Seismic Log Research Articles

Discrimination between porous zones and shale intervals using seismic logs

With recent advances in seismic processing and display capabilities, much emphasis has been placed on stratigraphic interpretation of seismic data. The shift from its structural counterpart began in the early ’70s when Peter Vail of Exxon presented several classic papers on seismic stratigraphy. (An excellent summary of this work can be found in AAPG Memoir 26, 1977). The original emphasis of seismic stratigraphic interpretation was on reflection‐character classifications and the identification of unconformities. This approach gained a large following during the mid‐70s. It was during this period that many geologists began to work more closely with seismic data. New interpretation techniques have greatly increased our understanding of depositional environments and lateral facies changes within underexplored portions of sedimentary basins. Fortunately, the advances in seismic stratigraphy have reduced the number of unsuccessful wildcat drilling locations. However, dry holes are still being drilled.

Subsurface Basin Analysis of Fault-Controlled Turbidite System in Bradano Trough, Southern Adriatic Foredeep, Italy

Subsurface data (seismic lines, wireline logs, cores, and drill cuttings) from intensive hydrocarbon exploration in the Pliocene-Pleistocene Bradano Trough were used in performing a three-dimensional basin analysis and in reconstructing the time-space evolution of the basin. A middle Pliocene sedimentary system characterizes the hydrocarbon-bearing sands of the major gas field of the Bradano Trough, the Candela field. This system includes two phases of deposition in a migrating basin. The first phase of deposition, caused by strong subsidence due to continental shortening, was accompanied by the influx of large quantities of clastic material. The outer part of the basin was filled with turbidites; the inner part is characterized by olistostromes (formed by gravitational sliding along the slope), which progressively prograded basinward. The fault-controlled bottom physiography acted as a channel for turbidity currents that deposited elongate bodies between the inner slope and the compressional faults growing outward. The coarser grained material, deposited in the trough by currents, onlapped the upthrown flanks of the faults, thus providing potential hydrocarbon traps. Turbidity currents carrying the finer grained material flowed over the fault scarps. In the meanti e, other external faults were activated. As the turbidity currents spilled over the former faults, deposition expanded laterally. Continuous basin subsidence kept pace with deposition, and superposed wedges accumulated to a thickness of at least 1,000 m (3,281 ft). The second phase of deposition is characterized by a decreasing rate of subsidence. Currents filled a flat trough with tabular turbidites, which accreted longitudinally. The sand bodies became more muddy-clayey in the distal southern part of the basin. At this point, the system consisted of a major fan-shaped sand body upcurrent; downcurrent, partially overlapping lobate sand bodies with large lobe fringes migrated transversally. The entire fan system was later blanketed with mud and finally with a transient carbonate platform, which preceded a new cycle of subsidence.

A multifunction sonde for horizontal borehole logging

A multifunction sonde designed to detect anomalous structure in horizontal gas drainage boreholes in coal has been developed. The sonde is manoeuvered in the borehole using the drill string and rig, and is remote-controlled to provide a semi-continuous suite of geophysical logs including seismic, caliper, natural gamma, spontaneous potential, resistivity, and survey. Logs are available in real time via a standard CCITT modem protocol. The seismic log is obtained from a piezoelectric source which can be pulsed at any rate from 1 to 30 Hz, detected by an array of six hydrophones with remotely switched gain in the range 24 to 90 dB, and bandwidth 300 Hz to 100 kHz. Selected sections of detected waveforms are sampled to 10 bit accuracy and averaged over up to 16,000 firings of the source providing good random noise rejection. Preliminary field testing of the sonde has shown that the combination of caliper, resistivity and gamma logs provide good correlations on basic structure. Performance of the seismic log was limited in gas-filled holes but where liquid-filled conditions were available both refracted path (normal velocity logging) and reflected path (sonic ranging) logs were obtained. Sonic ranging logs, allowing location of floor or roof of the coal seam at distances of up to 4 m from the borehole, have been demonstrated for crosshole logs and should also be possible from a single borehole.

