Sand Sheet Connectivity in a Low Net-to-Gross Setting: Remote Sensing-Based Modeling of a River Channel Network in a Terminal Fluvial System
Fluvial systems in a semi-arid endorheic basin setting terminate in an intricate network of narrow, shallow river channels and associated crevasse and terminal splays. In the fossil record their deposits are preserved as thin-bedded fluvial sandstone ribbons and sheets in a low net-to-gross succession. A recent re-perforation test in a depleted Rotliegend well (Dutch part of the NW European Gas Province) successfully produced 30 Mm3 gas from a single, less than 1-m-thick sheet sandstone. The success initiated the re-assessment of the reservoir potential of such fluvial sheets, which until then were discarded as low-permeable ‘waste zone’.
This paper presents a sedimentary architecture model, constructed from remote sensing data analyses, of the development in space and time of river channel sands and associated sheet deposits in a distal fluvial system of the Uyuni endorheic drainage basin in the Andean Altiplano of Bolivia. Salar de Uyuni in the basin center is the world’s largest salt pan with an area of ca. 12,500 km2. Main present-day fluvial inflow is from the SE, where the Río Colorado and Río Grande de Lípez drain the Cordillera de Lípez. Drainage area is approx. 9000 km2. The climate in the study area is semi-arid; total precipitation averages 184.5 mm/y whereas the evapotranspiration potential is 1300 mm/y. Rainfall is concentrated in the austral summer months December-March, and characterized by short, 24-48 hour periods of torrential rain.
Daily precipitation records of the study area in the period 1975-2010 were used to analyze changes between dry and wet periods in Landsat ETM+, PolSAR and InSAR imagery and yielded the visualization and volume quantification of changes in river morphology. Landsat data (spatial resolution 15 m) show that peak discharge in several consecutive days of high precipitation leads to abandoned channel re-activation and massive flood-outs over the floodplain area. Landsat spectral-band analysis combined with outcrop observation shows that a salt crust formed in the upper part of the soil in the dry period. This may have hampered flood-water infiltration into the floodplain and caused the large extent of flood-out. High-magnitude, low-frequency floods lead to crevasse splay formation, meander-bend cut-off and river avulsion. InSAR phase shift analysis yielded changes in elevation of the terminal fluvial system which were interpreted as crevasse splay emplacement in which sediment aggradation could be quantified.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California