history in hydrocarbon exploration
Various factors are widely recognised as important aspects of
hydrocarbon exploration, e.g.
- source rock
|Another important factor is the timing of hydrocarbon
generation in relation to the time at which structures were formed,
which is a central aspect of the concept of the "Petroleum
Petroleum system: basic
essential elements (source rock, reservoir, seal, overburden)
and processes (trap formation, generation-migration-accumulation
of petroleum) must occur in time
and space so that organic matter in a source rock can be converted
to a petroleum accumulation."
Magoon and Dow, 1994, AAPG Memoir 60
|For simple histories reconstructing the history of
hydrocarbon generation through time is relatively straightforward,
as illustrated by this example from the Gippsland basin.
But in other cases, the situation is less straightforward.
The section intersected in the Anglesea-1 well (Otway Basin) contains
two unconformities. Source rocks within the Cretaceous section
are within the oil window, but when was the oil generated?
How do we reconstruct a complete history when the preserved section
is incomplete? An example from the onshore UK.
History Reconstruction (THR)
THR relies on application of Apatite Fission
Track Analysis (AFTA®) and Vitrinite Reflectance (VR) to:
the timing of dominant episodes of heating and cooling that
have affected a sedimentary section
the paleotemperatures through the sequence
- and then to characterise
the mechanisms of heating and cooling
THR relies on measured thermal history parameters
a more complete thermal, maturation and burial / uplift history
In this way, the history of hydrocarbon
generation can be assessed within a consistent framework, constrained
by measured data.
This approach provides firm constraints
which must be honoured by any viable basin model - Constrained
|Applying this approach to the Anglesea-1 well reveals
that Early Cretaceous source rocks began to cool from maximum paleotemperatures
between 110 and 95 Ma. We can also detect a later event.
|In wells from the East Midlands of England we find
that Carboniferous source rocks began to cool from maximum paleotemperatures
in the Early Tertiary (between 65 and 50 Ma). A later cooling
episode is also revealed.
fission track analysis (AFTA®)
on analysis of radiation damage trails ("fission tracks")
within detrital apatite grains (extracted from sandstones and
other clastic rocks)
fission track is created by spontaneous fission of a single atom
Fission tracks can
be selectively dissolved and enlarged by etching in dilute nitric
|Etched spontaneous fission tracks in an apatite crystal.
|The number of tracks
in an apatite grain depends on:
- uranium content
Therefore in principle, if we measure the uranium content and
the number of tracks, we can measure the time over which tracks
have accumulated - the fission track age
|But if we measure fission track ages
in sub-surface samples, we find that at temperatures greater than
~70°C, the ages are progressively reduced to zero at around 120°C.
reduction in fission track age arises because the radiation damage
constituting each fission track is progressively repaired, at
a rate which increases with temperature. This repair is
manifested as a decrease in the length of individual tracks.
The proportion of
tracks intersecting the polished surface of an apatite grain depends
on the track length, and therefore as track length is progressively
reduced, so is the fission track age.
|Confined tracks in apatite.
grain fission track ages in a sample from a present-day temperature
of ~95°C show a clear and consistent variation with Cl content:
Chlorine content exerts
a significant control on fission track annealing rates in apatite,
as also seen in laboratory experiments:
fission track analysis (AFTA®)
AFTA is based on three types of measurement:
- fission track age
- confined track lengths
- chlorine content
content is measured in every grain in which either age or length
data is collected. Grain locations are recorded using computer-controlled
microscope stages, and archived for future reference.
modelling AFTA parameters through likely thermal history scenarios,
we can define the range of temperature-time conditions giving
predictions which are consistent with the observed data.
of chlorine content
Common detrital apatite grains in sedimentary
rocks from around the world show significant variation in chlorine content,
both within individual samples and between different samples.
- Ternary plot 4172 grains |
The kinetic model used to extract temperature-time solutions
from our AFTA data explicitly takes into account the effect of Cl content
on annealing rates.
Calibration of this kinetic model against data from simple geological
situations confirms the validity of the model.
Use of a kinetic model that does not take compositional effects into
account (above right) provides a very poor match with the calibration
response of VR, as expressed in the Burnham and Sweeney (1989)
model is very similar to the kinetics of fission track annealing
in durango apatite.
and Sweeney (1989) kinetic model has been calibrated against
data from similar geological situations, in similar fashion
Integration of AFTA and VR is useful in a number
of ways, e.g.:
- providing independent verification of paleo-thermal
- extending information from AFTA to units dominated
by fine-grained lithologies
- identifying multiple paleo-thermal episodes
pitfalls in using VR data
data from different laboratories vary appreciably in quality and
reliability. Our experience suggests that attribution
of modes - e.g. caved, indigenous and re-worked, solely
from the data is often erroneous. VR data produced on the
basis of identifying the indigenous vitrinite population on petrographic
grounds provides the most reliable data.
The results on the right are from a recent Geotrack
study. VR data supplied by a client suggest all units throughout
the well are currently at their maximum temperatures, whereas
AFTA and new VR data suggest that most units have been hotter
in the past.
Further investigation of the original VR dataset
revealed a number of populations within the measurements, as shown
on the right, including a population of higher reflectances, originally
interpreted as "re-worked" vitrinite, and a lower reflecting
population attributed to caved material. Equivalent maturity values
from SCI and Tmax data are highly consistent with these
"re-worked" vitrinite values.
The new VR values, the equivalent maturity values
from AFTA, SCI and Tmax data and the "re-worked"
vitrinite values from the original dataset reveal a well-defined
trend, as shown on the right, consistently higher than the values
originally attributed to the indigenous population. Thus, these
original values are clearly too low and values originally designated
as "reworked", together with the AFTA, SCI and Tmax
values, provide the most reliable indication of true maturity
levels in this well.
Similar observations are remarkably common, and illustrate
a general tendency amongst many geologists to underestimate the importance
of exhumation (or deeper burial). Thus, the reflectance population closest
to the value expected on the basis of the preserved section is most
commonly identified as the indigenous population. These observations
highlight the importance of combining information on palaeo-thermal
effects from different techniques, in order to provide a more objective
The occurence of three separate sub-parallel
trends denoting caved, in-situ and re-worked vitrinite, as in the earlier
analyses illustrated here, is highly unlikely, and more likely arises
because of measurement of macerals other than true vitrinite. The occurrence
of three sub-parallel trends is usually a reliable sign of maceral misidentification.