AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
A Multidisciplinary Assessment of Generation-Migration-Charge-Leakage-Seepage Processes, Gippsland Basin, Australia
(1) Energy Geoscience Group, GeoScience Victoria, Melbourne, VIC, Australia.
(2) CSIRO Petroleum, Kensington, WA, Australia.
(3) Pangean Resources LLC, Denver, CO.
The key hydrocarbon seepage processes in the onshore and offshore Gippsland Basin, Australia have been determined using 3D geochemical modeling, charge history fluid inclusion (QGF and FIS) analyses, fault and top seal integrity assessments and leakage and seepage mapping (using 3D seismic chimney mapping, soil-gas, water column geochemistry, radiometrics and synthetic aperture radar data). The basin was compartmentalized by primary fill-spill chains which controlled the distribution of a primary and pervasive oil charge in the Miocene, followed by a gas charge which displaced the previous oil charge in the western parts of the basin. Modeling, charge history, seal capacity and leakage-seepage investigations of traps located along the primary fill-spill chain in the western part of the basin, both offshore and onshore, have revealed that this region is the key fluid flow and leakage and seepage conduit in the basin.
Giant gas fields such as Barracouta, and all more basin-ward fields, were initially filled to spill with oil and subsequently with gas and evidence of either present day or paleo-leakage is lacking. However, some 20 km to the west along trend, the Golden Beach gas field reveals a complex history of charge and leakage. This trap was initially filled to spill with an oil column which subsequent gas charge displaced. The top seal was able to support only 19 m of gas (consistent with capillary measurements). As such, this trap is present as a transition zone which is relatively high-integrity for oil but low-integrity for gas. Total loss of seal capacity 15-20 km further west results in the formation of a region of strong seepage onshore, where long-lived seepage has resulted in aerially extensive seeps and seep fields which have prominent associated radiometric signatures and soil-gas anomalies.
Almost the entire hydrocarbon seepage inventory of the prolific hydrocarbon system within the Gippsland Basin has leaked along the migration corridor between the Golden Beach field and the terminal edge of seal onshore. The location of seepage within the Gippsland Basin has been dynamic through time. During the Miocene, when oil charge dominated, seepage was essentially restricted to the very margins of the basin. However, the establishment of a dominant gas charge in the Pliocene has caused the seepage windows to move basinward and has resulted in the loss of substantial hydrocarbon inventory from previously filled-to-spill accumulations.