Variability of Sand Bodies in Deltas

Harry H. Roberts, James M. Coleman

Deltaic systems display wide variability in sand-body geometry because of tectonic setting, marine energy regimes, proximity to source, climate, basin shape, and other less important variables. Many deltas contain the essential parameters that favor hydrocarbon generation, migration, and entrapment: multiple reservoir potentials, abundant source rocks, penecontemporaneous structure, numerous stratigraphic trapping mechanisms, and efficient pathways for fluid movement during burial and compaction.

River systems that carry large fine-grained sediment loads to low-energy tropical and subtropical receiving basins tend to develop thick vertical sequences characterized by stacked reservoir-quality sands encased in a matrix of highly organic shales. Differential compaction and mass-movement processes, typical of these delta types, result in abnormal thickening and displacement of reservoir-quality sands into deeper marine environments as well as rapid dewatering and compaction of clays, which favor fluid migration. In contrast, those river systems that debouch into marine basins characterized by high levels of wave and current activity usually show an abundance of widespread sand bodies, generally quartz rich and devoid of fine-grained matrix, but generally lack close proximity to so rce beds and are not characterized by an abundance of structural and stratigraphic traps. Most large-scale traps in these systems tend to form with subsequent tectonic movements, rather than by penecontemporaneous deformation.

Systematic studies of modern deltas from a variety of environmental settings have allowed the development of criteria to analyze reservoir geometry and quality, source rock potential, trapping mechanisms, and fluid migration pathways that can be extrapolated to the subsurface.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.