Browsing by Author "Mtelela, Cassy"

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    Application of U–Pb detrital zircon geochronology to drill cuttings for age control in hydrocarbon exploration wells: A case study from the Rukwa Rift Basin, Tanzania
    (The American Association of Petroleum Geologists, 2017-02) Hilbert-Wolf, Hannah; Roberts, Eric; Downie, Bob; Mtelela, Cassy; Stevens, Nancy J.; O’Connor, Patrick
    Precise dating and correlation of drilled wells through continental successions is challenging for hydrocarbon exploration, especially where preservation and recovery of age-diagnostic fossils is poor. As a complement or alternative to biostratigraphic dating we demonstrate the effectiveness of U–Pb geochronology via laser ablation–inductively coupled plasma–mass spectrometry on detrital zircon from well cuttings. In basins with syndepositional volcanic input, the youngest zircons in a stratigraphic interval can refine and serve as a proxy for the age of deposition. We demonstrate the reliability of this technique when applied to hydrocarbon exploration wells by analyzing drill cuttings through a continental interval of the Galula-1 well in the Rukwa Rift Basin, East African rift system, Tanzania, which previously yielded conflicting biostratigraphy results. The lower third of the well section reveals a late Miocene to Pliocene up-hole younging trend in the youngest detrital zircon populations, which matches new radioisotopic ages on volcanic tuffs from a correlative outcrop section. This is followed by an interval with recycled young zircons, followed by a zircon-free interval, interpreted to correspond to changes in magma composition of the nearbyRungwe volcanic province. This study provides the first radioisotopic age constraints for the Lake Beds in the Rukwa riftand demonstrates that sedimentation in the basin began by 8.7 Ma, critical for burial and thermal history modeling and establishing the probability of a working hydrocarbon system. Correspondence in age and zircon preservation between well and outcrop samples from the same intervals provides strong support for applying U–Pb detrital zircon geochronology to well cuttings, as a rapid, inexpensive approach for hydrocarbon exploration.
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    Combining detrital geochronology and sedimentology to assess basin development in the Rukwa Rift of the East African Rift System
    (EGU General Assembly 2015, 2015) Hilbert-Wolf, Hannah; Roberts, Eric; Mtelela, Cassy; Downie, Bob
    We have employed a multifaceted approach to sedimentary provenance analysis in order to assess the timing and magnitude of tectonic events, sedimentation, and landscape development in theWestern Branch of the East African Rift System. Our approach, termed ‘Sedimentary Triple Dating’, integrates: (1) U-Pb dating via LA-ICPMS; (2) fission track; and (3) (U-Th)/He thermochronology of detrital zircon and apatite. We integrate geochronology, thermochronology, and provenance analysis to relate the initiation of rifting events to regional dynamic uplift, sedimentation patterns, and interpret the far-reaching climatic and evolutionary effects of fluctuating rift flank topography in the Rukwa Rift, a segment of theWestern Branch. This work provides additional data to support the recent concept of synchronous development of the Western and Eastern branches of the East African Rift System 25 Ma, and better constrains the age, location and provenance of subsequent rifting and sedimentation events in the Rukwa Rift Basin. Investigation of well cuttings and outcrop samples from the Neogene-Recent Lake Beds Succession in the Rukwa Rift Basin revealed a suite of previously unrecognized tuffaceous deposits at the base of the succession. A population of euhedral, magmatic zircons from a basal Lake Beds tuff and Miocene-Pliocene detrital zircons from well cuttings suggest that Neogene rift reactivation and volcanism began 9-10 Ma. This timing is consistent with demonstrated rifting in Uganda and Malawi, as well as with the initiation of volcanism in the Rungwe Volcanic Province at the southern end of the Rukwa Rift, and the estimated development of Lake Tanganyika to the north. Moreover, there appear to be a suite of unconformity bounded stratigraphic units that make up the Lower Lake Beds succession, and detrital zircon maximum depositional ages from these units suggests episodic sedimentation in the rift, punctuated by long hiatuses or uplift, rather than steady subsidence and sedimentation. A distinct, upward-younging trend in detrital zircon populations associated with each stratigraphic interval suggests that volcanism was also episodic through the Late Miocene-Pliocene, and linked to periods of rifting and basin filling. Detrital zircon populations are dominated by Paleoproterozoic grains of the same age as the metamorphic Ubendian Belt that underlies the rift basin and forms the flanks. This provenance, volcaniclastic-dominated sedimentation, and clasts from the rift flanks suggest an internally draining basin and high rift flanks associated with the most recent rifting episode. There are also dominant populations of Neoproterozoic and Mesoproterozoic zircons, likely reworked from the underlying Cretaceous sandstones and derived from younger metamorphic terranes of the Ubendian Belt. Volcanic pulses associated with rifting are responsible for the young magmatic zircons, and suggest the initiation of a late Cenozoic rifting event, further constraining the timing of rifting and basin development in the Western Branch, as well as the timing of landscape change associated with erosion and uplift. Our dates additionally provide important temporal context for the rich vertebrate record described from the East African Rift, illuminating the tectonic backdrop of important large-scale faunal shifts in East Africa.
