EUSTASY: AN IMPORTANT TOOL IN THE EXPLORATION PROCESS
This special issue of Exploration Insights Magazine celebrates the release of a new version of our sequence stratigraphic model. Long-term subscribers will know that sequence stratigraphy, and, in particular, our proprietary eustatic model, forms the foundation of our approach to integrated regional/global geoscience and the set of products that we have produced to assist in basin screening and play fairway analysis.
Sequence stratigraphy is established as a primary interpretation methodology for petroleum geologists. Because this technique is based upon an understanding of the temporal and spatial relationship of sediments, we can recognize if, for example, they are physically connected. This is useful for estimating reservoir volume, and it is more effective than simple, potentially misleading, lithostratigraphic correlations. It is also an effective means of combining datasets, such as biostratigraphy, sedimentology, logs and seismic, within one integrated framework, and is a powerful means of predicting away from data. The geometries of sequence stratigraphy predict what facies may be expected up, down and laterally in the depositional system, so that the occurrence of reservoirs, source rocks and seals may be predicted regionally from relatively sparse datasets.
All these benefits are valid, regardless of whether a eustatic model underpins one’s view of sequence stratigraphy or not. However, if a eustatic model can be applied, then other powerful advantages accrue, including the development of a framework for detailed and precise correlation, mapping and isopaching, and the use of analogues and generic play concepts. Moreover, particular petroleum systems elements appear to be related to specific episodes of eustatic change, especially where the rate and amplitude of sea-level change is high. This means that certain stratigraphic intervals are especially prone to being a hydrocarbon habitat, adding a temporal dimension to the prediction of hydrocarbon occurrence.
Models of eustasy can never stand still, as new data emerge to refine them. In this issue, David Ray and Isobel Sides report on the work we have undertaken over the last few years to refine our eustatic model, in particular, the critical utilisation of a global stratigraphic database. This enhances our approach and provides a foundation to investigate the rate and amplitude of eustatic change. As discussed by Frans van Buchem, herein, these are important factors for predicting the stratigraphic distribution of play elements. Thomas Jewell and Roger Davies discuss examples from the Arabian Plate, where the predictive powers of sequence stratigraphy and an eustatic model lead to new stratigraphic play concepts. Over 80 years ago, the great geologist, Amadeus Grabau, was using a global database to develop his “pulsation theory” of Earth History (i.e. eustasy). We believe he would be delighted with the way in which eustatic models are now determined and employed in the hydrocarbon industry.
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