Achraf Bouaziz, Amina Mabrouk El Asmi, Ahmed Skanji, and Khaled El Asmi

Transcription

1 A new borehole temperature adjustment in the Jeffara Basin (southeast Tunisia): Inferred source rock maturation and hydrocarbon generation via one-dimensional modeling Achraf Bouaziz, Amina Mabrouk El Asmi, Ahmed Skanji, and Khaled El Asmi ABSTRACT In Tunisia, most borehole temperatures used to constrain the thermal histories of sedimentary basins were previously corrected using various methods, settobest-fitdomainsother than the Tunisian basins. This study aimed to propose a new method of borehole temperature correction suitable for the Jeffara Basin, southeastern Tunisia. A total of 92 temperature values including bottomhole temperatures (BHTs) and drill stem test (DST) temperatures were collected from 11 boreholes in the area. The correction method consists of plotting BHT versus DST. The derived equation BHT corrected = BHT uncorrected generates corrected temperatures close to formation temperatures. An error temperature curve following the equation y = x was calculated (x = depth in meters and y =temperature error in C). The error varies from +7.4 to +9.4 C ( to F) at depths below 274 m ( 898 ft). A temperature error (ΔT ) is proposed according to depth, and to the time elapsed since mud circulation ceased intervals. To test the method consistency, the Silurian Tannezuft hot shales Formation thermal history is simulated using the BasinMod 1-D software program. Two runs were tested based on the new method: (1) noncorrected BHT and (2) corrected BHT. The first run indicates that the Silurian source rock is at an early stage of the oil window. The second run shows that the Copyright The American Association of Petroleum Geologists. All rights reserved. Manuscript received August 7, 2014; provisional acceptance December 5, 2014; revised manuscript received January 9, 2015; final acceptance March 9, DOI: / AUTHORS Achraf Bouaziz Faculty of Sciences of Tunis, Department of Geology, University of Tunis El Manar, 2092 Tunis, Tunisia; achraf_bouaziz_02@yahoo.fr Achraf Bouaziz received his B.Sc. degree from the Preparatory Engineering Institute of Sfax in 2010, and bachelor of engineering in geosciences from the Faculty of Sciences of Tunis in He is currently pursuing an M.Sc. degree in hydraulic modeling and environment at the National Engineering School of Tunis. Amina Mabrouk El Asmi Faculty of Sciences of Tunis, Department of Geology, University of Tunis El Manar, 2092 Tunis, Tunisia; aminamabrouk@yahoo.co.uk Amina Mabrouk El Asmi graduated as an engineering geologist from El Manar University in She carried out her research at Kingston University, England, and received her Ph.D. in She specializes in geology, geochemistry, and chemostratigraphy. Currently she lectures at the Faculty of Sciences of Tunis. Ahmed Skanji ETAP, 54, Avenue Mohamed V, 1002 Tunis, Tunisia; ahmedskanji@gmail.com Ahmed Skanji holds bachelor s (2005) and master s (2008) degrees in geology from the Faculty of Sciences of Bizerte. After working as an instructor at the Faculty of Sciences of Tunis, he joined Entreprise Tunisienne d Activités Pétrolières (ETAP) in 2010 as an exploration geologist and is currently managing the southern Tunisia region. His projects include field studies and petroleum system analysis. Khaled El Asmi Faculty of Sciences of Bizerte, Department of Geology, University of Carthage, 7021 Zarzouna, Tunisia; kelasmi@yahoo.com Khaled El Asmi received his B.Sc. degree in geology in 1988 and his M.Sc. degree in 1991 from the University of Tunis. He specializes in geology and sedimentology. He currently lectures at the Faculty of Sciences of Bizerte, Tunisia. AAPG Bulletin, v. 99, no. 9 (September 2015), pp

