Seismic Acoustic Impédance: An Indicator for Deep-Water Depositional Processes

摘要

Deep-water siliciclastic sands have b??n deposited in pelagic basins by accumulations of epigenic clastic materials carried by turbidite flows from shelf or slope margins. The classic sequence stratigraphy model calls for the existence of basin-floor fans deposited during the lowstand systems tract (LST) within the depositional profile; however, alternative models suggest that turbidite sedimentation is limited to the slope area. The question that needs to be answered is that if there are indeed basin-floor fans, what is the depositional process controlling the formation of these fans, and where can one find reservoir sands within these fans. This paper discusses processes controlling the deposition of deep-water turbidite sands, and presents a case study in which seismic acoustic impedance data are used to reveal the deep-water depositional processes. These data suggest that the deposition of reservoir sands is accomplished by two major processes-downlapping and bac?stepping, as revealed by the analysis of seismic acoustic impedance volumes, and the subsequent integration of additional geosci-ences and engineering analysis. Backstepping sand bodies are associated with sea-level rising stages (corresponding to the LST), while downlapping processes are associated with sea-level retreat stages (corresponding to the highstand systems tract, or HST). The resultant depositional model allows one to successfully predict the occurrence of reservoir sands during development drilling. The model also provides useful guidance for deep-water exploration and exploitation that can be tested. These classic sequence stratigraphic concepts can be used to construct a chronostratigraphic framework. By applying advanced seismic technology, detailed lithostratigraphic correlation within each third-or fourth-order sequence can be performed to characterize the patterns of flow unit distribution, effectively enhancing the results of deep-water exploration and exploitation, especially in the frontier areas.