--> --> Abstract: Kinetics of the Opal-CT to Quartz Phase Transition Control Diagenetic Traps in Siliceous Shale Source Rock from the San Joaquin Basin and Hokkaido, by Danica Dralus, Kenneth E. Peters, Mike D. Lewan, Oliver Schenk, Michael Herron, and Kunihiro Tsuchida; #90124 (2011)

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Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA

Kinetics of the Opal-CT to Quartz Phase Transition Control Diagenetic Traps in Siliceous Shale Source Rock from the San Joaquin Basin and Hokkaido

Danica Dralus1; Kenneth E. Peters2; Mike D. Lewan3; Oliver Schenk4; Michael Herron5; Kunihiro Tsuchida6

(1) Stanford University, Stanford, CA.

(2) Schlumberger, Mill Valley, CA.

(3) U.S. Geological Survey, Denver, CO.

(4) Schlumberger, Aachen, Germany.

(5) Schlumberger, Cambridge, MA.

(6) Japan Oil, Gas and Metals Corporation, Chiba, Japan.

Porcelanite and chert originate from marine diatoms as diatomite, which undergoes diagenetic conversion of amorphous opal (opal-A) to cristobalite and tridymite (opal-CT) and finally quartz. Porosity decreases during this process, but permeability increases during transformation of opal-CT to quartz and results in stratigraphic traps for petroleum like those in recent discoveries in the San Joaquin Basin (Grau et al., 2003). The ability to accurately predict locations of these diagenetic traps would be a valuable exploration tool.

Ernst and Calvert (1969) determined zero-order kinetics for the opal-CT to quartz phase transition based on hydrothermal experiments using distilled water. However, the transition is a dissolution and re-precipitation process, where both the silica dissolution rate and solubility contribute to the rate of the reaction. We determined first-order kinetic parameters for the opal-CT to quartz transition based on hydrous pyrolysis of weathered Monterey Formation porcelanite from Lompoc, California, which also contained dolomite; and a Wakkanai Formation porcelanite from Hokkaido, Japan, which also contained quartz, albite, and some organic material. Temperatures were kept below the critical temperature of water and the aqueous solution was buffered so that final fluid pH values measured between 7.0 and 8.2. Under these conditions, the samples showed large variations in opal-CT to quartz conversion rates, where the rates of the Monterey and Wakkani conversions were approximately five times faster and three times slower, respectively, than that predicted by Ernst and Calvert.

We built a module in our petroleum system modeling software and used the hydrous pyrolysis kinetics to determine the depth of the opal-CT to quartz phase transition along a cross-section in the east-central portion of the San Joaquin Basin. The kinetics and software module may be useful to identify silica-phase-transition stratigraphic traps throughout the Pacific Rim where siliceous source rocks are common.