Abstract

The Badak and Nilam sandstone reservoirs consist of quartz arenites/wackes and felspathic arenites/wackes, increasing in porosity and permeability from the lower to the upper zones. The type of porosity is primary interparticle porosity which is partially filled with authigenic clay, detrital clay and secondary quartz.

Authigenic clay minerals and an intermediate to advance stage quartz overgrowth are the main cementing agents in these reservoirs: however, a small amount of carbonate and ferruginous cement was also observed.

Kaolinite, illite, smectite and chlorite clay minerals are distributed throughout all the reservoirs in variable amounts. Comparison of shallow and deeper reservoirs showed that diagenesis is the main factor controlling reservoir quality in these zones. A higher degree of post depositional compaction and quartz overgrowth, an increase in authigenic clay, and an increase in unstable minerals in the deeper reservoirs could be the most important factors causing porosity and permeability reduction with depth. The recycling process may have improved reservoir quality in the upper zones.

Knowledge of the kind of diagenetic minerals occuring within every zone could lead to a better selection of well bore fluid which would be compatible with the mineralogy of the pore minerals, so that the possibilites of formation damage could be reduced.