Common Depth Point Research Articles

Fluid Flow in Low Permeable, Porous Media

Migration of hydrocarbons deals with the subsequent movement of petroleum after expulsion from the source rock through water saturated reservoirs or through permeability created by fractures and faults. Although the underlying principles that control the fluid movement in porous media (reservoirs) are well understood by reservoir engineers, less is known about the flow characteristics in low-permeable, porous media, such as clays and shales. For flow considerations, the primary parameters are porosity, permeability and the fluid potential gradients. For clays and shales, these parameters are poorly known; yet these control the time periods during which fluid flow occurs in sedimentary basins (100 years to 100 million years). In this paper, I examine the parametric dependence of the time constantsof fluid flow in low permeability sediments on its porosity and permeability. This is accomplished in two parts. In the first part, a technique is presented to investigate the effect of fluid flow in shales which causes undercompaction and buildup of fluid pressures in excess of normal hydrostatic pressure. The technique is pre-drill in nature; it uses seismic velocity analysis of common depth point gather of surface seismic data and is based on the concept developed by Hottmann and Johnson (1965) and Pennebaker (1968). In the second part of the paper, the flow characteristics are discussed in the basin scale. I develop a model that describes the fluid flow in a continuously accreting and subsiding clastics basins, such as the Gulf of Mexico. I examine the pressure characteristics of such a basin by digital simulations and study the effect of the permeability variation of shales on the geologic time dependence of the fluid flux in the sediments, the basin subsidence rate and the pressure buildup with depth. The model incorporates both mechanical compaction and burial diagenesis involving smectite to illite conversion of shales. The latter is based on a kinetic theory of chemical reaction in which the time-temperature history is provided by the solution of an appropriate heat conduction equation. This model of geopressure has a practical application since it deals with the generation and maintenance of abnormally high fluid pressures over geologic time and impacts the hydrocarbon migration in clastic basins.

Seismic survey over a topographic scarp in the Snake River Plain

High-resolution seismic-reflection data were used to determine the subsurface structure and origin of a topographic scarp in the Snake River Plain, which has been ascribed to surface faulting by some investigators. The survey consisted of three lines crossing the scarp along the southern and eastern edges of Clay Butte near Mud Lake, Idaho. Considering the poor seismic-reflection reputation of thick basalt flows, the resulting common depth point seismic sections were of excellent quality. We conclude from these seismic sections that the Clay Butte scarp is not primarily the result of tectonic faulting. It is very likely the edge of a basalt flow and is covered with eolian sand which obscures the edge of the flow. The seismic data also reveal stratigraphic thickness detail on the scale of about 3 m from several reflection horizons.

Multichannel seismic evidence for a subcrustal intrusive complex under Oahu and a model for Hawaiian volcanism

Coincident multichannel seismic reflection and refraction data acquired during a wide‐aperture two‐ship experiment provide evidence for a complex crust‐mantle (C‐M) transition under Oahu, Hawaii. Several large‐aperture common depth point lines and three expanding spread profiles suggest the existence of an anomalously thick (3–6 km) C‐M transition zone underneath the volcanic ridge which extends for distances of 100 km to the north and south from the center of Oahu. The anomalous C‐M transition may represent a plutonic complex which intruded into the upper mantle and the lower crust in a 200‐km‐wide area centered at Oahu. The existence of such a large volume of intrusions near the base of the crust implies that the surficial expression of volcanism constitutes only a small fraction of the amount of melt generated at depth under the Hawaiian Islands. This interpretation is in accord with previous petrological models which predict trapping and accumulation of upwelling magma at and below the Moho. We have constructed a model which suggests that the interaction between the upwelling magma and the lithospheric flexural stress field may modulate the characteristic eruption history of Hawaiian volcanoes. In particular, the model for the plane stress field which accompanies the flexure of the oceanic crust around island chains indicates that the stress field under individual volcanoes varies considerably with its position relative to the tip of the chain. As a Hawaiian‐sized volcano develops, the magnitude of deviatoric compressive stresses under it is probably sufficient to block the conduits of the upwelling magma within the oceanic crust and to terminate eruptions. Further upwelling magma is predicted by the models to be ponded at the base of the crust. Resumption of posterosional volcanism seems to occur at a constant distance behind the center of active shield volcanism, as the horizontal compressive stresses along the axis of the chain are released. Observed orientations of dikes of this volcanic phase agree with the directions of the maximum calculated stresses. Our model implies that magma upwells over a 300‐km‐wide zone and that the oceanic plate may not be fractured under the islands.

