CRYSTALLITE SIZE IN ROCK SAMPLES

Abstract

A method comprises identifying a depositional form of a mineral phase in a sedimentary rock sample based on a measurement (a) of a parameter indicative of a size of crystallites of the mineral phase in the rock sample.

Claims

1. A method comprising identifying a depositional form of a mineral phase in a sedimentary rock sample based on a measurement of a parameter indicative of a size of crystallites of the mineral phase in the rock sample.

2. The method according to claim 1, wherein the parameter indicative of the size of crystallites of the mineral phase in the sedimentary rock sample is a parameter indicative of a mean size of crystallites of the mineral phase in the sedimentary rock sample.

3. The method according to claim 1 or claim 2, wherein the measurement of the parameter indicative of the size of crystallites of the mineral phase in the sedimentary rock sample is obtained by powder X-ray diffraction.

4. The method according to any preceding claim, wherein identifying the depositional form of the mineral phase in the sedimentary rock sample comprises: measuring a peak width of a peak associated with the mineral phase in an X-ray diffraction pattern obtained from the sedimentary rock sample; determining a value of the parameter indicative of the size of crystallites of the mineral phase in the sedimentary rock sample based on the measured peak width; and identifying the depositional form of the mineral phase in the sedimentary rock sample based on the determined value of the parameter indicative of the size of crystallites of the mineral phase in the sedimentary rock sample.

5. The method according to any preceding claim comprising determining an amount of the mineral phase in the sedimentary rock sample having the identified depositional form based on the measurement of the parameter indicative of the size of crystallites of the mineral phase in the sedimentary rock sample.

6. The method according to claim 5 comprising determining respective amounts of the mineral phase in the sedimentary rock sample having first and second depositional forms based on the measurement of the parameter indicative of the size of crystallites of the mineral phase in the sedimentary rock sample.

7. The method according to any preceding claim, wherein the mineral phase is quartz.

8. The method according to any preceding claim, wherein the sedimentary rock sample is a cuttings sample.

9. The method according to any preceding claim, wherein the steps of claims 1, 4, 5 or 6 are carried out by a computer.

10. A computer program comprising instructions which, when the program is executed by a computer, cause the computer to carry out the steps of the method of claims 1, 4, 5 or 6.

11. A computer-readable medium storing the computer program according to claim 10.

12. A method comprising determining an amount of excess quartz and/or an amount of terrigenous quartz in a sedimentary rock sample taking into account a measurement of an amount of quartz in the sedimentary rock sample and a measurement of a parameter indicative of a size of crystallites of quartz in the sedimentary rock sample.

13. The method according to claim 12, wherein the parameter indicative of the size of crystallites of quartz in the sedimentary rock sample is a parameter indicative of a mean size of quartz crystallites in the sedimentary rock sample

14. The method according to claim 12 or claim 13, wherein the measurement of the parameter indicative of the size of crystallites of quartz in the sedimentary rock sample is obtained by powder X-ray diffraction.

15. The method according to any of claims 12 to 14 comprising: measuring a peak width of a peak associated with quartz in a powder X-ray diffraction pattern obtained from the sedimentary rock sample; and determining a value of the parameter indicative of the size of crystallites of quartz in the sedimentary rock sample based on the measured peak width.

16. The method according to any of claims 12 to 15, wherein determining the amount of excess quartz in the sedimentary rock sample and/or the amount of terrigenous quartz in the sedimentary rock sample comprises taking into account a relationship between measurements of amounts of terrigenous quartz and measurements of the parameter indicative of the size of crystallites of quartz for sedimentary rock samples.

17. The method according to any of claims 12 to 16, wherein the sedimentary rock sample is a cuttings sample.

18. The method according to any of claims 12 to 17, wherein the steps of claims 12, 15 or 16 are carried out by a computer.

19. A computer program comprising instructions which, when the program is executed by a computer, cause the computer to carry out the steps of the method of claims 12, 15 or 16.

