Thin-Bedded Fluvial Sheet Sandstone as Secondary Reservoir Target: Outcrop Analogue Study of a Quaternary Semi-arid Fluvial System, Altiplano Basin, Bolivia

Donselaar, Marinus E.¹; Cuevas Gozalo, Margarita ²; Moyano, Sebastián ³
(1)Applied Earth Sciences, Delft University of Technology, Delft, Netherlands. (2) Brill, Leiden, Netherlands. (3) Facultad de Ciencias Naturales e Instituto Miguel Lillo, Universidad Nacional de Tucumán, Tucumán, Argentina.

The NW European Gas Province is a mature area with declining gas production. The secondary reservoir potential of Permian and Triassic thin-bedded fluvial sheet sandstone was investigated with the aim to postpone the end-of-field-life. In the past the stratigraphic intervals of thin-bedded fluvial deposits were discarded as ‘waste zone’ because of their low net-to-gross. However, a recent re-perforation test in a depleted Rotliegend well successfully produced 30 Mm³ of additional gas from a single sandstone bed. A comprehensive 3-D static reservoir architecture model of Rotliegend fluvial sheet sandstone in the Dutch part of the gas province showed the development in space and time of nested sandstone sheets at the end of widely spaced south-north elongated fluvial pathways. Challenge for a sustained cost-effective secondary target production is to assess the reservoir architecture of the sheet sandstone from wide-spaced well data.

The present outcrop analogue study deals with the quantification of reservoir units at the end of the Río Colorado in the Andean Altiplano endorheic drainage basin. The Río Colorado ends onto very-low gradient coastal plain fringing the Salar de Uyuni, the world’s largest salt pan with an area of ca. 12,500 km². Google Earth imagery shows an intricate network of former meandering river courses with multiple nodal and random avulsion points. Analysis of the size and shape parameters of the fluvial sandstone shows the development of river shape in time from straight to meandering (SI=1.3 to SI=2.0). In the downstream direction the river width and depth gradually decrease and the river ends in a terminal splay on the coastal plain. Crevasse-splays ‘wings’ extend on either side of the river; their density increases downstream to the point where crevasse splays form thin-bedded amalgamated sand sheets which merge with the terminal splay sand of the river.

Multiple river avulsions created a laterally extensive sheet of amalgamated channel fill, point bar and crevasse splay sand in an area with little vertical accommodation increase. The deposits show a low degree of heterogeneity. Lenticular channel infill is characterized by a basal layer of diatom-rich light-grey clay, overlain by thin very-fine sand to silt layers with parallel- and climbing-ripple cross-lamination. Point bar and channel infill sand form an elongate ribbon of connected sand. The 500-1500-m-wide crevasse splay sands effectively link up sand ribbons.

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.