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Copyright 2006, Society of Petroleum Engineers This paper was prepared for presentation at the 2006 Abu Dhabi International Petroleum Exhibition and Conference held in Abu Dhabi, U.A.E., 5–8 November 2006. This paper was selected for presentation by an SPE Program Committee following review of information contained in an abstract submitted by the author(s). Contents of the paper, as presented, have not been reviewed by the Society of Petroleum Engineers and are subject to correction by the author(s). The material, as presented, does not necessarily reflect any position of the Society of Petroleum Engineers, its officers, or members. Papers presented at SPE meetings are subject to publication review by Editorial Committees of the Society of Petroleum Engineers. Electronic reproduction, distribution, or storage of any part of this paper for commercial purposes without the written consent of the Society of Petroleum Engineers is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of where and by whom the paper was presented. Write Librarian, SPE, P.O. Box 833836, Richardson, TX 75083-3836, U.S.A., fax 01-972-952-9435.

Abstract The Upper Zubair Formation is a giant reservoir consisting of 350 ft of excellent quality sandstone. This main producing reservoir has a 100 thick tar mat historically encountered near oil water contact. Recently, a well completed in the crestal part of structure above tar mat zone and in good quality sand did not contribute to oil production and presence of tar was suspected. A re-look at identification, origin and distribution of the tar mat has become critical for formulating future development strategy of the reservoir. An appraisal well was drilled on the crest with a comprehensive data acquisition program including coring, fluid sampling and logging. Geochemical analyses of core plugs at every 4 ft were carried out along with fluid invasion studies. Observation from core material was compared with NMR logs to formulate criteria for identification of tar zones to supplement information from open hole and cased hole logs. NMR transverse T2 relaxation histogram was observed to be affected by fluid properties and poresize distribution. There is a consistent shortened T2 distribution along tar mats resulting in missing porosity compared to neutron-density derived porosity. Geochemical analyses of core material indicates high asphaltene zones having undegraded paraffin and are isotopically lighter with immature biomarkers and higher concentration of aromatics. The observations indicate early origin of tarmats due to biodegradation of immature oil at a lower reservoir temperature. Later, undegraded oil with undegraded paraffin profile has migrated in to the reservoir after biodegradation process has ceased. Biodegraded early origin of tarmat can not explain areal distribution over the field. Local compartmentalization of tar

mat from light oil leaking along faults leaving behind heavier fractions during geological history is the possible origin for tar occurring above tar window. Early high Asphaltene content from immature charge, Asphaltene precipitation due to later light oil migrating to trap and gravity segregation explains the occurrence of tarmat in tar window near oil water contact. Early charges/asphaltenes were bio-degraded and incorporated in the tar zone. Current study helps in identifying and mapping the tar and heavy oil zones in the reservoir. Distribution of tar mat of diverse origin needs to be understood well in advance to have realistic estimation of movable hydrocarbon.

Introduction Zubair reservoir is one of the prolific producing horizons in North Kuwait. It is overlain by three other producing reservoirs: Lower Burgan, Upper Burgan and Mauddud. The Upper Zubair sand horizon holds most of the inplace oil of the Zubair reservoir in estuarine channels. It is on continuous production since 1960. Presence of tar was initially detected from produced oil in some of the wells completed in Upper Zubair sand and visual inspection of core materials. Geochemical studies were subsequently undertaken to demarcate occurrence of tar in all the Zubair reservoirs. Tar zone in Upper Zubair Sand was considered to be structurally controlled and occurred in a specific depth window around oil-water contact. Later, presence of a thick zone of tar above the mapped window was detected from a crestal well completed in thick estuarine channels. Another well was drilled in the vicinity and full set of logging and coring followed by geochemical studies were carried out to understand the characteristics of this tar zone. Tar zones are identified with certainty from geochemical study of core material as zones having high asphaltene content and high extract yield. In absence of core, indirect methods: such as PNL, PLT, openhole logs of later drilled wells and production history were used to demarcate the tar zones. The massive sands of Z44CH and Z46CH are the main locales of tar. Smaller channels of other layers are devoid of tar above the historical tarmat.

