ABSTRACT: Chemostratigraphy of the Monterey Formation

Matthias D. Petersen

Chemical analysis of samples of the Monterey Formation provides a fresh look at Monterey stratigraphy. It can be used for correlation and dating of the formation and provides new insights into the distribution and origin of Monterey lithofacies. Combined with our knowledge of physical rock properties (fracturability, porosity, and permeability), this could also be used to evaluate Monterey Formation reservoir potential. The results of this work may have important implications for exploration and reservoir delineation within the Monterey Formation. The method is easily applicable because analyses are done on bulk cuttings.

Systematic chemical variations in the Miocene Monterey Formation in the Santa Maria and adjacent basins have been observed in 12 wells from on- and offshore California. Bulk rock variations of Al, Fe, Ti, Zr, Ca, and Mg in complete Monterey sections are shown to be systematic and controlled by large-scale events, such as sea-level changes and other oceanographic processes. These events affected sedimentation throughout the North Pacific. Our chemostratigraphic observations within the Monterey Formation correlate with the dated "Hiatuses NH2 to NH6," as seen in DSDP cores of the North Pacific and with the sea-level change-related unconformities observed in nearshore areas. Since these events are of large-scale control and are independent of local factors, they enable us to correlate ac oss depositional and diagenetic facies within the basin. Correlation of chemostratigraphic curves with events of known age enables us to date the Monterey sections, and lithofacies changes within time-equivalent stratigraphic sections (e.g., phosphate and dolomite contents) may be documented.

Of the 15 wells analyzed, 12 wells exhibit chemostratigraphic variations that are interpreted to represent the "typical Monterey stratigraphy." Variations in three other wells show tectonically modified sections.

AAPG Search and Discovery Article #90097©1990 Fifth Circum-Pacific Energy and Mineral Resources Conference, Honolulu, Hawaii, July 29-August 3, 1990