--> Evaluating the Controls of Opal and Clay Diagenesis in the Monterey Formation

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Evaluating the Controls of Opal and Clay Diagenesis in the Monterey Formation


The diagenetic conversion of opal to quartz in the Monterey Formation occurs variably both across basins and locally within reservoirs. Changes in diagenesis significantly affect reservoir properties including porosity and mechanical strength. Furthermore, the conversion of smectite to interlayered illite/smectite is concurrent with the diagenesis of opal, affecting how reservoirs will react to completion fluids. Consequently, evaluating the local degree of diagenesis of Monterey reservoirs is necessary for optimizing completion designs. Established opal diagenesis models for the Monterey Formation use the concentration of opal, availability of K+ ions, and the burial depth/temperature as controls on the rate of conversion. A recent thesis, Ijeoma 2014, challenged the existing diagenesis models by reporting heterogeneity of opal diagenesis within large sample volumes from a single stratigraphic section in the Casmalia Hills of the Santa Maria Basin. The high level of variability in apparent opal-CT crystallinity (and implied reservoir thermal history) of this data set suggests that additional variables are influencing the rate of opal diagenesis. This study investigated mechanisms for opal diagenesis: whether temperature and opal content are sufficient to explain the rate of opal diagenesis, or whether additional factors influence diagenesis as postulated by Ijeoma 2014. Samples were collected from the Chico Martinez Creek outcrops in San Joaquin Valley. The stage of opal and clay diagenesis was determined using XRD, thin section petrography, and SEM imaging. Samples from the Antelope member of the Monterey Formation contained opal-CT of varying crystallinity, smectite, and no carbonate minerals. The McDonald and Gould members contained quartz, interlayered illite/smectite, and variable carbonate concentrations. The conversion of opal to quartz, and that of smectite to interlayered illite/smectite, followed a down section trend across outcrops, as one would expect from a diagenetic process driven by geothermal gradients. A geothermal gradient of roughly 25-30 ºC/Km was calculated using the measured crystallinity of opal-CT from XRD and interpreted formation thickness from outcrops.