Areas Of Structural Complexity Research Articles

Quantitative Evaluation of Dynamic Time-Shifts for Correcting Seismic Data

Large lateral velocity changes, particularly those associated with sea-floor topography, can severely distort seismic stack sections, for example, under the submarine canyons in the Gippsland Basin in Bass Strait. Conventional replacement statics partially correct these problems, but are usually inadequate in situations where the sea-floor variation is severe or the water is deep. Dynamic corrections calculated by ray-tracing through the region of strong lateral velocity variation give improved results.Dynamic corrections derived by ray-tracing can be evaluated by analyzing t2 -y2 plots where t is time and y is offset within CMP gathers. This facilitates making the following comparisons: (1) the zero-offset time before and after dynamic correction; (2) the RMS velocities (calculated using zero-offset rays traced in the model) with normalized stacking velocities after dynamics; (3) hyperbolic moveout before and after dynamics. A similar evaluation of conventional replacement statics provides a reference for evaluating the ray-tracing derived ‘replacement dynamics’.Ray-tracing requires a subsurface model which can be difficult to construct in areas of structural complexity. This is often the case near submarine canyons. A simple model using water depth and a single marker horizon or datum when ray-traced to derive replacement dynamics gives significant improvement with respect to the conventional statics solution in submarine canyon areas. Errors in velocity can be reduced by as much as 50% and errors in hyperbolic moveout reduced by about 70%.

A test of airborne, side-looking synthetic-aperture radar in central Newfoundland for geological reconnaissance

Side-looking, C-band synthetic-aperture radar (SAR) penetrates cloud and fog, and operates day or night, to produce pseudo-three-dimensional terrain images with enhanced topography and surface roughness. The images, which have a 20 m resolution and cover large areas, have been used to map the regional trends, patterns of lineaments, and terrain types over a 6200 km2 area of complex lithology, structure, and drift cover. Four lineament classes are differentiated. Glacial trends are clear, and bedrock structures (faults, fractures, joints, foliation, and folded bedding) with relief expression at the surface show through the drift as lineaments. They accurately reproduce most known features when compared with bedrock and Quatenary geology maps. Hitherto unrecognized structural elements are revealed. Tones and textures reflect minute surface roughness variations useful in terrain classification. SAR wide-swath-mode imagery is thus a valuable complement to aerial photography, and is superior in revealing hummocky moraine, ribbed moraine, boulder fields and stony till. Wider use of this imagery is encouraged.

Use of High-Altitude Color Infrared Imagery in Structural Mapping of Monument Spring Area, West-Central Marathon Uplift, Brewster County, Texas: ABSTRACT

The Monument Spring area of the Marathon uplift, was analyzed using a Kelsh plotter and NASA high-altitude color infrared (CIR) imagery. Structurally, the Marathon uplift is a broad dome from which the Cretaceous cover has been eroded, exposing the Paleozoic structures. These Paleozoic rocks, ranging in age from Late Cambrian to Pennsylvanian, are complexly deformed and exhibit a variety of structural attitudes. The Marathon region was originally mapped extensively by P. B. King in 1937. The Monument Spring area contains two of the most prominent structural features described by King: the Marathon anticlinorium and the Pena Colorada synclinorium. These features are characterized by tight folds and thrust faults striking in a northeasterly direction. North-trending shears are also found within the area. Although there is a general agreement between present maps and King's original interpretation, the use of high-altitude specialized photography and quantitative Kelsh data provides additional information on the structural complexities of the area. This information, derived from the high-resolution model (obtained with the Kelsh Plotter) is perhaps not readily apparent in the field. The interpretations derived from this information are outlined in the detailed mapping of the Rock House Gap, Sunshine Spring area, and one additional area southeast of Monument Creek. End_Page 1438------------------------------ The high-resolution, infrared imagery and analytical photo-grammetric data in combination with more traditional geological information has proven to be a useful tool in this complex structural area. End_of_Article - Last_Page 1439------------

