Permeability-Porosity Patterns and Variations in Some Holocene Sand Bodies

Abstract

Sandstone reservoirs are the results of long and commonly complex histories of geologic evolution. The combined processes of deposition, burial, compaction, diagenesis, and structural deformation yield final reservoir bodies of widely varied geometries, permeability-porosity characteristics, and structural configurations that are difficult to predict. In unraveling the evolution of sandstone reservoirs it is necessary to have detailed knowledge of their initial depositional characteristics and of the postdepositional modifications impressed upon them. This knowledge can provide a rational basis in predicting the characteristics of reservoir bodies away from areas of data control. Little information pertaining to the reservoir characteristics of freshly deposited sand bodi s has been available. In an API sponsored study, permeability, porosity, and textural parameters were derived from 992 oriented and undisturbed sand samples of river bars, beaches, and dunes undergoing active sedimentation. River point-bar samples have permeabilities ranging from 4 md to more than 500 darcys and average 93 darcys. Porosities in the river point-bars range from 17 to 52 percent and average 41 percent. Beach sand samples have a permeability range of 3.6 to 166 darcys and average 68 darcys. Porosities in beach sands range from 39 to 56 percent and average 49 percent. Permeability values in dune sands range from 5 to 104 darcys and average 54 darcys. Dune sand porosities range from 42 to 55 percent and average 49 percent. Permeability values in river-bar sands are extremely varied in comparison to those of beaches and dunes. In river bars, permeability decreases systematically downstream and bankward. Although of low variability, permeabilities on beaches are low on the beach faces, high on t e beach crests, and variable on the beach-berm areas. Both river-bar and beach sands have well organized directional permeabilities, parallel with the length of the bodies in river bars and perpendicular to the length of the bodies in beaches. Dunes are characterized by low variability in permeability and porosity and show no significant patterns or trends. There is greater variability within bedding and lamination packets than between them. In addition the boundary conditions between bedding and lamination packets are important factors in determining the effective reservoir characteristics of sand bodies, to the extent that a bedding unit of higher permeability completely surrounded by units of lower permeabilities will not demonstrate its ultimate through-flow capabilities, but will have an effective permeability influenced by and largely determined by the lower permeabilities of the bounding units. River-bar sand bodies have significantly different arrangement and variability between bedding units from those of beaches or dunes. The ideal relations between permeability-porosity and textural parameters that have been set forth by various authors for artificially packed particles are demonstrated only slightly by these natural sands from different depositional environments. In all three depositional environments permeability increases with increase in grain size and porosity increases with increase in grain sorting. However, in river-bar sands permeability increases as grain sorting increases and porosity increases as grain size increases, just the opposite of the relations in beach-dune sands and in the artificially packed grain experiments. The underlying cause of these deviations is the different style of grain packing in the river-bar sands.