NATURAL FRACTURES OF THE SOUTHERN COOPER AND EASTERN WARBURTON BASINS, SOUTH AUSTRALIA

Abstract

A study of the southern Cooper and eastern Warburton Basins has resulted in classifying natural fractures and delineating regional fracture systems and orientations of open and partially open fractures. Firstly, natural fractures have been identified from selected lithological units in 44 wells of the southern Cooper Basin. Open fractures are developed mainly within the brittle Tirrawarra Sandstone. These open fractures are mostly high angle to subvertical, with measured apertures of up to 2 mm. Secondly, natural fractures have been characterised in Warburton Basin cores from 91 wells, 27 of which have dipmeter and/or FMS (Formation Micro- Scanner) logs. Fractures more commonly occur within brittle sandstone, dolomite, grainy limestone and ignimbrite.

Two systems of orthogonal, high-angle, regional fracture sets (four fractures) have been identified. They extend across local structures in both basins in South Australia. System I is comprised of a pair of orthogonal fractures, striking NNE–SSW (20–200°) and ESE–WNW (110–290°), while a second pair of orthogonal fracture sets, striking NE–SW (60–240°) and NW–SE (150–330°), characterises System II. Open, steeply dipping SW fractures striking WNW and NW within Systems I and II have been interpreted from core and FMS data in several wells. The results indicate that an optimum well trajectory designed to maximise intersection with open natural fractures should be 200–210° and 240–250°, and possibly also 270–290°. The deviation angle should be 30° from horizontal in the fracture zone due to the high-angle and subvertical fracture dips. A semi-quantitative estimate of fracture density from cores in both basins has been determined: the greatest fracture density is located within major fault zones and structural culminations.