ABSTRACT

Harmony Field in Clarke County, Mississippi, has produced approximately two million barrels of oil since its discovery in 1968. Production has been from oolitic grainstones in the upper Jurassic Smackover Formation.

The trapping mechanism at Harmony Field is a complex stratigraphic trap. Porous oolitic grainstones pinch out updip into tight carbonates and anhydrite. Structure contour maps on the top of the Smackover Formation indicate a low relief structural nose associated with the field. Additional structure maps contoured at the top of the Haynesville anhydrite, approximately five hundred feet above the top of the Smackover, reveal only regional southwesterly dip. An isopach map of the interval between the two structural markers shows a thinning of Haynesville section coincident with the field area. Evidence suggests, therefore, that the porous Smackover in Harmony Field was deposited with depositional relief above the surrounding sediments. This relief had been completely masked by the time the Haynesville anhydrite was deposited.

Stratigraphic and structural cross sections using the Haynesville anhydrite as datum indicate that the Smackover in Harmony Field consists of not one, but multiple, thin oolitic zones which are productive in various portions of the field. These zones grade laterally as well as updip into non-porous anhydritic carbonates.

The Smackover Formation is often considered to be a chronolithologic unit. In the Harmony Field area it is a lithostratigraphic unit, that is, a unit defined not by time but by a particular rock type, in this case, a porous limestone.

During upper Jurassic time, the Harmony Field area was near the updip limits of the Mississippi Salt Basin. Porous oolitic grainstones are interpreted as washovers or storm deposits into lagoons containing tight carbonate muds. Such a model can provide excellent exploration targets in similar areas within the Mississippi Salt Basin. As Harmony Field demonstrates, we are looking for features with little structural expression and great variations in rock type, true stratigraphic traps.