Sequence Stratigraphic Setting of Siliciclastic Source Rocks, Reservoir Rocks and Seals

John M. Armentrout

Sequence stratigraphy is the study of stratigraphic relationships within a time-stratigraphic framework. It combines detailed analysis of sedimentary facies and depositional geometries, and defines a hierarchy of stratigraphic units that stack into progressively larger scale cycles. Each depositional cycle consists of four phases of relative accommodation change which can be related to relative water level change, such as sea level. These four phases are rising, highstand, falling, and lowstand, and the rocks deposited during each phase are called systems tracts. Each systems tract has the potential to contain organic-rich rocks, and porous and permeable reservoir rocks. However, specific depositional environments within some systems tracts are likely to have higher proba ility as an effective source rock, seal or reservoir. For example, regionally extensive muds are deposited during transgression and effective seals may extend over broad areas. If sedimentation rates and biologic productivity rates are optimal, some outer shelf and upper slope areas of these muds may be organic rich and could yield significant hydrocarbons upon burial into the generative window. Reservoirs also occur in predictable patterns. High-energy nearshore environments produce porous and permeable sandstones, both within lowstand and highstand prograding depositional systems.

Systems tracts maps may be overlain one-on-the-other to assess the relative distribution of potential reservoir, seal or hydrocarbon source rock versus possible hydrocarbon traps. For example, reservoir sands occur within the basin floor thicks, lowstand prograding complexes, and the highstand prograding complexes. The regional mudstones deposited by transgressive systems tract provide the best top seal. The transgressive systems tract may overlies all of the updip ends of the underlying lowstand prograding complexes, providing top-seal. Because the lowstand prograding complexes are geographically restricted to re-entrants into the underlying slope, lateral seal may exist if the underlying slope is sand-poor. Such lateral and top-seal could provide a stratigraphic trap for the coastal plain or near-shore sandstones of the lowstand prograding complex. Sandstones of the highstand systems tract and the basin floor thick of the lowstand systems tract are often widely distributed and the potential for effective top-seal or lateral-seal is more problematic. For such highstand and basin floor reservoir targets structural closure is needed to form effective traps.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995