Tertiary structuring in southwest Queensland: implications for petroleum exploration

Abstract

Southwest Queensland is covered by sediments of the Eromanga Basin. Although deposition occurred during a structurally quiet time the basin shows extensive folding, faulting, uplift, and sequence truncation. This structuring is attributed to post-depositional (post-Cenomanian) tectonism involving essentially mild, east-west directed, basement compression. The detailed chronology of this tectonism is vague but uplift began during the Late Cretaceous and at least two discrete episodes of Tertiary folding are recognised. In addition to regional uplift, deformation involved the reactivation of pre-existing basement trends. The style of deformation varies systematically but is always dominantly compressional, although along the east-southeast oriented Gidgealpa-Merrimelia-Innamincka (GMI) trend wrenching is conspicuous. Across areas of shallow basement, deformation has been preferentially expressed as vertical displacements involving the formation of reverse fault bounded, tilted, blocks, pop-up blocks, and trap door structures. As hydrocarbon traps these structures have limited prospectivity. Formation of these structures post-dates the peak phase of hydrocarbon generation and critical closure is often fault dependent. Where sediments of the Adavale Basin are preserved Tertiary reactivation of Kanimblan structural trends has resulted in the development of folds within the overlying Eromanga Basin sequence. Folding may enhance pre-existing drape related closure. Where folding is associated with the rejuvenation of basement-related faults the trapping potential of the reactivated structures is often reduced. The common occurrence of multi-storey hydrocarbon pools not filled to spill-point, and located adjacent to reactivated fault escarpments provides strong evidence that Tertiary, reactivated, reverse faults do not seal. The most prospective structural targets appear to be those involving reactivated, large palaeo-highs. By absorbing much of the Tertiary deformation along their flanks the crestal integrity of the structure remains relatively intact. At the same time reactivated faults along the flanks provide conduits for vertical migration of hydrocarbons up into reservoirs lying in crestal locations. Uplift of the eastern margin established it as the major intake area for the recharge of the basin aquifers and the southwest flow of groundwater. The widespread formation of prominent anticlinal fold trends and reverse faults during the Tertiary established new barriers to ground water movement and horizontal migration of hydrocarbons. Tertiary uplift also retarded maturation along several key trends. Contemporary hydrocarbon drainage patterns were probably established during the Middle Tertiary and after peak hydrocarbon generation. Thus predictions of hydrocarbon drainage areas based on structural maps, such as the 'C' or 'P' horizons, can lead to erroneous conclusions. 'C-P' isopachs, corrected for post-depositional thinning, provide the best indication of drainage patterns during the critical period of peak generation.