Applications of Clastic Diagenesis in Exploration and Production
Many of the major factors that control diagenesis, such as detrital composition, fluid composition, and fluid flux, can be related directly or indirectly to physical and biological processes operating at the time of deposition. Each depositional environment produces a lithofacies with a specific limited range of physical and compositional characteristics that affect diagenesis. The concept of diagenesis as a function of facies is well illustrated by the second Frontier sandstone of the Moxa Arch, Wyoming.
The lower Frontier Formation on the Moxa Arch comprises sandstones and mudstones deposited in a delta/strand plain system on the western edge of the interior Cretaceous seaway. Depositional environments represented by the rocks include: marine shelf with sand ridges, marine shoreline, fluvial channels, and fluvial flood plain.
Marine sandstones are significantly more quartzose than fluvial units because of sorting within the delta and wave abrasion on beaches. Substantial input of silica-rich water expelled from the underlying Aspen Shale caused nearly complete cementation of the cleaner beach and backshore sandstones by quartz overgrowths. Fluvial sandstones contain less quartz and more chert grains and rock fragments than the marine sandstones, and as a result, were less affected by quartz cementation. In addition, temporary filling of pores by calcite prevented further irreversibly destructive diagenesis. As a result, fluvial sandstones are better reservoirs, even though they are compositionally less mature. Clay-rich sandstones of the lower shoreface, lower sections of the sand ridges, and muddy fine-grained fluvial sandstones have poor present-day porosity and permeability primarily because of compaction.
Fluid flux also appears to have played an important role in determining present-day porosity and permeability profiles. Because of a very low sandstone/shale ratio, fluid channeling in fluvial sandstones on the southern end of the Moxa Arch seems to have caused extreme leaching. The sandstone section here, although it is very thin, is very permeable. To the north, a higher sandstone/shale ratio appears to have permitted a lower fluid throughput per unit volume of sand. As a result, fluid channeling was not as severe, detrital and authigenic clays are more common, and the sandstone section is more homogeneous and of lower overall permeability.
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Clastic diagenesis has evolved from a very descriptive science to a much more process-oriented study. This evolution has been driven by the realization that many hydrocarbon reservoirs have significant diagenetic compotents directly affecting porosity and permeability characteristics. The prediction in time and space of reservoir characteristics affected by diagenesis can greatly reduce the risk in the search for hydrocarbon accumulations, particularly in subtle targets lacking pronounced structural expression. This publication contains three sections designed to increase understanding in the processes controlling clastic diagenesis: Conepts and Principles; Aspects of Porosity Modification; and Applications of Clastic Diagenesis in Exploration and Production. The first two sections deal with processes controlling various aspects of clastic diagenesis, and the third section applies these principles and observations to specific examples. Altogether, the three sections contain 22 chapters.