Three‐dimensional seismic monitoring of an enhanced oil recovery process

Seismic reflection data were used to monitor the progress of an in‐situ combustion, enhanced oil recovery process. Three sets of three‐dimensional (3-D) data were collected during a one‐year period in order to map the extent and directions of propagation in time. Acquisition and processing parameters were identical for each survey so that direct one‐to‐one comparison of traces could be made. Seismic attributes were calculated for each common‐depth‐point data set, and in a unique application of seismic reflection data, the preburn attributes were subtracted from the midburn and postburn attributes. The resulting “difference volumes” of 3-D seismic data showed anomalies which were the basis for the interpretation shown in this case study. Profiles and horizon slices from the data sets clearly show the initiation and development of a bright spot in the reflection from the top of the reservoir and a dim spot in the reflection from a limestone below it. Interpretation of these anomalies is supported by information from postburn coring. The bright spot was caused by increased gas saturation along the top‐of‐reservoir boundary. From postburn core data, a map of burn volume distribution was made. In comparison, the bright spot covered a greater area, and it was concluded that combustion and injection gases had propagated ahead of the actual combustion zone. The dim spot anomaly shows good correlation with the burn volume in distribution and direction. Evidence from postburn logs supports the conclusion that the burn substantially decreased seismic velocity and increased seismic attenuation in the reservoir. Net burn thicknesses measured in the cores were used to calibrate the dim‐spot amplitude. With this calibration, the dim‐spot amplitude at each common depth point was inverted to net burn thickness and a map of estimated burn thickness was made from the seismic data.

Geophysics in Australia

Geophysical observations began in Australia with the arrival of the first European explorers in the late 18th Century and there have been strong connections with European and North American geophysics ever since, both in academic and exploration geophysics. Government institutions, particularly the Bureau of Mineral Resources, have played a large part in the development of the subject in Australia, certainly more so than in North America. Academic research in geophysics has been dominated by that at the Australian National University. Palaeomagnetic research at the Australian National University has been particularly valuable, showing the large northerly drift of the continent in Cainozoic times as part of the Australia-India plate. Heat flow, electrical conductivity and upper mantle seismic velocities have been shown to be significantly different between Phanerozoic eastern Australia and the Western Shield. Geophysical exploration for metals and hydrocarbons began in the 1920s but did not develop strongly until the 1950s and 1960s. There are relatively few Australian geophysical companies and contracting companies, and instrumentation from North America and Europe have played an important role in exploration. Exploration for metals has been hampered by the deep weathered mantle over much of the continent, but the development of pulsed (transient) electromagnetic methods, including an Australian instrument (SIROTEM), has improved the situation. Geophysics has been important in several discoveries of ore-bodies. In hydrocarbon exploration the introduction of common depth point stacking and digital recording and processing in reflection surveys have played an important part in the discovery of offshore and onshore fields, as in other countries.

100% (single‐fold) vs. CDP (multifold) with today’s budgets

Because of recent and drastic reductions in exploration budgets, geophysicists need alternatives to the high costs of acquiring common depth point (CDP) data. One is readily available — the old 100% data, much of which is on file somewhere in the industry’s extensive archives. In areas of good record quality, the old data can be turned, via sophisticated modern processing methods, into useful seismic sections. Frequently, the cost will be just 5–10 percent of the cost of obtaining new CDP data.

Seismic Data Obtained Using .50-Caliber Machine Gun as High-Resolution Seismic Source

A seismic line across a shoestring sandstone served as a test of a .50-caliber machine gun as a high-resolution seismic source in exploring for shallow sandstones. The line crosses part of the Bronson-Xenia oil field, which produces from the Bartlesville sandstone of the Cherokee Group (Pennsylvanian) in Bourbon County, Kansas, at a depth of 622 ft (190 m). The reservoir, which has a flat base and an irregular top, may represent a superposition of fluvial sandstones. The seismic line clearly depicts a lenticular sandstone that is up to 56 ft (17 m) thick. Both the lateral extent of the sandstone body and a slight velocity pull-up of the underlying layers are evident. In addition to the target sandstone, two additional sandstone bodies may be present at the east end of the line, where well control is not available. Analysis of the seismic data indicates that the frequency of the .50-caliber source is from 30 to 170 Hz, with a dominant frequency of about 100 Hz. The high frequencies are retained deep into the section. Two shots per shotpoint are the minimum number necessary for acquiring high-quality data, and 12-fold is the minimum acceptable common-depth-point (CDP) coverage.

