Depth to Bedrock and Surface Casing

Janicki, Ed ^1
1 British Columbia Ministry of Energy Mines and Petroleum Resources, Victoria, BC, Canada.

Most jurisdictions in Canada and the United States responsible for regulating oil and gas activity require that surface casing be set within a competent formation, and deeper than the deepest useable aquifer. This requirement is intended to protect useable aquifers from contamination originating from oil and gas wells. Aquifers unprotected by fully cemented casing strings are vulnerable to hydrocarbons migrating up an unsealed annulus.

Data used to map depth to bedrock in British Columbia, Canada, shows that at least some wells have set their surface casing in unconsolidated glacial sediment instead of competent rock. A detailed examination of many wellfiles suggests that operators seldom display knowledge of depth to bedrock; and information on local aquifers is totally absent. Wellfiles of wells in the United States also generally show the same shortfall in information.

A preliminary depth to bedrock map for a portion of northeastern British Columbia (NTS map block 94A) is presented here. It was prepared using wellfile information, such as tour reports, and elog responses from wells drilled prior to 1960. Elogs for wells drilled before 1960 seem to be more diagnostic of the contact between consolidated and unconsolidated material than modern logs. Sometimes sample descriptions for the older wells are also very helpful. Data from oil and gas wells has been supplemented with depth to bedrock data from water wells supplied by the Ministry of Environment. The map will eventually be updated with hard data from shallow monitoring wells drilled specifically to map depth to bedrock.

This mapping is intended to serve several purposes. It will show where the bedrock surface is most deeply eroded; this should be useful for furthering the knowledge of local Quaternary stratigraphy. For the benefit of those looking for ultra-shallow gas (and drillers seeking to avoid blow-outs before surface casing is set), thick unconsolidated sediments close to Cretaceous gas fields might indicate the possibility of Quaternary gas. Hydrogeologists may be able to use thickness trends as indicators for possible aquifers. And perhaps most importantly, this map could provide operators with approximate depth to bedrock so they can design their drilling programs - and casing needs - to protect shallow aquifers.

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009