On: “Well logging—A 25‐year perspective,” by D. D. Snyder and D. B. Fleming (GEOPHYSICS, 50, 2504–2529, December 1985)

In reading the review by Snyder and Fleming in which they emphasized “those developments which we deem significant or which we have learned are held significant by our colleagues,” I was dumbfounded by the total absence of any mention of Humble Oil & Refining Co.’s leadership in the development of the two‐receiver, continuous velocity logging tool. That tool employed the techniques contained in W. D. Mounce’s patent number 2 200 476 on “Measurements of acoustical properties of materials” which was applied for in December, 1935, and approved in May, 1940. The Humble Seismic Velocity Logger was used to log a shallow core hole near Ozona, TX, on October 18, 1949, and the first deep hole velocity logging operation was achieved on March 5, 1951, in the Ida Rosenbaum no. 1 well in Washington County, Texas. The formal announcement of Humble’s success was in the paper “Seismic velocity logging” by W. D. Mounce, C. L. Hubbard, C. J. Charske, and H. P. Kuppers of Humble’s Geophysics Research Section, presented on November 20, 1951, at the Fifth Annual Midwestern Meeting of the SEG in Dallas, TX.

Integration of Seismic and Well Log Data Using Vertical Seismic Profile: ABSTRACT

Use of the vertical seismic profile (VSP) as the link between surface seismic data and advanced log evaluations makes it possible to calibrate seismic sections in terms of subsurface petrophysical parameters. A step-by-step procedure of (1) tying a surface seismic section to the borehole measurements via the VSP, (2) reprocessing the seismic section, and (3) propagating log information outward from the well using the calibrated seismic section is practical. In fact, if a VSP is run in the discovery well in a field, this procedure can be followed to obtain information about where to drill the first development well. If vertical seismic profiles are run in each subsequent development well, a continually updated reservoir description can be used to guide the development of the field. End_of_Article - Last_Page 144------------

Geophysical Exploration in Brazilian Continental Margin: History and State of the Art: ABSTRACT

Geophysical exploration by PETROBRAS started in 1954 in the onshore basins and in 1968 in the offshore basins of the Brazilian continental margin. The major problems that these basins share are: (1) short-range lateral velocity variations; (2) poor seismic data quality in many areas, especially on land, and (3) small traps with some degree of stratigraphic control. In the search for the solution to these problems, the best techniques available have been tried. CDP was introduced in the early 1960s; digital recording and processing in 1968; bright-spot methodology in 1973; trace inversion in 1976; 3-D migration in 1978; and image-ray depth migration in 1981. Facilities for computer-generated display for geophysical interpretation were made available in the early 1970s. Presently, an interactive interpretation mapping system with graphic stations is in use. Examples of techniques applied to exploration and field development activities include time-to-depth conversion, generation of seismic synthetic logs, and porosity prediction. Geophysics plays an important role in the exploration of the Brazilian continental margin, where recoverable volumes of oil have increased in onshore basins from 86.342 million BOE in 1954 to 2,132.81 million BOE in June 1983, and in offshore basins from 0.069 million BOE in 1968 to 1,626.73 million BOE in June, 1983. These volumes correspond to 246 bbl onshore and 520 bbl offshore per drilled meter for the same periods. End_of_Article - Last_Page 451------------

