Natural resources are extracted from reservoir fields in sedimentary formations that evolved as a result to multiple processes occurring over geologic times. These may include basin formation and development, accumulation of sediments, compaction and cementation, and post depositional processes (e.g., digenesis, compaction, cementation, dewatering) that resulting in regional or local alterations of material properties. These processes may also include cycles of tectonic loading and deformation, which may result in fracturing, pore pressure buildup or release, and stress development. In addition, there may also be effects associated to changes in pressure, temperature and the flow of fluids, including clay and kerogen maturation, generation and flow of hydrocarbons (e.g., gas and oil), water displacement, and mineralization of fractures, pores, and open interfaces. This results in a complex sedimentary system composed of interbedded lithofacies, whose properties may vary with location and with orientation to bedding. Furthermore, if the digenetic overprint is strong, the distribution of material properties may be weakly related to the depositional system. Thus, understanding the geological history and structural architecture of the system may not be conducive to understanding the distribution of material properties within the basin.
Examples of hydrocarbon producing systems with strong digenetic overprint are tight gas shale reservoirs. Typically, tight gas shale reservoirs are massive mudstone accumulations, with relatively simple structure (e.g., flat and continuous beds), and composed by end-member argillaceous, siliceous, calcareous facies, and a number of transitional facies. Argillaceous facies are primarily depositional. Siliceous and calcareous result from digenetic alterations, and their presence and abundance is related to abundance of biologic sources for these minerals. The result is a system that may be initially considered as a simple layered reservoir (e.g., a layer cake system) but because of the strong digenetic overprint and its localized nature, it is soon recognized to be a strongly heterogeneous reservoir (e.g., raisins in the pudding system). In the analogy, the pudding is pervasive but of poor reservoir quality, the raisins are few and far in between, but of high reservoir quality. Thus, identifying and tracking the raisins from the pudding, becomes the principal goal of tight gas shale exploration.