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This is primarily a study of the subsurface, geologic structure of oil fields in Louisiana within a coastal belt about 75 miles wide. For this report, 1,000 electrical logs of wells were studied; 123 were selected for reproduction in the cross sections. Most of the structures are the result of the rise of plastic salt columns from deep within the sedimentary layers of the earth's crust. The present depth of the top of the salt column determines the structural characteristics of the resulting domes.
This study is devoted particularly to the deep-seated domes in which salt has not been reached. Two common types of structures are found in this group.
The most common structural type consists of a region of domed sediments crossed by a series of normal faults creating a graben in the center of the domed layers. The graben of deep-seated domes is bounded by major faults. All observed grabens contain several faults, both major and minor, resulting in a complex mass of faulted blocks. Most of the major faults converge downward toward a zone believed to lie just above the head of the salt column. The dome-with-graben type of structure appears to be a fundamental type to which all of the others are related.
A less common type may be described as a dome offset by one or more major faults which have similar directions of strike and dip. In many such cases, it is believed that when additional wells are drilled, other faults will be found to complete a central graben.
No reverse faults were found. All of the structures contain normal faults. These faults may be broadly divided into major and minor types. Major faults characteristically increase in throw downward. The throws of major faults range from 100 to more than 900 feet. Observed dips of major faults range from 45° to 65°. The throw of these faults on any structure gives a rough approximation of the amount of structural relief.
Minor faults take only a minor part in the segmentation of the domes. Two distinctive types are recognized.
Radial minor faults extend radially from the center of doming.
Complementary minor faults are distinctive in that the throw decreases downward. Such faults create small grabens against major faults. Complementary faults relieve tension in the outer layers of the dome.
Other minor faults are less readily classified. They may be parallel and close to major faults or they may be branches or small divisions of major faults. In many, the throw is so small that they can not easily be traced.
The following theory is proposed to explain the development of the grabens. First, upward movement of a salt plug or column causes the deeply buried layers to be gently domed. Continued salt movement causes rupture in the form of a major, normal fault extending diagonally upward through the dome from the head of the salt plug.
Additional salt movement causes additional faults of the same kind but which in a general way are directed alternately opposite to, and parallel with, the first. The result is a complexly faulted
graben, with the later faults at higher levels within the graben. Continued salt movement results eventually in a shallow salt dome.
The present distribution of oil and gas on deep-seated domes is the result of two sets of conditions. The present general location of the oil and gas is related directly to the center of the original gentle dome created by the earlier movements of the salt. Into this simple domal trap, petroleum migrated from the entire deformed area of the dome.
The specific location of each individual trap for each sand is now dependent on the segmentation of the dome by faulting. In general, the petroleum will remain in its original geographic location, but with many complications added by the barriers developed by faulting and the further tilting of faulted blocks.
Original maps and sections were prepared for the following fields: Barataria, Bateman Lake, Bayou Sale, Erath, Gibson, Gillis-English Bayou, Grand Bay, Grand Lake, Horseshoe Bayou, Lafitte, Lake Long, Lake Mongoulois, North Crowley, Roanoke, St. Gabriel, West Gueydan.
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