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Hierarchical Stacking Geometries of Genetic Sequences: Implications for Predicting Geometry and Distribution of Reservoir Facies Tracts

Timothy A. Cross

Stratigraphic successions representing marine shelf to coastal-plain environments are successfully simulated by numerical models. The models are based upon the fundamental controls governing the development of stratigraphic sequences and stratal architecture in sedimentary basins: eustatic fluctuations, tectonic movement, quantity of sediment delivered to a basin, sediment compaction, and isostatic compensation.

The models show that the fundamental building block of marine shelf to coastal-plain stratigraphic sequences is the progradational event. Packages of lithologically diverse but temporally equivalent strata deposited during a progradational event are termed "genetic sequences." Genetic sequences are arranged systematically in three geometric patterns. Relative to the direction of progradation, these are forward-stepping (seaward), backward-stepping (landward), and vertical stacking.

Accompanying these geometric patterns are changes in thickness and lateral extent of similar facies tracts within separate genetic sequences. Shoreface sands are thin and widespread in forward-stepping phases, thick and laterally restricted in vertically stacked phases, and very thin and laterally restricted in backward-stepping phases. By contrast, facies representing fluvial and coastal-plain environments are thickest in backward-stepping phases and thinnest in forward-stepping phases. Different styles of fluvial architecture also are a direct response to the position of a genetic sequence within the hierarchical stacking pattern. Thick, multistoried channel belt sands occur in backward-stepping phases, and thin, multilateral channel belt sands occur in forward-stepping phases.

These variations in geometry and geographic distribution of similar facies tracts arise from geographic shifts of sites of maximum sediment accommodation potential. These shifts are deterministic and therefore predictable. Because the stacking patterns are expressed vertically as thinning and thickening patterns of successive genetic sequences, the geometry and geographic position of reservoir facies are predictable from geophysical well logs.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.