20. A computer-readable medium storing the computer program according to claim 19.

21. A method comprising determining a mechanical property of a rock sample taking into account: a respective amount of each of two or more constituent phases in the rock sample, the two or more constituent phases comprising a mineral phase; and a measurement of a parameter indicative of a size of crystallites of the mineral phase in the rock sample.

22. The method according to claim 21, wherein the parameter indicative of the size of crystallites of the mineral phase in the rock sample is a parameter indicative of a mean size of crystallites of the mineral phase in the rock sample.

23. The method according to claim 21 or claim 22, wherein the measurement of the parameter indicative of the size of crystallites of the mineral phase in the rock sample is obtained by powder X-ray diffraction.

24. The method according to any of claims 21 to 23 comprising: measuring a peak width of a peak associated with the mineral phase in a powder X-ray diffraction pattern obtained from the rock sample; and determining a value of the parameter indicative of the size of crystallites of the mineral phase in the rock sample based on the measured peak width.

25. The method according to any of claims 21 to 24, wherein: the mechanical property of the rock sample can be expressed in terms of a linear function of the respective amounts of each of the two or more constituent phases in the rock sample and the parameter indicative of the size of crystallites of the mineral phase in the rock sample; and the method comprises evaluating the linear function.

26. The method according to any of claim 21 or claim 25 comprising determining the respective amount of each of the two or more constituent phases in the rock sample, for example by X-ray diffraction.

27. The method according to any of claims 21 to 26, wherein the amount of a constituent phase in the rock sample is a parameter indicative of a mass of the said constituent phase in the rock sample, such as a mass or a mass fraction of the said constituent phase in the rock sample.

28. The method according to any of claims 21 to 27, wherein the mechanical property of the rock sample is an elastic modulus such as a Young's modulus, a reduced Young's modulus, a shear modulus or a bulk modulus, or a dimensionless mechanical property ratio such as a Poisson's ratio.

29. The method according to any of claims 21 to 28, wherein the mineral phase is quartz.

30. The method according to any of claims 21 to 29, wherein the rock sample is sedimentary rock sample.

31. The method according to any of claims 21 to 30, wherein the rock sample is a cuttings sample.

32. The method according to any of claims 21 to 31, wherein the steps of claims 21, 24 or 25 are carried out by a computer.

33. A computer program comprising instructions which, when the program is executed by a computer, cause the computer to carry out the steps of the method of claims 21, 24 or 25.

34. A computer-readable medium storing the computer program according to claim 33.

Description

FIGURES

[0092] Embodiments will now be described by way of example only, with reference to the Figures, in which:

[0093] FIG. 1 is shows scanning electron micrograph (SEM) images of thin sections of three different sedimentary rock samples (a), (b) and (c);

[0094] FIG. 2 is an example powder X-ray diffraction pattern for a sedimentary rock sample overlayed with reference peaks for quartz;

[0095] FIG. 3 is a plot of measured Si/Zr content ratio versus measured mean quartz crystallite size for a plurality of samples from four different sedimentary basins A, B, C and D;

[0096] FIG. 4 is a plot of experimentally measured reduced Young's modulus versus predicted reduced Young's modulus (not taking into account mean quartz crystallite size) for a plurality of siliciclastic sedimentary rock samples taken from a first region;

[0097] FIG. 5 is a plot of experimentally measured reduced Young's modulus versus predicted reduced Young's modulus (taking into account mean quartz crystallite size) for the plurality of siliciclastic sedimentary rock samples of FIG. 4;

[0098] FIG. 6 is a plot of experimentally measured reduced Young's modulus versus predicted reduced Young's modulus (not taking into account mean quartz crystallite size) for a plurality of siliciclastic sedimentary rock samples taken from a second region;

[0099] FIG. 7 is a plot of experimentally measured reduced Young's modulus versus predicted reduced Young's modulus (taking into account mean quartz crystallite size) for the plurality of siliciclastic sedimentary rock samples of FIG. 6;

[0100] FIG. 8 is a plot of amount of quartz versus experimentally measured reduced Young's modulus for the plurality of siliciclastic sedimentary rock samples taken from the first region used in the construction of FIGS. 4 and 5;

