Print this pageHierarchical 
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.