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Identification, Origin, and Distribution of Tarmats in Upper Zubair Sand Reservoir, Raudhatain Field, North Kuwait Shaikh Abdul Azim, Salah Al-Anzi, and Yahya Hassan, Kuwait Oil Co.; Stephen James, BP; and D. Mandal and Hamad Al-Ajmi, Kuwait Oil Co.

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Distribution of tarmat from the analysis seems more complex than can be explained from pure structure or stratigraphy. Occurrence of historical tarmat has better structural control as it is restricted to a depth interval. Similar tarmat has been seen in Minagish field1. The thick tar zone above the historical tarmat appears to have more geographical control and a combination of biodegradation of early immature oil and vertical migration has played important role on its origin. Geological Setting The Zubair Formation represents major clastic sedimentation during Barremian-Aptian time in North Kuwait. The sedimentation occurred as a low frequency clastic wedge ranging from lowstand, transgressive to high stand systems tract. Basinal mud rocks overlying a ramp carbonate setting of Ratawi Formation forms the base of Zubair Formation. Base of Zubair is a highstand systems tract abruptly overlying basinal Ratawi shales. Drowning of Zubair sedimentation process was initiated during early Aptian resulting in carbonate succession of Shuaiba Formation. The Zubair sedimentation occurred on low angle platform of North Kuwait. Raudhatain and Sabiriyah fields are the two major anticlines (Figure 1) holding hydrocarbons in clastics and carbonates of Cretaceous succession. The Zubair Formation consists of 1350 ft to 1400 ft of clastic section and is a commercial producer in Raudhatain field. The Formation becomes deeper water towards east with a north-south paleo shoreline. Current North Kuwait structures were inverted in the Cretaceous and had been the sites of the Jurassic trough2. Structure The reservoir structure at Zubair level is an asymmetric anticline with four-way dip closure in Raudhatain field. 2D seismic mapped a radial fault pattern over the field. More complex fault pattern is seen from high resolution 3D seismic (Figure 2). Main orientations of faults are NW-SE in the northern part of the field and SW-NE in the southern part. Subordinate E-W trending faults frequently act as major barriers as observed in southern part. Most of the faults can be traced from Mishrief to Ratawi Formations. Minor faults are not mappable from seismic due to weak Zubair reflectors in addition to contamination of signals with multiples from shallower horizons. All faults are normal faults with throw up to 120 ft. Gross thickness of the Zubair Formation does not show much variation over the field. It is vertically divided into six zones separated by significant marine flooding surfaces. The composite reservoirs and non-reservoirs are designated as Upper Zubair Shale (UZSH), Upper Zubair Sand (UZSD), Middle Zubair Shale (MZSH), Middle Zubair Sand (MZSD), Lower Zubair Shale (LZSH) and Lower Zubair Sand (LZSD) (Figure 3). The Shale reservoir units have more shale than sand. Another zone (Z52) was subsequently added as it had sparate fluid contact and presuure. The reservoirs have independent fluid contacts (Figure 4). Within this framework, the complex reservoir sedimentology and structure have given rise to the formation of some 11 independent oil reservoirs3, 4.

