Geology Reference
In-Depth Information
conformance of amplitude to structure on far-
offset seismic data,
Thick clean ss shows
Thick clean ss no shows
Thin clean sands no shows
Tight sands shows
Tight sands no shows
No sand
Hard streak (polarity issue)
Lignite
5%
2%
amplitude consistency within the mapped target
area, on gathers, far offset or far-angle stacks,
9%
AVO observations on gathers or offset/angle
stacks: checking that the AVO response is indeed
Class II and not affected by noise, inadequate
NMO correction, multiples and processing
artefacts,
19%
24%
10%
17%
14%
AVO behaviour is anomalous compared to the
same seismic event outside closure,
compatibility of observed AVO response with
prediction from well or modelling based on rock
physics.
Figure 10.39
Yegua trend: geological outcome in cases of failure
of AVO pay prediction (after Allen and Peddy,
1993
).
Tuning,
Interestingly, the success rate within Class II prospects
is higher than with Class III prospects, probably
because of the discrimination provided by the far stack
in the case of phase reversal signatures (
Chapter 7
).
The analysis of failures in the DHI consortium dataset
show many similarities with the Allen and Peddy
(
1993
) study; wet sands (49%), low saturation gas
(23%), tight reservoirs (11%) and no reservoir (17%).
Roden et al.(
2010
) and Forrest et al.(
2010
) echo a
conclusion made by Alexander and Lohr (
1998
) that
prospects with very good DHI signatures tend to be
over-risked whereas poor prospects tend to be under-
risked. High risk (
lithology effects
·
wet sands giving Class III AVO
·
sand quality (e.g. tight sands)
·
misidentification of coal signature
·
shale/shale AVO, possibly related to
anisotropy?
A more global approach to amplitude statistics has
been undertaken by the Rose & Associates DHI Con-
sortium (e.g. Roden et al.
2005
,
2012
; Forrest et al.,
2010
). Partner companies contribute to a database
which in 2012 comprised 217 wells, with roughly
equal numbers of successes and failures. Key param-
eters have been documented including geologic set-
ting, seismic and rock physics data quality, DHI
characteristics, pre drill risk estimates and drilling
results. The reservoirs range in age from Triassic to
Pleistocene. Most of the wells come from AVO Class
III (75%) or Class II (22%) settings. For the Class III
examples the most important DHI characteristics for
assessing the impact on chance of success were (in
order of importance):
<
20%) prospects tend to work only
5% of
the time, whereas mid range prospects
(25%
75% of the time.
In contrast to the Yegua example exploration
using seismic amplitudes in the area West of Shet-
lands appears to be more challenging (e.g. Loizou,
2005
; Lamers and Carmichael,
1999
). Of the 40 pro-
spects in which amplitude was a primary pre-drill
factor, around 25% proved to be discoveries. Most
of the failures are due to lack of a valid trap, with
the amplitude signatures principally being related to
lithological effects. Determining amplitude conform-
ance proves to be difficult in channelized sandstone
environments (Lamers and Carmichael,
1999
). Com-
parable success rates to the Yegua example have only
been attained on valid structural closures. It is evident
that successful exploration in this area requires a good
understanding of stratigraphy as well as defining a
valid trap in a good position to receive charge.
The presence of a DHI has implications for
volume calculations as well as risk; for example
limiting the area of the trap to the AVO anomaly
and/or to the thickness derived from net pay analysis.
-
60%) tend to be successful 35%
-
conformance of amplitude to structure on stacked
or far-offset seismic data,
phase or character change at downdip edge of the
anomaly,
amplitude consistency (uniformity) within the
mapped target area on stacked data,
flat spots,
AVO response anomalous compared to events
above and below.
For the Class II AVO prospects, the most important
characteristics were:
251
