DIAGENESIS AND RESERVOIR QUALITY, TOOLACHEE FORMATION, COOPER BASIN

Abstract

A preliminary petrological study of the Toolachee Formation in three Cooper Basin wells indicates that the intensity of diagenetic changes in the sandstones is the main factor influencing reservoir quality.

Reduction of primary porosity occurs as a result of silica cementation as quartz overgrowths, dissolution of quartz grains forming microstylolitic grain contacts, authigenic clay mineral formation accompanying breakdown of chemically unstable grains, and cementation by siderite. Secondary porosity is erratically developed as a result of dissolution of some unstable grains particularly in rocks showing an advanced state of diagenetic alteration. High permeabilities are obtained only from those sandstones where significant primary porosity is still retained.

Removal of chemically unstable grains during diagenesis has resulted in the formation of mineralogically mature sandstones, which consist almost entirely of quartz and clay minerals. The only other detrital components present are chemically resistant minerals such as muscovite or rock fragments composed of these resistant minerals.

A relatively simple relationship of increasing intensity of diagenetic processes with increasing depth of burial may exist for the Toolachee Formation in the Cooper Basin.