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Kinetics of the Opal-A to Opal-CT Phase Transition in Low- and High-TOC Siliceous Shale Source Rocks


Marine diatoms deposit biogenic silica as amorphous opal-A. These deposits interact with saturating aqueous solutions, transforming to microcrystalline opal-CT and eventually quartz through a series of dissolution and precipitation reactions. The mineralogical changes cause corresponding changes in rock properties such as porosity, permeability, and acoustic response. The enhanced permeability and preserved porosity during these transitions may result in formation of diagenetic hydrocarbon traps. Successful exploitation of diagenetic traps in oil and gas exploration requires an understanding of how quickly these phase transitions occur and how natural variations in rock composition affect the transition rates. In this study, the kinetics of the opal-A to opal-CT phase transition were determined through a series of hydrous pyrolysis experiments. Two diatomite samples from the Miocene Monterey Formation, California, were used, both from the same pedogenic weathering profile. The samples each comprise approximately 80 wt% opal-A, 10 wt% phyllosilicates, and 6 wt% quartz. However, they have different amounts of TOC (0.36 wt% and 4.65 wt%) and contain a thermally mature Type II kerogen. The samples were mixed with a buffered aqueous solution that ensured the fluid maintained pH 7 or greater, and the mixtures were pyrolyzed at multiple temperatures between 280°C and 330°C. The pyrolysis experiments sampled the transition from opal-A to opal-CT and showed that the conversion in the high-TOC sample was significantly delayed compared to the low-TOC sample at the same temperature. Data at multiple temperatures were combined to determine the activation energy and pre-exponential factor for the conversion of each of the two samples. These kinetics data, combined with knowledge of the local thermal history, allow prediction of the opal-A to opal-CT transition depth in a basin. The estimated transition depth can then be used to predict diagenetic trap locations or identify mineralogical sources of cross-cutting reflectors in seismic data. Low-TOC kinetics provide a baseline for these estimates, whereas high-TOC kinetics demonstrate the extent to which organic material affects the reaction rate in source rocks.