Seismic Reflection and Mineral Prospecting: ABSTRACT

Prospecting for minerals becomes increasingly difficult when targets lie at depth, or where basement expression is masked by thick sedimentary cover. Geophysical techniques such as gravity and magnetics are generally used as aids in reconnaissance, but rarely yield unambiguous solutions, while electrical methods suffer because the often highly conductive overburden limits current penetration. Recent advances in extending the resolving power of the seismic reflection method suggest that it is appropriate End_Page 977------------------------------ now to consider its use in mineral prospecting. Its applications can include indirect ore-detection methods such as: tracing intrabasement horizons; qualitative interpretation of zones, using or adapting ideas of seismic stratigraphy; mapping structural features such as faults or folds; interpolating between or extrapolating from existing drill holes. Direct detection may also be possible by using bright or dim spot techniques, or because some orebodies have characteristic reflection and diffraction responses. In contrast with petroleum exploration, where variations in velocity are taken as a guide to reflection response, the evidence indicates that density contrast is more likely to be the governing factor. Thus, for example, increasing substitution of pyrrhotitic ore into a country rock consisting of siltstone does not substantially alter an intrinsic rock velocity of 5.5 km/s. The density, on the other hand, may change from 2.7 to 4.5 t/m3. This is in agreement with the known relationship between velocity, density and mean atomic weight. Therefore, in metamorphic, igneous or mineral-bearing rocks, where porosity is low and exists mostly as microcracks, it is the variations in density which occur in predominantly monomineralic layers which may contribute to a significant reflec ion response. The thicknesses of such bands, the relative sizes of the targets and the degree of resolution sought of structural features requires the use of high resolution techniques and the recording of frequencies in excess of 200 Hz. Modeling of the responses of known orebodies confirms the notion that some may have characteristic seismic response. It is a useful approach which assists in the design of field surveys, particularly when one seeks to avoid spatial aliasing problems in areas of steep dip and structural complexity. Processing techniques suitable for a particular area may also be examined using modeling techniques. The use of common-depth-point stacking is inappropriate in structurally complex areas. However, as some form of stacking is necessary to yield adequate levels of signal above noise, the application of so-called slant-stacking techniques should be considered. These also alleviate problems associated with offset-dependent waveforms and reflection coefficients. Examples from various areas in Australia illustrate these concepts. End_of_Article - Last_Page 978------------

Discovery and Development of Nembe Creek Oil Field, Nigeria: ABSTRACT

The discovery well for the Nembe Creek field in the coastal swamp of the Niger delta was drilled in 1973. Average recoverable reserves have risen to over 600 million bbl after the drilling of 30 wells. Nearly one-third of the wells drilled have been proposed on the basis of direct reflection seismic support, principally in the form of seismic structure mapping and cross sections. In addition, lateral predictions on the basis of true amplitude impedance data have been made for two appraisal wells. The timely acquisition and interpretation of sufficient seismic lines in an area of complex structure, but good reflection quality, have permitted the drilling of long step-out appraisal wells, leading to early delineation of field limit and rapid growth of proved reserves. End_of_Article - Last_Page 502------------

Petroleum Source Beds on Continental Slopes and Rises

Continental slopes commonly are sites of high marine organic productivity and frequently contain reducing bottom conditions, quiet water, and intermediate sedimentation rates, all of which favor deposition of organic-rich sediments. These deposits typically have high percentages of aquatic organic matter with high petroleum yields as contrasted to relatively organic-lean shelf deposits which contain primarily low-yield terrestrial organic matter. Conversion of organic matter in potential source beds to oil and gas requires a combination of temperature and time. These variables are controlled primarily by the geothermal gradient, the rate of burial, and the age of the source interval. Most divergent margins need between 2 and 4 km of overburden for oil generation and from 3 to 7 km for gas generation. Typically cooler and younger convergent margins and deltaic margins must have even greater burial depth to achieve the same results. Continental margins, including present slopes and rises, can contain oil and gas source beds when minimum requirements of organic content, kerogen type, and thermal maturity are met. Migration and accumulation are most efficient, however, where reservoir sequences prograde over source beds in areas of structural complexity. Preservation of trapped petroleum requires effective seals and minimal structural readjustment after accumulation. All these conditions can be found on present slopes and rises, although they are not common, and must be considered as part of any economic evaluation of these largely untested deep-water realms.

