--> Fluvial Sandbody Connectivity: Observations From High Resolution 3-D Seismic of Dynamic Channel Fill Abandonment in the North Malay Basin

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Fluvial Sandbody Connectivity: Observations From High Resolution 3-D Seismic of Dynamic Channel Fill Abandonment in the North Malay Basin

Abstract

As common elements of fluvial systems, channel fills are greatly complicated by strong heterogeneity, but there are few studies concerning the segmentation effects generated by abandoned channel mudstone fills, owing in part to the lack of effective recognition in the subsurface. To analyze abandoned channel fills effect on reservoir connectivity, a series of low-gradient, mixed-load river systems in Pleistocene section across the North Malay basin, and associated depositional elements are imaged from 4000 km2 of offshore 3D seismic data from the Gulf of Thailand. In study area, abandoned channels result from tributary shifting processes at various scales and can be attributed to a combination of differences in the fluvial regimen, sediment accumulation rate, and local (autocyclic) to regional (allocyclkic)changes in accommodation on fifth to six order sequence cycles.: (1) Lateral chute cut-off abandoned channel. These formed in gradual abandonment processes during base level lowering as river currents gradually reoccupied old swales and flow in the main channel gradually decreased. The channel fill is completed by muds. The resulting sands would be low connected with upper seals. (2) Lateral neck cut-off abandoned channel. During base level lowering, rapid abandonment processes with entire meander loop cutoff, a channel between the two meanders formed. Sands deposited are very thin, the bulk of the package consisting of thicker muds and completely blocked off the connectivity of the sands. (3) Lateral main channel abandonment. These formed with gradualor rapid abandonment within regional transgressive system tract where the channel is abandoned during relative sea-level rise, causing clay deposits to fill the channel. For low sinuosity wide channels, the muds plug upper part of channel fill, for high sinuosity narrow channels the muds create a non-permeable partition among the sands. (4) Vertical coalescing flooding channels. This type of partitioning formed during base level elevation as the major channel axis broadens. Channel sands coalesce into siltier main axis channel sand. The final seal results from the transgressive muds. The observations demonstrate the variety of geometries for abandoned channel mudstone plugs and their partitioning effect in sandbodies. The resulting geometries are seismically discernable and with petrophysical validation. Such partitioning of fluvial sandbodies likely impact estimated reservoir volumes, and recovery factors.