana and the drifting of Madagascar as evidenced by pat- terns of sediments and structural features. The chaotic and discontinuous reflectors are characteristics features on the sediments pattern indicating a possible transitional setting following the breakup of Gondwana. From the Late Cretaceous, Block 5 sits in more stable subsiding sag as the consequence of the high thermal subsidence. The period displayed continuous parallel re- flectors with few markable faults. This was followed by the late post rift sedi- mentation that occurred after Middle Eocene Unconformity characterized by high wavy and sub parallel reflectors. The evolution of Block 5 through major tectonic events reveals a more complete petroleum system towards the south. Thus, Block 5 responded in both space and time to a complex interplay be- tween tectonics and sedimentation. This indicates that structural styles and associated features are potential control for hydrocarbon generation and mi- gration.
Crustal structure surrounding the northern Malawi rift and beneath the Rungwe volcanic province, East Africa
(Oxford, 2018-08-09) Borrego, D., Nyblade, A.A., Accardo, N.J., Gaherty, J.B., Ebinger, C.J., Shillington, D.J., Chindandali, P.R.N. Mbogoni, G., Ferdinand, R.W., Mulibo, G.D., O’Donnell, J. P. Kachingwe, M., and Tepp, G.
The crustal structure surrounding the northern Malawi rift and beneath the Rungwe Volcanic Province (RVP) has been investigated using teleseismic earthquakes recorded on SEGMeNT broad-band seismic stations to determine the extent to which the crust has been modified by Cenozoic rifting and magmatism. The SEGMeNT network included 57 broad-band seismic stations deployed in northern Malawi and southern Tanzania between August 2013 and October 2015. Estimates of crustal thickness, shear wave velocity and Poisson’s ratio have been obtained by modelling P-wave receiver functions using the H–k stacking method and jointly inverting receiver functions with Rayleigh wave phase velocities. These estimates are used to investigate the extent of magmatic modification to the crust, indicated by changes in Poisson’s ratio, and the geometry of crustal thinning along the northern margins of the Malawi rift and beneath the RVP. The average crustal thickness for the four stations in the RVP is 39 km, the average Poisson’s ratio is 0.28 (Vp/Vs = 1.83), and the average crustal shear wave velocity is 3.6 km s–1. Although the RVP has been a site of ongoing magmatism since at least 17 Ma and is associated with a pronounced low velocity zone in the mantle, our results show little evidence that the bulk composition or thickness of the crust beneath the RVP has been significantly modified by magmatism or extension. However, Poisson’s ratios of 0.29–0.31 (Vp/Vs = 1.85–1.91) at three of the stations in the RVP, where there is also no evidence for higher Vs, may indicate the presence of partial melt in the crust. The average crustal thickness of Proterozoic terranes surrounding the northern end of the Malawi rift ranges from 38 to 42 km. For most of the terranes, average Poisson’s ratios are between 0.25 and 0.26 (Vp/Vs = 1.73–1.76), with the exception of the Irumide Belt, which has an average Poisson’s ratio of 0.23 (Vp/Vs = 1.68). The average crustal shear wave velocities for all the terranes are either 3.6 or 3.7 km s–1. These results indicate a bulk felsic to intermediate crustal composition for all terranes, consistent with previous results, and reveal that there is little, if any, crustal thinning beneath the uplifted flanks of asymmetric basins within the northern Malawi rift or beneath the RVP. Consequently, crustal thinning in the northern Malawi rift must be highly focused beneath the centres of rift basin segments, consistent with models of rift flank topography and gravity observations.
Seismic evidence for plume- and craton-influenced upper mantle structure beneath the northern Malawi rift and the Rungwe volcanic province, East Africa
(Wiley, 2018-11-19) Grijalva, A., Nyblade, A.A., Homman, K., Accardo, N.J., Gaherty, J.B., Ebinger, C.J., Shillington, D.J., Chindandali, P.R.N. Mbogoni, G., Ferdinand, R.W., Mulibo, G.D., O’Donnell, J. P. Kachingwe, M., and Tepp, G
P and S wave tomographic models have been developed for the northern Malawi rift and adjacent Rungwe Volcanic Province (RVP) using data from the Study of Extension and maGmatism in Malawi aNd Tanzania project and data from previous networks in the study area. The main features of the models are a low-velocity zone (LVZ) with δVp = ~ 1.5–2.0% and δVs = ~ 2–3% centered beneath the RVP, a lower-amplitude LVZ (δVp = ~ 1.0–1.3% and δVs = ~ 0.7–1%) to the southeast of the RVP beneath the center and northeastern side of the northern Malawi rift, a shift of the lower-amplitude anomaly at ~ 10° to 11° to the west beneath the central basin and to the western side of the rift, and a fast anomaly at all depths beneath the Bangweulu Craton. The LVZ widens further at depths >~150–200 km and extends to the north beneath northwestern Malawi, wrapping around the fast anomaly beneath the craton. We attribute the LVZ beneath the RVP and the northern Malawi rift to the flow of warm, superplume mantle from the southwest, upwelling beneath and around the Bangweulu Craton lithosphere, consistent with high 3He/4He values from the RVP. The LVZ under the RVP and northern Malawi rift strongly indicates that the rifted lithosphere has been thermally perturbed. Given that volcanism in the RVP began about 10 million years earlier than the rift faulting, thermal and/or magmatic weakening of the lithosphere may have begun prior to the onset of rifting.
