The Homogeneity and Large Lateral Extent of Fluvial Sand Reservoirs Produced During the Falling-Stage Systems Tract: An Example from the East China Sea Continental Margin
Authors: Bartek Louis R. , Wellner Robert 
Published: April 2012

High-resolution (&lt;1 m) Geopulse seismic reflection (5,649 km) data collected during surveys of the East China Sea (ECS) continental margin have been used to reconstruct the shallow stratigraphic architecture of the ECS continental margin. Important boundary conditions for deposition on this margin include an extremely high sediment supply (relative to other margins) and a shallow and broad (~600 km) continental shelf that has a gentle gradient across the basin to the shelf edge. These conditions are rarely found on modern continental margins, but they were quite significant in ancient continental-margin stratigraphic sequences.

The seismic data and cores were collected from the ECS to begin to develop models of the spatial variability of reservoir facies that evolve in this type of system. These conditions are similar to those of the Yellow Sea continental margin, but the lateral extent of sandy falling-stage and transgressive deposits is an order of magnitude greater than that in the ECS. On the inner to middle portions of the ECS margin, we identified extensive areas outside several incised valleys that contain channelized seismic facies that are interpreted as fluvial successions deposited as sea level fell prior to the last lowstand. In inner shelf locales, the sandy facies are amalgamated, and very little (if any) of the fine-grained highstand strata lie between the sandy falling-stage deposits. In mid- to outer shelf locations, which are less frequently subaerially exposed, the sandy fluvial deposits lie above and between highstand silts and clays. The upper portions of these “interfluvial” fluvial deposits were reworked during the ensuing transgression. Above the transgressive surface, large tidal sand ridges were produced as the shoreline migrated landward. However, most of the lower fluvial sheet-sand facies are preserved in place. The tidal sand ridges and fluvial facies are downlapped by muddy highstand deposits.

Historical records suggest that the tremendous sediment load of the Yellow River caused the river to avulse over an area of nearly 1,000 km during the Holocene and to deposit the thick fluvial sand sheet in “interfluvial” areas. We suggest that as sea level fell in this area, the equilibrium point and bayline synchronously migrated seaward, and subaerial accommodation was created during the latter stages of highstands and the falling stage, in a manner similar to that proposed in published models. The high sediment supply of the area and increasing subaerial accommodation space provided an opportunity for deposition of the laterally extensive sandy fluvial facies we observed on seismic data. Low impedance contrast between seismic reflections within the laterally extensive fluvial deposits suggests that the composition of the lithofacies associated with the fluvial deposits is similar and very likely contains a high proportion of interconnected pore space, thus providing an excellent analog for large reservoirs of oil and gas found within fluvial sand deposits. This situation creates a laterally extensive braided-fluvial-sand-type reservoir (~1,000 km along strike, &gt;500 km down dip, between 10 and 40 m thick, and a reservoirrock volume of ~12,000 km3), with a potential for a stratigraphic seal that is close to hydrocarbon source rocks. Thus, the Pleistocene stratigraphy of the ECS shelf may serve as an analog for extensive fluvial sequences in the foreland basins of the Cretaceous Western Interior Seaway of the United States and the Carboniferous sequences of the eastern United States, and it may provide valuable insight into how best to develop and produce from these systems or other large fluvial sandstone reservoirs.