REVIEW OF LITERATUR.....

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Abstract Both core description and the log detection have evidenced the presence of bitumen inside the Bul Hanine Field (figure 1), which can be particularly abundant in some wells. This tar mat severely impacts reservoir production behaviour because it acts as a permeability reducer and a barrier to flow. Properly understanding its distribution and its propagation throughout the reservoir is then essential for the prediction of reservoir performance under various development plans, for instance when water flooding the field.

The objectives of this study were to: • Characterize the tar mat and understand its formation

mechanism. • Evaluate its occurrence in wells: type, thickness and

distribution, in the various rock types. • Propagate this distribution in a 3D reservoir model for

the entire field. Fulfilling these objectives has allowed more accurate

volumetric estimations, taking the tar mat into account in the dynamic reservoir modelling as well as in planning further development of the field.

Tar mat occurrence was investigated across more than 5400 ft of cores from 26 wells, 90 well logs and a large number of cuttings samples. Two tar mats were identified in the reservoir. The upper tarmat was formed in the crestal area at early stage of the oil charging (early phase segregation?). The second major one was formed at deeper depth.

The tar mat in the Jurassic reservoirs is composed of asphaltenes. Tar mat formation is explained as follows:

• A charging of oil, expelled from the Source Rock, followed by

• Gravity segregation of Asphaltene Precursor Entities (APE) within the oil column on top of permeability barriers and paleo-OWC,

• The precipitation of asphaltenes triggered by a secondary light oil charge.

The methods applied in this study include geochemical characterisation of the bitumen of the Bul Hanine Field, a quantification of the tar content in cores using simple techniques (optical observation, Rock-Eval, Iatroscan, image analysis), and extending this quantification through wireline data in non-cored wells and then, subsequently across the field. In the reservoir model, through the relationship between reservoir quality (rock-type) and bitumen content, the distribution of tar mats can be inferred and traced across the entire field. Introduction

Tar mat occurs in the Bul Hanine Field, particularly in Jurassic reservoir 1. Bitumen occurrence can be a problem due to its effect on oil in place calculation (since bitumen is not movable, it should then be removed from the volumetric calculation) and its impact on reservoir quality. Tar mat impacts on the development plans of an oil field when it behaves as a permeability barrier. Injecting water under the Tar mat might result in inadequate pressure support because of poor communication across the Tar mat 2.

For these reasons, it is important to know where Tarmat occurs in the field (both laterally and vertically), and what controlled its distribution. This information, supplemented by a good knowledge of compartmentalization of the field, could then be used to plan the location and design of peripheral field injectors and ensure optimum sweep efficiencies.

Objectives of This Paper

A series of investigations was carried out with the aim of data collection in order to:

• Identify the presence of solid bitumen in rock samples, • Suggest assumption on its origin to help in predicting

its occurrence in the field, • Give a quantitative estimation, • Describe and model its distribution in the reservoir, The detailed study of the tarmat in Bul Hanine Field was

carried out using standard techniques used in Organic Geochemistry.

Definition of Tar Mat The word Tar mat is used in this paper as a generic term for tarmat and bitumen.

The definition of Tar mat, as used in Organic Geochemistry, is given by Wilhelms et al. 3,4 and Bhullar et al5:

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Characterisation, Origin and Repartition of Tar Mat in the Bul Hanine Field in Qatar N.M. Jedaan, A. Al Abdulmalik, Qatar Petroleum; D. Dessort, Total; V.L.N. de Groen, Beicip-Franlab; C.J. Fraisse, Total; E. Pluchery, Beicip-Franlab.

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"Reservoir zone containing petroleum strongly enriched in asphaltenes relative to the related oil leg petroleum. Tar mats usually have a sharp boundary with the oil leg"

"Tar mats can best be described as compositionally sharply defined zones of petroleum columns often close to geological discontinuities including, but not limited to, oil-water contacts, which are enriched in asphaltenes relative to the oil leg up to concentrations of around 20-60wt.% of the C15+ fraction of petroleum. Although 'tar mat' is the historical term used for these petroleum reservoir zones, a more general term is viscous oil zone and the terms polar enriched and heavy oil tar zone have also been used. Tar mats generally underlie higher gravity petroleums and show distinct compositional contacts with the overlying oil column".

Some authors differentiate Tar mat and Bitumen on an API gravity and/or viscosity basis: Tar mat API degree would be less than 18 and its viscosity higher than 1000 Cp whereas bitumen API degree would be less than 8.5 or 10 and its viscosity above 10000 Cp. However API degree and viscosity measurements cannot be performed in the porous network; they can be performed only on produced fluids. Therefore this definition for Tar mat and bitumen is not applicable to the present study.

