Large Fault Blocks Research Articles

Surface‐to‐borehole fault imaging in a buried horizontal waveguide—An experimental study in the southern North Sea

One fifth of the United Kingdom’s energy is derived from the gas fields of the southern North Sea. The area is divided into large fault blocks, which are laced with minor faults. Surface reflection seismic fault maps of the main producing horizon are distorted by salt rollers or pillows in the overlying high velocity Zechstein evaporites. We have developed an inexpensive method of confirming the presence of minor faults in the relatively flat base of the Zechstein around a well that would simplify production planning. Deep well sonic logs indicate that seismic velocity drops abruptly beneath the base Zechstein and then rises gradually with depth. This profile is characteristic of a whispering gallery, or a buried, variable‐index beam waveguide. If base Zechstein faults scatter energy efficiently into the waveguide, then the faults might be mapped by moving a seismic source over the surface and recording the mode converted arrivals at a geophone within the guide. To test this possibility, over 3000 shots were fired at 27-m intervals in thirteen lines spaced by ≈500 m from a 4436-cu in. (0.075-cu m) marine air gun array into a 3 km deep triaxial VSP geophone, stationed beneath the Rotliegendes. Secondary arrivals were found to move out across the raw field geophone gathers in the manner expected of down‐going compressional to both guided compressional and guided shear vertical mode conversions [P-P and P-SV] at the base of the Zechstein. Prestack migration yielded a map of mode conversion activity at the edges of known faults with throws of 30 + m over an area of [Formula: see text]. We conclude that there is an extensive seismic waveguide beneath the Zechstein in the southern North Sea. Faults scatter down‐going P waves efficiently into the guide; noise levels are low at the bottom of a borehole. Vertical reverberation follows a surface source as it moves away from the wellhead. Horizontal waveguiding reduces geometric spreading loss. These factors make it possible to map small reservoir faults with high‐resolution at long ranges from an exploration well.

Footwall uplift associated with Late Jurassic normal faulting in the northern North Sea

Tilted fault-blocks in the North Sea show varying degrees of erosion. Such footwall erosion is predicted by the domino model of extension, in which rigid fault-blocks rotate as extension proceeds. Profiles of three North Sea fields (Snorre, Brent and Heather) are analysed to estimate the amount of uplift that accompanied Late Jurassic normal faulting, and these estimates are compared with the footwall uplift predicted by the domino model. Agreement between observed and predicted uplift is excellent, varying from >1 km at Snorre to c . 100 m at Heather. For a given amount of extension, large fault-blocks tend to be bounded by large faults and to show a greater degree of erosion. The Brent Province of the northern North Sea is an area of prolific hydrocarbon discoveries. Almost all of the major oil-fields occur in tilted fault-blocks which were formed by Late Jurassic extension and subsequently buried by Cretaceous-Tertiary thermal subsidence ( Badley et al. 1988 ; Marsden et al. 1990 ). The crests of the tilted blocks are typically eroded or degraded, with varying amounts of Middle-Lower Jurassic and Upper Triassic section removed. This footwall erosion is commonly ascribed in the literature to pre-rifting doming ( Ziegler 1982 ), heterogeneous stretching ( Coward 1986 ), local transpression ( Beach 1985 ) or even regional compression ( Frost 1987 ). The purpose of this paper is to show that the uplift and degradation is a direct consequence of the Late Jurassic normal faulting and fault-block rotation. The degree of uplift is related in a simple way to the size (across strike) of

Tectonic and volcanic events at the Jan Mayen Ridge microcontinent

Summary Two main tectonic phases were responsible for the formation of the Jan Mayen Ridge microcontinent: (1) the opening of the Norway Basin in late Palaeocene/early Eocene times, and (2) subsequent rifting within the Greenland margin by which complete separation was achieved in early Miocene times. During the first phase the eastern ridge flank developed as a volcanic passive margin. The initial break-up was associated with flexuring and the formation of sequences of eastward-dipping basalt flows, which are considered equivalent to similar features beneath the Vøring and Faeroe-Shetland marginal highs off Norway. Rifting along the Greenland margin during the second phase was accompanied by uplift, listric normal faulting and the formation of large extensional fault blocks. To the W and S of the ridge a flat volcanic marker of probable earliest Miocene age covers the subsided rift and masks the ocean-continent transition. It was formed by a volcanic event of large magnitude, either as submarine lava flows or as a sill complex.