[0101] FIG. 9 is a plot of experimentally measured reduced Young's modulus versus predicted reduced Young's modulus (not taking into account mean quartz crystallite size) for a plurality of siliciclastic sedimentary rock samples taken from a third region;

[0102] FIG. 10 is a plot of experimentally measured reduced Young's modulus versus predicted reduced Young's modulus (taking into account mean quartz crystallite size) for the plurality of siliciclastic sedimentary rock samples of FIG. 8;

[0103] FIG. 11 is a plot of amount of quartz versus experimentally measured reduced Young's modulus for the plurality of siliciclastic sedimentary rock samples taken from the third region used in the construction of FIGS. 9 and 10;

[0104] FIG. 12 is a plot of experimentally measured reduced Young's modulus versus predicted reduced Young's modulus (not taking into account mean quartz crystallite size) for a plurality of siliciclastic sedimentary rock samples taken from a fourth region;

[0105] FIG. 13 is a plot of experimentally measured reduced Young's modulus versus predicted reduced Young's modulus (taking into account mean quartz crystallite size) for the plurality of siliciclastic sedimentary rock samples of FIG. 10; and

[0106] FIG. 14 shows a computer processor in communication with a computer-readable medium storing a computer program comprising computer-executable instructions.

DETAILED DESCRIPTION

Rocks

[0107] Rocks are naturally-occurring composite materials. That is to say, rocks are not typically chemically or structurally homogeneous materials, but are instead aggregates of different phases having different chemical compositions and structures. For example, rocks typically include multiple different mineral or mineraloid (i.e. non-crystalline mineral-like substances, such as opal or obsidian) phases, and may also contain organic matter, as well liquids (such as water or hydrocarbons) trapped in pores.

[0108] Silica (i.e. SiO.sub.2) is a common component of rocks, particularly sedimentary rocks, and may be present in several different forms (e.g. as quartz, amorphous silica or cristobalite). Sedimentary rocks are rocks which were formed at or near the Earth's surface by the accumulation and lithification of material. The material from which sedimentary rocks formed may have been transported into a sedimentary basin from its surroundings by rivers or wind (i.e. allogenic material) or may have been generated where it is now found (i.e. authigenic material).

[0109] Accordingly, the silica present in sedimentary rocks can be classified in terms of its origin. Terrigenous or detrital silica in a sedimentary rock is silica incorporated in detrital sedimentary grains in the rock. Detrital sedimentary grains derive originally from weathered material transported into the basin from its hinterland by wind or water (i.e. it is allogenic in origin). Excess silica in a sedimentary rock is silica present in the rock not incorporated in detrital sedimentary grains. Excess silica in sedimentary rocks can be further categorised as biogenic-excess silica or authigenic-excess silica. Biogenic-excess silica was formed by (re)precipitation of silica liberated from prehistoric biota (such as diatoms, radiolaria, silicoflagellates and siliceous sponges) by chemical weathering. Authigenic-excess silica was formed by (re)precipitation of silica liberated from detrital sedimentary material by chemical weathering. Both biogenic-excess silica and authigenic-excess silica form part of the silica (typically quartz) cement or matrix material which surrounds and holds together the detrital grains in the rock.

[0110] Hydrocarbon explorers have found that the quality of hydrocarbon reservoirs in a region correlates with the type of silica deposits in that region. For example, relatively higher proportions of excess silica, in comparison to terrigenous/detrital silica, in the rock are associated with increased hydrocarbon production from hydrocarbon wells. Rock parameters such as the porosity and/or mechanical properties (e.g. Young's modulus) of rocks have also been found to depend on the relative amounts of terrigenous and excess silica, as well as biogenic-excess silica and authigenic-excess silica, in the rock. Material which is more brittle is easier to fracture, for example when forming hydrocarbon wells (such as lateral wells used to extract hydrocarbons from unconventional sources such as tight rock formations) by hydraulic fracturing. Knowledge of the silica content of rock samples extracted from a region can therefore help in identifying the best locations for drilling well bores and in determining the properties of subterranean rock strata which can be used, for example, in the development and interpretation of seismic models and in the calculation of rock strength, and therefore in the calculation of the pressure environments required to fracture rock (e.g. by hydraulic fracturing) or to maintain rock fractures (whether man-made or naturally occurring).

