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Thin section and SEM observations indicate that tight gas sands may be grouped into four broad categories based on pore geometry. These consist of (1) primary interparticle porosity; (2) primary interparticle porosity filled with authigenic minerals; (3) primary porosity reduced to narrow cracks with secondary honeycombed grains; and (4) intercrystalline porosity within a fine grained, elongate matrix. Type 1 porosity is common in conventional sands, types 2 and 3 are prevalent in tight sands, and type 4 is a rare class found in extremely tight rocks.
Reservoir property data on 51 sandstone core samples from the Mesaverde, Spirit River, and Frontier Formations led to an attempt to correlate reservoir parameters with pore geometry. The reservoir properties measured on these rocks under net confining stress include dry permeability, relative permeability, porosity to gas, and pore volume compressibility.
Results of the core analysis were combined with petrographic information to provide the following observations.
(1) Pore volume compressibility correlates well with pore geometry. Rocks containing types 1 and 2 pore structures are relatively incompressible owing to a rigid support framework of quartz sand grains in intimate contact. Clay linings on quartz overgrowths and weakly structured solution pores in the type 3 geometry cause moderate compressibility. Type 4 geometry, which occurs in matrix-supported rocks with rare quartz-grain contacts, is generally the most compressible of the 4 classes.
(2) Permeability correlates in general with the pore classes. Type 1 geometry is the most permeable and type 4 is generally the tightest.
(3) Porosity to gas did not correlate very well with pore geometry except on a sample-by-sample basis.
Development of a specific type of pore geometry in a tight sand is controlled by the original grain size and composition, depositional environment, and diagenetic history. It is usually difficult to isolate the effects of one factor from the others. The Mesaverde samples, however, were deposited from a single source into a variety of depositional environments, and underwent approximately uniform diagenesis. In this case, with the composition and diagenesis held constant, notable differences were seen in the pore geometry and reservoir properties that correlated quite well with depositional environment.
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