FORMATTION OF DETRITAL CLAY GRAIN COATS BY DEWATERING OF DEEP-WATER SANDS AND SIGNIFICANCE FOR RESERVOIR QUALITY

摘要

Well-developed detrital clay grain coats are observed in deep-marine sandstones of the Upper Cretaceous Springar Formation of the Voring Basin in the Norwegian Sea. The detrital clay coats form thin and compact rims on individual sand grains and meniscus-shaped bridges between grains. These well-developed coats are found in high density turbidites and proximal hybrid event beds with common to pervasive dewatering structures deposited in proximity to the base of a syndepositionally active basin high. Here, in one exploration well, detrital clay grain coats are common throughout a sandstone package 100 m thick. High-density turbidites and proximal and distal hybrid event beds drilled in mid-to distal-fan settings unaffected by seismically resolved seafloor topography show common dewatering features, but have only scattered detrital clay coats confined to individual dewatering pipes or dish structures. Hence, we propose that intense sediment dewatering has the potential to form detrital clay coats in deep marine sandstones by a combination of elutriation and reorganization of clays during fluid escape from sediment bodies with pore fluid pressures significantly higher than the hydrostatic pressure. In submarine fan systems, deposition of sediment with coeval trapping of large volumes of interstitial pore fluid is most likely to occur where gravity flows undergo rapid deceleration in response to an abrupt decrease in confinement or gradient. Such environments include the channel lobe transition and settings in proximity to seabed topography.