In environments such as the Malay Basin, Gulf of Mexico and Nile Delta, where rates of sedimentation are high, the sediments are young and with low geothermal gradients, pore pressure profi les through shale-dominated sequences can be estimated confi dently using seismic velocity, wireline sonic and resisitivity data.
Assumptions are that the basin is extensional and that the main mechanism of overpressure generation is under compaction as a result of ineffective dewatering, referred to as disequilibrium compaction (Hubbert and Ruby, 1959).
As porosity is reduced to low values as a result of mechanical compaction during burial and the temperature increases (for instance, above 70oC in some basins; Gulf of Mexico, Bruce 1984), mineralogical changes occur in the shales. These changes lead to the two main processes of SECONDARY OVER PRESSURE GENERATION, fluid expansion/volume change and load transfer (or framework weakening).
Fluid expansion/volume processes include dehydration reactions such as gypsum to anhydrite, and smectite to a more dehydrated form. Smectite to illite transformation produces water released and silica, which will tend to precipitate locally. The release of bound water into sediment pores is minor in terms of generating overpressure (Swarbrick, and Osborne 1998).
Maturation of hydrocarbons, particularly in the case of gas generation, PRODUCES RAPID VOLUME EXPANSION, reducing effective stress and INCREASING PORE PRESSURE.
And then it gets into the heavier stuff.
Another point to note in this item is that these conditions can lead to the standard logging and analysis potentially underestimating pore pressures and this is more than likely why we have seen the statement made that they had measured higher pore pressure than was expected.
" Conventional porosity-based pore pressure analysis using sonic/seismic velocity and resistivity data as a measure of porosity retention, underestimates the overpressure effect of these secondary overpressure mechanisms."
" empirical fits to local data, and results from our work in other basins, where we have attempted to quantify these mechanisms by integrating basin history with rock properties and temperature data."
Notice this last statement that introduces a large potential for error when trying to make allowance for these secondary mechanisms.
It seems logical to conclude that the ability to estimate accurately the GOR is compromised by these secondary pore pressure generation mechanisms within disequilibrium compaction reservoirs.
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