Sequence Stratigraphy in Fine-Grained Rocks: Beyond the Correlative Conformity…

K.M. Bohacs: ExxonMobil Upstream Research Company, and J.E. Neal, G.J. Grabowski, Jr.: ExxonMobil Exploration Company, Houston, Texas

Abstract

Mudrocks provide the source and seal of hydrocarbons and are key elements in reservoir models as baffles and barriers. Sequence stratigraphy provides an excellent framework within which to integrate the many scales of physical, chemical, and biological observations necessary to understand these rocks across the spectrum of depositional settings. Although flooding surfaces and sequence boundaries may be subtly expressed, they can be recognized through distinct changes observed in core, outcrop, well-logs, and on seismic data.

Flooding surfaces fundamentally record a critical increase in accommodation relative to sediment supply, commonly recorded by laterally extensive accumulations of authigenic and pelagic components, along with evidence of sediment starvation and low bottom-energy levels. Some may record minor erosion, reworking, and lag formation due to low sediment supply, but all are marked by a significant decrease in advected clastic input-- contrasting with sequence boundaries.

Sequence boundaries record a critical decrease in accommodation relative to sediment supply, commonly accompanied by an increase in depositional energy or a significant change in sediment supply, or both, over hundreds to thousands of square kilometers. This is recorded by regional erosional truncation with subsequent onlap, exposure, reworked fossils, decreased continuity at lamina to bedset scale, along with increased accumulations of advected clastics and fossils or secular changes in biogenic lithology.

Interactions of sediment supply and accommodation with pre-existing topography control the expression of depositional sequences. Marine environments tend to have the most widespread, gradually varying facies tracts, whereas paralic facies tracts tend to be most localized and abruptly changing. Lacustrine sequences vary according to lake-basin type, and range from very similar to shallow-marine siliciclastic sequences to very dissimilar.