Geology Reference
In-Depth Information
Diverse stratigraphy
Varying depositional
style, sand quality and
thickness
Sabine River
Yegua outcrop
Strandplain
Fluvial
Katy delta
Bar/strandplain
'mid-dip' shelf
Duval delta
Water sand
Gulf
of
Mexico
Gas sand
overpressured
Bar/strandplain
Rio Grande
Figure 10.37
The Yegua trend of onshore Texas and the first-order AVO model (redrawn after Allen and Peddy,
1993
).
mature gas play comprising shallow high-porosity
sands that when gas-filled give bright spots on
stacked sections and increasing amplitude with
offset on pre-stack gathers. Shale on brine sand
reflections generally have the opposite polarity to
shale on gas sand, and show decreasing amplitude
with offset (
Fig. 10.37
). Eighty-four wells were
drilled on AVO anomalies described from 2D data.
Commercial success rates improved dramatically,
from 5%
Commercial success
Gas shows at objective
No shows
30%
50%
20%
Figure 10.38
AVO results from the Yegua trend (after Allen
et al.,
1993
).
10% to 50% when AVO techniques were
employed (
Fig. 10.38
). This is encouraging, but of
course it does not follow that use of AVO will bring
similar improvements elsewhere. Interestingly,
around 10% of the AVO anomalies were considered
to have had low validity. In the majority of failure
cases sand was encountered (
Fig. 10.39
). Reasons
for failure included:
-
10.5.3 Risking, statistics and other
sense checks
Given the business importance of prospect risking
most oil companies do not tend to publish results.
Thus, the literature is fairly impoverished when it
comes to detailed case studies. Some general papers
have been published and a few observations are worth
highlighting.
An early example of the role of DHI informa-
tion in drilling results from the Yegua trend,
onshore Texas has been published by Allen and
Peddy (
1993
) and Allen et al.(
1993
). This is a
problems with 2D data quality
ยท
imaging effects, including out of plane effects,
migration/Fresnel zone issues and seismic
noise,
polarity and phase issues,
250
low gas saturation,