Stereographic Technique for Determination of Minor Fold Sense

A method using Stereographic projection permits accurate determination of minor fold sense in areas of structural complexity. This technique provides a simple and flexible field method to supplement routine mapping methods through structural analysis.

Regional Precambrian Tectonics and Stratigraphy of Rocky Mountains with Emphasis on Wyoming Province: ABSTRACT

The most ancient rocks of Precambrian age in the Rocky Mountains are in the Wyoming province. These rocks are in Wyoming and adjacent parts of Montana and South Dakota, and may include rocks older than 3 b.y. The older rocks of this province probably are granitized metasedimentary rocks that have been deformed severely and show no evidence of a regional structural pattern. Younger metasedimentary rocks, including iron formations, are in northeast-trending synclinoriums, but even these units are older than 2.5 b.y. Most areas studied in the Wyoming province have had an extraordinarily complex structural history and many areas show evidence of more than one episode of deformation; several areas have northwest fold systems and shear zones that disrupt earlier structure. The Wyoming province may be bordered on all sides by younger rocks or rocks affected by deformation after 2.5 b.y. Miogeosynclinal rocks of possible middle Precambrian age are along the southeastern border of the province. These rocks may be facies of metasedimentary rocks in Colorado that have been metamorphosed in an event of ~1.7 b.y. Evidence for middle Precambrian metamorphic events also has been found in South Dakota, Nebraska, and Montana. These events cannot be related to regional tectonic trends. For example, Colorado structural trends are east-northeast and northeast, whereas in the Dakotas and Nebraska trends are north-northwest. Precambrian rocks that are younger than middle Precambrian are primarily west and southwest of the Wyoming province. These include Beltian metasedimentary rocks in Idaho and Montana, and probable Beltian and younger sedimentary rocks in Utah and Nevada. In some areas Beltian metasedimentary rocks have been deformed several times; north-northwest to northwest structural patterns appear in many areas. Some Beltian metasedimentary rocks are probably older than ~1.2 b.y. Although fragmentary evidence based on partial mapping of the 10% outcrop area of Precambrian rocks in the Rocky Mountains does not allow establishment of firm regional tectonic patterns or make satisfactory correlations of ages and tectonics, evidence to date suggests a remarkably complete sequence of Precambrian rocks may be present from rocks older than 3 b.y. to rocks of late Precambrian age. End_of_Article - Last_Page 1902------------

La Salle Anticlinal Belt, Illinois: ABSTRACT

The La Salle anticlinal belt in Illinois extends from Rock River in the vicinity of Oregon, Ogle County, south-southeasterly to Wabash River near St. Francisville in Lawrence County, a distance of about 260 miles. The north end of the La Salle anticlinal belt is in an area of complex structure in which several structural axes converge. At the south it fades out into the Indiana part of the Illinois structural basin. The La Salle anticlinal belt is the site of one of the major oil accumulations of this country, the Southeastern Illinois oil field, located along its southernmost 65 miles. Total production to date has been 458,000,000 barrels of oil. The present production rate after 42 years of production is approximately 4½ million barrels a year. Approximately 19,307 oil- or gas-producing wells have been drilled in a total oil-producing area of 97,435 acres; of these approximately 10,878 wells are still producing. The principal producing sands are Mississippian and Pennsylvanian in age. Small amounts of oilhave been obtained from Devonian and Ordovician limestones in limited areas. Numerous dry holes have been drilled to the Devonian and deeper formations; a few reached the St. Peter sandstone. It seems probable that the La Salle anticlinal belt has already had the greater part of its drilling development. The most promising future possibilities appear to be for stratigraphic traps on the flanks. Secondary-recovery methods, both repressuring and water-flooding, which are yielding substantial quantities of oil in limited areas of the Southeastern Illinois field, have great possibilities for future expansion. End_of_Article - Last_Page 301------------