Overburden Compaction and Pseudo-Micritization: The Most Effective Pore-Deteriorating Processes in the Carbonate Reservoirs

Salih Saner, Lameed O. Babalola, and Waleed M. Abdulghani
Research Institute of KFUPM, Dhahran, Saudi Arabia

Pore geometry (amount, size, shape, distribution and interconnectivity) defines the petrophysical properties of carbonate reservoir rocks. The existing carbonate rock classification schemes are based either on mineralogical composition or texture of the rock matrix. Textural classifications, which partially reflect pore geometry, group carbonate rocks into different classes based upon relative amounts of depositional mud and framework fabric. Observations from the Middle Eastern carbonate reservoirs, as exemplified in this study, indicate that mud occurrence is not solely due to deposition. It may also be formed by post depositional compaction and micritization restructuring the pore configuration and deteriorating porosity in the granular carbonate rocks.

Cementation, dissolution, and micritization are the common processes during early diagenesis. Subsequent overburden causes mechanical and chemical deformations, thus reducing the intergranular porosity. Mechanical deformation includes grain elongation, cracking and disintegration. Chemical processes include pressure dissolution that causes the development of long grain contacts and micro-stylolites. The ratio of pore size to grain size decreases as effective overburden stress increases. Micritized grains are amalgamated, and incompetent grains are mechanically crushed disintegrating into the pore spaces and thus appearing like micrite. In case of widespread overburden effect, agglutinated micritized grains form a pseudo-mudstone (pseudo-micrite) where grain ghosts are hardly recognizable.

Pseudo-micritization and pore to grain size ratio have been used in this study to develop a scale for quantifying the overburden compaction effect on porosity and permeability measurements. Correlations observed between the overburden compaction effect and these reservoir properties indicate that if compaction and micritization are scaled, porosity and permeability could be better estimated from petrographic data.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005