Abstract

How compaction can yield a progressive decrease in the permeability and pore structure of carbonate grainstones is not well documented. In this study, the destruction of permeability by compaction relative to early cementation is examined in forty-five shallow-buried (30-440 m) grainstones with a range in permeability of 12 to 5180 md. Compaction and cementation indices calculated from thin-section point-count data indicate a group of grainstones dominated by cementation and another by compaction. Log-linear cross plots of permeability versus these indices suggest that permeability loss in these two groups occurs by different processes. The nature of those processes is elucidated by capillary pressure data and the distribution of cement and compaction fabrics as a function of permeability.

As cement abundance increases in the cement-dominated samples, a continuous distribution of cement develops along grain surfaces, which is followed by the development of partial and then complete pore-filling mosaics. Permeability is progressively reduced with this succession of cement fabrics and an increasingly greater percentage of pore space is positioned behind progressively smaller throats. Permeability loss results from porosity reduction, progressive constriction of pore throats, and increased tortuosity of flow paths. Once permeability is reduced to tens of millidarcies, effective pore radii are all less than 1 μm and many are less than 0.1 μm, but there is no evidence for complete blocking of all pore throats in this suite of samples. In absolute terms, the pore-lining cements reduce permeability by thousands of millidarcies whereas the pore-filling cements reduce permeability only by hundreds of millidarcies.

Mechanical compaction is the dominant cause of permeability reduction between 5000 and about 500 md in the compaction-dominated samples. Linear grain contacts and embayed grains become more abundant and close to overly close packing textures become more homogeneously distributed as permeability decreases. However, there is no concurrent change in the amount of pore space accessed by large or small throats, indicating that mechanical compaction does not uniformly constrict or seal pore throats. Instead, the data suggest that mechanical compaction reduces permeability by reducing porosity, lengthening pore throats, and increasing tortuosity.

At about 500 md the compaction-dominated trend steepens on the log-linear plot of permeability versus porosity, a change that corresponds to the development of grain-to-grain pressure solution. Unlike mechanical compaction, grain-to-grain pressure solution constricts and eliminates pore throats and isolates pore space, resulting in a loss of large throats and an increase in the amount of pore volume accessed by small throats. In contrast to cement-dominated samples, the most effective pore radii are still greater than 1 μm at tens of millidarcies of permeability. Such low amounts of permeability can be reached even when pressure solution is not pervasively distributed throughout a sample.

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