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The AAPG/Datapages Combined Publications Database
Houston Geological Society Bulletin
Abstract
Abstract: Delineation of a Diagenetic Trap Using P-Wave and Converted-Wave
Seismic
Data in the Miocene McLure Shale, San Joaquin Basin, CA
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By
EOG Resources Inc., Denver CO
North Shafter and Rose oil fields, located in California’s
San Joaquin basin, produce hydrocarbons from a subtle
stratigraphic trap within the Miocene Monterey Formation. The
trap-reservoir system was created during burial of a thick
diatomaceous shale sequence that forms various diagenetic
facies. Integration of well and 2D P-wave seismic
data shows that
a significant amplitude anomaly is present over both the reservoir
(quartz) and seal (Opal-CT) facies making delineation of
the updip edge problematic. The porosity of the Opal-CT and
reservoir quartz facies ranges from 24–50%.
Petrophysical analysis and seismic
modeling
result in the following
conclusions. Opal-CT and hydrocarbon-saturated quartz
have nearly the same acoustic impedance. Opal-CT is low density
while hydrocarbon-saturated quartz is low velocity. The presence
of gas-saturated oil in the quartz reduces the interval velocity in
a manner similar to the Gasmann effect in high-porosity sandstones.
The down-dip wet quartz interval is not associated with a
seismic
amplitude anomaly because its impedance is similar to
that of the bounding shales. Finally, converted-wave data, that
chiefly image lithology rather than fluids, can be used to delineate
the low-density Opal-CT from the higher density quartz.
Based on these conclusions, 2D converted-wave data were acquired to complement P-wave data. From these data sets the regional Opal-CT-to-quartz phase transformation boundary was mapped and a matrix of amplitude signature verses facies was constructed. This work formed the basis for mapping the hydrocarbon- saturated quartz facies.
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