ORIGIN OF QUARTZ CEMENT IN THE TIRRAWARRA SANDSTONE, SOUTHERN COOPER BASIN, SOUTH-AUSTRALIA

Quartz cement is the most abundant authigenic mineral in the fluvio-de ltaic Tirrawarra Sandstone and plays an important role in controlling reservoir quality, Quartz cement ranges from 0 to 19% and is controlle d by the original sandstone composition, Petrographic, fluid inclusion , electron microprobe and cathodoluminescence (CL) data from the quart z cement indicate multiple stages of cementation at different temperat ures and suggest more than one silica source, CL observations indicate up to six stages of quartz cement in some samples, The stages of quar tz cement can be classified into three zones: an innermost zone of bro wn-luminescing cement (Z1), a middle zone of bright blue-luminescing c ement (Z2) and an outer zone of bronm-luminescing cement (Z3), Dead oi l or bitumen is trapped between Z2 and Z3, indicating that Z3 formed a fter oil migration commenced, Measurements of homogenization temperatu res from fluid inclusions in quartz overgrowths indicate that quartz c ement precipitated over a temperature range of 65 to 130 degrees C. Z1 quartz cement formed between 65 and 80 degrees C and Z2 cement was pr ecipitated between 80 degrees C and 100 degrees C. Z3 quartz cement fo rmed later at temperatures around 130 degrees C, Microprobe analysis s hows a consistent variation in aluminum between each quartz cement zon e, The average aluminum content for Z1, Z2, and Z3 is respectively 237 , 538, and 58 ppm, fluid-inclusion precipitation temperatures and alum inum content have been used to help identify the silica sources for di fferent zones of cement. A likely source of silica for Z1 quartz cemen t is early alteration of feldspar grains. The bright blue CL color in the Z2 quartz cement is related to its high aluminum content, The sour ce of silica for this cement is likely to have been late dissolution o f feldspar grains by acidic fluids generated during kerogen maturation , Late quartz cement (Z3) has the lowest aluminum content, which is si milar to that of detrital quartz grains in the Tirrawarra Sandstone, C onsidering the temperature of precipitation, very low aluminum content , and the presence of Z3 cement in facies prone to stylolitization, th e silica source for the cement is likely to have been pressure solutio n of detrital quartz at stylolites and grain contacts.