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Chert Reservoirs of North America |
1 Department of Geological and Environmental Sciences, Stanford University, Stanford, California, 94305-2115; current address: Occidental of Elk Hills, Inc., P. O. Box 1001, Tupman, California, 93276-1001; caren_chaika{at}oxy.com
2 Independent Consultant, 2354 Moline Street, Aurora, Colorado, 80010; lorettaann53{at}hotmail.com
Caren Chaika received her B.A. degree in geology from Rice University in 1993 and her Ph.D. from Stanford University in 1998, where she combined geology and rock physics to better understand physical property changes during silica diagenesis. She has been a summer geologist for the Naval Research Laboratory at Stennis Space Center, Mississippi, and for the U.S. Geological Survey in Menlo Park, California. She is currently a production geologist with Occidental of Elk Hills, located near Bakersfield, California.Loretta Ann Williams has specialized in fine-grained carbonate and silica source rocks that serve as reservoirs. A past Sproule Award recipient, she holds a B.S. degree from SUNY-Stony Brook, an M.B.A. degree from University of Denver, and a Ph.D. from Princeton University. She served as a Stanford University postdoctoral research affiliate from 1981 to 1983 and as an exploration geologist for Champlin Petroleum from 1984 to 1986. Her career has spanned exploration, production, and remediation consulting. She is with PARSEC Group in Denver, Colorado.
The Miocene Monterey Formation, long known as the critical source rock in California, also includes significant fractured chert and porous diatomite reservoirs. What is not widely recognized is that there are high matrix porosity reservoirs within the opal-CT and quartz-phase rocks. Using density, porosity, and mineralogy data, we have identified two distinct groups of Monterey Formation reservoirs. group 1 porosity changes gradually during silica diagenesis-porosities of 55-70% exist not only in opal-A-dominated samples but also in samples that have undergone the transition from opal-A to opal-CT. In group 2, porosity decreases abruptly at the transition, and rocks below the transition are tight.
The main mineralogic difference between groups 1 and 2 is a higher clay content in group 1, resulting in different silica diagenesis pathways for the two groups. This led us to expect all San Joaquin basin samples to fall into group 1, and all coastal California samples to fall into group 2, because of the different depositional histories of the two areas. Although our prediction holds in most instances, we discovered that some San Joaquin basin samples exhibit group 2 characteristics. Therefore, it is important to use petrophysical, seismic, and/or geological information to determine if a Monterey Formation reservoir is likely to be a fracture-dominated type (group 2) or if there might be a matrix production component (group 1).
Once one has made this diagnosis, the characteristic density/porosity relationships of each group allow one to easily calculate an accurate matrix porosity.
This article has been cited by other articles:
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  R. J. Davies, N. R. Goulty, and D. Meadows Fluid flow due to the advance of basin-scale silica reaction zones Geological Society of America Bulletin, January 1, 2008; 120(1-2): 195 - 206. [Abstract] [Full Text] [PDF]
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R. J. Davies Differential compaction and subsidence in sedimentary basins due to silica diagenesis: A case study Geological Society of America Bulletin, September 1, 2005; 117(9-10): 1146 - 1155. [Abstract] [Full Text] [PDF]
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K. B. Follmi, C. Badertscher, E. de Kaenel, P. Stille, C. M. John, T. Adatte, and P. Steinmann Phosphogenesis and organic-carbon preservation in the Miocene Monterey Formation at Naples Beach, California--The Monterey hypothesis revisited Geological Society of America Bulletin, May 1, 2005; 117(5-6): 589 - 619. [Abstract] [Full Text] [PDF]
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 Carbonaceous and Phosphate-Rich Sediments of the Miocene Monterey Formation at El Capitan State Beach, California, U.S.A. Journal of Sedimentary Research, March 1, 2002; 72(2): 252 - 267.
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