Predicting lithology and transport properties from acoustic velocities based on petrophysical classification of siliciclastics

Geophysics ◽  
1994 ◽  
Vol 59 (3) ◽  
pp. 420-427 ◽  
Author(s):  
L. Vernik
Keyword(s):  
Transport Properties ◽  
Velocity Ratio ◽  
Clay Content ◽  
Unconsolidated Sediments ◽  
Velocity Versus ◽  
Critical Porosity ◽  
Porosity Reduction ◽  
Lithology Prediction ◽  
Acoustic Velocities

Based on the recently developed petrophysical classification of siliciclastics, which takes into account the amount of the volumetric clay content C and textural position of clay, it is shown that acoustic velocities can be fairly accurate tools in predicting lithology, porosity, and ultimately, transport properties of these rocks. Four major petrophysical groups of carbonate‐ and organic‐poor siliciclastics are distinguished: (1) clean arenites (C < 2 percent), (2) arenites and arkoses (C = 2–15 percent), (3) wackes (C = 15–35 percent), and (4) shales (C > 35 percent). The compressional velocity versus porosity relation for consolidated rocks in each of these groups is found to be linear with very high correlation coefficients. This allows for remarkably accurate porosity estimates or lithology prediction in consolidated siliciclastics from acoustic velocities compared to the widely used time average (Wyllie) equation or its improved modification (Raymer equations), both of which neglect textural factors, or recently proposed relations based on the critical porosity concept. The transforms proposed display fundamental trends subject to only a second‐order regional effects, such as details of mineralogy, grain size distribution, and authigenic clay development. These trends primarily reflect the processes of chemical diagenesis, including pressure solution, cementation, and mineral phase transformation. The processes of lithification of unconsolidated sediments by physical compaction and initial cementation are characterized by a steeper slope of the velocity‐porosity transform because of a more pronounced velocity increase compared to the porosity reduction at this stage. The use of the [Formula: see text] ratio versus velocity relation for lithology prediction is limited compared to the [Formula: see text] versus porosity plots; however, if both porosity and lithology are unknown, the velocity ratio can still be used for discriminating between predominantly grain‐supported reservoir rocks (clean arenite, arenite and arkose) and clay matrix‐supported (wacke, shale) rocks. Finally, a strong correlation between porosity and permeability of clean arenites is weakened somewhat in arenites. Nonetheless, even in the latter case, an order of magnitude accuracy in permeability assessment based on porosity can be achieved.

Wave Velocities in Sediments

1990 ◽  
Vol 195 ◽  
Author(s):  
Dominique Marion ◽  
Amos Nur ◽  
Hezhu Yin
Keyword(s):  
Pore Space ◽  
Low Frequency ◽  
Clay Content ◽  
Velocity Versus ◽  
Critical Porosity ◽  
Porous Sandstone ◽  
Linear Fit ◽  
Compressional Velocity

ABSTRACTSystematic relations between porosity and compressional velocity Vp in the three component (sand, grains, clay and brine) systems (1) porous sandstone, (2) sands, and (3) suspensions, were obtained using experimental data and models. In Cemented Shaley Sandstones Vp was found to correlate linearly with porosity and clay content. The velocities in clean sandstones are about 7% higher than those predicted by the linear fit, indicating that a small amount of clay significantly reduces the elastic moduli of sandstones.For uncemented shaley sand, a model for the dependence of sonic velocity and porosity on clay content and compaction was developed for sand with clay dispersed in the pore space and for shale with suspened sand grains. The model closely mimics the experimentally observed minimum for porosity and the peak in velocity versus clay content. The results explain much of the scatter in velocity data in-situ. Velocity in suspensions at ϕ = 39% of grains in brine is close to values predicted by the Reuss (Isostress) average. Velocity dispersion, as suggested by Biot (1956 a,b) is calculated and observed in coarser sediments such as sand, whereas velocities in the finer clay and silt follow Biot's low frequency value.In total, our results provide the complete dependence of velocity on porosity in brine saturated sediment with clays, ranging from pure quartz to pure clay and water. Our results also highlight the crucial role of the critical porosity ϕ at about 39%, and the transition from cemented to uncemented sands.

