Trend of Geothermal gradient from Bottom Hole Temperature studies in South Cambay Basin (Narmada Broach Block)

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1 10 th Biennial International Conference & Exposition P 386 Trend of Geothermal gradient from Bottom Hole Temperature studies in South Cambay Basin (Narmada Broach Block) Summary Sonam*, B.S.Dhannawa & Varun Kumar An endeavour has been made to ascertain the geothermal gradient and its trend in the South Cambay Basin. Knowledge of geothermal gradient is required for many oilfield applications viz. evaluating well logs, designing cementing programs, basin modeling for discerning source rock, to name a few. Bottom Hole Temperature (BHT) data from well logs of about 70 wells were collected, corrected for equilibrium temperature and interpreted to understand the trend of geothermal gradient. Regional geothermal gradient of the study area varies in the range from 27 C/km to 67 C/km and averages about 39 C/km. The subsurface gradient map shows that the gradients are relatively lower towards the depocentre and increases towards its eastern margin. An inverse relationship between the geothermal gradient and depth of basement (Deccan Trap) is observed i.e. geothermal gradient is more in areas where basement is at shallow level & vice-versa. Keywords: Bottom Hole Temperature, Geothermal Gradient, South Cambay Basin Introduction The recent thrust on energy has necessitated pooling of all the available G&G tools to analyze and understand the data available that can help in the future discovery of much needed oil and gas. It is widely accepted & well known that knowledge of subsurface temperature has a great bearing & is an important tool, input & determinant in oil exploration. It has a significant role in hydrocarbon generation and migration of hydrocarbons and associated pore fluids. Knowledge of geothermal gradient is required for many oilfield applications. Some of these applications include evaluating open and cased hole logs, designing cementing programs, basin modeling for discerning source rock, modeling steady and unsteady state of fluid and heat flows in the wellbore for designing thermal recovery projects, etc. Keeping this in view an endeavour has been made in the present work to ascertain the geothermal gradient of the South Cambay Basin. The geothermal gradient is the rate of change of temperature with depth in the earth. The temperature of the earth normally increases with depth, and as a result when a well is drilled it shows an increase in temperature with depth. The geothermal gradient usually holds a linear relationship with depth, but this holds good only in a completely homogeneous medium. The geothermal gradient is greatly affected by rock type and it s thermal conductivity; i.e. a rock with high thermal conductivity will show a low thermal gradient. Therefore the real temperature gradient in a well is not a straight line, but a series of gradient related to thermal conductivities of various strata. A good no. of sample points has been taken across the basin to understand the trend of geothermal gradient laterally and vertically. Interesting observations have been made and the regional trends are brought out. General Geological Setting of Study Area Cambay Basin is a tertiary intracratonic rift basin, lying between western and northwestern margin of Indian shield covering an area of about sq. km which is 450 km long and km wide in dimension (Sarraf et al. 2000). Upper Cretaceous-Lower Paleocene, tholeiitic continental flood basalts represented by Deccan Trap forms the technical basement over which more than 7-11 Km of tertiary & quaternary sediments have been deposited. These lava flows were the result of extensive volcanic activity, associated with the northward drift of the Indian plate near the end of the Mesozoic. They tend to thicken towards the centre of the Cambay Basin, possibly reflecting the onset of rifting (Yalcin et al., 1987). In the * Shed no. 26, Forward Base, ONGC, Ankleshwar. sonamgupta18@gmail.com

basinal part, the continental sediments corresponding to Olpad Formation are deposited over Trap.

The alternating pulses of regression and transgression of sea in the Basin have deposited the Cambay, Kanwa and Tarkeshwar argillaceous units separated by Hazad, Dadhar and Babaguru arenaceous units.

The Cambay basin is divided into four tectonic blocks based on recognizable basement fault system (Raju et al. 1983) and the study area falls in the Jambusar-Broach Block i.

