RE: Estimate of resource2 Nov 2023 01:53
Part 1
@Bluebay, @skittish – thanks for the images you’ve posted…all very thought provoking!
I don’t think the basement layer was reached at all on Tai-1. The thickness of the Karoo sequence will be broadly similar to that defined by the well log data on Ivuna-1, so only from drilling down through the Karoo have they discovered that the underling basement sediments are a lot thicker than expected. MWD should indicate another 100m or so to TD and the metamorphic debris being encountered just before the drilling stopped confirms this as basement sediment with the amount of fractured/weathered basement material likely increase too.
Looking at the seismic section in the CPR (fig 2.5, p19), the interpreted basement layer coded in purple must be the top basement sediments, since it does follow the overlying Karoo sequence in time. The bottom of the basement sediment sequence, should be following an unconformity surface with the ancient metamorphic basement and not following the same trend. It’s not been mapped probably because it’s poorly defined with lower signal to noise ratios below the very strongly reflective top basement. Combine that with the depth conversion using 2D derived data, which led to shallower estimates.
My reading of the Rukwa story from the CPR is:
Firstly, there’s an ancient basement which last underwent deformation 2.3 billion years ago and since that event, it’s considered that helium has been produced from radioactive decay ever since, all of it being trapped within the rock.
Fast forward 2 billion years later to when the basement sediments were laid down, say sometime between 360 – 290 million years ago before the Karoo formation was laid down in Permian times. The (hopefully) extensive weathering and fracturing of the top basement rock will act as a good conduit later on to get the helium out, but all the layers above, reservoirs and seals, right up to the Lake Bed Formations, were deposited while the helium was still trapped in the basement rock.
If TD is now 1550m, the assumed temperature gradient would not have produced temperatures at basement level sufficient to achieve helium closure – say around 55C instead of the 70C needed. It was only 40 million years ago that the tectonic activity and volcanic magmatism raised this temperature over the threshold and then helium began to be released into the pore spaces of the basement rock. Being in the so called ‘goldilocks zone’ meant that the prospect area is close enough to the mantle plume for heating but not too close to risk the helium being diluted by CO2.
So now the presence of an advective carrier in the form of gaseous nitrogen or groundwater was required to migrate the helium out of the pore spaces and upwards into the overlying strata from 40 million years ago until the present. The confirmation of helium shows in the Karoo and Lake Bed formation, strongly suggests that a carrier is present to successfully get the helium out of the base