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Summary The petrography and diagenesis of calcite cements in the Lower Jurassic, Bridport Sands (southern England) and Upper Jurassic, Viking Group sandstones (Troll Field, offshore Norway) have been investigated in order to assess their geometry and effect on hydrocarbon recovery. In the Bridport Sands, sediment texture and mineralogy controlled carbonate cementation. Clay-rich fairweather sediments were weakly cemented and are now compacted. Bioclast-rich storm deposits were stabilized mechanically by early fringing cements. During burial bioclasts and fringing cements were replaced or dissolved, and pores were filled by simultaneously precipitated ferroan calcite. Thus, cemented beds are laterally continuous for several kilometres in the Bridport Sands as a consequence of the sheet-like geometry of the storm beds in which they developed. In the Viking Group sandstones, carbonate cementation was controlled by rate of burial. Fringing cements formed locally during non-deposition or emergence. Cementation continued with non-ferroan calcite incorporating bacterially derived bicarbonate generated during prolonged residence in near surface zones of bacterial activity. Cement geometries will reflect the distribution of emergent surfaces and the longevity of residence near the surface. These two cases demonstrate the potential for laterally extensive carbonate cements to develop in shelf sandstones. The cements in these examples have different origins but in both cases their distribution is related to the episodic nature of deposition in the shelf environment.

Summary Diagenesis is the sum of those processes by which originally sedimentary clastic assemblages attempt to reach equilibrium with their environments. The subject has rapidly evolved, over the last 20 years, after several decades of routine petrographical analysis, to become a discipline of sophisticated analytical geochemistry. This progression may be traced through the evolution of depositional facies models, the growth of theoretical geochemistry, and the development and application of quantitative analytical techniques. Consequently, the study of diagenesis today involves the integration of data gathered from a range of interrelated disciplines. The publication in recent years of such integrated studies, particularly from the Texas Gulf Coast of the U.S.A., has led to a number of wide ranging models being established. These models allow an interpretation of the sequence of authigenic minerals in terms of their relationship to the depositional environment or surface chemistry (eogenesis), burial or subsurface conditions (mesogenesis) and weathering or reexposure to surface conditions (telogenesis). More specifically, once the pre-depositional controls on diagenesis have been established, it is possible to relate the inferred eogenetic mineral assemblages to exact geochemical sedimentary environments. Sedimentary mineral assemblages chemically are characterized by relative instability and so tend to interact with interstitial pore waters. Thus, in non-marine environments, mineral authigenesis may reflect arid-oxidizing, or wet-reducing pore-water conditions, and in the marine environment, either oxidizing or reducing pore waters. During mesogenesis different processes become important. Elevated temperatures add energy to the reacting system, lowering reaction barriers and increasing reaction rates. Furthermore, widespread pore fluid migration at depth, transporting large quantities of solute, is likely to impart major, regional diagenetic changes to sediments. That there is a remarkable degree of consistency in deep burial settings suggests that depth-related processes conform to a predictable pattern. Additional geochemical instability is introduced into the diagenetic system during uplift and exposure to the telogenetic realm. Minerals formed during burial, at elevated temperatures and pressures, and from concentrated formation waters, may become unstable in oxidizing, meteoric waters. Diagenetic research requires the complete dissection of sedimentary rocks and, subsequently, the quantitative chemical and mineralogical analysis of their individual components. Such an approach to diagenetic studies, when related to an assessment of their paragenesis, may eventually lead to a predictive, integrated model for the evolution of sedimentary clastic rocks.

Middle Jurassic sandstones contain a variety of authigenic clay materials. The origin and distribution of these are related to the influence of several factors, principally depositional pore-fluid chemistry, sandbody geometry and the migration of aggressive fluids. Pore-lining illite, pore-lining chlorite and pore-filling vermiform kaolinite in the Ravenscar Group mutually exclude each other because of depositional pore-water chemistry; seawater in the case of the illite, anoxic freshwater in the case of chlorite and oxygenated freshwater in the case of the kaolinite. Blocky pore-filling dickite occurs ubiquitously within the large connected sandbodies. Its origin may be related to the migration of aggressive fluids and its distribution to depositional sandbody geometry. Mixed-layer chlorite-vermiculite also occurs, and is believed to have formed from chlorite during Recent weathering. Similar patterns occur in Ninian Field Brent Group sandstones, although the situation is more complicated (in these sandstones the kandite subgroup minerals are undifferentiated in this paper). Here too, pore-lining illite occurs in marine sandstones and pore-filling vermiform kandites in non-marine sandstones. However, vermiform kandites also occur in the marine sandstones, perhaps due to freshwater-table development following progradation. The more blocky kandites occur in large connected sandbodies. Finally, a second phase of illitization occurs, postdating blocky kandites, perhaps caused by alkaline formation waters. The occurrence of chlorite in the Broom Formation is anomalous and its possible origin is discussed.

Cement-porosity relationships are described from the Lower Triassic Sherwood Sandstone Group and the Middle Jurassic Ravenscar Group in the United Kingdom. Calcite cemented sandstones display a variety of replacement textures, with preferential replacement of grains and of overgrowth faces with high freesurface energy. Dolomite and siderite cemented sandstones display similar textures but replacement is less specific and euhedral overgrowth surfaces are commonly embayed by carbonates. Examination of the more porous sandstones with the scanning electron microscope reveals a range of pitting and embayment textures in authigenic overgrowths and in detrital grains. These range from small 'v'-shaped notches and pits, through regular and irregular shaped embayments, into large depressions. These textures appear to be morphologically similar to the quartz surfaces seen in thin sections of carbonate cemented sandstones, and are interpreted to have been formed by the dissolution of pore-filling and grain replacive authigenic carbonates. This is confirmed by examination of experimentally exhumed overgrowth surfaces from carbonate cemented sandstones. These textures indicate that part of the intergranular porosity in these sediments is secondary in origin, and has been generated by the dissolution of carbonate cements. The identification of such textures may lead to a more confident interpretation of the nature of intergranular porosity in the subsurface.