Kevin M. Bohacs1,
J. E. Neal1
(1) Exxon Production Research Company, Houston, TX
Abstract: Source and seal sequence stratigraphy—the rest of the hydrocarbon system
Sequence stratigraphy benefits source and seal evaluation by revealing genetically related rocks, aiding correlation, and allowing prediction away from sample control. Expression of depositional sequences is a function of the interaction of sediment supply and accommodation with pre-existing topography. Sequences in marine environments tend to have widespread, gradually varying facies tracts, whereas paralic environments tend to have localized, abruptly changing facies tracts with a wide variety of lithologies. Sequences in lakes range from being very similar to shallow-marine siliciclastic sequences to very dissimilar. Sequence boundaries and flooding surfaces may be subtly expressed, but are readily recognizable through changes in lithology, grain size, body and trace fossils, organic matter content, natural gamma-ray activity, and stratal stacking patterns, and on reflection seismic data.
Flooding surfaces (parasequence boundaries) most generally register sharp increases in accommodation relative to sediment supply. This is recorded by laterally extensive concentrations of authigenic minerals and nodules, pelagic/air-fall components, burrowing/boring, early lithification, and by increased stratal continuity. Some may record minor erosion and lag formation due to low sediment supply, but all are marked by a significant decrease in advected clastic input— contrasting with sequence boundaries. Most log signatures used in "pattern" or "marker-bed" correlation are distal expressions of flooding surfaces. Downlap surfaces in seismic data portray mostly close convergence of distal flooding surfaces and little actual stratal termination.
Sequence boundaries record a sharp decrease in accom-modation relative to sediment supply, commonly accompanied by an increase in depositional energy (regional erosional truncation with sub-sequent onlap, exposure, reworked fossils, decreased lamina/bedset continuity) or a significant change in sediment supply (advected clastics and fossils, secular change in biogenic lithology), or both, over hundreds to thousands of square kilometers.
AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana