Bull 185 Rami M. Idan Bull. Iraq nat. Hist. Mus. (2017) 14 (3): 185-195 TOTAL ORGANIC CARBON (TOC) PREDICTION FROM RESISTIVITY AND POROSITY LOGS: A CASE STUDY FROM IRAQ Rami M. Idan Department of Geophysics, College of Remote Sensing and Geophysics, Al-Karkh University of Sciences, Baghdad, Iraq Email: ramisc3@kus.edu.iq Received Date:11.Octber.2016 Accepted Date: 22.Feberaury.2017 ABSTRACT The open hole well log data (Resistivity, Sonic, and Gamma Ray) of well X in Euphrates subzone within the Mesopotamian basin are applied to detect the total organic carbon (TOC) of Zubair Formation in the south part of Iraq. The mathematical interpretation of the logs parameters helped in detecting the TOC and source rock productivity. As well, the quantitative interpretation of the logs data leads to assigning to the organic content and source rock intervals identification. The reactions of logs in relation to the increasing of TOC can be detected through logs parameters. By this way, the TOC can be predicted with an increase in gamma-ray, sonic, neutron, and resistivity, as well as a decrease in the density log. In calculating TOC content, sonic/resistivity overlay technique was used. The results detected that the upper and lower parts (3300- 3460 and 3570-3700 respectively) of the formation were the principal source rock in this location. The TOC results from logs are ranged respectively from 1-6 and 1-4 wt % for the upper and lower parts from the formation. These results are compared with TOC from (58) samples of Rock -Eval Pyrolysis, which showed a close pattern of increasing and decreasing in TOC values. This comparison was made so as to enhance the results of this technique. In addition, this tool revealed the possible lithology of the studied intervals, where the logs originally would give an indication to the lithology, as such high TOC is significant to relatively low energy environments. TOC calculation showed that the upper and lower packages represent source-seal rocks, while the middle had good reservoir properties. This relation may indicate a locally stratigraphic trap, and a need for further detailed studies. Key words: Resistivity logs, Southern Iraq, TOC prediction, Zubair Formation, Δt log R. INTRODUCTION The assessment of source rocks for any basin studies in various geological settings needs to grow the uses of logs techniques to enhance the database, especially when geochemical information is restricted. These uses provide an integrated assessment of source rock ability for volumetric determinations. Source rocks are mainly formed of fine- grained sediments like mudstones and shale (Tissot and Welte, 1984). The important component of source rocks is the organic matter (OM), which expresses the Total Organic Carbon (TOC). The later have to be more than 1% to be worthy in the DOI: http://dx.doi.org/10.26842/binhm.7.2017.14.3.0185 http://dx.doi.org/10.26842/binhm.7.2017.14.3.0185 186 Total organig carbon (TOC) prediction from resistivity and porosity logs source rock studies (Hunt, 1996). In organic geochemical studies and petroleum exploration, the TOC is the main indicator of the quantitative parameters of any source rock, in addition to S2 and the qualitative once (Peters et al., 2005). Forasmuch, due to the value of TOC is the main parameter to expect the quantity and quality, this paper focuses on the method of how to get the real values or to be closer (Leckie et al., 1988 and Dymann et al., 1996). A number of logs data have been prepared and predestined to use in determining variations and absolute quantities of TOC. In this work, two main logs have applications in quantifying organic content from wire line logs. The resistivity and porosity, in addition to the gamma ray and calibre logs, are established for determining the TOC in the evaluated units as shown by (Passey et al., 1990). In south east in the Mesopotamian zone of Iraq in Euphrates subzone precisely, is the area of interest and is considered as one of the promising regions (Map 1). The case study was from southern oilfield, well X was chosen due to the integration of data. The total depth of this well is (4700 m) from sea level. The interval of interest is located from (3300-3700 m) which represents the Zubair Formation. The formation is comprised from alternation of sandstone, siltstone, and shale, representing the delta and pro delta facies, while limestone is restricted to the upper part, of the formation, which