TRAPPING MECHANISMS AND THE HYDROCARBON POTENTIAL OF THE EXMOUTH PLATEAU, WESTERN AUSTRALIA

An interpretation of the structural configuration and hydrocarbon potential of the Exmouth Plateau has been made utilizing the results of G.S.I's 1976 Scientific Investigation (No. 10 S.L.).Seismic sections suggest sediments down to at least four seconds (two way time) below the sea floor. A prominent regional unconformity separates faulted and dipping sediments from generally flat overlying beds.Eight horizons of apparent regional importance were picked and correlated to existing well control.Structure maps were prepared for the regional unconformity and three overlying horizons, including the interpreted top of the Barrow Group. Mapping showed the main potential hydrocarbon traps to be large fault block features associated with the regional culmination of the unconformity event, and broad anticlinal drapes at top Barrow Group level. Geological interpretation suggests that the fault blocks consist of late Middle to Upper Triassic sediments, probably sandstone and shale. These sandstones could be potential reservoirs for hydrocarbons generated at depth. Seismic evidence of hydrocarbon generation includes a probable gas chimney associated with the western bounding fault of the highest fault block.The present sea floor topographic high of the Exmouth Plateau is coincident with the large drape structure embracing a foresetting package within the Barrow Group. Velocity studies over the Plateau suggest a probable lack of sand in the upper part of the Barrow Group section. Sourcing of sands in the lower part is envisaged as originating from sediments in older, underlying fault blocks. The Triassic fault blocks are considered to have the higher potential for oil reserves, but recoverable volumes are not likely to be in the 'giant' category (500 million barrels).

Magnetic anomaly patterns on Mid-Atlantic Ridge crest at 26°N

Magnetic anomalies over ocean crust with ages between 0 and 3.5 m.y.B.P. have been identified on the Mid-Atlantic Ridge crest at 26oN. These anomalies indicate a spreading rate of 1.1 cm/yr on the west flank of the ridge and 1.3 cm/yr on the east flank. Computer modeling of the anomalies has shown that simple blocks of uniform thickness, whose width and direction of magnetization are obtained from the reversal time scale and whose intensities are chosen for best visual fit of the anomaly, are not capable of explaining the details of the observed magnetic anomalies. Shorter-wavelength variation in the intensity of magnetization parallel to the ridge crest can, however, explain the observed anoma- lies. A possible degradation of the intensity of magnetization within the axial anomaly is associated with large faulted blocks in the wall of the rift, which is the site of the Trans-

Deep-tow studies of the structure of the Mid-Atlantic Ridge crest near lat 37°N

A detailed study of the structure of the Mid-Atlantic Ridge median valley and rift mountains near lat 37°N (FAMOUS) was conducted using a deep-tow instrument package. The median valley may have either a very narrow inner floor (1 to 4 km) and well-developed terraces or a wide inner floor (10 to 14 km) and narrow or no terraces. The terraces appear to be non–steady-state features of the rift valley. The entire depth and gross morphology of the median valley may be accounted for by normal faulting, while volcanic relief contributes to the short-wavelength topography (<2 km). Most faults dip toward the valley axis an average of 50°, and the blocks are tilted back 2° to 3°. Fault dip is asymmetric about the valley axis. Active crustal extension in the inner floor and inner walls has the same sense of asymmetry as the local spreading rates, reaching a maximum of 18 percent. Thus, asymmetric spreading appears to be accomplished by asymmetric crustal extension on a fine scale as well as by asymmetric crustal accretion. Spreading is 17° oblique to the transform faults and shows no indication of readjusting to an orthogonal system, even on a fine scale. Eighty percent of the decay or transformation of median-valley relief into rift-mountain topography is accomplished by normal faults that dip away from the valley axis. Most of the outward-facing faulting occurs near the median-valley–rift-mountain boundary. Tilting of crustal blocks accounts for only 20 percent of the decay of median-valley relief. Most long-wavelength topography in the rift mountains has a faulted origin. As in the median valley, volcanic relief is short wavelength (<2 km) and appears to be fossil, originating in the median-valley inner floor. Bending of large faulted blocks toward nearby fracture zones suggests that spreading-center tectonics is affected by fracture-zone tectonics throughout the length of the rift in the FAMOUS area. Both the crustal accretion zone and transform fault zone are narrow, only 1 to 2 km wide, over short periods of time. In the course of millions of years, however, they apparently migrate over a zone 10 to 20 km wide.

