Subsurface Geological Features Research Articles

Evaluation of digitally processed geophysical data sets for the analysis of geological features in northern Ireland

Abstract The efficacy of conventional remotely-sensed satellite imagery for regional geological mapping is greatly reduced in northern Ireland due to the extensive vegetation cover. Imagery produced from digitally processed geophysical data sets is an extremely powerful technique for the acquisition of geological information in such terrain. Directional filtering of gravity and aeromagnetic data allows the major structures to be delineated clearly. Pseudo-illumination from the northwest shows the main Caledonian structures: Tempo-Sixmilecross Lineament; the Omagh Fault; the continuation of the Southern Uplands Fault and the en-echelon character of the Fair Head-Clew Bay Lineament. A synergistic display of the geophysical data sets shows the major lithological boundaries, variations within individual lithologies and subsurface geological features.

Areal 3-D Seismic Technique for Reservoir Delineation: Case History from Offshore Niger Delta

In the 1950s, early exploration period in the Niger Delta witnessed the use of 2-D (two dimensional) seismic reflection method which adequate for imaging large subsurface geologic features including growth faulting and roll-over anticlines. This technique involves the Common-Depth-Point method (CDP) which acquires a plane of seismic information in distance along the surface and in time into the geological section, and is used to improve the signal-to-noise (S/N) ratio, to remove multiples and consequently give a representation of the subsurface particularly if the data are collected up- or downdip. By mid-1980s, the obvious geological structures have, in general, been discovered and it became necessary to adopt a more sophisticated technique such as the 3-D (three dimensional) seismic method to delineate more subtle reservoirs and resolve complex fault patterns in order to aid exploration as well as facilitate efficient field development. The case history discussed in this paper involves the use of areal 3-D seismic method for delineating the reservoir characterization of the O-field located in a shallow water area of the western Niger Delta. The areal 3-D seismic technique is superior to the earlier CDP method in that a cube of seismic data can be collected in two dimensions inmore » space and one in time by a variety of techniques including the swath seismic shooting pattern adopted for gathering the 3-D data for the O-field's reservoir which involves the line of sources. The objective is to adequately sample the subsurface so that changes in various parameters such as the amplitude phase or power in the siesmic signal or velocity of propagation can be mapped areally and interpreted as an indication of changes in the physical properties of the rock matrix.« less

Hydrothermal vents in turbidite sediments on a Northeast Pacific spreading centre: organisms and substratum at an ocean drilling site

The interaction of substratum, organisms, and hydrothermal venting was studied at Middle Valley on the northern Juan de Fuca Ridge, where venting occurs through turbidite sediments. Presently, hydrothermal activity in Middle Valley is concentrated within the 300 × 800 m High Heat Flow Area vent field, where it is associated with 10–15 m high sulphide mounds topped by active chimneys. Information on the biological nature of the area was derived from video and photographic records of four 1990 dives by the submersible Alvin, samples collected by Alvin, and bottom photographs by a towed camera. Fieldwork was carried out prior to commencement of 1991 Ocean Drilling Program activities at this site. Despite the limited nature of the collections, the Middle Valley fauna is remarkably specious. We documented 18 vent species new to the Juan de Fuca Ridge. Most interesting were the bivalves, which are rare elsewhere on the ridge. Two new species of vesicomyid clams and a new mussel species were present in the collections, as were a previously known mussel (Idasola washingtonia) and the gutless bivalve Solemya johnsoni. Other notable features are the close proximity of vent and non-vent organisms and the evident influence of surficial and subsurface geological features on venting and organism distribution. The number of new species and the small area occupied by vent sites led us to recommend several precautionary measures to the 1991 drilling program to prevent adverse effects on the vent fauna.

Interpretation of complex structure by 3-D seismic modeling

Images are often used to identify objects. Thus the photograph in a passport serves to identify the true holder of the passport. Ideally, a geophysicist would like to devise a similar procedure aimed at identifying the subsurface geologic features from their image based on seismic measurements. Various methods of seismic migration have come a long way toward accomplishing this goal. However, none of the methods yields satisfactory results in the presence of lateral variations in velocity. This is true even when we assume the earth is a 2-D medium. For 3-D variation of velocity, which is frequently encountered in association with the occurrence of hydrocarbon deposits, the problem of imaging becomes much more involved.

