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Journal of Natural Gas Science and Engineering 36 (2016) 800e810

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Journal of Natural Gas Science and Engineering

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Methods for the evaluation of water saturation considering TOC inshale reservoirs

Baoying Zhang a, b, c, Jingling Xu a, b, c, *

a Key Laboratory of Geo-detection (China University of Geosciences, Beijing), Ministry of Education, Beijing 100083, Chinab State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing 100083, Chinac School of Geophysics and Information Technology, China University of Geosciences (Beijing), Beijing 100083, China

a r t i c l e i n f o

Article history:Received 20 July 2016Received in revised form5 November 2016Accepted 7 November 2016Available online 9 November 2016

Keywords:Organic-rich shale reservoirTOC correction methodWater saturation difference methodConventional water saturation

* Corresponding authors. China University of GeosciE-mail address: jlxu@cugb.edu.cn (J. Xu).

http://dx.doi.org/10.1016/j.jngse.2016.11.0231875-5100/© 2016 Elsevier B.V. All rights reserved.

a b s t r a c t

Water saturation is one of the most important parameters in petroleum exploration and development.However, its calculation has been limited by the insufficient logging data required by a new techniquethat further influences the calculation of the free gas content. Total organic carbon (TOC) is compre-hensively considered in this study because it can indirectly control the gas content and saturation. Afteranalyzing the relationship between TOC, core water saturation and conventional water saturation, twonew methods are proposed to improve the accuracy of water saturation estimates: the TOC correctionmethod and water saturation difference method, in which Archie water saturation, modified total shalewater saturation, and TOC are integrated. According to the case studies, the water saturation results fromthese two methods in shale reservoirs with different lithology are consistent with those from coreanalysis. It is concluded that these two methods can quickly and effectively evaluate the water saturationof shale reservoirs.

© 2016 Elsevier B.V. All rights reserved.

1. Introduction

The evaluation of water saturation (Sw) is still difficult regardingwell logging interpretation, especially for organic-rich shale res-ervoirs featuring low porosity and permeability. A water saturationmodel considering porosity and resistivity was first proposed forclean sand formation (Archie, 1942) and was named Archie for-mula. After that, a number of important water saturation modelsbased on traditional logging data for shale-bearing sands werederived, such as Simandoux model (Simandoux, 1963), modifiedSimandoux model (Bardon and Pied, 1969), modified Simandouxmodel (Fertl and Hammack, 1971), Indonesian model (Poupon andLeveaux, 1971), total shale model, modified total shale model(Schlumberger, 1972), dual water model (Bassiouni, 1994), anddispersed clay model (Schlumberger, 1989).

Though it leads to good results for clean sandstone reservoirs,the Archie formula does not work well in shale-bearing and het-erogeneous formations. Obviously, water saturation of tight gasreservoirs cannot be evaluated by the Archie formula (Miller andShanley, 2010). Meanwhile, current log-derived water saturation

ences, Beijing, 100083, China.

models also do not workwell in these types of reservoirs, especiallyin shale reservoirs with a high abundance of TOC. However, con-ventional water saturationmodels can still be applied in some shalereservoirs. Boyce and Carr (2009) used the Simandoux modelinstead of the Archie formula to evaluate thewater saturation of theMarcellus Shale. Wu and Aguilera (2012) applied the Archie for-mula to evaluate the water saturation in the Barnett Shale based onexperimental observations. Amiri et al. (2015) studied the watersaturation in the Mesaverde tight gas reservoir and compared theeight traditional water saturation models (total shale, modifiedtotal shale, dispersed clay, Indonesian, Simandoux, and modifiedSimandoux models by Fertl, Hammack and Fert) with cores fromtight shale-bearing sandstones, after which they proposed a cali-bration coefficient to reduce the predictive uncertainty of watersaturation. Vincent and Wladyslaw (2015) improved the accuracyof water saturation calculations by consideration the clay- and silt-bound water within the overall reservoir resistivity. The estimatesof water saturation in formations that are rich in silt and clay wereimproved significantly compared with the Archie equation in J- andK-Reservoirs. Nuclear magnetic resonance (NMR) data receivedmore interest for improving the accuracy of petrophysical param-eters. Tan et al. (2015) evaluated gas saturation through a porositycorrection model based on a core NMR experiment. However, it is

B. Zhang, J. Xu / Journal of Natural Gas Science and Engineering 36 (2016) 800e810 801

neither practical nor economic to run NMR logs on all wells becauseof its high cost (Merkel and Gegg, 2008). Alfred and Vernik (2012,2013) proposed a new petrophysical model that divided the sys-tem into organic and inorganic domains for organic shales, and thecalculated water saturation agrees well with the core measure-ments. They applied this method to shales in North America, andfound that it failed to obtain parameters of organic and inorganicdomains when the experimental data were limited.

