The mature Hewett Field area in the Southern Gas Basin has recently been the subject of escalated development targeted at a carbonate reservoir, the Zechsteinkalk. This gas reservoir is located at some 4600 ft TVDSS and has both variable porosity and permeability. In order to address the fundamental requirement of defining porosity distribution within the Zechsteinkalk between wells, seismic modelling has been conducted.

A large suite of synthetic seismograms, based not only on actual, but also on altered logs has been used to simulate the acoustic response of various layer thicknesses, and porosities positioned differently within the carbonate section. This simulation is known as Incremental Pay Thickness modelling. The results, together with the ties between well and seismic data, show the models to be usable on a purpose shot seismic survey.

By producing a work-station derived set of Zechsteinkalk amplitude maps, and conducting local character analysis, a proposed drilling target was identified as a poor location. Three good locations were postulated, based on the modelling and amplitude mapping, and subsequently drilled. Well results show the method to be successful in predicting porosity within the Zechstein to within +/− 3pu.

The amplitude maps have also been used quantitively to provide porosity data for volumetric gas-in-place calculation. The gas-in-place results are compatible with volume predicted from reservoir engineering pressure data.

In view of the drilling success, contribution to reserves definition and quick turnaround to run Incremental Pay Thickness modelling, it is concluded that the method should be considered for North Sea porosity studies, where immediate and reliable results are required.

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