Braided Stream Reservoirs
Authors: Richardson J.G. , Sangree J.B. , Sneider R.M. 
Published: December 2007

Technology Today Series articles provide useful summary informationon both classic and emerging concepts in petroleum engineering. Purpose: To provide the general reader with a basic understanding of a significantconcept, technique, or development within a specific area of technology.

Introduction

Two types of river deposits commonly are excellent reservoirs-braided andmeandering streams (see Fig. 1). Some of the world's larger sandstonereservoirs are composed of braided stream deposits. Much of the Prudhoe Bayfield on Alaska's North Slope and the Statfjord reservoirs in the Statfjord and Brent fields in the North Sea contain sands deposited by braided streams. Braided streams are complex networks of low-sinuosity channels that flow onalluvial plains with slopes greater than 1 1/2 to 21 [0.026 to 0.034 rad]. Discharge through the channels is normally low, but during a year sporadicfloods carry enormous volumes of water and sediments into the system. Asexisting channels are overloaded with deposits during a flood, new channels arecut and quickly filled with coarser debris. Repeated bar formation andbranching of channels produces the network of braided channels. The water thatspills over the channel banks spreads fine silt and clay particles that arethen deposited in abandoned channels and on the plain. During geologic time, the stream complex may move its location back and forth across the width of avalley floor many times. As the streams move into a new area during a flood, the channels will generally cut into and erode away previous deposits of clayand silt from the valley floor but may leave remnants of thicker shale layersin abandoned stream channels.

Braided Stream Reservoirs

Lateral continuity of the pay is usually good to excellent in braided streamreservoirs and vertical continuity is fair to good. The shale members left inabandoned stream beds are not continuous barriers to vertical flow but areusually small, isolated, randomly distributed bodies of shale or silt. Porosities and permeabilities are often quite high but vary Porosities andpermeabilities are often quite high but vary rapidly with depth, reflecting thedepositional cycles of the floods (see Fig. 2). Most braided channel depositsare composed of vertically stacked channel sequences. Permeabilities arehighest at the base of a channel deposit. Permeabilities are highest at thebase of a channel deposit. where coarser material is left, and decrease upwardfrom the base, reflecting the finer particle size. If an entire channelsequence is preserved and not eroded away by later channels, very thin shalelaminae observed in cores from the top of a channel may drastically reducevertical permeabilities measured in cores. However, these thin laminae extendonly a few inches laterally and thus have a limited effect on reducing verticalpermeabilities in the reservoir. Permeabilities and porosities usually do notcorrelate from well to well at porosities usually do not correlate from well towell at equivalent stratigraphic depths because they represent differentpositions in a bed. However, zonal averages of permeabilities positions in abed. However, zonal averages of permeabilities often correlate from well towell. The zones represent many cycles of deposition in the various channelsduring the various geologic periods of the deposition. Further, arithmeticaverages of permeabilities measured in wells for a zone often agree with thosecalculated from well tests. Thus, lateral flow is through zones of similarpermeability distributions (as opposed to randomly distributed permeabilitieswhere geometric averages would apply).

JPT

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