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The Impact of Inheritance on the Architecture of Natural Fracture Networks – A Multi-Scale Study From Southeastern Australia

Abstract

The study of fracture networks in outcrop provides critical insights for modeling fluid migration pathways in hydrocarbon and geothermal reservoirs and understanding their structural development. While brittle deformation is controlled by far-field stresses, fracturing can be significantly influenced by local stress rotation due to pre-existing fabrics and discontinuities in the host rock or in older underlying rocks. In this project we explore the mechanical controls on fracture localization and geometries in a rift basin that has been subjected to multiple phases of extension and compression. The Gippsland Basin of southeastern Australia is an ideal location for investigating the control of pre-existing basement structures on subsequent deformation in the overlying cover, because outcrops of both the folded and faulted Paleozoic basement and the unconformably overlying Mesozoic sedimentary cover are abundant along the coast. We map fractures using geophysical (gravity and magnetic) data, near-shore bathymetric maps, and outcrops, all of which combine to provide near-continuous, high-resolution data coverage along approximately 100 km of coastline. Fieldwork is aided by unmanned aerial vehicle (UAV) photogrammetry, which is used to trace and analyze fractures in 2D (on orthorectified aerial images) and 3D (on virtual outcrops). By using a semi-automated image analysis approach, we were able to increase the number of fractures mapped and analyzed over a larger scale. The workflow overcomes issues often encountered in fracture studies, namely the challenge of characterizing a 3D fracture network using 1D well data, sampling bias across multiple scales, and the resolution limit of seismic data. Preliminary results show that: (i) there is geometric similarity, or parallelism, between fractures in cover rocks in some areas and the structural grain of the underlying basement, and (ii) north-south trending fracture zones deviate from the general east-west trend of the Gippsland Basin rift system. These observations suggest that despite there being roughly north-south directed regional extension and compression, local fracture geometries reflect either varying structural domains in the underlying basement or deviated stresses around older fractures. The study area thus provides outstanding structural analogs for the many hydrocarbon reservoirs affected by reactivation and inheritance.