The Seismic Response Of High Velocity Calcite Cemented Sands And Removal Of Their Effects On Depth Mapping Of The Gidgealpa Field.
Honours Degree, 1990
South Australian Institute of Technology
The presence of calcite cemented zones within the Namur Sandstone Member of the Mooga Formation of the Eromanga Basin causes velocity pull-up effects leading to inaccuracies in depth conversion. The amount of pull-up depends on the volume of the calcite cement. For example, wells within the Gidgealpa Field South Dome show pull-up effects ranging from 0.2 milliseconds to 11.2 milliseconds two-way time. When the relief on the structures being mapped is similar in size to the resulting depth conversion errors, it becomes critical to recognise and compensate for these velocity anomalies.
The fact that calcite cemented sandstone thickness changes rapidly and somewhat unpredictably over short distances makes it difficult to recognise and quantify its effects from well data alone. Hence, recognising the presence of calcite cement on seismic data and using it as a guide to interpolate or extrapolate the well data intelligently becomes essential.
This study has developed a method for recognising the presence of calcite cemented sandstone and quantifying and removing its effects to produce accurate depth maps. The Gidgealpa Fleld South Dome has been selected as the study area because it has a known calcite cement problem and dense well and seismic control. Also, two high density, good resolution seismic experimental lines have been shot over the study area and provides new data for interpretation.
The presence of calcite cement can be recognised by one or a number of the following:
- the tendency for it to occur over structurally high areas,
- seismic response,
- well log response.
The seismic response of calcite cemented sandstone beds of various thicknesses and bed spacings have been modelled using the Sierra l-D modelling software QUIKLOG. Results of the modelling exercise have been used to aid interpretation.
The effect of calcite cement can be determined by calculating its contribution to the time map, velocity map or both. The proposed procedure for quantifying its effect is the Delta T method which accommodates corrections to both the time map and the average velocity map. The corrected maps not only appear smoother but the depth map produced using the preferred method agrees very well with well data.
It is proposed that this procedure could be used to map other fields in the Cooper/Eromanga Basins which have high velocity layers causing travel time distortions on the underlying horizons. An important restriction is that an adequate degree of well control is required.