Geology Reference
In-Depth Information
Table 10.3.
A checklist for assessing the validity of a DHI
Data quality
Is the seismic data quality good enough to interpret relative amplitudes (high signal-to-noise ratio, good event
continuity)?
Have the processing parameters been checked for processes that would significantly distort amplitude behaviour?
Are migration errors and other imaging artefacts (such as multiples) a significant issue?
Is there a high degree of confidence in the phase and polarity of the data (i.e based on well ties)?
If AVO is a critical element of the play:
can reflections be traced from near to far traces on seismic gathers?
Is residual moveout a problem?
Is the approach to AVO analysis appropriate for the given data quality?
Trap definition
Is the trap adequately defined and consistent with the expected style for this play?
Well control/analogues
Is there enough well data of good quality to calibrate the seismic data?
Have the logs been appropriately conditioned?
Have appropriate reflectivity/impedance models been generated for both pay and non-pay scenarios?
Is there good reason to believe that the assumptions in the modelling (including the low-frequency component in
seismic inversions) hold over the prospect area?
Amplitude relationships
Are there clear relative/absolute differences in amplitude across the prospect area that fit with the hydrocarbon rock
physics model, particularly from downdip to updip across the region of the contact and justifying the interpretation of
dim spots, bright spots and phase reversals, as well as other features such as low-frequency shadow zones and
velocity sags? Generally four-way dip traps show more DHI characteristics than stratigraphic traps.
Conformance of amplitude to structure
Do the amplitudes show a high degree of conformance with a particular depth contour?
Flat spot
If a flat spot exits, does it show discordance to stratigraphy?
Are apparent terminations present?
Does the flat spot have amplitude, polarity and AVO behaviour in accordance with the modelled hydrocarbon
response?
Confidence in the play
Confidence in the stratigraphic identification of seismic reflections is high.
Has the target been drilled before in this type of situation (i.e is there a good (local) analogy)?
Are there reliable statistics from well results available for the target of interest?
Is the effect of variation in lithology, fluid fill and bed thickness as a function of reflection angle well understood? Note
that a single well may provide only limited calibration; the chance of the lithology being significantly different from
that at the well may be quite high.
Rose,
2001
). In this case (
Fig. 10.36
), the axes are
defined as the knowledge of the play and the confi-
dence in recognition of a DHI. The checklist ques-
tionnaire would help in arriving at the appropriate
position in the matrix and the assigned probabilities
would be specific to a particular play, data types and
analytical techniques. Populating the matrix of course
requires access to data for a sample of prospects.
This is relatively straightforward for large operators
drilling a large number of wells but much more diffi-
cult for smaller companies.
10.5.2 A Bayesian approach to
prospect risking
The DHI risk is often used to lower the prospect risk
where a valid DHI is present but it can also be used to
increase the risk in situations where a DHI is absent,
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