NMR Logging tools providing better answers for hydrocarbon ...

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Copyright © 2007， JAPT

石油技術協会誌　第 72 巻　第 1 号（平成 19年 1月）JOURNAL OF THE JAPANESE ASSOCIATION FOR PETROLEUM TECHNOLOGY

VOL. 72, NO. 1（ Jan., 2007）

報　　　告

1.　Introduction

Nuclear Magnetic Resonance （NMR） is the only logging technique sensitive to pore-size , distribution and pore structure. However, quantitative interpretation of NMR data can become uncertain in carbonate rocks because of unaccounted diffusion coupling between existing pore scales.

NMR logging has be en used to assess the petrophysical parameters, such as irreducible bulk

volume, clay-bound water volume, movable bulk volume, effective porosity （φe） and total porosity （φt）, and permeability index（k）. All of these parameters are related to the amount and the distribution of the fluids within the pore structure. Thus, an understanding of how NMR measurements respond to variations of pore structure is essential to quantify petrophysical formation properties （Coates et al., 1999）.

Generally, the Western Desert is considered the future for the gas and oil development in Egypt. Khalda Petroleum Company has recently achieved the latest discoveries in Qasr, Hayat-Yasser and Selkit Oil Fields. Our study is concentrated on the Kharita Formation of Lower Cretaceous age in Selkit Oil Field in the north Western Desert. The Kharita Formation

* タンタ大学　Geology Department, Faculty of Science, Tanta University, Egypt.

** カルダ石油会社　Khalda Petroleum Company.*** 石油資源開発㈱　JAPEX Research Center, Japan Petroleum

Exploration Co., Ltd.

NMR Logging tools providing better answers forhydrocarbon bearing reservoirs, North Western Desert, Egypt

Mohamed R. Shalaby*, Abdelmoneim N. Abu Shady** and Osamu Takano***

（Received June 26, 2006；accepted January 12, 2007）

Abstract：　This study aims to evaluate the hydrocarbon potentiality of the Kharita Formation of Lower

Cretaceous age which is one of the most promising reservoirs in the Western Desert of Egypt. The conventional

well log data and the advanced Nuclear Magnetic Resonance （NMR） logging technique in addition to the Formation

MicroImager （FMI） have been collected from Selkit -1X Well and used for analysis and interpretation. The Density-

Magnetic Resonance （DMR） porosity can be used in volumetric calculations to provide more accurate reservoir

volume estimates than previously possible. Reservoir productivity, and fl uid contacts are the topmost objectives

in petrophysical analysis especially in hydrocarbon-bearing reservoirs. On the other hand, designing a wireline

formation testing job is very much dependant on knowing reservoir permeability and fl uid types.

The study indicates that the NMR has played an essential role with the help of FMI in reservoir characterization

of the Kharita Formation. These techniques have been used also to identify, quantify movable and immovable water

saturations, hydrocarbon fl uid typing, and it has better permeability estimation. Also, when integrated with bulk

density, data from both tools were used to enhance calculated total porosity and permeability.

The combination of NMR log derived permeability and other tools confi rm the obtained result and facilitate

the determination of accurate fl uid contact of the Kharita reservoir. It can be concluded that the NMR logging

combinations with other logs provides better answers to hydrocarbon-bearing reservoirs in today's search for

hydrocarbons.

Key words： NMR logging, FMI logging, Reservoir characterization, Kharita Formation, DMR porosity,

Hydrothermal fl uid typing

NMR Logging tools providing better answers for hydrocarbon bearing reservoirs, North Western Desert, Egypt

石油技術協会誌　72巻 1号（2007）

122

consists mainly of argillaceous and calcareous sandstone interbedded by thin to thick beds of shale and siltstone. Early Cretaceous age consists of four different formations arranged from top to bottom as the Kharita, Dahab, Alamein and Alam El-Bueib Formations where the Kharita Formation is the top most one. Fig. 1 shows the location map of the study area and Fig. 2 shows the simplified columnar section of the Lower Cretaceous in the Western Desert indicating the location of the Kharita Formation.

The potential reservoirs especially sandstones are widely represented throughout the Albian Kharita Formation in the Western Desert. The upper clastic and regressive series are referred to as the Albian Kharita Formation which consists of consolidated sandstone with occasional coal seams and minor intercalations of shale （Schlumberger 1984）.