The methods mentioned above are based on the conductivity ofthe formation or the use of new logging technologies. However,conventional or improved water saturation methods for consid-ering the conductivity of shale-bearing sand and clay are not alwaysfeasible in organic-rich shale reservoirs, especially in those withlow resistivity. Moreover, data obtained by the new logging tech-nology are usually limited. Hence, a newmodel for characterizationof water saturation in shale reservoirs, especially in organic-richshale reservoirs, is urgently needed, especially when data arelimited. In this study, two new methods are proposed to quicklyevaluate the water saturation in shale reservoirs, and case studiesare conducted in shale reservoirs with different lithology. Thewater saturation results from both rock-electric experiments andconventional logs integrating TOC are compared in the final section.

2. Calculation of water saturation

Shale reservoirs can be mainly divided into three parts: porosity

Fig. 1. Petrophysical model of the shale reservoir.

Fig. 2. The relationship among TOC, conventional water saturation and core watersaturation in the resistivity-increasing shale formation. The conventional water satu-ration method can evaluate the water saturation of a shale formation with a lowerabundance of TOC.

(organic porosity and inorganic porosity), kerogen, and matrix,including clay minerals (Fig. 1). Gas can accumulate in both inor-ganic and organic pores in the form of free gas and absorbed gas.Free gas generally fills the fractures and pores, whereas absorbedgas is normally absorbed by organic matter (kerogen) (Guo et al.,2015). The accuracy of the water saturation estimation de-termines whether we can achieve a more accurate free gas contentor not. The mineral compositions and presence of organic mattercan influence the distribution of pores and fluid saturation(Sondergeld et al., 2010).

2.1. Conventional water saturation calculation

The basis of the Archie equation (Archie, 1942), which is popu-larly applied to calculate the water saturation of clean, non-shale-bearing sandstone formations, is that the formation conductivityshould be a function of the conductivity of the fluid content in itspore space. It can be written in terms of conductivity, as shown inEquation (1):

Fig. 3. The relationship among TOC, conventional water saturation and core watersaturation in a resistivity-decreasing shale formation. There is a positive correlationbetween TOC and conventional water saturation, whereas there is a negative corre-lation between TOC and core water saturation. The difference of conventional watersaturation and core analysis of water saturation behaves regularly with TOC.

Fig. 4. The relationship between conventional water saturation and Sw_d.

Fig. 5. The situation of water saturation and gas saturation in a shale formation with increasing resistivity and TOC.

Ct ¼ Swn$Cw=F (1)

where Ct is the total conductivity, F is the formation factor, Cwequals the conductivity of the formation water, and Sw is the watersaturation. Equation (1) can be rewritten in terms of the resistivity,as displayed in Equation (2):

1Rt

¼ 4m$Snwa$Rw

(2)

wherem is the cementation factor that was generally considered toequal 2, n is the saturation exponent that was generally consideredto equal 2, a is the tortuosity factor that was generally considered toequal 1, 4 is the porosity, Rt is the true resistivity and is measured inohm$m and Rw is the formation water resistivity.

Simandoux considered another conductivity source arising fromclay in the calculation of water saturation, as seen in Equation (3):

Ct ¼ Swn$Cw=F þ X (3)

where X is the additional conductivity of clay. The modified totalshale equation (Schlumberger, 1972), which was developed fromthe Simandoux equation, is usually applied to the water saturationcalculation of shale formations. The equation can be written interms of resistivity, as shown in Equation (4):

1Rt

¼ fm$Snwa$Rw$ð1� VshÞ

þ Vsh$SwRsh

(4)

where Rsh is the resistivity of shales with units of ohm$m and Vsh isthe volume of clay.

Fig. 6. The relationship among TOC, core water saturation (Sw (core)), conventionalwater saturation (Sw (convention)) and difference of core water saturation and con-ventional water saturation (Sw (difference)).a. The relationship between TOC and water saturations in a resistivity-increasing for-mation. A negative correlation between TOC and Sw (convention); a positive correlationbetween TOC and Sw (difference); a negative correlation between TOC and Sw (core).b. The relationship between TOC and water saturations in a resistivity-decreasing for-mation. A positive correlation between TOC and Sw (convention); a positive correlationbetween TOC and Sw (difference); a negative correlation between TOC and Sw (core).

