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The AAPG/Datapages Combined Publications Database
Deltas generally are formed at river mouths during stillstands of sea level under conditions of cyclic transgression or regression. Consequently, they are rarely isolated phenomena, but form in multiples in a predictable fashion. Reservoir facies consist of continuous and discontinuous, bifurcating channel sandstones and delta-front sheet sandstones. The channel sandstones generally thicken downward at the expense of the underlying prodelta clays and may replace selected parts of the delta-front sheet sandstones.
The lithologic components of a deltaic complex are interrelated and are referred to collectively as one type of Genetic Increment of Strata (GIS). The GIS is a vertical sequence of strata in which each lithologic component is related genetically to all the others. It is defined at the top by a time-lithologic marker bed (such as a thin limestone or bentonite) and at the base by either a time-lithologic marker bed, an unconformity, or a facies change from marine to nonmarine beds. It generally consists of the total of all marginal marine sediments deposited during one stillstand stage of a shoreline, or it may be a wedge of sediments deposited during a series of cyclic subsidences or emergences. An isopach map of a GIS clearly shows the bifurcating trends of the individual distributari s and the shape of the delta, regardless of the variable lithology of the channel fills.
A Genetic Sequence of Strata (GSS) consists of two or more contiguous GIS's and, when isopached, clearly defines the shelf, hinge line, and less stable part of a depositional basin. An isopach map of the McAlester Formation of the Arkoma basin is a good example of a GSS. The oil-productive Booch sandstone is a good example of a deltaic complex occurring within a GIS of this GSS. The upper Tonkawa, Endicott, and Red Fork sandstones of the Anadarko basin are identified as deltaic accumulations within different GIS's.
A hypothetical model serves as a basis for establishing the criteria for (1) recognizing successive stillstand positions of a shoreline, (2) predicting paleodrainage courses, (3) predicting positions of a series of deltaic reservoirs, (4) locating isolated channel sandstone reservoirs, and (5) tracing related beach sandstone reservoirs.
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