Subsurface Study of Jennings Field, Acadia Parish, Louisiana

Abstract

The Jennings field is on a piercement-type salt plug, where shallow supercap production from upper Miocene sands was discovered in August, 1901. Flank production from Oligocene Marginulina-Frio sands was first obtained in 1929, but active development of the sands did not begin until 1936. The Marginulina-Frio sands are broken into many small blocks by faulting of Oligocene and post-Oligocene age. The Oligocene faulting occurred with few exceptions just after the beginning of Heterostegina time and it is buried under an unconformity near the base of the Heterostegina sediments. Later movements of the salt during the deposition of the Oligocene and Miocene sediments caused additional unconformities and thinning of the strata toward the salt plug. At present the salt plug has penetrated as far as the middle Miocene and is surmounted by thick cap rock which is in contact on the east half of the dome with a mineralized sand section in the Miocene. This mineralization is of major importance, and its origin and mechanics are discussed in some detail. It was formed during a period of seeming quiescence on the part of the salt, and, when upward movement was resumed, the salt was unable to break the bond between the cap rock and the mineralized sand. As a result the salt took the path of least resistance and bulged outward on the west flank where the associated sand beds are not so highly cemented. This suggests the manner in which salt spines and offset salt plugs originate. It is concluded that the salt bulge broke the continuity of the west flank sediments in fairly recent time and allowed the oil accumulations on that side of the field to migrate to the supercap sands.