ARBUES, PAU¹, and DONATELLA MELLERE^2
1University of Barcelona, Spain
²Statoil Petroleum Technology, Norway

Abstract: Architecture and stratigraphy of a slope valley fill: the O-Grao system, Ainsa Basin, Spain

The Ainsa Basin is dissected by a number of turbidite channels envisaged as potentially good reservoir-analogues for slope settings.

O-Grao is one of the sandier bodies of the Ainsa slope: it is up to 80 m thick, 3 km wide and it crops-out along a 6.5 km section. It consists of three channeled sandstone units (O-Grao 1, 2 and 3). O-Grao 1 and 2, are up to 15 m thick and at least 100 m wide. They are confined within an erosive, terraced and asymmetric surface. They consist of stacked and compensated scour and fill sandstone bodies up to 5 m thick and 100's m wide. The axial part of the erosional surface accommodates the thickest sandstone beds. Facies assemblage is dominated by massive to normal graded sandstone deposited from high-density turbidite flows, with local trough cross-strata. The scour and fill bodies are separated by 0.1-1 m thick thin-bedded intervals of burrowed to ripple-laminated sandstones and shale, possibly representing overbank deposits. The relative volume of the thin beds increases from base to top as well as from the axis towards the margin of the channeled system. O-Grao 3 is up to 15 m thick and more than 1600 wide. It consists of a net of closely spaced scour and fill units, some m deep and several tens of m wide, at places imbricated and mimicking units of lateral accretion.

The basal surface of erosion is interpreted as a submarine valley. The initial stage of infill occurred through highdensity turbidite flows powerful enough to create multiple episodes of scour and fill and occasionally threedimensional dunes. Successive flows infilled the newly generated topographic lows giving origin to compensate sandbodies which onlapped the valley margin. O-Grao 3 is not a confined system. Architectural analysis indicates that it forms a slightly sinuous channel belt. The large amount of granules and the high frequency of scours suggest that the channel belt was a more efficient conduit when compared to the underlying O-Grao I and 2, and likely it developed during a stage of major basinward shift of the coastline and of the feeder fluvial system.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas