Remote Sensing of Subsurface Fractures: A South Australian Case Study

Abstract

South Australia's Penola Trough was used as a natural laboratory for the detection of naturally occurring fractures, following an integrated methodology which included identification and interpretation of fractures in wellbore image logs and core, and the remote detection of fractures in a 3D seismic volume. In this study, electrical resistivity image logs from 11 petroleum wells were interpreted for structural features, with 508 fractures and 523 stress indicators identified. Stress indicators demonstrate a mean maximum horizontal stress orientation of 127°N in the Penola Trough. Two fracture types were identified: 1) 268 electrically conductive (potentially open to fluid flow) fractures with mean NW-SE strikes, and; 2) 239 electrically resistive (closed to fluid flow) fractures with mean E-W strikes. Core recovered from Jacaranda Ridge-1 shows open fractures are more rare than image logs indicate, due to the presence of fracture-filling siderite. Siderite is an iron-rich, electrically conductive cement that may cause fractures to appear hydraulically conductive in resistivity-based image logs. Fracture susceptibility plots created using the defined stress orientation, and previously derived magnitudes, illustrate that the majority of fractures detected are favourably oriented for reactivation under in-situ stresses. However, it is demonstrated that fracture fills exert a primary control over which fractures are open to fluid flow in the sub-surface. As natural fractures generally lie below the resolution of seismic amplitude data, seismic attributes were calculated from the 3D Balnaves/Haselgrove survey and mapped to the target Pretty Hill Formation to enhance observations of structural fabrics. Linear discontinuities likely to represent faults and fractures were identified with orientations consistent with natural fracture orientations identified in image logs, striking E-W and NW-SE. However, these are mostly limited in extent to zones around larger faults and so likely represent damage zones. Additionally, it is unlikely that a large proportion of these fractures are open to fluid flow, given observations from core and image logs. This limits possible fracture connectivity and, therefore, the possibility of significant secondary permeability in the Penola Trough. This integrated methodology provides an effective workflow for the remote detection of natural fractures, and for determining whether or not those fractures are hydraulically conductive.

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