Stratigraphy and sedimentology A tidally influenced deltaic system with high frequency change in sea level prevailed during the deposition of Zubair Formation5. The interval has been divided in to six major depositional packages (Z10-Z60) which are correlatable over the field due to areally extensive markers (Figure. 5). These depositional packages have been described in Appendix-A. The packages have been further subdivided into layers: the marine and shoreface units are easy to correlate while considerable uncertainty remains in estuarine units due to frequenting channel cutting and switching. Main producing intervals within the package are Z10, Z20, Z40 and Z60. There are four reservoir lithofacies within Zubair Formation: Cross-stratified Sandstone (Sx), Ripple/Laminated Sandstone (Sr/l), Bioturbated Sandstone (Sbl), and Carbonaceous Sandstone (Sc). These were deposited as estuarine channel fills within a tidal-dominated deltaic environment and as marine shoreface sand bodies6. The non-reservoir lithotypes consist of muddy sandstones, mudrocks, coals and, within the UZSH there are scattered minor limestone stringers, all with negligible porosity and permeability. The reservoir lithofacies have been genetically linked to Estuarine Channel fill Sandstones, Mouth Bar Sandstones, Delta Top Sandstones, Proximal Shoreface Sandstones and Distal Shoreface Sandstones. More detailed description on the genetic units is given in Appendix-B. The Upper Zubair Sand Reservoir It consists of reservoir layers Z36 through Z51 and the most arenaceous unit of Zubair reservoir. Extensive marine influenced estuarine channels are developed in the interval. Some of the channels are densely stacked with cross cutting geometry. Some of the incisions cut 60 ft in to underlying section. There is excellent vertical connectivity within reservoir leading to a single fluid contact. Extensive areal continuity of the reservoir is due to lateral coalescing of channels. Good reservoir continuity coupled with high porosity (>20 5) and permeability (up to 1.5 Darcies) enables the reservoir to be under active aquifer support with less than 600 psi pressure depletion over a production period of 46 years. Tarmats in Zubair Reservoirs Occurrence of tar in Zubair has been identified from production data, log, core and geochemical data. Initially, hydrocarbon staining in cores was used as guide for identifying tar zones. Hydrocarbon staining is expressed in three forms within the permeable sandstones: light, medium and dark. Iatroscan analysis reveals that the both the light and medium stained samples yield asphaltene percentages predominantly below 50% (98% and 92% of the light and medium stained samples, respectively). This sharply contrasts with the darkly stained samples, which overwhelmingly exhibit asphaltene percentages greater than 50% (89% of the dark stained samples), principally >80%. These darkly stained intervals therefore represent tar mats. The wells having Iatroscan analysis in cores of the Zubair interval to dearcate tar zones are shown in Figure 6.

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Tar mats Tar mats are seen in three principal intervals (Figure 7). These lie within the following zones (in descending stratigraphical order): Upper Zubair Sand (Z40): a thick (101ft), well-defined tar mat cuts across the stratigraphy within the main reservoir interval. In the crestal well RA-A, the top is best defined, where it occurs in core within the middle of a sandbody in Z44 at - 9345ft SS. The top can be projected across into the structurally deeper wells RA-B and RA-D, where it is constrained, but lies within a core gap in the stratigraphically higher Z46. The base is best defined in RA-B where it lies at -9446ft SS within Z36. Projected across to RA-D the base lies within a mudrock prone interval within Z44, below this unit the sandstones are unstained. In RA-A, a thin (<10ft) darkly-stained interval lies above a mudrock unit that defines the base of Z46. This may represent the development of a minor mat (related to the presence of the underlying mudrock unit), but, iatroscan analyses do not show the presence of significant asphaltenes at this level. Middle Zubair Sand (Z24): a thinner mat is present in the relatively thick channel sandbody which extends across the central and southern parts of the field. The top is best defined in core in RA-B at -9895ft SS where it occurs within the sandbody. The sandbody in RA-A lies entirely above the mat. In RA-C the top of the mat occurs in the uppermost part of the sandbody within a carbonaceous-rich sandstone package. In core, the main darkly stained interval extends down to -9932ft SS; below this level the core shows an apparently lighter stain with only patchily developed darker staining, and the iatroscan data shows variable asphaltene levels down to the base of the core at -9936ft SS. Lower Zubair Sand (Z10): within this, highly heterogeneous high asphaltene levels are present within many of the sandbodies in the cored intervals in RA-A and RA-C. None appear to be correlatable, but there is only core data for these two wells. In addition, there are thin (<5ft) apparently uncorrelatable high asphaltene levels within the more heterogeneous sections in Z22, Z24, Z26, Z28 and Z32. Tar above Upper Zubair Tarmat After identifying tar zone in Upper Zubair Sand interval of - 9345 ft to -9646 ft, wells were being completed in layers above this window. One of the crestal wells, RA-E (Figure 8) was completed in the thicker channel intervals ranging from - 9080 ft to -9291 ft. Production logging in May 2001 indicated zones in the interval -9175 to -9291 were not contributing to flow. These intervals show excellent log-derived porosity and permeability with very little residual water saturation. The sand bodies were individually tested and confirmed no flow to surface with recovery of heavy oil on reverse circulation. The well did not have any core and a single pressure reading to make any meaningful interpretation. An offset well, RA-F) was drilled with comprehensive data acquisition plan to understand the nature and distribution this heavy oil zone. The

heavy oil-interval was cored; tightly spaced pressure points and six fluid samples were taken. Complete suite of open hole logs including NMR were recorded. Plugs were cut at every 4 ft at well site to carryout geochemical and mud filtrate invasion analysis.

