Elastic anisotropy of the Middle Bakken Formation

The Bakken Formation consists of three members: The Upper Bakken and Lower Bakken are dark marine shales with high organic content, whereas the Middle Bakken consists of mixed carbonates and clastics and is the main reservoir unit, despite having low porosity and permeability. Dipole S-wave data acquired in a lateral well in the Middle Bakken Formation revealed this formation to be anisotropic. Backus upscaling of logs acquired in a nearby vertical pilot well in the same layers sampled by the lateral well gave estimates of the anisotropy that were too small to explain the S-wave anisotropy measured in the lateral well. The observed anisotropy was interpreted in terms of bedding-parallel compliant discontinuities such as microcracks and low-aspect-ratio pores. The presence of bedding-parallel microcracks and low-aspect-ratio pores may contribute to the permeability of the tight Middle Bakken reservoir, and the sensitivity of P- and S-wave velocities to the presence of microcracks and low aspect ratio pores suggested the use of sonic and seismic measurements for identifying the productive zones in the low-permeability Middle Bakken reservoir.

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P-to-S-wave velocity ratio in organic shales

Geophysics ◽

10.1190/geo2018-0723.1 ◽

2019 ◽

Vol 84 (6) ◽

pp. MR205-MR222 ◽

Cited By ~ 2

Author(s):

Sheyore John Omovie ◽

John P. Castagna

Keyword(s):

Wave Velocity ◽

Pore Space ◽

Velocity Ratio ◽

Organic Content ◽

Theoretical Modeling ◽

S Wave ◽

Lower Velocity ◽

Wave Velocities ◽

Ultrasonic Measurements

In situ P- and S-wave velocity measurements in a variety of organic-rich shales exhibit P-to-S-wave velocity ratios that are significantly lower than lithologically similar fully brine-saturated shales having low organic content. It has been hypothesized that this drop could be explained by the direct influence of kerogen on the rock frame and/or by the presence of free hydrocarbons in the pore space. The correlation of hydrocarbon saturation with total organic content in situ makes it difficult to separate these possible mechanisms using log data alone. Theoretical bounding equations, using pure kerogen as an end-member component without associated gas, indicate that kerogen reduces the P- and S-wave velocities but does not in general reduce their ratio enough to explain the observed low velocity ratio. The theoretical modeling is consistent with ultrasonic measurements on organic shale core samples that indicate no dependence of velocity ratios on the kerogen volume alone. Sonic log measurements of P- and S-wave velocities in seven organic-rich shale formations deviate significantly (typically more than 5%) from the Greenberg-Castagna empirical brine-saturated shale trend toward lower velocity ratios. In these formations, and on core measurements, Gassmann fluid substitution to 100% brine saturation yields velocity ratios consistent with the Greenberg-Castagna velocity trend for fully brine-saturated shales, despite the high organic content. These sonic and ultrasonic measurements, as well as theoretical modeling, suggest that the velocity ratio reduction in organic shales is best explained by the presence of free hydrocarbons.

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EXPERIMENTAL EVALUATION OF ULTRASONIC VELOCITIES AND ANISOTROPY IN THE TUSCALOOSA MARINE SHALE FORMATION

Interpretation ◽

10.1190/int-2019-0268.1 ◽

2020 ◽

pp. 1-62 ◽

Cited By ~ 2

Author(s):

Jamal Ahmadov ◽

Mehdi Mokhtari

Keyword(s):

Wave Velocity ◽

Mineralogical Composition ◽

Organic Content ◽

P Wave ◽

S Wave ◽

Velocity Anisotropy ◽

Wave Velocities ◽

Marine Shale ◽

P Wave Velocity ◽

Degree Of Anisotropy

Tuscaloosa Marine Shale (TMS) formation is a clay- and organic-rich emerging shale play with a considerable amount of hydrocarbon resources. Despite the substantial potential, there have been only a few wells drilled and produced in the formation over the recent years. The analyzed TMS samples contain an average of 50 wt% total clay, 27 wt% quartz and 14 wt% calcite and the mineralogy varies considerably over the small intervals. The high amount of clay leads to pronounced anisotropy and the frequent changes in mineralogy result in the heterogeneity of the formation. We studied the compressional (VP) and shear-wave (VS) velocities to evaluate the degree of anisotropy and heterogeneity, which impact hydraulic fracture growth, borehole instabilities, and subsurface imaging. The ultrasonic measurements of P- and S-wave velocities from five TMS wells are the best fit to the linear relationship with R2 = 0.84 in the least-squares criteria. We observed that TMS S-wave velocities are relatively lower when compared to the established velocity relationships. Most of the velocity data in bedding-normal direction lie outside constant VP/VS lines of 1.6–1.8, a region typical of most organic-rich shale plays. For all of the studied TMS samples, the S-wave velocity anisotropy exhibits higher values than P-wave velocity anisotropy. In the samples in which the composition is dominated by either calcite or quartz minerals, mineralogy controls the velocities and VP/VS ratios to a great extent. Additionally, the organic content and maturity account for the velocity behavior in the samples in which the mineralogical composition fails to do so. The results provide further insights into TMS Formation evaluation and contribute to a better understanding of the heterogeneity and anisotropy of the play.

