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Rift Structure: Models and Observations |
Department of Petroleum Geology, University of Brunei, Gadong 3186, Brunei Darussalam
Three different aspects of rift geometry and their impact on hydrocarbon exploration are examined: rift architecture, fault angle and basin inversion. Rift architecture is controlled by fault geometry, which itself is heavily influenced by pre-existing fabrics. At the largest scale, rifts are divided into segments tens to hundreds of kilometres long. They can be joined along offset segments, separated by rift jumps and gaps, or terminated in splays. Rift jumps and gaps provide the entry points for large rivers to enter rifts and create along-axis drainage systems, while rift offsets are areas where anomalous structural patterns develop in response to oblique extension. Within rift segments transfer zones between boundary faults mark important changes in rift geometry and are the preferred sites of coarse clastic sedimentation. In the Gulf of Suez, for example, the structures and the syn-rift reservoir units within two major transfer zones are associated with over four billion barrels of reserves out of a total of six billion barrels. The way individual tilted fault blocks terminate include strike ramps, forced folds, rhomb-blocks, transfer faults, and major and minor cross-strike faults. In areas of relatively poor seismic data quality the choice of fault block termination interpreted on geophysical maps can make a considerable difference to structural interpretation. There has been considerable debate as to whether rifts are composed of high-angle (45–75°) faults or a mixture of high- and low-angle faults. Low-angle faults occur in the sedimentary sequence of rifts, where they occur in a variety of structural settings. Such faults can have a significant economic impact in field development. The presence of low-angled basement faults is more problematic. They do apparently occur in rifts, both due to reactivation of older faults and as completely new faults. Rifts can evolve through a variety of low- and high-angle fault structures with time, which impacts trap creation and destruction. Rifts commonly undergo changes in their stress regime which may lead to basin inversion features. This affects trap development and destruction and can lead to a variable subsidence history for basins, as the basin bounding faults change their senses of motion. The recognition of inversion features in many rifts indicates that this is a very important aspect of rift history.
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