Constraints on the Evolution of Structural Compartments With Unconventional Reservoir Potential in the Southeastern Georgina Basin Area, Australia, From Adjacent Basement Exposures

Abstract

The Georgina Basin of central Australia contains an emerging Paleozoic unconventional hydrocarbon play. The basin initiated during Cryogenian intracontinental rifting, was incorporated into the early Cambrian ~2 × 106 km² intraplate Centralian Superbasin, and experienced contraction during the intracontinental Paleozoic Alice Springs Orogeny. Early stage exploration indicates that middle Ediacaran and Cambrian strata (Thorntonia Limestone, Arthur Creek Formation) in the SE Georgina Basin have the greatest source rock potential (Munson, 2014). The best play fairways are defined by fault-bounded domains but preliminary analyses suggest some, though not all, of these domains may be overmature. Existing structural interpretations are limited, as they do not incorporate constraints from adjacent kinematically equivalent basement exposures and are markedly two-dimensional (i.e., no assessment of oblique motion). Geological mapping in the basement provides constraints that would improve understanding of the basin's structural evolution, allowing better prediction of differential burial and sealing potential between play compartments, and reducing exploration risk. Major NW-SE structural lineaments in the basin continue along strike as faults exposed in the basement, previously interpreted as normal faults formed during Neoproterozoic NE-SW extension and inverted by Paleozoic NE-SW contraction. These faults are 10-m scale thick zones of quartz ± hematite breccia, subhorizontally overlain (where not tilted by Paleozoic thrusts) by lower Georgina strata lacking brecciation and veining. Microstructures and clast populations indicate cyclic cataclasis and hydraulic fracturing during overall oblique dilational strain. Rather than calling upon a new tectonic event, we tentatively interpret that fluid flux accompanied syn-rift Neoproterozoic faulting. These breccia zones dextrally offset NE-SW trending, SE-vergent thrust faults in the basin's margin, where the geometry of drag folds indicates dip-slip motion. These observations imply that (1) Neoproterozoic rifting may have been more NNE-SSW-directed than previously thought; (2) the early fault zones were significant fluid pathways; (3) an earlier stage of Paleozoic contraction formed SE-vergent thrusts (possibly reactivating the normal faults sinistrally); and (4) later Paleozoic NE-SW contraction reactivated the breccia zones as dextral-oblique faults expressed as open folds within the basin.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015