The Geometry, Distribution, and Development of Clastic Injections in Slope Systems

Abstract

An integrated three-dimensional seismic, well, and core study indicates the development of a series of slope channel and fan-depositional systems in the Upper Cretaceous interval of the Mly slope, Norwegian margin. Because of their sand content and occurrence in a mud-dominated succession, the slope-depositional systems manifest as high-amplitude reflection packages on seismic reflection data. The Upper Cretaceous depositional systems are flanked and overlain by two types of amplitude anomalies that display unusual geometries in cross section and plan view. The first type of anomaly is bedding discordant and crosscuts overlying reflections, dips 10–20, is as much as 100 ms high, and is typically developed at the margins of the slope systems. The second type of amplitude anomaly is bedding concordant, as much as 400 m (1312 ft) long in cross section, and is developed either halfway up or at the upper tips of the bedding discordant anomalies. In three dimensions, the steeply dipping anomalies developed at the margins of the slope-depositional systems form winglike structures that are elongate along the lengths of the slope-depositional systems. Based on their close spatial relationship to the Upper Cretaceous slope-depositional systems and inferred sand content, the bedding-discordant and bedding-concordant amplitude anomalies are interpreted as clastic dikes and sills, respectively, sourced from the Upper Cretaceous slope systems. Although the mechanism that caused initial overpressuring of the sand bodies is unclear, it is speculated that a combination of the migration of basinal fluids into the sealed depositional sand bodies and rapid burial of the sand bodies in low-permeability mudstones may have contributed. The development of the largest clastic dikes at the margins of the depositional systems suggests that differential compaction and forced folding adjacent to the buried depositional systems triggered remobilization and injection and the subsequent geometry and distribution of clastic injection features. The postdepositional remobilization and injection of clastic slope systems as exemplified in this study have important implications for hydrocarbon exploration and production in slope systems because this process has caused marked changes in primary reservoir geometry and has resulted in the development of clastic intrusions that are large enough to represent stand-alone exploration targets.