Characterising the structural, petrophysical, and geochemical properties of inverted fault zones

Abstract

It is widely recognised that inverted fault zones form economically significant structures for subsurface fluid exploration and production. Inverted fault zones are formed by the contractional reactivation and reversal of pre-existing extensional fault zones. Recognising the reverse-reactivation of normal faults in sedimentary basins is fundamental, as the reconfiguration of fault geometries has implications for overall basin geometry, sediment accommodation and supply, and fluid flow pathways. This is particularly important for understanding the modification or creation of petroleum system elements through time, which in turn allows for increased targeted exploration. Notwithstanding the broad economic relevance of inverted fault zones, integrated multi-scale (from micrometre-scale to outcrop-scale) studies on the structural, petrophysical, and geochemical properties of inverted fault zones within porous reservoir rocks are limited. This thesis characterises the structural, petrophysical, and geochemical properties of inverted fault zones from two localities, the Otway Basin (Australia) and Bristol Channel Basin (United Kingdom), in order to understand how inverted faults influence fluid flow at a range of scales. To address this, this thesis has two main topics of focus: (1) identify the influence of inverted faults on surrounding lithology by assessing the relationship between faults, damage zones around faults, and fractures related to fault growth; and (2) identify how subsurface fluids flow, interact, and modify their surrounds by assessing the geochemistry of fluids in fractures and thereby constraining the source, evolution, and migration of fluids preserved in fractures. An integrated, multi-scale approach is crucial for improving the prediction of subsurface fluid flow beyond the wellbore. In order to understand the influence of inverted faults on surrounding lithology, an inverted fault (Castle Cove Fault) in the Otway Basin, southeast Australia, is the focus of the first two chapters of this thesis. The geometries and relative chronologies of natural fractures adjacent to the Castle Cove Fault are investigated. Structural mapping in the hanging wall damage zone reveals three sets of shear fractures that are geometrically related to the Castle Cove Fault. Inversion of the Castle Cove Fault has resulted in the development of an extensive network of fractures and complex fold structures, and inversion would have subsequently improved the outcrop-scale permeability structure of the damage zone for fluid migration. At the micrometre-scale, the permeability structure has also been influenced by fault inversion. Petrophysical and petrographical analyses in the hanging wall damage zone show that microstructural changes due to faulting have enhanced the micrometre-scale permeability structure of the Eumeralla Formation. These microstructural changes have been attributed to the formation of microfractures and destruction of original pore-lining chlorite morphology as a result of fault deformation. Consequently, inversion has subsequently improved the micrometre-scale permeability structure of the damage zone adjacent to the Castle Cove fault plane. Characterisation of the permeability structure adjacent to reverse-reactivated faults at a range of scales will aid with predicting fluid flow associated with inversion structures. Structural and geochemical analyses in the next two chapters of this thesis aim to understand how subsurface fluids flow and characterise the source, evolution, and migration pathways of fluids preserved in inverted fault zones. The geochemical evolution of fluids precipitated as calcite and siderite-cemented concretions and fractures throughout the eastern Otway Basin have been investigated. Pore fluids were sourced from both meteoric water and sea water during the deposition of the Eumeralla Formation and pore fluid evolution was strongly influenced by diagenetic reactions and increased temperature during burial. Using a similar analytical approach, the geochemical evolution of fluids precipitated as calcite and gypsum-cemented fractures throughout the eastern Bristol Channel Basin have vibeen investigated. The main source of fluids were connate pore waters, which were altered by diagenetic reactions within their host lithologies and subsequently redistributed through migration along faults and their associated damage zones. Knowledge of the source, evolution, and migration pathways of these fluids provides valuable insights for understanding the development of inverted sedimentary basins through time. Consequently, integrated studies on the multi-scaled permeability structure of inverted fault zones and the fluids preserved within them will ultimately improve fluid exploration and monitoring strategies in sedimentary basins.