Large Sedimentary Basins Research Articles

Geology of Arctic Islands: ABSTRACT

The Arctic Islands present opportunities for study of the most completely undeveloped potential oil basin anywhere in the Western Hemisphere. Strata are End_Page 635------------------------------ well exposed over a very wide area. As a result, there is a sufficient variety of representative structural features and stratigraphic sections exposed at the surface so that it is possible to establish a reasonably accurate evaluation of the oil prospects of this large sedimentary basin. Normally, in the evaluation of a new sedimentary basin, it is necessary to drill many deep holes to arrive at a fair assessment of the stratigraphic section and the potential oil and gas reservoirs. In the Arctic Islands, however, the exposed stratigraphic sections are so numerous and so conveniently spaced that relatively modest sums of money spent in studying exposed sections can take the place of many millions of dollars spent in acquiring similar stratigraphic detail by drilling. For example, facies studies of the type commonly made between well borings for the purpose of outlining oil- and gas-producing trends can be made in the Islands relatively cheaply by studies of outcrops. As proof of the above concept, it is shown that hydrocarbon seepages are associated with what has been interpreted to be buried reef trends, and that reefoid and other related buried reservoirs can be identified by surface expression of deep-seated features. The writer's conclusion is that surface geology, supplemented with seismic and other geophysical methods, will reduce the number of dry holes drilled on the Arctic Islands far below that normally required in a new-basin area. End_of_Article - Last_Page 636------------

Gravity measurements in the north-eastern part of the Irish Sea

During the summer of 1961 124 new gravity measurements were made in the north (mainly north-east) Irish Sea with an underwater gravity meter. In north-west England, south-west Scotland, and Northern Ireland a regional rise in the Bouguer anomaly is observed on approaching the north Irish Sea; a high regional Bouguer anomaly of more than 40mgal is also characteristic of the Isle of Man. The author considers that this reflects either a crustal thinning of about 3 km or a denser crust (by about0 03g/cm 3 ) beneath the Irish Sea; denser basement rocks may in part be responsible. Over most of the sea area surveyed negative gravity anomalies of 20mgal to 40mgal amplitude are superimposed on the high regional Bouguer anomaly. Both the shape of the anomalies and their correlation with the surrounding coastal geology suggest that they are essentially caused by large sedimentary basins. The two main negative anomalies thus interpreted are ( a ) between the Point of Ayr, the Solway Firth, and Wigtown Bay (thought to be at least 2130 m [7000ft] deep), and (b) between south Cumberland and the Isle of Man (probably at least 2440m [8000ft] deep). Coastal geology suggests that Carboniferous and New Red Sandstone rocks form the major 'fill', although sediments of Old Red Sandstone or post-Triassic age cannot be ruled out. A negative anomaly of nearly 20mgal amplitude forms the south-eastern continuation into Luce Bay of the negative anomaly over the Stranraer New Red Sandstone basin. It closes to the south-east near the mouth of Luce Bay. As on land, it is interpreted as an asymmetrical basin of New Red Sandstone and earlier sedimentary infill with a steep (? faulted) northeastern margin, reaching a maximum depth of at least 1340 m (4400 ft). A single gravity traverse westwards from Peel reveals a sharp gravity drop of about 15mgal immediately off shore; this is interpreted as a sedimentary basin lying off the west coast of the Isle of Man. The Isle of Man thus appears to be an uplifted horst-like region of Lower Palaeozoic rocks with the northwestern and south-eastern coastlines controlled by the margins of steep-sided sedimentary basins of post-Lower Palaeozoic age.

Sulphur Isotope Geochemistry of Petroleum, Evaporites, and Ancient Seas: ABSTRACT

