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Understanding the processes that drive fluid flow in sedimentary basins has important implications for models of metallic ore and petroleum migration and accumulation in the subsurface. Potential drive mechanisms include gravity-driven flow, compaction-driven flow, and density-driven convection. In this study, we demonstrate that the brine hosted in Ordovician sandstone in the Illizi Basin in Algeria is genetically linked to Triassic-Liassic evaporites deposited >400 km to the north in the Berkine Basin. This observation confirms that long distance, lateral, brine migration has occurred within the basin in the past. We assess the hydrogeologic record preserved in aqueous fluid inclusions within the Ordovician sandstone, document a marked increase in formation water salinity during cooling and exhumation, and evaluate the drive mechanisms for late-stage remobilization of deep brines within the basin. It is hypothesized that the release of overpressure during exhumation of the Illizi Basin may have been a critical contributor to updip fluid flux. This model could be applicable to other exhumed basins worldwide.

The Eocene sandstones in the northern Dongying Sag, Bohai Bay Basin, China, are reservoirs for large accumulations of hydrocarbons. The sandstones are mainly lithic arkoses and feldspathic litharenites, texturally and compositionally immature. These sandstones have a wide range of porosity (0.4–37%) and permeability (0.004–6969 mD) and show an overall decrease in reservoir quality from 1500 m to 5000 m below sea level. The reduction in reservoir quality is a product of several digenetic processes; these include compaction, precipitation of dolomite and calcite in eodiagenetic stage; compaction, feldspar dissolution, precipitation of quartz cements and clays (kaolin and illite) and precipitation of ferrocalcite and ankerite in mesodiagenetic stage. Mineral distribution pattern and isotopic composition suggest carbonate cements in sandstones originate from sources outside the sandstones. Carbonate cementation, together with compaction reduced the sandstones’ porosity and permeability significantly. In a sandstone bed, marginal sandstones with distance to sandstone/mudstone interface less than one meter always have lower porosity than central sandstones. As burial depth exceeds 4000 m, marginal sandstones have very low porosities ( 2 m) can be potential effective reservoirs. Feldspar dissolution and precipitation of clays and quartz cements have little impact on absolute porosity. Mineral distribution pattern and quantitative data show that leached feldspars are the internal source of authigenic quartz and clays in sandstones, and the volume difference between feldspar secondary porosity and related authigenic cements is generally less than 0.25%. However, although there is little or no net import of matter to the sandstones, the pore architecture changes dramatically. Primary macropores are lost as clays and quartz precipitate while the proportion of microporosity increases, occurring mainly between clay crystals. The overall result is that permeability is significantly degraded.

Exotic minerals, i.e. those that do not have an obvious source of components within the host rocks, can be major porosity occluding cement and replacement phases in subsurface carbonate reservoirs. Here we present and discuss petrographic and fluid inclusion data from a number of petroleum systems. These have had different geological histories but a common factor is the presence of exotic mineral cements and late stage dissolution. The effect of these cements and late stage dissolution on reservoir quality is also considered. Petroleum filling does not appear to inhibit the precipitation of exotic cements in the way that it appears to inhibit the precipitation of calcite. Burial dolomite and anhydrite are the most important of these volumetrically but they are often accompanied by other late stage cements such as fluorite, kaolin, quartz, barite, celestite, sphalerite and galena, generally in much smaller quantities. In accounting for the concentration of the material contained in these cements compared with the host rocks, stylolitisation is a possible mechanism in some cases, but, in many others, import from sources external to the reservoir is required.

The relative importance of petroleum emplacement in inhibiting diagenetic processes and preserving porosity and permeability in Lower Cretaceous, Thamama Group (Kharaib Formation) carbonate reservoirs of Abu Dhabi, UAE, and in Callovian-Kimmeridgian carbonate reservoirs of the Amu Darya Basin in Uzbekistan and Turkmenistan, has been evaluated by combining geologic, petrophysical and geochemical data. When petroleum emplacement is synchronous with and prior to significant burial cementation in carbonates, primary petroleum inclusions are trapped in the cements. The process appears to be characterised by steep intra-field porosity-depth trends within a more gradual regional decline in porosity with depth. This has profound implications for the prediction of porosity in carbonate reservoirs. Reservoir quality is better in grainstones and packstones compared to adjacent wackestones and lime mudstones in the Kharaib Formation because of preserved macroporosity (intergranular, vuggy, mouldic); the pore system in the finer units is dominated by micropores. These features indicate a primary textural control on porosity and permeability. Within the grainstones and packstones, macroporosity is variably filled by late equant sparry calcite cements. Porosity and permeability variations in grainstones and packstones at a reservoir scale are therefore controlled by the variation in amount of equant sparry calcite cement. This in turn depends on the timing of the precipitation of this cement relative to petroleum emplacement, as shown by fluid inclusion data. Where petroleum emplacement has occurred relatively early, at migration foci, prior to significant burial cementation by equant sparry calcite, reservoir quality is preserved. Where it has occurred after significant burial cementation, reservoir quality has been destroyed. In the Amu Darya sequences, primary macroporosity is commonly preserved down to depths of 11,000 ft (3.5 km) with differences in the porosity and permeability characteristics of grainstones being controlled by variations in the amount of early, probably freshwater, cement and the extent of associated dissolution. Small volumes of burial cements do occur, but they do not contain petroleum inclusions. Consequently, there is no firm evidence that petroleum emplacement has inhibited diagenesis in this area. This part of the study has shown that it is not always possible to obtain conclusive evidence from the diagenesis to pin down the processes responsible for the preservation of reservoir quality and that petroleum filling may not always be the primary cause. The relationships documented here show that the ‘race for space’ between diagenetic waters and petroleum is a major control on reservoir quality in the Thamama Group carbonate reservoirs, but is not so important for the Jurassic carbonates in the Amu Darya basin.

This paper describes evidence for and against burial induced deformation of fluid inclusions in diagenetic quartz. Fluid inclusions in diagenetic minerals are prone to non-elastic deformation (stretching or leakage) if subjected to progressive burial in a sedimentary basin. If they deform, they cannot record mineral growth conditions correctly. A correlation between homogenization temperature and depth may be interpreted as indicating wholesale leakage or stretching. Experimental work and theoretical studies suggest that heating >200°C higher than the homogenization temperature is required to produce significant deformation, but it may not be possible to extend the results to natural conditions. The experiments further predict a correlation between homogenization temperature and inclusion size in a sample of inclusions that have undergone natural deformation. This is not observed in natural examples, but the absence of a correlation is not proof that leakage has not occurred. Several features of inclusion data in quartz cements do, however, argue against stretching or leakage. Homogenization temperatures commonly show narrow distributions and are compatible with independent data such as illite potassium-argon ages and petrography. Different generations of inclusions in a single sample set also retain homogenization temperature distributions compatible with their sequential origin. Taken as a whole, the body of evidence argues against widespread stretching or leakage of fluid inclusions in diagenetic quartz.