Identification of hydrocarbons in chalk reservoirs from surface seismic data: South Arne field, North Sea

Seismic data are mainly used to map out structures in the subsurface, but are also increasingly used to detect differences in porosity and in the fluids that occupy the pore space in sedimentary rocks. Hydrocarbons are generally lighter than brine, and the bulk density and sonic velocity (speed of pressure waves or P-wave velocity) of hydrocarbon-bearing sedimentary rocks are therefore reduced compared to non-reservoir rocks. However, sound is transmitted in different wave forms through the rock, and the shear velocity (speed of shear waves or S-wave velocity) is hardly affected by the density of the pore fluid. In order to detect the presence of hydrocarbons from seismic data, it is thus necessary to investigate how porosity and pore fluids affect the acoustic properties of a sedimentary rock. Much previous research has focused on describing such effects in sandstone (see Mavko et al. 1998), and only in recent years have corresponding studies on the rock physics of chalk appeared (e.g. Walls et al. 1998; Røgen 2002; Fabricius 2003; Gommesen 2003; Japsen et al. 2004). In the North Sea, chalk of the Danian Ekofisk Formation and the Maastrichtian Tor Formation are important reservoir rocks. More information could no doubt be extracted from seismic data if the fundamental physical properties of chalk were better understood. The presence of gas in chalk is known to cause a phase reversal in the seismic signal (Megson 1992), but the presence of oil in chalk has only recently been demonstrated to have an effect on surface seismic data (Japsen et al. 2004). The need for a better link between chalk reservoir parameters and geophysical observations has, however, strongly increased since the discovery of the Halfdan field proved major reserves outside four-way dip closures (Jacobsen et al. 1999; Vejbæk & Kristensen 2000).

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Sediments with gas hydrates: Internal structure from seismic AVO

Geophysics ◽

10.1190/1.1444462 ◽

1998 ◽

Vol 63 (5) ◽

pp. 1659-1669 ◽

Cited By ~ 193

Author(s):

Christine Ecker ◽

Jack Dvorkin ◽

Amos Nur

Keyword(s):

Internal Structure ◽

Wave Velocity ◽

Seismic Data ◽

Pore Space ◽

Rock Physics ◽

P Wave ◽

S Wave ◽

High Concentration ◽

Amplitude Variation With Offset ◽

Bottom Simulating Reflector

We interpret amplitude variation with offset (AVO) data from a bottom simulating reflector (BSR) offshore Florida by using rock‐physics‐based synthetic seismic models. A previously conducted velocity and AVO analysis of the in‐situ seismic data showed that the BSR separates hydrate‐bearing sediments from sediments containing free methane. The amplitude at the BSR are increasingly negative with increasing offset. This behavior was explained by P-wave velocity above the BSR being larger than that below the BSR, and S-wave velocity above the BSR being smaller than that below the BSR. We use these AVO and velocity results to infer the internal structure of the hydrated sediment. To do so, we examine two micromechanical models that correspond to the two extreme cases of hydrate deposition in the pore space: (1) the hydrate cements grain contacts and strongly reinforces the sediment, and (2) the hydrate is located away from grain contacts and does not affect the stiffness of the sediment frame. Only the second model can qualitatively reproduce the observed AVO response. Thus inferred internal structure of the hydrate‐bearing sediment means that (1) the sediment above the BSR is uncemented and, thereby, mechanically weak, and (2) its permeability is very low because the hydrate clogs large pore‐space conduits. The latter explains why free gas is trapped underneath the BSR. The seismic data also indicate the absence of strong reflections at the top of the hydrate layer. This fact suggests that the high concentration of hydrates in the sediment just above the BSR gradually decreases with decreasing depth. This effect is consistent with the fact that the low‐permeability hydrated sediments above the BSR prevent free methane from migrating upwards.

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Elastic reflectivity vectors and the geometry of intercept-gradient crossplots

The Leading Edge ◽

10.1190/tle38100762.1 ◽

2019 ◽

Vol 38 (10) ◽

pp. 762-769

Author(s):

Patrick Connolly

Keyword(s):

Elastic Property ◽

Elastic Properties ◽

Wave Velocity ◽

Seismic Data ◽

Measurement Errors ◽

Rock Physics ◽

P Wave ◽

S Wave ◽

P Wave Velocity ◽

Well Log Analysis

Reflectivities of elastic properties can be expressed as a sum of the reflectivities of P-wave velocity, S-wave velocity, and density, as can the amplitude-variation-with-offset (AVO) parameters, intercept, gradient, and curvature. This common format allows elastic property reflectivities to be expressed as a sum of AVO parameters. Most AVO studies are conducted using a two-term approximation, so it is helpful to reduce the three-term expressions for elastic reflectivities to two by assuming a relationship between P-wave velocity and density. Reduced to two AVO components, elastic property reflectivities can be represented as vectors on intercept-gradient crossplots. Normalizing the lengths of the vectors allows them to serve as basis vectors such that the position of any point in intercept-gradient space can be inferred directly from changes in elastic properties. This provides a direct link between properties commonly used in rock physics and attributes that can be measured from seismic data. The theory is best exploited by constructing new seismic data sets from combinations of intercept and gradient data at various projection angles. Elastic property reflectivity theory can be transferred to the impedance domain to aid in the analysis of well data to help inform the choice of projection angles. Because of the effects of gradient measurement errors, seismic projection angles are unlikely to be the same as theoretical angles or angles derived from well-log analysis, so seismic data will need to be scanned through a range of angles to find the optimum.

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Using Seismic Data to Predict Shale Pore Pressure and Overpressure Sweet Spots: A Case Study from the Lower Silurian Longmaxi Formation in Sichuan Basin, China

10.2118/208098-ms ◽

2021 ◽

Author(s):

Sheng Chen ◽

Qingcai Zeng ◽

Xiujiao Wang ◽

Qing Yang ◽

Chunmeng Dai ◽

...

Keyword(s):

Prediction Model ◽

Wave Velocity ◽

Shale Gas ◽

Seismic Data ◽

P Wave ◽

Formation Pressure ◽

S Wave ◽

P Wave Velocity ◽

New Formation ◽

Sweet Spots

Abstract Practices of marine shale gas exploration and development in south China have proved that formation overpressure is the main controlling factor of shale gas enrichment and an indicator of good preservation condition. Accurate prediction of formation pressure before drilling is necessary for drilling safety and important for sweet spots predicting and horizontal wells deploying. However, the existing prediction methods of formation pore pressures all have defects, the prediction accuracy unsatisfactory for shale gas development. By means of rock mechanics analysis and related formulas, we derived a formula for calculating formation pore pressures. Through regional rock physical analysis, we determined and optimized the relevant parameters in the formula, and established a new formation pressure prediction model considering P-wave velocity, S-wave velocity and density. Based on regional exploration wells and 3D seismic data, we carried out pre-stack seismic inversion to obtain high-precision P-wave velocity, S-wave velocity and density data volumes. We utilized the new formation pressure prediction model to predict the pressure and the spatial distribution of overpressure sweet spots. Then, we applied the measured pressure data of three new wells to verify the predicted formation pressure by seismic data. The result shows that the new method has a higher accuracy. This method is qualified for safe drilling and prediction of overpressure sweet spots for shale gas development, so it is worthy of promotion.

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Shear‐wave velocity and density estimation from PS-wave AVO analysis: Application to an OBS dataset from the North Sea

Geophysics ◽

10.1190/1.1444833 ◽

2000 ◽

Vol 65 (5) ◽

pp. 1446-1454 ◽

Cited By ~ 33

Author(s):

Side Jin ◽

G. Cambois ◽

C. Vuillermoz

Keyword(s):

Wave Velocity ◽

North Sea ◽

Random Noise ◽

P Wave ◽

S Wave ◽

Data Set ◽

Avo Analysis ◽

The North ◽

Avo Inversion ◽

The North Sea

S-wave velocity and density information is crucial for hydrocarbon detection, because they help in the discrimination of pore filling fluids. Unfortunately, these two parameters cannot be accurately resolved from conventional P-wave marine data. Recent developments in ocean‐bottom seismic (OBS) technology make it possible to acquire high quality S-wave data in marine environments. The use of (S)-waves for amplitude variation with offset (AVO) analysis can give better estimates of S-wave velocity and density contrasts. Like P-wave AVO, S-wave AVO is sensitive to various types of noise. We investigate numerically and analytically the sensitivity of AVO inversion to random noise and errors in angles of incidence. Synthetic examples show that random noise and angle errors can strongly bias the parameter estimation. The use of singular value decomposition offers a simple stabilization scheme to solve for the elastic parameters. The AVO inversion is applied to an OBS data set from the North Sea. Special prestack processing techniques are required for the success of S-wave AVO inversion. The derived S-wave velocity and density contrasts help in detecting the fluid contacts and delineating the extent of the reservoir sand.

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Identification of gas hydrate dissociation from wireline-log data in the Shenhu area, South China Sea

Geophysics ◽

10.1190/geo2011-0324.1 ◽

2012 ◽

Vol 77 (3) ◽

pp. B125-B134 ◽

Cited By ~ 16

Author(s):

Xiujuan Wang ◽

Myung Lee ◽

Shiguo Wu ◽

Shengxiong Yang

Keyword(s):

Wave Velocity ◽

Gas Hydrate ◽

Seismic Data ◽

Pore Space ◽

P Wave ◽

Well Log ◽

Hydrate Dissociation ◽

Free Gas ◽

P Wave Velocity

Wireline logs were acquired in eight wells during China’s first gas hydrate drilling expedition (GMGS-1) in April–June of 2007. Well logs obtained from site SH3 indicated gas hydrate was present in the depth range of 195–206 m below seafloor with a maximum pore-space gas hydrate saturation, calculated from pore water freshening, of about 26%. Assuming gas hydrate is uniformly distributed in the sediments, resistivity calculations using Archie’s equation yielded hydrate-saturation trends similar to those from chloride concentrations. However, the measured compressional (P-wave) velocities decreased sharply at the depth between 194 and 199 mbsf, dropping as low as [Formula: see text], indicating the presence of free gas in the pore space, possibly caused by the dissociation of gas hydrate during drilling. Because surface seismic data acquired prior to drilling were not influenced by the in situ gas hydrate dissociation, surface seismic data could be used to identify the cause of the low P-wave velocity observed in the well log. To determine whether the low well-log P-wave velocity was caused by in situ free gas or by gas hydrate dissociation, synthetic seismograms were generated using the measured well-log P-wave velocity along with velocities calculated assuming both gas hydrate and free gas in the pore space. Comparing the surface seismic data with various synthetic seismograms suggested that low P-wave velocities were likely caused by the dissociation of in situ gas hydrate during drilling.

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Converted-wave model building and imaging based on common-focus-point methodology

Geophysics ◽

10.1190/geo2019-0549.1 ◽

2020 ◽

Vol 85 (6) ◽

pp. U139-U149

Author(s):

Hongwei Liu ◽

Mustafa Naser Al-Ali ◽

Yi Luo

Keyword(s):

Elastic Wave ◽

Wave Velocity ◽

Seismic Data ◽

Model Building ◽

P Wave ◽

Rock Properties ◽

S Wave ◽

Converted Wave ◽

Seismic Images ◽

Focus Point

Seismic images can be viewed as photographs for underground rocks. These images can be generated from different reflections of elastic waves with different rock properties. Although the dominant seismic data processing is still based on the acoustic wave assumption, elastic wave processing and imaging have become increasingly popular in recent years. A major challenge in elastic wave processing is shear-wave (S-wave) velocity model building. For this reason, we have developed a sequence of procedures for estimating seismic S-wave velocities and the subsequent generation of seismic images using converted waves. We have two main essential new supporting techniques. The first technique is the decoupling of the S-wave information by generating common-focus-point gathers via application of the compressional-wave (P-wave) velocity on the converted seismic data. The second technique is to assume one common VP/ VS ratio to approximate two types of ratios, namely, the ratio of the average earth layer velocity and the ratio of the stacking velocity. The benefit is that we reduce two unknown ratios into one, so it can be easily scanned and picked in practice. The PS-wave images produced by this technology could be aligned with the PP-wave images such that both can be produced in the same coordinate system. The registration between the PP and PS images provides cross-validation of the migrated structures and a better estimation of underground rock and fluid properties. The S-wave velocity, computed from the picked optimal ratio, can be used not only for generating the PS-wave images, but also to ensure well registration between the converted-wave and P-wave images.

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The Acoustic Properties of Sandy and Clayey Hydrate-Bearing Sediments

Energies ◽

10.3390/en12101825 ◽

2019 ◽

Vol 12 (10) ◽

pp. 1825

Author(s):

Xiao-Hui Wang ◽

Qiang Xu ◽

Ya-Nan He ◽

Yun-Fei Wang ◽

Yi-Fei Sun ◽

...

Keyword(s):

Wave Velocity ◽

Gas Hydrate ◽

Acoustic Properties ◽

P Wave ◽

S Wave ◽

Hydrate Saturation ◽

New Apparatus ◽

Sandy Sediments ◽

Hydrate Bearing Sediments ◽

And Control

Natural gas hydrates samples are rare and difficult to store and transport at in situ pressure and temperature conditions, resulting in difficulty to characterize natural hydrate-bearing sediments and to identify hydrate accumulation position and saturation at the field scale. A new apparatus was designed to study the acoustic properties of seafloor recovered cores with and without hydrate. To protect the natural frames of recovered cores and control hydrate distribution, the addition of water into cores was performed by injecting water vapor. The results show that hydrate saturation and types of host sediments are the two most important factors that govern the elastic properties of hydrate-bearing sediments. When gas hydrate saturation adds approximately to 5–25%, the corresponding P-wave velocity (Vp) increases from 1.94 to 3.93 km/s and S-wave velocity (Vs) increases from 1.14 to 2.23 km/s for sandy specimens; Vp and Vs for clayey samples are 1.72–2.13 km/s and 1.10–1.32 km/s, respectively. The acoustic properties of sandy sediments can be significantly changed by the formation/dissociation of gas hydrate, while these only minorly change for clayey specimens.

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Pyrolysis-induced P-wave velocity anisotropy in organic-rich shales

Geophysics ◽

10.1190/geo2013-0254.1 ◽

2014 ◽

Vol 79 (2) ◽

pp. D41-D53 ◽

Cited By ~ 28

Author(s):

Adam M. Allan ◽

Tiziana Vanorio ◽

Jeremy E. P. Dahl

Keyword(s):

Wave Velocity ◽

Elastic Anisotropy ◽

Rock Physics ◽

Confining Pressure ◽

Source Rocks ◽

Acoustic Properties ◽

P Wave ◽

Velocity Anisotropy ◽

Geochemical Parameters ◽

P Wave Velocity

The sources of elastic anisotropy in organic-rich shale and their relative contribution therein remain poorly understood in the rock-physics literature. Given the importance of organic-rich shale as source rocks and unconventional reservoirs, it is imperative that a thorough understanding of shale rock physics is developed. We made a first attempt at establishing cause-and-effect relationships between geochemical parameters and microstructure/rock physics as organic-rich shales thermally mature. To minimize auxiliary effects, e.g., mineralogical variations among samples, we studied the induced evolution of three pairs of vertical and horizontal shale plugs through dry pyrolysis experiments in lieu of traditional samples from a range of in situ thermal maturities. The sensitivity of P-wave velocity to pressure showed a significant increase post-pyrolysis indicating the development of considerable soft porosity, e.g., microcracks. Time-lapse, high-resolution backscattered electron-scanning electron microscope images complemented this analysis through the identification of extensive microcracking within and proximally to kerogen bodies. As a result of the extensive microcracking, the P-wave velocity anisotropy, as defined by the Thomsen parameter epsilon, increased by up to 0.60 at low confining pressures. Additionally, the degree of microcracking was shown to increase as a function of the hydrocarbon generative potential of each shale. At 50 MPa confining pressure, P-wave anisotropy values increased by 0.29–0.35 over those measured at the baseline — i.e., the immature window. The increase in anisotropy at high confining pressure may indicate a source of anisotropy in addition to microcracking — potentially clay mineralogical transformation or the development of intrinsic anisotropy in the organic matter through aromatization. Furthermore, the evolution of acoustic properties and microstructure upon further pyrolysis to the dry-gas window was shown to be negligible.

