A classification scheme for compartments and seals is introduced and physico-chemical processes underlying their genesis and evolution are suggested. The sedimentary basin is viewed as a chemical reactor of epic scale constantly being driven out of equilibrium. As a result, it sustains a variety of important compartmentation and sealing phenomena. Practical implications of this can be obtained by building a comprehensive model accounting for operating physico-chemical processes and then developing computer codes to simulate it. We argue that this is feasible and can contribute greatly to the development of exploration, production, and resource assessment strategies.
Diagenesis deep in a sedimentary basin involves a number of strongly coupled reaction, transport, and mechanical (RTM) processes. When a coupled RTM system is driven sufficiently far-from-equilibrium, it can become organized in space or time in ways that have no direct relation to patterns imposed at the basin boundary or through sedimentary input. Rather, these patterns organize themselves. Our results to date suggest that many aspects of compartment and seal genesis and dynamics appear to be a manifestation of this far-from-equilibrium dynamic.
If sedimentation was very slow, then all fluids could escape and rock at depth would be porosity-free. But beyond some critical subsidence and burial rate, fluid can get trapped in compartments for appreciable times. This is because relatively uncompacted rock finds itself at appreciable depth. This rock is therefore far-from-equilibrium—a large free energy difference exists between the uncompacted and the compacted system due to the overburden stress.
A most dramatic manifestation of far-from-equilibrium conditions occurs when the system develops periodic or other oscillatory variations in space or time. A sequence of episodic fluid releases from an overpressurizing compartment can occur via a cycle of fracture generation and healing. Also, alternating layers of contrasting texture or mineralogy can develop to produce textural banding that has been found to be at the root of a number of pressure seals.
We set forth the general point of view that a basin is a far-from-equilibrium system capable of sustaining a variety of compartmentation phenomena. Compartments, banded seals, and episodic fluid migration and other phenomena key to petroleum exploration, production, and resource assessment are found to be consequences of the far-from-equilibrium basin dynamic.
To turn these general observations into practical strategies one must be more specific in correlating these phenomena with basin tectonic, thermal, and sedimentary history. The complexity arising from the many coupled RTM processes demands that this can only be done by the development of a basin RTM simulator, whereby we can
predict new compartment and seal types;
extrapolate findings from one basin to other basins;
predict the location, internal structure and extent, and contents of compartments;
understand episodic and other petroleum release phenomena in compart-mented basins for the exploration of petroleum expelled from compartments; and
design production strategies for compartmented reservoirs.
Figures & Tables
Basin Compartments and Seals
Basins worldwide exhibit an unexpected degree of hydrologic segregation. There can be regions of a sedimentary basin that are isolated from their surroundings by a relatively thin envelope of low-permeability rock with an interior of sufficiently high permeability to maintain a consistent internal hydrostatic fluid pressure gradient. These have been named pressure compartments. Presure compartments have several remarkable features, just one of which is that internal fluid pressures can greatly exceed or be significantly less than any regional topographically controlled hydrologic head or drain. This publication contains 30 chapters that take detailed looks at pressure compartments in general, and detail case studies of these compartments in specific basins, such as the Anadarko and Gulf of Mexico. The volume also looks at other considerations in sedimentary basins such as hydrodynamic and thermal characteristics, and mechanical properties of rock.