Santos Ltd requested the services of the author to undertake detailed petrological descriptions and a sedimentological interpretation of Late Carboniferous to Early Permian sediments of the lower Gidgealpa Group in the Big Lake Field. The study was designed to aid facies identification and to determine depositional and other factors controlling reservoir quality. This was achieved by reviewing the literature, available drill core, wireline well logs and routine core analysis data, and by describing and point counting thin sections of 41 drill core samples plus 8 drill cuttings samples selected from 12 wells in the Big Lake/Moomba Fields, besides conducting X-ray diffraction plots of certain samples. 'Merrimelia-type facies' occur within the Tirrawarra Sandstone, and therefore an informal stratigraphic unit called the Tirrawarra-Merrimelia Formation has been proposed for Big Lake. This informal unit includes the 'Tirrawarra Conglomerate'. A lithostratigraphic approach was employed to identify facies within the Tirrawarra-Merrimelia Formation. Problems with the identification of basement at Big Lake 46, 51 and Moomba 82 are related to the presence of weathered granite, and elsewhere have resulted from an oversimplification of the interpreted depositional environment. Twelve lithofacies have been recognised at Big Lake, with four unconformities that make lateral correlation across the field difficult. A thick alluvial fan sequence centred near Big Lake 30 has effectively subdivided the field. Documentation of the sedimentological characteristics of each lithofacies, isopach maps and cross sections provided a basis for interpretation of the depositional environments. Rippled siltstones in lithofacies 1 are probably related to a glaciolacustrine lower shoreface setting. Upper shoreface, delta front and distributary channels characterise lithofacies 2 and 8. Floodplain deposits in marshes, bays and abandoned channels are apparent in the muds and silts of lithofacies 3. Medium to coarse grained, planar bedded sublitharenites dominate in the vertically stacked bar forms deposited on a distal braidplain during lithofacies 4, 6 and 9. Medial braidplains are differentiated by an increase in the number of sandy conglomerates in lithofacies 5 and 10. In addition, there may have been input from the alluvial fan during deposition of lithofacies 5. Return of glaciolacustrine and deltaic facies may be represented by the shales of lithofacies 7. Sandy conglomerates are dominant in the proximal braidplain deposits of lithofacies 11 and in the alluvial fan sequence of lithofacies 12. A distinct change in depositional environment between lithofacies 11 and 12 may be associated with uplift along the Big Lake/Moomba fault. Sandstones are typically comprised of medium to coarse grained, moderately well to well sorted sublitharenites. There are rare litharenites and quartzarenites, and minor poorly to moderately sorted sandy conglomerates. Mineralogical maturity is related to the depositional environments, with immature sediments concentrated in lithofacies 5, 10 and 11 and 12. Feldspar contents have been reconstructed from the proportion of grain replacing kaolin and illite. Anomalously high gamma responses in the conglomerates are due to a combination of the abundance of zircons that occur as inclusions within altered muscovites, and the number of shale lithics. Sediment was derived from igneous, metamorphic and sedimentary terranes, possibly the Big Lake suite of granites and granodiorites, the Dullingari Group and Mooracoochie Volcanics of the Warburton Basin, and other metamorphic terranes rimming the Cooper Basin. Reworking occurred during advance and retreat of the ice sheet. Authigenic minerals are dominated by quartz and clay minerals that have reduced reservoir quality. Relative abundance of the clay minerals was predetermined by the detrital mineralogy and later diagenetic changes. Early diagenetic chlorite and chlorite-smectite (corrensite) that filled pores and replaced lithics and micas are most abundant in the southern part of the field. Kaolin replaced feldspars and precipitated within intergranular pores during a phase of meteoric flushing that also produced dissolution pores. Kaolinite was converted to dickite when temperatures were approximately 120 to 130 degrees C. Mechanical compaction ceased when silicification started at temperatures above 90 degrees C and chemical compaction became important. Distribution of quartz cement has been controlled by geothermal gradients and grain size. Silica was derived from dissolution associated with stylolites and feldspar alteration.