Early compaction history of marine siliciclastic sediments.

Abstract

Differential compaction occurs within many sedimentary settings, such as alluvial and deltaic deposition, but it is within the submarine fan environment where the process is most effective due to the very high depositional porosities of the muds found there. Additionally the grain size of siliciclastic sediments within the submarine fan environment varies rapidly both horizontally and vertically, and hence the effect of differential compaction control on the depositional geometry and arrangement needs to be examined and modelled. It is also important to ascertain the rate at which sediments compact when buried, and whether compaction is complete at the end of deposition or whether it requires additional time to achieve this state. Sea- floor topography can be created if the latter case is true, and could influence subsequent deposition. Alternatively, if sea-floor topography is not created, the major control upon subsequent deposition may be the compatibility of the underlying section. Both controls will favour deposition of successive coarse clastic units above areas of fine-grained sediments, i.e. sand above shale rather than sand above sand. The Palaeocene sediments of the Central North Sea In the Montrose - Arbroath area (Blocks 22/17 and 22/18) combined with outcrop studies In southern California and New Mexico, have been used to assess the control of differential compaction on sediment distribution in a deep-sea fan setting. Differential compaction affects the Montrose - Arbroath area on a variety of scales. Firstly, differential compaction of the entire Palaeocene section across the underlying Forties - Montrose High induces structure. At a smaller scale, differential compaction may form a considerable control upon the spatial distribution of submarine fan channels and lobes that form the reservoir section throughout the area, and therefore the areal distribution of the oilfields themselves. Finally differential compaction may effect the distribution pattern of individual turbidites within such channel systems, thus forming a fine control upon the distribution of sands and shales within the reservoir. Fieldwork on submarine fan deposits in southern California has highlighted further complications to differential compaction that need to be addressed during the modelling process. Sedimentary processes such as basal loading and slumping are highly common in such deposits, and both can effect the compactional process to differing degrees. Results obtained from the modelling of stratal patterns observed in New Mexico provide information on the timing of differential compaction. It is suggested that compaction of sediments, even during early burial, requires a time interval often greater than the period of deposition, resulting in post-depositional compaction and the production of near-surface overpressure.