3-D modelling of a reef-fault block structure

Darran J. Edwards

The study presented here revolves around a 3-D zero-offset survey shot over a reef-fault block model using the physical modelling facilities at The University of Calgary. Dimensions and velocities for the model are taken from Keg River patch reefs or bioherms flanking the eastern Peace River Arch, northwestern Alberta. These reefs developed typically on or above prominent basement structures, which are thought to be fault-controlled in this area. 2-D seismic data from the Panny Field are available for interpretation, and a schematic 'reef-fault block (horst)' geological cross-section is presented here. Direct identification of such faulting on 2-D data is shown to be difficult. Erosion of such uplifted fault blocks produced basal clastic (Granite Wash) fault-scarp deposits in adjacent margins. The main objective of the Panny model is to image such a clastic layer beneath the reef. In real data, identification of any regular trends of these deposits and correlation with overlying bioherms would substantiate any theory on the tectonic control of such build-ups.

Such a scenario is used as the geological template for the physical model construction. Physical modelling is described in terms of 3-D zero-offset acquisition, processing and interpretation. A number of vertical and horizontal sections are displayed. Timeslice investigation shows that the areal extent of the basal clastic fault block is particularly well imaged, even beneath a large part of the reef. 3-D numerical modelling of a similar structure has also successfully delineated the regular margins of this layer, thereby identifying fault positions.

Another aim of the model is to uncover interpretation pitfalls with 2-D data. Through numerical modelling it can be shown that certain anomalies observed on physical-model seismic sections are sideswipe reflections. Therefore, this study shows that, in dealing with 2-D data reefs may be inferred along lines shot away from the actual reef. This may frequently result in false imaging on 2-D data.