Pressure Stratigraphy in Deep Marine Sediments
Kenneth E. Williams1 and Martin D. Matthews2
1 Knowledge Systems, Inc, Sugar Land, TX
2 Knowledge Systems Inc, Stafford, TX
The pressure stratigraphy within individual overpressured compartments is a result of the depositional system and its burial history. These are defined by the lithostratigraphy, biostratigraphy, seismic, and sequence stratigraphy of the sediments within that pressure cell. A pressure stratigraphic system is defined on the top by a sealing facies. The rate of pressure change within the seal unit is commonly greater than the lithostatic gradient, most often due to pressure bleed-off. The base of the system is defined by the next pressure system. Laterally it is defined by faults (sudden changes) or by facies changes (usually more gradual and operationally defined). Internally there commonly is a pressure gradient that is less than in the overlying seal and is bounded between lithostatic and hydrostatic gradients.
These pressure units are dynamic through time. The topmost unit (seal) consists of low permeability units that are correlateable within a mappable area and are roughly parallel to conventional stratigraphic or biostratigraphic divisions over these distances. However, the age of pressure sealing units often differ from existing zonations based on other criteria (faunal marker or sequence, first sand, seismic event, etc). They are often closest to units determined by sequence stratigraphy.
Vertical and horizontal compartment boundaries can be aerially mapped. The permeability with a seal changes across the area depending on its sedimentation and diagenetic characteristics. Forward modeling of these pressure systems allows the mapping of the permeability changes. Examples of the use of pressure stratigraphy are derived from the deepwater Gulf of Mexico basin.