Interactive Interpretation of Morphology of Upper Tertiary Contourites, Porcupine Basin, Offshore Ireland

D. Bradford Macurda, H. Roice Nelson

Contourites are deposits formed by deep-water currents that flow parallel to contours in the continental slope and rise. These currents are episodically accelerated by sharp differences in temperature and salinity. In some settings, the accelerated currents have sufficient velocity to transport sand, which could form petroleum reservoirs.

A workstation-based, seismic-stratigraphic study of a series of Miocene-Pliocene sequences in the eastern portion of Porcupine basin, offshore Ireland, reveals the stratigraphic complexities and evolution of a contourite dune. Five seismic facies are present: parallel reflectors, which onlap the continental slope; a mound, with internal prograding oblique reflectors that dip to the southwest; a channel axis, oriented northwest-southeast; a natural levee, on the southwest side of the channel; and parallel or hummocky reflectors, downdip toward the basin center. Morphologically the mound, comprised of prograding clinoforms and channel and levee facies, resembles the point bar, channel, and levee of a terrestrial river meander system. The length of the mound exceeds 50 km; channel widths are as great as 8 km. Internally the mound is comprised of at least six sequences that were deposited episodically. Current flow was southeast, parallel to contours. Interactive mapping and contouring of seismic sequences, using paper and a seismic workstation, allow the shifts of directions of progradation and main depocenters to be detailed through the Miocene-Pliocene interval. Mapping of seismic facies suggests possible variations in environments and lithofacies.

Ongoing investigations of contourites suggest that possible end members are morphologically complex and variable. By understanding their variability, the role of contour currents in modifying submarine fans or possibly forming sandy facies capable of reservoiring hydrocarbons can be determined.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.