The Monterey formation has a wide variety of lithologies (diatomites and diagenetically derived cherts, porcelanite, organic-rich mudstones, phosphatic, and carbonate rocks) representing the effect of tectonic, oceanographic, and climatic events. Although the formation has been studied extensively, the effects of lithology on saturation and pore size distributions are still poorly understood. This lack of understanding stems partly from a paucity of data and partly because models built for conventional siliciclastic reservoirs are not applicable. In this study we investigate the differences between biogenic and detrital silica as well as effects of additional influx of clay and carbonate material in the biogenic lithology. Recognizing such differences can help better analyze and interpret NMR, sonic, porosity, and resistivity logs.

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