AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Stratigraphic Architecture of a Structurally Confined, Ponded Submarine Fan: An Outcrop Study of the Guaso I Turbidite System (Ainsa Basin, Southern Spanish Pyrenees)
(1) Dept. of Geology and Geol. Engineering, Chevron Center of Research Excellence at the Colorado School of Mines, Golden, CO.
(2) Chevron ETC, San Ramon, CA.
(3) Chevron Gulf of Mexico Business Unit, Covington, LA.
Ponded, structurally confined submarine fan systems are common features near earth’s continental margins. These systems often form significant hydrocarbon reservoirs, and they can occur in a wide variety of tectonic settings, including salt-withdrawal mini-basins, extensional-contractional systems (e.g., Mississippi fan fold-belt, and offshore Nigeria), transtensional regions, foreland basins, and fore-arc basins. The Eocene Guaso turbidite system crops out in the Ainsa basin, a piggyback basin within the South Pyrenean foreland basin system. Outcrops of the Guaso I (GI), a fourth-order cycle within the Guaso turbidite system, reveal a structurally confined, distributive submarine fan.
GI outcrops provide a rare opportunity to document stratigraphic architecture, as well as proximal-to-distal changes in gross thickness, net-sand thickness, lithofacies associations, and reservoir quality for a structurally confined submarine fan. The strata in the proximal slope depositional setting are dominated by mass-transport-deposits (MTDs), sandy debrites, mud-filled channels, and occasional pebble- or sandstone-filled channels. Net-sand values are relatively low (< 10 m) in this area, and gross thickness increases downslope. The medial slope setting contains bedded mudstones, as well as sandstone-filled channels and levees. Net-sand and gross thickness values are higher than in the proximal slope setting. Near the basin’s depocenter, the GI is comprised of mudstone overlain by a thin MTD, which is in turn overlain by ~ 80 m of vertically connected sandstone (with some minor siltstones). The vertical succession of this dominantly sandstone package is: (1) basal lobes; (2) distributary channels and lobes; (3) interbedded very-fine-sandstones and siltstones. This region has the highest net-sand and gross thickness (~ 150 m) values of the entire system. Near the distal and lateral fan margins, there are minor sandstone-filled channels and thin, tabular sandstone beds intercalated with laminated mudstone. This depositional region has the lowest net-sand values (1-3 m) of the entire GI system.
Stratigraphic stacking patterns document that the GI fan increased in depositional area through time. As depositional area increased, successive lobes and channels increased in the degree of compensational stacking. Data collected in this study can be used to predict architectural and facies patterns in ponded strata of structurally confined turbidite systems in the subsurface.