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

Abstract

The diagenetic conversion of opal to quartz in the Monterey Formation occurs variably both across basins and locally within reservoirs. Changes in Previous HitdiagenesisNext Hit significantly affect reservoir properties including porosity and mechanical strength. Furthermore, the conversion of smectite to interlayered illite/smectite is concurrent with the Previous HitdiagenesisNext Hit of opal, affecting how reservoirs will react to completion fluids. Consequently, evaluating the local degree of Previous HitdiagenesisNext Hit of Monterey reservoirs is necessary for optimizing completion designs. Established opal Previous HitdiagenesisNext Hit 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 Previous HitdiagenesisNext Hit models by reporting heterogeneity of opal Previous HitdiagenesisNext Hit 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 Previous HitdiagenesisNext Hit. This study investigated mechanisms for opal Previous HitdiagenesisNext Hit: whether temperature and opal content are sufficient to explain the rate of opal Previous HitdiagenesisNext Hit, or whether additional factors influence Previous HitdiagenesisNext Hit as postulated by Ijeoma 2014. Samples were collected from the Chico Martinez Creek outcrops in San Joaquin Valley. The stage of opal and clay Previous HitdiagenesisNext Hit 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 Previous HitcarbonateNext Hit minerals. The McDonald and Gould members contained quartz, interlayered illite/smectite, and variable Previous HitcarbonateTop 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.