Structural reservoir quality

Shallow water carbonate (reefs carbonate muds) Reservoir quality governed by diagenetic processes and structural history (fracturing). Prolific production from karstified carbonates. High and early water production possible. Dual porosity systems in fractured carbonates. Dolomites may produce H S. 

The maps most frequently consulted in field development are structural maps and reservoir quality maps. Commonly a set of maps will be constructed for each drainage unit. 

Reservoir quality maps are used to illustrate the lateral distribution of reservoir parameters such as net sand, porosity or reservoir thickness. It is important to know whether thickness values are isochore or isopach (see Figure 5.46). Isochore maps are useful if properties related to a fluid column are contoured, e.g. net oil sand. Isopach maps are used for sedimentological studies, e.g. to show the lateral thinning out of a sand body. In cases of low structural dip (<12°) isochore and isopach thickness are virtually the same. 

FIGURE 3.7 Diagram showing effects of sedimentary structures and textures on the flow of fluids in a point-bar sandstone reservoir. The cross-bedded unit is coarser grained and is inferred to have better reservoir properties (i.e., permeability) than the overlying rippled unit (A). Uneven advance of injected fluids illustrating permeability variations results from differences in reservoir quality. (Modified after Ebanks, 1987.)... 

The best reservoir quality potential expected for the Serraria Formation is in structural blocks in the distal and middle domains affected by porosity enhancement through extensive feldspar and carbonate cement dissolution in connection with the post-rift exposure and telogenetic infl ux of meteoric waters. 

The oil in this sand is accounted for by a structural high with gentle relief and permeability development. The initial iso-water cut lines showed that the reservoir quality varied from zero initial water cut in the structural high in the area of the waterflood pilot to over 50 per cent in the north end of the field. Initial water cuts in the polymer flood pilot area were 20 to 30 per cent. Similarly, individual pattern waterflood recoveries varied from about 100 bbl/ acre-ft in the north to about 300 bbl/ acre-ft in the structural high. The polymer flood area was of average quality for the field. 

Growth faulted deltaic areas are highly prospective since they comprise of thick sections of good quality reservoir sands. Deltas usually overlay organic rich marine clays which can source the structures on maturation. Examples are the Niger, Baram or Mississippi Deltas. Clays, deposited within deltaic sequences may restrict the water expulsion during the rapid sedimentation / compaction. This can lead to the generation of overpressures. 

A reservoir cover is a structure installed on or over the surface of the reservoir to minimize water-quality degradation. The three basic design types for reservoir covers are the following ... 

The bifurcation mechanisms for formation of multi-slip fault zones suggest that maximum fault zone thickness will often correspond to the strike-normal distance between the traces of two overlapping slip surfaces (Fig. 2c). Fault overlaps and their breached equivalents occur on faults of all sizes as do, by implication, paired and multi-slip surface fault zones. Complex and paired slip surface fault zone structures will occur on scales below that resolvable by even high quality seismic data (lateral resolution is no better than 50-100 m at North Sea reservoir depths). The possible impact of sub-seismic complexity and paired slip surfaces on connectivity and sealing across faults offsetting an Upper Brent type sequence are briefly considered below. 

The eastern area of Block 30/9 is structurally not as well constrained as the western area because of the seismic data quality. In addition, the hydrocarbons are located in geologically more heterogeneous reservoir units (channels). Consequently, the sealing evaluation performed is more uncertain, and should be used as a guideline rather than an exact measure of the sealing potential. The reservoir is thinner and less shaly, and if sealing is to occur then cataclasis and/or diagenesis would have to play a more important role. The lowest values of SGR are found on the lower parts of the faults (Lower Ness Fm). 

Another possible explanation of the low GORs in the Field is the imperfect seal over the reservoir. Shows of petroleum resembling the Snorre population have been found several hundreds of metres over the reservoir interval (Caillet 1993 Leith et al. 1993 Leith Fallick 1995), even though the field probably has never reached fracture pressure. It is clear that the poor cap rock quality on the relatively shallow and underfilled Snorre structure makes this a perfect candidate for a Type 3 trap as explicitly classified by Sales (1997), i.e. the GOR of the Snorre petroleum charge is determined more by the cap rock properties than by SR quality and its kinetic properties. The oil inferred by Leith et al. (1993) may have leaked from the reservoir as the cap rock section has very low pore entry pressures and micro fracturing is also observed in the cap rock section. One can therefore speculate that the leakage may be a prerequisite to maintain black oil in the Snorre Field. 

The combustible part of petroliferous beds, unlike that of hard coal and oil shale, is only weakly bound to the solid mineral skeleton and becomes very mobile when heated. During in situ combustion of crude oil in a petroliferous sand only 2.3% of the substance, by volume, participates in the process of burning [28]. This fact makes it obvious that the petroliferous formation, when regarded as a combustible substance, has very unique characteristics. Porosity, permeability, structure of pore space, quality of the crude, the degree of its saturation with gas, also the presence of connate water are some of the factors upon which self-ignition of the oil reservoir will depend. 

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