ABSTRACT: Petrophysics of Gas Reservoirs, Otway Basin, South Australia

ALEXANDER, ELINOR M., South Australian Department of Mines and Energy, Adelaide, S.A., Australia

In 1987 the first deep commercial gas was discovered in the South Australian portion of the Otway basin at Katnook 1. Production commenced in early 1991 after sufficient reserves were proved in Katnook and nearby Ladbroke Grove fields to supply 22.5 petajoules (600 million cu m) of sales gas over 15 years. Gas is produced from the distal, low-sinuosity fluvial Pretty Hill Sandstone (Lower Cretaceous Otway Group), deposited under seasonally frigid conditions (paleolatitude of approximately 70 degrees S).

Pretty Hill reservoirs are mineralogically complex and consist of fine- to medium-grained quartz-feldspar-lithic (volcaniclastic) sandstone with patchy carbonate, and laumontite cement and disperse chlorite and kaolinite. Reservoir depths range from 2548 m (Ladbroke Grove 1) to 2859 m (Katnook 3) and reservoir quality depends strongly on the timing of growth of authigenic minerals and the rigidity of matrix and cement under burial conditions.

Special core analysis (SCAL) was done on a suite of 350 core plugs, with particular attention to multisalinity conductivity measurements and mercury porosimetry. SCAL results were integrated with comprehensive wireline log suites for each of the five wells studied.

Preliminary results show: porosity is secondary; permeability, under overburden pressure conditions, is correlated with grain size; rapid and sever fluctuations in porosity and permeability are caused by diagenetic effects; elevated clay cation exchange capacity is due to isopachous chlorite and possibly also to laumontite (a zeolite); mercury injection proceeds stepwise, successively occupying two to three different pore-throat geometries.

The latest interpretation and relationship to wireline logs are presented and discussed.

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