Abstract
Sandstone reservoirs are the results of long and frequently complex histories of geologic evolution. The combined processes of deposition, burial compaction diagenesis and structural deformation yield final reservoir bodies of widely varying geometries, permeability-porosity characteristics, and structural configurations that are difficult to predict. In unraveling the evolution of sandstone predict. In unraveling the evolution of sandstone reservoirs, it is necessary to have detailed knowledge of their initial depositional characteristics and of the post-depositional modifications impressed upon them. This knowledge can provide a rational basis in predicting the characteristics of reservoir bodies away from areas of data control. To present, little information pertaining to the reservoir characteristics of freshly deposited sand bodies bas been available. In an API-sponsored study, permeabilities, porosities, and textural properties were derived from 992 oriented and properties 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 darcies and average 93 darcies. Porosities in the river point bars range from 17 to 52 percent and average 41 percent. Beach sand samples have a permeability percent. Beach sand samples have a permeability range of 3.6 to 166 darcies and average 68 darcies. Porosities in beach sands range from 39 to 56 Porosities in beach sands range from 39 to 56 percent and average 49 Percent. Permeability values percent and average 49 Percent. Permeability values in dune sands range from 5 to 104 darcies and average 54 darcies. Dune-sand porosities range from 42 to 55 percent and average 49 percent. Permeabilities in river-bar sands are extremely Permeabilities in river-bar sands are extremely variable compared with 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 the beach crests, and variable on the beach berm areas. Both river-bar and beach sands have well organized directional permeabilities, parallel to the length of the bodies permeabilities, parallel to 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 permeability 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 a significantly different arrangement and variability between bedding units than do beaches or dunes.
The ideal relationships between permeability-porosity and textural characteristics permeability-porosity and textural characteristics that various authors have set forth for artificially packed particles are only weakly demonstrated by packed particles are only weakly demonstrated by these natural sands from various depositional environments. In all three depositional environments, permeability increases with increase in grain 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 relationships 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.
Introduction
Permeability and porosity are important characteristics of sand reservoir bodies; their magnitudes, patterns, and variabilities significantly influence the migration, accumulation, and distribution of fluids and gases in the reservoirs, and just as significantly determine the ability of reservoirs to release their fluids and gases to production stimulation.
SPEJ
P. 229
Fort-Dauphin beach sands, south Madagascar: natural radionuclides and mineralogical studies