2 ACKNOWLEDGEMENTS The authors greatly acknowledge the Entreprise Tunisienne d Activités Pétrolières (ETAP) for providing necessary data to accomplish the present study. The authors are very grateful to the honorable reviewers Mohammed S. Ameen and Matthias Grobe for their valuable revisions and recommendations. The authors are thankful to the journal editor Michael L. Sweet for his support. In addition, we thank AAPG consulting geologist Frances Plants Whitehurst for her keen input on several technical and editorial issues, which led to considerable further improvement. The authors would also like to extend their thanks to the AAPG Bulletin for publishing this paper. EDITOR S NOTE Color versions of Figures 1, 3, 5 7, 10 13, and can be seen in the online version of this paper. source rock is mature and even at a late mature stage toward the north. Oil expulsion has occurred even at an oil saturation expulsion threshold of 5%. The results explain gas condensate production in the study area. INTRODUCTION Borehole temperatures are commonly used for calculations of heat flow and calibrations of basin models. These temperatures are considered valuable inputs to reconstruct the thermal history of sedimentary basins and to assess the maturity of their source rock intervals. Borehole temperatures are mainly of two types: (1) bottomhole temperatures (BHT) and (2) drill stem test (DST) temperatures. The BHTs are widely available and are measured at different depths in each drilled borehole inside the drilling mud, which is not in temperature equilibrium with the surrounding formations. Equilibrium takes a few days to establish but BHTs are measured a few hours after the mud circulation has ceased, and therefore BHT values do not reflect the real formation temperatures, and discrepancies may reach C (50 59 F) (Deming, 1989). This difference depends on the depth, thermal diffusivity of the drilled rock, and the time elapsed since mud circulation ceased (TSC). The BHTs can be used to draw a geothermal gradient map only if they were readjusted and corrected from the effect of mud circulation. The DST temperatures are not as widely available as BHTs. They are directly measured in the trapped fluids using a maxima thermometer during the test. They better reflect the real temperatures and are considered as the most representative and most reliable to estimate the geothermal gradient. Most borehole temperatures, used to model thermal histories of Tunisian basins, are corrected using different approaches. For instance, Waples and Ramly (2001) and Waples et al. (2004) proposed equations for correcting log-derived BHTs. These latter equations were calculated through applications in the Malay Basin, the Gulf of Florida southeast of Mexico, and the Danish Central Graben in the North Sea. Correction equations calibrated and adjusted to a given basin, considering its local geology and specific geodynamic, can only improve the results and yield better fits (Waples et al., 2004). In addition, the Waples equations are only functional at specific depth intervals and are related to the characteristics of the three aforementioned studied basins. Therefore, we believe that previous reconstructions of thermal histories could have been underestimated because of the application of temperature corrections that were not suitable for the Tunisian basins. This may have resulted in an incorrect 1650 New Bottomhole Temperature Adjustment in the Jeffara Basin, Southeast Tunisia

3 assessment of source rock maturity and hydrocarbon generation and expulsion. In this study, we propose a new method of BHT correction from the effect of mud circulation applied to a Tunisian basin and test its validity using onedimensional (1-D) modeling. Data used are from a proven and productive hydrocarbon province in Tunisia, the Jeffara Basin. GEOLOGICAL SETTING The Jeffara Basin is located in southeastern Tunisia, within the northern part of the Ghadames Basin (Figure 1). It includes two geographical and geological provinces: (1) a western part, which contains the broad Dahar plateau associated with high structures of the Telemzane and Bounemcha; and (2) an eastern Figure 1. Location map of the Jeffara Basin and geological provinces of southern Tunisia (digital elevation model inspired from shuttle radar topography mission data). Inset: Satellite image of the Maghreb region (northwest Africa) showing the location of the study area (Google Earth). BOUAZIZ ET AL. 1651

