Reservoir Architecture of Fluvial Channel Networks: Outcrop Analogue of a Semi-arid River System, Altiplano Basin, Bolivia
Permian and Triassic fluvial successions in the NW European Gas Province contain numerous sandstone beds less than 1 m thick, in an overall low net-to-gross setting. The aim of the present paper is to assess the reservoir potential of such deposits for secondary recovery by analyzing the sedimentary architecture of a reservoir-analog outcrop in an endorheic drainage basin in the Andean Altiplano of Bolivia. Size, shape, connectivity and potential reservoir volume of sand bodies were established in a meandering channel network in the distal part of the modern Río Colorado, which ends on a mudflat at the edge of the Salar de Uyuni, the world’s largest salt pan with an area of ca. 12,500 km2.
Absolute age dating of the river deposits with the optically stimulated luminescence (OSL) method, combined with detailed mapping of the channel patterns, reveals that an intricate network of cross-cutting channels, in an area of over 400 km2, formed by multiple nodal and random river avulsions in the last 4000 y. Sand deposits concentrate in point bars, channel fills, crevasse and terminal splays, in a background of floodplain silt. Channel width gradually decreases downstream from 65 m to less than 10 m within 28 km. The channel ends in terminal splays on the mudflat. Because of the semi-arid climate, channels are ephemeral. Therefore, the typical mud- and organic channel infill (“clay plug”) is lacking. Instead, the avulsed channels show various stages of sandy sediment infill. Sedimentation in the avulsed channels takes place by re-activation during peak flow, and by lateral infilling with crevasse-splay sand. Complete channel fills are up to 2 m thick and characterized by thin very-fine sand to silt layers with parallel lamination and climbing-ripple cross-lamination overlying diatom-rich light-grey lacustrine clay. Channel-infill forms an elongate ribbon sand body which connects successive point-bar sand deposits. Crevasse-splays extend on either side of the channels; their size and number increases downstream to the point where crevasse splays form thin-bedded amalgamated sand sheets which merge with sandy terminal splays. The amalgamated sheets are 1-2 km2 in area and effectively link up with adjacent channel trajectories.
The study shows that thin-bedded fluvial sands may constitute sizable reservoirs because of their large areal extent and good lateral connectivity.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California