The Dirkala South Oil Discovery: Focussing on Cost-Efficient Reservoir Delineation

Abstract

The Dirkala Field is located in the southern Murta Block of PEL's 5 and 6 in the southern Cooper and Eromanga Basins. Excellent oil produc­tion from a single reservoir sandstone in the Juras­sic Birkhead Formation in Dirkala-1 had indicated a potentially larger resource than could be mapped volumetrically. The hypothesis that the resource was stratigraphically trapped led to the need to define the fluvial sand reservoir seismically and thereby prepare for future development.A small (16 km2) 3D seismic survey was acquired over the area in December 1992. The project was designed not only to evaluate the limits of the Birkhead sand but also to evaluate the cost effi­ciency of recording such small 3D surveys in the basin.Interpretation of the data set integrated with seismic modelling and seismic attribute analysis delineated a thin Birkhead fluvial channel sand reservoir. Geological pay mapping matched volu­metric estimates from production performance data. Structural mapping showed Dirkala-1 to be opti­mally placed and that no further development drill­ing was justifiable.Seismic characteristics comparable with those of the Dirkala-1 Birkhead reservoir were noted in another area of the survey beyond field limits. This led to the proposal to drill an exploration well, Dirkala South-1, which discovered a new oil pool in the Birkhead Formation. A post-well audit of the pre-drill modelling confirmed that the seismic response could be used to determine the presence of the Birkhead channel sand reservoir.The acquisition of the Dirkala-3D seismic survey demonstrated the feasibility of conducting small 3D seismic surveys to identify subtle stratigraphically trapped Eromanga Basin accumulations at lower cost and risk than appraisal/development drilling based on 2D seismic data.

The Birkhead and Murta formations—organic geochemistry and organic petrography of Mesozoic fluvio-lacustrine source rocks in the Eromanga Basin, central Australia

“…Our findings accord with the previously assigned meandering, high-sinuosity, deltaic to fluvio-lacustrine conditions (e.g. Veevers 1984;McIntyre et al 1989;Lanzilli, 1999) that characterized the "Lake Birkhead" floodplain (Mackie and Gumley 1995;Boult et al 1997). However, the brackish paleo-depositional conditions inferred by Burger (1986) are not confirmed by our study.…”

“…During that particular period, the input of organic matter from higher land plants and fresh water algae increased and its preservation potential likely improved. While the resulting floodplain of "Lake Birkhead" (Mackie and Gumley 1995;Boult et al 1997) was fluvial, paleo-depositional conditions may have been temporarily brackish at the connection between the Eromanga Basin and the Carpentaria Basin (Burger 1986). The Birkhead Formation can be subdivided into three units, each measuring about 30 m in thickness (Paton 1986):…”