Pore-Pressure Prediction in Overconsolidated Shales

Abstract

Abstract 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 overpressured 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.