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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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Elastic properties of continental carbonates: From controlling factors to an applicable model for acoustic-velocity predictions

Geophysics ◽

10.1190/geo2017-0344.1 ◽

2019 ◽

Vol 84 (1) ◽

pp. MR45-MR59 ◽

Cited By ~ 8

Author(s):

Jean-Baptiste Regnet ◽

Jérôme Fortin ◽

Aurélien Nicolas ◽

Matthieu Pellerin ◽

Yves Guéguen

Keyword(s):

Aspect Ratio ◽

Elastic Properties ◽

Carbonate Rocks ◽

Effective Medium ◽

Acoustic Velocity ◽

Controlling Factors ◽

S Wave ◽

Wave Velocities ◽

Diagenetic Alteration ◽

Continental Carbonates

We have provided new insights into the controlling factors of elastic properties in continental carbonate rocks and introduced an applicable model for acoustic-velocity predictions in such a medium. Petrophysical properties (porosity, permeability, P- and S-wave velocities) from laboratory measurements have been coupled with thin-section observations and characterizations, and X-ray diffraction (XRD) analyses. A major achievement is the establishment of the link between the mineralogical composition and the P- and S-wave velocity dispersion at a given porosity. This reflects the subtle interplay between physicochemical and biological precipitation of continental carbonates, which can also be associated with a strong influence of detrital mineralogical inputs. The result is a mineralogical commixture, coupled to a wide array of pore types inherited from the strong ability of carbonate rocks to undergo diagenetic alteration. The proposed model takes into account the elastic moduli of the minerals, porosity, and pore shape, and it is based on the effective medium theory. We have considered the case in which the medium contained randomly oriented pores with different aspect ratios. Overall, the fit between the predicted trends and the experimental data is fairly good, especially for calcite and quartz matrix mineralogy. The results are even better when considering mineralogy inferred from XRD data, although in some case, and despite the aspect ratio variation in both simulations, the model fails to accurately predict the P-wave velocities. This probably means that another factor is at stake beside mineralogy. This can be explained by the limitation of the effective medium approach, which oversimplifies the reality and fails to account for the variability of some aspect ratio from one inclusion to another.

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Determination of elastic anisotropy of rocks from P- and S-wave velocities: numerical modelling and lab measurements

Geophysical Journal International ◽

10.1093/gji/ggu332 ◽

2014 ◽

Vol 199 (3) ◽

pp. 1682-1697 ◽

Cited By ~ 19

Author(s):

Tomáš Svitek ◽

Václav Vavryčuk ◽

Tomáš Lokajíček ◽

Matěj Petružálek

Keyword(s):

Numerical Modelling ◽

Elastic Anisotropy ◽

S Wave ◽

Wave Velocities

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Influence of fluids on VP/VS ratio: increase or decrease?

Geophysical Journal International ◽

10.1093/gji/ggy518 ◽

2018 ◽

Vol 216 (3) ◽

pp. 2037-2043 ◽

Cited By ~ 1

Author(s):

Nicolas Brantut ◽

Emmanuel C David

Keyword(s):

Aspect Ratio ◽

Poisson’S Ratio ◽

Subduction Zones ◽

Compressible Fluids ◽

Poisson's Ratio ◽

Modulus Ratio ◽

S Wave ◽

Wave Velocities ◽

Fluid Properties ◽

Solid Bulk

SUMMARY The evolution of the ratio between P- and S-wave velocities (VP/VS) with increasing fluid-saturated porosity is computed for isotropic rocks containing spheroidal pores. The ratio VP/VS is shown to either decrease or increase with increasing porosity, depending on the aspect ratio α of the pores, fluid to solid bulk modulus ratio ζ and Poisson’s ratio ν0 of the solid constituents of the rock. A critical initial Poisson’s ratio ν0, crit is computed, separating cases where VP/VS increases (if ν0 < ν0, crit) or decreases (if ν0 > ν0, crit) with increasing porosity. For thin cracks and highly compressible fluids, ν0, crit is approximated by $0.157\, \zeta /\alpha$, whereas for spherical pores ν0, crit is given by 0.2 + 0.8ζ. When ν0 is close to ν0, crit, the evolution of VP/VS with increasing fluid-saturated porosity is near neutral and depends on subtle changes in pore shape and fluid properties. This regime is found to be relevant to partially dehydrated serpentinites in subduction zones (porosity of aspect ratio near 0.1 and ζ in the range 0.01–0.1), and makes detection of these rocks and possibly elevated fluid pressures difficult from VP/VS only.

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