Robert Kidney1, John Arestad2, Anne Grau1, and Robert Sterling1
Search and Discovery Article #20012 (2003)
*Adapted from an oral presentation at the AAPG’s annual convention, 2003, Salt Lake City, Utah, May, 2003. A companion article, entitled “Success! Using Seismic Attributes and Horizontal Drilling to Delineate and Exploit a Diagenetic Trap, Monterey Shale, San Joaquin Valley, California,” is Search and Discovery Article #20011 (2003).
1EOG Resources, Inc, Denver, CO ([email protected])
2ExplorTech, Littleton, 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 the burial process of a thick diatomaceous shale sequence that forms various diagenetic facies. Integration of well and 2-D 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 50% to 24%.
From petrophysical analysis and seismic modeling the following conclusions can be drawn. The Opal-CT and hydrocarbon-saturated quartz have nearly the same acoustic impedance. The Opal-CT is low density while the 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 Gassmann effect in high porosity sandstones. The down-dip wet quartz interval is not associated with a seismic amplitude anomaly since its impedance is similar to the bounding shales. Finally, converted-wave data, which primarily images lithology rather than fluids, can be used to delineate the low density Opal-CT from the higher density quartz.
Based on the above conclusions, 2-D converted-wave data were acquired to complement the p-wave data. From these data sets the regional Opal-CT to quartz phase transformation boundary was mapped and a matrix of amplitude signatures versus facies was constructed. This work then formed the basis for mapping the hydrocarbon saturated quartz facies.
Figure 1. Regional P-wave seismic amplitude anomaly with discovery well. Early drilling that targeted the P-wave seismic amplitude anomaly resulted in McLure penetrations of both the trap and reservoir facies.
Figure 5. P- and S- wave seismic modeling, with Gassman effect in the P-wave model, which shows both mineralogical and fluid effects, while only the mineralogical changes are shown in the S-wave model.
Figure 9. Regional conversion in lower interval of opal CT to quartz on P-wave and C-wave seismic profiles. The two lines are from North Shafter Field and the northern boundary of North Shafter Field.
Figure 10. Regional conversion in upper interval of opal CT to quartz on P-wave seismic profiles. One profile is from the northern boundary of North Shafter Field, and the other is from the north boundary of Rose Field.
An integrated study of the Miocene Monterey Formation in the San Joaquin Basin resulted in discovery of oil in its McLure Member and in the subsequent development of North Shafter and Rose oil fields (Figure 1). Integration of density vs. sonic data from well logs with 2-D (P-wave) seismic data does not delineate the reservoir from the updip seal. The two corresponding reservoir and trap facies (hydrocarbon-bearing quartz and Opal-CT, respectively) have essentially the same impedance (velocity x density). On the other hand, the impedance of the downdip, water-bearing quartz stratigraphic section is similar to that of the bounding shales.
Opal-CT stratigraphic section is low-density.
Hydrocarbon-saturated, quartz section has low velocity.
C(converted)-wave data images lithology rather than fluids.
Data from the seismic program shows that the gas-saturated oil in the quartz section in both an upper and a lower interval reduces the interval velocity in a similar fashion to the Gassmann effect in sandstones with high porosity (Figures 4 and 5). There is no seismic anomaly associated with the water-bearing quartz section in both intervals. Converted-wave data delineates the Opal-CT section (with low density) from the quartz section (with higher density).
In effect, the P-wave data define the downdip water-bearing interval, and the converted-wave data defines the updip diagenetic seal formed by the Opal-CT interval (Figures 7, 8, 9, and 10). The region in between these two areas is the hydrocarbon-bearing reservoir interval.
Amplitude anomalies of the upper interval and the lower interval are downdip from the regional (Figures 11 and 12). The seismic-anomaly map (Figure 1) presents a composite of both intervals (Figure 12).