ABSTRACT: Methods for Comparing and Correlating Surface Outcrop Data to Subsurface Geophysical Logs

Steven Vealey

Geophysical and petrophysical data gathered directly from the outcrop can be displayed graphically so as to emulate suites of geophysical well logs, including gamma-ray, bulk density, and sonic transit time. The outcrop data can then be compared directly to subsurface well logs and included in regional log correlations. This method is useful for tying surface geology to the subsurface and for anticipating well log response in areas where subsurface control is sparse.

A hand-held gamma-ray scintillation counter is carried along an outcrop traverse to produce the gamma-ray curve. This can be plotted in the field and used immediately in log correlation. Density data are developed from outcrop hand specimens (<200 g), which are weighed and measured for grain volume and pore volume. From these, a table of grain density, bulk density, and porosity can be calculated. Sonic transit time data are developed mathematically from the density values. The sonic and density data may be used to produce a synthetic seismic trace.

When displayed as log-curves, these gamma, density, and sonic data approximate very closely the character of nearby subsurface geophysical logs. These comparisons have been used to tie the surface outcrop of the Tensleep Sandstone at Alcova, Wyoming, to the subsurface Tensleep oil reservoir of South Casper Creek, Wyoming, three townships away. This method has aided in the characterization of the South Casper Creek reservoir by directly relating the oil field to a surface outcrop analog.

AAPG Search and Discovery Article #91002©1990 AAPG Rocky Mountain Section Meeting, Denver, Colorado, September 16-19, 1990