[0111] The mineralogical composition of rock samples extracted from hydrocarbon wells can be determined precisely in the laboratory, for example using methods such as quantitative X-ray diffraction (QXRD). The presence of organic phases in the rock can also be determined using combustion or pyrolysis analysis methods (e.g. using a LECO instrument for combustion analysis or a Rock-Eval instrument for pyrolysis analysis). The origins of mineral phases (such as quartz) in rocks can also be studied by analysis of thin sections or using isotope geochemical methods. However, alternative and improved methods for determining the depositional form of minerals such as quartz in rock samples would not only yield valuable information about geologic history but could also help in our understanding of the geomechanical properties of sedimentary rocks, enabling improvements in geomechanical prediction.

Terrigenous Quartz and Excess Quartz Content

[0112] It has been found that the terrigenous/detrital quartz content of sedimentary rocks correlates with the abundance of the trace element zirconium (Zr) in the rock. For example, RATCLIFFE et al., Unconventional Methods For Unconventional Plays: Using Elemental Data To Understand Shale Resource Plays, Part 2, PESA News Resources, April/May 2012 (which is hereby incorporated by reference in its entirety) identified a positive linear relationship between the terrigenous (i.e. terrestrial) quartz content and the zirconium content of shale rock samples extracted from the Haynesville formation in the United States of America. This trend is understood on the basis that zirconium in sedimentary rocks derives predominantly or entirely from detrital fragments of older rocks (and, in particular, zircon-containing rocks) incorporated in the sedimentary rock and that the abundance of zirconium in sedimentary rocks could not have been enriched significantly by biological activity, nor by (re)precipitation of dissolved silica from ancient bodies of water. Accordingly, it can be assumed that sedimentary rock material containing zirconium is terrigenous in origin. The abundance of zirconium in sedimentary rocks, and in particular the ratio of silicon to zirconium (Si/Zr) content in sedimentary rocks, can therefore be used as a proxy for quantifying the amount of the silica in the rock which is terrigenous in origin (i.e. which derived from the basin hinterland). Moreover, since, by definition, all of the quartz in a sedimentary rock is either terrigenous or excess in nature (in particular, because excess quartz is defined as being that quartz which is not terrigenous in origin), the abundance of zirconium in sedimentary rocks can be used to quantify the amount of excess quartz present in the rock.

[0113] The present inventors have found that the amounts of terrigenous and excess quartz in sedimentary rock can also be determined by considering the size of quartz crystallites in the rock. For example, FIGS. 1 (a) to (c) show scanning electron micrograph (SEM) images (at the same magnification) of thin sections taken from three different sedimentary rock samples. Going from (a) to (c), the observed quartz crystallite size increases while the amounts of authigenic (i.e. excess) quartz identified decreases. In general, larger quartz crystallites have been found to be associated with more terrigenous deposition and smaller quartz crystallites have been to be associated with more authigenic deposition.

[0114] The size of quartz crystallites can be estimated based on the broadening of peaks measured in a powder X-ray diffraction (XRD) pattern obtained from a sample. For example, FIG. 2 shows an example powder XRD pattern obtained from a sedimentary rock sample. Reference peaks associated with quartz are superimposed on the sample diffraction pattern. The significant overlap of the peaks in the measured XRD pattern with the reference peaks indicates that quartz is indeed present in the sedimentary rock sample. In addition, the broadening of the measured peaks relative to the sharp reference peaks provides a measure of the size of the quartz crystallites present in the sample.

