Analyzing Applicability of Lopatin’s Method and Estimation of Total Organic Carbon Content (TOC) through Conventional Log Responses:

A case study of Mumbai Offshore Basin, Western India.

Sunit K Biswas+, Ravinder Ariketi and Bijaya K Behera

Gujarat State Petroleum Corporation Ltd., Gandhinagar, Gujarat.

Abstract. N.V. Lopatin has developed a method for quantifying thermal maturation of kerogen with help of time and temperature as main parameters. Lopatin’s time-temperature index of maturity (TTI), later modified by Waples, helps to delineate the oil generation process and the subsequent cracking of oil to methane, by means of making an oil generative window. Lopatin’s method has been used in the present study to understand the burial history, thermal maturity and hydrocarbon generation potential of the study area. The results of the study carried out on one of the drilled well reveals that the Panna Formation, which acts as one of the major source rock, is not matured enough to generate hydrocarbon. The predictive ability and accuracy of the Lopatin’s method in hydrocarbon generation modeling depends largely on the accuracies in time and available temperature data. Thereby, it advocates the comparison of the estimated TTI values (derived from Lopatin’s method) with the measured Vitrinite Reflectance (Ro) values. In the present study, the results could not be validated with any Ro data due to the absence of the same. Thereby, taking this study one step forward, a quantitative approach has also been made to calculate the Total Organic Carbon content (TOC) and maturity of organic matter based on Resistivity, Sonic and Density logs overlay. The obtained Level of Maturity (LOM) and TOC data also indicates that the source rock is not matured enough to generate hydrocarbon. However, with TOC being as high as 3.3%, it is considered as a substantial potential source rock.

Keywords: Lopatin’s Method, Time-Temperature Index (TTI), Burial History, Logs overlay, TOC.

1. Introduction N. V. Lopatin1 has developed a method of quantification of thermal maturation of kerogen by

incorporating time and temperature as an inherent factors. Lopatin’s time-temperature index of maturity (TTI) later modified by Waples2, helps to delineate the oil generation process and the subsequent cracking of oil to methane. In the present study, a part of Mumbai Offshore Basin, Western India (Fig. 1) has been taken up in an attempt to analyze the applicability of Lopatin’s TTI method in this area; to evaluate the source rock by means of its Level of Maturity (LOM) and to estimate quantitatively the Total Organic Carbon Content (TOC) through conventional log (resistivity, sonic and density) responses.

The Mumbai Offshore Basin is the southern extension of Cambay Basin and encompasses the Gulf of Cambay and the adjoining shelf part of Arabian Sea3. This basin has acquired its primary structural and stratigraphic features from events associated with plate movements that occurred from latest Paleozoic time to the present4. The study area is located on the southern Saurashtra coast near the mouth of Gulf of Cambay in the Arabian sea. The stratigraphy of the study area commences with the Tertiary rocks, deposited over thick flood basalt rocks of Deccan Trap Formation3, 4. The early Tertiaries (Paleocene – Lower Eocene), are named as Panna Formation and comprises of shale, sandstone, siltstone and a section of coal-shale alternation. The thick shale facies within this formation, representing restricted marine to lagoon environment, have been identified as the possible source for the petroleum system in this area4. + Corresponding author. Tel.: + (91 79 66701014); fax: + (91 79 23236375). E-mail address: (sbiswas@gspc.in).

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2011 International Conference on Environment and BioScience IPCBEE vol.21 (2011) © (2011) IACSIT Press, Singapore

Fig. 1: Location map of the study area.

2. Methodology

2.1. Burial History An exploratory well, which penetrated all the geological formations of interest, has been taken up in the

present study. Table 1 shows the age and thickness data used to construct the burial-history curves.

Formations Top Bottom Present Thickness

Age Range (Ma)

Chinchini 0 829 829 0-10 Unconformity 829 829 0 10-10

Tapti 829 1184 355 10-15 Unconformity 1184 1184 0 15-15

Mahim 1184 1479 295 15-25 Unconformity 1479 1479 0 25-25

Daman 1479 1741 262 25-30 Hiatus 1741 1741 0 30-33

Mahuva 1741 1868 127 33-36 Hiatus 1868 1868 0 36-38

Pipavav 1868 2070 202 38-43 Hiatus 2070 2070 0 43-51 Panna 2070 2265 195 51-67

Basement Not Reached - >67 Table 1 – Data used to generate burial-history curves for Well-X.

The average surface temperature has been taken as 200C and the present-day geothermal gradient has been considered as 450C/km. Further, the depositions of the sediments have been considered continuous. The TTI curves have been plotted by using Lopatin’s method4

2.2. Log Evaluation (Δ log R technique) and TOC Calculations Numerous techniques have been developed to identify source rock from well logs5. In this study, LOM

and TOC have been computed by using available basic logs (Resistivity, Density and Sonic). This parameter

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used to calculate the TOC. The algebraic expression for the calculated ∆ log R5 from the sonic/resistivity and density/resistivity overlays is

∆ log R Sonic = log10(R/Rbaseline) + 0.02x(∆t – ∆tbaseline) ∆ log R Density= log10(R/Rbaseline) - 2.50x(ρb-ρbbaseline) Where the Rbaseline is 8 ohm.m, ρbbaseline is 2.5 gm/cc and ∆tbaseline is 85 µsec/ft. and ∆ log R is the

separation measured in logarithmic resistivity scale and is used to calculate the TOC as shown TOC = (∆ log R) x 10 (2.297 – 0.1688 x LOM) LOM has been calculated from the obtained TTI values using figure 2.

