Reservoirs Mesozoic

Figure 11. Relation of oil reservoirs (solid patterns) in Pennsylvanian Tensleep Sandstone to carbonaceous shale beds of the Permian Phosphoria Formation and to inferred maximum depth of burial (thickness of Mesozoic rocks, in kilometers) shown by solid lines and supplementary 2.5- and 3,5-km thickness shown by dashed line.

Of the two main episodes of oil source-rock deposition, the Palaeozoic accounts for only 10—15% of the total reserves of conventional oils (Bois et al. 1982).The smaller size of the Palaeozoic oil reserves compared with those of the Mesozoic is probably mainly attributable to escape of the older oils from reservoirs due to later tectonic events and to various degradation processes (as described in Section 4.5.6). The surviving Palaeozoic oils are found in stable continental platforms that have escaped significant tectonic activity. The relationship of relative sea level to the occurrence of oil source rocks is reflected in the fact that most oil is derived from marine sources. In comparison, lacustrine source rocks are usually of much more restricted... 

The chemical form of sulphur is important in determining the availability of metal ions in marine environments. In area I of Fig. 5.34 sulphur is generally in the form of S042 and nickel is available for bonding (as Ni2+), but vanadium is unavailable. Oils from source rocks deposited under these conditions, such as those from Mesozoic and Tertiary reservoirs in the Uinta Basin (USA), have V/(V + Ni) values <0.1 and S content <1%. In area II sulphate is again the major form of sulphur, while Ni2+ and V02+ (vanadyl) are available for bonding, but the vanadyl species may... 

Norphlet formation waters produced from three Fairway Field wells were analysed for major components and for selected minor cations as part of normal production operations (Table 1). The samples were taken from platform separators during production. Multiple samples from individual wells taken over a period of several years show only minor variations in composition. The waters are extremely saline brines with total dissolved solids between 300 and 373 g/1. Chlorine is the dominant anion in these brines, and sodium (71.6-89.8 g/1), calcium (33.0-41.6 g/1) and potassium (12.9-17.6 g/1) are the major cations. Norphlet formation waters at Fairway are also characterized by relatively high concentrations of Mg, Ba, Sr, Fe, Mn, Li and Pb. Brines of similar composition are produced from deep Norphlet reservoirs and other Mesozoic strata of onshore Mississippi (Carpenter et al. 1974 Kharaka et al. 1987). 

Claypool, G. E. Mancini, E. A. 1989. Geochemical relationships of petroleum in Mesozoic reservoirs to carbonate source rocks of Jurassic Smackover Formation, southwest Alabama. American Association of Petroleum Geologists Bulletin, 73, 904-924. 

Horstad, I. 1995. Petroleum migration, filling and biological degradation in Mesozoic reservoirs in the Northern North Sea. Ph.D. dissertation. University of Oslo, Oslo. 

Depression where there is a south-facing wedge of productive sandy horizons in certain parts of the Triassic and Permo-Triassic (Fig. 2.4). Along this wedge zone of reservoir rocks we may observe lithological traps in the basal rocks of the Mesozoic complex. It is even possible to locate stratigraphic traps resulting from the closure of reservoir rocks by internal unconformities. 

The main factors controlling a sandstone compaction are shales diagenesis and subsequent overpressures, the presence of evaporite formations which permit heat loss due to their high thermal conductivity, and early quartz overgrowths which resulted in a solid sandstone skeleton before Mesozoic subsidence. These overgrowths have impeded further reservoir compaction. Mapping such early quartz overgrowth devel-... 

The theoretical subsidence in various areas over time can be reconstituted with reference to the Hercynian unconformity taken for a zero level by adding successive layers of Mesozoic sedimentary layers. Since the Silurian source rocks (and, occasionally, the Devonian) closely underlie the Hercynian unconformity in the northern Sahara, the subsidence depth relative to the Hercynian surface (the bottom of a Triassic reservoir) is in fact the depth of source rock subsidence for a given area. 

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