Computer Graphics as a Tool for Correlation of Thematic Mapper Satellite and Subsurface Geologic Data in Hydrocarbon Exploration: ABSTRACT

Computer technology, particularly high-speed graphics workstations, have made it possible to graphically combine and visualize large data bases and multispectral satellite data. This presentation focuses on a tool that attempts to extract information concerning spectral components of satellite data by graphically correlating raster-format satellite data with vector-format maps of surface and subsurface geologic data. Thematic mapper (TM) satellite data have been graphically correlated with topographic data and subsurface data from maps constructed using Petroleum Information (PI) data for the Patrick Draw area, southwestern Wyoming. The study area is on the eastern margin of the Rock Springs uplift and extends into the northern Washakie basin and southern Red Desert basin. Subsets of the TM satellite data were selected for a 40-km/sup 2/ area around Patrick Draw, Table Rock, and Desert Springs fields. Formation tops were extracted from the PI data base and were used to create structure contour maps of the region. Perspective plots and block diagrams were constructed to show interrelationships of surface and subsurface geologic features. Imate profiles were taken through each spectral band of TM satellite data, and cross sections were constructed along identical sections from subsurface data. The image profiling technique facilitates correlation and identification of componentsmore » in satellite data that may relate to occurrence of oil and gas. Thus, graphical correlation applied in this manner may act as a useful data integration tool in the search for oil and gas prospects.« less

Fundamentals of 3-D migration, Part 1

This is a tutorial on 3-D migration of seismic data. Subsurface geological features of interest in hydrocarbon exploration are three dimensional in nature. Examples are salt diapirs, overthrust and folded belts, major unconformities, and deltaic sands. The 2-D seismic section is a cross‐section of the 3-D seismic wave field. It contains signals from all directions, including out‐of‐plane of the profile (sideswipe energy). This sideswipe energy can only be handled by 3-D migration of 3-D seismic data.

Gas Injection With Radioactive Tracer To Determine Reservoir Continuity-East Coalinga Field, California

Radioactive tracers injected with natural gas revealed that desaturated intervals were often continuous over wide areas of the field; those intervals could greatly influence an injection project by acting as thief zones for injected water. The tracer study suggested generally better reservoir sand continuity than could be inferred from a related outcrop study. Introduction A waterflood pilot project, to start in 1974, has been planned for the Temblor Zone II reservoir of the East planned for the Temblor Zone II reservoir of the East Coalinga field, Calif. This proposed five-spot pattern drive project has required data on reservoir continuity and fluid distribution that have not been readily available from a study of a nearby outcrop nor from modem indirect geological methods., Engineering and geological studies indicated that this gravity drainage reservoir consists of complex channel sand deposits having uncertain interwell continuity complicated by oil desaturated intervals (intervals having a gas saturation and a low gravity-drainage residual oil saturation). Many of these sands contain sufficient movable oil associated with gas intervals to be included in a supplemental oil recovery project. A geological outcrop study was carried out approximately 2 miles northwest of the project study area to develop a conceptual picture of the Temblor Zone II reservoir. To learn more about this reservoir and to follow up the outcrop study, a series of reservoir characterization tests was undertaken. The tests consisted of injecting gas with radioactive tracers into the desaturated intervals. After a study of the out-crop, it was concluded that the subsurface geological features associated with channel boundaries would limit reservoir continuity to the channel width of 300 to 500 ft in all major sands, with better continuity along the trend of the channels. This restricted continuity would severely limit an injection project, but the results of the gas tracer tests indicate that the effective reservoir continuity is much greater than the continuity inferred from the outcrop. Method of Testing Gas tracer surveys were carried out successfully in Temblor Zone II sands, which had desaturated intervals with considerable continuity, Fig. 1 is an oil saturation profile based upon core analysis data for the gas injection well. The oil saturations within the sands having no desaturation have been reduced by the flushing action of the mud filtrate during coring. Some desaturated intervals are shown to overlie movable oil intervals and some desaturated intervals cover entire sand members. The sand in this reservoir is unconsolidated, so in obtaining all rock data, modem procedures for handling unconsolidated core were procedures for handling unconsolidated core were used and rock properties were measured under confining pressure. The well was completed with casing set through the Temblor Zone II formation so that individual sands could be isolated by tubing and packers in the overbore for each of the injection test. packers in the overbore for each of the injection test. The well completion consisted of perforating the desaturated intervals in four sands. To insure that the injected gas had been injected into the right sand, normal radioactive gas injection profile surveys were run at the conclusion of each phase of the testing before the packers were moved. The injection tests were conducted in separate phases using methane containing a radioactive tracer phases using methane containing a radioactive tracer at the constant rate of 500 Mcf/D. JPT P. 1251

