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AAPG Bulletin, V.
Geometric models of porosity reduction by ductile grain compaction and cementation
1University of Texas at Austin, Bureau of Economic Geology, Box X, University Station, Austin, Texas; present address: BP America Production Company, 501 Westlake Park Blvd., Houston, Texas 77079; [email protected]
2University of Texas at Austin, Petroleum and Geosystems Engineering, 1 University Station, Austin, Texas; [email protected]
Pore-volume reduction of sediments by plastic deformation during compaction and by cementation of grains has been evaluated for different proportions of ductile and hard grains. We represent the compaction behavior of grains with a purely geometric model, which uses the cooperative rearrangement algorithm to produce dense, random packings of partly interpenetrating spheres. We varied the fraction of grains assumed to be ductile and the radius of the rigid core of the ductile grains. The predicted relationship between the fraction of ductile grains in the sediment and the porosity after compaction agrees well with previously published experimental data in the literature. The radius of the rigid core of the ductile grains is an effective way to represent different kinds of ductile material, ranging from brittle (rigid radius 0.9) to extremely ductile (rigid radius 0.7). We simulated quartz cementation in our compacted rock by adding isopachous cement. Cement thickness was reduced on the smaller grains and increased on the larger grains to account for presumed export of pressure-dissolved material from finer grained regions and the import of material into coarser grained regions. These simulations yield descriptions of pore-scale geometry resulting from processes common in sandstones. Modeled pore geometry provides insight into transport properties of such rocks. For example, the models predict, to within a factor of five, the permeability of samples of tight-gas sandstones having little intragranular porosity.
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