A guide to current uses of vertical seismic profiles

Vertical seismic profiles (VSPs) are small‐scale seismic surveys in which geophones are lowered into a well to record waves traveling both down into the earth (direct waves from the surface source and downgoing multiples) and back toward the surface (primary reflections and upgoing multiples). VSPs thus contain information about the reflection and transmission properties of the earth with a coverage that depends upon the geometry of the VSP experiment and the structure near the well. This article describes the uses of VSPs in seismic exploration that have been published in the last three years and is designed to complement the more detailed surveys by Hardage (1983) and Balch and Lee (1984). When the earth is horizontally layered, the well is vertical, and the source is close to the wellhead, upgoing and downgoing waves recorded by the VSP travel vertically, and the VSP can be used to calibrate surface seismic sections by providing the time‐to‐depth curve and allowing a detailed analysis of reflection and transmission properties of the earth at a given location. These applications rely heavily on signal processing to separate the upgoing and downgoing waves and to study their relationships to data recorded at the surface. When the earth varies laterally or when the source is offset from the well, the VSP can be used to complement surface surveys by providing high‐resolution images of structure near the well. Current work has concentrated on forming images from the reflected waves by the methods of common‐depth‐point (CDP) stacking and migration. Tomographic methods for inverting the traveltimes and amplitudes of transmitted waves are also being developed and will become important when downhole arrays and powerful downhole sources are available. The most significant advance in the next few years, however, will be the development of a reliable three‐axis tool with internal devices for determining both the orientation of the tool and the quality of its coupling to the borehole wall.

North Atlantic Transect: A wide‐aperture, two‐ship multichannel seismic investigation of the oceanic crust

The North Atlantic Transect experiment collected a unique set of two‐ship, multichannel seismic data across oceanic crust spanning almost 200 m.y. in age from the Blake‐Bahama Plateau, off the U.S. East Coast, to the Mid‐Atlantic Ridge near 23°N. A large, synthetic receiving array was simulated by having two ships, each equipped with a large air gun source array and a multichannel streamer, steam in line maintaining a fixed separation while alternately firing their air guns and recording arrivals generated by shots from both ships. More than 3800 km of wide‐aperture common depth point (CDP) data with a total synthetic aperture of more than 10 km were collected along the transect. At intervals along this wide‐aperture profile, 11 expanding spread profiles were also acquired on oceanic crust of various ages: in the Jurassic and Cretaceous quiet zones, on magnetic anomalies MO, 34, 26, and 5, in the Mid‐Atlantic Ridge rift valley, on the Blake‐Bahama Plateau, and north of the Puerto Rico Trench on the Antilles Outer Ridge. These data have provided some remarkable new images of the oceanic crust. A very continuous Moho reflection is observed along most of the western half of the transect. The character of the Moho reflection and its approximate depth appear to vary only slightly over distances of several tens of kilometers, although major changes in crustal thickness and seismic stratigraphy are observed along the profile. Near the Blake‐Spur fracture zone southwest of Bermuda the Moho gradually shallows over a distance of several tens of kilometers. Beneath the fracture zone itself the Moho abruptly rises to less than half its normal depth, providing the most convincing evidence yet for anomalously thin crust beneath even small‐offset fracture zones. An intracrustal reflection, called horizon “R,” is identified about 1.8–2.0 s beneath the sediment‐basement interface but well above the Moho reflection. Although this event is extremely variable laterally, the total thickness of the crust and the thickness of the unit between horizon “R” and the Moho appear to thin progressively toward fracture zones. An expanding spread profile on 118‐m.y.‐old crust southwest of Bermuda indicates a surprisingly thick (∼8 km) crustal section and the presence of a low‐velocity zone near the base of the crust. Although the analysis and interpretation of the transect data will continue, these results convincingly demonstrate the ability of modern, multichannel seismic techniques to map variations in oceanic crustal structure with a high degree of spatial resolution over thousands of kilometers.

