Murta Formation Seal Analysis: Seal Capacity and Geometry of Subunits M5 and M1, Southern Cooper/Eromanga Basin, South Australia Bianchi-Rossi, Anthony
Honours Degree, 2003
University of Adelaide
The Murta Formation in the Southern Eromanga Basin was deposited in a pro-delta lacustrine environment during the Cretaceous. Mercury Injection Capillary Pressure (MICP) data indicates that the basal subunit M5 within the Murta Formation over the Milpera Horst can support an oil column of over 580 metres (1900 feet), before threshold pressure is exceeded and the seal fails. Scanning electron microscopy data suggest that this subunit has only minor variations in sand composition across the study area, which would not substantially affect seal quality. No secondary porosity and little intra-granular porosity were visible. X Ray Diffraction analysis indicates that the dominant mineralogies are quartz, detrital kaolin, illite and siderite cement. Despite the high seal capacities and favourable mineralogies there is oil found above the M5 in subunit M2. Micro-faulting, which juxtaposes shales against interbedded storm beds which characterise subunit M5, is the mechanism to explain the oil found above in subunit M2. Oil probably travelled along the erosive bases of subunit M4, interpreted to be formed by a fluvial incision, and the interbedded storm beds are interpreted to be likely leak zones (within subunit M5).