Thomas Flottmann¹, David S Warner², David J Campagna³, Richard R Hillis⁴

(1) Santos Ltd, Brisbane, Qld, Australia
(2) Santos Ltd, Adelaide, South Australia, Australia
(3) Fracture Quest LLC, Williamsburg, VA
(4) National Centre Petroleum Geology and Geophysics, Adelaide, South Australia, Australia

ABSTRACT: Horizontal microfracturing and intracontinental inversion, Cooper and Eromanga Basins, Australia

We present evidence for genetic links between regional basin-inversion, changes in stress regime and the formation of microfractures in the Cooper and Eromanga basins, Australia. The vertically stacked Cooper/Eromanga basins formed during the Carboniferous/Permian and Jurassic-Cretaceous and are Australia's main onshore oil and gas producers. Inversion during the Oligocene to Recent stripped up to several hundred metres of sediment cover off some parts of the basins. At microscopic-scale drillcore from the Cooper-Eromanga Basin shows conspicuous subhorizontal, mode 1 microfractures. These natural fractures contain clay mineralisation and occur exclusively in grain-supported sandstones suggesting that stress concentrations at grain boundaries contribute to fracture formation. In most cases horizontal fractures are closed and may not contribute to bulk permeability. The overall in-situ stress regime in the basins is transitional between strike-slip faulting (sH>sv>sh) and reverse faulting (sH>sh>sv). Minimum horizontal stress (sh) and vertical stress (sv) are of similar magnitude, but the maximum horizontal stress (sH) is significantly larger than sh and sv. The formation of horizontal fractures requires a reverse fault stress regime (sH>sh>>sv). Occurrence of horizontal microfractures coincides broadly with areas of greatest inversion. This suggests that the formation of horizontal microfractures is driven by a feedback mechanism between far field stresses, which cause localised intracontinental inversion leading to the stripping of overburden, which in turn reduces the magnitude of vertical stress. Inversion related overburden stripping may be sufficient to facilitate reverse fault stress conditions (sH>sh>>sv) thus prompting the formation of horizontal microfractures within inverted parts of the basin.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado