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Most reservoir properties, ranging in scale from megascopic to microscopic, can be ultimately traced to environmental variations within deposi-tional systems. Each depositional environment produces sand bodies which display a certain size and shape, and which exhibit a characteristic range in mineral composition, sedimentary structures, and textures. Following deposition, diagenetic processes affecting reservoir quality of siliciclastic rocks normally proceed along different paths that are directly or indirectly related to environmentally-controlled differences in composition and physical characteristics (Figure 4-1).

The effect of depositional facies on reservoir quality in siliciclastics is, obviously, most pronounced in relatively shallow reservoirs. In such rocks, reservoir quality (particularly permeability) is generally controlled by lithofacies (Figure 4-2), which are a product of the depositional environment (e.g., Weber, 1980; Clark and Reinson, 1990). As noted by Weber (1980), in many reservoirs, “it is often possible to work out the train of events leading to present rock properties and to relate these properties to the original sedimentologi-cal characteristics.”

The influence of depositional facies in not restricted to shallow burial depths. In many (but not all) deeply-buried reservoirs affected by a moderate to heavy diagenetic overprint, the relative quality of each facies does not change significantly (e.g., Weber, 1980; Harms and others, 1981; Lindquist, 1983; Schotchman and Johnes, 1990). As noted by Weber (1980), even in deeply-buried sandstones, “one often finds the same general contrasts in permeability in the reservoir that existed in the original sandstone body but with an enhancement of the ratio between maximum and minimum permeabilities.” Permeability trends

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