[0115] More particularly, the mean size, τ, of crystalline domains of a particular mineral in a sample which coherently scatter X-rays during powder XRD analysis can be determined based on the broadening of a peak in the powder XRD pattern using the Scherrer equation,

[00005]$\tau =\frac{K\lambda }{\beta \cos \theta },$

where K is a dimensionless shape factor (which typically has a value of about 0.9), λ is the X-ray wavelength, β is the line broadening at full-width half-maximum (FWHM) of the peak at a scattering angle, 2θ, in the XRD pattern associated with the mineral, and θ is the corresponding angle of diffraction. It will be appreciated that, at least in theory, the mean coherently-scattering crystalline domain size, τ, is not the same as the mean mineral crystallite size. However, τ does define a lower bound on the mean crystallite size and, in practice, the mean crystallite size is generally found to correlate with τ such that, to a good approximation, τ can be used as a proxy for the mean crystallite size. It will be appreciated, however, that peak broadening only takes place, and therefore the Scherrer equation is only applicable, when the mean crystallite size is less than about 10000 Ångströms.

[0116] Those skilled in the art will appreciate that, although the Scherrer equation presented above relates to the broadening of a single XRD peak, the mean coherently-scattering crystalline domain size can be determined on the basis of a plurality of peaks associated with the mineral in question. For example, if the profile coefficients (i.e. FWHM or β) are parameterized as a function of

[00006]$\frac{1}{\cos \theta },$

crystallite size information can be extracted across the full range of 2θ, provided that enough peaks are present in the diffraction pattern for a robust assessment.

[0117] XRD analysis software, suitable for use in determining the value of τ, is commercially available. One example is JADE available from Materials Data Inc, CA, USA.

[0118] Measurements carried out on reference samples (e.g. lanthanum hexaboride (LaBs) powder (e.g. NIST line position and line shape standard SRM 660C)) can be used to subtract instrument contributions to the peak broadening.

[0119] Those skilled in the art will further appreciate that there are alternative methods for determining crystallite size information from powder XRD measurements. For example, . . . .

[0120] In addition, the inventors have found that the Si/Zr ratio, and therefore the ratio of excess quartz to terrigenous quartz, in sedimentary rock samples correlates with measurements of τ. For example, FIG. 3 shows how the Si/Zr content ratio varies as a function of mean quartz crystallite size (determined on the basis of XRD τ measurements) for a plurality of sedimentary rock samples extracted from four different sedimentary basins A, B, C and D. The size of the sample points is proportional to the total carbonate content of the rock samples. As can be seen, there is a negative linear relationship between the Si/Zr ratio and the quartz crystallite size. This relationship is even stronger when high-carbonate-containing samples are excluded (i.e. essentially limiting the results to siliciclastic rock samples).

[0121] Accordingly, measurements of the mean quartz crystallite size (i.e. τ) can be used to identify the presence of terrigenous or excess (i.e. authigenic) quartz in sedimentary rock samples. Importantly, the amounts of terrigenous and excess (i.e. authigenic) quartz in a sample can also be quantified based on the measured mean quartz crystallite size (which provides a measure of the relative proportions of terrigenous and excess quartz in the sample) and a measurement of the total amount of quartz in the sample (which may be determined using quantitative XRD or spectroscopic techniques such as infra-red spectroscopy or X-ray fluorescence). For example, a measured mean quartz crystallite size can be compared to a trend line for the region in which the rock sample was obtained (e.g. as shown in FIG. 3) to determine the relative proportions of terrigenous quartz and excess quartz in the sample (i.e. the ratio of terrigenous quartz to excess quartz in the sample). The absolute quantities of terrigenous quartz and excess quartz in the sample can then be calculated based on these relative proportions (i.e. the ratio) and a measurement of the total amount of quartz in the sample. The skilled person will appreciate that this analysis is suited to automation and implementation in computer software (for example, computer software 102 stored on a computer-readable medium 101, for execution by a computer processor 100, as shown in FIG. 14).

Rock Mechanics

[0122] Rock mechanics is the study of the mechanical behaviour of rocks and rocks masses, i.e. the mechanical response of rocks to applied forces. Of particular concern in hydrocarbon exploration is the determination of the mechanical properties of subsurface rocks and, in particular, of subsurface sedimentary rock strata, which determine the rock's response to both natural environmental and artificially applied forces.

