ABSTRACT

Siltstones, mudstones and shales have been studied mainly with regard to clay mineralogy and general transport-deposition. Recent studies on deepwater deposits from cores and outcrops have shown that fluid flow properties of deepwater reservoirs are greatly affected by the presence of finer-grained deposits in the reservoir. Initial analysis indicates that the majority of these finer-grained deposits have a large silt component and are closer to siltstones rather than mudstones, commonly called shales. Studies on these deposits have indicated that they are often comprised of graded fine silt laminae sandwiched between films of clay minerals, quartz dust and organics. To date, little attention has been given to their characteristics resulting from different depositional processes. Internal characteristics, based on the different types of transport and the resulting structures after compaction and diagenesis, are poorly understood. Depending on the transport-deposition process, the architecture of the deposit will have different 3D extents and continuity. Stratigraphic prediction of the position and dimensions of the fine-grained beds can indicate whether the deposits will be a barrier or a baffle to fluid flow or a possible reservoir for natural gas. Minor variations in depositional style and other characteristics can result in more differences than presently assumed.