[SUBJECT TO REVISION]

DISCUSSION OF THIS PAPER IS INVITED. It should preferably be presented in person at the Colorado Meeting, September, 1918, when an abstract of the paper will be read. If this is impossible, then discussion in writing may be sent to the Editor, American Institute of Mining Engineers, 29 West 39th Street, New York, N. Y., for presentation by the Secretary or other representative of its author. Unless special arrangement is made, the discussion of this paper will cloose Oct. 1, 1918. Any discussion offered thereafter should preferably be in the form of a new paper.

Possible Existence of Deep-seated Oil Deposits on the Gulf Coast

THE discovery of oil in 1901 on the Spindletop dome, Texas, inaugurated a new industry on the Gulf Coast, an industry which has grown with the discovery of successive fields, until today it engages the services of thousands of workers and employs enormous capital. New fields are being discovered from time to time and doubtless some still remain to be found, though of late years discoveries have become more infrequent. Nowadays several hundred dry holes are drilled each year in a fruitless and blind effort to discover new fields, for as yet geologic science has developed no effectual method of locating the coastal oil deposits in advance of drilling.

Moreover, despite the occasional discovery of new fields, the total production of the Gulf Coast is today no greater than it was in 1906, for the added production of the new fields has been offset by the rapid decline and more or less complete exhaustion of some of the older ones. Careful geologic work within the fields has in some cases increased the production temporarily, but has developed no really new supplies. The Gulf Coast oil industry seems to have passed its period of greatest expansion and to be declining at a fairly steady rate, and this condition is naturally viewed with alarm by the more farsighted operators.

In my opinion, the time has come for the adoption of radical and aggressive methods of prospecting, and a fraction of the money wasted yearly in drilling shallow wells in hopeless locations might well be devoted to this purpose. Many facts lead me to believe that all the salt-dome oil has had a common origin; that it has migrated up from considerable

depth along lines of structural weakness; and that a deep well, properly located, stands an excellent chance of discovering the parent reservoir and thus of developing new and probably great supplies. This paper is presented as a discussion-necessarily hypothetical and based largely on personal opinion-of the possibility of encountering deep-seated oil deposits beneath the salt domes.

SALT-DOME STRUCTURE

All of the oil produced in the coastal region of Texas and Louisiana is probably associated with salt domes, though in Goose Creek, Edgerly, and one or two other fields no salt has yet been actually penetrated. As a result of the innumerable wells that have been drilled on the various domes, it is now known that a typical salt dome consists of a very thick mass of pretty pure rock salt, generally almost flat-topped, but sloping abruptly away from the rim on every side. The flat top of the salt is generally covered by rock 25 to several hundred feet thick, and consisting chiefly of limestone, dolomite, anhydrite, or gypsum, with generally more or less sulphur. The sediments above the salt are slightly domed and those on the sides of the salt mass generally slope at angles of 30° to 60°, and in some fields at even greater angles. These sediments consist of sand, gravel, shale, and gumbo, arranged in beds so lenticular and irregular that they can seldom be correlated from one well to another.

Of the many salt domes already discovered on the Coastal Plain, there are no two whose structures are identical. Each has its individual peculiarities of size, height, steepness, character of cap rock, and wealth or absence of oil. Some of the domes, like the phenomenally rich Spindletop, are only a few hundred acres in extent; others, like Humble, Damon Mound, and some of the salt islands of Louisiana, cover several square miles. Most of these domes are roughly circular in outline, but some, like South Dayton and Blue Ridge, are greatly elongated. Many of the domes form more or less distinct mounds at the surface, the highest rising 100 ft. or so above the surrounding plain, but others are overlain by sunken areas or lakes, and still others by perfectly level country in which no clue to the structure can be obtained. The cap rock of some mounds, like Sulphur and Bryan Heights, contains commercial quantities of sulphur, whereas that of others contains only traces. Gypsum and anhydrite are almost the sole constituents of some cap rocks, whereas in others limestone and dolomite greatly predominate. Pyrite is common in nearly all, and in one or two, galena, sphalerite, chalcopyrite, etc., have been found in small quantities.

Finally, the salt itself is physically different in different mounds; in most of them it is hard and well crystallized, but in others it is soft, granular, and almost incoherent. As may be seen in the salt mines at

Petite Anse and Grand Cote Islands, La., the salt is characterized by peculiar gray streaks or markings. These do not seem to have any regular trend, and are often contorted and twisted into the most fantastic shapes (see Fig. 1).

FIG. 1.-GRAY MARKINGS ON SALT, AS FOUND AT PETITE ANSE AND GRAND COTE ISLANDS.

The salt-dome oil occurs under conditions just as irregular and impossible to determine in advance of drilling as the structure of the dome itself. The oil at Spindletop occurs chiefly in a hard cavernous limestone layer which forms the cap rock of the dome. At Welsh, Saratoga, and many other fields, the oil is found in the loose sands above the cap rock. At Anse la Butte, Damon Mound, and some other fields, it occurs partly or wholly in the deeper and sharply dipping sands which lie on the flanks of the domes and below the level of the cap, while in the Humble field it is found under all three conditions. In only one dome, Belle Isle, La., has oil been found within the salt itself, and in this locality the salt contains much gas under enormous pressure and also a small quantity of high-grade paraffin oil.

As few of the domes are more than a mile across, it is evident that the productive area is very limited in extent, though where oil occurs on the flanks of the salt mass the field is somewhat larger. The largest

field, Humble, is, however, less than 2 miles square. Salt-dome oil thus occurs under conditions very different from those in the Appalachian fields, where the oil is found in well defined gently folded sands traceable over large areas. In the Gulf Coast fields the wells are more expensive and less likely to find oil, and though their production may be enormous for a short period their decline is usually rapid.

Of the four or five dozen salt domes now known in Texas and Louisiana, less than two dozen have produced oil in commercial quantities and less than one dozen have become really important fields. Many are perfect blanks as far as production is concerned, and though apparently similar to richly productive domes and characterized by apparently identical surface indications, they may yield only a puff of gas while drilling and a showing of oil so slight that it may have found its way into the well from the surface machinery. These conditions suffice to emphasize the practical importance of arriving at a solution of the origin of the domes and their associated minerals, and of determining, if possible, the ultimate source of the oil and the conditions which have controlled its migration and accumulation.

PRESENT METHODS OF EXPLORATION AND DEVELOPMENT

The search for oil on the Gulf Coast at present consists of two phases, the search for new and hidden salt domes, and the drilling of old and plainly visible domes which have yet produced no oil. As the occurrence of oil on the dome is very irregular, two dozen or more wells may be drilled before commercial supplies are found. As already stated, these supplies may be phenomenally rich considering the area involved, but they soon become exhausted and the search must then be resumed.

Owing to the flat and monotonous topography of the Coastal Plain and to the fact that the oil-bearing rocks are overlain by a thick and structureless mantle of Pleistocene deposits, ordinary methods of geologic field work are of no avail. Some domes are marked by prominent and unmistakable hills, but others appear only as low and inconspicuous mounds. The so-called gas mounds, and even large ant hills, have sometimes been confused with true domes, and wells have been drilled on them. Other domes have no topographic reflection whatsoever and the search for such domes therefore resolves itself into a search for surface indications of oil; gas seeps, the so-called paraffin-dirt, oil or asphalt exudations, sulphur or salt springs, and mud volcanoes are commonly regarded as the best indications. Even if they lead to the discovery of a new salt dome, however, there is, of course, no assurance that the dome will prove productive of oil.

Quite recently several different and as yet untried methods for locating