The Monterey Formation of the Santa Ynez Unit, Part I: Stratigraphy, Outcrop Gamma-Rays, and Cyclicity

J. R. Schwalbach, D. F. Lockman, and K. M. Bohacs

Monterey reservoirs challenge conventional analytical techniques because of their widely varying, thinly-bedded lithotypes and dependence on fractures for economic production. Our studies employ a multi-faceted approach linking outcrops, cores, well logs, borehole images, and production data.

Excellent outcrops of reservoir facies at Shell Beach and Mussel Rock have been correlated to subsurface stratigraphy using spectral gamma-ray surveys. Gamma-ray components vary with rock composition. Uranium correlates with organic matter (r²=0.80), potassium and thorium are generally reliable detritus indicators (r²=0.75), and thorium is concentrated in many volcanic ash beds.

Gamma-ray signatures also reveal key stratigraphic surfaces. We project chronostratigraphy established at the outcrops into the subsurface using the gamma-ray logs, guiding regional and field correlations. Chronostratigraphy is critical for establishing facies distribution and depositional models because lithofacies are time-transgressive. Mineralogic data derived from strip samples of continuous cores calibrate the well-log models that portray lithotype variation. The lithotypes depend on the relative abundance of the three main sediment components in these deep-basin environments (fine-grained detritus, carbonate, and biogenic silica), and reflect Miocene paleoceanography.

Monterey strata stack in repetitive packages at various scales Understanding the controls on the cyclic packages facilitates the prediction of rock properties away from the wells. Independent age control allows us to test the approximate frequency of these cycles versus projected orbital parameters.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California