CHAPTER 1 THE DEVELOPMENT, SIGNIFICANCE, AND FUTURE POTENTIAL OF THE OVERTHRUST BELT

• Jurisdiction: United States

Overthrust Belt--Oil and Gas Legal and Land Issues
(Nov 1980)
CHAPTER 1
THE DEVELOPMENT, SIGNIFICANCE, AND FUTURE POTENTIAL OF THE OVERTHRUST BELT
Vincent Matthews III
Amoco Production Company
Denver, Colorado

Introduction

During the past five years the Utah-Wyoming Overthrust Belt has evolved from a driller's graveyard to the most significant, new petroleum province since the discovery of Prudhoe Bay over a decade ago. The widely ranging estimates of the potential resources yet to be found in the Overthrust Belt attest to the play's infancy and complexity.

Geological Definition

The Overthrust Belt is a descriptive geological term applied to the series of extremely complex, folded and faulted rocks that have resulted from the movement of one part of the earth's crust up and over another part.

This narrow belt of intense deformation extends along the entire length of western North America from Alaska through Mexico (Figure 1). The forces that caused the formation of the Overthrust Belt in the western United States are a consequence of continental drift. During the time that the Overthrust Belt was forming, the North American continent was drifting westward away from the mid-Atlantic ridge. As the continent moved westward it continually overrode the floor of the Pacific Ocean along its entire western margin. The forces associated with this overriding at the continental margin caused the Overthrust Belt to form parallel to the western coast of North America.

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Prior to the overthrust event, the sedimentary rocks in the western United States were essentially horizontal as shown in Figure 2. These rocks were broken or "faulted" along the places labeled Crawford, Absaroka and Darby-LaBarge. The rocks were then forced or "thrust" along these faults up and over one another as shown in Figure 3. Thus, the name "Overthrust". The amount of movement along these faults is quite large, totalling as much as two miles in a vertical sense, and as much as 10 or 15 miles in a horizontal sense. The total amount of horizontal movement along all of these faults across Utah and Wyoming is on the order of 60 miles. Obviously, movements of this magnitude cause severe distortion within the crust. The overthrusting in Wyoming occurred in a place where the right combination of geological conditions happen to be present to create oil and gas pools.

The four major factors that are normally needed to create a commercial accumulation of oil...