Masters Thesis

Stratigraphic and diagenetic comparisons of the Monterey Formation, Point Reyes and Monterey areas, California

The Miocene Monterey Formation is a deep marine deposit characterized by a high content of biogenic silica and organic matter. The biogenic silica is derived mainly from diatoms. Rock types include diatomaceous rocks and their diagenetic equivalents- chert, porcelanite and siliceous mudstone. The Monterey Formation is the source and reservoir rock for most of the oil and gas resources in California. Although the Monterey Formation in many other places in California is composed of three distinctive members, calcareous, phosphatic, and siliceous, the younger siliceous part is the most extensive facies and is the only member well exposed at Point Reyes and the Monterey area. I have examined the stratigraphic and diagenetic features of the sub-members of the siliceous part of the Monterey Formation at tw o locations: Pt Reyes and the Monterey area. My methods of study included XRD and petrographic analyses to qualitatively determine the major silica phases of Monterey Formation samples. I drew cross sections at eight localities to calculate the minimum thicknesses of the Monterey sub-members, and constructed three stratigraphic columns. My XRD and petrographic analyses show that the Monterey Formation at Point Reyes is composed of opal-CT cherts, opal-CT porcelanites, and opal-CT and quartz mudstone, and the section is thicker and more siliceous than the Monterey rocks of Monterey area. In the Monterey area, the Monterey Formation is composed of opal-A to opal-CT diatomites, opal-CT porcelanites, and opal-CT and quartz mudstone. For my calculations I used 25°C, 50°C and 55°C geothermal gradients; for Pt Reyes this gave approximately 1138 to 2000 m of minimum burial depth, and 283 to 391 m of erosion; for the Monterey area the results were 1139 to 2000 m of minimum burial, and 259 to 1120 m of overburden pressure. The data suggests that burial and diagenesis at Point Reyes occurred before the overlying Santa Margarita sandstone was deposited as evidenced by an erosional unconformity between Monterey and overlying sandstone. In contrast, in the Monterey area, diagenesis occurred after Santa Margarita sandstone deposition as evidenced by the absence o f an erosional unconformity and the lack of enough overburden provided by the entire Monterey section. The Monterey Formation in both locations are not efficient petroleum sources like in southern California in part because of insufficient burial depth.

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