Field data

The main producing reservoir of the Bul Hanine Field is the Late Jurassic, which lies at an average depth of 2,332 meters. The thickness varies from 75 meters to 160 meters and contains more than 90% of the field reserves.

At bottom of a Tar mat occurs near the Free Water Level and at other instances inside the oil column and represents a restriction to the permeability from core data. The Tar mat varies in thickness and restriction from one location to the other and shows a dip in a northerly direction. It probably acts as a local restriction to vertical permeability, but laterally is discontinuous in nature. It is most well developed in the south where the reservoir quality is best and becomes more patchy and thinner towards the North.

The fluids in Bul Hanine Field contain generally hydrogen sulphide and carbon dioxide in various quantities.

Samples and Analytical Program The analytical program of the figure 2 was applied on the samples:

• Optical observation and image analysis of thin sections using Jmicrovision® software (figure 3),

• Rock Eval of reservoir rock (quality and quantity of bitumen),

• Quantity and gross composition of organic extract using Iatroscan,

• Detailed analysis of selected extracts using Gas Chromatography / Mass Spectrometry (GCMS).

One oil sample was analysed for the occurrence of the Thiadiamondoids series. This series of Sulfur-bearing compounds is specific of Thermochemical Sulphate Reduction (TSR) which produces acid gas in carbonate reservoirs 6-8.

Optical Study of Thin Sections Reflectant Bitumen or Asphaltenes fills an important part of the porous network (figure 4). Measurement of the reflectance by white reflected light gives a maturity between 0.76% and

0.83% Ro eq., after correction using the Jacob’s formula. This maturity corresponds to the first half of the “oil window”.

The observation shows that a part of the bitumen is not soluble in organic solvents, even after a prolongated extraction. From the optical study it can be concluded that the Tarmat:

• Is not kerogen, • Is not the result of biodegradation of oil because

bitumen deposit formed by the biodegradation are usually completely soluble in organic solvents,

• Doesn’t undergo thermal alteration.

Composition of Oil and Bitumen The gross and detailed compositions of the extracted bitumen and oil sample (figure 5) are key parameters to determine the origin of Tar mat.

There is an excess of distillate C15- in the oil (42% of the whole oil) compared to type II-S oils of the same maturity in the Gulf (typically ~25% of distillate at 0.75 % Ro eq., data from unpublished internal studies, Total). Similarly there is a large excess of asphaltenes in the reservoir extracts (> 50%) compared to the oil (2%). Finally asphaltenes in bitumen are hydrogen-rich, showing that they were not formed by thermal alteration of oil in reservoir (pure pyrobitumen are usually hydrogen-lean).

Oil & Bitumen alteration From GCMS data:

• Oil is not biodegraded or paleobiodegraded, • Oil is not altered by gas or water washing, • Bitumen extracted from the reservoir does not show

any proof of biodegradation, • Molecular and isotope data on oil samples do not show

evidence of secondary cracking in reservoir. Nevertheless oil was altered by TSR as showed by the

occurrence of the Thiadiamondoids series in the oil sample. Thiadiamondoids series is specific of TSR which produces acid gas (H2S and CO2) in carbonate reservoirs

6-8.

Source & maturity of Oil and Bitumen Molecular fingerprinting of heavy biomarkers in the produced oil and organic extracts are similar. They are typical of the type II-S Source Rock.

The maturity of oil samples and bitumen extracts are close to 0.75% Ro eq. (first half of the “oil window”).

Comparison of Rock-Eval VI and Iatroscan data Rock-Eval & Iatroscan analyses were performed on the same crushed sample. These techniques gave similar results in term of bitumen quantity (figure 6). Nevertheless it can be noted that the extraction yield measured by Iatroscan is not perfectly correlated to the Total Organic Carbon (TOC) given by the Rock-Eval. The samples having high TOC values show the lowest extraction yield. We suggest that high TOC values are associated to the occurrence of insoluble bitumen.

After extraction, insoluble Bitumen or Asphaltenes fills an important part of the porous network (figure 7). The quantity of non-extractible bitumen in porosity varies according to a very wide range (from 0% to 100% of the total bitumen).

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Comparison of Rock-Eval / Iatroscan methods and image analysis (JMicrovision®) The TOC (% weight) and the image processing data (%Area) are corrected by taking into account the density of the reservoir rock and the bitumen.