Pre-Pennsylvania Paleotectonics--Key to Basin Evolution and Petroleum Occurrences in Paradox Basin: ABSTRACT

A large northwest-trending fault block composed of late Precambrian through Mississippian rocks is exposed in the core of the San Juan Mountains near Silverado, Colorado. The fault block was formed prior to Ignacio (Late Cambrian) time when younger Precambrian quartzite were extensively down-faulted into the older Precambrian basement complex. The structure stood as a high topographic feature during Ignacio to Late Devonian time, but was large buried by the upper Elbert Formation. Renewed activity occurred in Ouray (latest Devonian or earlier Mississippian) time, when tidal flats developed on the high flanks of the fault block while normal marine waters moved into the graben. The entire structure was high during the Early Mississippian, for the Leadville Formation is pres rved only as tidal-flat dolomites and weathering residual blocks within the regolithic Molas Formation above. Pennsylvania and later movement occurred along the graben, as Hermosa and Cutler strata are now involved in the graben. With this paleotectonic feature as a model, other areas are more readily understood. A similar ancient fault block is present south of Ouray, Colorado, and extends northwest into the subsurface of the eastern Paradox basin. This structure joins a major northwest-trending pre-Pennsylvanian fault system that flanks each of the major salt anticlines which parallel the adjacent Uncompahgre uplift. Isopachous and lithofacies studies reveal that these structural lineaments were already present in Late Cambrian time, and actively controlled sedimentation through Mississippian time. It is possible that the closely related Uncompahgre uplift had a similar early history. Pre-Pennsylvanian reservoir facies are best developed along the high flanks of the faults. Late Devonian McCracken sandstones occur in linear bars along the structures, and crinoidal biogenic banks, which are associated with all Leadville production, also occur on the shallower structural flanks. Where paleotectonic relief was too high, however, pre-Pennsylvanian rocks are missing either because of non-deposition or subsequent erosion. The down-faulted paleotectonic troughs were the site of thick Pennsylvanian salt deposition. When Middle Pennsylvanian to Early Permian clastic wedges from the Uncompahgre uplift initiated salt flowage by differential loading, the fault blocks acted as buttresses which deflected the plastic salt upward. Consequently, the salt anticlines grew along the linear trends created by the Precambrian through Mississippian faults. End_of_Article - Last_Page 1572------------

STRUCTURAL TRENDS IN CENTRAL WESTERN WYOMING

Research Article| January 01, 1949 STRUCTURAL TRENDS IN CENTRAL WESTERN WYOMING LELAND HORBERG; LELAND HORBERG UNIVERSITY OF CHICAGO, CHICAGO, ILL.; UNIVERSITY OF KENTUCKY, LEXINGTON, KY.; SHELL OIL COMPANY, BAKERSFIELD, CALIF. Search for other works by this author on: GSW Google Scholar VINCENT NELSON; VINCENT NELSON UNIVERSITY OF CHICAGO, CHICAGO, ILL.; UNIVERSITY OF KENTUCKY, LEXINGTON, KY.; SHELL OIL COMPANY, BAKERSFIELD, CALIF. Search for other works by this author on: GSW Google Scholar VICTOR CHURCH VICTOR CHURCH UNIVERSITY OF CHICAGO, CHICAGO, ILL.; UNIVERSITY OF KENTUCKY, LEXINGTON, KY.; SHELL OIL COMPANY, BAKERSFIELD, CALIF. Search for other works by this author on: GSW Google Scholar GSA Bulletin (1949) 60 (1): 183–216. https://doi.org/10.1130/0016-7606(1949)60[183:STICWW]2.0.CO;2 Article history received: 28 May 1941 first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation LELAND HORBERG, VINCENT NELSON, VICTOR CHURCH; STRUCTURAL TRENDS IN CENTRAL WESTERN WYOMING. GSA Bulletin 1949;; 60 (1): 183–216. doi: https://doi.org/10.1130/0016-7606(1949)60[183:STICWW]2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The area extending southeastward from the south end of Teton Basin, Idaho, to the northern part of Green River Basin, Wyoming, exhibits two types of Laramide structure modified by Tertiary high-angle faults. Included within the area are parts of the Teton, Gros Ventre, Hoback, and Snake River ranges.Structure in the Teton and Gros Ventre ranges consists of broad, asymmetrical anticlines, a major northeast-dipping thrust fault, and three large fault blocks. Structures in the Hoback and Snake River ranges include close folds of Appalachian type and low-angle, southwest-dipping thrust faults. Initial Teton-Gros Ventre structures probably were formed during an early phase of Laramide orogeny and, as buttresses, controlled the trend and character of the later Hoback-Snake River structures. Laramide movements appear to have begun in the Late Cretaceous and continued at least into late Eocene time. Tertiary faults of Basin-Range type and with north-south trends transect older structures and are responsible for the east scarp of the Teton Range, small fault blocks in Jackson Hole, and the western boundary structure of the northern Hoback Range.Regional stratigraphic relations had an important effect on both the trend and type of Laramide deformation, but this influence cannot be recognized in the Basin-Range faulting. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Notes on Rocky Mountain Thrust Faults: ABSTRACT

