UK oil & gas firm gleans more insight into Namibian asset’s hydrocarbon prospectivity
UK-based oil and gas company Tower Resources has undertaken a basin and thermal maturity study within its operated license PEL 96 in Namibia. This has significantly progressed the AIM-listed player’s understanding of the hydrocarbon prospectivity of the license.
The UK player highlights that the results of its recently completed geochemical and thermal maturity basin modelling study have positive implications for the prospectivity of the license PEL 96 in Namibia, covering offshore blocks 1910A, 1911 and 1912B. These are being incorporated into a play fairway analysis covering the full license area. Back in 1994, the 1911/15-1 well, which was drilled by Norsk Hydro within the Dolphin Graben, encountered several potential source horizons in both the Upper and Lower Cretaceous, providing the most valuable and well-documented calibration point for the basin modelling.
For Tower Resources, this well has been “critical” in identifying the most regionally geologically consistent input parameters for the computer models and has provided “crucial” geochemical data for optimising the basin modelling inputs and ensuring a very close fit between the output predictions of the computer models and the real-world measurements from the wells and seismic data. The company underscores that there is “clear evidence” of a working petroleum system present in PEL 96.
According to the company, the purpose of the basin modelling analysis was to assess three critical elements of the hydrocarbon charging system, including providing an evaluation of the thermal maturity of the main known source rocks and their areal extent; estimating the generative capacity of the mature source rocks within the generative kitchens within PEL96 and volumes of hydrocarbons generated; and assessing the timing of generation/expulsion of hydrocarbons, phase of hydrocarbons expelled and map main migration pathways for hydrocarbons, and to assess volumetrics of the principal fetch areas for each exploration target.
Furthermore, the presence of an oil-mature early Cretaceous and/or potentially older source rock is proven by the presence of a light – thermally mature – oil which was recovered from core samples taken from an Albian-aged carbonate in the exploration well 1911/15-1. The well drilled an intra-basinal high within the central part of the Dolphin Graben. As the basin modelling outputs are consistent with the observed well data, this implies that this oil represents hydrocarbons that have migrated from a nearby mature source kitchen within the adjacent half-grabens during Oligo-Miocene times.
In addition, the UK firm explains that oil samples recovered from 1911/15-1 have biomarkers indicative of a lacustrine source – of likely syn-rift origin – which implies Barremian and/or older early Cretaceous source rocks. Tower says that its new technical work, especially the detailed migration pathway analyses, suggests the presence of only residual hydrocarbons was due to trap breaching, caused by later structural movement and tilting, which spilled any accumulation to the East.
Additionally, the company underscores that the presence of an oil-mature early Cretaceous and/or potentially older source rock is proven by the presence of -what it deems to be – compelling seismic evidence of widespread direct hydrocarbon indicators in the form of large gas chimneys, where gas is observed to be migrating up and along deep-seated faults from half graben depocenters, and also from the presence of seismic anomalies indicating shallow gas accumulations above and in the vicinity of the gas chimneys.
Tower Resources outlines that these shallow gas anomalies and gas chimneys are particularly prevalent along the western margin of the Dolphin Graben and directly along mapped major migration pathways, and are associated with major extensional faults that extend into the deepest parts of the half-graben basins.
What initial conclusions can be drawn?
Tower Resources lists eight main conclusions of the basin modelling study. The first one states that the results of the basin modelling analysis, integrated with seismic sequence stratigraphic interpretation of the extensive 2D seismic data sets, indicate that oil-mature Lower Cretaceous source-prone intervals belonging to the early Cretaceous syn-rift phase of the basin are widespread within PEL 96 and present over much of the Dolphin Graben throughout PEL 96.
Additional sub-basins are also present in the western part of PEL 96 that may extend the presence of source-prone early Cretaceous, with the potential for additional hydrocarbon generation in this area.
The second conclusion underlines that Lower Cretaceous source-prone intervals are calculated to sit within the oil window over a large part of the Dolphin Graben within PEL 96, reaching Main and Late Oil Maturity Windows over much of the area, while the third one specifies that the timing of the source-prone intervals entering the main and late oil generation windows calculates to be mid-Tertiary (Oligocene) to present-day.