Seismic reflections from a gas‐water contact

Using the Biot theory, we have performed calculations to show that viscous fluid flow affects seismic wave amplitudes and reflection coefficients at a gas‐water boundary in a porous sand reservoir. Our formulation of the boundary value problem for fluid‐filled porous rocks is applicable at all angles and follows parallel to the classical reflection and transmission problem solved by Knott, Zöeppritz, and others for two elastic media in perfect contact. It is pointed out that there are two major differences between the two: (1) in the present case, we deal with the propagation of inhomogeneous waves, and (2) there are interference fluxes of energy between various types of waves. The latter exists only at an oblique incidence. In our model, loss of seismic energy is mainly due to mode converted type II waves of Biot at the gas‐water contact which propagate away from the boundary in the manner of a diffusion process at low frequencies. Both seismic energy loss and reflection coefficients are studied over a wide range of frequencies and angles of incidence. We find that for a porous unconsolidated sandstone [Formula: see text] [Formula: see text] [Formula: see text] [Formula: see text] [Formula: see text] the energy loss associated with a single gas‐water boundary is approximately 2.5 percent at 100 Hz for P‐waves incident at 30 degrees and that the loss increases with frequency as [Formula: see text]. Further, at a given frequency, the loss of energy is minimal at normal incidence (∼1.2 percent at 100 Hz) and maximal (∼6 percent at 100 Hz) in the neighborhood of the critical angle (∼42 degrees). We find that the inelastic losses are significantly higher at the frequencies at which seismic logging tools operate (50 percent at 10 kHz near the critical angle). As far as the amplitude reflection coefficients are concerned, we find that the effect of the fluid flow across a single gas‐water boundary is to reduce the reflection coefficient with increasing frequency (when the wave is incident from the gas saturated rock) at all angles of incidence. In the exploration frequency band, the reduction is about 1.5–3 percent, whereas the reduction is approximately 40–50 percent in the logging frequency band. For high frequencies (10 kHz and higher) classical interpretation breaks down, and hence it appears that for frequencies at which the seismic logging tool operates corrections should be made to the classical theory.

Field appraisal with three‐dimensional seismic surveys offshore Trinidad

A consortium comprising Texaco Trinidad Inc., Trinidad and Tobago Oil Company Ltd., and Trinidad‐Tesoro Petroleum Company Ltd. commenced exploration in the South East Coast Consortium block offshore Trinidad in 1973. After four years of intensive exploration, a gas/condensate discovery was announced in early 1977 on the Pelican prospect. Later that year, in anticipation of the possible future need to site drilling/production platforms, a three‐dimensional (3-D) seismic survey was recorded over the prospect. This survey resulted in improvements in seismic record quality, multiple attenuation, and fault resolution. A coordinated geologic‐geophysical interpretation based on the 3-D seismic survey, a reevaluation of log correlations, and the use of seismic logs differed significantly from earlier interpretations. Because of this, it is anticipated that development of the field will need to be initiated in a different fault block from that previously envisioned. A second 3-D survey contiguous to the Pelican survey was recorded in 1978 over the Ibis prospect. Results show significant data enhancement in the deeper part of the section and improved fault resolution relative to previous two‐dimensional (2-D) control. The 3-D interpretation has revealed a much more complex fault pattern than originally mapped. Separate fault blocks will have to be individually evaluated, thus greatly increasing exploration risk.

Stratigraphy and Depositional Mosaics of Lower Clear Fork and Wichita Groups (Permian), Northern Midland Basin, Texas

The stratigraphy, depositional environments, and reservoir potential of the Wichita Group and overlying Lower Clear Fork (Permian) in the northern Midland basin are described on the basis of seismic interpretations, log correlations, and lithologic analyses. The Wichita in this area is divided into a lower member of slope to basinal limestones and shales, and an upper member of shelf dolomites, varicolored shales, and evaporites. Only a starved lower Wichita section is present in areas to the south. Fusulinid evidence and physical correlations suggest that the Wolfcampian-Leonardian boundary occurs within the Wichita and ascends stratigraphically toward the north. These data, in conjunction with the upward-shoaling aspect of the Wichita, establish a progradational relatio between shelf facies of the upper member and slope to basinal facies of the lower member. The Lower Clear Fork beneath the Tubb member records continued southward progradation of similar shelf facies. These lithologies are represented in the basin by limestones and shales beneath, and possibly including lowermost Dean sandstones. Depositional systems of the Wichita and pre-Tubb Clear Fork sections are carbonate-dominated, rimmed shelves adjoining deeper basins. Periods of rapid progradation and offlapping shelf-edge accretion were cyclic, being coincident with periods of low sea level. The Tubb section records a change to a mixed siliciclastic and carbonate, rimmed shelf system. Deposition of shelf Tubb and basinal Dean facies was related to a mosaic of cyclic, reciprocal siliciclastic-carbonate sedimentation. Rapid shelf outbuilding occurred at low sea-level stands during the siliciclastic phases of sedimentation. The formation of the shallow North platform was more a consequence of constructional-sedimentary processes during the late Wolfcampian and early Leonardian than of structural influences. Over much of the northern Midland basin study area, potential stratigraphic hydrocarbon reservoirs are in Wichita and Lower Clear Fork strata. Such reservoirs occur as separate offlapping shelf-marginal (dolomitized) oolite grainstone shoals rather than organic buildups as was described in earlier studies.

