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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
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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