The Hydrodynamics of Fields in the Macedon, Pyrenees and Barrow Sands, Exmouth Sub-Basin: Identifying Seals and Compartments

Abstract

The Barrow Group strata (Macedon, Pyrenees and Barrow sands) of the Exmouth Sub-Basin host significant accumulations of gas and liquid hydrocarbons. There is currently production from the Macedon sands at Enfield and ongoing development drilling at Stybarrow. Active appraisal and exploration is underway, including the mutli-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 (FWL?s), and the likelihood of compartmentalisation within the discoveries. Whilst the region is faulted with a predominant southwest-northeast 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 intraformational seals have been breached. Thus, top and bottom seal capacity is likely not limiting pool size. 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 FWL?s to have a high confidence. Pressure data from the hydrocarbon phases indicate that in some cases, fault zones effectively compartmentalize a field into multiple pools. The Macedon Gas Field, on the eastern edge of the play fairway, marks a change in the trapping character with intraformational and fault seals having been breached resulting in a single continuous gas pool despite internal structural complexity.