[0123] For example, geophysicists use the mechanical properties of subsurface rocks in the development and interpretation of seismic models. Knowledge of the mechanical properties of subsurface rocks is also required for accurate calculations of rock strength and of the pressure environments required to fracture rock (e.g., by hydraulic fracturing) or to maintain rock fractures (whether man-made or naturally occurring). Drilling engineers may also use knowledge of rock properties to avoid accidental fracture of rocks (for example, to reduce the risk of a blowout in an overpressured formation).

[0124] As discussed in more detail below, Young's modulus and Poisson's ratio are two important rock parameters used in the design, formation and maintenance of wells used in hydrocarbon exploration. An understanding of the fracture behaviour of subsurface sedimentary rocks is especially useful in unconventional hydrocarbon exploration (for example, for hydraulic fracturing of rocks in lateral wells). For example, Young's modulus can be used to calculate the fracture width which can be achieved using hydraulic fracturing processes, and the fracture width correlates with the hydrocarbon production achievable in a well.

[0125] The mechanical properties of rocks extracted from hydrocarbon wells can be measured precisely in the laboratory, for example using indentation-based mechanical testing techniques. However, the mechanical testing of core samples by standard laboratory methods can be time-consuming. Accordingly, alternative methods for determining the mechanical properties of rocks are being developed. Of particular importance are modelling methods which can be used to estimate the mechanical properties of a rock sample based on its composition.

Elastic Constant Modelling

[0126] As discussed hereinabove, rocks are composite materials. That is to say, rocks are not chemically or structurally homogeneous materials, but are instead aggregates of different mineral or mineraloid phases having different chemical compositions and structures. The mechanical properties of rocks, therefore, depend on the particular rock constituent phases and, in some cases, their relative arrangements.

[0127] For example, a shale rock typically comprises a plurality of layered clay (i.e. silicate) mineral sheets held together by a chemically and structurally distinct mineral matrix. The mineral matrix typically comprises a mixture of randomly oriented matrix mineral crystals, such as crystals of quartz, feldspar, calcite, dolomite, pyrite, etc. Shale rock may also include organic material, such as kerogen, bitumen and pyrobitumen.

[0128] The present inventors have found that rock mechanical properties can be predicted by considering the detailed composite nature of rock. For example, a rock mechanical property, P.sub.Rock, for a given rock sample can be modelled as a linear function of the mass fraction, W.sub.X, (e.g. in mass %) of each constituent component (i.e., phase) X in the rock,

P.sub.Rock=I++a.sub.AW.sub.A+a.sub.BW.sub.B+a.sub.CW.sub.C+ . . . ,

where the constant (i.e. intercept) I and the coefficients a.sub.X, associated with each component (i.e. phase) X, are determined by fitting the equation to experimental data, and the sum is taken over all components X=A, B, C . . . in the rock. The value of the intercept and the coefficients can be determined based on a multiple linear regression analysis using, for example, a least squares method (as implemented in commercially-available software such as Microsoft Excel, available from Microsoft Corporation, WA, USA). Once a model has been fit by determining the values of the intercept and the coefficients, the equation can be used to predict the value of P.sub.Rock for an unknown rock sample based on measurements of the amount of each component (i.e. phase) in the rock. The skilled person will appreciate that such calculations are suited to automation and implementation in computer software (for example, computer software 102 stored on a computer-readable medium 101, for execution by a computer processor 100, as shown in FIG. 14).

