Changes

Rarely do deltas conform perfectly to these end-members. In general they are transitional, giving rise to complexity and variability in the geometry and reservoir heterogeneities of resulting sandstone bodies (e.g., Sneider et al.<ref name=pt06r130>Sneider, R. M., Tinker, C. N., Meckel, L. D., 1978, Deltaic environment reservoir types and their characteristics: Journal of Petroleum Technology, Nov., p. 1538–1546.</ref>).

Rarely do deltas conform perfectly to these end-members. In general they are transitional, giving rise to complexity and variability in the geometry and reservoir heterogeneities of resulting sandstone bodies (e.g., Sneider et al.<ref name=pt06r130>Sneider, R. M., Tinker, C. N., Meckel, L. D., 1978, Deltaic environment reservoir types and their characteristics: Journal of Petroleum Technology, Nov., p. 1538–1546.</ref>).

[[Distributary mouth bar]]s and channel deposits (Figure 3h) comprise the best reservoir quality bodies within a delta system. The general upward-coarsening character of distributary mouth bars tends to produce sandstone bodies that have their greatest [[permeability]] at the top. Conversely, distributary channel sandstone bodies are usually upward-fining and have their greatest permeability at the base.<ref name=pt06r130 /> Preferred orientation of flow may be expected to follow paleochannel trends.

[[Distributary mouth bar]]s and channel deposits ([[:file:lithofacies-and-environmental-analysis-of-clastic-depositional-systems_fig3.png|Figure 3h]]) comprise the best reservoir quality bodies within a delta system. The general upward-coarsening character of distributary mouth bars tends to produce sandstone bodies that have their greatest [[permeability]] at the top. Conversely, distributary channel sandstone bodies are usually upward-fining and have their greatest permeability at the base.<ref name=pt06r130 /> Preferred orientation of flow may be expected to follow paleochannel trends.

Distributary mouth bars typically contain a high percentage of interstratified clay that reduces [[Directional permeability|vertical permeability]]. Hartman and Paynter<ref name=pt06r50>Hartman, J. A., Paynter, D. D., 1979, Drainage anomalies in Gulf Coast Tertiary sandstones: Journal of Petroleum Technology, Oct., p. 1313–1322.</ref> document an example of such behavior from a Gulf Coast deltaic reservoir undergoing natural [[Drive mechanisms and recovery#Water drive|water drive]]. After several years of production, the better quality channel sands watered out, whereas oil remained in the poorer quality delta fringe deposits. In this field, bypassed oil was accessed by recompleting wells only in the delta fringe interval.

Distributary mouth bars typically contain a high percentage of interstratified clay that reduces [[Directional permeability|vertical permeability]]. Hartman and Paynter<ref name=pt06r50>Hartman, J. A., Paynter, D. D., 1979, Drainage anomalies in Gulf Coast Tertiary sandstones: Journal of Petroleum Technology, Oct., p. 1313–1322.</ref> document an example of such behavior from a Gulf Coast deltaic reservoir undergoing natural [[Drive mechanisms and recovery#Water drive|water drive]]. After several years of production, the better quality channel sands watered out, whereas oil remained in the poorer quality delta fringe deposits. In this field, bypassed oil was accessed by recompleting wells only in the delta fringe interval.