2 basinal part, the continental sediments corresponding to Olpad Formation are deposited over Trap. The area initiated by the marine incursion from the south during the early Eocene and remained as a narrow elongated Basin till date i.e. Gulf of Khambhat. The alternating pulses of regression and transgression of sea in the Basin have deposited the Cambay, Kanwa and Tarkeshwar argillaceous units separated by Hazad, Dadhar and Babaguru arenaceous units. Cambay shale is the main source rock in the area, Hazad sands are the reservoir and Kanwa and Telwa and intervening shales within Hazad are the cap rock in the area. The Cambay basin is divided into four tectonic blocks based on recognizable basement fault system (Raju et al. 1983) and the study area falls in the Jambusar-Broach Block i.e in southern part of the NNWtrending Cambay Basin which is bounded by the transverse fault zone of Mahi in the north and Narmada river in the south (Mathuria T. K. et al. 2011) Figure 1: Location map of study area (Raju et al. 1993) Data correction for equilibrium temperature: Measured subsurface formation temperature from open hole logs is always lower than the true or static formation temperature. During the drilling of oil wells, a large quantity of mud is circulated in the borehole to facilitate the drilling, evacuate the cuttings and stabilize the hole. The influence of this circulation and other drilling effects like thermal properties of the drilling fluid, nature of heat exchange between borehole fluid and the formation, duration of drilling; provides a non-equilibrium temperature at the time of temperature measurements. Numerous methods have been adopted in the past to correct the logged bottomhole temperature & estimate real formation temperatures. These methods include those of Hyodo et al (1994), Middleton M. F. (1982), Dowdle W.L. and Cobb W. M. (1975), etc. The most common method and the one used here is the Horner plot proposed by Dowdle W.L. and Cobb W. M. (1975). The Horner method, plots the measured temperature (at a given depth) from each of several logging runs, against log(t/(t+t)). The parameter t represents the length of time that the borehole was subjected to the cooling effects of the fluid, and T represents the time after circulation that the borehole has had to partially reheat. The best fitted straight line results from the plot is extrapolated to cut the temperature axis where logt/(t+t) equals to zero, reflects the true formation temperature at that particular depth. We have assumed mean surface temperature to be 25 C. Geothermal gradient determination: Assuming that there is a linear relationship between temperature and depth, the equation of the straight line can be expressed by a regression formula based on type of available data as (Nwankwo et al. 2009): Methodology Data Collection: The efforts for determining geothermal gradients began with compilation of information of exploratory wells drilled in the study area. Bottom hole temperature used for this analysis were taken from log headers collected during well logging. More than 100 exploratory & development wells in Narmada Broach block of South Cambay Basin were taken, evaluated and analyzed for the study. However, fewer than 70 exploratory wells were found to have suitable technical conditions (depth range, time elapsed between cessation of mud circulation and temperature measurement, sufficient number of log run, etc.) which could potentially help in eventual geothermal measurements. The geothermal gradient (G) is determined from the given formula: Where; x= temperature, y = depths in metres Results & Discussion Computed geothermal gradient in different boreholes along with well details (such as depth drilled, maximum borehole temperature recorded, etc.) is presented in Table- 1. These wells are widely distributed in South Cambay Basin and extend from shallow eastern margin to major depocentre towards west. The geothermal gradient of the 2

3 study area shows wide variation. It ranges from 27 C/km Figure 2: Geothermal gradient of some representative wells 3

Temperature data in trap section indicates higher gradients and can be seen in Figure 2 (BH-26, BH-44, BH-47, BH-67, etc.).

4 to 67 C/km with an average of about 39 C/km. Trend of temperature variation with depth in respect of selected wells is presented in figure 2. It is very obvious from the trend that geothermal gradient is not uniform through the entire borehole. Temperature data in trap section indicates higher gradients and can be seen in Figure 2 (BH-26, BH-44, BH-47, BH-67, etc.). It is observed that the rate of change of temperature with depth in sedimentary section is lower as compared to the basement igneous rocks. Some wells are projected to E-W cross section line and their corresponding geothermal gradient is plotted as shown in Figure 3. This figure depicts that the gradient profile follow the same trend as the depth to basement. There is a general increasing trend of geothermal gradient as we move from western to eastern margin of the basin. The geothermal gradient is more in areas where basement is at shallow level & vice-versa. Figure 4 shows the location of the wells used in the study and their corresponding geothermal gradient contour map. The contour map shows that the gradients are relatively lower at the depocentre of the basin compared to the eastern marginal part where geothermal gradients are high. Low gradient of 27 C/Km is observed in Dahej & North Harinagar fields which lie more or less in the western and central part in Broach depression while higher gradient of 67 C/km in Karjan, Karvan and Padra fields which lies in the eastern margin of the basin. The gradients of Padra, Karjan and Karvan field in general are on the higher side as Deccan trap has been encountered at shallow depth. These differences in geothermal gradients may reflect changes in thermal conductivity of rocks, groundwater movement and endothermic reaction during diagenesis. The higher gradient values observed in the marginal part of the basin may have resulted in high heat flow due to tectonic activities in the basin. Table 1: Well Details and their calculated geothermal gradient 4

permission to publish the paper. Authors are grateful to Shri S. K. Das, ED-Basin Manager, WON Basin and Dr. M. C.

