Anadiagenetic Versus Epidiagenetic Abnormal Pore Pressure: Relationship Between Pressure Origin, Timing, and Methods of Prediction
Oscar L. Jensen
The principles of pore pressure occurrence and prediction are based primarily on petrophysical characteristics of shaly sequences in which continuous compaction produces a "normal" vertical fluid-pressure gradient and, congruently, undercompaction due to entrapped formation water reveals abnormally high pressures. These premises have yielded satisfactory results only in relatively young, rapidly subsiding basins of continuous and abundant fine clastic sedimentation. In mature or more complex geological provinces, the conventional pressure-prediction models have shown serious limitations. Reasons for these failures have been attributed to the heterogeneity of the lithologic columns, to the variations in their mineral composition (which restrain the delineation of the local "normal trend" that most prediction methods require), and to the fact that acoustic velocity (a common porosity-pressure tool) is more a function of effective stress rather than porosity. However, the main reason for the insufficiency of these models lies in the unawareness that the petrophysical characteristics, chosen as indicators, do not always reflect the presence of hydrologic anomalies, as well as the common disregard of postcompaction processes.
Accordingly, a necessary distinction between syncompaction (anadiagenetic) and postcompaction (epidiagenetic) pressuring processes is proposed. In anadiagenetic pressuring, the rock preserves petrophysical features which are proper of younger diagenetic stages, and the existing or relict effective stress (^sgrr) is the maximum stress (^sgrm) ever experienced by the rock-fluid system (^sgrr = ^sgrm). These pressures can be predicted by conventional methods, as in the case of the Cenozoic of the Gulf Coast and the Niger delta. In epidiagenetic pressuring, the maximum effective stress endured by the system exceeds the existing effective stress (^sgrm > ^sgrr), in which the increase in pore pressure is not neutralized by a change in interstitial volume due to the intrinsically irrevers ble nature of the compaction process. Examples of this are found in the Late Jurassic of the North Sea Viking graben, where overpressures are associated with normal to high formation densities (a consequence of postcompaction cementation), and in the deep Jurassic of Mississippi, in which normal and abnormal pressures concur with abnormally high secondary porosities which are, by definition, epidiagenetic.
AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.