Time-depth Relationships Research Articles

Establishing Time-Depth Relationships Constrained by Modes of the Reservoir Architecture

Time-depth relationships (TDRs) can connect seismic and wireline logs, both essential characterization data of reservoirs. The seismic well tie is always a complex work on account of the complicated reservoir structures. Since seismic and logging data are responses of reservoir architectures, the seismic well tie can be efficiently improved constrained by the reservoir architectures. This study adopts a clastic reservoir as the study area. Three architecture modes (i.e., normal cycle mode, inverse-normal cycle mode, and homogeneous-normal cycle mode) are summarized based on combinations of architecture elements. For the generation of the synthetic seismograms, optimized wavelets (i.e., wavelet A, wavelet B, and wavelet C) are suitable for the wells belonging to normal cycle mode, inverse-normal cycle mode, and homogeneous-normal cycle mode, respectively. Precise TDRs are established by matching the synthetics and seismic traces. Wells belong to the same architecture mode and have similar TDRs. The two-way travel time is shortest in the same depth interval of homogeneous-normal cycle mode compared to other architecture modes.

Wireline distributed acoustic sensing allows 4.2 km deep vertical seismic profiling of the Rotliegend 150 °C geothermal reservoir in the North German Basin

Abstract. We performed so-far-unprecedented deep wireline vertical seismic profiling at the Groß Schönebeck site with the novel method of distributed acoustic sensing (DAS) to gain more detailed information on the structural setting and geometry of the geothermal reservoir, which is comprised of volcanic rocks and sediments of Lower Permian age. During the survey of 4 d only, we acquired data for 61 source positions using hybrid wireline fiber-optic sensor cables deployed in two 4.3 km deep, already existing wells. While most of the recorded data have a very good signal-to-noise ratio, individual sections of the profiles are affected by characteristic coherent noise patterns. This ringing noise results from incomplete coupling of the sensor cable to the borehole wall, and it can be suppressed to a large extent using suitable filtering methods. After conversion to strain rate, the DAS data exhibit a high similarity to the vertical component data of a conventional borehole geophone. We derived accurate time–depth relationships, interval velocities, and corridor stacks from the recorded data. Based on integration with other well data and geological information, we show that the top of a porous and permeable sandstone interval of the geothermal reservoir can be identified by a positive reflection event. Overall, the sequence of reflection events shows a different character for both wells explained by lateral changes in lithology. The top of the volcanic rocks has a somewhat different seismic response in both wells, and no clear reflection event is obvious at the postulated base of the volcanic rocks, so that their thickness cannot be inferred from individual reflection events in the seismic data alone. The DAS method enabled measurements at elevated temperatures up to 150 ∘C over extended periods and led to significant time and cost savings compared to deployment of a conventional borehole geophone string. This wireline approach finally suggests significant implications for observation options in old wells for a variety of purposes.

A comparative analysis of time–depth relationships derived from scientific ocean drilling expeditions

Time–depth relationships (TDR) are required for correlating geological information from drill sites with seismic reflection profiles. Conventional time–depth domain conversion is implemented using P-wave velocity data, derived from downhole sonic logs, calibrated with vertical seismic check-shots. During scientific ocean drilling expeditions, immediate seismic correlation is carried out using laboratory velocities measured on recovered core material. As these three velocity measurements vary significantly in signal frequency, resolution and acoustic pathways, they carry potential for substantial TDR differences and consequent miscorrelation to seismic profiles. Our analytical work uses the comprehensive scientific ocean drilling dataset to quantify these differences in core-seismic integration. TDRs are calculated and compared at sites where check-shot, sonic log, and laboratory velocity measurements cover the same depth segments of the drill hole. We find that the maximum differences between the TDRs (TDR diffmax ) reach up to 55%, which can cause fundamental errors in the seismic correlation. No direct relationship to porosity and bulk density of the cored material is observed. Instead, higher TDR variability is found at sites with carbonate content > 70%, particularly with coarser grain texture. Sites containing primarily igneous and siliciclastic sequences show less than 10% TDR diffmax . This semi-quantitative criterion indicates that downhole logging should be conducted during drilling expeditions, especially at sites with carbonate sequences, or low core recovery, to ensure accurate core-seismic integrations.

