Fluid Flow in Compacting and Aggrading Sedimentary Basins

John M. Sharp, Jr., P. E. Blanchard

Fluid flow in a compacting and aggrading sedimentary basin occurs in three separate but interacting hydrogeologic regimes: meteoric, overpressured, and thermobaric. In the meteoric regime, recharge by precipitation occurs at topographic highs and the ground water moves away from these recharge areas in response to the force of gravity. Most of the flow occurs in local flow systems, which discharge at rivers. Regional flow systems also are present in which flow is downdip and discharge occurs by cross-formational flow near the shoreline. The overpressured regime occurs in the part of the basin undergoing sediment deposition and compaction. The fluids in this regime move in response to excess fluid pressures created by compaction disequilibrium and, possibly, mineral transf rmations, aquathermal pressuring, and hydrocarbon maturation. Numerical modeling indicates that fluid movement is vertical in isolated or low-permeability sediments whereas lateral movement may occur in continuous high-permeability sediments. Major fault zones enhance vertical fluid flow. The thermobaric regime occurs in that part of the basin where pressures and temperatures are sufficient for metamorphic reactions to release fluids, especially water and carbon dioxide. This regime is not well known, but it may be important in deep-basin diagenesis. Mass and energy transfer in excess of what is possible by diffusion and conduction is apparently required for diagenesis, petroleum migration, and ore formation. Forced convection (advection) has generally been considered the dominant proces of enhanced energy and mass transfer. However, theoretical calculations and circumstantial field evidence indicate that free convection may also be important.

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