Early January 2023, the Australian Junior Oil Company Invictus Energy completed the drilling of the Mukuyu 1 well, and subsequent sidetrack, in the Cabora Bassa Basin of Northern Zimbabwe.
Invictus Energy consider the well a geological success, encountering gas and oil in multiple reservoir zones in Triassic Karoo Supergroup sediments, proving a working petroleum system, along with potentially opening a new play in the region.
Repeated equipment failures hindered the acquisition and retrieval of a fluid sample, required under the Australian Securities Exchange Ltd (ASX) rules to declare a formal discovery.
Based on the current data, independent resource estimates suggest the Cabora Basin Basin may hold in excess of 5.5 billion barrels of oil equivalent across several prospects. Invictus Energy plans to integrate their findings from the well to develop a more refined understanding of the basin, along with updated resource estimates, intending to spud Mukuyu 2 in Q3 2023, along with potentially attracting partners to continue the exploration effort across the licence.
Context
The Cabora Bassa Basin is one of several rift basins found across Southern Africa. These formed during a series of tectonic events along the margins of Gondwana from the Late Carboniferous to Jurassic, within which a mixture of marine, marginal marine, to continental sediments were deposited as part of the Karoo Supergroup. These basins were subsequently uplifted and partly eroded.
Outcrop and shallow borehole studies elsewhere in the region have indicated the key elements of a working petroleum system, including the presence of well-developed Permo-Triassic Karoo Group sandstones and organic-rich shales 40-60 km to the west of the licence area. Based upon these analogues, pre-drill estimates were that the Mukuyu targets may be relatively sand-prone, with possible sealing and trap containment issues.
A database consisting of legacy 2D seismic from earlier exploration by Mobil in the 1990’s was supplemented by a new survey acquired by Polaris Natural Resources in 2021. This revealed a series of 3- and 4-way closures across the licence area. Quantitative analysis also suggests the presence of flat spots and other direct hydrocarbon indicators. The Mukuyu 1 well spudded on the 26th September 2022, and was targeting a series of stacked reservoir units in various Formations of the Karoo Supergroup.
Read also: How Stryde enabled new African opportunities to be explored
Well results
The drilling was dogged by adverse hole conditions, repeated equipment failures, and logistical issues, but was completed without any safety incidents. Despite this, Invictus Energy believes it has strong petrophysical evidence, and supporting data, to suggest multiple pay zones were encountered during drilling.
Mukuyu 1 ST1 identified at least 13 potential hydrocarbon bearing sandstone zones, with porosities of up to 15%, and gas saturations of up to 90% in the Triassic Upper Angwa Formation, their primary target. A shallower 10-15 m sandstone unit with complex mineralogy and gas indications was encountered in the shallower Post Dande Formation. Repeated reservoir, seal, and source packages with hydrocarbons were encountered until total depth of the well at 3603 m MD in the Upper Angwa Formation.
The Lower Angwa Formation remains untested. The well encountered a series of Triassic-aged non-marine, fluvial, fluvio-deltaic, pluvial, and lacustrine clastic sediments. Further palynological analysis will be undertaken to refine the age and environments of deposition. Invictus Energy liken these to the Karoo Supergroup strata found in the Ambilobe, Sakamena, and Majunga Basins of Madagascar.
The wells encountered considerably thicker shale sequences in the Upper Angwa Formation than predicted pre-drill, which has in turn decreased seal risk within these units. Overall, these units were thicker, and therefore deeper, than prognosed. Further analysis is pending the analysis of core data, where a total of 24 cores were taken in the reservoir, source, and seal intervals. Larger sidewall cores were taken, which may allow the retrieval of native fluids from the reservoir units, to better calibrate petrophysical models and highlight potential pay characteristics.
Initial core analysis has revealed strong fluorescence in sidewall cores taken from 2407 m MD onwards in the Pebbly Arkose Formation. Lower fluorescence and higher gas concentrations were noted in Mukuyu 1 ST1 compared to Mukuyu 1, which the company interprets may relate to a paleo-fluid contact, or that there is possibly considerable variation in reservoir and fluid properties across the broader Mukuyu structure. Post-well pressure analysis of pressure data suggests the potential for significant gas columns in the Pebbly Arkose and Upper Angwa Formations.
Way forward
The next phase of the exploration of the Cabora Bassa Basin will build upon the integration of the well and seismic data to refine the geological structure of the licence, refine basin modelling, updated resource estimates, and allow the high-grading and ranking of prospects.
Invictus Energy is uniquely positioned in owning the licence across the geological limits of the Cabora Bassa Basin. They interpret a “string of pearls’ along the basin margin, analogous to the Lokichar Basin in Kenya, and the Albertine Graben in Uganda, themselves proving muil-billion barrel potential in East African Rift systems.
In a recent podcast, Invictus Energy Managing Director Scott Macmillan noted that the emphasis of the next exploration phase will be upon data gathering, cost consciousness, reducing uncertainty, gaining new information, extending and drilling play fairways, and unlocking the prospectivity of the basin.
In the same interview, he suggests acquiring infill 2D surveys across other parts of the basin, which is anticipated to start in Q2 2023. Longer term, a 3D seismic survey could be used to conduct seismic geomorphology studies to delimit fluvial channel bodies, and aid exploration and development well placement. Upgrades will also be made to the mud gas system and other equipment on the Exalo 1 rig, which Invictus Energy have retained for 12 months following the TD of the Mukuyu 1/ST1 wells.
Invictus Energy intends to spud Mukuyu 2 in Q3 2023, along with potentially attracting partners to continue the exploration effort across the licence. The exact location is yet to be determined, pending further subsurface work and permitting.
Thanks to Scott Macmillan, Managing Director of Invictus Energy, for his input and permission to publish this article.
References
Catuneanu, O., Wopfner, H., Eriksson, P.G., Cairncross, B., Rubidge, B.S., Smith, R.M.H., and Hancox, P.J., 2005. The Karoo basins of south-central Africa. Journal of African Earth Sciences, 43, 211-253.
Cryton Phiri, C., Wang, P., and Nyambe, I.A., 2016 Geology and potential hydrocarbon play system of Lower Karoo Group in the Maamba Coalfield Basin, southern Zambia. Journal of African Earth Sciences, 118, 245-262.
Davidson, I., and Steel, I., 2017. Geology and hydrocarbon potential of the East African continental margin: a review. Petroleum Geoscience, 24(1), 57-91.
Oesterlen, P.M., and Millsteed, B.D., 1994. Lithostratigraphy, palaeontology, and sedimentary environments of the western Cabora Bassa Basin, Lower Zambezi Valley, Zimbabwe. South African Journal of Geology, 97(2), 205-224.
