SEISMIC AMPLITUDE ANALYSIS: AN END TO FIELD APPRAISAL?

Abstract

The success of lateral prediction techniques based on seismic reflection amplitude analysis has had a significant impact upon recent appraisal and development planning strategies in the Coodwyn Gas Field, offshore north-western Australia.

The Coodwyn structure is one of a series of major tilted fault blocks on the Rankin Trend. The gently dipping reservoir sequence of Late Triassic to earliest Jurassic age is truncated by a major erosional unconformity and is overlain by sealing Cretaceous sediments. It is situated some SO kilometres west- south-west of the producing North Rankin Gas Field, to which it bears a striking resemblance in structural form and reservoir stratigraphy. Eight appraisal wells have been drilled in and around the field since its discovery in 1971. The most recent appraisal drilling campaign was designed to test a possible northern extension of the field within a stratigraphically younger reservoir sequence than that previously seen. The success of this campaign was such that the northern Coodwyn reservoirs are now being evaluated as possible candidates for development from a Coodwyn Platform to provide gas for the North West Shelf Project - one of the largest and most ambitious natural resource developments yet undertaken in Australia.

During the latest campaign it was confirmed that seismic reflection amplitudes at the Main Unconformity were directly related to the lithology and fluid content of the subcropping reservoir sequence. This has allowed the gas-bearing sands to be mapped across the field with far greater confidence than was previously possible, obviating the need for further appraisal drilling. In fact, Coodwyn -10, a well proposed to intersect the unappraised upper F sands, was not drilled because of the confidence placed in the amplitude map.

The amplitude map was used extensively during the 1986 drilling campaign, for refining the structural interpretation of the field, and during the recent Goodwyn Field development planning for the targeting of notional development wells from possible platform locations.