11th Biennial International Conference & Exposition

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1 Evlution of Cretceous Sediments for Orgnic Richness nd Level of Mturity using Petrophysicl dt D. Sriknt* nd P. Shnmugm Petroleum Engineering Progrm, Deprtment of Ocen Engineering, Indin Institute of Technology Mdrs, Chenni , Indi Corresponding uthor emil*: srikntchowdhry@gmil.com Key words: petroleum explortion, sedimentry sis, orgnic richness, level of orgnic mturity, porosity, wireline logs, Rmnd su-sin, Indi Astrct A quntittive estimtion of source rock prmeters ws ttempted using experimentl TOC dt, porosity nd resistivity log dt cquired in the well sections of Periypttinm, Perungulm, Rmnvlsi, Rjsingmnglm nd Uchipuli in the Rmnd su sin. Cretceous sedimentry succession ws nlyzed for orgnic richness nd level of mturity using log R technique nd crossplots for six explortory wells in this su-sin. The interprettion of results of oth %TOC nd LOM recognized tht only Andimdm Formtion consists of high vlues of TOC rnging from 4 to 6% nd LOM from 8.6 to 12.0, indicting mtured nture of orgnic source rocks in the Rmnd su-sin. The study lso found n excellent mtch etween mesured TOC nd log TOC therey suggesting tht log TOC cn e sfely relied upon for estimting orgnic mturity in the Rmnd su-sin. Increse in formtion resistivity, decrese in formtion density, increse of neutron porosity, increse in sonic trnsit time nd increse in gmm ry ctivity were oserved in mny wells ginst the source rock rich Andimdm formtion. All the log recorded prmeters hve responded to the mtured source rock in the formtion. Introduction Esy methods of finding oil nd gs pools hve sustntilly reduced nd dvnced explortion methods of oil nd gs reserves from deeper sediments involving more complex geology hve come into picture from the pst decde. More dvnced technologies oth in the explortion nd production domins hve come into existence; one such technology is evlution of sediments for orgnic richness nd level of orgnic mturity from porosity nd resistivity logs. The chief constituent of the orgnic mtter i.e. totl orgnic cron TOC hs een widely used to ssess the potentil of hydrocron resources in the sedimentry sins. Trditionlly geochemicl method is employed for determintion of source rock prmeters nmely TOC, S1, S2, T mx nd VR o. This method is time consuming, cost intensive nd cn e mesured t smpled intervls. This method is lso error prone if sufficient cre is not tken during smpling. An lterntive to the geochemicl method of determintion of source rock prmeters ws developed y erlier petrophysicist. Among severl reported methods ville for estimtion of TOC, four methods re populr nd widely used: 1 directly y regression of core TOC with core ulk density; 2 Δ logr technique; 3 sed on the petrophysicl response model; nd 4 using n rtificil neurl network. The generl conundrum of geoscientists in oil industry is to identify possile hydrocron resource plys. To ccomplish this tsk, determining the potentil orgnic shle sections is one of the primry exercises nd gin doing it inexpensively due to lck of redily ville informtion for estimtes. There re mny methods ville for quick estimtion of TOC y interprettion of vrious electricl logs. One such method is log R developed y Pssey et l which hs een commonly used to determine totl orgnic cron richness TOC y cross-plotting sonic logs DT nd log nturl logrithm of resistivity dt. This method llows orgnic richness to e ssessed in wide vriety of lithologies nd mturities using common well logs. In wter sturted, orgnic-len rocks, the two curves prllel or overlie ech other llow the shle clcultion line to e determined. However, in either hydrocron reservoir rocks or orgnic-rich shle sections, seprtion log R etween the curves occurs. The seprtion in orgnic-rich intervls results from two effects: the porosity curve responds to the presence of low-density, low-velocity Kerogen, nd the resistivity curve responds to the formtion fluid. In mture source rocks the mgnitude of the resistivity increses ecuse of the presence of generted hydrocrons. By cross-plotting multiple wells in sin this technique cn provide reltive informtion for entire shle section of n re/sin. This method requires simple cross-plot nd log