5 shows that the gradients are relatively lower towards the depocentre and increases towards its eastern margin. Acknowledgement Figure 3: Cross section showing geothermal gradient from western to eastern part of the Basin The authors express their thanks to ONGC for providing infrastructure and giving permission to publish the paper. Authors are grateful to Shri S. K. Das, ED-Basin Manager, WON Basin and Dr. M. C. Kandpal, GM, Block Manager- I for facilitating publication of this paper and for their encouragement and support. Authors acknowledge the helps rendered by their seniors and colleagues for the technical suggestions and helps during their work. References Biswas S. K., Rangaragu M. K., Thomas J., Bhattacharya, S. K., (1994), Cambay-Hazad (!) petroleum system in the South Cambay Basin, India, in Magoon, L.B., and Dow, W.G., eds., The petroleum system from source to trap: AAPG Memoir, no. 60, p Chen Z., Osadetz K. G., Issler D. R., Grasby S. E., (2008), Hydrocarbon migration detected by regional temperature field variation, Beaufort-Mackenzie Basin, Canada; AAPG Bulletin Vol. 92 pp Davis R. W., (2012), Deriving geothermal parameters from bottom-hole temperatures in Wyoming; AAPG Bulletin Vol. 96, pp Dowdle W. L., Cobb W. M., (1975), Static Formation Temperature from well logs- an empirical method; J. Petrol. Tech., 27, pp Figure 4: Contour map showing geothermal gradient in the study area Conclusions Regional geothermal gradient vary clearly from well to well and with depths. It ranges from 27 C/km to 67 C/km. It is observed that geothermal gradient depends on thickness of sedimentary column and thus depth to basement. A generalized correlation is established between the geothermal gradient and depth of Deccan trap. An inverse relationship between the geothermal gradient and depth of basement (Deccan Trap) is observed i.e. geothermal gradient is more in areas where basement is at shallow level & vice-versa. The subsurface gradient map Gomes A. J. L., Hazma V. M., (2005), Geothermal Gradient and Heat Flow in the state of Rio De Janerio; RBGf 23 (4), pp Grisafi T. W., Rieke H. H., (1973), Approximation of Geothermal Gradients in Northern West Virginia using Bottom Hole Temperatures from Electric Logs; AAPG Bulletin. Hyodo M., Takai K., Takasugi S., (1994), Evaluation of curve fitting method for estimating the formation temperature from logging data; preceedings of the 90 th SEGJ Conference, Jam L. P., Dickey P.A., Tryggvason E., (1969), Subsurface Temperature in South Louisiana; AAPG Bulletin Vol. 50, pp

6 Kundu J., Wani M. R., (1992), Structural Styles and Tectono-Stratigraphic Framework of Cambay Rift Basin, Western India; Indian Journal of Petroleum Geology, 1 (2), pp Kutasov I. M., Eppelbaum L. V., (2010), A new method for determining the formation temperature from bottom hole temperature logs; Journal of Petroleum and GasEngineering Vol. 1(1), pp Yalcin M. N., Welte D. H., Misra K. N., Mandal S. K., Balan K. C. Mehrotra K. L., Lohar B. L., Kumar S. P., Misra G. S., (1987), 3-D computer aided basin modelling of Cambay Basin, India - a case history of hydrocarbon generation, in Kumar R., Dwivedi P., Banerjie P. V., Gupta V., eds., Petroleum geochemistry and exploration in the Afro-Asian region: Balkema, Rotterdam, p Mathuria T. K., Julka A. C., Dimri P. K., Pandey P. B., (2011), Search and Discovery Article # 10326, Middleton M. F. (1982), Bottom-hole temperature stabilization with continued circulation of drilling mud, Geophysics, 47, pp Mukharjee M. K., (1981), Evolution of Ankleshwar Anticline, Cambay Basin, India; AAPG Bulletin, Geologic Notes. Nwankwo C.N., Ekine A. S., (2009), Geothermal gradients in the Chad Basin, Nigeria, from bottom hole temperature logs; International Lournal of Physical Sciences Vol. 4(12), pp Nwachukwu S. O., (1976), Approximate Geothermal Gradients in Niger Delta Sedimentary Basin; AAPG Bulletin Vol. 60, pp Raju A.T.R., Srinivasan S., (1983), More Hydrocarbon from Well Explored Cambay Basin, Petroleum Asia Journal, pp Raju A.T.R., and Srinivasan S., (1993), Cambay Basin petroleum habitat, in Biswas S.K., Alok D., Garg, P. Pandey J., Maithani A., and Thomas, N.J., eds., Proceedings of the Second Seminar on Petroliferous Basins of India, v. 2: Indian Petroleum Publishers, Dehra Dun, pp Sarraf S.C., Ray D. S., Kararia A. D., Lal N. K., (2000), Geology, sedimentation and petroleum system of Cambay Basin; Petroleum Geochemistry and Exploration in Afro Asian Region, pp

Determination of Geothermal Gradient in the Eastern Niger Delta Sedimentary Basin from Bottom Hole Temperatures

Journal of Earth Sciences and Geotechnical Engineering, vol. 4, no. 3, 2014, 109-114 ISSN: 1792-9040 (print), 1792-9660 (online) Scienpress Ltd, 2014 Determination of Geothermal Gradient in the Eastern