The determination of the constant phase of seismic wavelet using automatic seismic-well tying

ABSTRACTSeismic wavelet estimation is the bedrock of seismic-well tying and seismic inversion but remains a challenge. Huge amounts of effort have been expended on seismic wavelet estimation and determining the amplitude and phase spectrum is a time-consuming task. In this article, we develop a workflow to determine automatically the constant phase of an estimated wavelet. This workflow begins with statistical wavelet estimation and seismic-well tying. We then extract a new seismic wavelet with a constant phase by using the well and seismic data together. To obtain the best phase for the extracted wavelet using well and seismic data, we rotate the phase of the wavelet by a user-defined increment and perform automatic seismic-well tying for each phase-rotated wavelet. The phase that reaches the maximum correlation coefficient between the synthetic and seismic data is regarded as the best phase for wavelets in each iteration. We next update the time–depth relation using the best seismic-well tie (the maximum correlation coefficient). We repeat the wavelet estimation using well and seismic data, phase rotation, automatic seismic-well tying and time–depth updating until the difference between wavelets, and time–depth relationships, in the current and previous iterations is below a user-defined threshold.

Effects of near surface lithology on velocity modelling and time–depth relationships in the Cooper–Eromanga–Lake Eyre Basin

Structural closures on the western flank of the Patchawarra Trough in the Cooper–Eromanga Basin are truly low relief; drilling opportunities regularly target hydrocarbon columns of similar magnitude to the uncertainty of depth prediction. Improving the accuracy and precision of depth prediction will reduce risk for drilling opportunities, and improve drilling success rates. A detailed study of the near surface geology (surface to ~500 m depth) of the western flank of the Patchawarra Trough has been undertaken to better understand the effect of observed geological variations of the near surface on depth prediction at deeper target levels. The stratigraphic interval investigated includes the top of the Eromanga Basin and the entire Lake Eyre Basin, which is sparingly studied and routinely overlooked in the statics and velocity modelling process. This study analysed recently acquired cased-hole sonic logs in conjunction with gamma logs and mudlog data to map out the observed geological variations, and construct a 3D velocity model of the near surface. Variations of layer thickness and seismic velocity were input into Monte Carlo simulations to investigate sensitivities of each formation on two-way travel time and depth prediction. This investigation has found that velocity variations of the Weathered Winton Formation, and thickness variations of the Namba Clastics have the greatest impact on imaging of structures at depth. Independently, these have the potential to completely conceal or create structures in the time domain. Continued efforts in improved understanding of the near surface will subsequently lead to enhanced imaging of structures, which can then be used in the mapping of structural closures in petroleum exploration and development.

The high resolution sedimentary filling in Qiongdongnan Basin, Northern South China Sea

Abstract The high density (2 km × 6 km), high resolution (19 sequence boundaries) seismic reflection profiles and 42 industrial wells (including VSP data, gamma ray, sonic and density logs) are now available in Qiongdongnan Basin to the northwest of South China Sea. Then we explored the time–depth relationships of strata, obtained the compaction curves of shale and sand, and recovered the original sediment thickness. Furthermore, we calculated the sediment budget and analyzed the deepwater sedimentation processes. The thickest sediment of Qiongdongnan Basin is more than 16 km. The temporal–spatial distribution of sedimentation is characterized by three stages i.e. syn-rift stage (45–21 Ma), early post-rift stage (21–10.5 Ma) and late post-rift stage (10.5–0 Ma). The distribution of sedimentation in deepwater is mainly controlled by the sediment source and transportation routes. The NE trended, S-shaped central canyon is ~ 430 km in length and inferred to firstly develop at 10.5 Ma in the east of Qiongdongnan Basin, then evolve westwards and form at ~ 5.5 Ma in the west. The canyon was significantly filled by sediment in 5.5–4.2 Ma and 4.2–3.8 Ma, during which the sediment can be transported from west to the Baodao and Changchang Sags in the east. Four stages of sedimentary accumulation in Qiongdongnan Basin were identified. They are 86 m/myr in 45–36 Ma, 200–300 m/myr in 36–15.5 Ma, 90–130 m/myr in 15.5–5.5 Ma and 150–630 m/myr since 5.5 Ma. It is markedly different from that of Yinggehai–Song Hong basin in some periods, i.e. the early Oligocene (36–30 Ma) and the early to middle Miocene (21–10.5 Ma), which suggest the Qiongdongnan Basin may have other provenances besides the Red River, such as Hainan Island and Indochina.

