The need for an improved model of the Tirrawarra Oil Field reservoir led to a characterisation project jointly undertaken by SANTOS Ltd and the Bureau of Economic Geology. The objectives of the project were to describe and characterise the geological complexity of the field and within this framework to quantify the residency of the original and remaining oil resource.A detailed facies based flow-unit model was developed, within which petrophysical parameters and pay distribution have been mapped. Four principal facies were identified from which six flow units were characterised in detail. The environments of deposition were shown to control the detailed fabric of the rock, which in turn controls reservoir properties and productivity. The reservoir is interpreted to have been deposited as a braid delta, which has been reworked by a lacustrine shoreface transgressing and regressing across the area in response to rapid and frequent changes in lake level. Consequently, the two main flow units are relatively uniform in character, with much of the production-reducing shale being removed. The new reservoir model provides a greatly improved explanation of field performance, particularly that which was previously considered anomalous. This attests to the reliability of the model for use in reservoir engineering studies and the identification of opportunities for incremental development of the large oil resource remaining in the field.Under-developed portions of the field are now being assessed for miscible flood expansion with the aim of future growth in both oil production and reserves.

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractThis paper describes an unusual well design that has been implemented in a mature onshore oil field to minimise the cost of development drilling, improve well performance and utilise existing infrastructure.