Banded Diagenetic Pressure Seals: Types, Mechanisms, and Homogenized Basin Dynamics
The structure, mechanisms of formation, and key role of diagenetically banded pressure seals are reviewed. A difficulty in predicting the genesis time and location of these seals is that they are apparently affected by local stresses and fluid pressures, but, in turn, they affect the basin-scale distribution of these factors. We show that this very formidable multiple-scale basin modeling problem can be solved via a computational homogenization technique.
Banded pressure seals are classified in two complementary ways. First we distinguish those generated through self-organization from those directly of sedimentary origin. They are then classified according to the specific processes (pressure solution, nucleation, diffusion, flow, coupled mineral reactions, and grain comminution) by which they emerge. The argument is made that feedback in the network of reaction, transport, and mechanical processes underlies the development of many diagenetic seals.
The technical problem of the need to simulate submeter-scale banding phenomena in basin-scale models is introduced and addressed. A computational homogenization scheme is shown to simultaneously capture phenomena on these two scales by treating them both in a way that preserves their characteristics. We believe that this approach is a great advance in basin modeling more generally in that it allows one to capture phenomena on multiple spatial scales arising from complex sedimentary features and their reworking through diagenesis.
The following are demonstrated by simulations carried out for pure quartz sandstones undergoing stress-induced reactions, diffusion, and fluid flow. Seals are very likely to form within finer grain beds or at some specific depth within macroscopically uniform grain-size sediments. The depth and the time needed to form a seal depend upon the intensity of overpressure, fluid flow, geothermal gradient, subsidence velocity, grain size, the sediment sequence, and tectonic environment. High overpressure or higher geothermal gradient tends to decrease the depth of the seal. Lower fluid pressure is favorable for the differentiation of the rock texture (and the formation of the seal). Once a seal starts to develop, the process is self-accelerated and self-enhanced and the sediments become more heterogeneous and anisotropic.
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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.