SARA Analysis Soxhlet extraction was carried out on 83 core samples of well RA-F to remove hydrocarbons and tar. The extracts were quantified and analyzed. Forty-eight of the extracted samples contained in excess of 2.0% extractable organic material by weight, with most of the samples being higher than 6.0%. The extracts of each of the forty-eight contained >50% asphaltenes with most above 85%. The relatively large extractable organic content and high asphaltene content verifies that these core samples contain substantial quantities of tar. Four additional samples contained extract with >50% asphaltenes. However, these four samples contained less than 0.35% extractable organics and, therefore, do not contain large quantities of tar. Two samples displayed large proportions of polars in the extract; however, these samples contained less than 0.1% extractable organic material and are not significant. The remaining 29 samples contained from 0.11 to 1.23 extractable organic materials, with most being greater than 0.4%. These samples contain significant quantities of saturates (33-52 %) and aromatic (25-31%) hydrocarbons and do not represent tar- rich samples. Figure 9 shows the percent extract and saturated, aromatic, resin (polar) and asphaltene distribution vs. depth. The tar- rich interval in the core is clearly defined by the high extractable hydrocarbon content and asphalt-rich extract. This section is from 9765-9975 ft with two small tar-free intervals at about 9931ft and 9963ft.

Carbon Isotope Analysis Stable carbon isotope analysis was performed on 82 of the core extracts A plot of the carbon isotopic composition vs. asphaltene content (Figure 10) reveals that the asphalt-rich samples are about 0.5 ‰ less enriched (lighter or more negative) than the samples with lower asphalt contents. As the asphalt fraction is usually more enriched that the other fractions, this is an unusual result.

Detailed Composition of Extracts Detailed analysis was carried out on fifteen samples representing each type of sample (asphalt-rich, resin-rich, etc.) and each interval. Whole oil gas chromatograms show a typical distribution of normal paraffins and low pristane/n-C17 ratios (0.21-0.32). The paraffin distributions and pristane/n- C17 ratios are consistent with undegraded oils. The whole oil chromatograms contain a large peak between C23 and C24 and a group of peaks at about C26. These components, which do not appear in the saturated or aromatic chromatograms, are from the resin (polar) fraction (Figure 11). The components may represent contamination from drilling fluid or other sources. The saturated and aromatic fractions of the fifteen samples were analyzed by gas chromatography. The saturated fractions display a typical distribution of normal paraffins and low pristane/n-C17 ratios that are consistent with undegraded

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oil. The aromatics display a significant increase in apparent concentration in the tar-rich intervals as indicated by an increase in peak amplitude in the tar-rich intervals. Biomarker GCMS analysis of the saturated fractions revealed a high degree of similarity among the selected samples. The samples display low Ts/Tm ratios and significant quantities of homomoretanes (Figure 12), which are consistent with lower maturity or early generation oil. Identification of Tarmats In addition to geochemical analysis of cores, tars have been identified from other indirect methods: Resistivity logs Presence of immobile tar prevents any mudfiltrate invasion in to the formation around the well bore. Thus, formation resistivities from different depths of investigation do not show any change and all the curves lay on each other, while there is clear separation of curves in light hydrocarbon bearing intervals. Tar zones in RA-E show very little resistivity separations in the tar zone from 9405 ft to 9600 ft (Figure 13). Micro-resistivity log having depth of investigation few millimeters reads resistivity of invaded zone close the borehole. It usually reads lower value than the deeper resistivity tools in oil zones drilled with a mud of lower salinity filtrate than formation water. Due to lack of invasion in tar zone, the micro-resistivity log would have same or higher resistivity than deeper resistivity logs. The water saturation of invaded and uninvaded zones would overlie over the tar zone. The criterion is useful to identify tar in many wells but is ambiguous in zones with low porosity with residual oil saturation which has similar response. Resistivity profiles may be used to detect tar zones with ease where tar is the main component but the method fails where porous intervals are partially saturated with tar, oil and water. The tar window in Upper Zubair Sand have microresistivity reading similar to shallow resistivity but lower than deep resistivity and has considerably higher water saturation in it.

