The hydrodynamics of fields in the Macedon, Pyrenees, and Barrow Sands, Exmouth Sub-basin, Northwest Shelf Australia: identifying seals and compartments ^*

Abstract

The Barrow Group strata (Macedon Member, Pyrenees Member, and Barrow Group sandstones) of the Exmouth Sub-basin host significant accumulations of gas and liquid hydrocarbons. There is currently oil production from the Macedon sandstone at the Enfield Field and ongoing development drilling at the Stybarrow Field. Active appraisal and exploration is underway, including the multi-field Pyrenees Development. In the course of assessing these discoveries, BHP Billiton and its joint-venture partners have undertaken a hydrodynamic study in order to better understand the sealing mechanisms, the position of free-water levels (FWLs), and the likelihood of compartmentalisation within the discoveries.

Whilst the region is faulted with a predominant south-west-north-east grain, the potentiometric gradient is surprisingly flat indicating that the individual sands are hydraulically well connected. Other than the Macedon Gas Field, there is no pressure data that indicate intra-formational seals have been breached. Thus, top and bottom seal capacity is probably not limiting the pool sizes. Rather, structural spill points and fault seal capacity appear the significant factors in determining pool geometry, with the underlying aquifer being regionally connected around fault tips.

On the field-scale, the flat hydraulic gradient allows for the calculated FWLs to have a high confidence. Pressure data from the hydrocarbon phases indicate that in some cases, fault zones may compartmentalise a field into multiple pools. These areas are then targeted for additional focused geological analysis to reduce uncertainty in field compartmentalisation. The Macedon Gas Field, on the eastern edge of the play fairway, marks a change in the trapping character with intra-formational and fault seals having been breached resulting in a single continuous gas pool despite internal structural complexity. Stybarrow and Laverda-Skiddaw clearly occur as separate accumulations and the Stybarrow data define a single oil column in contrast to the potentially compartmentalized Laverda-Skiddaw field. Stybarrow represents an anomalously large oil column relative to other fields in the area and it is located on the low hydraulic head side of a sealing fault.