4 part, which contains the coastal Jeffara plain (Figure 1). The Jeffara Basin was highly deformed during the pan-african orogeny ( Ma) (Craig et al., 2008). This orogeny is interpreted by Bertrand and Caby (1978) to be a result of a continental collision between the rigid West African craton and the East Saharan African craton. The basin structuring started with the compressive Taconic orogeny (Middle Ordovician), which generated significant uplifts followed by erosion, as highlighted by gaps and discordances (Beuf et al., 1971). This orogeny resulted in the creation of the east-west Telemzane arch, located north of the Ghadames Basin and where Upper Triassic sediments (Azizia Formation) overlie Paleozoic systems (Ordovician and Cambrian). This was followed by the Caledonian orogeny, which occurred in the Late Silurian-Early Devonian (Echikh, 1998). Later, the Hercynian orogeny (Late Carboniferous) took place and was characterized by a polyphase compressive structuration with first signs occurring at the top of the Devonian (Bouaziz, 1995). This orogeny, well expressed in the northern part of the Ghadames Basin, caused the reactivation of preexisting structures (the Telemzane arch and the north-northwest south-southeast faults; Bouaziz, 1995) and the deformation of Paleozoic rocks followed by intensive erosion (Echikh, 1998). Within the Ghadames Basin, the Upper Triassic sediments lie, progressively from south to north, on Paleozoic sediments with an angular unconformity. During the Late Permian Triassic, northwest-southeast extension produced major thickness and facies variations on either side of tilted blocks delimited by normal faults (Boudjema, 1987). The basin s configuration was then reactivated through the Early Cretaceous orogenic Austrian tectonic phase (Bouaziz et al., 2002). This was followed by the Late Cretaceous-late Eocene compressive Pyrenean orogeny, leading to the accentuation and remodeling of the Austrian structures (Boudjema, 1987). The Atlasic compressive phase took place during the mid-tortonian and was responsible for reversal reworking and thrusting of deeply rooted major faults as several Hercynian normal faults were locally inverted during this latest orogenic phase (Echikh, 1998). LITHOSTRATIGRAPHY Lithostratigraphic series in the Jeffara Basin range from Paleozoic to Cretaceous (Figure 2). The Paleozoic starts with the Cambrian System ranging from 800 to 1000 m (2624 to 3280 ft) thick and made of quartzites and sandstones and some volcanic rocks (Mejri et al., 2006). Ordovician rocks Figure 2. Stratigraphic column showing the lihostratigraphic series encountered in the Jeffara Basin New Bottomhole Temperature Adjustment in the Jeffara Basin, Southeast Tunisia

5 are sandstone and shale ( m [ ft]). The Silurian rocks are divided into two formations: the Tannezuft Formation (up to 600 m [1968 ft] thick), which is mainly radioactive organic matter-rich black shales, and the Acacus Formation(800m[2624ft]),whichconsistsof alternating shales and very fine-grained sandstones. Four formations represent the Devonian: (1) the Tadrart Formation: sandstone with shaly intercalations; (2) the Ouan Kasa Formation: limestones, dolostone, and sandstone; (3) the Aouinet Ouenine Formation: intercalated marlstone and sandstone; and (4) the Tahara Formation: interbedded sandstones and silty pyritic shale (Mejri et al., 2006). The Carboniferous is 450 to 550 m (1476 to 1804 ft) thick and is fossiliferous sandy limestone and anhydrites. Permian rocks are absent in the study area as a result of a hiatus in sedimentation. The Mesozoic series in the Jeffara Basin start with the Lower Middle Triassic Bir Mastoura, Bir El Jaja, Ouled Chebbi, and Kirchaou Formations, mainly composed of continental sandstone, conglomerates, and clays. The Upper Triassic (Azizia and Adjaj Formations) is composed of evaporitic and finegrained siliciclastic deposits. The Jurassic is characterized by a carbonate sequence that is a few meters thick (the B Horizon, 20 m [65 ft]), a thick Lower Jurassic evaporitic sequence (Abreghs Formation), a mainly marine Middle Jurassic carbonates sequence and an Upper Jurassic mixed facies (Bouaziz et al., 2002). The Cretaceous represents a general gradation from lagoonal and continental facies to more neritic facies (Mejri et al., 2006). It includes the continental Asfer and Sidi Aïch Formations at the base, successively overlain by the marine Orbata, Zebbag, and Aleg Formations (Figure 2). PETROLEUM SYSTEM The main hydrocarbon source rock for Paleozoic and Triassic reservoirs in southern Tunisia is the Silurian Tannezuft Formation (Ghenima, 1995; Acheche et al., 2001; M. Saidi, 2001, personal communication). The formation represents a regional transgressive facies where an anoxic environment prevailed, favoring the accumulation and preservation of organic matter. This unit is composed of radioactive black shales at thebase( hot shales ) with sandstone at the top. Thickness varies from 10 to 70 m (32 to 229 ft) with an average of 30 m (98 ft) (Mejri et al., 2006). These hot shales are easily identifiable on electrical logs because of their high radioactivity and high organic matter content. They are characterized as being excellent source rocks with total organic carbon (TOC) content ranging from 2% to 16% and a hydrogen index (HI) of mg hydrocarbon (HC)/g TOC, the organic matter is type II, and the thermal maturity of this source rock varies from immature to mature (M. Saidi, 2001, personal communication). The Ordovician (Jeffara and Bir Ben Tartar Formations), Silurian (Acacus Formation), and Triassic (Trias argileux gréseux inférieur [TAGI] Formation) sandstones (Figure 2) constitute the primary reservoir targets in the basin. DATA SET AND WELL LOCATIONS Data from 11 wells (Figure 1) located in the Remada and the Dehibat regions (southeast Tunisia) were provided by the Tunisian Company of Petroleum Activities (ETAP). The initial data base included 92 temperature values (76 BHTs and 16 DSTs) measured from well log runs and tests (Table 1). The DST temperatures are available from four wells (W-5, W-6, W-7, and W-9) and range from 46.7 to Table 1. Temperature Data Availability* Wells BHT DST W-1 Available Not available W-2 Available Not available W-3 Available Not available W-4 Available Not available W-5 Available Available W-6 Available Available W-7 Available Available W-8 Available Not available W-9 Available Available W-10 Available Not available W-11 Available Not available *BHT = bottomhole temperatures; DST = drill stem test temperatures. BOUAZIZ ET AL. 1653

6 Table 2. Well Surface Temperatures Calculated with Formula by Barker (2000) and Applying Altitude Correction Wells Latitude (DD) Surface Temperatures without Altitude Correction ( C) Ground Level (m) Altitude Correction ( C) Surface Temperatures ( C) W W W W W W W W W W W C ( to F). Only three wells have two or three consistent DST temperature measurements at different depths, whereas the fourth well has only one DST temperature value. All of the used wells offer at least two BHT measurements at various depths. The BHT values ranges between 42 and 99.5 C (107.6 and F). Surface temperature (ST) data are useful for the calculation of geothermal gradients for each well. Because of the lack of accurate surface temperature data in this study area, we used the Barker s (2000) formula to calculate ST via latitude measurements, ST = L L 2, where L = latitude in degrees and ST = surface temperature in C. In addition to this, an altitude correction of 6.5 C/km (43.7 F/mi) was applied to the calculated temperature value. Table 2 shows the ST values, which range from 16.8 to 19 C (62.24 and 66.2 F). UNCORRECTED REGIONAL GEOTHERMAL GRADIENT For comparative reasons, we chose to estimate a geothermal gradient based on uncorrected BHT data that we termed uncorrected geothermal gradient. The regional geothermal gradients were mapped using the kriging interpolation method. This method is well adapted to data showing irregular trends in their spatial distribution. The uncorrected geothermal gradient varies from 22 to 31 C/km (71.6 to 87.8 F/mi) with an overall increasing southwest-northeast trend (Figure 3). BOTTOMHOLE TEMPERATURE DATA: PROPOSED CORRECTION METHOD As mentioned earlier, the previous conventional datacorrectionusedintunisiaandinthejeffara Basin is that of Waples, using specific equations that are known to be most applicable at a specific depth range only. In this study, we propose the linear cross-plot (X-Y plot) as a correction method, where X is the uncorrected BHT and Y is DST temperature (Table 3). It was already specified that DST temperatures are considered the most representative and most reliable borehole temperatures, therefore the BHT-DST plot will be generating a linear equation of Y = ax + b, which will be used for correction. The cross-plot graph (Figure 4) shows that the measured BHT values between 52 and 80 C (125.6 and 176 F) exhibit a conspicuous deviation from DST measured at the same depth, indicating that their correction from the effect of cooling generated by the mud circulation is required New Bottomhole Temperature Adjustment in the Jeffara Basin, Southeast Tunisia

7 Figure 3. Uncorrected geothermal gradient map of the Jeffara Basin in C/km. Table 3. DST and Uncorrected BHT Values Measured at the Same Depth* Depth (m) DST ( C) Uncorrected BHT ( C) *DST = drill stem test temperatures; BHT = bottomhole temperatures. Thus, all of the uncorrected BHT values are produced in the following equation: BHT corrected = 1.036BHT uncorrected (1) where 5.9 in C [42.62 F] and without unit. These processes involve not only heat exchange between the atmosphere and the subsurface but also heat transfer in response to changes in topography and/or the local to regional hydrological regime. All temperatures (uncorrected BHT, corrected BHT, DST) were plotted against depth generating different linear equations. Figure 5 shows the effect of BHT correction using our BOUAZIZ ET AL. 1655

8 where D is the depth in meters, is in C (98.15 F), and is the slope of the straight line ( C/m). CORRECTED REGIONAL GEOTHERMAL GRADIENT Figure 4. Cross-plot graph drill stem test (DST) temperature versus uncorrected bottomhole temperature (BHT). proposed method, where a deviation is clearly visible between the uncorrected and corrected BHT. At a given depth in the Jeffara Basin, we propose the mathematical relation using the following linear equation: BHT corrected = 0.019D (2) A second geothermal gradient map (Figure 6) was established using the corrected BHT values. The corrected geothermal gradient values range between 25 and 38 C/km (77 and F/mi) from southwest to northeast. The study area as a whole shows an average geothermal gradient of about 31.4 C/km (88.52 F/mi). A high geothermal gradient is shown in the northern part of the Jeffara Basin, where W-1, W-2, and W-5 were drilled. Such increase could be the result of brittle extensional tectonics that affected the Jeffara plain during the Permian-Triassic (Figure 7; Raulin et al., 2011) promoting fluid flow and generating convection process. ERROR ESTIMATION Error estimation on uncorrected BHT and recommended correction value is calculated following the equation: ΔT = BHT corrected BHT uncorrected (3) The error temperature curve (ΔT ) is plotted versus depth (Figure 8) and adjusted according to a polynomial law equation Figure 5. Deviation between the uncorrected bottomhole temperatures (BHTs) and those corrected by the cross-plot method depending on depth. DST = drill stem temperature. y = x (4) where x = depth in meters and y = temperature error in C. The error is found to ranges from +7.4 to +9.4 C ( to F) at depths between 247 and m ( 810 and ft), suggesting that at depths greater than 247 m ( 810 ft), avalue 7.4 C ( F) and should be added to all uncorrected BHT values. From this plot, we tried to generalize the BHT temperature correction in the study area and its surroundings. First, we selected all BHT values with TSC, which were later grouped into TSC intervals. The TSC averages have been selected according to the availability of data which are 5, 8, 13, and 18 hr New Bottomhole Temperature Adjustment in the Jeffara Basin, Southeast Tunisia

9 Figure 6. Corrected geothermal gradient map of the Jeffara basin in C/km. Thereafter, each series of interval graphs ΔT ðtscþ was plotted as a function of depth (Figure 9). These graphs ΔT ðtscþ = f ðdþ were adjusted according to a logarithmic function. The derived equations (Table 4) were used to develop curves and to estimate the temperature error ΔT according to depth and to TSC intervals (Figure 10). For an average of TSC = 5 hr, the error remains constant independently of the depth at which BHT is measured. However, the error varies for higher TSC intervals. To get an approximate value of ΔT correction at a given depth where both uncorrected BHT and TSC are available, it is sufficient to project this latter on the established curve. In this case study, the Horner plot temperature correction method cannot be applied because of the lack of mud circulation time values and the unavailability of at least three BHT measurements at a given depth. ASSESSMENT OF THE PROPOSED BHT CORRECTION VIA ONE-DIMENSIONAL BASIN MODELING We used 1-D basin modeling to assess the impact of BHT correction on the thermal maturation and the petroleum potential of the source rock within the Jeffara Basin. The same 11 wells were selected for 1-D modeling using the BasinMod software of Platte Rivers Associates (Inc.). Input required to constrain the geological model include the lithology, BOUAZIZ ET AL. 1657