Stress sensitivity of sandstones

Geophysics ◽  
1996 ◽  
Vol 61 (2) ◽  
pp. 444-455 ◽  
Author(s):  
Jack Dvorkin ◽  
Amos Nur ◽  
Caren Chaika
Keyword(s):  
Elastic Modulus ◽  
Effective Stress ◽  
Elastic Moduli ◽  
Hydrostatic Stress ◽  
Clay Content ◽  
Stress Sensitivity ◽  
High Porosity ◽  
Effective Pressure ◽  
Critical Porosity ◽  
Difference Formula

Our observations made on dry‐sandstone ultrasonic velocity data relate to the variation in velocity (or modulus) with effective stress, and the ability to predict a velocity for a rock under one effective pressure when it is known only under a different effective pressure. We find that the sensitivity of elastic moduli, and velocities, to effective hydrostatic stress increases with decreasing porosity. Specifically, we calculate the difference between an elastic modulus, [Formula: see text], of a sample of porosity ϕ at effective pressure [Formula: see text] and the same modulus, [Formula: see text], at effective pressure [Formula: see text]. If this difference, [Formula: see text], is plotted versus porosity for a suite of samples, then the scatter of ΔM is close to zero as porosity approaches the critical porosity value, and reaches its maximum as porosity approaches zero. The dependence of this scatter on porosity is close to linear. Critical porosity here is the porosity above which rock can exist only as a suspension—between 36% and 40% for sandstones. This stress‐sensitivity pattern of grain‐supported sandstones (clay content below 0.35) practically does not depend on clay content. In practical terms, the uncertainty of determining elastic moduli at a higher effective stress from the measurements at a lower effective stress is small at high porosity and increases with decreasing porosity. We explain this effect by using a combination of two heuristic models—the critical porosity model and the modified solid model. The former is based on the observation that the elastic‐modulus‐versus‐porosity relation can be approximated by a straight line that connects two points in the modulus‐porosity plane: the modulus of the solid phase at zero porosity and zero at critical porosity. The second one reflects the fact that at constant effective stress, low‐porosity sandstones (even with small amounts of clay) exhibit large variability in elastic moduli. We attribute this variability to compliant cracks that hardly affect porosity but strongly affect the stiffness. The above qualitative observation helps to quantitatively constrain P‐ and S‐wave velocities at varying stresses from a single measurement at a fixed stress. We also show that there are distinctive linear relations between Poisson’s ratios (ν) of sandstone measured at two different stresses. For example, in consolidated medium‐porosity sandstones [Formula: see text], where the subscripts indicate hydrostatic stress in MPa. Linear functions can also be used to relate the changes (with hydrostatic stress) in shear moduli to those in compressional moduli. For example, [Formula: see text], where [Formula: see text] is shear modulus and [Formula: see text] is compressional modulus, both in GPa, and the subscripts indicate stress in MPa.

An approximation for the Xu‐White velocity model

Geophysics ◽  
2002 ◽  
Vol 67 (5) ◽  
pp. 1406-1414 ◽  
Author(s):  
Robert G. Keys ◽  
Shiyu Xu
Keyword(s):  
Clay Content ◽  
Velocity Model ◽  
Compressional Wave ◽  
Effective Medium ◽  
Critical Porosity ◽  
Shear Moduli ◽  
Effective Medium Method ◽  
Computationally Intensive ◽  
Medium Method ◽  
Rock Bulk

In 1995, S. Xu and R. E. White described a method for estimating compressional and shear‐wave velocities of shaley sandstones from porosity and shale content. Their model was able to predict the effect of increasing clay content on compressional‐wave velocity observed in laboratory measurements. A key step in the Xu‐White method estimates dry rock bulk and shear moduli for the sand/shale mixture. This step is performed numerically by applying the differential effective medium method to the Kuster‐Toksöz equations for ellipsoidal pores. This step is computationally intensive. Using reasonable assumptions about dry rock elastic properties, this step can be replaced with a set of approximations for dry rock bulk and shear moduli. Numerical experiments show an extremely close match between velocities obtained with these approximations and velocities computed with the differential effective medium method. These approximations simplify the application of the Xu‐White method, and make the method computationally more efficient. They also provide insight into the Xu‐White method. For example, these approximations show how the Xu‐White model is related to the critical porosity model.

