RECENT DISCOVERIES IN THE PYRENEES MEMBER, EXMOUTH SUB-BASIN: A NEW OIL PLAY FAIRWAY

Abstract

Recent drilling by BHP Billiton Pty Ltd in WA-155-P(1) and WA-12-R, on behalf of its partners Apache Energy Ltd and INPEX ALPHA LTD, has resulted in the discovery of four oil fields in the southern Exmouth Sub-basin, namely Ravensworth, Crosby, Stickle and Harrison. These discoveries, together with the earlier discoveries made by West Muiron–5 and Pyrenees–2, define the Early Cretaceous Pyrenees Member play fairway.

The Pyrenees Trend play was first conceived in 1999 following appraisal of the Macedon gas field (Keall, 1999), but the concept remained dormant until the integration of geological information with high quality 3D seismic data led to the recognition of hydrocarbon related seismic attributes in the postulated play fairway.

Ravensworth–1 intersected a 37 m gross oil column below a 7 m gas cap in high quality Pyrenees Member sandstones beneath the regionally significant Intra- Hauterivian Unconformity. Ravensworth, located on a northeast–southwest trending fault terrace, is a complex structural-stratigraphic trap that relies on separate top, base and cross-fault seals. High quality 3D seismic data coupled with recent interpretation techniques were integral to its discovery. In particular, the quantitative interpretation of seismic amplitude populations was a key factor in decreasing exploration risk.

The Ravensworth discovery was followed by successful exploration wells on the adjacent Crosby, Stickle and Harrison fault terraces. Four appraisal wells have since been drilled at the northern ends of the main discoveries.

The oil in the Pyrenees Member discoveries is biodegraded, moderately viscous (8–11 cp) and heavy (18–19° API gravity). Methane-dominated gas caps were intersected in Ravensworth–1, West Muiron–5 and Pyrenees–2.

The recent drilling and coring campaigns by BHP Billiton and others in the Exmouth Sub-basin have significantly advanced knowledge of the stratigraphy and depositional environments of the late Tithonian to early Berriasian Macedon, Muiron and Pyrenees Members of the lower Barrow Group. The lower Barrow Group is a third order sequence deposited rapidly in marine to fluviodeltaic environments in response to the breakup of Gondwana and the onset of active rifting along the West Australian margin.

BHP Billiton and its joint venture partners are assessing the commercial viability of the Pyrenees Trend discoveries.