2000 AGI
Interpreting Siliciclastic Sequence Stratigraphy from Well Logs

The concept of sequence stratigraphy is arguably the most revolutionary and unifying one in the study of basin-scale sedimentary successions. It involves analysis of sedimentation patterns present in marine stratigraphic successions that were deposited in response to cyclic variations in relative sea level. Sequence-stratigraphic analysis, in marked contrast to lithostratigraphic interpretation, requires recognition and correlation of key chronostratigraphic surfaces (sequence boundaries) in order to construct a genetic stratal framework. This powerful model thus enables prediction of the spatial and temporal distribution of cyclic stratal patterns regionally within basins, as well as subregionally within large oil fields. The modern sequence-stratigraphic model is grounded in the application of many fundamental geologic concepts (pre-1970), the development of multichannel seismic imaging (late 1960’s and 1970’s), and the recognition of the relation between stratal architecture and relative sea-level change (post-1980). Recent work has demonstrated the applicability of high-resolution sequence stratigraphy to well logs, cores, and outcrops, thus making this method of analysis practical for most stratigraphic studies. Specifically, the methodology permits prediction of reservoirs, traps, seals, and hydrocarbon-migration pathways.

For readers who are interested in gaining more insight into the sequence-stratigraphic concept than this module can present, Wilgus and others (1988), Van Wagoner and others (1990), Emery and Myers (1996), and Posamentier and Allen (1999), among other authors, provided comprehensive discussions of the sequence-stratigraphic model. For the sake of brevity, the following discussion presents a generalized view of the model.

 

Relative Sea Level

Cyclic change in relative sea level is the primary control on the depositional architecture and stratal-stacking patterns in sedimentary basins. Two factors, each varying independently through time, are the principal controls on relative-sea-level change: eustasy and subsidence rate. Eustasy (=global sea level) and subsidence (or uplift) rate together control accommodation, the amount of space available for sediment accumulation. When the rate of eustatic fall exceeds the rate of subsidence, accommodation decreases, and a fall in relative sea level occurs. Conversely, rise in relative sea level, marking an increase in accommodation, results from either a higher rate of subsidence relative to that of eustatic fall, or subsidence without eustatic change.

The rate of sediment supply to the basin and basin physiography are additional factors that influence relative sea level, and they largely control the stratal geometry of the basin fill. Change in relative sea level exerts a profound influence on the vertical and lateral relations of lithofacies, depositional facies, and, therefore, the well log signatures of these strata. Moreover, it directly controls formation of the key sequence-stratigraphic surfaces that bound systems tracts: sequence boundary, transgressive surface, and maximum flooding surface.