--> Abstract: Two Patterns of Porosity Reduction During Silica Diagenesis in the Monterey Formation, California, by C. Chaika; #90920 (1999).

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CHAIKA, CAREN
Occidental of Elk Hills, Inc., Tupman, CA

Abstract: Two Patterns of Porosity Reduction During Silica Diagenesis in the Monterey Formation, California

There are two distinct patterns of porosity reduction in reservoir rocks of the Monterey Formation, California, as they undergo silica diagenesis from opal-A through opal-CT to quartz. The most important difference between the two groups is that rocks in one group (Group 1) retain higher porosity than rocks in the other group (Group 2) at the same diagenetic grade. Whether a given reservoir has characteristics of Group 1 or Group 2 is primarily-- but not entirely-- geographically constrained. Therefore, correct volumetric estimations for a given reservoir require knowing whether the siliceous rocks have Group 1 or Group 2 characteristics.

Analysis of core data from wells six reservoirs in the San Joaquin Valley and outcrop data from two localities along the California coast show several ways to discriminate between these two patterns of porosity reduction. One way to recognize the two groups is that they separate along two bulk density- porosity trends. Furthermore, in Group 1 grain density increases with decreasing porosity, the fraction of opal-A plus opal-CT decreases while the fraction of non-silica minerals (and quartz, if present) increases, and the opal-A/opal-CT transition has a smooth decrease in porosity. In these rocks, porosity reduction appears to be due to increased amounts of non-silica minerals. In Group 2, grain density decreases with decreasing porosity, the amount of opal-CT (and quartz, if present) increases while the fraction of nonsilica minerals is relatively constant, and the opal-A/opal-CT transition is marked by an abrupt decrease in porosity. In these samples, porosity is reduced predominantly through addition of silica to the rocks.

AAPG Search and Discovery Article #90920©1999 AAPG Pacific Section Meeting, Monterey, California