Fluids in the Upper Continental Crust: Static or Dynamic?

University Of Oklahoma Norman Campus, Norman OK

Investigators

Abstract

9909277 Deming A major area of uncertainty in our knowledge of crustal hydrodynamics concems the temporal and spatial scales over which overpressures in the continental crust are generated, maintained, and dissipated. At the present time there is no consensus in either academic or industry circles to several fundamental questions. What is the cause of overpressuring in older sedimentary basins which have not undergone active sedimentation for hundreds of millions of years? Do pressure seals exist? If they exist, how low must rock-permeability be to form a seal? If pressure seals exist, are they created by diagenetic alteration, or are they in fact gas capillary seals? The Anadarko Basin in southwestem Oklahoma will be used as a natural laboratory to study the conditions under which high fluid pressures may be generated and contained. The Anadarko Basin is anomalous, because it contains extensive areas of abnormally high fluid pressures, yet it has been tectonically quiescent for more than 100 Ma. The problem will be approached from a conceptual framework which postulates two end-member conceptions of crustal hydrogeology. Static hypotheses explain the existence of high fluidpressures in the crust by invoking pressure seals, rock units that are essentially impermeable over an appropriate time scale. Dynamic hypotheses explain the existence of overpressures in the crust as a transient imbalance caused by a geologic process (e.g., hydrocarbon generation) creating overpressures faster than they may be dissipated from low-permeability rocks. To distinguish between these alternative hypotheses, or to determine if they are in fact end-member behaviors of a continuum, well log and core data will be gathered and used to characterize the thermal, hydrologic, and geologic nature of the basin. Models of pressure generation and dissipation will then be used to test different hypotheses and discriminate between contrasting visions of basinal fluid regimes. Obtaining an understanding of how overpressures form and dissipate in the upper continental crust could lead to a better understwiding of many important physical, chemical, and mechanical processes (e.g., diagenesis, compaction, hydrocarbon generation and migration, seismicity) that take place in the continental crust as well as an understanding of how the physical properties (e.g. porosity, permeability) of crustal rocks vary in time, space, and scale.