Late Cretaceous palynological correlation and environmental analyses of fluvial reservoir facies of the Tuna Field, Gippsland Basin, southeast Australia

The Tuna Field in the Gippsland Basin southeastern Australia hosts Late Cretaceous fluvial siliciclastic oil and gas reservoirs that have proved difficult to correlate seismically. Previous correlations of the reservoir facies have been achieved using a combination of well data (wireline log, cuttings and core) and qualitative palynological data. This study integrates new detailed facies and palynological analyses that improve reservoir correlation across the Tuna Field. Specifically, investigation of the Campanian sections in Tuna-1 and Tuna-4 wells has resulted in a revision of the age assigned to the strata. In addition, the facies analyses of these two wells, supported by wireline log interpretation from several wells in the Tuna Field suggests that the reservoir intervals represent a low and high sinuosity fluvial depositional system. The revised palynological dating was enhanced by quantitative palynology leading to the identification of a biostratigraphically significant vegetation event within the Tubulifloridites (al. Tricolporites) lilliei Zone. The palynological data further shows that the regional vegetation was dominated by tree species attributable to the Podocarpaceae, Araucariaceae, Proteaceae and Nothofagus spp. (ancestral type). However, this canopy was not as stable as previous authors have suggested, and the vegetation events that help in the correlation further suggest times of small-scale climate variation within the Campanian.