Fig. 2: Correlation of TTI with Ro and LOM2, 6.

3. Results The burial history curve along with 100C isotherms and TTI curves for Well-X, is shown in Figure 3.

The highest TTI value attained in this well is 12.53 indicating the corresponding LOM value to be ~8.7 and Vitrinite Reflectance (Ro) value of 0.62. The TOC values obtained through the log overlays (Fig. 4) are given in Table 2, which shows maximum value of 3.3 (the other higher values indicates presence of coal in the section).

Depth Delta T LLD RHOB TOC

2150.0 88.2 10.9 2.4 1.2 2155.0 85.7 17.3 2.6 2.5 2160.0 91.5 7.1 2.4 0.2 2165.0 90.0 13.5 2.4 2.3 2170.0 87.1 12.0 2.5 1.4 2175.0 86.0 9.5 2.6 0.3 2180.0 85.7 9.2 2.6 0.1 2185.0 91.2 9.1 2.5 1.0 2190.0 100.2 11.0 2.5 3.3 2195.0 86.2 9.1 2.5 0.2 2199.0 119.9 24.1 2.1 9.7 2200.0 95.9 8.1 2.5 1.4

Table 2 – Calculations for obtaining TOC values from Log Responses. 49

Fig. 3: Burial History Curves for all horizons, Well – X.

Fig. 4: Graphical representation of TOC calculations through log overlays.

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4. Conclusion A general picture of the burial history, thermal maturity and hydrocarbon generation potential of

petroleum systems in the study area has been developed using the Lopatin’s “time-temperature” maturity index. The results reveal that Panna Formation has attained TTI values lesser than the value for onset of oil generation (Table 3). In other words, in this part of the area, this formation is not matured enough to generate hydrocarbon. Therefore, “Kitchen area” for this source rock may lie further towards the deeper parts of the basin. The predictive ability and accuracy of the Lopatin’s method in hydrocarbon generation modeling depends largely on the accuracies in time and available temperature data. Thereby, it advocates the comparison of the estimated TTI values (derived from Lopatin’s method) with the measured Ro values. In the present study, the results could not be validated with any Ro data due to the absence of the same, however, LOM and Ro was calculated based on the obtained TTI values. LOM values were further used to establish the TOC data. The obtained LOM and TOC data also indicates that the source rock is not matured enough to generate hydrocarbon. However, TOC being as high as 3.3%, it may be noted that it is considered as a substantial potential source rock. This Δ log R technique is very much useful, where the geochemical data are not available. The out comings of this analysis are corroborated by the results of studies carried out in the adjacent and nearby areas3, 7.

Stage TTI Ro TAI

Onset of Oil Generation 15 0.65 2.65 Peak oil generation 75 1.00 2.90 End of oil generation 160 1.30 3.20 Upper TTI limit for occurrence of oil with API gravity <40 ~ 500 1.75 3.60 Upper TTI limit for occurrence of oil with API gravity <50 ~ 1,000 2.0 3.70 Upper TTI limit for occurrence of wet gas ~ 1,500 2.2 3.75 Last known occurrence of dry gas 65,000 4.8 >4.0 Liquid sulfur in Lone star Baden 1 (below dry gas limit) 972,000 >5 >4.0

Table 3 – Correlation of TTI with Important Stages of Oil Generation2.

5. Acknowledgements Authors are thankful to the Managing Director, GSPCL for his kind approval for this technical

presentation.

6. References [1] N. V. Lopatin, 1971, Temperature and geologic time as factors in coalification (in Russian): Akad. Nauk SSSR Izv.

Ser. Geol., no. 3, p. 95-106.

[2] D. W. Waples, 1980, Time and Temperature in Petroleum Formation: Application of Lopatin’s Method to Petroleum Exploration, AAPG Bull. V. 64, pp. 916-926.

[3] D. N. Basu, A. Banerjee, and D.M. Tamhane, 1980, Source areas and migration trends of oil and gas in Bombay Offshore Basin, India: American Association of Petroleum Geologists Bulletin, v. 64, no. 2, p. 209–220.

[4] C. J. Wandrey, 2004, Bombay geologic province Eocene to Miocene composite total petroleum system, India: U.S. Geological Survey Bulletin 2208-F, 20p.

[5] Said A. Aly, Hossam A. M. Ali, Nasser M. H. Abou Ashour and Mohamed M. El-Gezeiry, 2003, Resistivity, Radioactivity and Porosity logs as tools to evaluate the organic content of abu roash “f” and “g” members, north western desert, Egypt: Egyptian Geophysical Society, EGS Journal, vol. 1, No.1, 129-137.

[6] A. Hood, C. C. M. Gutjahr, and R. L. Heacock, 1975: Organic metamorphism and the generation of petroleum. AAPG Bulletin, v. 59, p. 986-996.

[7] M. K. Samanta, 2000, Cambay-Kalol (!) petroleum system in north Cambay Basin, India [abs.]: American Association of Petroleum Geologists Bulletin, v. 84, no. 9, pp 1395.

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