Different discoveries in the study area proved that the Kharita Formation is considered one of the most important reservoirs in the Western Desert. Varying lithology of the Kharita Formation especially the presence of siltstone within the sandstone reservoir

makes it difficult to be interpreted using conventional well log data. Therefore, latest and advanced logging techniques have been used to improve the reservoir interpretation and also to increase the estimated productivity of the concerned reservoirs. The Kharita Formation in Selkit Oil Field in the north Western Desert has been evaluated using standard open-hole measurements for porosity and water saturation estimation, beside the latest NMR technology where the two measurements complement each other. The Density-Magnetic Resonance （DMR） is a new well logging technique for identifying and evaluating hydrocarbon-bearing reservoirs. It is based also on the equations recently derived by Freedman et al.（1997） and it is similar to the Neutron-Density cross-

plot porosity. The equation and the method can be applied to reservoirs with gas condensate or light oil. The DMR porosity can be used in volumetric fluid estimation of the Kharita Formation in Selkit-1X Well in the study area. It is evaluated also using Formation MicroImager （FMI） which helps to identify the presence of hydrocarbon in the formation.

2. Comparison between NMR and conventional tools

The conventional porosity tools, such as neutron, bulk density, sonic and also the resistivity tool are influenced by all components of a reservoir rock （Bassiouni, 1994, Kenyon 1997 and Freedman et al.,

1998）. They are more sensitive to matrix materials than to pore fluids and are highly affected by the borehole size and mudcake. They cannot be regarded as true fluid-logging devices and the distinguishing Fig. 1　Location map for the study area

Fig. 2　 Simplified columnar section of Lower Cretaceous Formations （Modified after Schlumberger 1984 & 1995）.

J. JAPANESE ASSOC. PETROL. TECHNOL. Vol. 72, No. 1（2007）

123Mohamed R. Shalaby, Abdelmoneim N. Abu Shady and Osamu Takano

of light oil, medium-viscosity oil and heavy oil is still impossible.

Because only fluids are visible to NMR, the porosity measured by an NMR tool contains no contribution from matrix materials and does not need to be calibrated to formation lithology （Vinegar, 1995）. Therefore, NMR logging is effective and provides three types of information. These are the quantities, properties of the fluid in the rock and the sizes of the pores that contain these fluids.

As known that NMR porosity is essentially matrix independent, the NMR tools are sensitive only to pore fluids. The difference in various NMR properties such as relaxation times （T1 and T2） and diffusivity （D） among various fluids makes it possible to distinguish （in the zone of investigation） among bound water,

movable water, gas, light oil, medium-viscosity oil and heavy oil （Morriss et al., 1994, Prammer 1994, Akkurt et al., 1995, Prammer et al., 1995, 1998, Mardon et al., 1996, Looyestijn and 1996, Coates et al., 1999）.

3.　Advantages of NMR logging tool

3.1 NMR porosity and permeabilityNMR porosity measurements on rock cores can be

measured with sufficiently short TE and sufficiently long TW to capture all the NMR-visible porosity. The data acquisition parameters include inter-echo spacing （TE） and polarization time （TW）. TE is the time

between the individual echoes in an echo train. TW is the time between cessation of measurement of one echo train and the beginning of the measurements of the next echo train （Coates et al., 1999）. NMR relaxation properties of rock samples are dependent on porosity, pore size, pore-fluid properties and mineralogy. The NMR estimate of permeability is based on theoretical models that show permeability increases with both increasing porosity and increasing pore size （Timur, 1967, 1968）.

3. 2 NMR T2 distributionThe set of all the decay constants forms the

decay spectrum or transverse-relaxation time （T2） distribution. In water saturated rocks, it can be proven mathematically that the decay curve associated with a single pore will be a single exponential with a decay constant proportional to pore size. The small pores have short T2 value and large pores have long T2

values （Kenyon 1992, Chen et al., 1999, Quintero et al., 1999, Godefroy et al., 2001, Allen et al., 2001 and Toumelin et al., 2002）. Because T2 values can be related to pore sizes, T2 value can be selected below which the corresponding fluids are expected to reside in small pores and above which the corresponding fluids are expected to reside in larger pores. This T2 value is called T2 cutoff （Timur, 1967 and Coates et al., 1999）. The T2 cutoff can be determined with NMR measurements but although the capillary pressure, lithology and pore characteristics all affect T2 cutoff values, common practice can establish local standard values for T2 cutoff for each field. There are no hard and fast steps between clay bound, capillary bound and producible, often there is overlap. Typically, clay bound occupies the lowest part of the T2 spectrum, followed by capillary bound and, finally, producible.

4.　Methods of study

Hydrocarbon typing is performed using two different techniques, Time Domain Analysis and Enhanced Diffusion Method.

4.1 Time Domain Analysis （TDA）TDA is an outgrowth of the Differential Spectrum

Method （DSM） which uses the difference between two waiting time spectrum （TWshort & TWlong） to qualitatively investigate the existence of gas and light oil in rock pore （Fig. 3）.

DSM indicates that the echo train recorded after the short TW contains almost all of the water signals but only some of the light oil signals. However, the echo train recorded after the long TW contains all of the signals from both the water and the light oil that are present （Akkurt et al., 1995 and Prammer et al., 1995）.

In TDA, the two echo trains used before to compute the DSM can be subtracted from one another where the resulting echo train is examined as TDA. Therefore, the detection of only light hydrocarbon components can be created by taking the difference between the T2 distributions computed separately from the echo trains. TDA starts by resolving the exponential decays associated with light hydrocarbons （oil and/or gas）, thereby confirming the presence of these fluids, and then provides estimates of the fluid volumes. TDA is a more robust process than DSM.

With TDA, the subtraction is performed in the



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ordinary time domain instead of the T2 domain. The TDA method has two key advantages over the DSM, 1） the difference between the two echo trains is calculated in the time domain, so the difference is more robust. The difference is then transformed to a T2 distribution, 2） TDA provides better corrections for under-polarized hydrogen （T1）, and for hydrogen-index effects （Hi）.

Therefore, TDA relies on the fact that different

fluids have different rates of polarization, or different T1 relaxation times. The T1 of both gas and light oil is normally much longer than the T1 of water as

Fig. 3　Time Domain Analysis （TDA）. Gas, light oil, water and some medium viscosity oils exhibit significant diffusion induced relaxation when they are in a gradient magnetic field. For these fluids, T2diffusion, the relaxation time constant associated with the diffusion mechanism becomes an important tool for their detection. Clay-bound water, capillary-bound water, and movable water occupy different pore sizes and locations. Hydrocarbon fluids differ from brine in their locations in the pore space usually occupying the larger pores. They also differ from each other in viscosity and diffusivity. NMR logging uses these differences to characterize the fluids in the pore space. In general, bound fluids have very short T1 and T2 times, along with slow diffusion （small D）. Free water commonly exhibits medium T1, T2 and D values. Hydrocarbons, such as natural gas, light oil, medium-viscosity oil, and heavy oil, also have very different NMR characteristics. Natural gas exhibits very long T1 times but short T2 times. Lighter oils are highly diffusive, have long T1 and T2 times.

Phase T1 T2

Hydro-carbon

Gas ＞ 3.0 sec. 40 – 64 msec.

Oil 1 – 3 sec. ＞ 300 msec.

Water Water 0.5 – 1.0 sec. 200 – 500 msec.

Table 1　Different fluids having different relaxation times



shown in Table 1.4.2 Enhanced Diffusion Method （EDM）EDM is used to determine volumes of light-to

intermediate viscosity oils when a T2 distribution from a long TW, long TE measurement is available from a gradient-field NMR tool. For fluids in rock pores, three independent relaxation mechanisms are involved such as the apparent, bulk, surface-induced, and diffusion-induced mechanisms. Fig. 4 shows the relation between apparent and surface relaxation times for the self-diffusion relaxation of water.

As seen from this figure that the T2DW＝50 msec represents the absolute upper bound for the measured T2 of water, and all T2 relaxation times associated with water will be less than or equal to T2DW. Thus, a 2DW. Thus, a 2DW

residual intermediate crude oil with a relaxation time longer than T2DW can be unambiguously identified as hydrocarbon.

