3-D Mapping and Correlation of Intraformational Seals Within the Latrobe Group in the Nearshore Gippsland Basin

Abstract

During its assessment of the nearshore Gippsland Basin (within 25 km of the coastline), the CarbonNet project has identified interbedded coals and shales of the Middle Eocene Lower N. asperus Zone as the key seal for upper Halibut reservoirs. This interval corresponds to the (Bartonian) T2 basal zone of the coal seams within the onshore coal-bearing Traralgon Formation which is widespread within the nearshore region of the Gippsland Basin. Previous hydrocarbon exploration shows that the T2 sequence is the intraformational topseal to several intra-Latrobe oil accumulations in the nearshore area, and that distinct pressure and salinity differences exist across this aquitard. Hence, the T2 represents a sub-regional seal, and is shown to be one of a set of backstepping subregional seals throughout the Bass Strait petroleum province. This family of seals offers additional trapping opportunities for future oil exploration in the offshore Gippsland Basin. A detailed correlation between nearshore and onshore wells has been carried out using existing well and 3D seismic data to define 3D geometry and continuity of the T2 units. The onshore Traralgon Formation ties to the offshore Burong Formation in the Barracouta gas field and small oilfields in the nearshore western Gippsland Basin. Seismic attribute extracts are presented as maps of coal quality and facies demonstrating the aspect ratio and lateral extent of coal depocentres, as well as details of the fluvial inputs, channel geometries, and clastic depocentres. Seal capacity of these intraformational seals is defined as a minimum by the observed hydrocarbon columns but also by MICP data which suggests seal potential well in excess of the proven columns. The critical constraints on trap capacity appear to be fault-related, and depend on the time scale. For petroleum, where multi-million year trapping is required in order for oil to be still present today, very efficient trapping is required with essentially no fault transmissibility. For CO₂ storage over many thousands of years, slow seepage through faults and offset baffles may be acceptable, especially where it leads to additional dissolution into the active aquifer which is sweeping fluids from onshore to offshore Correlation of the T2 sequence and the definition of fairways where there is suitable seal potential is crucial to assess CO₂ storage potential and intraformational hydrocarbon trapping over the next 50 years of Gippsland Basin activity.

AAPG Datapages/Search and Discovery Article #90217 © 2015 International Conference & Exhibition, Melbourne, Australia, September 13-16, 2015