Prediction of Reservoir Quality in Deeply Buried Sandstone Reservoirs

K. Bjorlykke

When exploring for oil reservoirs that have been buried to more than about 4 km depth, reservoir quality is one of the most important factors that goes into the risk analyses. Porosity may vary from less than 10% to about 20% porosity. Particularly in offshore basins, this means that diagenetic factors alone can determine if a prospect is economical or not.

Porosity reduction during deep burial is mostly due to chemical compaction (pressure solution) and precipitation of pore-filling cement, commonly quartz. Modeling of fluid flow in sedimentary basins can constrain the supply of dissolved solids by advection. At burial depths below 4 km the porosities of shales are normally low (<5%), limiting the potential for further compaction-driven pore-water flow. Pore-water flow in the deeper parts of sedimentary basins is sufficiently low that diagenetic reactions involving large volumes of rocks can be taken to be close to isochemical.

Precipitation of illite depends on the presence of Al-precusor minerals like kaolinite and smectite and on a source of potassium, usually k-feldspar. Preservation or destruction of porosity during deeper burial depends, then, mainly on the primary mineral composition, textural relationships (i.e., clay coatings) and on temperature. The vertical variations in porosity that can be observed in cores and on well logs are evidence of strong control by sedimentary facies and primary mineralogy: this variation cannot be predicted by normal basin modeling.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995