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The AAPG/Datapages Combined Publications Database

Pacific Section of AAPG


Field Guide to the Monterey Formation between Santa Barbara and Gaviota, California, 1994
Pages 85-94

Natural Gamma-Ray Spectrometry of the Monterey Formation at Naples Beach, California: Insights into Lithology, Stratigraphy, and Source-Rock Quality

Kevin M. Bohacs, Jon R. Schwalbach


Natural gamma-ray spectrometry is a powerful addition to the traditional lithostratigraphic analysis of mudrocks. Gamma-ray data can reveal many aspects of mudrock deposition: lithologic variation, organic-matter content, hydrocarbon source quality, bedding thickness, stratal stacking patterns, depositional environments, and significant stratigraphic surfaces and packages and their regional distribution. It is especially valuable in studying the Monterey Formation with its complex composition and lithologic variations. Our studies of the Monterey Formation show that:

1) potassium and thorium are reliable indicators of detritus content (r3 = 0.75 v. Al2O),

2) high levels of thorium occur coincident with volcanic ash beds,

3) uranium correlates with total-organic-carbon content (TOC, r3 = 0.80), and

4) K/U correlates well with hydrocarbon source quality measured by Alumina/TOC.

Gamma-ray spectra also portray the relative proportions of hemipelagic and pelagic components in these rocks and can reveal the depositional environment when considered along with the stratal stacking patterns (Bohacs, 1990).

The data obtained from an outcrop gamma-ray survey is analogous to the data obtained from the NGT or spectral gamma-ray well log from boreholes. This enables a detailed calibration of well-log response with numerous outcrop samples and provides a powerful correlation and interpretation tool for integrating outcrop and subsurface data. Many important stratigraphic surfaces have distinct spectral gamma-ray signatures. This is a critical element for stratigraphic analysis and developing depositional models: the surfaces approximate time lines whilst lithologic packages often range widely in age. For example, the very organic-rich phosphatic shales at Naples Beach span several million more years than the most enriched rocks at Point Pedernales (Bohacs, 1993); see also Hornafius (1991) for other examples.

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