Computation of residual statics using projectors

The residual statics problem, as we know, is treated as the solution of one linear system of equations. If we assume that the static corrections are “surface consistent,” then we know that time shifts of each trace can be written as the sum of three terms [Formula: see text] where i = 1, …, [Formula: see text] is the shot position index with [Formula: see text] the number of shot positions, j = 1, …, [Formula: see text] is the receiver position index with [Formula: see text] the number of receiver positions, k = 1, …, [Formula: see text] is the common‐depth‐point (CDP) position index with [Formula: see text] the number of common depth points, [Formula: see text] = correction for ith shot position, [Formula: see text] = correction for jth receiver position, and [Formula: see text] = correction for each trace in the kth CDP gather. For every pair (i, j) we have one equation. We write system (1) in matrix form as [Formula: see text] where [Formula: see text] is the vector of unknown parameters; and [Formula: see text] is the vector which consists of the time shifts obtained by crosscorrelation of each trace in CDP gather with the corresponding reference trace.

Phase‐coherency filtering of reflection seismic data

In addition to the standard techniques of stacking and digital band‐pass filtering for enhancing signal‐to‐noise (S/N) ratio for common‐depth‐point (CDP) reflection seismic data, a number of alternative or supplementary methods are available: (1) weighted mixing of adjacent traces, (2) semblance weighting of multichannel data based upon interchannel semblances (Neidell and Taner, 1971), and (3) filtering with the output energy filter (Robinson and Treitel, 1980). Each approach has advantages and disadvantages.

Some interesting seismic noise

The seismic data used in geophysical interpretation is normally restricted to common depth point stacked seismic sections. The quality of the seismic section is dependent upon the design of the recording system and the skill of the seismic processor. The system (vessel, source and cable for marine work) is designed to minimize known sources of noise or at least record them so that they can be later attenuated. A major part of the processor’s function is to recognize and attenuate this noise; he may also be called upon for suggestions to improve the recording system.

A case study in seismic diffraction near a Gulf Coast salt dome

The diffraction response from a sequence of truncated dipping beds is examined. It is shown that standard velocity analysis and interpretation can emphasize the diffracted energy in the common‐depth‐point (CDP) stack. This energy can then produce a distorted image in the poststack migration of the data. In our case this distortion took the form of smaller estimates for the structural dip, which was independently confirmed by well control. Wave‐equation synthetics are also used to support the role of truncated dipping beds as strong diffractors. It is shown that the diffraction from these beds will give rise to certain identifiable characteristics in the CDP data. These characteristics are demonstrated by examining the ideal case of a truncated reflector with dip. It is shown that in the presence of dip, a proper choice for the stacking velocity will lead to the enhancement of the reflected energy at the expense of the diffraction. These stacking velocities can then be used to improve the interval velocity estimates for migration. In this study the application of these ideas brought the structural dip as derived from the surface seismics into good agreement with the well data. In comparison with model data, there was also some evidence that the gas‐water contact could be identified in the reprocessed migration.

Direct estimation of shear‐wave interval velocities from seismic data

Conventional velocity analysis of seismic data is based on normal moveout of common‐depth‐point (CDP) traveltime curves. Analysis is done in a hyperbolic framework and, therefore, is limited to using the small‐angle reflections only (muted data). Hence, it can estimate the interval velocities of compressional waves only, since mode conversion is negligible when small‐angle arrivals are concerned. We propose a new method which can estimate the interval velocities of compressional and mode‐converted waves separately. The method is based on slant stacking or plane‐wave decomposition (PWD) of the observed data (seismogram), which transforms the data from the conventional T-X domain into the intercept time‐ray parameter domain. Since PWD places most of the compressional energy into the precritical region of the slant‐stacked seismogram, the compressional‐wave interval velocities can be estimated using the “best ellipse” approximation on the assumption that the elliptic array velocity (stacking velocity) is approximately equal to the root‐mean‐square (rms) velocity. Similarly, shear‐wave interval velocities can be estimated by inverting the traveltime curves in the region of the PWD seismogram, where compressional waves decay exponentially (postcritical region). The method is illustrated by examples using synthetic and real data.

