ABSTRACT Drilling activity in the Deep Tuscaloosa Gas Trend has experienced a significant decline since reaching a peak in 1980. An important factor in this decline is the current oversupply of gas, but, in addition, the Trend has entered a mature stage of development. The deep Tuscaloosa Gas Trend in Louisiana is approximately 25 miles in width and extends northwesterly from Lake Borgne on the east to the Texas state line, a distance of nearly 200 miles. Since the Trend discovery in 1975, exploratory drilling has discovered 24 fields. Proved and potential reserves are estimated at five trillion cubic feet of gas and 100 million barrels of condensate. Virtually all the significant reserves have been established in the vicinity of Baton Rouge, Louisiana. Reservoirs in the deep Lower Tuscaloosa Trend occur in an Upper Cretaceous wedge of clastics, which rapidly expands downdip from a carbonate shelf break of Early Cretaceous age. The occurrence of commercial oil and gas in the Trend is dependent on many factors. Foremost are paleoenvironment, syndepositional tectonics, source rock, original sand mineralogy, and burial diagenesis. Reservoir distribution, thickness, and quality are closely related to the depositional environment. Commercial discoveries have been made in deep Lower Tuscaloosa sandstones deposited in fluvial, distributary channel, distributary-mouth bar, offshore bar, and prodelta environments. Three types of productive structures have been mapped in the deep Lower Tuscaloosa. The most common structures are east-west trending closures on the downthrown side of large down-to-the-basin growth faults, which developed in response to sediment loading. A second type of structure, which is low-relief with only minor faulting, occurs north of the expanded trend. The third type of structure is defined by uplift and closure related to salt piercement. The total organic carbon content of the Lower Tuscaloosa shales ranges between 0.5 and 2.3 wt.% and averages 0.9 wt.%. The very mature kerogen is a mixture of oil and gas prone types. Reservoir sandstones reflect a wide range of lithologies and textures. Features that many of the reservoirs share include a high quartz and low feldspar content and ubiquitous, grain-rimming authigenic chlorite. The high quartz content, indicative of a mature mineralogy, is caused by extensive dissolution of metastable, silicate grains. Although local variations are encountered, two diagenetic sequences are recognized. The sequences are similar in several respects. In both areas, physical compaction, accompanied by precipitation within the pore system of authigenic clay, was followed by syntaxial quartz overgrowths. This episode of porosity reduction was temporarily reversed by a process of dissolution and replacement of metastable grains. The sequences differ in the amount and type of authigenic clay, the degree of quartz cementation, and the quantity, quality, and genesis of the secondary porosity. Primary, intergranular pore space normally dominates the pore system in the deep Lower Tuscaloosa reservoirs. Secondary porosity, created by grain dissolution and replacement, is also common and, in many reservoirs, comprises 20% of the total porosity. Reservoir porosities exceeding 20% are common where the primary and secondary pore systems are combined. It is anticipated that additional fields will be discovered in the deep Tuscaloosa Gas Trend. An awareness of the factors that contribute to oil and gas accumulations in the deep Tuscaloosa will aid in their discovery, development, and exploitation.

Abstract The value of geothermal data as an aid to the oil geologist remains to be demonstrated, at least insofar as the direct discovery of individual oil pools is concerned. While Van Orstrand demonstrated years ago, in his work on the Salt Creek field of Wyoming, that in some instances isothermal contours below the surface of the earth quite faithfully reflect the structural conditions and, more recently, observations on salt domes in Texas have demonstrated temperature conditions that would be recognized as anomalous by any geologist familiar with the area, the facts that no data are available in advance of drilling, that even after wells are drilled significant measurements can be secured only occasionally, and that in spite of the years of work that have been devoted to the study of earth temperatures we are still far from a complete understanding of their significance, have combined to dull the interest of those who must measure their effectiveness in terms of new oil-field discoveries and who have, therefore, concentrated their attention on methods that are more speedy, more definite and less dependent on a multitude of circumstances than is the measurement of the geothermal gradient. On the other hand, the science of petroleum geology, during the past few years, has left the narrow lane which led only to the discovery of obvious anticlinal structures, along which it traveled for decades after the anticlinal theory was first announced, and to-day geologists recognize that all facts which may throw additional light upon either the

