85 TO 100 MILLION YEAR OLD SOUTHERN HIGH LATITUDE RIVER ENVIRONMENTS: THE KEY TO UNDERSTANDING GIPPSLAND GAS RESERVOIRS

The southern high latitude of Early Cretaceous (140 to 100 Ma) Australia was situated at least 65° S. The climate varied from arid to humid. The strata of the Strzelecki Group and Emperor Subgroup in the region were examined to determine their diagenetic and environmental history. Typically the sediment consisted of sub-angular to sub-rounded, compacted grains composed of detrital quartz, volcanolithic fragments, feldspar and mica. SEM analyses indicated grains coated in diagenetic swelling-chlorite clay with little to no original porosity, which suggests diagenetic alterations soon after burial.

The onshore Early Cretaceous (Strzelecki Group) siliciclastic sediment was deposited in a non-marine, fluvial environment. Typical facies included fining upward sequences of gray/green, “pepperpot” textured sandstone, overbank sediment, floodplain deposits and coal. Previous literature suggests Early Cretaceous deposition in a fluvio-lacustrine environment; however, this work suggests a predominantly fluvial environment, which fluctuated between periods of high and low-sinuosity. The offshore mid-Cretaceous (Emperor Subgroup) strata are host to tight gas reservoirs. These non-marine facies are typified by “pepperpot” textured sandstone, floodplain deposits, soil horizons and coal. This “pepperpot” texture is due to a volcanic litharenite source similar to the underlying Early Cretaceous. The previous literature suggests the mid-Cretaceous strata in the Gippsland Basin was deposited in a lake; however, data from Longtom 2-ST1 and Garfish-1 suggests that at least part of the Emperor Subgroup was deposited in a fluvial environment with lower-sinuosity and higher-sinuosity units.

It is likely that the changes in fluvial style from Early and mid-Cretaceous of the Gippsland Basin relates to a combination of climate and/or tectonics. The climate was driven by orbital parameters on the order of 20 to 30 kyr. Seasonality was driven by flooding and dry periods, which was indicated by the alternating of high and low-sinuosity units in Boundary Creek-1A.