ON INDUCED ELECTRICAL POLARIZATION AND GROUNDWATER

Geophysics ◽  
1968 ◽  
Vol 33 (5) ◽  
pp. 805-821 ◽  
Author(s):  
René Bodmer ◽  
S. H. Ward ◽  
H. F. Morrison
Keyword(s):  
Induced Polarization ◽  
Unconsolidated Sediments ◽  
Frequency Effect ◽  
Polarization Method ◽  
Near Surface ◽  
Frequency Effects ◽  
Santa Clara County ◽  
Potential Recharge

Clay horizons and other clay‐bearing unconsolidated sediments are potential sources of induced‐polarization anomalies. If such anomalies may be detected above system noise, the induced‐polarization method may be of value for in‐situ classification of unconsolidated sediments encountered in hydrological projects. One such project exists in Santa Clara County where near‐surface unconsolidated sediments are frequently considered as potential recharge areas. Of four areas surveyed with induced‐polarization apparatus in Santa Clara County, only two yielded significant frequency‐effect anomalies, and in each of these two the frequency effects were of the order of 3 percent. These anomalous frequency effects may be related to clayey gravels. The dipole‐dipole array, with spreads of 10 ft and 20 ft, was typically used in the study.

Remote acoustic classification of marine sediments with application to offshore Newfoundland

1983 ◽  
Vol 20 (7) ◽  
pp. 1195-1211 ◽  
Author(s):  
N. A. Cochrane ◽  
A. D. Dunsiger
Keyword(s):  
Marine Sediments ◽  
Classification Scheme ◽  
Spatial Coherence ◽  
Unconsolidated Sediments ◽  
Limited Area ◽  
Acoustic Source ◽  
Shallow Marine Sediments ◽  
Correlation Properties ◽  
Acoustic Classification

Shallow marine sediments can be remotely classified by the spatial correlation properties of their seismic reflection signatures provided one uses a highly repetitive broadband acoustic source. A classification scheme defined by three spatial coherence parameters is shown capable of automatically differentiating between several formations of unconsolidated sediments in a limited area of offshore Newfoundland. The consistency and generality of the technique are explored and comparisons with standard echogram interpretation are made.

Monitoring the changes in the microstructure and the elastic and transport properties of Eagle Ford marl during maturation

Geophysics ◽  
2018 ◽  
Vol 83 (5) ◽  
pp. MR263-MR281 ◽  
Author(s):  
Krongrath Suwannasri ◽  
Tiziana Vanorio ◽  
Anthony Clark
Keyword(s):  
Transport Properties ◽  
Clay Content ◽  
Confining Pressure ◽  
Time Lapse ◽  
Ex Situ ◽  
Minor Role ◽  
Peak Oil ◽  
High Permeability ◽  
Geochemical Properties ◽  
Organic Porosity

Organic-rich marl is one of the best unconventional reservoirs because of its high calcite and low clay content leading to relatively high permeability and fracability. However, how its stiff pores and relatively high permeability affect the changes in its microstructure and elastic and transport properties during maturation remains a research interest. We have induced ex situ maturation of organic-rich marl core plugs by conducting confined pyrolysis in fine steps across the maturity windows from immature through the early-peak oil, late oil, wet gas, and finally, the dry gas window. This was performed under high and low confining pressures on different samples to investigate the role of confining pressure during maturation. After each pyrolysis, we monitored the changes in microstructure, porosity, velocity, permeability, and geochemical properties. The results indicate increasing porosity, decreasing velocity, and increasing permeability as the maturation progresses. The time-lapse scanning electron microcopy images reveal the progressive development of secondary organic porosity at the expense of kerogen volume. Most of the changes in the acoustic velocity and permeability occur in the late oil window and are concurrent with the generation of connected secondary organic porosity. The total organic carbon (TOC) and Rock-Eval results indicate that most of the generated hydrocarbons immediately exit the samples during pyrolysis so that the generation of microcracks from pore-pressure buildup is unlikely. Rather, secondary organic porosity is the main microstructural change, and the amount of depleted TOC can be used as a proxy to predict the increase in porosity and the changes in the velocity and permeability. Finally, confining pressure plays a minor role in the evolution of the elastic and transport properties of organic-rich marl.

