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Using Basement Exposures to Better Understand Influence of Deep Structure on the Structural Evolution of Intracontinental Basins with Unconventional Reservoir Potential

Abstract

Structural grain in the continental crust can strongly influence the surfaces along which seismicity occurs and faults initiate, and therefore the structural evolution within and underlying intracontinental basins. Continental interiors are likely to be anisotropic during horizontal tension because their tectonic history inherently includes convergence or transcurrence between crustal terranes. Transcurrence, and most angles of oblique convergence, produce a steep structural grain. Crust with steep planar fabric is strongly anisotropic for most horizontal tension directions (<40° or >60° from the fabric's strike). If the basement is exhumed deeper and more ductile crust, 1km to 10km scale coeval structural domains are likely, possibly creating domains of different anisotropy that influence structural compartmentalization in the basin. Timing and overprinting relationships between structural anisotropy domains is best determined using basement exposures. Studies of outcropping basement adjacent to potentially productive but underexplored basins therefore have potential to improve forward models of a basin's structural evolution. This in turn can assist explorers in defining the architecture, orientation and timing of structural domains in the basin – especially critical for unconventional reservoirs – with relatively low cost, prior to seismic acquisition and drilling. The southern Georgina Basin of central Australia, one of several remnants of a ∼2 x 10 km Neoproterozoic intraplate basin, is a natural laboratory for such studies. Neoproterozoic and Cambrian strata, correlated between the basins, contain under-explored hydrocarbon resources including unconventional reservoirs. The southern Georgina Basin and underlying Paleoproterozoic basement shared two major periods of deformation: Neoproterozoic NE-SW extension forming NW-SE grabens, and Devonian contraction. Prior work along the basin's southeastern margin indicates that, here, the adjacent exposed basement is in a similar orientation to that at the time of basin formation. New mapping and structural analyses in this area have documented penetrative steep fabrics, with km-scale structural domains partly distinguished by foliation trend (NW, N, or NE). The pattern of Devonian deformation in the basement and the cover sequences cannot be explained solely by the inversion of Neoproterozoic normal faults, suggesting that basement anisotropy influenced how shortening was accommodated.