2.2. New methods for water saturation calculations

Conventional water saturation equations, such as the Siman-doux equation, modified Simandoux equation, total shale equationand modified total shale equation, are derived from the conduc-tivity of matrix rock and clay. They may be available in some shaleformations, such as resistivity- and TOC-increasing ones (re-sistivity-increasing formation, Fig. 2). However, organic matter inshale reservoirs has poor conductivity, whereas associated pyrites

B. Zhang, J. Xu / Journal of Natural Gas Science and Engineering 36 (2016) 800e810 803

have high conductivity. Conventional water saturation models areinapplicable in organic-rich shale intervals. Gas saturation in thoseintervals is high because the increased mature kerogen generatesmore hydrocarbons that contribute to the restored gas in pores(resistivity-decreasing formation, Fig. 3).

2.2.1. TOC correction methodGas saturation receives contributions from gas in organic mat-

ter, matrix and clay. Hence, we try to take total organic carbon intoaccount, and a modified factor is helpful in the water saturationcalculation because fluid saturation is related differently to theinorganic and organic sections. The calculation of gas saturationincludes two parts, gas filling inorganic pores and that fillingorganic pores. Therefore, the water saturation calculation equationfor shale reservoirs can be expressed as

Fig. 7. The relationship between TOC, core water saturation and conventional watersaturation from Equation (9).

Fig. 8. The situation of water saturation and gas saturation in a shale formation with decreasaturation and conventional water saturation.

Sw ¼ 1� Vg inorg

Vpor� Vg org

Vpor(5)

where Vg_inorg is the gas volume for the inorganic section, Vg_org isthe gas volume for the organic section, and Vpor is the pore volumefor the shale reservoir. However, these two parts are hard to obtain.Generally, conventional water saturation is greater than core watersaturation (Fig. 4), so a modified correction factor is needed in shalereservoirs because of the existence of organic matter. Hence, thewater saturation in shale reservoirs can be expressed as in Equation(6):

Sw ¼ Sw con � Sw d (6)

where Sw_con is calculated with conventional water saturationequations mainly representing the influence of the inorganic sec-tion if the abundance of TOC is low (Fig. 4); Sw_d is the watersaturation influenced by the existence of the organic matter inshale reservoirs (Fig. 5).

As a good indicator of organic matter in shale reservoirs, TOCcan be introduced into the water saturation calculation to explorethe influence of the organic section. Many studies show that thereis a linear relationship between TOC and gas content, and reservoirswith a high abundance of TOC generally possess a high gas contentand gas saturation. Therefore, TOC shows a negative correlationwith the core water saturation. Using TOC as another source forwater saturation, Equation (6) can be written as Equation (7).

Sw ¼ Sw con � Y$TOC (7)

where Y is an unknown variable. As Fig. 6 shows, there is a negativecorrelation between TOC and core water saturation, a positive ornegative correlation between TOC and conventional water satura-tion and mainly a negative correlation between TOC and core watersaturation in organic-rich shale reservoirs (Fig. 6b). Additionally,there is a positive correlation between the conventional watersaturation difference and core water saturation. It is assumed that

sing resistivity and increasing TOC. There is an obvious difference between core water

Fig. 9. The selection of a conventional water saturation problem. The siltstone-silt limestone in the formation greatly induces a change in resistivity and provides the selectedproblem for the conventional water saturation model. The conventional water saturation model, whose water saturation results are greater than those of core analysis, is preferredwith an added modified factor.

Fig. 10. Conventional water saturations behave regularly for the difference with corewater saturation.

B. Zhang, J. Xu / Journal of Natural Gas Science and Engineering 36 (2016) 800e810804

there is a linear relationship between Sw_d and Sw_con following thechange in TOC. Then, Equation (7) can be approximately expressedas the following:

Sw ¼ Sw con � ðx$TOCÞ$Sw con (8)

where x is a constant. Equation (8) can be rewritten as Equation (9)(which is known as the water saturation of the TOC correctionmethod):

Sw ¼ Sw con$

�1� TOC

TOCx

�(9)

where TOCx is defined as a constant value of total organic carbon (%)and is determined by Fig. 7 and (1�TOC/TOCX) is the modifiedfactor. Fig. 7 shows that there is a good linear relationship betweenTOC and (1�SW_core/SW_con), and the coefficient of determination R2

is 0.9677, which means that Equation (9) is available; the stronglinear relationship of Sw_d and TOC$Sw_con is confirmed by x(Equation (8)). There is a good linear relationship between thewater saturation of the conventional part and that of the influencepart of the organic matter following the change in the TOC value tosome extent.

