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Because of the possible economic importance of the arched Ellenburger group, the identification of its subdivisions by insoluble residues and general character of well cuttings has been attempted with encouraging results. To facilitate correlation of the Pennsylvanian and lower Permian strata, classification by "series" is proposed, based on unconformities and faunal changes of wide regional importance. Rearrangement of certain formations and groups is also advocated. Former stratigraphic names are retained where possible even though redefined or changed in rank. Cross sections and maps reveal the Concho arch as an imposing northwest trending structural feature, denuded in the Llano uplift area. Uplift and erosion of Ordovician beds along this axis evidently began during pre-Mississippian time. Beds from Barnett to Strawn in age show marked thinning and considerable truncation as this regional feature is approached. Attention is also given to the evidence of intermittent growth of the Ouachita-Marathon Mountains, the Electra and Muenster arches. The Bend flexure and other large structural features of this region are described. Regional conditions affecting oil and gas migration and accumulation are discussed. These include the progressive development of structural trends and local folds, overlap of structurally high reservoirs by apparent source beds, differential pressures resulting from wedge-shaped overburden, distribution of sedimentary material, and the development of local sandstone or limestone reservoirs and stratigraphic traps. A "compaction-hydraulic theory" of oil migration is favored as the main cause of movement of oil into and laterally within the reservoir toward areas of less overburden and pressure.

A clearer picture of when and how the Llano uplift attained its present segmented domal form is sought by interpretation of data from various outcrop and subsurface studies. A series of thickness and structural maps have been prepared. These indicate that broad initial warping of the Concho arch, the uptilted southeast part of which forms the present Llano uplift, occurred during the interval between deposition of Ellenburger and Mississippian rocks. Pronounced development of the northeast, southeast and southwest flanks of the Llano uplift took place during Medial Pennsylvanian (Lampasas-Strawn) time when the deep Strawn and Kerr basins were formed as foredeeps paralleling the Ouachita orogenic system. Much rifting accompanied or followed these subsidences, particularly where the connecting trough crossed the central and eastern parts of the Llano region. Considerable folding and elevation occurred along northeast-southwest trends at the close of Lampasas and probably at intervals during Strawn time. Maximum uplift and erosion evidently centered in northwest Mason County, Canyon sediments having been deposited on Cambrian rocks in this area where the extensive Bend flexure-Richland Springs and Concho axes intersect. Development of a pronounced west flank and general erosion of Paleozoic rocks of the Llano uplift awaited regional elevation to the east and subsidence to the west, mainly during Permian and Triassic time. Periodic accentuation of the Concho-Llano-San Marcos structural axis may be attributed to the presence of a relatively buoyant or stable region in the earth's crust. However, the absence of positive tendencies along this trend during the Cambrian and Early Ordovician seems to contradict this theory. Deltaic loading, particularly by Lower Pennsylvanian sediments in the Ouachita and Marathon regions, may have brought compensating positive tendencies to the broad intervening region, thereby influencing subsequent diastrophic and depositional developments. End_of_Article - Last_Page 2626

Obviously, the study of successive controlling geologic conditions and influences has many direct applications to the problems of the oil geologist with his duties of interpretation and prediction. The Carboniferous sediments of the Mid-Continent region were deposited upon a basement which was exceedingly well base-leveled but very complex structurally due to 10,000 feet of structural change during earlier Paleozoic, and far greater changes during pre-Cambrian times. Accumulation occurred mainly in shallow seas, hence practically at sea-level and without appreciable regional dip. Great inequalities in thickness required extensive structural changes within the basin areas, and the arrival of vast quantities of coarse material indicates equally extensive and concurrent uplift of areas around the basins. Such interrelation of negative and positive movements furnishes the basis for certain important deductions. Six successive groups of sediments are discussed as to character, location, thickness, and probable source. The oscillatory and migratory tendencies of these basins are noted and attendant structural effects discussed. Analysis of the various types of structure of this region is attempted, and maps are submitted showing net results of structural changes since (1) early Pennsylvanian and (2) Jurassic times. It is concluded that unequal subsidence and diastrophism caused by subcrustal pressures associated with successive basin subsidences best explain the ordinary oil field type of structure and the successive rejuvenations. The Ouachita geosyncline and synclinorium are considered the dominant features of the region, and a diagram is included indicating the successive developments in this area of greatest deposition, structural change, and erosion.