2 clcultion mthemtics to provide geoscientist sufficient dt to esily nd quickly determine potentil orgnic shle sections. Across n re, these log cross-plots prop up the correltion nd mpping of orgnic-rich shle sections nd llow the geoscientist to quickly determine high prospective res for further study. The min ojectives of this study re to qulittively estimte totl orgnic cron TOC, level of mturity LOM nd S2 source rock prmeters for lte Cretceous successions in Periypttinm, Perungulm, Rmnvlsi, Rjsingmnglm nd Uchipuli fields in the Rmnd su sin Swnson et l y using log R technique of Pssey et l. The results were verified with the l determined prmeters. The min dvntge of this method ws cost enefit, continuous mesurement nd quick delivery of results. Conceptul Model of Orgnic-Rich Rocks Source rocks re commonly shles nd limemudstones tht contin significnt mount of orgnic mtter. Non source rocks lso contin orgnic mtter ut the mount is generlly not significnt i.e., less thn 1 wt. %. The usul method for ssessing the richness nd mturity of source rocks is through vriety of lortory nlyses. This pper discusses the effect tht orgnic mtter hs on the response of common well logging tools nd proposes n esily implemented curve overly method tht is clirted for orgnic richness nd mturity. For the purpose of this work, orgnic rich rocks re ssumed to e composed of three components Fig. 1: 1 the rock mtrix, 2 the solid orgnic mtter, nd 3 the fluids filling the pore spce. Non-source rocks re composed primrily of only two components: the mtrix nd the fluid filling the pore spce Fig. 1A. In immture source rocks, solid orgnic mtter nd rock mtrix comprise the solid frction nd formtion wter fills the pore spce Fig. 1B. As the source rock mtures portion of the solid orgnic mtter is trnsformed to liquid or gseous hydrocrons which move into the pore spce, displcing the formtion wter Fig. 1C. The effects of these physicl trnsformtions on the porosity nd resistivity log responses re the suject of this work. Fig. 1. Schemtic of solid nd fluid components in source nd non-source rocks fter Pssey et l., Dt Source nd Methods A complete set of log dt utilized in this work ws otined from the Ntionl Oil Compny ONGC. Interctive Petrophysics softwre version IP 4.1 ws used to generte logr seprtion etween resistivity nd sonic log when overlin nd determine totl orgnic cron richness TOC. TOC present in the source rock effects the response of severl types of logs, especilly incresing trend oth in sonic trvel time nd formtion resistivity. In orgnic len rocks the log scles re djusted so tht the sonic nd resistivity curves re overlin on ech other. In orgnic rich rocks, due to the presence of Kerogen sonic log reds higher trvel time nd resistivity log reds higher resistivity ecuse Kerogen is more resistive compred to formtion wter which is present in the pores of the formtion.rock-evl pyrolysis ws used to determine the type nd mturity of the orgnic mtter nd to detect petroleum potentil in sediments. A seprte trck hs een generted to compre TOC derived from logs nd experimentlly derived TOC dt. The lgeric expressions for the clculted ΔlogR from the Sonic Vs Resistivity, Neutron Vs Resistivity nd Density Vs Resistivity overlys re given s follows: ΔlogR Sonic = log10 R/ R-seline Dt Dtseline ΔlogR Neutron = log10 R /R-seline φn φn-seline ΔlogR Density = log10 R/ R-seline 2.5 ρ ρ-seline The ΔlogR seprtion is linerly relted to the TOC content nd is function of mturity. The empiricl eqution for clculting TOC content in orgnic rich rocks from ΔlogR is: TOC = ΔlogR LOM, where TOC is the totl orgnic cron content wt % nd LOM is the mesured level of mturity. LOM is otined from the vitrinite reflectnce Vro or therml ltertion index y using the mturtion indictors. Using Fig. 2 the ΔlogR seprtion cn e trnsformed directly to TOC if the mturity cn e determined or estimted. In prctice, LOM is otined from vriety of smple nlysis e.g. vitrinite reflectnce, therml ltertion index, or T mx, or from estimtes of uril nd therml history. In this study, Pssey et