Optical methods give different results than Rock-Eval TOC and Iatroscan, because:

• The Rock-Eval and Iatroscan methods measure the residual oil in porosity. This is not the case with optical methods,

• The Rock-Eval and Iatroscan methods use ~10 grams of crushed rock, giving an averaged quantity of organic matter. These methods are well designed to measure the global bitumen content in rock samples. On the opposite the measurement of the % bitumen by image processing is performed on a size-limited, 2D sample surface: this method is best designed to measure the bitumen content at the pore scale.

Consequently the results obtained by the Rock-Eval or Iatroscan methods cannot give the same results, even on the same sample (figure 8).

Tar mat repartition in porosity according to the rock type 60 core samples from 4 wells were taken in the Tar mat layers for Rock-Eval & Iatroscan analyses. Thin sections were prepared on the same samples and image analysis was performed using JMicrovision ® software.

TOC, quantity and composition of organic extract and the relative quantity of Bitumen obtained by the image processing were compared. These results were tentatively correlated to the rock type characterized for each sample (figure 9).

The main conclusion is: there is no obvious correlation of the bitumen content with the rock type whatever the method used to quantify the bitumen even bitumen is more frequently localized in grainstones.

Vertical Distribution of Tarmat The vertical distribution of the bitumen was studied in detail according to the analytical results.

Selected geochemical logs are showed (figure 10). Arbitrarily, the zones with bitumen were defined when the bitumen occupies more than 40% of porosity. In this figure the following observations can be made:

• The bitumen deposits corresponds to an excess of asphaltenes in the extracts, confirming that the tar mat are mainly precipitation of asphaltenes from the oil,

• The bitumen deposits are localised in layers of high porosity / permeability surmounting zones of lower porosity / permeability. This observation shows the importance of local contrasts of petrophysical properties for the bitumen formation,

• The distribution of the insoluble bitumen does not correlate with the total quantity of bitumen in general. That could mean that the insolubilisation and the asphaltene precipitation are independent phenomena.

Origin of Tarmat & scenario of formation The analytical results show that biodegradation, “in reservoir” maturation, evaporation of light ends from oil, water washing

did not trigger the precipitation of asphaltenes. These results let us to propose the following mechanism of Tarmat formation (figure 11):

• Oil was expelled from the Source Rock and filled the reservoir,

• Gravity segregation of Asphaltene Precursor Entities (APE) within the oil column

• Precipitation of asphaltenes near to the paleo-OWC or above permeability barriers was triggered by a secondary light oil charge filling the reservoir or change in P&T conditions.

• TSR (or vulcanization?) altered the fluid & bitumen, the consequence could be the insolubilisation of the polar compounds and the production of acid gases H2S and CO2.

Cross Ether Reticulation was proposed by Walters et al.9 to explain the occurrence of non soluble bitumen in petroleum fields such as Tengiz (Kazakhstan). Cross Ether Reticulation can be caracterised by a high Oxygen Index. However it cannot explain the occurrence of insoluble bitumen in Bul Hanine Field because the Oxygen Index of this material is very low.

TSR and/or a phenomenon similar to the vulcanization (Cross Linking of Polars by Sulfur and production of H2S) could explain the formation of variable quantity of insoluble bitumen in reservoirs filled with sulphur-rich oils. The vulcanization of asphaltenes is a chemical reaction rather badly known.

Bitumen distribution at the field scale It is important to model the bitumen distribution because:

• Since bitumen is not movable during production, it should then be removed from the volumetric calculation,

• Since bitumen affects permeability, it should be included in the properties of the reservoir model to be taken into account during the dynamic flow modeling.

Logs of bitumen occurrence detected along the wells were loaded into Petrel and then upscaled in the cells of the model. Three different interpretations of bitumen detection were calculated and loaded in Petrel for the volumetric purpose (pessimistic, medium case and optimistic bitumen detection) corresponding to cases that are further used for 1P-3P volumetric calculation. As an example the figure 12 shows the three different upscaled bitumen logs in a selected well.

The bitumen where modeled using the SIS methodology, using well data, vertical proportion curves and variograms. The same variogram parameters were used to simulate bitumen for the median or 2P case and for the 1P and 3P cases.

The figure 13 shows the results of bitumen distribution in a layer and on a cross section. Cumulated height maps where calculated based on the bitumen distribution (figure 14). The model shows that the bitumen distribution is very different across the different intervals.

Conclusion

Tarmat can behave like a permeability barrier. They should be removed from the volumetric calculation.

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The analytical results let us to propose the following mechanism of Tar mat formation in Bul Hanine Field.

