Petrophysical Evaluation of Lacustrine Shales in the Cooper Basin, Australia

Vallee, Melissa

While the petrophysical evaluation of conventional reservoirs has become routine, the assessment of gas shales in the Cooper Basin has required the use of the latest logging technology and core analysis techniques. Estimation of the gas content contained within the connected porosity of these mudrocks and formulation of an effective production strategy requires the integration of data from multiple disciplines including organic geochemistry, tight rock analysis, organic petrology and geomechanics. Although the Cooper Basin has been explored and exploited for more than 40 years, the current logging suite for conventional sandstones still only comprises a handful of logs (gamma ray, resistivity, neutron-density, sonic), sometimes formation tests and rotary side wall cores and usually involves one or two service companies. By way of comparison, during the drilling of six wells used to assess the unconventional gas potential of the Nappamerri Trough in the Cooper Basin in the East-Central Australia, a diverse suite of specialised petrophyisical imaging and mineralogy logs was acquired; three hundred feet of core have been cut, and eight service companies engaged to undertake core analyses, including retort analyses for fluid saturation, geomechanical tests, crushed bulk density, pressure decay permeability, X-Ray Diffraction (XRD), organic petrology, RockEval, and high resolution computer tomography CT scans in order to determine the organic content, mineral content, porosity, permeability, amount of adsorbed gas and geomechanical properties of the Murteree Shale. These lacustrine mudrocks were characterized by a simple mineralogy and homogeneous lithofacies, demonstrating that diagenesis has had a strong impact on the mineralogy of the clays and the development of siderite. Free gas storage capacity was associated with inorganic porosity developed within intercrystalline pore spaces of diagenetically altered clays rather than organic derived porosity. The shale gas potential of the basin is related to the dynamics of the basin-centered gas system developed within the Nappamerri Trough, kerogen type, thermal maturity and a spectrum of inorganic and organic derived poro-perm networks that are both variable and complex. The purpose of this paper is to illustrate how the Santos petrophysics team has adopted and adapted the different techniques and technologies in the market to evaluate a challenging and still not well understood lacustrine shale play such as found in the Cooper Basin.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013