Controls on Sequence Architecture in Lacustrine Basins- Insights for Sequence Stratigraphy in General

Bohacs, K.M. : ExxonMobil Upstream Research Company; J.E. Neal, G.J. Grabowski, Jr.: ExxonMobil Exploration Company; D.J. Reynolds: ExxonMobil Upstream Research Company; and A.R. Carroll: University of Wisconsin-Madison

Abstract

The sequence-stratigraphic approach of looking at a hierarchy of rock packages bounded by various surfaces works very well in lake strata. The expression of depositional sequences, however, varies as a function of the lake depositional system - just as shallow marine-carbonate sequences look different from shallow-marine-siliciclastic sequences. Contrasts among lake and marine systems make it inappropriate to directly apply one unmodified marine sequence-stratigraphic model to all lake systems. Indeed, one lacustrine model is not applicable to all lake-basin types.

Contrasts of sequence expression among lake-basin types arise from several key attributes: Lake level and sediment supply are commonly linked closely in lake systems (most marine models assume no linkage); lake shorelines commonly move basinward by a combination of progradation and desiccation. In addition, the character of a lake is fundamentally controlled by the relative rates of potential accommodation change and supply of sediment plus water, giving rise to three distinct lake-basin types: overfilled, balanced-filled, and underfilled.

These differences strongly influence the occurrence, distribution, and character of hydrocarbon source, reservoir, and seal lithologies. Sequence boundaries vary from non-existent or minimally developed, through extensive erosion and incised-valley formation, to large basinward shifts and widespread exposure. Flooding surfaces are enhanced as they are commonly coincident with decreased sediment supply.

Lowstands vary from aggradational stacks of basin-floor turbidite parasequences to basin-center evaporites surrounded by extensive desiccation surfaces. Transgressive systems tracts vary from thin and shale prone to thick and coarse-clastic prone. Highstand systems tracts range from obliquely progradational clastic shoreline parasequences to aggradational carbonate shoreline parasequences.

Successful exploration and production in lake basins requires attention to these variations.