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

Abstract

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 stick 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 system 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 probability 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 area of these muds may be organic rich and could yield significant hydrocarbons upon burial into the generative window. 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 themore » basin floor thicks, lowstand prograding complexes, and the highstand prograding complexes. The regional mudstones deposited by transgressive systems tract provide the best top seal. 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.« less