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    Geologic review of hydrocarbons potential of the Rufiji Basin, Tanzania
    (Springer, 2022-04-15) Sabuni, Rachel; Mtelela, Cassy; Kagya, Meshack
    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.
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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.
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    Lithological and facies analysis of the Roseneaath and Murteree shales, Cooper Basin, Australia
    (Elsevier, 2017) Jadoon, Quaid Khan; Roberts, Eric M.; Henderson, Bob; Blenkinsop, Thomas G.; Wust, Raphael A.J; Mtelela, Cassy
    Unconventional shale plays have received marked attention over the last five years because of their economic potential for hydrocarbon generation, and yet they are amongst the least understood of all clastic sedimentary rock systems. The Cooper Basin is one of the largest Gondwana intracratonic basins of Australia, extending from northern South Australia into southwestern Queensland and covering approximately 130,000 seq. km. The basin may be prospective for shale gas, particularly within the lacustrine shales of the Permian Murteree and Roseneath formations. This study investigates lithological characteristics of these two units in relation to reservoir evaluation. Core samples representing the Dirkala-02 and Moomba-46 wells were used for petrographic analysis. A combination of wireline log analysis, thin section petrography, X-ray diffraction and pyrolysis analysis was used to define and characterize four distinct lithofacies within the Roseneath and Murteree shales: siliceous mudstone, organic siliceous mudstone, calcareous siliceous mudstone, and silty siliceous mudstone. The siliceous mudstone and organic siliceous mudstone are the most common. Diagenetic sideriteoccurs in all lithofacies. A conceptual depositional model is developed for deposition of the Roseneath and Murteree shales. Wireline-log cross plots were interpreted and utilized in the construction of electrofacies. The study was concentrated on the northern portion of the basin between the Nappameri and Patchawarra Troughs in order to understand the nature of lithofacies and variability in reservoir architecture, which was controlled by relative lake-level fluactuation. The results of this study will aid in the evaluation of shale gas potential for this portion of the basin, as well as a better understanding of shale gas opportunities in the Cooper Basin more generally.
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    Oligocene Termite Nests with In Situ Fungus Gardens from the Rukwa Rift Basin, Tanzania, Support a Paleogene African Origin for Insect Agriculture
    (2016-06-22) Roberts, Eric M.; Todd, Christopher N.; Aanen, Duur K.; Nobre, Tânia; HilbertWolf, Hannah L.; O’Connor, Patrick M.; Tapanila, Leif; Mtelela, Cassy; Stevens, Nancy J.
    Based on molecular dating, the origin of insect agriculture is hypothesized to have taken place independently in three clades of fungus-farming insects: the termites, ants or mbrosia beetles during the Paleogene (66–24 Ma). Yet, definitive fossil evidence of fungus-growing behavior has been elusive, with no unequivocal records prior to the late Miocene (7–10Ma). Here we report fossil evidence of insect agriculture in the form of fossil fungus gardens, preserved within 25 Ma termite nests from southwestern Tanzania. Using these well-dated fossil fungus gardens, we have recalibrated molecular divergence estimates for the origins of termite agriculture to around 31 Ma, lending support to hypotheses suggesting an African Paleogene origin for termite-fungus symbiosis; perhaps coinciding with rift initiation and changes in the African landscape.
