Abstract: The Use of Porosity - Effective Stress Relationships in Mudrocks

HARROLD, TOBY W.D., R.E. SWARBRICK, and N.R. GOULTY

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. Analysis of mudrock porosity values in overpressured wells shows that this is often not the case. Two scenarios exist to explain the origin of the measured overpressure in the section cut by the well:

Firstly the high pressure measured in the reservoirs is derived from the adjacent mudrocks which are overpressured by disequilibrium compaction or some fluid expansion mechanism (e.g., gas generation). The second situation arises where the fluid in the reservoirs is derived from deeper in the basin and arrives at the section via vertical and lateral transfer. Until an equilibrium is achieved, the aquifer pressure exceeds that of the adjacent mudrocks.

A 700m interval in one case well shows the mudrock porosity and hence effective stress remain constant, while in the aquifers, pore pressure increases and effective stress decreases rapidly over the same section, thus ruling out disequilibrium compaction. The constant log response in the mudrocks 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.

Analysis of effective stress and porosity data coupled with an understanding of overpressure generating mechanisms will help quantify seal integrity and therefore the trapping potential for hydrocarbons.

AAPG Search and Discovery Article #90942©1997 AAPG International Conference and Exhibition, Vienna, Austria