Mudstone porosity and clay fraction in overpressured basins

Abstract

This thesis demonstrates the use of a mixture of standard and novel petrophysical techniques to estimate physical parameters of mudstone and explores the use of a generic, clay fraction-dependent compaction model in the context of pore pressure evaluation. Mudstones are often highly heterogeneous, yet many authors use a single compaction trend to describe their behaviour. Previous work has shown that the rate of a mudstone's compaction with vertical effective stress is a function of its clay fraction, the proportion of the sediment matrix with a particle diameter of less than 2μm. This observation forms the basis of the generic mudstone compaction model used in this thesis. The use of the generic compaction model is explored in two case studies using characterised mudstone samples and wireline log data from the Gulf of Thailand and Gulf of Mexico. Further mudstone samples from the Central North Sea were characterised. An error analysis showed that the compaction model can provide estimates of pressure to within ±1.8MPa at a burial depth of 3km (equivalent to ±0.5ppg mudweight) when the input parameters are constrained to an attainable level. In both cases studied, standard methods of analysis could not provide reasonable estimates of pressure in mudstone using wireline resistivity and porosity log data compared to pressure measurements in associated sand bodies. The deep sediments of the two wells studied from the Gulf of Thailand are overconsolidated with respect to their current stress state. The generic compaction model was used to determine that the overconsolidated sediments were uplifted by 1,300m and have been reburied beneath 900m of sediment that now overlies a regional unconformity. The generic compaction model was used in conjunction with an artificial neural network technique for the characterisation of mudstones from wireline data to determine pressure estimates in the mudstones of three deepwater wells in the Gulf of Mexico. A pressure transition zone in one well was shown to be associated with a 10% increase of mudstone clay fraction within the zone compared to surrounding rocks. In both case studies disequilibrium compaction was identified as the key overpressure generation mechanism.