Major gas reserves have been discovered in the past 5 years in Cretaceous sandstones and conglomerates at depths of 3,000 to 10,000 ft (915 to 3045 m) within Alberta and British Columbia, Canada. Discovery of these new reserves resulted from a joint geological/ petrophysical/reservoir engineering effort that utilized rock/fluid data from cuttings, cores, well logs, and drillstem and production tests. A key element in the exploration search was the rapid detection and characterization of reservoir-rock properties from well cuttings, especially of the low-permeability rocks (1 md to a few microdarcies). The methods used to establish and to characterize the reservoir potential of thick, multiple sandstone and conglomerate intervals are illustrated.
Rock studies of greater than 10,000 ft (3045 m) of conventional cores integrated with petrophysical studies of well logs and core analyses, which in turn were compared with drillstem tests (DTS's) and production tests in more than 200 wells, provide the basis for establishing reservoir rock potential. Porous rocks are subdivided into three categories.
Type I rocks are capable of gas production without natural and/or artificial fracturing.
Type II rocks are capable of gas production when interbedded with Type I rocks or with natural and/or artificial fracturing.
Type III rocks are too tight to produce at commercial rates even with natural or artificial fracturing.
Type I rocks are subdivided into four classes, with four air-permeability ranges: "A" (greater than 100 md), "B" (10 to 100 md), "C" (1 to 10 md), and "D" (+/- 0.5 to 1 md).
Criteria to identify a rock's reservoir potential from well cuttings or conventional and sidewall cores are based on examination of dry, freshly broken fragments with a binocular microscope at 20x magnification. A rock's reservoir potential is established on the basis of estimates of
size, volume, and distribution of visible pores,
particle size and distribution, and
type and amount of cements and pore-filling material and degree of consolidation (the way fragments break).
Geologists and engineers can make rapid, accurate estimates of reservoir-rock potential of unknown porous intervals with the help of a number of visual aids. These aids include
plastic trays of cuttings-size rock chips crushed from conventional cores of known rock/pore types, porosities, and permeabilities;
color photographs of freshly broken surfaces of these rock/pore types taken at 20x magnification; and
thin-section microphotographs and scanning electron microscope (SEM) photographs (at 100 to 5,000x magnification) of the rock chips and their pore casts.
Methods and procedures described in this paper continue to be used in western Canada as well as the U.S. for delineation of exploration opportunities, identification of bypassed pays in old wells, interpretation of well logs, and evaluation of intervals for completion.