The Penola Trough is an elongate, Late Jurassic to Early Cretaceous, NW-SE trending half graben filled mainly with synrift sediments of the Crayfish Group. Katnook-1 discovered gas in the basal Eumeralla Formation, but all commercial discoveries have been within the Crayfish Group, particularly the Pretty Hill Formation. Recent improvements in seismic data quality, in conjunction with additional well control, have greatly improved the understanding of the stratigraphy, structure and hydrocarbon prospectivity of the trough. Strati-graphic units within the Pretty Hill Formation are now mappable seismically. The maturity of potential source rocks within these deeper units has been modelled, and the distribution and quality of potential reservoir sands at several levels within the Crayfish Group have been studied using both well and seismic data. Evaluation of the structural history of the trough, the risk of a late carbon dioxide charge to traps, the direct detection of gas using seismic AVO analysis, and the petrophysical ambiguities recorded in wells has resulted in new insights. An important new play has been recognised on the northern flank of the Penola Trough: a gas and oil charge from mature source rocks directly overlying basement into a quartzose sand sequence referred to informally as the Sawpit Sandstone. This play was successfully tested in early 1994 by Wynn-1 which flowed both oil and gas during testing from the Sawpit Sandstone. In mid 1994, Haselgrove-1 discovered commercial quantities of gas in a tilted Pretty Hill Formation fault block adjacent to the Katnook Field. These recent discoveries enhance the prospectivity of the Penola Trough and of the Early Cretaceous sequence in the wider Otway Basin where these sediments are within reach of the drill.

Geochemical analysis, petrographic examination and wireline log interpretation have identified intervals within the lower Eumeralla Formation of the Otway Group (Early Cretaceous) with good source potential. The sequence has a maximum penetrated thickness of 260 m and consists of thinly interbedded coal and siltstone deposited in peat swamp and lacustrine environments. Vitrinite is the dominant maceral present in the coal although the proportion of more oil-prone liptinite commonly exceeds 10 per cent. This is consistent with the intermediate Type II/ III kerogen composition indicated by Rock-Eval and is comparable with data from proven terrestrial oil-productive source rocks in the Gippsland and Cooper Basins. The siltstone is organically-lean but has some algal input. Algal-rich lacustrine source rocks could be developed nearer the basin centre.Regional stratigraphic, structural and thermal modelling studies highlight the exploration potential of the Chama Terrace in the northwest Otway Basin. Structures on the terrace are ideally situated to receive a hydrocarbon charge from mature Eumeralla Formation source rocks in fault blocks on the downthrown side of the Tartwaup Hingeline.Seismic mapping of offshore permit EPP 24, and adjacent onshore permit PEL 40, has delineated several large fault blocks where Crayfish Subgroup (Otway Group) reservoir is juxtaposed against, and sealed by, the lower Eumeralla Formation sequence. Drilling scheduled for late 1992 will determine the credibility of the Eumeralla-sourced play and provide a key test of the ultimate hydrocarbon potential of the Otway Basin.

Commercial volumes of hydrocarbons have been discovered in Early Cretaceous synrift sediments of the Crayfish Group of the Penola Trough, western Otway Basin. Four other such depocentres are recognised using available seismic and drilling data. A standard nomenclature is proposed for the major structural elements observed in this part of the Otway Basin.Various tectonic models proposed for the Early Cretaceous rift history of the Otway Basin are reviewed and tested using true dip analysis of seismic events and sandbox modelling experiments. Results support a generally north-south direction for rift extension in the Tithonian-Barremian. This implies that the ENE trending Robe Trough is the headwall rift compartment (although with 20°obliquity), whereas the NW trending Penola Trough is a more oblique or transtensional rift compartment.

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Origin Energy Resources Limited has been an active explorer of the onshore Otway Basin in southeastern Australia for over a decade. Since 1989, the company has acquired 1,893 km and 358 sq km of 2D and 3D seismic data respectively, and participated in the drilling of 21 exploration and appraisal wells, 18 of which it operated, at a net cost of over $25 million.The primary exploration targets have been fluvial sandstones developed axially in a series of elongate half grabens that formed in initial response to the separation of the continents of Australia and Antarctica in the Late Jurassic to Early Cretaceous. The most significant of these depocentres, in terms of established hydrocarbon potential, is the highly faulted, NW–SE trending Penola Trough where these reservoirs are informally referred to as the Sawpit Sandstone and the Pretty Hill Sandstone.As the company’s geological understanding of the Penola Trough improved over time, so its exploration efforts were rewarded with new commercial discoveries at Haselgrove, Haselgrove South and Redman, and through successful appraisal of Ladbroke Grove. With respect to the application of advanced technology, 2D and 3D seismic are now subjected to Prestack Depth Migration to improve imaging of the subsurface, and Amplitude Versus Offset, Seismic Variance, Seismic Attribute, Fault Analysis and 3D Visualisation processing and/or software are also used in an integrated manner to facilitate interpretation of these data sets. Nuclear magnetic resonance logs are run in most wells to characterise gas-bearing reservoirs.Technology, respect and persistence have all played key roles in optimising the exploration process. So too have corporate teamwork and collaborative research with representatives from academic institutions and government agencies, and with industry consultants and contractors. The complex interplay between many of these success factors is illustrated by way of a case history, specifically that of the discovery and commercialisation of a low quality gas resource at Ladbroke Grove.In the fullness of time, the company is confident that further exploration success in the onshore Otway Basin will see new market opportunities addressed in terms of both gas and electricity supply. It is also hopeful that oil will eventually be found in commercial quantities.