The low-angle thrust faults of the central Rocky Mountains are of two types. The faults along the Idaho-Wyoming border were formed when the exceptionally thick Paleozoic and Mesozoic strata of the Cordilleran geosyncline were intensely compressed during End_Page 132------------------------------ the Laramide revolution. Movement on these faults, which do not involve the crystalline rocks of the basement complex, is believed to have been dissipated at depth by shearing along bedding planes. The name geosynclinal thrust fault is proposed for faults of this type. In contrast, the low-angle thrust faults of central Wyoming and southern Montana were produced by the further compression of large faulted blocks previously uplifted during an earlier stage of the Laramide revolution. The crystalline cores of these uplifts contributed large quantities of detritus to the Paleocene strata forming the foot-wall blocks of some of the thrusts. A study of the structures of the basement complex indicates that the present uplifts occupy the sites of pre-Cambrian mountain ranges. Hence, this group of thrust faults borders areas that have been recurrently structurally positive throughout geologic time. The name geanticlinal thrust fault is proposed for thrust faults of this type. End_of_Article - Last_Page 133------------

Fault Troughs

Fault troughs may be classed as superficial or profound. Superficial fault troughs result from (1) earthquakes, or (2) volcanic activity, or (3) tension. Tensional fault troughs are due to differential uplift, rotational strain, or the tilting of large fault blocks. The differential uplift may be caused by igneous intrusions and by the relative displacement of large rigid fault blocks underlying sedimentary formations. The faults associated with very large fault troughs extend to profound depths and reach the zone of flowage. Evidence bearing on their origin is less conclusive than that for superficial troughs. Several factors have probably entered into their formation, but tension, resulting in normal faulting and an upward tilting of the inclosing masses, is probably most important. Extrusion of lavas and increase in density, due to expulsion of gases and the crystallization of magmas, have played a more or less important part in some instances. Large troughs have probably developed in part along pre-existing faults. Illustrations are given of the different types of fault troughs.

Geology and the Petroleum and Natural Gas Possibilities of Palestine and Sinaitic Peninsula

The presence of asphalt and oil showings in the vicinity of the Dead Sea and near Hasbeya, west of Mount Hermon, has long been known; and these, together with the asphalt-rock occurrences, have led many to believe commercial oil deposits might be found in Palestine. The so-called asphalt rock or bituminous limestone has no relationship to the occurrence of oil. The asphalt and oil seepages, where present along fault lines, signify nothing as regards commercial oil where they appear, except as they are suggestive that, in localities where traps occur which have not been disturbed by faulting, oil may be present in commercial quantity. Although all of Palestine is faulted into large fault blocks, within such blocks there are thirty-seven structural domes which might act as raps for oil and gas; but where igneous intrusions occur with such structures, it is less likely that they will produce. Promising domes occur in Palestine west of the Jordan and the Dead Sea, with the possible exception of the basalt-covered areas. From the Jordan and the Dead Sea eastward to the Hedjaz Railway, because of the basalt intrusions and exposure of lower beds by uplift, there is very little promise, though some folds are probable in this area. Of a total of 12,700 square miles in Palestine proper, more than 300 square miles offer structural conditions warranting tests for oil and gas. This offers an unusually high percentage of possible oil-bearing area, considering the small size of the state--about the size of Vermont. The Sinaitic Peninsula also offers possibilities. The formations range from Eocene to Precambrian. Cretaceous limestones and marls form the great bulk of the beds to be penetrated, and next below is the Nubian sandstone and several hundred feet of Carboniferous sandstones, limestones, and shales. There are eight possible oil horizons, half of which have a good chance to produce. With ample reservoir beds, with ample source material, and with good structural traps, we see every reason for the development of an oil supply in Palestine. We are anxious to see this development undertaken because it would speed the upbuilding of Palestine as nothing else can.