Abstract: Episodic Overpressure as a Primary Control of Diagenetic Fluids

Richard E. Swarbrick

Modeling of pore pressure histories in several contrasting basins has shown that variable rates of burial can lead to periods of excess pore pressure (overpressure) interspersed with normal (hydrostatic) pressure. The amount of excess pressure increases as the basin evolves and lower permeabilities are experienced in the deeper part of the basin. Complementary studies of petroleum and brine inclusions found in late diagenetic quartz cements from reservoir sandstones in these basins, and dated from radiometric ages of coeval authigenic illite, indicate hydrostatic pressures at the time of entrapment. The age dates for the diagenesis also coincide with periods of hydrostatic pressure from the pore pressure modeling. Furthermore, the diagenesis, which can be accompanied by migration of h drocarbons into the reservoir, immediately postdates high overpressure modeled for the field and/or the adjacent depocenter where the hydrocarbons are assumed to have been generated

A model is proposed whereby periods of rapid burial of the basin and disequilibrium compaction lead to regional overpressure (i.e., the entire sedimentary section including both mudrocks and reservoirs is influenced by overpressure) followed by progressive dissipation of the overpressure when burial is less rapid. Pressure dissipation occurs first in the most permeable units, primarily the reservoir sections, which then become the focus of formation fluids (which can be brines and hydrocarbons) being "drained" from adjacent mudrocks, which are slower at reaching pressure equilibrium. This episode of differential pressure between mudrocks and reservoir is characterized by local overpressure and is the timing for the diagenesis and also of hydrocarbon migration, where suitable mature so rce rocks exist.

AAPG Search and Discovery Article #90982©1994 AAPG International Conference and Exhibition, Kuala Lumpur, Malaysia, August 21-24, 1994