Several of the existing pore pressure estimation methods are based on establishing a normal compaction trend curve with depth. The rationale is that as fine grained sediments are buried with depth the increase in overburden stress compact the sediments therefrom the normal compaction curve can be developed. Common methods using a normal compaction trend as the basis is the equivalent depth method, or the Eaton ratio method which uses sonic, density or resistivity logs as inputs. When fluid overpressure develops caused by other sources than mechanical compaction, such as fluid mechanical entrapment, rock molecular transformation, migration or hydrocarbon generation, Bowers method can be applied. Bowers extended the above mentioned methods to calculate over pressure where sediments are unloaded during pore pressure build up.
However, these methods require that you establish a normal compaction curve for sediments without overpressure. In many onshore basins, such as the Mid-Alberta basin, which has thick eroded sections and therefore the sediments are over-consolidated, it is not possible to establish a normal compaction curve. To be able to predict the pore pressure in over-pressured Mid-Alberta shales, we developed a pore pressure prediction method based on over-consolidation theory. We tested the method with sonic logs and the method gave promising results for establishing pore pressures in the over-consolidated Alberta basin.
When clays are buried the overburden stress increases and the shale is compacted. This behaviour follows the Terzaghi (1929) stress relationship (Equation 1) between total stress (compressive stress developed by the sediment load), effective stress (as can be seen as the the sum of stresses acting on the grain to grain contacts), and the pore pressure.
Where is the total stress, s is the effective stress and PP is the pore pressure. During deposition will increase. When is increased and at the same time the fluids are allowed to escape, will increase while porepressure remains hydrostatic. The increase in will compact the sediment and reduce its porosity. Clay rich sediments with a uniform lithology will reduce porosity with depth as long as the fluids are allowed to escape. Numerous normal compaction curves versus effective stress or depth are given for different sedimentary basins and rock types (Rieke and Chillingarian, 1974; Baldwin and Butler, 1985). On the other hand if depositional rate is fast, the low permeability in clay rich sediments restricts the upward flow and creates overpressure (pressure above a normal hydrostatic gradient). This overpressure will hinder the compaction and stop the porosity reduction with depth trend. This trend deviation or stopped compaction can be used to estimate the amount of overpressure. Eaton (1975) proposed the Eaton ratio method to estimate overpressure on the form:
Where PP is the pore pressure, is the total vertical stress, Pn is the normal or hydrostatic pore pressure, and Aobs is the observed attribute, and Anorm is the attribute when the pore pressure is hydrostatic, and x is an empirical fitting constant for sonic compressional velocity data (x = 3) or restitivity data (x = 1.2). Since porosity is rarely measured directly, indirect measurements of porosity from logs such as acoustic velocity or restitivity data is used. The Anorm for the given depth of interest is not an actual measurement and the normal compaction trend has to be extrapolated for the depth interval where overpressure is encountered. The equivalent depth method also uses a reference normal compaction curve similar to Eaton's ratio method. Here the procedure is to compare the Aobs with the depth at which A norm would be at the normal compaction curve and use this value to determine the pore pressure (Ham, H.H. 1966).