The Gravity Effect of the Monterey Formation’s Low-Density Rock and Its Removal by Gravity Stripping, San Joaquin Basin, California: A Work in Progress

H. T. Angell¹, R. M. Negrini¹, and B. M. Hirst^2
1Department of Physics and Geology, California State University Bakersfield, Bakersfield, CA
²Strata Geophysical, Inc, Bakersfield, CA

The gravity effect of the shallow, low-density diatomaceous rock of the Monterey Formation in the southwestern San Joaquin Valley can have amplitudes and wavelengths large enough to disguise the gravity anomalies of deeper structures such as faults, folds, or bodies of anomalous density. In this study we remove the effect of the low-density rock using the gravity stripping method (AKA geologic stripping method) to create a resultant gravity map which will be used to detect previously obscured anomalies. The density contrast, thickness, and lateral extent of the low-density rock was identified and mapped using more than 500 bulk density logs acquired from several oil companies and installed in a Landmark OpenWorks^TM project at the CSU Bakersfield GeoTechnology Training Center. The density contrast was determined by comparing these logs to a “type” density log from a well not containing the low-density rock. These data will be used to build a three-dimensional gravity model of the low-density rock. The modeled gravity field associated with the low-density rock will then be subtracted from the existing USGS gravity data for this portion of the basin to produce a new gravity map. The new resultant gravity map may show anomalies that were previously obscured.

AAPG Search and Discovery Article #90904©2001 AAPG Pacific Section Meeting, Universal City, California