1 BMR lournal of Australian Geology & Geophysics. 12. 5- 12 © Commonwealth of Australia 1991 Observations on the geological origin of the 'C' horizon seismic reflection, Eromanga Basin I.H. Lavering) Seismic exploration throughout the Eromanga Basin has identified several regionally-extensive seismic reflection horizons. The ' C' horizon, at the boundary of the Wallumbilla and Cadna-owie Forma- tions, is one of the most significant. A difference in the petrophysical properties of these two formations is evident from sonic, density, gamma ray and resistivity well log data, and indicates that the amplitude of the 'C' horizon reflection is related to a sequence of low- Introduction The Cooper and Eromanga Basins of central and eastern Australia (Fig. 1) have been Australia's most active areas for onshore petroleum exploration and development. The early discovery of Cooper Basin gas and oil fields in the 1960s and 1970s and subsequent discovery of hydrocarbons over a significant area of the Eromanga Basin by a number of exploration groups have contributed to the rapid development of the region . The geological character of the Eromanga Basin has con- tributed to a lag between hydrocarbon discovery in the Cooper Basin (1964, Gidgealpa gas) and Eromanga Basin (1978 , Strzelecki oil) (Sprigg, 1986). o I) 14/0/2 Figure l. Location map of the Cooper and Eromanga Basins. Initial perception of the Eromanga Basin's prospectivity focused on the relatively uniform character of the basin ' s stratigraphy and limited structuring, as evident from early seismic reflection data. The 1964 Coopers Creek seismic survey in the central part of the basin indicated that there were several major seismic reflection horizons in the Cooper Basin and overlying Eromanga Basin sequences (Smith, 1983). Three of the seismic reflection events were proved by later drilling to be major geological boundaries: a Permo-Carboniferous uncon- formity ('Z' horizon), a near top Permian event (,P' horizon) , • Petroleum Resource Assessment Program, Bureau of Mineral Resources, Geology & Geophysics, Canberra ACT 2601. density (undercompacted) shales in the basal part of the Wallumbilla Formation. The properties of the shales appear to be a consequence of rapid subsidence (' undercompaction' ) and burial. The empirical re- lationships between the ' C' horizon reflection amplitude, formation density and reflection coefficient are discussed, and geological im- plications for petroleum prospectiveness of the Eromanga Basin are outlined. and the ' C' horizon, which is a high amplitude reflection generated near the boundary of the Wallumbilla Formation! Bulldog Shale and Cadna-owie Formations (Fig. 2). The Wallumbilla-Bulldog is a shale-rich marine sequence and the Cadna-owie is a nearshore to marginal-marine sequence of sandstone, siltstone and shale. EROMANGA BASIN AG E WEST I EAST SEISMIC HORIZONS Late Winton Formation and superficial deposits Cretaceou s Mackunda Formation Allaru Mudstone I All aru Mudstone Toolebu c fm Oodnadatta Fm I Toolebuc Fm Earl y Wallumbilla Fm C oorikiana Sst I Wallumbilla fm Cretaceous Bulldog Shale I 'C ' Wvandra Sandstone Member MI. Anna Ss. Mbr I Cadna- owie Formation Murta Member Algebuckina Mooga Fm I Westbourne Fm Sandstone Mbr I Adori Sst Jurassic Birkhead Formation Poolowanna Hutton Sandstone Formation Windorah Formation ' p' Permo- Pedirka Ba sin Cooper Basin ·Z· Triass ic 14/0/3 Figure 2. Regional stratigraphic relationships in the Eromanga Basin. BMR regional seismic data were collected during a multi- disciplinary study of the eastern part of the basin between 1980 and 1983. Interpretation of the data, and of wireline log data from petroleum exploration wells , indicates that the large amplitude of the 'C' seismic horizon reflector is the product of a marked contrast in acoustic impedance between the basal part of the Wallumbilla Formation and the upper part of the Cadna- owie Formation. Although the 'C' horizon reflection has been the subject of general discussion (Smith, 1983; Moore & Pitt, 1984), the geological factors which appear to affect its charac- ter and amplitude have not been specifically discussed. The lower part of the Wallumbilla Formation is rich in organic matter and corresponds to a low interval velocity zone evident on sonic log data (Moore & Pitt, 1984). This zone of low interval velocity causes much of the acoustic impedance contrast between the formations. If the 'low velocity' shale was absent from the sequence, a much smaller amplitude 'C' horizon reflection would be evident. The purpose of this paper is therefore to outline factors affecting the amplitude and continuity of the 'C' horizon and the geological sequence from which it originates .