Kevin J. Thomas¹, James P. Evans¹, Zoe K. Shipton¹

(1) Utah State University, Logan, UT

ABSTRACT: Stratigraphic and Structural Heterogeneities in Faulted Aeolian Sandstone from Borehole Geophysics and Cores, San Rafael Swell, Central Utah

When modeling reservoirs, borehole geophysics are often used to identify stratigraphic changes and permeability differences. Fault zones may also appear on well-logs. Fault zones and stratigraphic changes are incorporated differently in reservoir models. Therefore, a technique to identify sub-seismic faults from well-logs is required to distinguish between stratigraphically- and structurally-derived permeability variations. Approximately 300 meters of core, along with well logs, have been collected from five boreholes through a fault zone in the Jurassic Navajo Sandstone at two sites where fault displacement is at or below current seismic resolution limits. This allows us to study how sub-seismic faults directly affect well logs in aeolian sandstones. Detailed stratigraphic logging of the cores reveals four distinct microfacies, A) highly cross-stratified composed of alternating layers of finer (0.125 mm) and coarser (0.250 mm) grains in approximately equal amounts; B) cross-stratified units in which coarser grains dominate; C) cross-stratified units in which finer grains dominate; D) poorly cross-stratified to massive finer-grained units. Mini-permeameter measurements deliver an average permeability of about 850 millidarcies (md) for type-A units, about 1300 md for type-B units, about 700 md for type-C units, and about 250 md for type-D units. The fault zone is characterized by cataclastic grain size reduction and an average permeability of about 60 md. These results can be used to calculate bulk permeabilities for unfaulted sequences and compare them to faulted intervals. This quantitative data will allow us to use well logs from aeolian systems to differentiate permeability variations caused by stratigraphic versus structural changes.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado