AVO analysis for high amplitude anomalies using 2D pre-stack seismic data

Amplitude variation with offset (AVO) analysis is an 1 efficient tool for hydrocarbon detection and identification of elastic rock properties and fluid types. It has been applied in the present study using reprocessed pre-stack 2D seismic data (1992, Caulerpa) from north-west of the Bonaparte Basin, Australia. The AVO response along the 2D pre-stack seismic data in the Laminaria High NW shelf of Australia was also investigated. Three hypotheses were suggested to investigate the AVO behaviour of the amplitude anomalies in which three different factors; fluid substitution, porosity and thickness (Wedge model) were tested. The AVO models with the synthetic gathers were analysed using log information to find which of these is the controlling parameter on the AVO analysis. AVO cross plots from the real pre-stack seismic data reveal AVO class IV (showing a negative intercept decreasing with offset). This result matches our modelled result of fluid substitution for the seismic synthetics. It is concluded that fluid substitution is the controlling parameter on the AVO analysis and therefore, the high amplitude anomaly on the seabed and the target horizon 9 is the result of changing the fluid content and the lithology along the target horizons. While changing the porosity has little effect on the amplitude variation with offset within the AVO cross plot. Finally, results from the wedge models show that a small change of thickness causes a change in the amplitude; however, this change in thickness gives a different AVO characteristic and a mismatch with the AVO result of the real 2D pre-stack seismic data. Therefore, a constant thin layer with changing fluids is more likely to be the cause of the high amplitude anomalies.

AVO Analysis of a Weak BSR on the Hikurangi Margin, New Zealand

<p><b>Gas hydrates occur in deep, cold areas on the Hikurangi margin, New Zealand, generally at water depths of ≥ 600m and ≤ 8oC temperature. In these areas elevated hydrostatic pressures and low temperatures create stable conditions for hydrate formation. The occurrence of Bottom-Simulating Reflections (BSRs) is known to indicate the Base of the Gas Hydrate Stability (BGHS) zone, below which solid hydrates cannot exist due to increasing temperatures of sediments. BSRs in most settings worldwide are thought to be largely caused by free gas at the base of the gas hydrate stability zone. They are characterized by a large negative reflection coefficient due to significant decrease in P-wave velocity attributed to the presence of gas below the BSR. On the Hikurangi margin however, many BSRs appear relatively weak. This study presents the results of Amplitude Variation with Offset (AVO) analysis of a weak BSR beneath Puke Ridge, a thrust ridge on the accretionary wedge east of Gisborne, North Island. Rock-physics modelling is used to interpret the findings.</b></p> <p>The 05CM04 seismic line has been processed by preserving the amplitude and care has been taken to not bias the variation of reflectivity coefficient with offset. The zero-offset reflection coefficient or AVO intercept (A) is in the range of -0.008 to - 0.015 and the AVO gradient (B) is between -0.015 and -0.03.</p> <p>Rock-physics modelling was employed to determine the possible concentrations of gas and hydrate that can yield the observed reflection coefficients. Negligible hydrate saturation above with a patchy gas distribution of 3% saturation beneath the BSR might explain this pattern. An alternative end-member estimation of 13% saturation of hydrate in a frame-supporting model with no gas beneath it could generate the observed reflection coefficient but it is geologically unlikely. Synthetic modelling reveals that the low reflectivity of the BSR could also be due to the presence of thin layers of more concentrated or evenly distributed gas but this scenario is considered to be geologically unlikely.</p> <p>BSRs beneath some thrust ridges in the southern Hikurangi margin, appear as a series of clearly separated bright spots, which indicate free gas accumulations which when connected mimic the geometry of the seafloor. The most likely lithologic explanation for these high amplitude patches within weak BSRs, is the concept of segmented BSRs which is also seen in the Gulf of Mexico. The bright ―gas‖ anomalies are inferred to correlate with sand-rich high permeability layers while the weak BSR could be due to low saturations of gas in clay-rich low permeability layers. The weak BSR beneath the Puke Ridge is indicative of low and patchy gas saturations in low-permeability reservoir rocks while high amplitude patches found in this area may indicate high-permeability sands that may be attractive reservoir rocks for future gas hydrate production.</p>

AVO Analysis of a Weak BSR on the Hikurangi Margin, New Zealand

<p><b>Gas hydrates occur in deep, cold areas on the Hikurangi margin, New Zealand, generally at water depths of ≥ 600m and ≤ 8oC temperature. In these areas elevated hydrostatic pressures and low temperatures create stable conditions for hydrate formation. The occurrence of Bottom-Simulating Reflections (BSRs) is known to indicate the Base of the Gas Hydrate Stability (BGHS) zone, below which solid hydrates cannot exist due to increasing temperatures of sediments. BSRs in most settings worldwide are thought to be largely caused by free gas at the base of the gas hydrate stability zone. They are characterized by a large negative reflection coefficient due to significant decrease in P-wave velocity attributed to the presence of gas below the BSR. On the Hikurangi margin however, many BSRs appear relatively weak. This study presents the results of Amplitude Variation with Offset (AVO) analysis of a weak BSR beneath Puke Ridge, a thrust ridge on the accretionary wedge east of Gisborne, North Island. Rock-physics modelling is used to interpret the findings.</b></p> <p>The 05CM04 seismic line has been processed by preserving the amplitude and care has been taken to not bias the variation of reflectivity coefficient with offset. The zero-offset reflection coefficient or AVO intercept (A) is in the range of -0.008 to - 0.015 and the AVO gradient (B) is between -0.015 and -0.03.</p> <p>Rock-physics modelling was employed to determine the possible concentrations of gas and hydrate that can yield the observed reflection coefficients. Negligible hydrate saturation above with a patchy gas distribution of 3% saturation beneath the BSR might explain this pattern. An alternative end-member estimation of 13% saturation of hydrate in a frame-supporting model with no gas beneath it could generate the observed reflection coefficient but it is geologically unlikely. Synthetic modelling reveals that the low reflectivity of the BSR could also be due to the presence of thin layers of more concentrated or evenly distributed gas but this scenario is considered to be geologically unlikely.</p> <p>BSRs beneath some thrust ridges in the southern Hikurangi margin, appear as a series of clearly separated bright spots, which indicate free gas accumulations which when connected mimic the geometry of the seafloor. The most likely lithologic explanation for these high amplitude patches within weak BSRs, is the concept of segmented BSRs which is also seen in the Gulf of Mexico. The bright ―gas‖ anomalies are inferred to correlate with sand-rich high permeability layers while the weak BSR could be due to low saturations of gas in clay-rich low permeability layers. The weak BSR beneath the Puke Ridge is indicative of low and patchy gas saturations in low-permeability reservoir rocks while high amplitude patches found in this area may indicate high-permeability sands that may be attractive reservoir rocks for future gas hydrate production.</p>

De-risking Appraisal Phase in the Recent Gas Discovery in Banggai-Sula Basin, Central Sulawesi: Application of AVO Analysis and Pre-stack Inversion in the Sub-ophiolite Reef Carbonate Play

Following the success in the exploration drilling campaign in the last few years, Pertamina EP puts the recently discovered Wol Structure into the appraisal stage. The exploration wells Wol-001 and Wol-002 were spudded in 2017 and 2019 respectively, and both flowed a significant gas rate from an excellent reservoir of Miocene Reef of Minahaki Formation. A good understanding of the reservoir distribution was essential in such a stage. Therefore, a proper reservoir characterization was then carried out for further appraisal purposes. Using the improved quality data from the latest 5D interpolation-PSDM as input, integration of amplitude versus offset (AVO) techniques and rock physics analysis was conducted to investigate the hydrocarbon extent. The AVO class IIp was observed at the boundary between overlying Kintom Shale and gas saturated Minahaki limestone. It is indicated by a positive intercept (Ro), decreased amplitudes with offsets, and negative amplitudes in the far offsets. This polarity reversal characteristic is clearly seen from both AVO modeling and actual CDP in the well locations. Several CDPs inside and outside the closure were also examined to check the consistency. The slice of partial stack volumes has also exhibited a similar trend within the closure where class IIp is suggestive. Since the AVO attributes such as intercept and gradient solely were not able to visualize the reservoir extent properly, the pre-stack seismic inversion was performed to obtain a more accurate reservoir distribution through quantitative interpretation. A cross plot of P-impedance (Ip) over S-impedance (Is) differentiates the gas zone clearly from the wet linear trend. A depth slice at GWC (gas water contact) level describes that most of the Wol Structure is gas-saturated including the newly identified closure in the northwest. It is a three-way dip closure formed by limestone that was dragged upward by a thrust fault. Interestingly, it has a similar AVO response to the main Wol Structure which suggests a gas-bearing reservoir. This work brings an added value to the use of AVO analysis and pre-stack inversion for hydrocarbon mapping for appraisal purposes. Not only it has largely reduced the subsurface uncertainty, but also revealed an upside potential that is worth considering in future exploration.