10 Figure 7. Crustal section across the Tataouine subbasin showing deep faults affecting Paleozoic basement built from the geological map of Zouari et al. (1987) and modified by Raulin et al. (2011, used with permission of Elsevier). thickness, and age, which were gathered from well logs and final geological well reports. Other input, such as temperature and geochemical data, were included in the model to simulate the maturation history and the timing of hydrocarbon generation. BURIAL HISTORY Figure 11 shows the maturation history and burial curve for the W-6 well (chosen here as an example). The first phase of subsidence extends from the Ordovician to the Carboniferous but is found to be intersected by two compressive phases: (1) the Taconic ( Ma) and (2) the Caledonian ( Ma) (Echikh, 1998). During the Devonian, the overall rate of subsidence remained important but Figure 8. Error curve of corrected temperatures by cross-plot method versus depth. slightly decreased during the Carboniferous (the base of the Ordovician sediments reached a depth of 1700 m [ 5577 ft] around 300 Ma). The sediment burial curve shows a remarkable increase during the Silurian ( Ma) with an average of 15 m/m.y. (49 ft/m.y.) From the Late Silurian, the sediment burial decreased probably because of the Caledonian compressive phase. By the end of the Carboniferous, the sediment burial decreased in response to the Hercynian orogeny that affected the region at 271 Ma (Permian) and induced a significant erosion reaching 1000 m (3280 ft). The effects of this erosion are more important in the north of the study area and diminish toward the south where the Silurian section tends to be more complete. A second phase of subsidence was developed during the Triassic Cretaceous ( Ma) with an average of 16 m/m.y. (52 ft/m.y.) because of extensional movements that affected the region during the Late Triassic Early Jurassic rifting. The Silurian source rock reached a maximum depth of 2800 m ( 9186 ft) by the end of the Cretaceous (68 Ma). A Paleocene-early Eocene hiatus ( Ma) was followed by the Pyrenean compression phase (45 42 Ma), which resulted in a slight erosion of the Upper Cretaceous series ( 9 m/m.y.[ 30 ft/m.y.]). The maximum burial depth was reached, toward the south of Remada (Figure 1), at the end of the Cretaceous just prior to the Alpine uplift New Bottomhole Temperature Adjustment in the Jeffara Basin, Southeast Tunisia

11 Figure 9. Temperature error versus depth for a time elapsed since mud circulation ceased (TSC) average of 5, 8, 13, and 18 hr. PRESENT AND PALAEO-HEAT FLOW Uncorrected BHT, corrected BHT, DST, and thermal maturity data (vitrinite reflectance R o )isameasure of the percentage of incident light reflected from a polished surface of vitrinite and it is used as an indicator of the level of organic maturity. The T max,the temperature at which the maximum release of hydrocarbons from cracking of kerogen occurs during pyrolysis, indicates the stage of maturation of the organic matter. The T max, gathered from well reports, were used to constrain the thermal history of the Jeffara Basin. The present-day heat flow was determined on the basis of our knowledge of the geothermal gradient (using subsurface temperature data) and the thermal conductivity available as the default in the software (using mixing of pure lithology parameters). The modeled wells were calibrated using the transient heat flow model (constant over time). Accurate calibration was obtained by adjusting calculated to measured thermal maturation parameters. To better highlight the importance of BHT correction and its effect on the petroleum potential, two scenarios were tested. The first incorporated uncorrected BHT data and the second was based on corrected BHT data. The impact of correction is Table 4. BHT Correction Equations According to Depth and TSC* TSC Interval (hr) Average of TSC (hr) Equations From 4 to ΔT = ln(d) From 7.4 to ΔT = ln(d) From 11.5 to ΔT = ln(d) More than ΔT = ln(d) *TSC = time elapsed since mud circulation ceased; D = depth in meter; BHT = bottomhole temperatures. BOUAZIZ ET AL. 1659

12 Figure 10. Curves of temperature error estimation ΔT versus depth and time elapsed since mud circulation ceased (TSC). Figure 11. Burial history for W-6 well. Devonian formations (Tadrart, Ouan Kasa, Aouinet Ouenine and Tahara) and Carboniferous formations are absent as they were eroded by the Hercynian orogeny. Upper Jurassic formation and Cretaceous formations are also lacking as they were eroded during the Pyrenean and the Atlasic phases New Bottomhole Temperature Adjustment in the Jeffara Basin, Southeast Tunisia

