Data obtained from recent intensive and deep drilling on piercement salt domes of southern Louisiana demonstrate that many salt domes of that area exhibit structural configurations which differ considerably from those conforming with generally accepted concepts of salt-dome growth and from results of scale-model studies.
The domal cores of many of the salt-dome structures of the lower Gulf Coast are composed of intrusive salt and contorted, brecciated, intrusive shale, of deep-water marine facies of diverse age, which is encountered stratigraphically and structurally far above its normal position. Where present, this intrusive deep-water marine shale, termed “diapiric shale” in a system of terminology proposed herein, performs the same function as does the salt core itself in providing the upstructure seal against which the less disturbed bedded sands and shales pinch out and terminate. The diapiric shale is interpreted as having breached, just as has the salt, the overlying sediments as a result of the impetus imparted to it by the buoyant energy of the associated diapiric salt. A study of the Valentine dome of southeastern Louisiana presents a good example of the nature of the domal core of many of the salt domes of this region; the diapiric shale component of this domal core has actually flowed higher than has the diapiric salt, showing its intrusive nature and demonstrating that it has not merely been dragged upward by the intrusive salt core.
Evidence is presented which suggests that some salt-dome structures have been developed by differential vertical movement, at various times, of localized parts of the domal core, as well as by a shift in locus of the center of uplift.
Recent deep drilling on the flanks of three prominent piercement salt-dome features of coastal Louisiana has shown them to be components of a much larger deep-seated parent mass of intrusive salt, for which the term “salt massif” is proposed. This salt massif has an area of 200 square miles, as mapped at a depth of 20,000 feet, and contains 265 cubic miles of salt down to that depth. If this massif is projected downward with vertical sides from that depth to 50,000 feet, it would contain 1,400 cubic miles of salt. There is reason to believe that this salt massif is substantially larger than can now be proved by data obtained from drilling between it and nearby structures. The recognition of this type of major structure is important, as it marks as potentially productive many areas now considered to be off-structure or synclinal because of their apparent interdomal positions.
One of the perplexing problems encountered in the exploration and development of oil and gas fields associated with piercement salt-dome structures in southern Louisiana is the common restriction of the major reserves of a field to a single flank or segment of the structure. There appears to be no consistent preferential geographic location of these prolific segments, and their apparently random positions on the individual structures contribute greatly to the exploration problems of piercement salt domes.
The writers believe that the causative factors of this phenomenon are closely related to the complex growth history of each dome. Detailed studies allow the reconstruction of older, fossil structures, many of them geographically discordant with the present structures, that explain the selective nature of petroleum accumulations. Examples of the relation between fossil structures and areas of present accumulation are illustrated by studies of the Weeks Island, Cote Blanche Island, Iowa, and Lake Washington fields.
Salt-dome structures of southern Louisiana, as they now exist, represent the end products of complex sedimentary and structural histories, and substantial amounts of as-yet-undiscovered oil and gas await discovery in fossil fields whose existence is obscured by present structure.