in a Geotrack report?
ANGLESEA-1: a THR case study
typical Geotrack Report contains much more than just basic AFTA data
(fission track age, track lengths). What we aim to do is to synthesise
temperature-time solutions from individual AFTA samples with vitrinite
reflectance data and/or other types of maturity data to provide a thermal
history framework for the entire sampled section. AFTA results from
multiple samples within a well are combined to provide the best constraint
on the timing of cooling, while the variation of maximum paleotemperatures
with depth provides insights into mechanisms of heating and cooling.
Where applicable, estimates of paleogeothermal gradients and amounts
of eroded section can be obtained.
This Information Sheet
summarises the information which is provided in a Geotrack Report for
a Thermal History Recontruction study of a typical well. The example
is based on the Anglesea-1 well, located in the Otway Basin, Southeastern
samples were analysed from this well. Thermal history parameters
are extracted from the data using detailed kinetic modelling.
(see Geotrack Information Sheet 99/2) This defines the range of
temperature and time conditions consistent with the measured data.
solid line defines the profile expected if samples throughout
the section are currently at their maximum temperature since deposition,.
The data plot well above the profile, showing that the section
has been hotter in the past.
The question is - WHEN?
burial history derived from the preserved section (used together
with the present gradient to predict the profile shown in the
Figure to the left) contains two unconformities, one in the Late
Cretaceous and one in the Tertiary.
each AFTA sample, data suggest two major episodes of heating
and cooling. Boxes define the maximum or peak paleotemperatures
and timing of cooling determined from the AFTA data in each
of timing information from the three AFTA samples defines
two key paleo-thermal episodes: cooling began some time
between 110 and 95 Ma in the earlier episode and between
60 and 10 Ma in the later episode.
linear nature of paleotemperature profiles derived from
the AFTA and VR data suggests that heating in both episodes
was due chiefly to deeper burial.
analysis of paleotemperature data provides rigorous constraints
on paleogeothermal gradients and removed section. The contoured
regions define the range of allowed values (within ±2
sigma limits) for each episode.
the best-fit values of removed section and paleogeothermal
gradient, we can reconstruct thermal histories for each
preserved unit. The Early Cretaceous section reached maximum
temperatures during the mid-Cretaceous episode.
burial and uplift history is reconstructed by adding in
the removed section during each episode estimated from the
the reconstructed thermal histories, the maturity of each unit
through time can be predicted. Early Cretaceous units reached
maximum maturity values within the oil or gas windows during the
mid-Cretaceous episode, while Tertiary units remain immature.
Only early traps that were available when hydrocarbons were generated
will be prospective in this region.
variation of maturity with depth at the present day predicted
from the reconstructed thermal histories matches the data, as