The Quitman field is a faulted anticline trending northeastward approximately 7 miles, and reflecting more than 300 feet of closure. It represents an excellent example of a composite graben in which intensive faulting at shallow depths becomes simplified at lower levels. This condition is attributed to termination of lesser faults at the base of grabens. Below such points, the magnitude of the continuing fault is diminished by an amount equivalent to the throw of the cancelled one. This pattern, which appears to be the result of deformation in which the primary stress is vertical, also has been observed in other structures of similar stress conditions such as deep seated salt domes. However, detection of the displacement relationships, made possible by the excellent correlation at Quitman, may be obscure in fields where less accurate correlation is possible.
The main oil accumulation at Quitman has been found in the deeper structure where sands of the Paluxy formation have been proved productive between the subsea depths of 5,652 feet and 5,909 feet. In general, the structure at this depth is modified by faults of relatively small displacement which are the deep remnants of the shallower intercalated graben pattern. Increased fault displacements occur where main faults intersect below the Paluxy reservoir. The sands are extremely erratic in distribution suggesting original deposition by aeolian agencies and redistribution through current action under shallow-water conditions.
Shallow oil accumulations have been discovered in the thin sub-Clarksville and "Harris" sands of Eagle Ford age. These reservoirs are found at depths ranging from 3,500 to 3,865 feet, subsea, and are more complexly faulted than the deeper beds since some faults displacing the Eagle Ford reservoir sands terminate at fault intersections above the Paluxy formation. The oil accumulation in the sub-Clarksville member appears to extend over the entire field and may be more extensive than the Paluxy reservoir. The Harris sand appears to be productive only in the higher structural parts of upthrown fault blocks and the individual oilreservoirs in adjacent fault blocks may be separated by barren areas. The down-dropped central graben appears to be barren throughout the extent of the field.
Figures & Tables
Structure of Typical American Oil Fields: A Symposium of the Relation of Oil Accumulation to Structure
Modern petroleum geology in the United States had its beginning in the first decade of the 20th Century when the U.S. Geological Survey began mapping the structure of the rocks in and near old fields in order to discover the various types of structural conditions under which oil and gas are trapped. Structural geology has evolved as a branch of the broader science far more rapidly than have methods of mapping the attitude of rocks at the surface. This volume, published in the late 1920s, was designed to afford authoritative and modern descriptions of the structure of typical oil fields in the United States. Each of the 39 fields contained here is described by an author who is intimately familiar with the available data. The relationship of structure at the surface and at depth for different terranes is clearly set forth wherever the strata are not parallel. The volume concludes with a summary paper on the role of geologic structure in the accumulation of petroleum. Fields include: Florence, Colorado; Stephens, Arkansas; Kevin-Sunburst, Montana; Bradford Pennsylvania; and Salt Creek, Wyoming.