13 obvious as it systematically results in an increase of the heat flow. Figure 12 represents a map drawn using uncorrected BHT temperatures, where the heat flow values range between 49 and 84 mw m 2. After correction, heat flow values increase considerably and range from53.4to91mw m 2 (Figure 13) with a general south-north increasing trend. In the W-2 well, the heat flow is equal to 78 mw m 2 using uncorrected BHTs and it reaches 91 mw m 2 using corrected BHTs. The same conclusion can be drawn for the W-5 well where the heat flow rises from 63 mw m 2 with uncorrected BHTs to 71 mw m 2 based on corrected BHTs (Figure 14A, B). The top depth of the Lower and Middle Triassic aquifer in Jeffara Basin increases from northeast to southwest (from 865 m [ 2837 ft] in W-5 to 1684 m [ 5524 ft] in W-10) with a low dip toward the south. The aquifer displays a homogeneous lithology, consisting of sandstone with interbedded claystones. In addition, Ben Dhia (1987) pointed to the occurrence of a major southwest-northeast trend of groundwater flow and a southeast-northwest trend of salinity gradient (Figure 15; more than 40 g/l [1.35 oz/gal] around W-1 and W-2). The aquifer is sandier toward the southeast as compared to the northwest. Toward the northwest of the study area, there is a change in facies coupled with the salinity Figure 12. Uncorrected heat flow distribution map at present day of the Jeffara Basin. BOUAZIZ ET AL. 1661

14 Figure 13. Corrected heat flow distribution map at present day of the Jeffara Basin. increase (Ben Dhia, 1987). These observations are in a good agreement with the heat flow trends established based on corrected BHTs (Figure 13). For instance, the high heat flow shown in the northern part of the study area could be responsible for the increase in salinity, which in turn could be related to salt dissolution. TANNEZUFT HOT SHALES MATURATION The kerogen within the Tannezuft hot shales was modeled as type II with specific kinetic parameters measured on selected source rock samples and provided by ETAP. Geochemical properties of the Tannezuft hot shales were assigned considering an initial total organic carbon (TOC) equal to 9% and an initial HI of 450 mg HC/g TOC. The R o and T max data, provided by ETAP, were also incorporated into the model. Two maturity maps of the Tannezuft hot shales source rock were established through 1-D modeling. Using uncorrected heat flow data, the Silurian shales are modeled to be at two different stages of thermal maturity (Figure 16): an immature to early mature stage around W-5, W-6, W-7, and W-8 wells ð0.63% < R o < 0.67%Þ located in the southeast of the Jeffara Basin; and a mid-mature stage 1662 New Bottomhole Temperature Adjustment in the Jeffara Basin, Southeast Tunisia

15 Figure 14. Examples of maturity calibration by BasinMod software performed in the Jeffara Basin. (A) Based on uncorrected bottomhole temperature (BHT) data. (B) Based on cross-plot corrected BHT data. BOUAZIZ ET AL. 1663

16 Figure 15. Salinity map of the Triassic aquifer in southern Tunisia (Ben Dhia, 1987, used with permission of Elsevier). ð0.7% < R o < 0.88%Þ to the north (W-1, W-2, W-3, and W-4 wells) and to the southwest (W-9, W-10, and W-11 wells). Using corrected heat flow data, the immatureearly mature zone has less geographical extent as compared to the previous test. The Silurian hot shales are immature (R o = 0.66%) around W-7 well (Figure 17) and are mid-mature ð0.7% < R o < 0.88%Þ in the northern part around W-1, W-3, W-5, and W-6 wells and in the southern parts around the W-8, W-9, W-10, and W-11 wells. To the north and within the area neighboring W-2 and W-4 wells, the Silurian hot shales are predicted to be within the gas window ð1.15 < R o < 1.16%Þ. This late mature level of thermal maturity was not reached when testing the uncorrected heat flow. Such difference will no doubt affect hydrocarbon generation and expulsion processes. HYDROCARBON GENERATION AND EXPULSION Most wells, modeled in this study, show comparable results, so only a few wells will be used as examples to illustrate the timing of hydrocarbon generation and expulsion. Based on uncorrected heat flow data, hydrocarbon generation from the Silurian hot shales began at 421 and 206 Ma in the W-9 and W-6, respectively, and continues to the present day. In all wells, either gas or liquid hydrocarbons 1664 New Bottomhole Temperature Adjustment in the Jeffara Basin, Southeast Tunisia