[0129] The inventors have found that this method of calculating mechanical properties based on rock compositional measurements can produce results which agree well with experimentally-measured mechanical properties of rock samples. For example, FIG. 4 illustrates the correlation between (a) experimentally measured values of the reduced Young's modulus, E*, for a plurality of sedimentary rock samples taken from a particular geological formation and (b) corresponding values predicted using multiple linear regression based on a compositional analysis of the same samples. The reduced Young's modulus is defined according to

[00007]$E^{*}=\frac{E}{\left(1-v^2\right)},$

where E is Young's modulus and v is Poisson's ratio. E* is measured experimentally using impulse hammer analysis and has previously been found to correlate with scratch-test measurements of the unconfined compressive strength of rock samples (see, for example, GRAMIN, P, et al., Evaluation of the Impulse Hammer Technique for Core Mechanical Properties Profiling, presented at the International Symposium of the Society of Core Analysts, Snowmass, Colorado, USA, 21-26 Aug. 2016, which is hereby incorporated by reference in its entirety). For FIG. 4, the predicted values of E* were calculated according to

E.sub.Sample*=I+a.sub.QuartzW.sub.Quartz+a.sub.FeldsparW.sub.Feldspar+a.sub.CarbonateW.sub.Carbonate+a.sub.Clay/OMW.sub.Clay/OM

using mass fractions of quartz, feldspar, carbonate, and clay minerals and organic material in combination (i.e. W.sub.Quartz, W.sub.Feldspar, W.sub.Carbonate and W.sub.Clay/OM) measured by XRD and combustion analysis and corresponding coefficients determined by multiple linear regression. The values of the intercept and the coefficients used in FIG. 4 are presented in Table 1.

TABLE-US-00001 TABLE 1 Parameter Value I 13.624 α.sub.Quartz −0.032 α.sub.Feldspar 0.115 α.sub.Carbonate −0.116 α.sub.Clay/OM −0.197

[0130] As can be seen in FIG. 4, there is good agreement between the measured and predicted data (for example, a coefficient of determination, R.sup.2, of about 0.858 is achieved).

[0131] The inventors have, however, found that the model can be improved by taking into account the mean quartz crystallite size, τ.sub.Quartz, when calculating E*. For example, FIG. 5 shows the results obtained using the same experimental data as FIG. 4 when the predicted values of E* are instead calculated according to

E.sub.Sample*=I+a.sub.QuartzT.sub.QuartzW.sub.Quartz+a.sub.FeldsparW.sub.Feldspar+a.sub.CarbonateW.sub.Carbonate+a.sub.Clay/OMW.sub.Clay/OM

[0132] To generate FIG. 5, the intercept and the coefficients of the model were refit to the experimental data and are presented in Table 2. As can be seen in FIG. 5, the inclusion of the mean quartz crystallite size in the calculation resulted in an improved R.sup.2 value of 0.871.

TABLE-US-00002 TABLE 2 Parameter Value I 6.803 α.sub.Quartz 0.100 α.sub.Feldspar 0.051 α.sub.Carbonate −0.050 α.sub.Clay/OM −0.111

[0133] FIGS. 6 and 7 show the results of taking into account the mean quartz crystallite size when predicting E* in the same way for sedimentary rock samples taken from a second formation. In particular, FIG. 6 compares measured E* values with those predicted not taking quartz crystallite size into account, while FIG. 7 shows the corresponding results when the calculation of E* does take the mean quartz crystallite size into account. In this case, the predictive model takes into account the presence of quartz, feldspar, calcite, dolomite, ankerite, clay minerals and organic matter phases. As can be seen, the R.sup.2 value is improved from 0.443 to 0.747 by including the quartz crystallite size in the calculation.

[0134] As explained hereinabove, mean crystallite size is only measurable from a powder XRD pattern when the crystallite size is less than about 10000 Å. Accordingly, although without wishing to be bound by theory, the inventors posit that the inclusion of the quartz crystallite size may result in greater improvements in the predictive power of the model when fit to data obtained from rock samples including smaller quartz crystallites, and that the improvement in the predictive power is lower when the model is fit to data obtained from rock samples including larger quartz crystallites, for example more quartz crystallites greater than 10000 Å in size. This may account for the relatively larger improvement in R.sup.2 between the models in FIGS. 6 and 7 as compared to the relatively smaller improvement in R.sup.2 between the models in FIGS. 4 and 5.