The fourth conclusion confirms that migration pathways have been analysed and mapped, thus, the main foci of migration out of the source kitchens have been identified and the volumes of hydrocarbons generated in each fetch area have been quantified. Tower Resources elaborates that the basin has undergone a significant young structural tilting event (Mio-Pliocene age) which has re-orientated several of the major migration pathways.
As a result, migration maps needed to be constructed for both the present-day and also back-strip basin configuration to map the main migration pathways at a time pre-dating the tilting and coinciding with the onset of main oil generation (mid-Oligocene).
The fifth conclusion highlights that source kitchens have been calculated and mapped within PEL 96. Several of these have configurations that would focus any hydrocarbon migration towards several of the key prospects and play fairways. In general, the results show that Dolphin Graben is more mature towards the north and therefore the main volumes of migrations generated and migrated are in the northern parts of the basin.
Based on the sixth conclusion, the fetch areas that focus migration towards each of the main prospects are calculated as having generated oil within different volumetric ranges. While Alpha Prospect fetch area is calculated to have generated in the range of 45 to 79 billion bbls of oil, Gamma Prospect fetch area is calculated to have generated in the range of 15 to 23 billion bbls of oil.
On the other hand, the seventh conclusion indicates that Phoenix High play fairway is calculated to have generated in the range of 45 to 59 billion bbls of oil. Tower Resources underscores that the ranges in volumes of hydrocarbons generated per fetch area are in part a reflection of the late tilting of the basin which causes a reorganisation of the migration pathways from Oligocene to present-day and which changes the areal extent of each fetch area over time; thus changing the volume of hydrocarbons capable of being focused towards each prospect with time.
The last conclusion shows that the potential for stratigraphic traps with major regional onlaps of the main Cretaceous reservoir targets is identified on the seismic data to be located directly on several of the main migration pathways out of the generative kitchens.
The company claims that potential stratigraphic traps are recognised in western areas of PEL 96 along the eastern flanks of the Alpha and Gamma highs in both Lower and Upper Cretaceous reservoir-target sequences and have an up-dip towards the ocean (west) architecture, similar to recent major discoveries by TotalEnergies in South Africa.
Aside from this, potential stratigraphic traps are also recognised in central areas of PEL 96 along the western flanks of the Phoenix High, and potentially the Elephant High slightly to the South. These have an up-dip to the coast (east) architecture similar to some of the major recent discoveries further south in Namibia.
Taking steps to unlock full prospectivity potential
Currently, Tower Resources is undertaking an oil seep analysis to accompany the basin modelling work, and a review of the existing volumetric data on the leads that have already been identified, in a bid to enable a revised evaluation of the prioritisation, and volumetrics associated with those leads.
The company emphasises that the results of the recent technical work will not only highlight the potential of this asset but also provide “a good foundation” in ensuring the remaining initial exploration phase of the PEL 96 work programme, including the acquisition of new 3D seismic data, is sufficiently optimised and targeted to unlock the full prospectivity of the license.
Jeremy Asher, Tower’s Chairman and CEO, commented: “We are excited by the results of the basin modelling work and its indication of the prospectivity of Tower’s license in Namibia. It explains neatly the results of the Norsk Hydro well, the source of the lacustrine oil found within it, and the reasons why that oil found its way into that well and subsequently migrated away from it.
“The conclusions indicate the potential for either of the giant billion-barrel-plus structures in the west of the license to be charged; furthermore, the migration pathways, coupled with the recent impressive industry successes in drilling stratigraphic plays in the Orange Basin to the south, enhance our interest in the similar stratigraphic leads that we interpret on the flanks of the Alpha Prospect structure in particular.”
The results of the basin modelling work came months after Tower Resources completed an institutional placing of up to $6 million. The proceeds from this placing were to be used to fund work programme commitments for the company’s licenses in Cameroon, Namibia, and South Africa.