7 Sequence Sets and Composite Sequences

Abstract

ABSTRACT This chapter presents definitions, recognition criteria, and examples of sequence sets and composite sequences within a sequence-stratigraphic framework. This stratigraphic scale provides useful insights into shale-gas and tight-liquid plays with mudstone reservoirs that can be profitably grouped into four families based on stratal stacking at the sequence-set scale. Depositional sequences stack in progradational, aggradational, retrogradational, or degradational patterns to form sequence sets—exactly analogous to the stacking patterns of parasequences within depositional sequences discussed in Bohacs et al. (2022a, Chapter 6 this Memoir). Successions of sequence sets accumulate between lower-order sequence boundaries to form lower-order composite sequences containing lowstand, transgressive, and highstand sequence sets. Each of the component, “higher order,” sequences has all the stratal attributes of a depositional sequence, including constituent parasequences and systems tracts, that play a dominant role in controlling the distribution of reservoir, source, and sealing mudstones. Nonetheless, the relative development (thickness and character) of systems tracts in higher-order sequences is strongly influenced by the lower-order stacking pattern of those sequences. Thus, lowstand systems tracts tend to be better developed in depositional sequences within lowstand sequence sets, transgressive systems tracts are better developed in transgressive sequence sets, and so forth in each respective portion of the composite sequence. These repeated stacking patterns of strata and surfaces enable prediction of lithofacies character and distribution, both away from sample control and below the resolution of typical seismic-reflection data. For example, the most widespread, fine-grained, and biogenically dominated strata in the proximate shelfal areas of a composite sequence tend to occur near the top of the transgressive sequence set. Large-scale sequence-set stratigraphic analysis indicates that mudstone reservoirs do not occur randomly but have a repeated and predictable distribution within one of four families—and that such analysis is essential for understanding the localized variations in reservoir potential and distribution. The shared attributes within each family provide objective criteria for selecting appropriate analogs among mudstone reservoir plays and highlight the utility of conducting a basin-to-play–scale stratigraphic analysis.