The vast majority of the oil sands occur at depths beyond the limits of economical open pit mining. Research has been underway for a number of years on the recovery of the deeper oil by underground mining or in situ techniques. Owing to difficulties likely to be encountered in developing and maintaining production openings in the McMurray oil sands formation, it has been 3uggested that the underlying Devonian limestones might be used to provide primary access. This paper outlines the problems likely to be encountered in creating excavations in the McMurray 011 sand formation and assesses the potential for recovery of the oil.

INTRODUCTION

The Alberta oil sand deposits contain one of the world's largest reserves of crude oil in place. The Alberta Energy Resources Conservation Board estimates that the oil sands contain 800 billion to 1,000 billion barrels of oil in four major deposits located in the Peace River area, Wabasca, Cold Lake and Athabasca regions. Only a small percentage of these reserves are recoverable as the commercial technology proven to date requires surface mining of the deposits. This is feasible within a portion of the Athabasca deposit located in northeastern Alberta where the overburden is no thicker that 60 m (Fig. 1).

Two surface mining operations are producing from the Athabasca deposit and a third one is proposed. Great Canadian Oil Sands Ltd. commenced production in 1968 and an expansion is underway to boost production from 45,000 to 58,000 barrels per day. Sync rude Canada Ltd. started production in 1978 and is scheduled to produce at the rate of 109,000 barrels per day by early 1980, with full capacity of 129,000 barrels per day by 1982/83. The Allsands Project Group have made application to produce at the rate of 140,000 barrels per day. Research has been underway for a number of years on the recovery of the deeper oil by underground mining or in situ techniques. Undoubtedly, these techniques offer the greatest potential for extraction of oil from deep oil sand deposits.

This was brought into focus at a symposium held in May 1978 organized by the Alberta Oil Sands Technology Authority (AOSTRA). Some of the authors referred to the difficulties likely to be encountered in developing and maintaining production openings in the McMurray oil sands formation. Patching, Jeremic and Stimpson (1978) and Stephenson (1978) suggested that the more competent Devonian Limestone Formations beneath the oil sands might be used to provide access for underground mining and in situ oil extraction. Unfortunately, there is little published information on the nature, occurrence, properties and mechanical behaviour of the Devonian Limestone Formations. Mossop (1978), when considering the surface topography of the limestones, concluded that they were commonly irregular in form, possibly making excavation at or near the base of the "reservoir" very difficult. The purpose of this paper is to review the potential for the mining of oil sands by hydraulic and in situ methods using the underlying limestone formations for access.

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