Effects of Contemporaneous Structural Movement on Sedimentation at Thornwell Field, Jefferson Davis and Cameron Parishes, Louisiana

Abstract

Abstract The lower Miocene and Frio trends of southwestern Louisiana include some of the more prolific gas-productive units of the Gulf Coast province. Thornwell field, also known as South Thornwell, lies along the boundary between the two trends in Jefferson Davis and Cameron Parishes. The field was discovered in 1942 by Cities Service Oil Company. The discovery well produced gas and condensate from a Planulina palmerae sandstone at 9.605 ft. Subsequent drilling led to the discovery of deeper gas- condensate reservoirs in the Discorbis gravelli, Bolivina perca, Marginulina vaginata, Marginulina howei, and Miogypsinoides A sections. The productive stratigraphic section consists of alternate sandstone and shale: its thickness more than doubles from northwest to southeast across the field. This thickening takes place across a series of contemporaneous faults. Prior to Camerina time, contemporaneous faults probably were not in existence, and the thickness of the pre-Camerina section is related to the structural position of individual sedimentary units on the dome. Beginning with Camerina time, contemporaneous faulting took place. The stratigraphic sections are relatively thin and the sandstone poorly developed in the structurally high fault blocks, whereas the section is thicker and the sandstone better developed in each successively lower fault block. Thornwell is a domal structure with a complex fault pattern consisting of a large normal northeast-striking fault that enters the field from the southwest and branches into smaller radial faults toward the northeast. The structurally highest fault block is the northwestern block. Toward the southeast, each block is successively lower around the structure in a clockwise direction. Thornwell field has not been drilled completely, but enough now is known of the field to make a reasonable estimate of ultimately recoverable reserves—approximately 600 billion cu ft of gas and 15 million bbls of condensate. Production has been established in at least 10 sandstone reservoirs. Until recently, the southern flank had not been tested, but drilling since 1960 on this flank extended production to Lakeside field on the south. The pre-Camerina, or Cibicides hazzardi and Marginulina texana, sections have been penetrated only in the structurally highest fault block where no standstone was found. Both of these sections may prove to have productive sandstone in the lower fault blocks if they can be reached by the drill.