The EDM technique provides good determinations of intermediate oil volumes when the secondary inter-echo spacing is sufficiently large for self-diffusion to dominate the water decay and in cases in which the self-diffusion coefficient for the oil is smaller than the self-diffusion coefficient for water. In order to get an accurate oil volume, the oil must decay at a rate slower than T2DW, and the characteristic oil T2 distribution 2DW, and the characteristic oil T2 distribution 2DW

needs to be uni-modal

5.　Results and interpretation

Ou r study aimed to evaluate hydrocarbon potentiality of the Lower Cretaceous Kharita Formation and the data is collected from Selkit-1X

Well in the Western Desert. Fig. 5 reflects the well log data which have been represented by GR, density, neutron and resistivity logs.

The GR log in Track 1 helps to indicate the reservoir interval in the Kharita Formation from depth 9,525 to 9,630 ft. This interval constitutes the all pay zone containing both types of fluids which are hydrocarbon and water. This is also observed from the density-neutron cross-plot （Track 2）. There is a good crossover separation between the neutron-density logs indicating the presence of reservoir in the same interval from depth 9,525 to 9,630 ft. This Fig. 5 shows the logs over the expected hydrocarbon bearing interval in the Selkit-1X Well. The deep invasion is observed in this well therefore the density-neutron overlay might not always be sufficient to identify hydrocarbon bearing intervals.

The resistivity logs in Track 3 reveal that the reservoir contains two different types of fluids. In the upper part from depth 9,525 to 9,575 ft, the reservoir is expected as hydrocarbon bearing where there is a big separation between the three resistivity logs and the deep resistivity logs give very high values reaching to more than 1,000 ohm/m. The fluid contact is expected to be at the depth 9,575 ft from resistivity logs. In the lower part from depth 9,575 to 9,630 ft, the resistivity logs are reversed where the deep resistivity log become lower than the flushed zone resistivity. This may indicates the presence of water in the lower part.

Fig. 6 shows the pressure depth relationship which indicates the presence of two different fluids in the Kharita Formation in the study well. It is observed

Fig. 4　Enhanced Diffusion Method （EDM） for viscosity differentiation



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from this figure that the hydrocarbon is observed in the upper part with gradient 0.327 （psi/ft） and water is in the lower part with a gradient 0.436 （psi/ft）. The hydrocarbon is interpreted as oil and the expected oil water contact （OWC） is at depth 9,575ft from the plot with good agreement and confirmation with log data.

Wireline Formation Tester surveys can be time

consuming and the optimization of the acquisition procedure is therefore recommended. Using the field example, the Kharita Formation in Selkit-1X Well, it is possible to show how NMR log data can be used to optimize this acquisition and how the information provided by NMR logs and Formation Testers complement each other so that their combination enhances and confirms the estimation of downhole fluid properties.

The complete NMR log data evaluation of the Kharita Formation in Selkit-1X Well is presented in Fig. 7. The TDA permeability in Track 2 reflects an increase in the upper part to reach the maximum value in the reservoir zone. Below the estimated oil water contact the TDA permeability records very low values reaching to zero in most intervals. There is good confirmation between the TDA permeability and the resistivity data presented in the same track. Therefore,

Fig. 5　Expected hydrocarbon bearing interval in Selkit-1X Well

Fig. 6　Estimated fluid contact in the studied interval



the comparison of NMR log derived permeability with other logs is in good agreement.

To positively identify the light oil bearing intervals in this well, the volume of oil seen by the density-neutron tool combination in Fig. 5 is compared to the volume of oil seen by the NMR and FMI tools in Fig. 7. Therefore, the light oil bearing interval can be identified easily in Figure 7 from depths 9,525 to 9,575 ft from the deficit in the figure. The T2 distribution over the oil zones indicates an oil signal with an average T2 in the range of 400 ms. The improved determination of the T2DW line is easily recognized in Track 3 in Fig. 7.

In Track 4 in Fig. 7, Density Magnetic Resonance （DMR） porosity is illustrated indicating the estimated

porosity and the type of fluid detected in the Kharita Formation in Selkit-1X Well. The result obtained

reveals high porosity values in the upper zone of the formation with clear agreement with the result of TDA permeability in Track 2, Fig. 7. The reservoir interval contains different types of fluids as indicated from NMR logging combination with density log.

Oil is the main hydrocarbon detected with considerable percentage while other fluids can be detected such as the irreducible capillary bound water and the bound volume of water. The existence of hydrocarbon can be detected also by FMI in Track 5 which facilitates and confirms the hydrocarbon observed from other tools. FMI in Track 5 can indicate also the presence of OWC at depth 9,575 ft with good agreement with Fig. 6. Therefore, FMI tool is very useful for verification of NMR analysis.

