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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