A method to analyze and verify deep crustal reflections offshore Costa Rica

A multichannel seismic reflection profile across the oceanic crust seaward of the Middle America Trench off the Nicoya Peninsula, Costa Rica, shows discontinuous, low‐frequency events at 6.5 to 7.0 s. These events might first be interpreted as reflections from the Moho. However, careful analysis of the seismic data suggests that these events represent three‐dimensional (3-D) scattered energy from the rough basaltic basement. Velocity analysis indicates that root‐mean‐square (rms) velocities for these deep “reflection events” are too low to emanate from the Moho. Also, the ghost separation caused by the streamer depth decreases for increasing record time, suggesting that incident angle for these “reflections” increases with time. Furthermore, these events are approximately 13 dB stronger than would be expected for a Moho reflection. Common‐depth‐point (CDP) stacking and velocity filtering were used to attenuate the scattered noise and sideswipe from the basalt. The results show a 21 dB total reduction of scattered energy. However, Moho reflections still cannot be discerned. The results suggest (1) ambient noise after processing is 20 dB below the expected Moho level and is not a factor in detection of the Moho; (2) Moho reflectivity may be smaller than 0.1 (reflectivity is calculated from assumed velocities and densities) and could be as small as 0.05 (the detection threshold); (3) the Moho may not be a discrete reflector and may therefore represent a transition zone; and (4) Moho events may be disorganized by transmission through rough basalt so the CDP stacking process is not effective.

Seismic Structure and Stratigraphy of Northern Edge of Bahaman-Cuban Collision Zone

Common-depth-point (CDP) seismic reflection data in the southwestern Bahamas reveal the northern edge of the tectonized zone that resulted from the late Mesozoic-early Cenozoic collision of Cuba and the Bahamas. Two seismic facies are present: a basin facies and a shallow-water carbonate-platform facies. In Santaren Channel, between Cay Sal and the Great Bahama Bank, a 5-sec thick group of coherent flat-lying reflections is inferred to represent an accumulation of deep-water basinal carbonate deposits approximately 10 km thick. At the southern end of Santaren Channel and in Nicholas Channel toward the west, the basinal carbonate section thins abruptly and overlies zones that lack coherent reflections. These structureless zones, which are attended by positive gravity anoma ies, are inferred to represent shallow-water carbonate-platform materials. Neither seismic facies has associated short wavelength magnetic anomalies. A 10-km broad anticline occurs at the south end of Santaren Channel. Platform carbonates in the core of this structure overlie Early Cretaceous and older basinal carbonate deposits and are onlapped by Late Cretaceous and Cenozoic basinal facies. The structure is inferred to be a hanging-wall anticline at the northern limit of the Cuban fold-thrust belt formed in the Late Cretaceous. In eastern Nicholas Channel, a 40-km broad, tilted block of platform carbonate material appears to have been uplifted prior to the latest Cretaceous. This feature is onlapped by Upper Cretaceous and Cenozoic deposits. A depositional-erosional carbonate-platform edge of Early Cretaceous age occurs in western Nicholas Channel at its juncture with the Straits of Florida. An Early Cretaceous platform margin at this location indicates that a deeper water embayment extended northward into the Straits of Florida, around northern Cay Sal Bank, and back into Santaren Channel during the Early Cretaceous.

Seismic Lithology and Pitfalls in Seismic Porosity Detection: ABSTRACT

One common method of searching for anomalies associated with increased porosity is to integrate the stacked traces to obtain an acoustic section or, if a suitable relation between density and velocity is known, a interval velocity section. Lateral variations in the color-plotted displays can then be interpreted in terms of porosity variation associated with inferred velocity variation. Buried in this procedure is the assumption that the stacked section represents reflectivity at normal incidence. In the developing technique of seismic lithology, where variations in reflection amplitude with source receiver offset are used to infer lithologic properties, examples are accumulating that show dramatic changes in reflection amplitude with offset within common depth point gathers. Consequently, after stacking, the stacked section will be poorly correlated with normal incidence reflectivity. Recent development in lithologic analysis have led to a procedure that allows the normal incidence reflectivity to be estimated from the offset variation. The results of this procedure show greater continuity and geologic interpretability. More important, upon trace integration, the resultant impedance sections show greater stability leading to a higher confidence level in potential porosity anomalies. End_of_Article - Last_Page 842------------