In Fig. 7 , the parameters for TDA and EDM analysis in Tracks 4 and 6 are estimated using

Fig. 7　Complete evaluation using NMR logging



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empirical relations and used in the processing models. To minimize uncertainties, it is recommended to get those parameters as correct as we can from PVT, NMR Core/Fluid analysis. This figure shows a comparison between TDA and EDM processing where both confirm the existence of light oil in the reservoir section and prove the processing and interpretation done with conventional data.

6.　Summary and conclusionsThe advanced NMR logging tool is used in

combination with FMI and the conventional tools to evaluate the hydrocarbon productivity of the Kharita Formation in Selkit-1X Well in the north Western Desert. NMR logging tool is one of the most advanced and effective logging tool where it can provides information about the quantities and properties of the fluid in the rock and also the sizes of the pores that contain these fluids.

The Lower Cretaceous Kharita Formation in Selkit-1X Well is studied using these combination techniques. The NMR tools are sensitive mainly to pore fluids and it is essentially matrix independent. In our study, NMR logging has great effect on the evaluation of sandstone of the Kharita reservoir. The light oil bearing interval can be identified exactly with accurate depth with their average T2 value. The oil water contact can be detected exactly from the available data. The presence of FMI techniques makes it possible to confirm the result obtained from other tools. It is found that the information provided by NMR logs, FMI and Formation Testers complement each other. Therefore, their combination enhances and confirms the volumetric estimation of different fluids and properties in the reservoirs. It is also recommended to use these combinations for providing better answers in the reservoirs suffering from complicated interpretation.

References

Akkurt, R., Vinegar, H.J. Tutunjian, P.N., and Guillory, A.J., 1995：NMR logging of natural gas reservoirs, paper N, in 36th annual logging symposium transactions： Society of Professional Well Log Analysts, 12p.

Allen, D. F., Boyd, A., Massey, J., Fordham, E. J., Amabeoku, M. O., Kenyon, W. E. and Ward, W. B.,

2001：The practical application of NMR logging in carbonates： 3 case studies, paper K, in 42nd Annual Logging Symposium Transactions, Society of Professional Well-Log Analysts.

Bassiouni, Z., 1994：Theory, measurements and interpretation of well logs, SPE Textbook Series, V. 4.

Chen, S., Fang, S., Georgi, D., Salyer,J, and Shorey, D.,1999：Optimization of NMR logging acquisition and processing, SPE-56766 , in SPE Annual Technical Conference and Exhibition Proceedings, Society of Petroleum Engineers.

Coates, G. R., Xiao, L., and Prammer, M. G., 1999：NMR Logging, Principles and Appl ications , Halliburton Energy Services, Houston, 234p.

Freedman, R., Boyd, A., Gubelin, G., McKeon, D., Morriss, C., and Flaum, C., 1997：Measurement of total NMR porosity adds new value to NMR logging, paper O, in 38th annual logging symposium transactions, Society of Professional Well Log Analysts, 14 p.

Freedman, R., Minh, C.C., Gubelin, G., Freeman, J.J., McGinness, T., Terry, B., and Rawlence, D., 1998： Combining NMR and density logs for petrophysical analysis in gas-bearing formations, paper II, in 39th annual logging symposium transactions, Society of Professional Well Log Analysts, 14 p.

Godefroy, S., Fleury , M., Deflandre, F .,and Korb, J. P., 2001：Temperature effect on NMR surface relaxation, SPE-71700 , in Annual Technical Conference and Exhibition Proceedings, Society of Petroleum Engineers.

Kenyon, W.E., 1992：Nuclear magnetic resonance as a petrophysical measurement, Nuclear Geophysics, 6（2）, 153 – 171. Later revised and published in 1997 as Petrophysical principles of applications of NMR logging, The Log Analyst, 38（2）, 21 – 43.

Kenyon, W. E., 1997：Petrophysical principles and applications of NMR logging, The Log Analyst, 38（2）, 21 – 43.

Looyestijn, W., 1996：Determination of oil saturation from diffusion NMR logs, paper SS, in 37th annual logging symposium transactions, Society of Professional Well Log Analysts, 14 p.

