Integrated Seals Analysis of the Gidgealpa Oilfield, South Australia

James Underschultz¹, L.C. Johnson¹, P. Boult², Mark Lisk¹, and Anthony Gartrell^1
1 CSIRO Petroleum (ARRC), Perth, Australia
² Primary Industries and Resources, South Australia, Adelaide SA, Australia

The Gidgealpa Oil Field of the Eromanga Basin is characterised by stacked hydrocarbon accumulations with variable column heights. The Hutton Sandstone forms the main reservoir at Middle Jurassic level, with the overlying Birkhead Formation forming a secondary reservoir, waste zone, and top seal. A palaeo-oil column identified in the Hutton reservoir indicates that the Birkhead seal was breached, with the free water level rising more than 20m to its present position. While lineaments and CO2 migration related diagenetic zones exist across the structural closure, the principle control on column height is considered to be variable top seal capacity. The significant loss of oil prompts questions both of transient seal efficiency and the exploration potential of remigrated oil.

An integrated seals analysis involving, structural restoration, fluid inclusion, capillarity, and hydrodynamic methodologies was conducted on the field to further constrain: 1) the key seal risks/controls over the field's fill history; and, 2) the current distribution of hydrocarbon saturation in the reservoirs waste zone and top seal. The results show the Lower Birkhead waste zone (below the top seal) to have patchy oil saturation that is subject to reservoir compartmentalization and pressure depletion on production. There is evidence that above the seal, vertical oil leakage of the palaeo-oil column has created residual pospectivity in the Upper Birkhead sands. A significant volume of oil must have been lost through the Birkhead Formation into the overlying Namur aquifer system where some was trapped, but most remigrated out of the study area.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005