It is a well established fact that finding rates of natural gas in western Canada have declined over the last thirty or so years. However, statistical analyses of the declining trends have, to date, failed to provide a generally acceptable estimate of the conventional natural gas resources of the Western Canada Sedimentary Basin. This paper provides an explanation of why decline analysis appears to provide a valid method for estimating the conventional crude oil resources of the basin while the same is not true for natural gas. The paper proposes solutions to the problems encountered in the natural gas data and provides an estimate of the conventional natural gas resources of the basin by statistical analyses of the declining finding rate trends.
The Western Canada Sedimentary Basin (WCSB) occupies an area of 1.4 million square kilometres of southwestern Manitoba, southern Saskatchewan, Alberta, northeastern British Columbia and the south-west corner of the Northwest Territories (Figure 1). Petroleum resources of the basin range from natural gas at the light end of the spectrum through conventional crude oil to bitumen. Conventional natural gas is that portion of this spectrum which exists in solution in crude oil or in the gaseous state in reservoirs having a permeability greater than about 0.1 md. Unconventional natural gas resources are those contained in low permeability reservoirs (tight gas), those associated with coal deposits (coalbed methane) and those contained in shale deposits (shale gas).
Costs of extraction and transportation and natural gas prices are important determinants in the assessment of recoverable natural gas resources. However, consideration of the economic aspects of natural gas recovery is avoided by assuming a high natural gas price, in the order of $3.00 per mcf, so that estimates of recoverable resources are not constrained by price considerations.
The Canadian Gas Potential Committee (CGPC) in its recent inaugural report1, focused on conventional natural gas in the WCSB. However, an analysis such as that reported in this paper was not included.
Definitions for categories of resources are taken from the Petroleum Society Monograph Number 1, Determination of Oil and Gas Reserves2. The definitions of primary interest in this paper are as follows:
Resources of conventional natural gas are the total quantities of gas that are estimated, at a particular time, to be contained in, or that have been produced from, known accumulations, plus those estimated quantities in accumulations yet to be discovered.
Discovered resources of conventional natural gas, which also may be referred to as initial volumes in place, are those quantities of gas that are estimated, at a particular time, to be initially contained in known accumulations that have been penetrated by a wellbore. They comprise those quantities that are recoverable from known accumulations and those that will remain in known accumulations, based on known technology under specified conditions that are generallyaccepted as being a reasonable outlook for the future.
Undiscovered resources, which may also be referred to as future initial volumes in place, are those in-place quantities of gas that are estimated, at a particular time, to exist inaccumulations yet to be discovered.