A STRATIGRAPHIC CASE HISTORY USING THREE‐DIMENSIONAL SEISMIC DATA IN THE GULF OF THAILAND *

AbstractA three‐dimensional marine seismic survey was conducted in the Gulf of Thailand to aid in the development of a gas field indicated by three wildcat wells. The results and interpretation reported previously demonstrated improved fault resolution and better structural definition.Five successful appraisal wells have now been drilled, and these show that most of the sands have limited extent. Widespread character changes in the seismic data also support stratigraphic variations in many of the sands.Several new methods of 3D stratigraphic interpretation have been developed while investigating the depositional history of this area. Anomalous seismic amplitudes, tied to sands penetrated by wells and mapped from SeiscropTM horizontal sections in time and depth, have indicated the distribution of bars and channels. Horizon Seiscrop sections, each sliced through a single bed, have been used to delineate these depositional features directly. G‐LOGTM sections, displaying seismic logs derived by rigorous wave equation inversion, confirm the existence of these features. Sands greater than 10 m thick have proved mappable.

How to Correct Seismic Data to Improve Seismic Impedance Logs: ABSTRACT

Besides geometrical and velocity effects, two global parameters may disturb the comparison between acoustic logs recorded in a borehole and those derived from seismic data: (a) the amplitude spectrum of the basic seismic wavelet, which controls the power of resolution; and (b) its phase characteristics that monitor the resemblance. The latter is of major importance in stratigraphic interpretation of seismic impedance logs (recognition of geologic transitions). The most important causes of distortions can sometimes be determined and compensated. The seismic source signature needs to be recorded, analyzed, and processed to determine the seismic wavelet. Phase distortions due to recording equipment (geophones and laboratory) are very important, and need to be corrected. Trav l into the earth introduces multiples and absorption. Multiples can be dealt with in the classic way with a few precautions. Then an estimation of the amplitude spectrum of signal and noise at a given time-depth provides one with a quantitative estimate of the best achievable resolution of the actual data, and the absorption curve in amplitude spectrum using the recorded source spectrum. A good match between experiments and theory leads to the estimated impulse response of absorption at a given time, and reversely, to an optimized (eventually time-varying) correction operator. Further conventional deconvolution processes should not be used. Nevertheless, when distortions cannot be corrected, and if well data are available, a deconvolution program controlling both amplitude and phase can be used interactively to achieve an equivalent result. End_of_Article - Last_Page 980------------

Study of Subtle Traps Using Horizontal Seismic Sections: ABSTRACT

A three-dimensional seismic survey, after proper design, data collection, and data processing, yields a three-dimensionally migrated data volume. Horizontal, or SeiscropTM, sections sliced from this data volume provide a direct horizontal view of the subsurface from which structural interpretation can be straightforward. In the absence of structure, Seiscrop sections display stratigraphic or paleogeomorphic features directly. However, structural deformation can be removed from the data by flattening. Horizon Seiscrop sections, sliced from the flattened volume, permit stratigraphic and other depositional features to be recognized and studied in detail without the confusion of structure. Using horizontal seismic sections primarily from the Gulf of Thailand, a variety of small and subtle traps have been identified. These include small fault traps, sand channels, and sandbars. The acoustic nature of these features has been further studied using seismic logs, derived by wave equation inversion. Reservoirs thicker than 30 ft (9 m) have proved mappable. End_of_Article - Last_Page 907------------