3 l ws dopted for plotting Δ log R vs TOC Fig. 2 nd to determine LOM level of orgnic mturity type nd clcultion of S 2 mg HC/g Rock Fig. 3. TOC delt log R Fig. 2. Level of orgnic mturity LOM derived y cross plotting logr vs. TOC wt % S 2 mghc/g ROCK LOM 6 LOM 12 LOM 11 LOM 10 LOM 9 LOM 8 LOM 7 LOM <6 LOM 7 LOM 8 LOM 9 LOM 10 LOM 11 LOM TOC wt.% Fig. 3. Determintion of S2 mg HC/g Rock from the plot of TOCwt% nd LOM The interctive petrophysics softwre includes the TOC module tht computes the wt% weight percentge of orgnic contents from density reltionships nd/or logr Delt log resistivity methods. 14 different density reltionships re used to predict TOC nd re comined through verging to produce the finl TOC. Alterntively logr cn e used with Sonic, Density nd Neutron logs with n verge or weighted verge to produce the finl TOC. The density nd Neutron porosity equtions in Phi/Sw module hve een modified to include the wt% nd volumes of TOC Kerogen to llow the correct determintion of porosity nd hydrocron volume in the pore spce. A TOC wt% Eqution nd Kerogen Minerl hve een dded to the Minerl Solver to include the results from the TOC module into the model. Results The present study minly focused on eight explortory wells drilled into cretceous succession Andimdm, Sttppdi, Bhuvngiri formtions in strtigrphic order in the Rmnd su-sin for evluting its source potentil y determining TOC, LOM nd S2 vlues. TOC log is generted y overlying porosity nd resistivity logs. This log consists of 12 trcks nmely gmm ry log trck green, depth trck lck, resistivity trck red, porosity logs trck density ornge, neutron violet nd sonic pink, resistivity nd porosity overly trck nd TOC trck red. logr is otined from the seprtion etween resistivity red nd porosity logs density, neutron nd sonic, preferly sonic log lue since it is not ffected y d hole conditions. In well PE#AX, TOC log ws generted y overlying porosity nd resistivity logs Fig.4. The log revels tht the Andimdm Formtion m contins excellent TOC 4 6% oth from well logs nd l mesurements. There is reltive increse of resistivity nd porosity ginst Andimdm Formtion compred to overlying Sttpdi Formtion. logr is ~2 in the Andimdm Formtion wheres in the Sttpdi Formtion it is less thn or equl to 1. Figure 5 shows the level of orgnic mturity LOM derived y cross plotting logr vs. TOC wt %, nd S 2 Fig. 5 from cross plot of TOC nd LOM. Both TOC nd clculted S 2 vlues re in good greement with the mesured dt from core/cutting smples throughout the study intervl. The fine grined sediments within this sequence hve good hydrocron generting potentil S 2 = 3 mg HC/g rock preferly gs s LOM = 11.2.

4 Source rock evlution from Petrophysicl dt Fig. 4. TOC log generted from the wireline logs using Interctive Petrophysics softwre for well PE#AX. C1gmm ry, C2-depth, C4-deep resistivity, C5-porosity logs, C7- logrt- t, C8- logrt-rho,c9- logrt-nphi,c10toc derived from vrious porosity logs,c12- TOC derived from well logs nd l. Fig. 5 nd. LOM derived from the crossplot of TOC nd log R nd determintion of S2mg HC/g Rock from the plot of TOCwt% nd LOM of Andimdm formtion for the well PE#AX. In well UP#CX, TOC log ws generted for the depth intervl from 2000 to 3000m which includes Nnnilm, Kudvsl, Bhuvngiri, Sttpdi nd Andimdm formtions Figs. 6 nd 7. It is seen tht ginst the Andimdm formtion the increse in resistivity nd increse in porosity in ll the three porosity logs is evident due the increse in the presence of Kerogen compred to the overlying formtions. In Bhuvngiri formtion, logr nd TOC re present in the rnge of 0 to 0.5 nd 2 to 3 wt% which indictes fir to good concentrtion of orgnic mtter. S2 nd level of orgnic mturity LOM indicte 4.9 nd 7.5 respectively Figs. 8 nd. In spite of sufficient orgnic mtter richness, there is no hydrocron genertion potentils since the qulity of orgnic mtter is poor. In Andimdm formtion, logr nd TOC re present in the rnge of 1 nd 3 wt% which indictes good concentrtion of orgnic mtter. S2 nd level of orgnic mturity LOM indicte 2 nd 11 respectively Figs. 9 nd. The orgnic mtter within this sequence cnnot generte hydrocrons ecuse of low vlue of S2. The lithology of Bhuvngiri nd Nnnilm formtions depth intervl m is predominntly sndstone, nd shows considerle vrince etween log nd l mesured TOC vlues. Fig. 6. TOC log generted from the wireline logs using Interctive Petrophysics softwre for well UP#CX Bhuvngiri. C1-gmm ry, C2-depth, C4-deep resistivity, C5-porosity logs, C7- logrt- t, C8- logrtrho,c9- logrt-nphi,c10-toc derived from vrious porosity logs,c12- TOC derived from well logs nd l. Fig. 7. TOC log generted from the wireline logs using Interctive Petrophysics softwre for well UP#CX Andimdm. C1-gmm ry, C2-depth, C4-deep resistivity, C5-porosity logs, C7- logrt- t, C8- logrtrho,c9- logrt-nphi,c10-toc derived from vrious porosity logs,c12- TOC derived from well logs nd l