represents transgressive phase deposits (Buday and Jassim, 1980). It is divided into three units; Upper, Middle, and Lower Zubair Formation. The shale packages become thinner toward the western area of the study while the coarse clastic packages become thinner within and toward the eastern parts (Jassim and Goff 2006). Such differences in the thickness of the packages must be due to progradation of the delta sand bodies. On the other hand, Euphrates Subzone lies in the west of Mesopotamian zone. It is the shallowest unit but has thicker Quaternary deposits compared with the Tigris Subzone (Aqrawi et al., 2010). MATERIALS AND METHODS Logs of resistivity, porosity, gamma ray, and calibre were converted from hard formula into digital data to be possibly used in calibration by Excel software. This operation was made by the didger program. Primarily, the logs of resistivity (ILD) and the sonic (Δt) are superimposed to be in one harmonic track, where each 50 µsec/ft from Δt equal one logarithmic ILD cycle (Passey et al., 1990). The gamma ray (GR) log is synchronous to delineate the shale base line. As shown in detail in Diagram (1) (Shayesteh, 2011), the ILD and Δt baselines (which represent the overlapping between them in non-source, clay rich rocks based on relatively high values of GR) specified and appointed to equal 30.71 ohm.m and 81.43 µsec/ft respectively at the interval 3500-3550 m. Then the equation (1) was run in Excel software as below: Δ log R = (R/R baseline) +0.02(Δt - Δt baseline) ............ (1) Where: R is the resistivity measured from log, Δt is the transit time from log, R baseline (30.71 ohm.m) is the resistivity corresponding to the Δt baseline value (81.43 µsec/ft) when the curves overlap in non-source, clay rich rocks, and 0.02 is based on the ratio of (50) µsec/ft for each resistivity cycle. The resulted Δ log R, as well, entered to the Excel software to calculate the total organic carbon (TOC) as shown in equation (2). TOC = (Δ log R)*10 exp (2.297 - 0.1688 * LOM)............ (2) Where: TOC is the total organic carbon measured in wt%, LOM is the level of organic maturity. Depending on the calculated Ro and comparing with (Hood et al., 1975); the LOM value 187 Rami M. Idan is eight (8) for this research as shown in Table (1) and Diagram (2), which means of Ro had ranged from 0.5-0.6 depending on (Jarvie, 1991), the other numbers are constant. In each practical study, the resulted values have to be realized and ensured for their certainty, wherefore; fifty eight (58) samples of cutting were analyzed to determine the geochemical parameters of Rock-Eval pyrolysis techniques. The samples were selected as far as possible from the shale intervals of different depths as shown in Table (1). These data were inverted from the South oil company (SOC) in Basra governorate. Finally, all the collected logs data were used to describe the organic richness in the interval of interest. Furthermore, the TOC results from Rock - Eval pyrolysis were represented in a log curve and superimposed with TOC resulted from equations, as in Diagram (3). Table (1): The organic geochemical parameters obtained from Rock-Eval Pyrolysis with calculated Ro according to the equation of Jarvie (1991). No. Depth TOC S2 Tmax Cal. Ro No. depth TOC S2 Tmax Cal. Ro 1. 3306 1.3 1.11 429 0.562 30. 3528 0.74 1.03 433 0.634 2. 3312 0.96 0.55 429 0.562 31. 3532 0.83 0.87 434 0.652 3. 3316 0.92 1.03 429 0.562 32. 3537 0.51 0.37 441 0.778 4. 3324 1.07 1.05 432 0.616 33. 3542 0.58 0.38 429 0.562 5. 3332 1.65 2.88 425 0.49 34. 3550 0.66 0.66 440 0.76 6. 3338 0.6 0.79 424 0.472 35. 3554 0.76 0.58 431 0.598 7. 3346 1.24 1.4 430 0.58 36. 3562 1.28 1.25 425 0.49 8. 3358 0.97 1.66 422 0.436 37. 3567 0.45 0.38 435 0.67 9. 3366 0.74 0.5 430 0.58 38. 3570 0.54 0.59 438 0.724 10. 3388 0.4 0.2 435 0.67 39. 3578 0.58 0.24 429 0.562 11. 3392 0.86 0.39 431 0.598 40. 3582 0.67 0.48 435 0.67 12. 3399 0.49 0.25 439 0.742 41. 3588 1.19 0.9 433 0.634 13. 3408 0.92 0.57 429 0.562 42. 3592 1.52 0.27 427 0.526 14. 3414 0.46 0.14 423 0.454 43. 3598 0.5 0.17 429 0.562 15. 3422 1.01 1.31 432 0.616 44. 3604 0.54 0.41 440 0.76 16. 3430 0.87 1.07 433 0.634 45. 3610 0.5 0.23 440 0.76 17. 3435 0.82 0.59 428 0.544 46. 3618 0.76 0.59 434 0.652 18. 3442 0.74 0.43 427 0.526 47. 3624 0.72 0.53 430 0.58 19. 3456 1.02 0.83 433 0.634 48. 3628 0.53 0.31 432 0.616 20. 3470 0.85 0.96 435 0.67 49. 3636 0.75 0.41 432 0.616 21. 3476 1.13 1.71 431 0.598 50. 