2.2.2. Water saturation difference methodThe selection of conventional water saturation equations is

important in the new method that is introducing a modified factor.Because conventional water saturation must be larger than corewater saturation, and TOCx is a positive constant. Hence, in partic-ular, the water saturation from the modified total shale equation

will be much smaller or larger than core water saturationwhen thismethod is applied to other shale reservoirs with high or low re-sistivity formations (Fig. 8). Fig. 9 illustrates the comparison ofwater saturation from the Archie equation (Sw-Archie) and themodified total shale equation (Sw modified total shale) with coreanalysis. The Sw modified total shale cannot represent the influ-ence of water saturation in the inorganic part of the reservoirbecause many of the core water saturation dots are bigger than the

B. Zhang, J. Xu / Journal of Natural Gas Science and Engineering 36 (2016) 800e810 805

calculated ones. Thus, Sw-Archie is chosen to calculate watersaturation for this well (Fig. 9). Using Equation (9) the Sw-TOC isachieved and the error is reduced in the water saturation evalua-tion of shale reservoirs.

To minimize the selection problem of the conventional waterequations, we substituted Equation (8) into Equation (10). Equation(10) is based on the difference between core water saturation and

Fig. 12. Lithology of the Long

Fig. 11. The relationship among TOC, core water saturation and conventional watersaturation from Equation (11).

conventional water saturation regardless of whether conventionalwater saturation is larger or smaller than core water saturationsince they still behave regularly for determining the difference(Fig. 10). We named this method the water saturation differencemethod, as seen in Equation (10).

Sw � Sw con ¼ x$TOC$Sw con þ k (10)

where x and k are positive or negative constants, respectively.Fig. 11 shows that we can achieve x and k at a high precision, withR2¼ 0.9976. In addition, water saturation of a shale reservoir can bewritten as in Equation (11):

Sw ¼ Sw con þ x$TOC$Sw con þ k (11)

We can obtain the gas saturation from Equation (12)

Sw þ Sg ¼ 1 (12)

where Sg is the gas saturation.

3. Case study and results

3.1. A case in a resistivity-decreasing formation

Well J4 is located in the Jiaoshiba section in southeasternSichuan Basin, which is one of the most important petroliferousbasins in China. It is a well in a resistivity-decreasing formationwhere the resistivity of the black shale is generally smaller than

maxi-Wufeng formation.

Fig. 14. Plot showing the relationship between the bulk density (DEN) and totalorganic carbon (TOC) in the Longmaxi-Wufeng formation.

B. Zhang, J. Xu / Journal of Natural Gas Science and Engineering 36 (2016) 800e810806

that of the other shale formation when TOC is increasing. Thedrilling aims to evaluate the hydrocarbon abundance of organic-rich shales of the Longmaxi-Wufeng formation, which includes agas saturation calculation, fracture identification, mineral compo-nent calculation as well as lithology and lithofacies analyses.Among these calculated parameters, the gas saturation and shalereservoir evaluation are the most important because they arecritical for evaluating the free gas content.

The thick Longmaxi-Wufeng formation was deposited in apassive continental margin environment during the early Silurian,corresponding to a major global transgression (Su et al., 2007). TheLongmaxi-Wufeng formation is underlain by shallow Ordovicianmarine limestone and overlain by the mid-Silurian Luoreping for-mation of light gray to yellow siltstone, as seen in Fig. 12. Manyresearchers have studied the geologic origin of the Longmaxi-Wufeng formation, and many achievements have been made interms of determining the geochemical characteristics, pore struc-ture and reservoir physical properties (Liang et al., 2012; Huanget al., 2012; Tian et al., 2013, 2015; Wang et al., 2013; Pan et al.,2016). Organic pores are well developed in the Longmaxi-Wufengformation deposits due to the high maturity of organic matter aswell as the generation and expulsion of hydrocarbons (Liang et al.,2016).

3.1.1. Porosity and total organic carbon calculationPorosity is one of the most important parameters in accurately

estimating water saturation. Conventional calculation methods arenot applicable in tight reservoirs. An empirical relationship be-tween porosity (POR) and acoustic (DT) is introduced due to thegood linear relationship between them in the Longmaxi-Wufengformation (Fig. 13).

