External Geometry of Reservoir Rocks

The external geometry of reservoir rocks is largely determined by the depositional environments, but faulting and diagenesis may define the lateral or vertical extent of a reservoir.

Fluvial sandstones typically represent point bar sequences in a meandering river system. The lateral accretion of the point bar will deposit a sandstone layer extending to the width of the meander belt in the valley. The thicknesses of channel sandstones are limited by the depth of the river. The primary thickness at the time of deposition is, however, reduced by 10–30% or more by compaction. The overbank muds will become tight shales which will reduce the vertical permeability. Fluvial channels are characterised by fining-upwards sequences with the highest permeability near the base.

This makes it more likely that water will break through along the basal part and the oil will be by-passed in the finer-grained upper part during production. Braided stream facies will tend to have higher sand/shale ratios and will normally have better lateral and vertical permeabilitites on a larger scale. The ratio between the intervals with high enough porosity and permeability to be produced (net or pay), and the total sequence (gross), will be mostly determined by the primary facies relationships.

The net/gross ratio is often taken to be approximately equal to the sand/shale ratio but even at moderate burial depths many sandstones are not reservoir rocks, due to poor sorting or carbonate cement. Aeolian dunes also have specific external geometries and there are many different types. Here the net/gross will be very high. Aeolian sand is often reworked by transgressions, accumulating as marine sediments in drowned topographic depressions (valleys).

Marine sandstones deposited as delta mouth bars, shoreface accretion and barrier islands have thicknesses controlled by the wave energy (wave base depth). In protected environments, particularly interdistributary bays, the shoreface sandstones may be very thin. Each shallow marine unit has a limited thickness controlled by fair-weather wave base. Local subsidence or transgressions can increase the thickness of these sand deposits.

The tidal range is very important in determining the thickness and the length of tidal channel sandstones. Tidal channels and also fluvial channels in deltas tend to be oriented perpendicular to coastlines.

Drilling into shallow marine sandstones, it would be very important to determine whether it was a barrier island which would represent an elongated reservoir parallel to the coastline, or a tidal sandstone which tends to be oriented perpendicular to the coastline. In some cases dipmeter logs could help to determine the orientation of cross-beddding and progradation direction of sand bars. Turbidites may be laterally very extensive, but may also be confined to narrow submarine channels. In either case they may form very thick sequences because there is ample accommodation space.

We may have very thick sequences of stacked sandstone reservoir rocks in slope and deepwater facies and this may compensate for the lower porosity and permeability compared to beach deposits. Turbidites and fluvial sandstones form finingupwards units while marine shoreface and mouth bar sandstones are coarsening-upwards. This becomes very significant during production because oil and gas will be concentrated in the upper part. Coarseningupwards sandstones therefore have the best properties for flow of oil and gas during production.