POROSITY AND PERMEABILITY OF RESERVOIRS AND CAPROCKS IN THE EROMANGA BASIN, SOUTH AUSTRALIA

1986 ◽  
Vol 26 (1) ◽  
pp. 202
Author(s):  
D.I. Gravestock ◽  
E.M. Alexander
Keyword(s):  
Grain Size ◽  
Gamma Ray ◽  
Size Variation ◽  
South Australia ◽  
Clay Content ◽  
Effective Porosity ◽  
Overburden Pressure ◽  
Porosity Reduction ◽  
Porosity And Permeability ◽  
Eromanga Basin

When effective porosity and permeability are measured at simulated overburden pressure, and grain size variation is taken into account, two distinct relationships are evident for Eromanga Basin reservoirs. Reservoirs in the Hutton Sandstone and Namur Sandstone Member behave such that significant porosity reduction can be sustained with retention of high permeability, whereas permeability of reservoirs in the Birkhead Formation and Murta Member is critically dependent on slight porosity variations. Logging tool responses are compared with core-derived data to show in particular the effects of grain size and clay content on the gamma ray, sonic, and density tools, where clay content is assessed from cation exchange capacity measurements. Sonic and density crossplots, constructed to provide comparison with a water-saturated 'reference' reservoir, are advantageous in comparing measured effective porosity from core plugs at overburden pressure with porosity calculated from logs. Gamma ray and sonic log responses of the Murta Member in the Murteree Horst area are clearly distinct from those of all other reservoirs, perhaps partly due to differences in mineralogy and shallower depth of burial compared with other formations.

Velocity anisotropy in shales: A petrophysical study

Geophysics ◽  
1997 ◽  
Vol 62 (2) ◽  
pp. 521-532 ◽  
Author(s):  
Lev Vernik ◽  
Xingzhou Liu
Keyword(s):  
Seismic Anisotropy ◽  
Elastic Anisotropy ◽  
Clay Mineralogy ◽  
Fluid Phase ◽  
Black Shales ◽  
P Wave ◽  
Velocity Versus ◽  
Softening Effect ◽  
Porosity Reduction ◽  
Wide Range

Using ultrasonic velocity and anisotropy measurements on a variety of shales with different clay and kerogen content, clay mineralogy, and porosity at a wide range of effective pressure, we find that elastic anisotropy of shales increases substantially with compaction. The effect is attributed to both porosity reduction and smectite‐ to‐illite transformation with diagenesis. A means of kerogen content mapping using velocity versus porosity crossplot for shales is shown. Matrix anisotropy of shales dramatically increases with kerogen reaching the maximum values of about 0.4 at total organic carbon (TOC)=15–20%. A strong chemical softening effect was found in shales containing even minor amounts of swelling (smectite) clay when saturated with aqueous solution. This effect results in a significant P‐wave anisotropy reduction as compared to dry and oil‐saturated shales. Since mature black shales are normally oil wet, this effect can only have a local significance restricted to the wellbore wall. Accurate measurements of phase velocities, including velocities at a 45° direction to the bedding plane, allow us to immediately calculate elastic stiffnesses and anisotropic parameters. Intrinsic (high pressure) properties of shales display an ε > δ > 0 relation. Introduction of the bedding‐parallel microcracks in overpressured shales results in a δ decrease when fully fluid saturated and a δ increase when partially gas saturated, with a characteristic effect on the shape of the P‐wave velocity surface at small angles of incidence. Filtering the contribution of the intrinsic anisotropy of shales, it is possible to estimate the pore fluid phase, microcrack density, and aspect ratio parameters using seismic anisotropy measurements.

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