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The Use of Porosity - Previous HitEffectiveNext Hit Stress Relationships in Mudrocks

HARROLD, TOBY W. D.

Fluid pressure measurements are not possible in mudrocks because their permeability is too low. It is generally assumed that the pore pressures in mudrock sections follow the profile defined by measurements made in the reservoir units. I have examined shale porosity values in overpressured wells from SE Asia, where the overpressuring mechanism has been reported as disequilibrium compaction, to test this assumption.

Shale porosity was derived from sonic velocities and correlated with variation in mudrock lithology using gamma ray, resistivity, neutron and density logs. Previous HitEffectiveNext Hit stress was derived from the shale porosity by comparison with normal compaction relationships applicable to hydrostatic pressured mudrocks and those overpressured by disequilibrium compaction. For the adjacent reservoir units, the Previous HiteffectiveNext Hit stress equals the overburden stress minus the pore pressure. The shales appear to have higher Previous HiteffectiveNext Hit stress values than the reservoirs: if this is the case the fluid pressures in the shales are less than in the reservoirs. This difference would imply that the mechanism for generating overpressure could not be disequilibrium compaction alone.

In one well, a 700m interval shows the shale porosity and hence Previous HiteffectiveNext Hit stress remains constant, whilst in the aquifers, pore pressure increases and Previous HiteffectiveTop stress decreases rapidly over the same section. The constant log response in the shales rules out fluid expansion generated within this interval. The overpressured fluids must therefore be derived from greater depths, and introduced preferentially into the aquifer via vertical and/or lateral transfer. Other wells show similar observations.

AAPG Search and Discovery Article #91021©1997 AAPG Annual Convention, Dallas, Texas.