ABSTRACT: Time and Temperature of Hydrocarbon Migration and Fracture Development in the Palm Valley Gas Field, Central Australia

HAMILTON, JOE, P. EADINGTON, M. E. LISK, CSIRO Division of Exploration Geoscience, Sydney, New South Wales, Australia, and N. MILNE, Magellan Petroleum Australia, Brisbane, Australia

The Palm Valley gas reservoir comprises fractured sediments of the Ordovician Larapinta Ciroup in the Armadeus basin, central Australia.

Gas drainage efficiency is largely controlled by an extensive fracture network in the otherwise low permeability reservoir rocks of the Pacoota Sandstone, Horn Valley Siltstone, and lower Stairway Sandstone. As part of the present studies to investigate the timing of the fracture network, a stable isotope and fluid inclusion study of diagenetic cements and mineralized fractures from Palm Valley 7 core was undertaken.

Fluids in fractures were derived from at least two sources. Basinal brines that had prior contact with Precambrian evaporites (Bitter Springs Formation) comprise one fluid source while low salinity waters of meteoric origin and derived from host sandstone porosity comprise another. These fluids resulted in ankerite, barite and quartz formation in fractures and in host sandstones. Temperatures of 90-115 degrees C prevailed at this time which coincides with maximum burial depth, crudely estimated to have occurred at about 340-260 Ma.

Hydrocarbon migration was in part synchronous with fracture development. Earlier fractures provided a migration pathway for liquid hydrocarbons and wet gas of low maturity. Subsequently, wet gas of low to relatively high maturity migrated through ester fractures. Temperatures continued to rise and dry gas was generated which displaced the wet gas now only observed in fluid inclusions.

The timing of the continued temperature rise and the subsequent decline in temperature together with the inferred subsequent erosion of about 3 km of sediment overburden is coincident with the Alice Springs orogeny of Late Carboniferous to Early Permian age.

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