Investigations, areal and engineering geology; Folsom Dam Project, central California

The case of Folsom Dam emphasizes the need for geologic reasoning in the site location, planning-design, and construction phases of multi-purpose projects. Construction afforded an unusual opportunity to evaluate current exploration techniques in highly fractured and deeply weathered granitic and metamorphic rocks.The project totals 4.8 miles of dam and consists of: an overpour gravity section with partially enveloping earthfill wing dams on the American River; Mormon Island earthfill dam across an ancestral channel; eight smaller earthfill saddle dams on tributary drainages; two diversion tunnels; and appurtenant structures.The main dam and most of the saddle embankments are founded on quartz diorite (Upper Jurassic). Metamorphic rocks of the Amador group (Middle-Upper Jurassic) form the foundation at the Mormon Island site. In addition to weathered bedrock, overburden consists of Recent mining debris and Pliocene (?)-Pleistocene channel gravels. The quartz diorite is extensively fractured by a complicated joint system; locally the closely spaced joints simulate a shear or fracture zone.The erratic weathering of quartz diorite constitutes a serious foundation and excavation problem. Commonly outcrops proved to be residual boulders underlain by highly weathered rock to considerable depths. The composition and origin of minerals in the fresh and weathered stages (slight, moderate, highly) of quartz diorite are tabulated for use in predicting the conditions at new sites involving similar rock; megascopic and microscopic criteria for subdividing the weathered rock are correlated with their respective engineering-physical properties. The most important factors controlling the depth and extent of weathering are: spacing of joints and fractures; degree of microfracturing; and the physiographic history of the area.Geologic investigations included a variety of subsurface techniques, i.e., borings, man-sized openings, geophysical surveys, and use of a bore hole camera. Several limitations became apparent with conventional exploration techniques; data from man-sized openings supplemented the cored borings.At the main dam, foundation excavation was performed in three contract stages; conditions exposed at subsequent levels aided in estimating the final foundation. The extremely varied excavation and final foundation rock are recorded in an exploded stage diagram; actual subsurface geologic features of typical lift surfaces are correlated with logs of cored borings and resistivity survey data. Three sizeable fault zones traverse the foundation; special dental treatment and added excavation exposed unquestionable foundation rock.The core trench of many earthfill dams was grouted on the split-method pattern. Adequate grouting of the highly weathered quartz diorite (moderately permeable) proved difficult due to its inhomogeneity and clayey-filled fractures. At Mormon Island, innumerable springs issued from the schistose foundation and necessitated a grout curtain to a depth of 60 feet.Sources of construction materials consisted of: (1) natural aggregate and pervious fill from alluvium deposits; (2) impervious earthfill borrowed selectively from outcrops of highly weathered quartz diorite; and (3) rip-rap of quartz diorite from the main dam, power house and tailrace excavations.To gain additional power head, the natural channel was lowered and the power house placed in a deep excavation. Geologic problems inherent to the excavation included design of a 250 foot open-cut slope and treatment of fractures and joints in a rock ledge between the spillway channel and partially underground power plant.