CRETACEOUS OF THE EROMANGA BASIN — IMPLICATIONS FOR HYDROCARBON EXPLORATION

The Cretaceous sequence in the Eromanga Basin covers nearly one-fifth of the Australian mainland and has a maximum thickness of about 2000 m. The lowermost prospective unit is the Neocomian Cadna-owie Formation, which commonly contains hydrocarbon shows and recently yielded oil in Merrimelia 15. Likely source rocks include organic-rich sediments at the base of the overlying Bulldog Shale and Wallumbilla Formation. Future commercial production from the Cadna-owie Formation will be hampered by fine grain size and carbonate cements, resulting in low porosity and permeability. In southwestern Queensland, the Cadna-owie Formation appears to have been locally dissected by prominent channels. If sand-infilled, these channels will represent an interesting stratigraphic play, although they are likely to be mudstone-and diamictite-infilled submarine canyons, similar to those in the Tertiary Latrobe Group, Gippsland Basin. As such, they represent one component of a complex hydrocarbon play that has yet to be evaluated.A third, stratigraphically higher exploration target is the middle Albian Coorikiana Sandstone. Although the marine mudstones which encase the Coorikiana Sandstone are only poor to fair source rocks, hydrocarbon shows are common in the unit, which yielded 9900 m3 of gas per day when tested in Strzelecki 8. The potential of the unit is limited by poor reservoir quality and marginal oil maturity.The Toolebuc Formation, of middle-late Albian age, is the fourth promising target for significant hydrocarbons. The formation consists of organic-rich mudstone which is oil-shale bearing over large parts of Queensland. Minor oil and gas shows, attributed to early generation from a very rich source rock, suggest that oil production from the Toolebuc Formation may be possible if naturally fractured shale reservoirs can be located. Overall, the Cretaceous sequence in the southwestern Eromanga Basin is considered to have a modest oil and gas potential which should be evaluated in the course of drilling for deeper targets.It is emphasised that knowledge of Cretaceous and Cainozoic stratigraphy is essential in understanding the evolution of the Eromanga Basin. Rapid deposition of up to 2000 m of Cretaceous sediments enabled generation of hydrocarbons in the Jurassic sequence, beginning in the latest Cretaceous and continuing today. Although early, syn-sedimentary structuring and mild epeirogeny are evident in Cretaceous sediments, of far greater significance is a major phase of Early Tertiary folding, due probably to the effects of continental breakup and collision. This greatly enhanced pre-existing structures, produced some very large anticlines, and caused up to 800 m of the Cenomanian Winton Formation to be eroded from elevated areas. A complex interplay between timing of structural growth and maturation of the sequence is thus considered to be a major control on the present distribution of hydrocarbons in the Eromanga Basin.