--> Macro- to Microscale Sedimentary Fabric: Features that Inhibit or Enhance Fluid Flow, by D. L. Siron and M. P. Segall; #90986 (1994).

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Abstract: Macro- to Microscale Sedimentary Fabric: Features that Inhibit or Enhance Fluid Flow

D. L. Siron, M. P. Segall

Detailed investigations of sedimentary structures should be incorporated with geophysical data, stratigraphic information and hydrologic parameters for a comprehensive evaluation of how fluids are transported within the subsurface. Shallow subsurface sediments (<200 m) on the South Carolina coastal plain evolved from the deposition of marine (carbonate to siliciclastic) and fluvial sequences that were altered subsequently by diagenetic processes. Each unit is characterized by unique sedimentary structures that will facilitate or impede fluid transport.

The uppermost fluvial sediments (0-20 m) are characterized by high clay/sand ratios and variable grain-size (clay-size to cobbles) that are depositionally inherited. These deposits are spatially and temporally discontinuous, and as such produce highly variable fluid transport pathways at the macroscale. Mesoscale sedimentary structures such as convoluted bedding, sand and silt interlaminations, biomineralized burrows and graded bedding are discontinuous at the cm-scale. Syn- and post-depositional lateritic weathering resulted in mineralogic alteration that can enhance local microporosity by up to 40%. Grain dissolution produces interconnected micropores 2-4 microns in diameter. Underlying the fluvial sediments are homogeneous, well-sorted shallow shelf sands characterized by low clay/ and ratios. Low concentrations of smectite do not impede fluid flow. These sediments have not been modified diagenetically and maintain most of their depositional characteristics.

The underlying carbonate sediments have been lithified partially by authigenic calcite and zeolite crystallization. Primary sedimentary structures such as silt and sand interlaminations, load structures and small-scale cross bedding are diagenetically overprinted so that little effective microporosity and micropermeability are maintained. This study provides evidence for the influence of sedimentary structures and fabric on the transport of fluids. These characteristics are controlled by depositional environment and post-depositional processes.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994