Quantitative Modelling of Depositional Processes and Sedimentary Facies of Shallow Water Deltas

Abstract

Shallow water delta depositional system has recently been identified as a major lithological and/or tight reservoir play. Due to its extremely heterogeneous nature conventional seismic data does not have the resolution to adequately characterize the thin-bedded (from cm to m scales) reservoir sandbodies in such shallow water deltaic systems. We used the forward stratigraphic modelling program, Sedsim, to model fine-scale sedimentary heterogeneities of modern deltas in in the Poyang Lake, southern China and a Triassic deltaic sandstone sequence in the Ordos Basin, western China. A series of shallow-water deltas are developed in the Poyang Lake, providing an excellent modern analogue for understanding key depositional processes that control deltaic development and evolution. We simulated the evolution of the shallow-water deltas over the past 1200 years. The simulation indicates that the delta plain is dominated by distributary channel sandbodies of cm-dm scales and thins towards the lake center, whereas the delta front thickens lake-center-wards, and the frequency of lake level oscillations appears to be a significant controlling factor in determining of the deltaic architecture within the deltaic systems. The Triassic Yanchang Formation in the Ordos Basin is the most important large-scale lithological and tight reservoir sequence in China. We simulated the Chang-6 to Chang-8 members (231.0 Ma and 217.5 Ma) over an area of 295 km x 495 km with a spatial resolution of 5 km and a temporal resolution of 50 ka and two nest models with 1 km spatial resolution. The simulation matches the known depositional thickness and facies derived from logs and cores and sedimentary heterogeneities down to sub-meter scale. The palaeogeography plays an important role in governing the deltaic migration, while the high frequency lake level oscillation and sediment supply control the fine scale facies variations. The delta plain is dominated by distributary channel sand-bodies, while the delta front is characterized by sheet-like sandstone. The sand-rich delta-plain and delta-front have been identified as the best reservoir properties with a porosity range of 7–13% and a corresponding permeability range of 0.3 mD to 1 mD.

AAPG Datapages/Search and Discovery Article #90217 © 2015 International Conference & Exhibition, Melbourne, Australia, September 13-16, 2015