SEDIMENTARY ROCKS/Sandstones, Diagenesis and Porosity Evolution 145 Figure Diagenetic sequence deduced from thin section and scanning electron microscopy (SEM) analysis, Brent Group, North Sea Adapted from Eglington G, Curtis CD, McKenzie DP, and Murchison DG (1985) Geochemistry of buried sediments Philosophical Transactions of the Royal Society of London, Series A: Mathematical and Physical Sciences 315 shown in Figure 7, although here the duration of events has been constrained using additional data from geochemical, isotopic, and petrographical analyses There are many descriptive terms used to refine the qualitative description of diagenesis Thus, it is possible to read of ‘early carbonate’, ‘mesogenic quartz’, ‘burial cements’, ‘late ferroan dolomite’, and so on It is all too easy to be confused by the plethora of terms, particularly when some are contractions of long, although better, descriptive terms For example, ‘early carbonate’ is often used to describe calcite or dolomite that precipitated soon after the deposition of a sand, before significant compaction, and whilst the sand was still in contact with surface or near-surface formation water Mineral Dissolution Dissolution of either grains or cements in a sandstone leads to the development of secondary porosity In the 1970s and 1980s, many publications suggested that mineral dissolution was a key process whereby significant porosity could be created at depth Such porosity could then be occupied by petroleum A range of dissolution mechanisms were proposed to explain this More recently, new work has indicated that none of these mechanisms is likely to be capable of Figure Diagenetic sequence calibrated to absolute time using geochemical and isotopic analysis in addition to ventional petrographical observations, Ula Formation, North Sea From Kupecz JA, Gluyas JG, and Bloch S (1997) Reservoir Quality Prediction in Sandstones and Carbonates, American Association of Petroleum Geologists’ Memoir 69 Tulsa: American Association of Petroleum Geologists generating significant secondary porosity in the deep subsurface That is to say, secondary porosity is rarely so extensive as to significantly improve reservoir quality Many minerals will dissolve during deposition and subsequent diagenesis The only requirement is that the connate (formation) water that surrounds the grains is undersaturated with respect to the mineral in question However, proof that a particular mineral has dissolved during diagenesis is often more difficult to come by Grains that have partially dissolved are positive proof that secondary porosity has been created, as is mouldic porosity within otherwise tight rock (Figure 8) However, so-called oversized pores are commonly cited as evidence for the complete dissolution of grains and, although such claims are sensible, proof of secondary porosity creation is lacking Advocates of secondary porosity often claimed the wholesale dissolution of mineral cements (particularly calcite) during deep burial, rendering once cemented, low-porosity sandstones highly porous and permeable Popular amongst the various processes invoked for such widespread dissolution was appeal to