Pressure Stratigraphic Sequences for Basin Analysis
Use of pressure stratigraphic sequences in basin modeling is shown to improve structural definition, including identification of lateral compartment boundaries, resulting in better basin analysis, which supports better risk assessment of hydrocarbon prospects.
There are four identifiable units in a complete pressure stratigraphic sequence. Unit I, a resistivity cap, is often observed immediately above an increase in overpressure, where resistivities are higher than would be expected for normally pressured rocks. It is commonly “chattery” and is more variable than the resistivities in the rocks below it. The cap may be either static or dynamic. Dynamic seals are those where pressure bleed-off has occurred, and the seal has migrated deeper in the section through time. The high resistivities in either static or dynamic seals are the result of precipitation of cement due to the changing pressure and temperature conditions that occur in the transition zone.
In Unit II, the pressure sealing interval, the rate of pressure change can either increase (IIa) or decrease (IIb) with increasing depth. Permeability is low enough to retard movement of interstitial water to a rate less than is required to maintain equilibrium with the overlying hydrostatic section. In a Unit III pressure compartment, vertical rock permeabilities are high enough for pressures within the compartment to have roughly equilibrated, so the rate of pressure increase is the same as a hydrostatic gradient. Within any unit in a pressure sequence, smaller-scale pressure cells commonly exist where lateral pressure transfer or hydrocarbon buoyancy effects are observed.
Pressure stratigraphic sequences vary laterally in a different way than typical stratigraphic zones. Equivalent overpressure values can occur in different stratigraphic units in different pressure compartments, and may cross unit boundaries laterally, depending on how permeability varies as facies change across any area in any stratigraphic interval. Velocities also vary strongly in response to pressure changes. The selection of basin modeling units based on pressure stratigraphy allows a closer tie to the seismic data used to build the structural surfaces used in modeling.
Finally, because hydrocarbons migrate from high to low pressure under the influence of gravity, use of pressure stratigraphic sequences in basin modeling allows for better definition of migration pathways.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009