requires samples of sandstone or coarse siltstone - unwashed,
or washed and dried cuttings, whole core, sidewall cores or outcrops.
Apatite is among the most resistant of accessory minerals and
is common in sediments and many basement rock types. Sandstones,
coarse silts, sandy or silty shales and a range of igneous and
metamorphic rock types are suitable for analysis. In sandstones,
it is advisable to collect the most mineralogically immature units,
and in general we find that 80 to 90% of sandstone samples collected
on this basis contain sufficient apatite for analysis.
In a downhole AFTA study, each
cuttings or core sample can be composited over a depth interval
representing a range in downhole temperature of not more than
about 5°C, corresponding to a depth range of about 150m for a
typical geothermal gradient of 30°C/km. A single sample
should not be composited across
an unconformity or major stratigraphic boundary. If core is used,
we do not need an integral solid piece of core. As the sample
will be crushed anyway, we can use offcuts, rubble or scraps left
behind from previous sampling.
Samples from depths where present
downhole temperatures exceed about 110°C are not generally useful
for AFTA (corresponding to about 3400m for a typical geothermal
gradient of 30°C/km).
A composite sample of about 1
kg (2lb) is required. There may be situations where a greater
or lesser amount is needed eg if the sample is a clean quartzose
sand, a larger sample may be required. In contrast, for a less
mature sandstone, or a granitic rock, about 500g may yield enough
apatite. It is better to err on the side of too much rather than
Sample Packing and shipping
information is available.
Sampling strategy depends on the
problems to be addressed. We recommend that you contact us to
discuss your specific requirements. It is always very useful,
where possible, to review vitrinite reflectance (VR) data before
planning an AFTA program. Geotrack provide a VR service and we
are happy to evaluate existing data before recommending an optimum
sampling strategy for AFTA. If VR data indicate that the section
has been hotter in the past, this can help target the optimum
AFTA samples to obtain the timing of maximum paleo-temperatures.
Some examples of sample strategy to address specific problems
are given below.
Constraining paleogeothermal gradient,
uplift and erosion:
If a key problem is constraint
on paleogeothermal gradients, eg to determine the amount of section
removed by uplift and erosion, a vertical sequence of AFTA and
VR samples is required through the largest possible section in
a well, ie from near surface to TD, bearing in mind the maximum
present down-hole temperature limit for AFTA. Between 4 and 8
AFTA samples may be needed in this case, depending on the quantity
and quality of the VR data.
Constraining time of maximum paleotemperatures:
If the problem is to determine
when a section cooled from its maximum paleotemperatures, the
optimum samples for AFTA would come from sections where the VR
is about 0.7% or more, and where the downhole temperature is less
than about 90°C if possible. A single sample from this setting
may be sufficient to provide a tight constraint on timing, but
2 or 3 samples over a limited range of depths would be advisable.
If little is known of the thermal
history, we would advise 6 to 8 AFTA samples spread over the largest
possible vertical section in the well, from near surface down
to a depth where present day temperatures are about 110°C. All
major stratigraphic units should be sampled, and a complete VR
profile should be collected if possible.
The application of AFTA in frontier
basins utilising either outcrop samples or samples from seismic
shot holes is extremely cost-effective in defining thermal history
constraints early in an exploration program. The number of samples
needed to provide a thermal history framework will depend on the
complexity and size of the area. We recommend that you discuss
specific details with us before collecting samples for AFTA.
A certain amount of basic information
is required to produce a comprehensive interpretation report;
e.g. general stratigraphic data (depths to formation tops), sample
depths and stratigraphic ages, estimates of present-day downhole
It is also useful to have some
knowledge of the location of the samples in relation to known
regional structure. We highly recommend obtaining vitrinite reflectance
data from the area of an AFTA survey. In our reports, we routinely
integrate AFTA interpretations with vitrinite reflectance results
to provide the most rigorously constrained thermal history available
with existing technology.