5 Source rock evlution from Petrophysicl dt Fig. 8 nd. LOM derived from the crossplot of TOC nd log R nd determintion of S2mg HC/g Rock from the plot of TOCwt% nd LOM of Bhuvngiri formtion for the well UP#CX. Fig. 9 nd. LOM derived from the crossplot of TOC nd log R nd determintion of S2mg HC/g Rock from the plot of TOCwt% nd LOM of Andimdm formtion for the well UP#CX. In the well RV#FX, TOC log ws generted from 2000 to 3050m which covers the entire Cretceous section Fig. 10. There is shrp increse in the resistivity from 2 to 10 ohmm ginst the Andimdm formtion compred to the overlying formtion. However, porosity hs not pprecily incresed ginst the Andimdm formtion proly due to over mturtion of the source rock. The upper Cretceous formtions contin TOC from 1%, logr 0 to 1, S2 0 nd LOM 12 which indictes fir concentrtion of orgnic mtter ut no hydrocron genertion potentil Figs. 11 nd. Andimdm Formtion in the depth intervl from m shows logr 1.5, TOC 2, S 2 0 nd LOM 12.5 which indictes good concentrtion of orgnic mtter nd the entire orgnic mtter is converted to hydrocrons Figs. 12 nd. L mesured TOC coincide with log derived vlues from m; l vlues re slightly lower from m; l vlues re less thn hlf of log vlues in the intervl from m nd coincide closely from m depth intervl with log derived vlues. Fig. 10. TOC log generted from the wireline logs using Interctive Petrophysics softwre for well RV#FX. C1gmm ry, C2-depth, C4-deep resistivity, C5-porosity logs, C7- logrt- t, C8- logrt-rho,c9- logrt-nphi,c10toc derived from vrious porosity logs,c12- TOC derived from well logs nd l. Fig.11 nd. LOM derived from the crossplot of TOC nd log R nd determintion of S2mg HC/g Rock from the

6 plot of TOCwt% nd LOM formtion for the well RV#FX. of upper Cretceous Fig. 12 nd. LOM derived from the crossplot of TOC nd log R nd determintion of S2mg HC/g Rock from the plot of TOCwt% nd LOM of Andimdm formtion for the well RV#FX. Discussion Wireline logs re useful to identify source rocks nd serve s n indictor for the source rock potentil provided tht the source rocks hve minimum thickness nd sufficiently rich in orgnic mtter. Wireline methods for estimting orgnic mtter content hve the dvntge of economy, redily ville sources of dt nd the continuous smpling of verticlly heterogeneous shle section. However, the l mesured geochemicl nlysis generlly offer specific dvntge in mking direct mesurement of orgnic content ut the extent nd nture of geochemicl smpling provide the gretest limittion to quntittive pprisl of orgnic content. Cutting smples re usully collected over n intervl of out 10 m. They represent n verge ssessment of orgnic content for the studied intervls in this study. However, the results of wireline logging technique hve dequtely highlighted the limittions inherent in quntittive determintion. Therefore, the correltion etween the results from l derived geochemicl nlysis nd log technique is importnt to prove the successfulness of wireline technique in evluting the orgnic richness in ny study. These limittions will e risen from the sence of solute vlues definle for log reding of orgnic mtter. Accordingly, the well logging methods tht involve direct clirtion with core/cutting dt re only relile within loclized re or susin. The interprettion results of oth %TOC nd LOM nd lithofcies llow to recognize different orgnic fcies in the present study. The strtigrphiclly oldest Andimdm Formtion consists of predominntly lterntions of minor snd nd mjor shle lterntions s chrcterized y high gmm nd high resistivity. Also, %TOC derived nd mesured show high vlues rnging from 4 to 6% nd LOM from 8.6 to There is lso excellent mtch etween mtoc mesured TOC nd dtoc derived TOC in the Andimdm Formtion of ll the study wells. The high %TOC nd LOM >9 indictes mtured nture