Huang et al. (2015) studied the calculation of TOC in the SichuanBasin and proposed the empirical bulk density method given thegood linear fit between TOC and bulk density (DEN) (Fig. 14).

Fig. 15. Plot showing a linear relationship between TOC and�1� SW core

SW con

�(Equation

3.1.2. Water saturation evaluationWith the methods mentioned above, the water saturation of the

modified total shale equation is chosen as conventional watersaturation, and according to Equations (9) and (11), the corre-sponding constants are obtained from Figs. 15 and 16, respectively.The water saturation equations for the Longmaxi-Wufeng forma-tion are determined by:

Fig. 13. Plot showing the relationship between the acoustic (DT) and porosity (POR) inthe Longmaxi-Wufeng formation.

Sw TOC ¼ Sw con � TOC6:2708

$Sw con (13)

Sw difference ¼ Sw con � 0:1671$TOC$Sw con þ 2:2482 (14)

From Figs. 15 and 16, we know that there are good linear fitsbased on Equations (9) and (11). TOC correction and water

(13)). Water saturation can be calculated by the TOC correction method.

Fig. 16. Plot showing a good linear relationship between the water saturation differ-ence and TOC,Sw_con (Equation (14)). Water saturation can be accurately calculated bythe water saturation difference method.

B. Zhang, J. Xu / Journal of Natural Gas Science and Engineering 36 (2016) 800e810 807

saturation difference methods can quickly and effectively evaluatethe gas saturation of the Longmaxi-Wufeng formations.

In Fig.17, water saturation inWell J4 is calculated using Equation(13) for the green curve and Equation (14) for the blue curve. Theshale formation is recognized by high gamma ray (GR) and lowcompensated neutron (CNL), as seen in the yellow section in thefifth column. TOC and porosity were accurately determined usingthe methods mentioned above. Sw_con, which is calculated usingthe modified total shale equation, is much larger than core watersaturation (Sw_core) due to the high conductivity of the shaleformation. However, the water saturation values show regulardifferences. There is little distinction between these two methodsin this case according to the last column in Fig. 17. The results of the

Fig. 17. Water saturation evaluation in organic-rich shale reservoirs in Well J4. Conventioncurve) is not available for the Longmaxi-Wufeng formation. The results of water saturationdifference method (Sw-difference in blue curve) are in good agreement with core water satureferred to the web version of this article.)

water saturation calculated by the new methods are in goodagreement with the core water saturation.

3.2. A case in a resistivity-increasing formation

We applied the newmethods to wells in a resistivity-increasingformation in which the resistivity of the black shale is generallyhigher than that of another shale formationwhen TOC is increasing.

We have successfully evaluated the water saturation of conti-nental shale in the Dongyuemiao Member of the Lower JurassicZiliujing Formation in the eastern margin of Sichuan Basin. The li-thology of Well A1 is mainly mudstone, black shale (intercalated bythin layers of siltstone and limestone), and argillaceous limestone.

al water saturation calculated by the modified total shale equation (Sw_con in greenusing the TOC correction method (Sw-TOC in green curve) and the water saturationration. (For interpretation of the references to colour in this figure legend, the reader is

B. Zhang, J. Xu / Journal of Natural Gas Science and Engineering 36 (2016) 800e810808

The heavy limestone composition in black shale leads to theobvious change in resistivity of the Dongyuemiao Member (Fig. 9).Hence, water saturation in this area is hard to evaluate. Shi et al.(2014) applied modified total shale model to the water saturationevaluation in this section, but the results were still not accuratecompared to core water saturation because of the increasing re-sistivity of shale reservoir. In Well A1, the porosity and TOC arecalculated using RBF network method, Sw-TOC is based on theArchie equation, and Sw-difference is based on the modified totalshale equation. Both results of the water saturation can largelyreduce the errors compared with core water saturation.

The methods mentioned above can also be applied to thecalculation of water saturation in oil shale reservoirs. The oil shalereservoir ofWell L6 of the lower 3rdmember of Shahejie Formationin Bonan Subsag, Bohai Bay Basin, mainly develops mudstone, limemudstone, calcareous shale, and argillaceous limestone (Zhang,2012; Zhao et al., 2012; Huang and Chai, 2014). Fig. 18 shows thatthe results of the TOC correction and water saturation differencemethods are in good agreement with the core water saturation,while the result from the modified total shale equation has a largeerror compared with the core water saturation.

4. Discussion

TOC plays an important role in the calculation of parameters ofshale reservoirs, and water saturation can be influence by the ex-istence of TOC.

