The Geophysical History of Darrow Dome, Ascension Parish, Louisiana

Abstract

Abstract This paper outlines the geophysical investigation of the area covering theDarrow salt dome, Louisiana. Surveys with the refraction seismograph andtorsion balance failed to disclose the dome, but reflection dip shooting wassuccessful and was confirmed by later drilling. Introduction The Darrow salt dome and oil field lies in western Ascension Parish, Louisiana, on the outside of a sharp bend and on the east bank of theMississippi River. The dome occurs about 30 miles south of Baton Rouge and 80miles northwest of New Orleans. Its geology, shape and structure have been ablydescribed in detail by Carroll E. Cook and, except for the general features, will not be repeated here. The purpose of this paper is to record some of theinteresting and pertinent geophysical data contributing to the discovery of thedome and eventually the oil field. The salt dome is circular in outline and comes to within about 4625 ft. ofthe surface. At this depth the diameter of the salt plug is approximately 4800ft. The dome has only a thin cap rock of less than 75 ft. Some oil has been found and is being produced in Miocene sands above the caprock, but most of the field's production comes from a narrow ring of wells aroundthe east and south flanks of the dome. Producing horizons on the flank arefound at 5670 to 5840 ft. in the Miocene and 6890 to 6985 ft., 7028 to 7060 ft.and 8260 to 8270 ft. in the Oligocene. The deepest well that has been drilledis the Humble Oil and Refining Company's Community B-5, on the south flank ofthe structure, which reached a depth of 10,013 feet. The size and shape of the field and the area of production are indicated onFig. I. The line showing the area of production encloses all wells nowproducing and all abandoned producers, both on the top and flank of the dome.This line of production is shown on all subsequent geophysical maps as a directmeans of comparing the early geophysics to later drilling development. T.P. 1495