[0135] In addition, although taking into account the mean quartz crystallite size may in some cases only result in modest improvements in the R.sup.2 value in terms of prediction of mechanical properties such as E*, the mean quartz crystallite size can also be used to evaluate the impact of the quartz depositional form on mechanical properties. For example, FIG. 8 is a plot of the “amount” of quartz versus measured E* for the samples taken from the first geological formation used in the construction of FIGS. 4 and 5. FIG. 8 shows two sets of data (a) and (b). In data set (a), the “amount” of quartz plotted is the measured mass % of quartz in each sample. In data set (b), the “amount” of quartz plotted is the measured mass % of quartz in each sample multiplied by the normalized mean quartz crystallite size. As can be seen in FIG. 8, there is a significantly stronger correlation (in terms of an increased R.sup.2 value) between the amount of quartz present in the samples and the measured E* values when the mean quartz crystallite size is taken into account. As the mean quartz crystallite size is related to the depositional form of the quartz, this indicates that the depositional form impacts the mechanical properties of the rock.

[0136] FIGS. 9 and 10 show the results of taking into account the mean quartz crystallite size when predicting E* for sedimentary rock samples taken from a third formation. In particular, FIG. 9 compares measured E* values with those predicted not taking quartz crystallite size into account, while FIG. 10 shows the corresponding results when the calculation of E* does take the mean quartz crystallite size into account. The predictive model takes into account the presence of quartz, feldspar, carbonate, clay minerals and organic matter phases. As can be seen, the R.sup.2 value is improved from 0.812 to 0.930 by including the quartz crystallite size in the calculation.

[0137] For example, FIG. 11 is a plot of the “amount” of quartz versus measured E* for the samples taken from the third formation used in the construction of FIGS. 9 and 10. FIG. 11 shows two sets of data (a) and (b). In data set (a), the “amount” of quartz plotted is the measured mass % of quartz in each sample. In data set (b), the “amount” of quartz plotted is the measured mass % of quartz in each sample multiplied by the normalized mean quartz crystallite size. As can be seen in FIG. 11, there is a significantly stronger correlation (in terms of an increased R.sup.2 value) between the amount of quartz present in the samples and the measured E* values when the mean quartz crystallite size is taken into account. As the mean quartz crystallite size is related to the depositional form of the quartz, this indicates that the depositional form impacts the mechanical properties of the rock.

[0138] FIGS. 12 and 13 show the results of taking into account the mean quartz crystallite size when predicting E* for sedimentary rock samples taken from a fourth formation. In particular, FIG. 12 compares measured E* values with those predicted not taking quartz crystallite size into account, while FIG. 13 shows the corresponding results when the calculation of E* does take the mean quartz crystallite size into account. The predictive model takes into account the presence of quartz, feldspar, carbonate and clay minerals. As can be seen, the R.sup.2 is improved from 0.535 to 0.7389 by including the quartz crystallite size in the calculation.

[0139] Although the preceding results relate to quartz in particular, it will appreciated that similar methods could be used to take into account the mean crystallite size of other crystalline phases (e.g. calcite) present in rock samples. In order to account for the mean crystallite size of multiple crystalline phases in the rock, the mechanical property, P.sub.Rock, of the rock sample may be modelled as a linear function of (a) the mass fraction, W.sub.X, (e.g. in mass %) of each phase, X=A, B, C . . . , in the rock and (b) the mean size of crystallites, τ.sub.X, of each phase in the rock as

P.sub.Rock=I+a.sub.AT.sub.AW.sub.A+a.sub.Bτ.sub.BW.sub.B+a.sub.Cτ.sub.CW.sub.C+ . . . .

[0140] Where a mean crystallite size of a given phase is not to be taken into account, the corresponding value of τ.sub.X could be set to unity.

[0141] It will be understood that the invention is not limited to the embodiments described above and various modifications and improvements can be made without departing from the concepts described herein. Except where mutually exclusive, any of the features may be employed separately or in combination with any other features and the disclosure extends to and includes all combinations and sub-combinations of one or more features described herein.