Reconnaissance mapping for petroleum exploration began in the early 1920s along the Mexia-Talco fault line in Hopkins County, Texas. Drilling to test the Woodbine sand, which took place largely between 1925 and 1933, has thus far been unsuccessful. Since the discovery in 1936 of the Sulphur Bluff field, subsequent Paluxy testing resulted in the discovery of Mitchell Creek field in 1948 and the small Birthright field in 1954. In 1958, production from the Smackover limestone of Jurassic age was established on the Sulphur Bluff fault. These fields are located on en échelon faults, each of which has a corresponding back fault, resulting in a series of grabens.
Most of the wells along the fault line in western Hopkins County and Delta County have been drilled since 1940, and in nearly all instances, electric logs were run. Because of the availability of electric logs, more accurate interpretation of the geologic history of the west area is possible than in the east half of Hopkins County.
The area has a flat to rolling topography. It is drained on the north by Sulphur River and on the south by the tributaries of White Oak Bayou and Caney Creek. Along Sulphur River in the extreme western part of Hopkins County and in Delta County, much of the land is very flat and covered with secondary material.
Jurassic, Cretaceous, and Tertiary sediments have been penetrated by the drill in this area, making possible relatively detailed lithologic descriptions and interval measurement. Careful comparisons of the intervals and sections show their relative thickening and thinning and reveal the timing of the faulting. These comparisons show that there were movements along the faults from early Cretaceous time. However, these movements were not continuous, but were interrupted by periods of quiescence when little or no movement took place.
The surface map shows the front faults, back faults, and graben areas in the east half of Hopkins County. For convenience in mapping the surface faults, the Midway formation has been divided into three parts in this paper, instead of the customary two. In Delta and extreme western Hopkins counties, the Peerless fault is mapped on surface outcrops and the upthrown structure contoured on the top of the caprock of the Nacatoch sand.
The front faults and complementary back faults forming the grabens are also shown on Pecan Gap structural maps and Goodland structural maps for both the east and west parts of Hopkins County and southern Delta County. The text explains how these maps were made and points out salient features and differences.
The grabens are contoured where subsurface control is available on the Pecan Gap maps. In the Sulphur Bluff field the downthrown beds are complexly folded into several small irregular highs and lows. The Goodland map on identical areas, but on the upthrown side of the fault, shows a long regular structure. A comparison of the structural highs on the Sulphur Bluff and Hatchetville faults shows that there was pronounced uplift from early Cretaceous time at Sulphur Bluff, while the area at Hatchetville appears to have been structurally low for a long period. This accounts for the large accumulation of oil in the Sulphur Bluff field.
A cross section extending north and south across the Sulphur Bluff field graphically shows the front and back faults with their accompanying grabens.
The present small highs on the Posey and Peerless faults in the western half of the county actually decrease with depth. This accounts for the absence of oil from the Paluxy formation in the western half of the county.
The long periods of quiescence along the entire fault zone in northern Hopkins and Delta counties after basal Austin chalk time probably accounts for the absence of any oil or gas accumulations in the Woodbine and sub-Clarksville sands on the upthrown side of the south front faults.
Future oil possibilities from new sands are not overly favorable at this time. However, Woodbine accumulations in the graben areas are a definite possibility.
The Preston syncline in western Hopkins and Delta counties appears to be due to fairly late sinking. From Fredericksburg to Pecan Gap time, this syncline was shallow and at times possible non-existent.