Lithospheric modification by extension and magmatism at the craton-orogenic boundary: North Tanzania Divergence
(Oxford, 2019-12-10) Tiberi, C., S. Gautier, C. Ebinger, S. Roecker, M. Plasman, J. Albaric, J. D´everch`ere, S. Peyrat, J. Perrot, R. Ferdinand Wambura, M. Msabi, A. Muzuka, G. D. Mulibo, G. Kianji
We present a joint analysis of newly acquired gravity and teleseismic data in the North Tanzanian Divergence, where the lithospheric break-up is at its earliest stage. The impact of a mantle upwelling in more mature branches of the East African Rift has been extensively studied at a lithospheric scale. However, few studies have been completed that relate the deep-seated mantle anomaly detected in broad regional seismic tomography with the surface deformation observed in the thick Archaean Pan-African suture zone located in North Tanzania. Our joint inversion closes the gap between local and regional geophysical studies, providing velocity and density structures from the surface down to ca. 250 km depth with new details. Our results support the idea of a broad mantle upwelling rising up to the lithosphere and creating a thermal modification along its path. However, our study clearly presents an increasing amplitude of the associated anomaly both in velocity and density above 200 km depth, which cannot be solely explained by a temperature rise. We infer from our images the combined impact of melt (2–3 per cent), composition and hydration that accompany the modification of a thick heterogenous cratonic lithosphere are a response to the hot mantle rising. The detailed images we obtained in density and velocity assert that Archaean and Proterozoic units interact with the mantle upwelling to restrict the lithosphere modifications within the Magadi–Natron–Manyara rift arm. The composition and hydration variations associated with those units equilibrate the thermal erosion of the craton root and allow for its stability between 100 and 200 km depth. Above 80 km depth, the crustal part is strongly affected by intruding bodies (melt and gas) which produces large negative anomalies in both velocity and density beneath the main magmatic centres. In addition to the global impact of a superplume, the velocity and density anomaly pattern suggests a 3-D distribution of the crust and mantle lithospheric stretching, which is likely to be controlled by inherited fabrics and enhanced by lateral compositional and hydration variations at the Tanzanian craton-orogenic belt boundary.
Kinematics of Active Deformation in the Malawi Rift and Rungwe Volcanic Province, Africa
(Wiley, 2019-07-22) Ebinger, C. J., Oliva, S.J., Pham, T., Peterson, K., Chindandali, P., Illsley‐Kemp, F., Drooff, C., Shillington, D.J., Accardo, N.J., Gallacher, R.J., Gaherty, J., Nyblade, A.A., and Mulibo, G.
Although the deep, wide basins of the Western rift, Africa, have served as analogues for the evolution of half‐graben basins, the geometry and kinematics of the border, intrabasinal, and transfer fault systems have been weakly constrained. Despite the >100‐km‐long fault systems bounding basins, little was known of seismicity patterns or the potential for M > 7.5 earthquakes. Using our new local earthquake database from the 2013‐2015 Study of Extension and maGmatism in Malawi aNd Tanzania (SEGMeNT) seismic array (57 onshore, 32 lake‐bottom stations) and TANGA14 (13 stations), we examine the kinematics and extension direction of the Rungwe Volcanic Province and northern Malawi rift. We relocated earthquakes using a new 1‐D velocity model and both absolute and double‐difference relocation methods. Local magnitudes of 1,178 earthquakes within the array are 0.7 < ML < 5.2 with a b‐value 0.77 ± 0.03, and magnitude of completeness ML 1.9. Focal mechanism solutions for 63 earthquakes reveal predominantly normal and oblique‐slip motion, and full moment tensor solutions for ML 4.5, 5.2 earthquakes have centroid depths within 2 km of catalog depths. The preferred nodal planes dip more than 40° from surface to >25‐km depths. Extension direction from local earthquakes and source mechanisms of teleseismically detected earthquakes are approximately N58°E and N65°E, respectively, refuting earlier interpretations of a NW‐SE transform fault system. The low b‐value indicating strong coupling across crustal‐scale border faults, border fault lengths >100 km, and evidence for aseismic deformation together indicate that infrequent M > 7.5 earthquakes are possible within this cratonic rift system.

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