Fig. 19a is a cross plot of core water saturation (Sw_core) and

Fig. 18. Water saturation evaluation in organic-rich shale reservoirs in Well L6. The results omethod are in good agreement with core water saturation.

conventional water saturation (Sw_con). Conventional watersaturation of Well A1 is based on the Archie equation and that ofWell J4 is based on the modified total shale equation. It shows thatthe results of conventional water saturation are generally largerthan of core water saturation and that all of them are scatteredabove the 45� line.

Fig. 19b is a cross plot of core water saturation (Sw_core) andwater saturation calculated using the TOC correction method (Sw-TOC) (Equation (12)). The Sw-TOC of Well A1 is based on the Archieequation and that of Well J4 is based on the modified total shaleequation. It is observed that the results of water saturation (Sw-TOC) are distributed closely along the 45� line. There is a goodrelationship between core water saturation and Sw-TOC because ofthe coefficient of determination, R2 ¼ 0.4913 for Well A1 andR2 ¼ 0.8696 for Well J4.

Fig. 19c is a cross plot of core water saturation (Sw_core) andwater saturation calculated using the water saturation differencemethod (Sw-difference) (Equation (14)). Water saturations of WellsA1 and J4 are both based on the modified total shale equation,which shows that the results of water saturation using Equation(14) are distributed closely along the 45� line. There is a goodrelationship between core water saturation and Sw-differencebecause of the coefficient of determination, R2 ¼ 0.87.

From the comparison of Fig. 19a ~ c, it is known that watersaturation in organic-rich shale reservoirs can be effectively eval-uated with the TOC correction and water saturation differencemethods, no matter that the resistivity is increasing or decreasingwhen TOC is increasing and ignoring the lithology, such as silty

f water saturation using the TOC correction method and the water saturation difference

Fig. 19. Plots showing the results of water saturation by different methods.a. The cross plot of core water saturation (Sw_core) and conventional water saturationusing the modified total shale equation (Sw_con). The results of Sw_con are generallyscattered above the 45� line, and Sw_con is much larger than Sw_core;b. The cross plot of core water saturation and water saturation using the TOC correctionmethod (Sw-TOC). The results of Sw-TOC are distributed closely along the 45� line,indicating that the new method can effectively evaluate the water saturation inorganic-rich shale reservoirs;c. The cross plot of core water saturation and water saturation using the water satu-ration difference method (Sw-difference). The results of Sw-difference are distributedclosely along the 45� line, implying that the new method can effectively evaluate thewater saturation in organic-rich shale reservoirs.

B. Zhang, J. Xu / Journal of Natural Gas Science and Engineering 36 (2016) 800e810 809

shale or calcareous shale. In addition, the water saturation differ-ence method is more effective than the TOC correction method. Insummary, the accuracy of water saturation can be greatly improvedby these new methods.

5. Conclusions

(1) Kerogens greatly influence gas saturation in organic-richshale reservoirs. TOC is critical for calculating the gas satu-ration of shale reservoirs, and there is a regular relationshipamong TOC, conventional water saturation and core watersaturation.

(2) Considering the shale gas that exists in the organic sectionand the inorganic section, a modified factor

�1� TOC

TOCx

�is

introduced for correction and a new method called the TOCcorrection method is proposed. We can calculate gas or oilsaturation more accurately for shale reservoirs, especially fororganic-rich ones.

(3) According to the relationship between conventional watersaturation and core water saturation, we develop a methodfor the water saturation calculation considering the differ-ence between core water saturation and conventional watersaturation. This difference shows a regular relationship withTOC in organic-rich shale reservoirs.

(4) TOC correction and water saturation difference methods canquickly and effectively evaluate water saturation in organic-rich shale reservoirs ignoring the lithology and conductivityof the formation. The water saturation difference methodoutweighs the TOC correction method regarding the accu-racy of the water saturation calculation.

Acknowledgments

This study was financially supported by 1) the National NaturalScience Foundation of China (Grant No. 41302107), 2) the Ministryof Land and Resources special funds for scientific research in thepublic cause (Grant No. 201311107), 3) the Fundamental ResearchFunds for the Central Universities (Grant No. 2652011282) and 4)the CNPC Innovation Foundation (Grant No. 2012D-5006-0103).Wethank Elsevier language editing for improving our manuscript andthank the editors and anonymous reviewers for their careful re-views and detailed comments that helped to substantially improvethe manuscript.

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