Field Appraisal with 3-D Seismic Surveys, Offshore Trinidad: ABSTRACT

The Southeast Coast Consortium block offshore eastern Trinidad contains four structures in each of which hydrocarbons have been discovered. Field appraisal to date had included two 3-D seismic surveys: the Pelican structure was surveyed in 1977 and, following its success, the Ibis 3-D survey was conducted in 1978. The 3-D migrated data over both the Pelican and Ibis structures exhibit significant enhancement in the deeper part of the section and also improved fault resolution relative to previous 2-D control. On the north flank of the Pelican structure, primary dips have been recognized for the first time. As a consequence, estimated reserves have been increased by approximately 20%, thus substantially affecting development economics. Furthermore, the interpreted position of reserves in the minor reservoir sand has shifted from one fault block to another as a result of the revised fault interpretation. The acoustic nature of the stratigraphic boundary which may delimit this reservoir has been clearly demonstrated using seismic logs. Also the extent of this boundary has been manned from Seiscrop SUP>TM sections. The major change resulting from interpretation of the 3-D data at Ibis has been the definition of a much more complex fault pattern affecting all the reservoirs. This will have a significant impact on future field development plans. The 3-D seismic surveys are a useful tool for prospect evaluation in this area offshore Trinidad and should be considered prior to commitment to expensive offshore development programs. End_of_Article - Last_Page 929------------

Effects of Fluid Saturation on Waves in Porous Rock and Relations to Hydraulic Permeability

Abstract Detailed and very accurate measurements of compressional Vp and shear Vs wave velocities and their attenuation in dry, partly saturated and fully saturated clay-bearing sandstone were carried out at 20, 145, and 198°C, under variable confining and pore pressures. The velocity results show that water saturated rock has high Vp and relatively low Vs not only at room temperature but also at 145 and l98°C. Varying the pore pressure reveals that the transition from the fully saturated to the dry state involves a gradual transition of Vs and a very irregular transition of Vp – with a minimum in Vp relative to either the dry or the fully saturated cases. The attenuation of S and P waves – Qs−1 and Qp−1, respectively - shows similar patterns: in dry rock, both Qs−1 and Qp−1 are low. In fully saturated rock, Qp−1 is low and Qs−1 is high. In partly saturated rock, Qs−1 changes gradually with saturation, but Qp−1 shows a very strong maximum-suggesting strong energy dissipation. The same behavior is observed at 145°C, with the steam/hot-water transition. It was also found that attenuation of shear waves increases with permeability in some sandstones, consistent with squirting attenuation mechanism. The results suggest diagnostic characteristics for determining the degree of saturation in situ from seismic logs, with particular applicability to evaluation of steam content in geothermal reservoirs and of gas content in oil reservoirs.

SEISMIC STUDIES ON ANTARCTIC ICE SHELVES

Three methods have been employed at Little America V to determine seismic velocity as a function of depth in the Ross ice shelf: (1) refraction‐shooting interpreted by means of the Herglotz‐Bateman‐Wiechert integral, (2) seismic logging of a deep borehole, (3) Robin’s formula relating P wave velocity to the density and temperature measurements made in the borehole. Results of the three methods are compared, discrepancies are discussed, and the preferred P and S velocity‐depth profiles are obtained. From the profiles and the measurements of density in the borehole, a complete determination of the elastic properties of the ice shelf is possible. Small velocity differences are obtained at depth by the methods of refraction‐shooting and borehole‐logging. The effects of crystal orientation on seismic velocity is discussed and found to be sufficient to account for the differences. A new determination of the variation of seismic velocity in ice as a function of temperature is made on the basis of recent experimental work.