Near Surface Velocity Estimation Using GPR Data: Investigations by Numerical Simulation, and Experimental Approach with AVO Response

The velocity of near-surface materials is one of the most important for Ground-Penetrating Radar (GPR). In the study, we evaluate the options for determining the GPR velocity to measure the accuracy of velocity approximations from the acquired GPR data at an experimental site in Hangzhou, China. A vertical profile of interval velocities can be estimated from each common mid-point (CMP) gather using velocity spectrum analysis. Firstly, GPR data are acquired and analyzed using the popular method of hyperbola fitting which generated surprisingly high subsurface signal velocity estimates while, for the same profile, the Amplitude variation with offset (AVO) analysis of the GPR data (using the same hyperbola fitting method) generate a more reasonable subsurface signal velocity estimate. Several necessary processing steps are applied both for CMP and AVO analysis. Furthermore, experimental analysis is conducted on the same test site to get velocities of samples based on dielectric constant measurement during the drilling process. Synthetic velocities generated by AVO analysis are validated by the experimental velocities which confirmed the suitability of velocity interpretations.

Identification of nonstructural type traps within the limits of northern side of Dnieper-Donets Depression according to the data of AVO analysis and seismic inversion

Nonstructural type traps in the sedimentary cover of the northern side of the Dnieper-Donets depression are poorly studied by seismic methods due to many factors among which are the following: complicated geological structure and not so high quality of given data of geological-geophysical studies of last years. Identification of lithologically screened gas-saturated object has been demonstrated based on the studies of elastic dynamic characteristics by the methods of AVO-analysis and elastic seismic inversion. Acoustic and elastic properties have been analyzed in the wells with cross-dipole real and synthesized acoustics. Gas-saturated intervals have been identified based on the ratios VP /VS and acoustic impedance. According to AVO-studies within the northern side of DDD in the stratum of productive horizons of the moscovian horizon a positive answer has been obtained to the question «if AVO analysis works correctly in general within the limits of the northern side of DDD deposits». Determining factors that influence on the result during the application of AVO-analysis at the most part of gas condensate field of the northern side of DDD are effective thicknesses, depths of occurrence, lithology, poro-sity and quality of given seismic data and data from geophysical surveys of wells. Taking into account minor effective thicknesses, small values of porosity, significant depths of occurrence of productive layers typical for the northern side of DDD, the majority of gas-saturated intervals are not at all identified in the wave field by dynamic characteristics of seismic signal. All the seismic anomalies analyzed had effective thicknesses from 6 to 20 m. Within the limits of deposits of the northern side of DDD, AVO-anomalies of the 2nd class are modeled. Along with positive experience of identification of dynamically cont-rasting objects in the wave field limitations to use methods of AVO-analysis and elastic seismic inversion are generalized for the northern side shown in particular and the whole Dnieper-Donets depression.

IDENTIFICATION OF GAS HYDRATES AND BOTTOM SIMULATING REFLECTORS IN FAR OFFSET SEISMIC IMAGES

The presence of marine gas hydrates is routinely inferred based on the identification of bottom simulating reflectors (BSRs) in common depth-point (CDP) seismic images. Additional seismic studies such as amplitude variation with offset (AVO) analysis can be applied for corroboration. Though confirmation is needed by drilling and sampling, seismic analysis has proven to be a cost-effective approach to identify the presence of marine gas hydrates. Single channel far offset seismic images are investigated for what appears to be a more reliable and cost-effective indicator for the presence of bottom simulating reflectors than traditional CDP processing or AVO analysis. A non-traditional approach to processing seismic data is taken to be more relevant to imaging the gas/gas hydrate contact. Instead of applying the traditional CDP seismic processing workflows from the oil industry, we more carefully review the significant amount of information existing in the data to explore how the character of the data changes as offset angle increases. Three cases from different environments are selected for detailed analysis. These include 1) stratigraphy running parallel with the ocean bottom; 2) a potential bottom simulating reflector, running parallel to the ocean bottom, and cross-cutting dipping reflections, and 3) a suspected thermal intrusion without a recognizable bottom simulating reflector. This investigation considers recently collected multi-channel seismic data from the deep waters of the central Aleutian Basin beneath the Bering Sea, the pre-processing of the data sets, and the methodology for processing and display to generate single channel seismic images. Descriptions are provided for the single channel near and far offset seismic images for the example cases. Results indicate that BSRs related to marine gas hydrates, and originating due to the presence of free gas, are more easily and uniquely identifiable from single channel displays of far offset seismic images than from traditional CDP displays.