Occurrence of Cemented Reservoir Roof Rock And Geopressure Caprock and its Implication in Petroleum Geology and Geohydrology

Abstract

Abstract Diagenesis comprises processes of transformation through which, as a function of pressure, temperature and depth, compact rocks are formed from unconsolidated sediments. Interspace in such consolidating sediments generally is filled with aqueous solutions, by far exceeding in quantity local accumulations of oil and gas. These interstitial solutions, their composition, movement, and mixing are important factors in diagenetic processes. The latter alter sandstones and clay minerals, both equally important in petroleum geology and geohydrology. Of great significance is the occurrence of cemented layers which can readily be located and correlated by means of conventional well logs. Such cemented barriers occur as roof rocks, especially in structurally high positions in both oil or water bearing reservoir rocks, and as caprock sealing geopressured zones. Distribution of these cementing materials can be related to hydrodynamic conditions, planning of completions and the presence of abnormal pressure conditions. pressure conditions. General Discussion Compaction of argillaceous sediments causes movement of large amounts of aqueous solutions through the subsiding sedimentary column. While moving through sandstones and shales, the composition of these solutions may have changed many times considering the geologic time factor involved. Furthermore, any movement of formation waters across a sand/shale boundary will result in changes in the brine concentration in the sandstone. This is caused by the "salt filtering" effect of various clay minerals in respect to electrolytic solutions. As early as 1933, it was suggested by Schlumberger et al that clays and shales act as semipermeable membranes and in 1947 the salt filtering properties of clays were shown experimentally by Buneev et al. A recent and excellent review of the relative factors influencing membrane filtration effects in geologic environments has been given by Berry 1969.