JABIRU FIELD — HORST, SUB- HORST OR INVERTED GRABEN?

With control from only the Jabiru 1A discovery well and a grid of seismic lines 3 km apart, original oil- in- place at Jabiru was estimated by the operator as 620 MMBBL (100 MkL). With a 3- D seismic grid and three more wells, the original oil- in- place estimate was reduced to 45 MMBBL (7 MkL). Subsequent drilling of three more wells has enabled this estimate to be increased to about 110 MMBBL (17 MkL). Production from the field is currently at a rate of about 43 000 BOPD (6800 kL/d), from the discovery well and the latter three wells. Seismic data across the Jabiru structure is poor and lacks character at the reservoir level, with the result that interpretation is partly dependent on the interpreter's preferred structural model. Initially, interpretation of a broad horst, up to 3 km across, seemed appropriate. Now most of the oil in the structure is thought to be in a narrow sub- structure, about 500 m wide, believed by some to be a sub- horst. The existence of a sub- horst is difficult to justify structurally, and a third possibility, in which the sub- horst was a terrace of a now- inverted graben, is proposed. The development of the graben and its subsequent inversion would most likely result from strike- slip and dip- slip motion along an underlying northeast- trending fault. The strike- slip is interpreted as right- lateral during the Late Jurassic, and left- lateral during Early Cretaceous and Late Miocene to Holocene. Such strike- slip has not previously been recognised in the Jabiru area. Consideration of structural models has been useful in predicting the potential of areas of very poor data at Jabiru. The concept of balanced sections is helpful in determining the model which best fits the data and is geometrically feasible.