ABSTRACT: Analyzing Fractures and Lineaments to Determine the Direction of a Horizontal Well, Sand Wash Basin, Northwest Colorado
Northwest/southeast is the direction to drill if you are drilling a horizontal well in the Sand Wash basin of northwest Colorado. This interpretation is based on fault, fracture, and lineament analysis of the area.
In northwest Colorado recurrent movement along preexisting zones of weakness in the Precambrian occurred throughout geologic time. This movement played a major role in the development of most northeast/southwest and northwest/southeast-trending faults, shears, and fracture systems (manifested as lineaments). The development of these structures are from pure and simple shear stresses. The structures have a regularity and linearity that cannot be attributed to random deformation.
The orientation of the structural trends generally fits the theoretical angles associated with rock failure in a horizontal compression system. Stress orientations indicate that the primary stress direction is northeast/southwest and, therefore, tensional. As a result, northeast/southwest fractures are tensional--i.e. more open than extensional fractures--and they can be conduits for hydrocarbons. This interpretation is based on: (1) most stream drainages in the area trend northeast/southwest; (2) the best fit to a stress diagram of lineaments, faults, and fractures is northeast/southwest structures are tensional; (3) northwest/southeast-trending faults show compressional characteristics; and (4) the dominant fractures in coals, from oriented core, trend northeast/southwest.
Drilling northwest/southeast would allow for maximum exposure to the most permeable, open (tensional) fractures. The specific direction (northwest or southeast) would depend on local factors such as dip and surface location.
AAPG Search and Discovery Article #91002©1990 AAPG Rocky Mountain Section Meeting, Denver, Colorado, September 16-19, 1990