• Oil was expelled from the Source Rock and filled the reservoir,

• Asphaltene Precursor Entities (APE) were gravity- segregated within the oil column,

• Precipitation of asphaltenes near to the paleo-OWC or above permeability barriers was triggered by a secondary light oil charge filling the reservoir or change in P&T conditions.

• TSR (or vulcanization) altered the fluid & bitumen, triggering the insolubilisation of the polar compounds and the production of acid gases H2S and CO2.

The Tar mat is localised in layers of high porosity / permeability surmounting zones of lower porosity / permeability (paleo OWC can behave as permeability barrier for asphaltenes). This observation shows the importance of local contrasts of petrophysical properties for the bitumen formation and accumulation.

Cumulated height maps where calculated based on the bitumen distribution. The model shows that the bitumen distribution is very different across the different intervals.

Acknowledgements The authors would like to thank Qatar Petroleum, Total and Beicip-Franlab managements for granting permission to publish this paper. Special thanks are also extended to colleagues for their support. References 1. Munn, D. and Jubralla, A.F.: “Reservoir Geological Modeling of

the Arab D Reservoir in the Bul Hanine Field, Offshore Qatar: Approach and Results”. SPE paper n°15699, 1987.

2. Al-Ajmi, H., Brayshaw, A.C, Barwise, A.G and Gaur, R.S.: “The Minagish Field Tar Mat, Kuwait: Its Formation, Distribution and Impact on Water Flood”. GeoArabia, Vol. 6, No. 1, 2001.

3. Wilhelms, A and Larter, S.R.: “Origin of tar mats in petroleum reservoirs. Part I: introduction and case studies”. Marine and Petroleum Geology, Volume 11, Issue 4, August 1994, Pages 418-441.

4. Wilhelms, A and Larter, S.R.: “Origin of tar mats in petroleum reservoirs. Part II: formation mechanisms for tar mats”. Marine and Petroleum Geology, Volume 11, Issue 4, August 1994, Pages 442-456.

5. Bhullar, A.G., Karlsen, D.A., Lacharpagne, J.-C. and Holm, K.: “Reservoir screening using Iatroscan TLC-FID and identification of palaeo-oil zones, oil–water contacts, tar-mats and residual oil saturations in the Frøy and Rind petroleum accumulations”. Journal of Petroleum Science and Engineering 23 p 41–63, 1999.

6. Charrié-Duhaut, A., Lemoine, S., Adam, P, Connan, J. and Albrecht, P: “Abiotic oxidation of petroleum bitumen under natural conditions”. Organic Geochemistry, Volume 31, Issue 10, October 2000, Pages 977-1003.

7. Dessort, D. Montel, F. and Caillet, G.: “Organic geochemistry of oils and condensates associated to sour gas in Gulf”. GEO2004, Bahrein, march 7-10, 2004.

8. Dessort, D. Caillet, G., Lescanne, M., Insalaco, E. and Montel, M.: “Geochemical characterization and interpretation of Khuff Reservoir Fluids, North Dome”. GEO2006, Bahrein, March 27- 29, 2006.

9. Walters, C.C., Kelemen, S.R., Kwiatek, P.J., Pottorf, R.J., Mankiewicz, P.J., Curry, D.J. and Putney, K.: “Reactive polar precipitation via ether cross-linkage: A new mechanism for solid bitumen formation”. Organic Geochemistry 37, pp 408-427 (2006).

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Figure 1: Location map of the Bul Hanine Field.

Figure 2: Analytical program.

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Figure 3: Bitumen quantification by colour thresholding and extraction (JMicrovision Software).

Figure 4: Optical study and reflectance measurement of bitumen

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Figure 5: gross and detailed compositions of the extracted bitumen and oil sample

Figure 6: Relationship between Rock-Eval TOC and Extractible Organic Matter (EOM%).

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Figure 7: Microscope observation of bitumen after solvent extraction

Figure 8: Example of "patchy" bitumen (A) and dispersed bitumen (B) (in red) obtained by image analysis.

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Figure 9: Attempt to correlate the bitumen occurrence and the rock type.

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Figure 10: Geochemical Logs showing the bitumen layers and the petrophysical properties of the reservoir samples. B = Bitumen; TOC = Total Organic Carbon; EOM = Extractible Organic Matter

B

B

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Figure 11: Proposed mechanism for the Tar Mat formation in Bul Hanine field.

Figure 12: different upscaled bitumen logs in a selected well Figure 13: Bitumen distribution and cross section.

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Figure 14: Cumulated Bitumen height.