Permeability and Porosity Estimation by Electrofacies Determination
Low-permeability seals associated with abnormal pressures are most commonly identified by examining vertical pressure profiles and noting the depths at which a major change in the pressure gradient occurs. Alternatively, zones of very low permeability that may act as fluid seals may be identified on the basis of core analyses and well tests. Often, however, there are an insufficient number of direct pressure or permeability measurements to adequately identify the depth and lateral extent of these seals. A method has been developed for estimating porosity and permeability through the use of wireline logs. Multivariate statistical techniques are used to segment the logs and group the segments into electrofacies types. Application of this technique to 18 wells within the St. Peter Sandstone of the Michigan basin shows that the electrofacies characterization reflects both the hydraulic and diagenetic characteristics of the formation. Six electrofacies types have been identified, one of which has characteristics similar to those found within seals in other basins.
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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.