A procedure for testing the significance of orbital tuning of the martian polar layered deposits

Layered deposits of dusty ice in the martian polar caps have been hypothesized to record climate changes driven by orbitally induced variations in the distribution of incoming solar radiation. Attempts to identify such an orbital signal by tuning a stratigraphic sequence of polar layered deposits (PLDs) to match an assumed forcing introduce a risk of identifying spurious matches between unrelated records. We present an approach for evaluating the significance of matches obtained by orbital tuning, and investigate the utility of this approach for identifying orbital signals in the Mars PLDs. Using a set of simple models for ice and dust accumulation driven by insolation, we generate synthetic PLD stratigraphic sequences with nonlinear time–depth relationships. We then use a dynamic time warping algorithm to attempt to identify an orbital signal in the modeled sequences, and apply a Monte Carlo procedure to determine whether this match is significantly better than a match to a random sequence that contains no orbital signal. For simple deposition mechanisms in which dust deposition rate is constant and ice deposition rate varies linearly with insolation, we find that an orbital signal can be confidently identified if at least 10% of the accumulation time interval is preserved as strata. Addition of noise to our models raises this minimum preservation requirement, and we expect that more complex deposition functions would generally also make identification more difficult. In light of these results, we consider the prospects for identifying an orbital signal in the actual PLD stratigraphy, and conclude that this is feasible even with a strongly nonlinear relationship between stratigraphic depth and time, provided that a sufficient fraction of time is preserved in the record and that ice and dust deposition rates vary predictably with insolation. Independent age constraints from other techniques may be necessary, for example, if an insufficient amount of time is preserved in the stratigraphy.

Origin and significance of intra-basement seismic reflections offshore western Norway

We use 3D seismic data to image a series of enigmatic, SW-dipping reflection packets within pre-Mesozoic crystalline basement offshore western Norway. Based on their low-angle dip and complex reflection wave-train our preferred interpretation is that the reflection packets are the seismic expression of mylonitic zones generated by nappe emplacement during the Caledonian orogeny. Late Jurassic faults truncate and offset these reflection packets by several hundred metres, suggesting that these faults did not exploit pre-existing basement weaknesses. Our observations suggest that older basement fabrics may not always play a significant role in determining the geometry of later fault systems. Supplementary materials: Details of the time-depth relationships from wells that have been used to depth-convert interpretations (S1), a supplementary figure showing an uninterpreted version of the seismic profile presented in Fig. 2 (S2) and a throw v. distance plot for fault F1 at the structural levels of the basement reflection and intra-basement reflection 1 are available at www.geolsoc.org.uk/SUP18683 .

Hyperbolic Velocity Model

Asymptotically bounded velocity profiles describe the vertical velocity variations in compacted sediments in a more realistic way than unbounded velocity models, and allow presenting the subsurface by a smaller number of thicker layers. The first and the simplest asymptotically bounded model is the Hyperbolic velocity profile proposed by Muscatin 1937, and our paper is an extension of this early study. The Hyperbolic model has an advantage over other bounded models: The velocity increases with depth and approaches the limiting value with a more smooth and gradual rate. We derive the time-depth relationships, forward and backward transforms between the instantaneous velocity profile and the effective models (average, RMS and fourth order average velocities), study the trajectories for pre-critical and post-critical curved rays and derive the equations for traveltime, lateral propagation and arc length. We compare the ray paths obtained with the Hyperbolic model and with the other bounded velocity profiles.

Reconciling sonic logs with check-shot surveys: Stretching synthetic seismograms

Every seismic interpretation project needs to begin with an attempt to tie seismic reflectors to geologic units via synthetic seismograms. However, the synthetics normally do not tie character-wise to the seismic, and require stretching (occasionally squeezing). Check-shot surveys are frequently used in an effort to determine the amount of stretch necessary. However, the time-depth relationships from check shots usually differ by alarming amounts from the time-depth relationships from sonics. There is usually no discernible pattern to the mismatches. The interpreter knows that both check shots and compressional acoustic (sonic) logs are subject to error, so there is no way to know which to trust. Since no interpretation can ever be more credible than the well-seismic tie it is based on, this problem is worthy of considerable attention. The reasons that check shots do not tie sonics are many, but may be grouped into three categories: Therefore, a three-stage correction process is required to remedy this three-stage problem: (1) purify the logs; (2) reject check shots …

Velocity–depth and time–depth relationships for a decompacted uplifted unit

ABSTRACTA linear instantaneous velocity model is used to describe the velocity variations in an uplifted unit that has been partly decompacted as a result of the reduction in overburden that often accompanies uplift. The model results in a series of equations for deriving values for the function parameters in the velocity–depth and the time– depth domains and for carrying out time‐to‐depth conversions. The formulation uses the base of the unit as a reference level to generate the reference datum from a combination of the depth of the base of the unit and a parameter that represents the decompaction factor.