It has now been established that considerable sulphur isotope fractionation occurs in the biological sulphur cycle and that the bacteria] reduction of sulphate, which leads to the enrichment of S34 in sulphate and its depletion in sulphide, is largely responsible for the wide fluctuations in isotope ratio which occur in marine sediments. In this regard, present-day ocean water sulphate is remarkably uniform in sulphur isotope content, both in depth and in geographical location at a value of ^dgr = S34 = +20 (20 parts per mil enriched in S34 with respect to sulphur in meteorites) and provides a base level in isotopic ratio from which fractionation can be reckoned. However, in dealing with ancient sediments and petroleum, we need to know the S34 content of the ancient oceans or seas. Recently (Thode and Monster, 1963) a study of the sulphur isotope distribution in the marine evaporites of some ten sedimentary basins of several continents was carried out. From this study it has been possible to estimate the sulphur isotope ratio for the various ancient oceans and to establish the pattern of change throughout geological time. The pattern of change for petroleum sulphur appears to be parallel with that for the evaporites and ancient seas. However, the petroleum sulphur is, in general, depleted in S34 by about 15^pmil with respect to the contemporaneous gypsum anhydrite deposits. This displacement of ~15°^pmil in the S34 content, which is about the isotope fractionation expected in the bacterial reduction of sulphate, is strong evidence that sea water sulphate is the original source of petroleum sulphur and that it is first reduced by bacterial action in the shallow muds before being incorporated into the petroleum. The lack of any sulphur isotope fractionation in the plant metabolism of sulphate would seem to rule out plant sulphur as a major source of petroleum sulphur. Since the ^dgrS34 values for petroleum pools in a given horizon, e.g., Devonian (D-2), are fairly uniform over a large sedimentary basin and since these values vary from one horizon to another depending on the S34 content of the contemporaneous seas, sulphur isotope studies should be useful in solving migration problems. End_of_Article - Last_Page 2076------------

Geological Outline of Libya: ABSTRACT

Cambrian and Ordovician sandstones and quartzites cover the major part of Libya. During Silurian (Gothlandian) time a generally north-south trend from Tibesti through Gargaf to Garian divided the country in two sedimentary provinces, commonly differentiated as Western-Libya and Eastern-Libya. Western Libya: Divided by a west-southwest--east-northeast ridge in two distinct sedimentary and structural basins which are known to some of the authorities as the and the Mourzouk basins. Marine sediments were deposited in both basins during entire Paleozoic time. At the end of Carboniferous the Hercynic orogeny was very active in the northwestern part of the Gadames basin, where Upper Carboniferous and Permian sediments produced successive transgressions and unconformities. From Upper Carboniferous to Lower Cretaceous gentle subsidence movements governed the deposition in both of the western basins and produced gradation from shallow marine, to lagoonal and to continental deposits. Eastern Libya: The deposition in the eastern portion of Libya was probably active for the greater part of Paleozoic time. Early Paleozoic sediments are preserved in several areas and also in the Kufra basin, to the south. From Permian to Jurassic time this part End_Page 1245------------------------------ of Libya was possibly uplifted with the exception of the Northern Cyrenaican area. The Upper Cretaceous produced a large widespread transgression in all of the northern portion of Libya, over an eroded peneplaned surface. The deposition was thin and of a stable shelf type with the exception of northern Cyrenaica. During Paleocene time Western Libya remained stable; however a tilting in the eastern part of Libya produced a large sedimentary basin in the Syrte area, which continued subsiding until Miocene time with its hinge line along the Hun-Misurata main fracture system. Large portions of Libya are still to be tested by drilling, with the oil strikes localized in general as follows: (1) in the two western Paleozoic basins; (2) in the eastern Syrte Tertiary basin where several unconformities in Upper Cretaceous and Eocene time combined with various movements, resulted in effective favorable conditions for oil accumulation. End_of_Article - Last_Page 1246------------

BASIC THEORY OF THE MAGNETO‐TELLURIC METHOD OF GEOPHYSICAL PROSPECTING

From Ampere’s Law (for a homogeneous earth) and from Maxwell’s equations using the concept of Hertz vectors (for a multilayered earth), solutions are obtained for the horizontal components of the electric and magnetic fields at the surface due to telluric currents in the earth. The ratio of these horizontal components, together with their relative phases, is diagnostic of the structure and true resistivities of subsurface strata. The ratios of certain other pairs of electromagnetic elements are similarly diagnostic. Normally, a magneto‐telluric sounding is represented by curves of the apparent resistivity and the phase difference at a given station plotted as functions of the period of the various telluric current components. Specific formulae are derived for the resistivities, depths to interfaces, etc. in both the two‐ and three‐layer problems. For two sections which are geometrically similar and whose corresponding resistivities differ only by a linear factor, the phase relationships are the same and the apparent resistivities differ by the same proportionality constant which relates the corresponding true resistivities. This “principle of similitude” greatly simplifies the representation of a master set of curves, such as is given for use in geologic interpretation. In addition to the usual advantages offered by the use of telluric currents (no need for current sources or long cables, greater depths of investigation, etc.), the magneto‐telluric method of prospecting resolves the effects of individual beds better than do conventional resistivity methods. It seems to be an ideal tool for the initial investigation of large sedimentary basins with potential petroleum reserves.

