Is There A Relationship Between Coal Thickness and Sandstone Reservoir Distribution In The Mid-Patchawarra Formation, Tirrawarra Field, Cooper Basin?
Honours Degree, 2000
University of South Australia
Petroleum exploration in the coal prone Patchawarra Formation of the Cooper Basin, central Australia, requires a thorough understanding of the relationships between sandstone reservoir geometry and stacking patterns, and coal thickness and distribution. As seismic surveys are the principal tool used in exploration, and because tick coals dominate the seismic record in the Patchawarra Formation, being able to demonstrate that a relationship exists between coal thickness and sandstone reservoir distribution could prove to be a powerful tool in exploration and development. As coal thickness is a function of peat compaction history, initially a valid peat to coal compaction factor needs to be determined. This may also aid checking the validity of stratigraphic correlations.
Based on 72 wells from the coal measure successions of the mid-Patchawarra Formation in the Tirrawarra Field, various sedimentary bulk decompaction factors were trialled. The results show that optimum decompaction values to apply were coal 10:1, carbonaceous shale 6:1, shale 2:1, siltstone 1:1, and sandstone 1:1. These decompaction factors allowed stratigraphic correlations to be done without radical variation in genetic interval isopachs. This approach could then be used to obtain a clearer view of sandstone stacking relationships above and below coals.
To test whether a relationship may exist between thick sand bodies and thick underlying coals, an exhaustive series of cross-plots between key coal markers and all other intervals in the succession was performed, but produced few statistically significant relationships. This involved plotting coal thickness vs genetic interval thickness, net sand thickness and san percentage, as well as other stratigraphic relationships, then applying regression analysis.
Of the statistically significant relationships, only comparing the VC40 coal thickness to the overlying VC30 siliciclastic interval net sand and sand percentage could be explained geologically by the compaction of the VC40 coal creating accommodation space in the VC30 interval. Other statistically significant relationships exist for pairs of intervals; however, they overlap the VU45 unconformity, making geological justification for the relationship difficult. These findings suggest that there are generally no statistically significant geological relationships between coal thickness and sand distribution in the Tirrawarra Field dataset.
The use of seismic amplitude data from 2D surveys over the Tirrawarra Field to obtain coal amplitudes, which could then be used to predict possible relationships with overlying sandstone thickness, was also shown to have severe limitations. Problems involving the reliability of the seismic amplitudes include; critical tuning thickness, masking by coal multiples, transmission losses, and significant average well deviation. The lack of velocity contrast between the sands and other siliciclastic rock types makes them effectively indistinguishable from each other on seismic sections, and therefore it is difficult to know that silts and shales are not being identified instead of sands.
Given that establishing a relationship between sandstone distribution and coal thicknesses appears to be unsuccessful, it is necessary to examine other possible geological rather than solely quantitative relationships. It can be demonstrated that the primary depositional environment overlying the coal is the key factor determining sandstone body thickness and distribution. In the Tirrawarra Field area, the predominance of crevasse splay sediments above the coals, characterized by narrow elongate but relatively thick splay distributary channels, and more extensive and thinner splay lobes, is thought to be the key control on sand distribution. This is supported by comparison with modern analogues from similar depositional settings in Western Siberia and studies of a Permian coal mine in Australia.