Imaging the Overturned Limb of a Footwall Syncline in the Western Canadian Fold and Thrust Belt

Abstract

The Canadian Cordillera borders the North West corner of the North American Craton. It is made up of two distinct geological domains. The most westerly one consists of a series of four exotic terrains that have been accreted on to this craton. They are made up of a complex mixture of sedimentary, igneous and metamorphic rocks and they range in age from Pre-Cambrian to Cenozoic. The most easterly domain is the Western Canadian fold and thrust belt and it results from rocks being deformed by the compressional Laramide Orogeny. This area is dominated by Mesozoic and Paleozoic sedimentary rocks which have been deformed by shallow decollement thrust faulting and folding. Understanding the footwall structures beneath faults has been a major challenge to oil and gas exploration in fold and thrust belts. The seismic line LOFF8 is a fifty kilometer long experimental seismic line shot to test the validity of imaging sub thrust structures. A geological cross section was drawn over the structure using surface geology mapping, cutting samples, dipmeter and wireline logs. The cross section was then balanced and a velocity model was derived. This model was input into a seismic full wave form modelling package to evaluate the seismic response to the actual acquisition parameters. These results were used to guide the resulting depth migration and interpretation of LOFF8. In this interpretation we will show an example of a steep, overturned fold limb imaged on a seismic line, in a sub thrust position. The offsetting wells confirm this interpretation. The fold had amplitude of 3 kilometres before it was faulted by a later thrust fault with 20 kilometers of dip slip movement. We will illustrate the evidence used to support the imaging of the sub thrust structure and present some guide lines for approaching the interpretation of similar structures around the world.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016