The use of wider spacing and the consequent drilling of fewer wells appear to be a promising procedure for necessary and important reductions in the cost of producing oil. It is believed that petroleum geologists and petroleum engineers have a very important role to play in studying the various characteristics of the producing reservoir as a basis for a suitable and efficient development and production program.

A clearer concept of the growth of the Llano uplift and related principal structural features of central Texas is sought through use of the present increased store of surface and subsurface geological data. Generalized thickness maps have been prepared as an aid to analysis of time, place, and amount of major uplifts and subsidences, some of which were intense, localized tectonic movements, others widespread epeirogenic adjustments. The most obvious major tectonic feature in central Texas is the Llano uplift where the pre-Cambrian basement complex crops out in an area 40 miles wide and 70 miles long. During much of early Paleozoic time this area was a seaway and for long intervals thereafter it was alternately above and below sea-level. Major diastrophic changes occurred after Early Ordovician and before Late Pennsylvanian, by which latter time the pre-Cambrian surface had become at least 10,000 feet higher in the western part of the Llano uplift than beneath the flanking Fort Worth and Kerr basins. Erosional losses indicate that uplift accounted for about one-third and subsidence two-thirds of the vertical movement in these structural adjustments. Many pronounced structural features observed in the Llano uplift show a surprising lack of parallelism with the main northwest axial trend. Instead, a north to north-northeast trend is common, thereby dividing this uplift into several major segments which are bounded by relatively steep dips and extensive normal faults with vertical displacement as great as 3,000 feet. These major intersecting tectonic features appreciably affected only those beds that are older than the Lazy Bend group (restricted) of the Strawn series. Thereafter structural trends developed mainly along northwest trends. Mississippian outcrops in the Llano region transgress the truncated Ordovician Ellenburger group. Drilling has shown an increasing loss of section west of the Llano uplift so that, as a result of both erosion and non-deposition, Upper Pennsylvanian (Canyon) marine sediments locally overlap Cambrian rocks in and near northeast Menard County. Farther west and northwest, Middle Pennsylvanian beds rest on truncated Mississippian and Ordovician or older rocks in a large region, heretofore called the Concho arch, where local as well as regional tectonic features had developed mainly along trends varying from north-northeast to northwest. Thin Middle Pennsylvanian marine sediments of the Lampasas and Strawn series deposited across this base-levelled region are chiefly limestones and shales o the platform type in contrast to thick basinal type deposits on the east and south. The northwest part of the Llano area evidently remained somewhat above sea-level during most or all of the Strawn time. However, at least 2,000 feet of Upper Pennsylvanian and Lower Permian sediments are assumed to have been deposited across the Llano region, judged by their thickness and marine character, also by projection of regional dip, in near-by outcrops. Regional upward tilting toward the east, followed by extensive erosion, has brought prominence to the Llano uplift. As disclosed by drilling, a regional downward tilting toward the west has given the eastern part of the Concho platform the appearance of a major arch, commonly referred to as the Bend arch or flexure. The much larger Concho arch and broad Concho platform between the Bend axis and the Midland basin lost much of their original dominant position as a result of this regional tilting which began in Late Pennsylvanian, but took place mainly during Permian time. Subsequently, epeirogenic movements, including continental emergence after the Lower Cretaceous epoch, have permitted erosion of all sediments and re-exposure of pre-Cambrian basement in the Llano area, now known geogr phically as the Central Mineral region.