ABSTRACT

Stratigraphic traps in Eocene, Oligocene, and Miocene sand-shale sequences on low relief structures are becoming increasingly important drilling objectives in the Louisiana Gulf Coast. A survey of Southwest Louisiana oil pools, exclusive of piercement salt domes, discloses over 50 in which lateral stratigraphic variations are in some degree responsible for entrapment.

Except for those sands associated with piercement salt domes, no "pure" stratigraphic traps were found, and all should be classified as combination traps according to A. I. Levorsen's classification of reservoir traps. The relative importance of structure and stratigraphy in the combination traps studied was recognized and it was felt that combination traps require further classification. The term mechaniform or mechaniform Combination trap is proposed for those combination traps in which structural influence dominates, and the term permeaform or permeaform Combination trap is proposed for those combination traps in which stratigraphic influence dominates.

Geologic maps, electrical log cross sections, and production data from 14 pools are used to illustrate and document both mechaniform and permeaform combination traps. These data also demonstrate the value of net sand isopachous mapping in locating exploratory and development wells.

Regional stratigraphic trap prospecting is accomplished best by superimposing net sand isopachous contours on subsurface structure maps contoured on the same sand. This approach is illustrated by geologic maps of the Lewisburg Field Area, St. Landry Parish.