ABSTRACT: Coorong Lacustrine Carbonates as Holocene Analogues of Ancient Evaporite-Associated Hydrocarbon Source Rocks

McKIRDY, DAVID M., DIANNE PADLEY, and FIONA MACDONALD, University of Adelaide, South Australia, Australia, JOHN K. WARREN, National Centre for Petroleum Geology and Geophysics, Adelaide, South Australia, Australia, ANTHONY J. HAYBALL, South Australian Department of Mines and Energy, Eastwood, South Australia, Australia, and CHRISTOPHER C. VON DER BORCH, Flinders University, Bedford Park, South Australia, Australia

Scattered along the first interdunal corridor landward of the Coorong Lagoon, within the coastal Pleistocene beach-dune ridge system of southeastern South Australia, is a chain of small, ephemeral, evaporitic lakes. Total organic carbon contents of the Holocene lacustrine carbonates range from less than 0.5% to as high as 12% in one indistinctly laminated mudstone unit with a mineralogy dominated by low-Mg calcite and aragonite. This organic-rich unit, in places up to 1 m thick, is present in at least six lakes south of Salt Creek. It was deposited under restricted anoxic bottom conditions, a result of density stratification which developed when the lakes were first cut off from the main Coorong Lagoon. The dispersed organic matter comprises degraded filamentous cyanobacteria, remains of the lipid-rich, green alga Botryococcus braunii, and cuticle from the grass-like metaphyte Ruppia sp.. Rock-Eval pyrolysis of core samples from Pellet Lake, North Stromatolite Lake and South Stromatolite Lake confirms the oil-prone character of its type I-II protokerogen (hydrogen index = 420-830). Yields of extractable geolipids are in the range 10,000-15,000 ppm (dry sediment weight). Aliphatic hydrocarbon signatures are dominated by highly branched isoprenoids (HBI: 2,6,1 0-trimethyl-7-(3-methylbutyl)dodecane and one of several isomeric C25H50 alkenes); C21-C33 n-alkanes with a marked odd/even predominance (maximum at C27); C27-C31 hopanoid triterpenes; and D-2 sterenes (C27 >C29 >C28). High concentrations of C20-C22 and C25 HBI hydrocarbons have previously been observed in h persaline Holocene sediments from Abu Dhabi and Shark Bay, Western Australia; and in the Rozel Point asphaltic crude oil of Miocene evaporitic

lacustrine source affinity from Utah. These unusual compounds appear to be of bacterial origin.

Organically leaner carbonates deposited during the shallowing perennial and ephemeral phases of the lakes' history define an inverse relationship between hydrogen index and organic carbon content. This feature is attributed to differences in the extent of bioturbation (pelletisation) of the carbonate muds by brine shrimp, gastropods and ostracods, that in turn reflect the fluctuating oxicity of the sediment-water interface.

Coorong-type lacustrine sequences are susceptible to flushing by oxic groundwater. Nevertheless, ancient oil-prone source beds deposited in similar shallow-water evaporitic environments do occur in the McArthur basin (middle Proterozoic), Australia and the Kwanza basin (Cretaceous), Angola.

AAPG Search and Discovery Article #91015©1992 AAPG International Conference, Sydney, N.S.W., Australia, August 2-5, 1992 (2009)