A geophysical study of the Manila Trench, Luzon, Philippines: 2. Fore arc basin structural and stratigraphic evolution

The pattern of sediment distribution within the Manila Trench subduction zone west of Luzon exerts a strong influence on the tectonic evolution of the accretionary prism and the fore arc basins of this convergent plate margin. Multichannel common depth point seismic reflection data reveal up to 2600 m of turbidite sediments overlying a thin pelagic/hemipelagic sediment layer and oceanic basement in the northern part of the Manila Trench. These turbidites were transported from the uplifted collision zone of Taiwan, southward along the Manila Trench by gravity‐controlled processes. Similarly, thinner turbidite deposits in the southern end of the trench were transported northward from the collision zone involving the North Palawan block and the central Philippines. Sedimentation within the Manila Trench is dominated by sources at the ends of the trench with longitudinal transport along the trench axis. In contrast, the fore arc basin system of the Manila Trench receives sediment through submarine canyons incised into the West Luzon continental slope. The rate of sediment supply to the fore arc basin system is not directly related to the rate of sediment supply to the trench. The size of the accretionary prism varies significantly along strike of the trench and it strongly correlated with the local thickness of turbidite sediments within the trench. A thickness of more than 200 m of trench turbidites is probably required before accretion and uplift become the dominant process affecting the fore arc region. This generalization may apply to other convergent plate margins as well. The uplift rate of the accretionary prism probably varies dramatically along the trench and is largely independent of variations in the local sediment accumulation rates in the fore arc basins. This variability results in the formation of several different types of fore arc morphology along the Manila Trench. The overall seismic stratigraphy of the West Luzon Trough fore arc basin is remarkably uniform, with seismic sequences continuous for up to 110 km along strike. The unconformities that bound these seismic sequences are probably the result of a complex interplay of relatively long‐term (approximately 10 Ma) uplift of the accretionary prism and relatively short‐term (approximately 1–2 Ma) variations in sediment supply to the fore arc basin, modulated by eustatic sea level changes.

Enhancement of transient electromagnetic soundings: A metallic model study

Transient electromagnetic (TEM) soundings can be enhanced by additions of matched filtered signals from different coil separations. Results of laboratory metallic model studies are used to demonstrate this technique. A short pulse of current is transmitted from one coil. For each of the signals received in the other coil, the half‐space response (surface wave) is subtracted from the total signal to determine the reflected signal. These reflected signals are passed through matched filters corresponding to the proper separation and a possible layer depth. All of the matched filtered signals corresponding to a particular possible depth are added, and the summed output is stored. The filtering and summing process is repeated for a set of possible interface depths. The sum of matched filtered signals for the correct depth has a peak value of 1 and is symmetric about the peak. We believe that short‐range early‐time values of the transient response can be used to estimate the conductivity of the first layer. This estimate is used for numerical computations of half‐space responses. We have used several magnetic dipoles to represent the finite‐sized coils to show good agreement between theoretical and experimental half‐spaces. Theoretical reflection signals for matched filters can be computed numerically. The stacking of matched filtered signals is an electromagnetic analog of the common‐depth‐point (CDP) seismic technique. It should reduce the effects of nearsurface inhomogeneities and improve the resolution. As an incidental experiment, we show comparisons of the reflected signal and the “image transmission.” The reflected and transmitted signals are essentially the same and support the concept of hybrid‐ray theory for electromagnetic sounding.

The common depth point stack

The Common Depth Point (CDP) Method is a seismic data acquisition and processing technique which transforms field recordings from seismic surveys into pseudo-cross-sectional images of the earth's geologic layering beneath the survey line. Such images are the primary geophysical analysis tools used by explorationists to pinpoint likely drilling locations for oil and gas reservoirs. The CDP Method has evolved from an elementary data enhancement concept in the analog seismic era of the 1950s to a highly sophisticated digital imaging and parameter estimation technique in the 1980s. This paper reviews the history of the method, and its underlying mathematical-physical basis in terms of simple computer simulations of reflection seismic experiments. The method is illustrated with actual examples from several geologic provinces, and the paper closes with a discussion of some limitations of the CDP Stack and possible future directions in seismic imagery.