PELTON, WILLIAM H., ALAN L. KLAWITTER, Phoenix Instruments, Inc., Denver, CO; and THOMAS L. THOMPSON, Thompson's Geo-Discovery, Inc., Boulder, CO

ABSTRACT: Natural Field Tilt Angle EM for Mapping Fracture Zones in Hydrocarbon Exploration

A new geophysical technique uses three components of the Earth's natural electromagnetic (EM) field to rapidly map fracture zones. The weaker vertical electromagnetic field caused by the conductivity contrast between fracture zones and unfractured rock is more difficult to measure than the horizontal EM fields. As a result, the tilt angle component of the magnetotelluric (MT) measurement is often noisy and ignored. A typical MT measurement takes many hours to acquire, is expensive, and as a result, stations are usually widely spaced.

This new method uses innovations in coil design and processing to permit data acquisition in minutes rather than hours. As a result, measurements cost less per site, and can be more densely spaced, thereby allowing the accurate delineation of fault zones.

The extensional San Luis Valley is an intermontane basin located in south-central Colorado and is separated from the Sangre de Cristo Mountains to the east by the Sangre de Cristo fault. Our survey traverse showed a large vertical field EM response over the northern Sangre de Cristo fault.

A reconnaissance test over a hydrocarbon bearing fracture zone was conducted across the Buck Peak oil field near Craig in NW Colorado. The survey indicated a fracture zone in the vicinity of the fractured Niobrara reservoir, which is cored by a thrust fault.

This new geophysical exploration methodology is a powerful tool for rapid regional and local delineation of faults and fault systems, with two orders of magnitude less cost than 3D seismic.

AAPG Search and Discovery Article #90915©2000 AAPG Rocky Mountain Section, Albuquerque, New Mexico