Mardon, D., Prammer, M.G., and Coates, G.R., 1996：



Characterization of light hydrocarbon reservoirs by g radient -NMR wel l logging, Magnet ic Resonance Imaging, 14（7/8）, 769 – 777.

Morriss, C.E., Freedman, R., Straley, C., Johnston, M., Vinegar, H.J., and Tutunjian, P.N., 1994：Hydrocarbon saturation and viscosity estimation from NMR logging in the Belridge diatomite, paper C, in 35th annual logging symposium transactions, Society of Professional Well Log Analysts, 24 p. Later published in 1997, The Log Analyst, 38（2）,44 – 59.

Prammer, M.G., 1994：NMR pore size distributions and permeability at the well site, SPE-28368, inSPE annual technical conference and exhibition proceedings, v. omega, Formation evaluation and reservoir geology, Society of Petroleum Engineers, 55 – 64.

Prammer, M.G., Mardon, D., Coates, G.R., and Miller, M.N., 1995：Lithology-independent gas detection by gradient-NMR logging, SPE-30562, in Annual technical conference and exhibition proceedings, v. omega, Formation evaluation and reservoir geology, Society of Petroleum Engineers, 325 – 336.

Prammer, M.G., Bouton, J., Chandler, R.N., Drack, E.D., and Miller, N.M., 1998：A new multiband generation of NMR logging tools, SPE-49011, in 1998 SPE annual technical conference and exhibition proceedings, v. omega, Formation evaluation and reservoir geology, Society of Petroleum Engineers, 237 – 244.

Quintero, L, Boyd, A. Gyllensten, A., and El Wazeer, F., 1999：Comparison of permeability from NMR and production analysis in carbonate reservoirs, SPE-56798, in Annual Technical Conference and Exhibition Proceedings, Society of Petroleum Engineers.

Schlumberger ; 1984：Well Evaluation Conference, Egypt 1984, 250p.

Schlumberger ; 1995：Well Evaluation Conference, Egypt 1995, 87p.

Timur, A., 1967：Pulsed nuclear magnetic resonance studies of porosity, movable fluid and permeability of sandstones, SPE-2045, Society of Petroleum

Engineers, in 42nd Annual Meeting Preprint. Later published in 1969, Journal of Petroleum Technology, 21（6）, 775 – 786.

Timur, A., 1968：Effective porosity and permeability of sandstones investigated through nuclear magnetic principles, paper K, in 9th annual logging symposium transactions, Society of Professional Well Log Analysts, 18 p., Later published in 1969, The Log Analyst, 10（1）, January-February, 3 – 11.

Toumelin, E., Torres-Verden, C., and Chen, S., 2002：Quantification of multi-phase fluid saturations in complex pore geometries from simulations of NMR measurements, SPE-77399, in Annual Technical Conference and Exhibition Proceedings, Society of Petroleum Engineers.

Vinegar, H. J., 1995：NMR log interpretation with examples, chapter 8, in Georgi, D.T., ed., Nuclear magnetic resonance logging short course notes, in36th annual symposium, Paris, France, Society of Professional Well Log Analysts, variously paginated.

要　　　旨

NMR検層を用いた炭化水素貯留層評価：エジプト西部砂漠の例

M. R. シャラビー・A. N.アブシャディ・高野　　修

エジプト西部砂漠において最も有望な貯留層と考えら

れている下部白亜系 Kharita層の貯留層評価を目的とし

て，NMRと他の通常検層項目の組み合わせを主体とし

た検層解析を行った。NMR検層は岩石構成粒子には反

応せず，孔隙流体のみに反応することから，可動水飽和

率，不動水飽和率，炭化水素流体タイプ，浸透率などの

情報を容易に得ることができる。さらに密度検層などと

の組み合わせによって全孔隙率，浸透率を得ることがで

きる。本スタディでは，Selkit-1X坑井の Kharita層の砂

岩層について，NMRと FMI,密度，比抵抗検層の組み合

わせによる解析を行い，軽質油胚胎層準と油水接触面を

正確に特定することができた。本解析結果は，NMR検

層と従来の通常検層項目の組み合わせによる解析が，有

効かつ効率的な油層評価手法であることを示している。

NMR Logging tools providing better answers for hydrocarbon ...

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