Reducing the Structural Uncertainty in Poor 2-D Seismic Data, Gambier Embayment, Otway Basin, Australia — A Minimum Strain Approach

Peter Boult², Brett Freeman¹, Graham Yielding¹, and Dave Quinn^1
1Badley Geoscience Ltd, Hundleby, United Kingdom
²Petroleum & Geothermal Group, PIRSA, Adelaide, SA, Australia

In faulted reservoirs, structural uncertainty arises from two major sources: systematic error of the seismic method and human error of the interpreter.

When the distance between faults along a sample line is greater than the distance between lines, the pattern of faulting is usually clear. The error in lateral positioning of structures is then similar to the error in the vertical dimension, and both are dominantly systematic. This is typical of good 3D seismic data.

For 2D data, poor 3D data, and situations when faults are more closely-spaced than lines of the interpretation sub-grid, the pattern of faulting becomes much more interpretive. The effects of systematic errors become second-order relative to those inherent in the interpreter’s “model”.

The Otway Basin is a passive margin displaying multiple rift events and a complex history of faulting. The Gambier Embayment, within the basin, contains what we now recognise as two vintages of faults with a small angular offset of trends. The older, deep-seated, faults have displacement increasing with depth, and a younger set have their displacement maxima at a higher level in the overburden.

In the original structural model (work by a 3rd party) the distinction between these two sets was not made, and consequently prospect maps were largely erroneous. In this new work we quantify the relative uncertainty by assuming that the fault configuration should represent a minimum volume strain given the interpreted, local, offsets. By mapping the displacement patterns on the fault planes from the original structural model we show that the displacement gradients, locally, are unrealistically high. Using the regions of implied high displacement gradient as an indicator for higher uncertainty we were able to break out the faults into two self-consistent sets each of lower implied volume strain.

AAPG International Conference and Exhibition, Cape Town, South Africa 2008 © AAPG Search and Discovery