of orgnic source rocks in the Andimdm Formtion in the Rmnd su-sin. The overlying Sttpdi nd Bhuvngiri formtions re dominntly shle fcies with minor sndy fcies occurring in upper prt of Bhuvngiri Formtion. However, the mtoc nd dtoc show low vlues rnging from 0-1%, while LOM rnges from 1-3. The very low vlues of LOM suggest immture nture of modertely rich orgnic source rocks occurring in the Bhuvngiri nd Sttpdi formtions. While Nnnilm Formtion consists of reservoir sndy fcies in its upper prt, it contins very poor TOC nd LOM vlues indicting no source potentil of orgnic fcies in this formtion. The mesured TOC vlues from core/sidewll core nd ditch cuttings nd log derived TOC following were compred for ll the study well sections in the Rmnd susin. For ll the seven explortory wells, we found firly good correltion etween logderived TOC vlues with mesured TOC vlues from side wll cores, conventionl cores or cutting smples. However, in wells RV#FX nd RS#GX, we found mismtch etween the two vlues t certin depth intervls. The sediment smples re predominntly cuttings nd thus often suject to downhole contmintion, therey giving spurious mesured TOC vlues. Therefore, this mismtch cn e ttriuted to the nture of smples. However, in most cses, the mtch is very good nd this hs enled to pply the log R technique in wells for which no smples re ville. Also, the ility to perform the log R technique t well site using properly-scled sonic nd resistivity curves further provides n dvntge for identifying orgnic rich intervls to e smpled with side wll coring tool. Therefore, this study further proposes tht logderived TOC cn e relied upon for evluting

7 orgnic source potentil in the Rmnd su-sin, wherever mesured dt is not ville or unrelile due to smple contmintion. Summry nd Conclusion Geochemicl prmeters viz., TOC, S 2 nd LOM were derived from wireline log dt using the principle of log R developed y Pssey et l., 1990 for eight explortory wells of Rmnd susin of Cuvery sin. The results were vlidted with l derived prmeters. The concentrtion of TOC, S 2 nd LOM for different litho units of Cretceous sediments enled us to derive the following conclusions: log R technique is useful to evlute %TOC nd orgnic mturity for 8 explortory wells. References is no dditionl cost impliction for dt cquisition. Once the method is estlished it cn e used quickly nd relily for lrge well sections in the entire su-sin. [1] Jinjing L S, Chen Z G, Zhng G, Guo J, Xu Y. The upper limit of mturity of nturl gs genertion nd its impliction for the Ycheng formtion in the Qiongdongnn Bsin, Chin. Journl of Asin Erth Sciences : log R seprtion shows direct reltion with %TOC, while comprison of mtoc mesured TOC nd dtoc derived TOC found closely mtching for most depth intervls in ll well sections. However, mismtch found for few depth intervls in wells RV-FX nd RS-GX cn e ttriuted to smple contmintion due to uphole cvings. The good mtch found etween mtoc nd dtoc in most depth intervls in ll 8 well sections suggests tht dtoc cn e sfely relied upon for estimting %TOC nd LOM in the Rmnd su-sin. Log derived TOC dtoc hs dvntge over mtoc s it cn e derived t the well site nd cn e used redily for plnning side wll core especilly in source rich intervls, if side wll core pertins to evlute source potentil generlly done in shle gs explortion. mtoc is cost intensive s it involves collection of smples, pcking them, trnsporting them to the centrl loction nd mking elorte mesurements which is time consuming. The input log dt for dtoc method is ny how recorded in ll the drilled wells. There [2] Wples, D.W. Geochemistry in Petroleum Explortion Interntionl Resources Development Corportion, Boston, U.S.A. [3] Pssey Q R., Creney S, Kull J B., Moretti, F J., Stroud J D. A Prcticl Model for Orgnic Richness from Porosity nd Resitivity Logs. AAPG Bulletin, 74 12, [4] Meter B L, Nederlof M H. Identifiction of Source Rocks on Wireline Logs y Density/Resistivity nd Sonic Trnsit Time/Resistivity Cross-plots. AAPG Bulletin 1984: 682. [5] Mendelson J D nd Toksoz M H. Source rock chrcteriztion using multivrite nlysis of log dt SPWLA 26th Annul Logging Symposium, pper U.U. [6] John H M. Petroleum Geochemistry nd Geology; nd edition W.H. freemn nd Compny, New York, U.S.A.