Seismic stratigraphy and thermal considerations in overpressure analysis

The depth at which the pressure gradient exceed hydrostatic gradient is defined as the on-set of overpressure. The overpressure zone started within fine sediments and is associated with high rate of sedimentation. The principal components in the development of overpressure are lithology and principal stress. The properties of the soft and fine sediments, of low<br>permeability over extensive areas with thick intervals allows for seismic facies interpretation of the top of overpressure. However, presence of pressure compartments due to faults,<br>differential compaction and localised rock facies vary the depth to top of on-set overpressure.<br>The on-set of overpressure may occur near stratigraphic maximum flooding surface (Figures 1<br>and 2). It has been established that the top of overpressure and the transition zones are readily predicted by seismic data. The problem arise when the drilling progress into the hard overpressure zones where the formation pressure registered during drilling are likely to be higher that the anticipated pressure from seismic. Limitation of well control data and poor seismic reflection data pose constraints in the estimation of overpressure. Incorporating thermal analysis and pressure trend curves in estimation of overpressure in deeper intervals will complement the seismic data for prediction of the pressure. The under prediction of overpressure occurs within the shallow reservoir is uncommon, but it occurrence may surprised drillers. This occurrence of abrupt pressure increase at shallow depth within the overpressure zones is because, of inflationary pressure or the centroid effect. The pressure in the sand bodies is in disequilibrium and of higher pressure than the surrounding shale at shallow depth. This is observed when the sand aquifer is dipping at certain angles. In this situation a pressure-temperature assessment shall provide an estimate<br>of the pressure. The observed pressure-temperature gradient, superimposed on the velocity profile can produce a better estimation of the pressure within the overpressure zones.<br>This technique forms a comprehensive method in estimation top of overpressure and hard overpressure and also the prediction of overpressure within the deep reservoirs. By<br>applying the techniques of seismic velocity inversion and Interval Pressure-Thermal Gradient modeling, provide a robust solution into quantification of the pressure in the deep reservoir. The seismic and thermal data could be integrated to estimate the pressure and predict the pressure at deeper reservoirs. The estimated trend from thermal analysis would complement seismic data in the prediction of overpressures (Figure 3). In fact, in thermal models, the estimated top of over pressure and hard overpressure are shallower than those of seismic prediction and in addition the value of pressure is higher in the estimated trends than those of seismic predicted, for deep reservoirs.