Gas for the Future

The growth of sales of natural gas has been one of the outstanding industrial developments of the United States during the past 25 years. During this period, the estimated reserves of natural gas have been increasing at an even more rapid rate. Pipe-line projects approved by the Federal Power Commission indicate a sudden increase in the use of natural gas for the period just ahead. To maintain the comfortable ratio of gas reserves to sales enjoyed by the industry in the past, important additional reserves will have to be discovered. The large sedimentary basins, remnants of more extensive areas of deposition, are the loci of geological conditions necessary for important accumulations of oil and gas. In the successful exploration of these basins, the role of the geologist in interpreting well data for the correlation of deeply buried strata far from outcrop and in directing exploration to areas of favorable depositional facies of potential reservoir beds will be of the utmost importance. The status of exploration and prospects for further discoveries of 33 important basin areas in the United States are briefly discussed in the paper. On the basis of available data, of those basins which are adequately explored so that geologic conditions are fairly well established, the Anadarko, Midland, and Gulf embayments offer the greatest promise of discovery of substantial reserves. Of these, the Gulf embayment, defined as including the area from beyond the Rio Grande to the east side of the Mississippi delta and including the tidelands area, is believed to have the greatest prospects. Accordingly, it appears that in the future, as at the present time, more than half of the gas reserves of the United States will be in fields situated in Texas.

Gravity anomalies due to extensive sedimentary beds

Research Article| October 01, 1944 Gravity anomalies due to extensive sedimentary beds CLAUDE W. HORTON CLAUDE W. HORTON Search for other works by this author on: GSW Google Scholar GSA Bulletin (1944) 55 (10): 1217–1228. https://doi.org/10.1130/GSAB-55-1217 Article history received: 10 Sep 1943 first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation CLAUDE W. HORTON; Gravity anomalies due to extensive sedimentary beds. GSA Bulletin 1944;; 55 (10): 1217–1228. doi: https://doi.org/10.1130/GSAB-55-1217 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu nav search search input Search input auto suggest search filter All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Isostatic anomalies result from various two-dimensional structures such as large sedimentary basins and geosynclines. It is shown that a basin should be surrounded by a “halo” of positive anomalies and that the areal extent of the sedimentary layers and the dip of the lower contact are more important than thickness in producing the isostatic anomaly. The computations are applied to a coastal plain in order to interpret Vening Meinesz' findings that on many offshore profiles the anomaly increases algebraically away from the coast. Considerable computations are necessary before one can determine the amount of time lag between the deposition of sediments and the attainment of complete isostatic compensation. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not currently have access to this article.

Outline of Chinese Geology

The structural framework of Greater China consists of three positive areas and the geosynclines that separate and enclose them. These are compared with analogous features of eastern North America. A thick succession of Paleozoic formations accumulated in a great trough which extended northeast and southwest across China proper. At the end of Ordovician time this trough was restricted on the north and Silurian, Devonian and Mississippian strata are unrepresented in north China. The Pennsylvanian and Permian seas, however, again extended into that area. Eastern Asia was uplifted at the end of Triassic time and subsequent sedimentation was almost entirely non-marine. Mountain-forming movements, which began in mid-Pennsylvanian time and have continued with modifications to the present, produced a series of shifting sedimentary basins in which all later formations accumulated. The structure of central Asia is dominated by curving east-west mountain arcs but the coastal region is characterized by northeast-southwest trending structures. Throughout much of China proper these two systems have produced a complicated interference pattern. Diastrophism that affected central and eastern Asia since the Paleozoic era was so profound that most parts of China are intensely folded and faulted. Five large sedimentary basins are present, however, two in Chinese Turkistan, one in Tibet, and two in western and northern China proper.