Sedimentology and stratigraphy of the late Cenozoic lake beds succession, Rukwa Rift Basin, Tanzania: implications for hydrocarbon prospectivity

Mtelela, Cassy (2016) Sedimentology and stratigraphy of the late Cenozoic lake beds succession, Rukwa Rift Basin, Tanzania: implications for hydrocarbon prospectivity. PhD thesis, James Cook University.

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Abstract

The late Cenozoic Lake Beds succession forms the most significant strata for economic and academic aspects in the Rukwa Rift Basin, yet represents the least studied and most poorly understood sedimentary package in the basin. Deposition of the Lake Beds succession resulted from the tectonic development of the modern East African Rift System, and thus represents an important archive of the associated environmental and climate changes in eastern Africa, as well as a record of floral and faunal evolution during this time. Economically, the Rukwa Rift Basin was subject to brief, but unsuccessful hydrocarbon exploration during the late 1980s. The results of these investigations were inconclusive, but indicated potential source and reservoir rocks for hydrocarbon in the different portion of the basin stratigraphy, underlying the Lake Beds succession. However, hydrocarbon exploration in the basin has been hampered by a poor understanding of the basin stratigraphy and sedimentology. For instance, the upper half of the rift stratigraphy (including the Lake Beds succession) was long considered to be too young for hydrocarbon generation and accumulation. However, recentl discoveries of significant hydrocarbon accumulations in the Neogene rift basin strata in Lake Albert (Uganda) and Turkana (Kenya) have prompted renewed hydrocarbon exploration in the Rukwa Rift Basin by Heritage Oil, but with a focus on the potentially correlative Late Cenozoic Lake Beds succession. This project was, therefore, developed to conduct the first detailed investigation on Lake Beds stratigraphic and sedimentology, via outcrop-based lithofacies mapping, sedimentary petrology, X-ray power diffraction mineralogy, provenance analysis, geochronology, sequence stratigraphy and re-examination of existing legacy exploration well-cuttings.

The results demonstrate a complex stratigraphy and depositional history for the Lake Beds, and are used to: establish a formal stratigraphic framework; reconstruct tectonic, paleoenvironmental and palaeoclimatic trends in the basin through time; and assess the hydrocarbon prospectivity of these strata. Two new formations, herein named the Malangali and Ilasilo formations, are defined. The Malangali Formation represents a previously unrecognized late Miocene to Pliocene unit in the RRB, which is subdivided into the lower Mpona and upper Hamposia members. The Mpona Member records a myriad of volcanically influenced alluvial, fluvial and lacustrine depositional environments recording the initiation of Neogene rifting and volcanism in the RRB. A major unconformity and provenance shift marks the transition between the two members. The Hamposia Member is characterised by well-developed paleosols and large sandy channel systems interpreted to record a semi-arid depositional environment and preserving a previously unknown fauna of this age dominated by fishes, turtles, crocodyliformes, hippopotami, and large ungulates. Overlying the Malangali Formation, is a thick succession of Late Quaternary volcanic-rich lacustrine dominated strata. Above this, the upper 100 m of the basin stratigraphy is widely exposed across the RRB and formally defined in this study as the Ilasilo Formation. Detailed sedimentologic, geochronologic and sequence stratigraphic analysis of these strata reveal eight discrete depositional sequences that record significant tectonic and climate-driven lake cyclicity in the Rukwa Rift Basin during the Quaternary, including +/-70 m lake level fluctuations that occurred over the last 50 ka. Lake cyclicity and sedimentation were also strongly influenced by periodic, intense volcanism in the nearby Rungwe Volcanic Center, which lead to rapid transitions between overfilled and underfilled lake basins.

This sedimentologic and stratigraphic investigation indicates positive characteristic for hydrocarbon source, reservoir and seal rock capacity in the late Cenozoic Lake Beds Group. A synthesis of facies analysis, sandstone petrography and well-cuttings examination indicates that base level cyclicity of Lake Rukwa during late Cenozoic resulted in deposition of alternating profundal lacustrine deposits that are organic-rich (diatomaceous) and fluvial deltaic sandstone and conglomerate dominated deposits that are highly porous and permeable, respectively. Furthermore, lithologic, petrographic and mineralogy studies presented herein indicate that the uppermost portion of the strata (the Ilasilo Formation) is dominated by less porous and impermeable volcanic units that are identified as potential seal rocks. These preliminary findings encourages follow up exploration techniques across the basin, as well as sampling and analysis of the subsurface portion of the strata and underlying older units to fully appreciate the hydrocarbon prospectivity of the Rukwa Rift Basin, and in particular, the late Cenozoic Lake Beds Group as a complete "hydrocarbon kitchen".

Item ID: 48569
Item Type: Thesis (PhD)
Keywords: Africa, Cenozoic, facies analysis, hydrocarbon, lake beds, neogene, Pleistocene, quaternary, Rukwa Rift Basin, Rukwa, sedimentology, stratigraphy, Tanzania
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Publications arising from this thesis are available from the Related URLs field. The publications are:

Chapter 1: Mtelela, Cassy, Roberts, Eric M., Downie, Robert, and Hendrix, Marc S. (2016) Interplay between structural, climatic and volcanic controls on Late Quaternary lacustrine-deltaic sedimentation patterns in the Western Branch of the East African Rift System, Rukwa Rift Basin, Tanzania. Journal of Sedimentary Research, 86 (10). pp. 1179-1207.

Chapter 2: Mtelela, Cassy, Roberts, Eric M., Hilbert-Wolf, Hannah L., Downie, Robert, Hendrix, Marc S., O'Connor, Patrick M., and Stevens, Nancy J. (2017) Sedimentology and paleoenvironments of a new fossiliferous late Miocene-Pliocene sedimentary succession in the Rukwa Rift Basin, Tanzania. Journal of African Earth Sciences, 129. pp. 260-281.

Appendix 1: Roberts, Eric M., Todd, Christopher N., Aanen, Duur K., Nobre, Tânia, Hilbert-Wolf, Hannah L., O'Connor, Patrick M., Tapanila, Leif, Mtelela, Cassy, and Stevens, Nancy J. (2016) Oligocene termite nests with in situ fungus gardens from the Rukwa Rift Basin, Tanzania, support a paleogene African origin for insect agriculture. PLoS ONE, 11 (6). pp. 1-17.

Date Deposited: 20 Apr 2017 01:21
FoR Codes: 04 EARTH SCIENCES > 0403 Geology > 040301 Basin Analysis @ 30%
04 EARTH SCIENCES > 0403 Geology > 040310 Sedimentology @ 35%
04 EARTH SCIENCES > 0403 Geology > 040311 Stratigraphy (incl Biostratigraphy and Sequence Stratigraphy) @ 35%
SEO Codes: 85 ENERGY > 8501 Energy Exploration > 850103 Oil and Gas Exploration @ 50%
97 EXPANDING KNOWLEDGE > 970104 Expanding Knowledge in the Earth Sciences @ 50%
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