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Petroacoustics and composition of presalt rocks from Santos Basin

The Leading Edge ◽

10.1190/tle38050342.1 ◽

2019 ◽

Vol 38 (5) ◽

pp. 342-348 ◽

Cited By ~ 3

Author(s):

Guilherme Fernandes Vasquez ◽

Marcio José Morschbacher ◽

Camila Wense Dias dos Anjos ◽

Yaro Moisés Parisek Silva ◽

Vanessa Madrucci ◽

...

Keyword(s):

Clay Minerals ◽

Chemical Elements ◽

Pore Space ◽

Rock Physics ◽

P Wave ◽

Rock Properties ◽

Tectonic Activity ◽

S Wave ◽

Wave Velocities ◽

Lacustrine Carbonate

The deposition of the presalt section from Santos Basin began when Gondwana started to break up and South America and Africa were separating. Initial synrift carbonate deposits affected by relatively severe tectonic activity evolved to a lacustrine carbonate environment during the later stages of basin formation. Although the reservoirs are composed of carbonate rocks, the occurrence of faults and the intense colocation of igneous rocks served as a source of chemical elements uncommon in typical carbonate environments. Consequently, beyond the presence of different facies with complex textures and pore geometries, the presalt reservoir rocks present marked compositional and microstructural variability. Therefore, rock-physics modeling is used to understand and interpret the extensive laboratory measurements of P-wave velocities, S-wave velocities, and density that we have undertaken on the presalt carbonate cores from Santos Basin. We show that quartz and exotic clay minerals (such as stevensite and other magnesium-rich clay minerals), which have different values of elastic moduli and Poisson's ratio as compared to calcite and dolomite, may introduce noticeable “Poisson's reflectivity anomalies” on prestack seismic data. Moreover, although the authors concentrate their attention on composition, it will become clear that pore-space geometry also may influence seismic rock properties of presalt carbonate reservoirs.

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Predicting reservoir quality in the Bakken Formation, North Dakota, using petrophysics and 3C seismic data

Interpretation ◽

10.1190/int-2020-0007.1 ◽

2020 ◽

Vol 8 (4) ◽

pp. T851-T868

Author(s):

Andrea G. Paris ◽

Robert R. Stewart

Keyword(s):

North Dakota ◽

Seismic Data ◽

Rock Physics ◽

P Wave ◽

Well Logs ◽

Reservoir Quality ◽

Bakken Formation ◽

S Wave ◽

Converted Wave ◽

Wave Velocities

Combining rock-property analysis with multicomponent seismic imaging can be an effective approach for reservoir quality prediction in the Bakken Formation, North Dakota. The hydrocarbon potential of shale is indicated on well logs by low density, high gamma-ray response, low compressional-wave (P-wave) and shear-wave (S-wave) velocities, and high neutron porosity. We have recognized the shale intervals by cross plotting sonic velocities versus density. Intervals with total organic carbon (TOC) content higher than 10 wt% deviate from lower TOC regions in the density domain and exhibit slightly lower velocities and densities (<2.30 g/cm3). We consider TOC to be the principal factor affecting changes in the density and P- and S-wave velocities in the Bakken shales, where VP/ VS ranges between 1.65 and 1.75. We generate the synthetic seismic data using an anisotropic version of the Zoeppritz equations, including estimated Thomsen’s parameters. For the tops of the Upper and Lower Bakken, the amplitude shows a negative intercept and a positive gradient, which corresponds to an amplitude variation with offset of class IV. The P-impedance error decreases by 14% when incorporating the converted-wave information in the inversion process. A statistical approach using multiattribute analysis and neural networks delimits the zones of interest in terms of P-impedance, density, TOC content, and brittleness. The inverted and predicted results show reasonable correlations with the original well logs. The integration of well log analysis, rock physics, seismic modeling, constrained inversions, and statistical predictions contributes to identifying the areas of highest reservoir quality within the Bakken Formation.

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