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    PRELIMINARY SEDIMENTOLOGY AND STRATIGRAPHY OF THE ENIGMATIC MIDDLE LAKE BEDS SUCCESSION (PLEISTOCENE?) IN THE RUKWA RIFT BASIN, TANZANIA
    (College of Natural and Applied Sciences, University of Dar es Salaam, 2018-05) Mtelela, Cassy
    This paper presents sedimentology and stratigraphic overview of a series of previously unrecognized lithostratigraphic unit in the Rukwa Rift Basin, Tanzania. Based on the cross-cutting relationship and pilot detrital zircon geochronology, these isolated, enigmatic strata are herein informally termed the middle Lake Beds (mLB). Facies analysis reveals that the mLB units were deposited within a complex array of depositional environments, ranging from alluvial to fluvial channels, floodplains and shallow lakes. The mLB are characterised herein as isolated outcrop exposures with uncertain age relationships, but which can be divided into five distinctive lithostratigraphic units herein identified as informal members (A-E). These are:(1)at least 36 m-thick lacustrine (limestone) unit (member A); (2) a thin (<15 m-thick) volcaniclastics succession of alluvial sandstones, siltstones, mudstones and thin conglomerates (member B); (3) well-sorted siliciclastic fluvial sandstones (member C); (4) tuffaceous/ash-rich siliciclastics fluvial sandstones and conglomerates (member D); and (5) thin lacustrine volcaniclastics siltstone unit, termed member E. Deposition of the mLB was most likely transpired during tectonically active periods, at times contemporaneously with explosive and effusive volcanism associated with the Rungwe Volcanic Province. The identification of the mLB unit provides an important insight into revising the Lake Beds stratigraphy, and establishing a formal nomenclature of this uppermost mega-sequence in the basin.
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    Sedimentology and paleoenvironments of a new fossiliferous late Miocene-Pliocene sedimentary succession in the Rukwa Rift Basin, Tanzania
    (elsevier, 2017-01) Mtelela, Cassy; Roberts, Eric M.; Hilbert-Wolf, Hannah L.; Downie, Bob; Hendrix, Marc S.; O’Connor, Patrick M.; Stevens, Nancy J.
    This paper presents a detailed sedimentologic investigation of a newly identified, fossiliferous Late Neogene sedimentary succession in the Rukwa Rift Basin, southwestern Tanzania. This synrift deposit is a rare and significant new example of a fossiliferous succession of this age in the Western Branch of East Africa Rift System. The unit, informally termed the lower Lake Beds succession, is late Miocene to Pliocene in age based on cross-cutting relationships, preliminary biostratigraphy, and U-Pb geochronology. An angular unconformity separates the lower Lake Beds from underlying Cretaceous and Oligocene strata. Deposition was controlled by rapid generation of accommodation space and increased sediment supply associated with late Cenozoic tectonic reactivation of the Rukwa Rift and synchronous initiation of the Rungwe Volcanic Centre. The lower Lake Beds, which have thus far only been identified in three localities throughout the Rukwa Rift Basin, are characterized by two discrete lithologic members (herein A and B). The lower Member A is a volcanic-rich succession composed mostly of devitrified volcanic tuffs, and volcaniclastic mudstones and sandstones with minor conglomerates. The upper Member B is a siliciclastic-dominated succession of conglomerates, sandstones, mudstones and minor volcanic tuffs. Detailed facies analysis of the lower Lake Beds reveals various distinctive depositional environments that can be grouped into three categories: 1) alluvial fan; 2) fluvial channel; and 3) flood basin environments, characterized by volcanoclastic-filled lakes and ponds, abandoned channel-fills and pedogenically modified floodplains. Member A represents a shallow lacustrine setting filled by tuffaceous sediments, which grade up into a system of alluvial fans and high-energy, proximal gravel-bed braided rivers. An unconformity marks the contact between the two members. Member B shows an upward transition from a high-energy, gravel-bed braided river system to a sandy braided river system with increasingly abundant floodplain deposits and well-developed paleosols. Vertebrate fossils are sparse in member A, but common in member B, preserved both within pedogenic soil horizons and as isolated elements and microsites within fluvial channel facies associations. Faunal remains include fishes, turtles and crocodylians, along with well-preserved mammal cranial and post-cranial remains. In addition, freshwater gastropod shells are locally present in member A and continental trace fossils, including abundant fossilized termite nests, are present in both members