Regional Fractures II: Fracturing of Mesaverde Reservoirs in the Piceance Basin, Colorado (1)

Cretaceous strata in the east-central Piceance basin of northwestern Colorado were subjected to several phases of stress due to horizontal tectonic compression, burial, and uplift. These rocks contain a regional set of west-northwest extension fractures, observed in abundant vertical and deviated core from the U.S. Department of Energy's Multiwell Experiment and Slant Hole Completion Test wells. Time-depth relationships, fracture orientation, and fluid inclusion analyses indicate that the fractures formed at about 36-40 Ma, during a phase of increased Laramide west-northwest compression, and in a pressure-temperature regime compatible with the geologically reconstructed maximum burial depths. This regional fracturing is an example of load-parallel extension fracturing and basinwide dilatancy at depth, under conditions of high pore pressure and anisotropic, tectonically created, horizontal stress.

Tectonism, Subsidence, and Fracturing of Mesaverde Reservoirs in the Piceance Basin, Colorado: ABSTRACT

Cretaceous strata in the Piceance basin of northwestern Colorado were subjected to several phases of tectonic, burial, and uplift stress. However, extensive data on fractures in core from the US Department of Energy's Multiwell Experiment (MWX) wells suggest that most of the fractures in the Cretaceous strata in this part of the basin originated during one episode of stress. Time-depth relationships, fracture orientations, and fluid inclusion analyses all indicate that fracturing occurred about 35-40 m.y. during Laramide west-northwest horizontal compression. Most Cretaceous rocks at the MWX site contain a single set of west-northwest extension fractures. Isochore calculations indicate trapping pressures around 325 bars for fluid inclusions in 16 samples of quartz and calcite mineralization from the fractures, suggesting that mineralization occurred in a pressure-temperature regime compatible with the reconstructed burial depths of 10,000-12,000 ft. Younger episodes of stress are recorded in both the post-Cretaceous strata and in the less deeply buried Cretaceous strata near the basin boundaries. However, the effects of these younger stresses are not evident in deeply buried Mesaverde reservoirs at the MWX site in the east-central part of the basin. Moreover, if the orientation of the horizontal compressive stress rotated significantly during the late Laramide, as suggested more » by some authors, its effects are not apparent in the Cretaceous formations examined. Most measurements of ancient and present-day stress in Mesaverde strata at the MWX site indicate only west-northwest Laramide compression. « less

Predicting Interval Transit Time for Synthetic Seismograms from Nuclear Well Logs: ABSTRACT

Sonic logs commonly were not included as part of the logging program on older wells. In certain areas, wells are drilled with air. The absence of drilling fluid in the borehole eliminates the possibility of recording a sonic log. In other areas, sonic logs are either of poor quality or, for one reason or another, omitted from the logging program. A method has been developed to predict interval transit times using nuclear well logs. It involves combining neutron, density, and gamma-ray log measurements into a log of predicted interval transit times referred to as a synthetic sonic log. The method involves a combination of commonly used and accepted well-log interpretation techniques. It effectively accounts for lithology, shale, porosity and hydrocarbon effects. The method requires only 3 parameters, which may be selected based on the well-log data. The synthetic sonic log agrees well with actual sonic-log measurements over a wide variety of geographic areas and borehole depths. The method is effective for formations commonly encountered in geophysical well logging including salt, shale, sandstone, and carbonate. The synthetic sonic log is an excellent substitute for use in generating synthetic seismograms and establishing time-depth relationships. End_of_Article - Last_Page 246------------

A REGIONAL INVESTIGATION TO DETERMINE TIME-DEPTH AND VELOCITY-DEPTH RELATIONSHIPS APPLICABLE IN CENTRAL AUSTRALIA

Simple time-depth and velocity-depth relationships have been derived in the Western Great Artesian Basin which can be employed with seismic time information in the prediction of depth and instantaneous velocity.These relationships have been formed by using data from calibrated sonic logs and by combining a similarity between Acheson's velocity-depth equation and the “theoretical one-sixth power laws”.Variations, corresponding to horizons encountered, in the parameter “a” the equation t = aZ.83 enable the prediction of depths to the horizons C, P and Z.