A Key Test of Otway Basin Potential: The Eumeralla-Sourced Play on the Chama Terrace

Abstract

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.

Geological structure and kinematics of normal faults in the Otway Basin, Australia, based on quantitative analysis of 3‐D seismic reflection data

“…The Otway Group is a succession of fluvio‐lacustrine sediments, up to 8000 m thick, that accumulated in graben and half‐graben structures during the first rifting event (Late Jurassic – Early Cretaceous). It also contains large amounts of volcaniclastic material derived from the east (Kopsen & Scholefield, ; Tupper et al ., ).…”

“…A thin basal unit, the Massacre Shale, passes upwards into the argillaceous sandstones of the Pebble Point Formation (Fig. , Tupper et al ., ). The Pebble Point Fm.…”

“…The strata of the Otway Basin can be divided into five unconformity‐bound successions (Tupper et al ., ): the Otway, Sherbrook, Wangerrip, Nirranda and Heytesbury Groups (Fig. , Perincek & Cockshell, ).…”

Subseismic pathway prediction by three-dimensional structural restoration and strain analysis based on seismic interpretation

“…The stratigraphy of the study area can be divided into eight supersequences that are combined here as five lithostratigraphic groups; they are as follows, from oldest to youngest: the Otway, Sherbrook, Wangerrip, Nirranda, and Heytesbury Groups (Figure 2; Tupper et al, 1993). The stratigraphic horizons interpreted in the seismic volume belong to the youngest four of the five groups (Figure 2).…”