3640 0.74 0.45 437 0.706 22. 3482 0.82 0.93 433 0.634 51. 3644 0.95 0.54 438 0.724 23. 3488 1.9 3.56 431 0.598 52. 3650 1.8 0.48 441 0.778 24. 3493 3.17 10.61 431 0.598 53. 3656 0.64 0.16 439 0.742 25. 3500 1.17 0.98 430 0.58 54. 3666 1.1 0.36 428 0.544 26. 3506 1.16 1.13 434 0.652 55. 3676 1.03 0.38 430 0.58 27. 3512 0.85 1.16 432 0.616 56. 3682 1.52 0.45 440 0.76 28. 3516 0.64 0.37 434 0.652 57. 3688 1.55 0.66 435 0.67 29. 3524 1.54 1.01 431 0.598 58. 3695 2.6 1.75 435 0.67 188 Total organig carbon (TOC) prediction from resistivity and porosity logs RESULTS The main results of this paper are: 1. The TOC can be calculated from the right integration of porosity and resistivity logs. These results are supported by comparison with TOC from the Rock-Eval pyrolysis. 2. TOC is a good indicator for the lithology of different intervals. As well as, the studied section is divided into three rock packages in respect to the TOC, and as a result, to the lithology. These three rock packages started in 3300- 3460m (160m), 3460-3570m (110m), and 3570-3700m (130m) from top to bottom respectively. 3. Similar to these studies can be used and applied in the promising areas that lack geochemical data. 4. The correlation between the TOC results from log and core analyses is important to determine and prove the success of the logs techniques in organic geochemistry evaluation. In well X, the correlation between the calculated and the measured TOC reflects a good obvious similarity between them (Diag. 3). Thus, the overlay proved real tools for quantitative determination of TOC in this well. DISCUSSION Wire line logs of sonic and resistivity overlays are applied for the certain interval of interest (Zubair Formation) in the well X. One overlay is drawn for the well, combining both of the calculated TOC values and the GR log (Diag. 3). This type of presentation may help in identifying the organic rich rocks and evaluating the organic richness in a whole formation or intervals that have no enough data to study the geochemical properties. The sonic - resistivity overlays of the formation can be presented as in (Diag. 3).This overlay reflects the dominant of a good Δ log R separation with high percentage of the TOC (wt %) in the upper (3300-3460m) and lower (3570-3700m) parts of Zubair Formation. The calculated TOC range between 1-6 wt % in the upper formation, while 1- 4 wt % in the lower, indicating the prevalence of organic matter in these two intervals. This result refers to good source rock quantity parameters, which may indicate a possible source rock, and can provide oil and/or gas to the nearby reservoir rocks. The middle Zubair Formation (3460-3570m) showed a decrease in the TOC values. This may indicate good reservoir characteristics in the studied area as shown in (Diag. 4). Rock characteristics are dominantly solid and have good total porosity as expected by the company of porosity, resistivity, and Gamma ray, in addition to calibre log; this may mean the consistence of pure sandstone as assigned by (Idan et al., 2015). The resulting ideas may suggest three different rock packages, the middle of them diagnosed to be the cleaner sandstone reservoir trapped between two shale-rich intervals acting as source-seal stratigraphic trap or complete petroleum system as explained in (Idan, 2012). This conclusion may be enhanced by several later studies for the promising area, concerning with reservoir characterization (porosity, permeability, and water saturation) and oil habitat. CONCLUSIONS Logs method is considered as the up-to-date to the identification and quantification source rock. The evaluation method primarily starts by exposing the responses of the logs GR, Δt, and ILD, in addition off course to the neutron and density, to increasing TOC. 189 Rami M. Idan Map (1): The tectonical zonation and the area of study representing the target oil fields (Al-Ameri et al., 2011). Increase of GR, Δt, neutron, resistivity and decrease of density may indicate increasing in TOC but this is not necessarily always true in all cases (Diag. 4). Porosity/resistivity tool shows that logs can be used to identify organic-rich formations. As in this case, the calculated TOC ranged from 1-6 wt %. This result is close to the Rock-Eval analysis which means that the studied interval is considered as source-seal rock in the study area. To detect that log, analysis may really evaluate and be applied for quantitative determination of TOC, it is essential to correlate with Rock-Eval pyrolysis data. Results from the overlay showed a generally accepted compatible with core data in estimating TOC in this area (Diag. 3). As a result, these calculations can be used in intervals that lack of geochemical data to obtain an overview in exploration. ACKNOWLEDGMENT The South Iraqi Oil Company (SOC) is acknowledged for the supply of logs and rocks samples analyses as well as other information related to this paper. 