Sedimentary Architecture and Connectivity of Reservoir-Quality Facies in Fluvial Overbank Successions

The distribution of sand bodies within fluvial overbank settings is strongly controlled by processes that dictate the style and frequency of overbank flooding via the breaching of levees, the generation of crevasse splays, and the development of minor distributary channels. In particular, size, longevity, spatial distribution and style of connection of splays to primary channels govern the distribution of sand bodies within overbank settings. The presence of reservoir-quality facies in otherwise mud-prone fluvial overbank successions is important as such sand bodies can provide significant connectivity between neighboring major channel elements in an avulsion-prone channel belt.

Although prediction of 3D architecture and overbank connectivity is crucial for reservoir prediction in low net:gross floodplain settings, typical well spacing in hydrocarbon provinces (several km) is too great and the total number of wells too few for the development of such predictive models. Likewise tertiary splay and minor distributary channels (≤3m thickness) are below the vertical resolution of seismic data.

Results from 2 subsurface case studies demonstrate 3D architecture of reservoir-quality overbank deposits in low net:gross settings. The first, studying secondary and tertiary fluvial elements deposited in interseams of the Late Permian Rangal Coal Measures (Bowen Basin, Queensland), utilizes a suite of densely spaced well data from a database of well logs acquired during coal prospecting at the South Blackwater Mine. Where well-spacing was too great to allow accurate correlation and hence modeling of tertiary channel elements, dimensions from modern analogues (Cumberland Marshes, Saskatchewan, and the Ob River, Siberia) were used as a proxy to produce stochastic models of potential sand body distributions. The second case study utilizes a 3D seismic cube from the Australian NW Shelf, and takes the form of a 3D architecture of mappable surfaces using analogue and modeling to predict distributions of sub-seismic features.

An integrated workflow is presented for the modeling of sub-seismic features using a combination of well, analogue and seismic data to model potential distributions of sub-seismic reservoir quality deposits. Preservation and connectivity of tertiary channels is shown to vary as a result of channel thickness and width, channel % of infill and frequency of crevassing. These in turn are governed by allogenic controls, most notably accommodation space.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California