17 Figure 16. Uncorrected thermal maturity map of Silurian Tannezuft hot shales in the Jeffara Basin in % R o. have been generated. Cumulative generated quantities range from 1.6 to 35.4 mg HC/g TOC. However, at a saturation expulsion threshold (Satex) of either 5% or 10%, the Tannezuft Formation source rock is found to be incapable of expelling hydrocarbons. The saturation expulsion threshold, expressed in percent of total porous volume of rock, is defined as the quantity of generated hydrocarbon beyond which any produced excess will be expelled from the source rock (Cooles et al., 1986; Mackenzie and Quigley, 1988). Based on corrected heat flow data, the hydrocarbon generation from the Silurian source rock started at 422 Ma in W-9 and 280 Ma in W-6. All wells have produced either oil or gas. Cumulative generated quantities range from 4 to 68 mg HC/g TOC. The source rock started expelling hydrocarbons only at the W-2 and the W-4 wells (Satex = 5% or 10%). In these wells, gas and liquid hydrocarbon expulsion is considered significant with a maximum during the Late Cretaceous for the W-4 well attaining 51 mg/g TOC at a Satex of 5% (Figure 18). These results were mapped to show the spatial distribution of hydrocarbon expulsion in the Jeffara Basin at a Satex of 10% (Figure 19) and where the area around wells W2 and W4 is very pronounced. The thermal and maturation history results of the Silurian source rock in the Jeffara Basin using BOUAZIZ ET AL. 1665

18 Figure 17. Corrected thermal maturity map of Silurian Tannezuft hot shales in the Jeffara Basin in % R o. corrected heat flow via corrected BHTs are different from those obtained by Ferjaoui et al. (2001). For instance, the maturity map generated in the present study (after heat flow correction) shows that the Silurian source rock is late mature in the northern part of the Jeffara Basin (Figure 17). Whereas, it has been reported to be at a mid-mature stage, at the same location based on Ferjaoui et al. s study. In addition, results derived from this study in terms of thermal maturity of the Tannezuft Formation source rock are consistent with the geochemical findings concluded by Rezouga et al. (2012) for the southeastern part of Tunisia showing a possible hydrocarbon migration path from W-2 and W-4 well locations (where the Silurian source rock is found to be in a late mature stage in this study, Figure 17) to the W-6 well locations. This is further supported by the fact that the W-6 well is producing light oil and condensate gas. It is most likely that this well was supplied by gas condensate from the late mature area located in the northern part of the Jeffara Basin, as identified in this study. CONCLUSIONS Cross-plot BHT temperature correction proposed for the Jeffara Basin has been proven to be 1666 New Bottomhole Temperature Adjustment in the Jeffara Basin, Southeast Tunisia

19 Figure 18. Expelled hydrocarbon interval and quantity in mg/g total organic carbon (TOC) in W-4 at a saturation expulsion threshold of 5%. reasonably efficient. Its validity check was performed through the 1-D Jeffara Basin modeling and through comparisons of results using uncorrected temperatures, on one hand and proposed corrected data, on the other hand. Heat flow adjustments on corrected BHTs using the crossplot method indicate that the northern part of the study area has higher heat flow ð91 mw m 2 Þ than that obtained using uncorrected data ð84 mw m 2 Þ, influencing thus the thermal maturity of the Silurian source rock. In the northern part of the study area (W-2 and W-4 wells), the Silurian source rock is in a late mature stage and has reached the gas window. This source rock generated liquid hydrocarbons and is currently producing gas and condensate. At the W-2 and W-4 wells, the Tannezuft (hot shale) have expelled oil and gas since the Late Cretaceous (51 mg/g TOC). The heat flow correction has influenced the maturity of the Silurian source rock, especially through the appearance of an area where the source rock is at a late mature stage, and also through an increase in the amounts of generated hydrocarbons, which are estimated to be five times higher than those obtained based on uncorrected data. In addition, the hydrocarbon expulsion at the W-2 and W-4 wells was significant and could have occurred even at a saturation expulsion threshold of 5%. The maturity results for the Tannezuft hot shales are not in complete agreement with the previously published maturity map of Ferjaoui et al. (2001) but concur with the study by Rezouga et al. (2012). BOUAZIZ ET AL. 1667

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