ABSTRACT

Quartz cement is the most abundant authigenic mineral in the fluvio-deltaic Tirrawarra Sandstone and plays an important role in controlling reservoir quality. Quartz cement ranges from 0 to 19% and is controlled by the original sandstone composition. Petrographic, fluid inclusion, electron microprobe and cathodoluminescence (CL) data from the quartz cement indicate multiple stages of cementation at different temperatures and suggest more than one silica source. CL observations indicate up to six stages of quartz cement in some samples. The stages of quartz cement can be classified into three zones: an innermost zone of brown-luminescing cement (Z1), a middle zone of bright blue-luminescing cement (Z2) and an outer zone of brown-luminescing cement (Z3). Dead oil or bitumen is trapped b tween Z2 and Z3, indicating that Z3 formed after oil migration commenced. Measurements of homogenization temperatures from fluid inclusions in quartz overgrowths indicate that quartz cement precipitated over a temperature range of 65 to 130°C. Z1 quartz cement formed between 65 and 80°C and Z2 cement was precipitated between 80°C and 100°C. Z3 quartz cement formed later at temperatures around 130°C.

Microprobe analysis shows a consistent variation in aluminum between each quartz cement zone. The average aluminum content for Z1, Z2, and Z3 is respectively 237, 538, and 58 ppm. Fluid-inclusion precipitation temperatures and aluminum content have been used to help identify the silica sources for different zones of cement. A likely source of silica for Z1 quartz cement is early alteration of feldspar grains. The bright blue CL color in the Z2 quartz cement is related to its high aluminum content. The source of silica for this cement is likely to have been late dissolution of feldspar grains by acidic fluids generated during kerogen maturation. Late quartz cement (Z3) has the lowest aluminum content, which is similar to that of detrital quartz grains in the Tirrawarra Sandstone. Consi ering the temperature of precipitation, very low aluminum content, and the presence of Z3 cement in facies prone to stylolitization, the silica source for the cement is likely to have been pressure solution of detrital quartz at stylolites and grain contacts.