190 Total organig carbon (TOC) prediction from resistivity and porosity logs D ia g r a m ( 1 ): S o n ic /R e si st iv it y o v e rl a y , sh o w in g Δ l o g R s e p a ra ti o n i n t h e o rg a n ic r ic h i n te rv a ls . G a m m a R a y ( in A P I u n it s) i s c o n te m p o ra n e o u s to f in d o u t th e s h a le b a se l in e i n t h e s tu d ie d c a se . 191 Rami M. Idan Diagram (2): Level of Organic Maturity or Metamorphism (LOM) explains how the LOM value has been chosen depending on the Vitrinite reflectance (Ro), which calculated as Jarvie (1991), modified from Hood et al. (1975). 192 Total organig carbon (TOC) prediction from resistivity and porosity logs D ia g r a m (3 ): r e p re se n ti n g t h e r e su lt e d T O C f ro m l o g o v e rl a p p e d b y t h e T O C f ro m R o c k - E v a l p y ro ly si s to c o m p a re t h e r e su lt s. 193 Rami M. Idan Diagram (4): Full set of resistivity, porosity, GR, and calibre of the interested interval, showing the petrophysical properties of the formation. Note that middle section (3460-3570m) behaves relatively different than upper and lower parts. 194 Total organig carbon (TOC) prediction from resistivity and porosity logs LITERATURE CITED Al-Ameri, T.K., Zumberge, J. and Markarian, Z.M. 2011. Hydrocarbons in The Middle Miocene Jeribe Formation, Diyala Region, NE Iraq. Journal of Petroleum Geology, 34(2): 199 – 216. Aqrawi, A., Goff, J., Horbury, A. and Sadooni, F. 2010. The Petroleum Geology of Iraq, Scientific press ltd, 1st edition, 424pp. Buday T. and Jassim S.Z. 1980. The Regional Geology of Iraq Stratigraphy and Palaeogeography. State Organization for Minerals, Baghdad, Iraq, 445pp. Dymann, T.S., Palacos, J.G., Tysdal, R.G., Perry, W.J. and Pawlewicz, M.J. 1996. Source Rock Potential of Middle Cretaceous Rock in South Western Montana. AAPG Bulletin, 80: 1177-1184. Hood, A., Gutjahr, C. C. M. and Heacock, R. L. 1975. Organic Metamorphism and the Generation of Petroleum. AAPG Bulletin, 59(6): 986-996. Hunt, J.M. 1996. 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(2017) 14 (3): 185-195 دراسة حالة :التنبؤ بكمية المادة العضوية الكلية من خالل مجسات المقاومية والمسامية من العراق رامي محمود عيدان جامعة الكرخ للعلوم, بغداد, العراق ,اءلية التحسس النائي والجيوفيزيك ,قسم الجيوفيزياء 162211111: تأريخ القبول 1620126122 :تأريخ االستالم الخالصة X للبئر تم تطبيق معلمات مجسات المقاومية والمجس الصوتي ومجس اشعة كاما لغرض تحديد كمية المادة , في شبه نطاق الفرات العائد لنطاق سهل مابين الرافدين لقد ساعد التفسير الرياضي للمجسات .العضوية الكلية في تكوين الزبير جنوبي العراق فأن , لكاضافة لذ. في كشف عن كمية المادة العضوية وانتاجية الصخرة المصدرية لمجسات قاد الى المحتوى العضوي وتحديد اعماقه في ا التفسير الكمي لمعلومات يمكن الكشف عن العالقة بين استجابة المجسات لزيادة كمية المادة . الصخرة المصدرية بهذه الطريقة ممكن استنتاج المادة العضوية عند . العضوية من خالل المعطيات الرقمية وبالتاكيد انخفاض قرائة مجس , زيادة المجسات كاما والصوتي والنيتروني والمقاومية الصوتي في حساب كمية المادة / ومة القد استخدمت تقنية تراكب مجسي المق. الكثافة و 0643-0033)العضوية وقد اظهرت النتائج ان الجزء العلوي والسفلي من التكوين لقد . هما الصخرة المصدرية الرئيسية في هذا الموقع( على التوالي 0753-0533 للجزئين % وزن 6-1و 4-1مية المادة العضوية من المجسات ما بين تراوحت ك عينة 75ومن ثم قورنت هذه النتائج مع نتائج تحاليل . العلوي والسفلي على التوالي صخرية لنفس التكوين تم تحليلها بجهاز تقييم الصخور المصدرية والتي اظهرت نمطا جائت هذه المقارنة . ية المادة العضويةمقاربا للزيادة والنقصان على حد السواء في كم باالضافة لذلك فقد كشفت هذه الطريقة عن الصخارية . لتعزيز نتائج تقنية المجسات حيث اعطت المجسات بصورة متأصلة توقع , المحتملة للفترة العمقية موضوع الدراسة داللة حول الطبيعة الصخرية للتكوين حيث يعتبر المحتوى العالي من المادة العضوية اظهرت حسابات كمية المادة العضوية ان الحزمة . على بيئات الترسيب الهادئة نسبيا غطاء بينما مثلت الحزمة الوسطية -الصخرية العليا والسفلى تمثل صخور مصدر هذه العالقة قد تدل على وجود مصائد طباقية . صخرة مصدرية ذات خواص مكمنية جيدة .لية كثيرة الحقامحلية تحتاج الى دراسات تفصي