Can Lithology Be Predicted From Stratal Architecture?An Empirical Approach To The Sequence Stratigraphy Of The Plio-Pleistocene, Gulf Of Mexico

Abstract

In many basins around the world the exploration focus is increasing on slope and basin sediments as potential reservoir targets. The stratal architectural models of Posamentier and Vail1 are often used to predict reservoir facies in these illusive deep water environments. The value of the models is in their prediction, primarily from seismic data, of stratigraphic geometries that define a sequence and its component depositional systems tracts. A genetic relationship is inferred between each systems tract and eustatic sea level and expected facies associations. Unfortunately these simplistic predictive models fail to account for the complex interactions between the various forcing functions other than eustatic sea level that influence sediment distribution. As a result, the predicted lithologies and those actually encountered while drilling are often different. In this study an empirical methodology was developed based on digital integration of seismic with lithoIogic paleontologic and oxygen isotope data within a computer workstation environment. Climatic/eustatic cycles of the Plio-Pleistocene defined for the northern Gulf of Mexico were tied to their associated positions in each well by means of graphic correlation of fossil and oxygen isotope data. A eustatic sea level curve was then derived independent of seismic geometries and directly compared to Iithofacies and depositional systems tracts observed in wells and contrasted with the model predictions. Results show that significant differences exist between model predictions and actual observations Additional factors such as climatic variation, sedimentary process, salt tectonism and basin geometry are critical. The combination of these factors with eustatic sea level fluctuation caused wide variation in the timing and processes of sedimentation. A multidiscipline approach to sequence stratigraphic analysis applying an empirical methodology, provides a more realistic understanding of stratal architecture and a more reliable framework within which to make sediment predictions. INTRODUCTION The primary objective of this project has been to test the eustatic depositional systems tract models, particulary in the slope-to-basin environments. Relative sea level changes, regardless of whether they are driven by eustasy, climate, tectonics or sedimentation rates, are thought to control the distribution of sands on the slope.2–5 The basic hypothesis to be tested by this study is that siliciiclastic slope deposits accumulate during sea level low stands. The existing models infer sea level position based on geometric pattern relationships, subdividing the seismic stratigraphic sequence into its component lowstand, transgressive and highstand systems tracts. The Plio-Pleistocene interval experienced numerous fluctuations in glacio- eustatic sea level that can be directly identified through independent means (oxygen isotopes). The occurrence of deep water sands can then be correlated within this eustatic framework to gain a more accurate understanding of the relationship between eustatic sea level fluctuation, sedimentary processes, climatic variations and salt tectonics and the resulting lateral and vertical facies distributions within a given sequence. Once achieved, this framework provides a constrained analysis of some of the basic assumptions embodied in the current sequence stratigraphic models.1,6

Terrigenous Clastic Depositional Systems

“…For example, in the Quaternary Mississippi system, with its glacially influenced continental fluvial sediment source, the transgressive phase is accompanied by highest rates of sediment discharge and turbidite deposition in fan and delta-fed apron systems (Kolla and Perlmutter 1993;Self and Scott 1993). To the extent that proxies, such as eustatic sea-level change, provide a unique definition of the supply variables, use of such a proxy may improve predictive or explanatory capabilities.…”

“…Significant volumes of sand may continue to be reworked from the shelf, sourcing sandy facies within the retrogradational slope apron (Saito et al 1989;Kolla and Perlmutter 1993;Self and Scott 1993). 8).…”