Abstract Oil fields of the world with recoverable reserves more than 500 million bbl and gas fields with reserves more than 3.5 Tcf are analyzed to determine what characteristics they have in common and, on the other hand, reasons why some of these fields have unusual characteristics. Giant hydrocarbon accumulations require that there be a giant trap, formed more or less concurrently with generation of the hydrocarbons from the organic source materials, and abundant source beds rich in organic matter. Marine sediments are dominant as source beds, but non-marine beds also preserved the organic materials which supplied the hydrocarbons to many giant fields. Although argillaceous sediments generally trapped more organic matter, carbonates also can be sources. The reservoir rocks of most giant fields are very porous and permeable, but there are notable exceptions; in the exceptions, total reservoir volume compensates for deficiency in reservoir quality. The reservoir rocks must be interconnected with channels of migration, or carrier beds, from the source beds. An effective seal must be present to prevent escape of hydrocarbons from the reservoir; the most efficient seal is provided by evaporites. Evaporites, in addition to sealing many important reservoirs, also may be the primary agent responsible for the development of the structures of many large fields— generally through either diapirism or décollement. Un-conformities have had an important role in some fields in aiding trap development and in bringing carrier beds into juxtaposition with others through which hydrocarbons may migrate. Giant traps caused by lateral facies changes, changes in reservoir matrix, postdepositional dia-genetic changes, and paleogeomorphic factors are few, but their absence does not mean that they are scarce; most exploration has been directed toward the structural types of trap, and searches for other types of traps have been few. Though most giant-field reserves are in Mesozoic and Tertiary rocks, there is no preferred age of source beds or reservoir rocks. The important factor is the time when, during the sedimentational cycle of a basin, the largest amounts of organic matter were buried and preserved. It makes little difference if this time was Cambrian or Pleistocene. Higher than normal geothermal gradients probably resulted in greater efficiency of hydrocarbon generation in certain basins. The sources of gas include the same types of materials that generated liquid oil, but they also include other materials of vegetal origin that do not contribute significantly to the formation of oil. Hence gas can be derived from a greater variety of source materials. Volumes of gas and oil generally are inversely related to increasing depth. Downward increase of temperature results in the "phasing out" of oil and the dominance of gas. Liquids disappear and are replaced by gas in the range 5,640-8,380 m (18,500 27,500 ft); the actual depth depends on the geothermal gradient present in the field under study. The depths given are not necessarily the present overburden thicknesses; many reservoirs are affected by weight of overburden of an earlier geologic time, which today may no long exist because of subsequent uplift and erosion. We present for consideration a basin classification having different spatial relations with ciatons and continental borders, and having different evolutionary development. The type of basin having the largest number of giant fields is the basin which is, or was, a downwarp toward an oceanic area. Intermontane basins, developed either as geosynclinal-type basins between rising geanticlines or in transverse downwarps, generally are smaller in size, but many such basins contain clusters of very large fields. Interior cratonic areas exposed to long periods of erosion and tectonic and/or epeirogenic activity also contain numerous large fields, but a smaller percentage of giant fields. Giant hydrocarbon accumulations require that there be a giant trap, formed more or less concurrently with generation of the hydrocarbons from the organic source materials, and abundant source beds rich in organic matter. Marine sediments are dominant as source beds, but non-marine beds also preserved the organic materials which supplied the hydrocarbons to many giant fields. Although argillaceous sediments generally trapped more organic matter, carbonates also can be sources. During the past 15 years, deeper drilling and exploration of new basins have led to the discovery of an increasing percentage of Mesozoic vs. Tertiary giant fields. The greatest future prospects for discovery of giant fields are (1) in the continental shelves of the world; (2) onshore in Asia; (3) unexplored basins containing mainly continental strata; and (4) all areas of the world where deliberate exploration for obscure traps has not been carried out—i.e., stratigraphic, unconformity-associated, and paleogeomorphic traps.

Abstract Paleozoic rocks range in thickness from 2,800 ft (850 m) near the crest of the Findlay arch to 13,000 ft (3,960 m) at the Ohio and West Virginia border. All systems of the Paleozoic Era are present. Regional dip on top of the monoclinal basement structure is east and southeast, increasing from 40 ft (12 m) per mile in central Ohio to 150 ft (45 m) per mile in southeastern Ohio. Oil and gas are produced from rocks in all Paleozoic systems except the Permian. Most present oil and gas production is from stratigraphic traps in the Silurian sandstones of eastern Ohio and from Cambrian dolomite in the shallow Appalachian basin of central Ohio. Pennsylvanian, Mississippian, and Devonian rocks will continue to be explored but will not add significantly to reserves. New Silurian ''Clinton" fields will be found, as indicated by the recent (1968) discovery of the Clays- vilie field in Guernsey County. Cambrian-Ordovician strata in the deeper part of the basin of eastern Ohio are relatively unexplored and have the greatest potential for adding to reserves. The increasing volume of geophysical and geological information should aid in discovery of the deeper stratigraphic and structural traps believed to be present in the area.

Abstract Fractured crystalline basement reservoirs (basement) on the UK Continental Shelf (UKCS) and the Norwegian Continental Shelf (NCS) have been underexplored. Over the last 12 years, Hurricane Energy has deliberately set out to explore basement potential by exploring and appraising the Rona Ridge, West of Shetland; and the Rona Ridge Lancaster Field is now being progressed towards being the first UK basement field development. The Norwegian basement play is also recognized as a potentially material resource through serendipitous oil discoveries, with the 16/1-15 well, drilled in 2011, being the first successful full-scale basement test on the NCS. Building on this success, the 2018 Rolvsnes appraisal well (16/1-28 S) has demonstrated the significant basement potential of the extensive Utsira High and confirmed the materiality of a Norwegian basement play. The Rona Ridge and Utsira basement discoveries are used as a comparison with two other, yet to be evaluated, prospective basement plays on the NCS, with the objective of establishing technical subject areas where future UKCS and NCS collaboration would aid in accelerating understanding of the UKCS–NCS basement play.