Self Potential (SP) The SP sonde measures the potential developed due to salinity contrast in permeable beds. Given ample salinity difference between mud filtrate and formation water, SP is a measure of permeability. When formation water is more saline, as in the case of North Kuwait reservoirs, more negative SP is observed in more permeable reservoirs. Presence of tar reduces permeability of a reservoir consequently affecting SP character. SP gives higher readings in tar zones of RA-F (-100 mv Z44CH and Z46CH) compared to light oil zones with similar petrophysical characteristics (-145 mv in Z48CH: Figure 13). SP has been qualitatively used to demarcate tar zones in current study. Limitation of usage of SP is in low permeable zones where it would show similar character of tarmat as immobile light oil.

Nuclear Magnetic Resonance (NMR) NMR technology is found to be a useful supplement to means available for tar detection. It has been used in the Zubair well

RA-F to characterize fluid in zones completely filled with tar (above OWC) and in zones partially filled with tar (below OWC). The main output of NMR logging is Transverse relaxation distribution (T2) is sensitive to several geological and petrophysical parameters and thus provides understanding on pore texture along with saturating fluid. Surface relaxation, Bulk fluid relaxation and molecular diffusion mechanisms control fluid relaxation in rock pore space. The total Decay Rate is given by:

1/T2=1/T2s+1/T2b+1/T2d Where 1/T2 is total transverse relaxation time, 1/T2s is surface relaxation, 1/T2b is the bulk fluid relaxation and 1/T2d is the diffusion relaxation. Useful information can be obtained on tar and heavy oil from understanding the bulk fluid relaxation. Tar bulk relaxation corresponds to very short T2s and at ambient temperature, typical tar falls outside measurement range of NMR and doesn’t contribute to porosity signal. As observed in RA-F, the T2 distribution has a lower value in tar zone than light oil zone (Figure 14). Also, the porosity from conventional Neutron-Density measurement is compared with NMR porosity and the deficit can be used as the indicator of tar (Figure 15). NMR is promising in identifying tarzones and would be used in future wells. The technology is proved to be elsewhere in Middle Eastern Carbonates9. Residual Oil Saturation (Sor) Tarmats have been mapped from high residual oil saturation/unswept oil zones in wells drilled during later part of field life (after 1980) when substantial water movement has occurred (Figure 16). Log derived oil saturation is unusualy high in tar zone of RA-E and RA-F above the historical tarmat window. Production Logging (PLT) PLTs are routinely carried out for surveillance activity. Lack of contribution or low contribution from channel sands have been used to infer presence of heavy oil. PLT interpretation is ambiguous in poorer quality rocks. Layer correlation was found to be useful supplement. Detection of heavier oil and tarmat is difficult to be distinguished from PLT alone.

Origin of Tarmats Origin of tar in Upper Zubair Sand appears to be a combination of processes over geologic time. Early hydrocarbon filling and Bio-degradation Tar mats are commonly associated with oils derived from low siliciclastic source rocks. The initial charge from a carbonate source rock has high asphaltene content, partly as a result of its organic matter composition, and partly because of the low temperatures at which such source rocks expel most of their petroleum potential. At low maturity (onset of petroleum expulsion), oils typically have high asphaltene contents. The burial history of Zubair reservoir and generation and expulsion history of the source (Figure 17) indicate that expulsion and migration of hydrocarbons into the reservoir began about 70MYBP. At that time, the reservoir temperature was about 40-50°C, a temperature favorable for biodegradation of the trapped oil. About 35MYBP, the

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reservoir temperature of the reservoir reached about 70°C, a temperature above which biodegradation of oil becomes insignificant. Thus, hydrocarbons entering the trap after about 35MYBP would be undegraded. The unusual isotopic data, where the high-asphalt tars are isotopically lighter than the low asphalt extracts is consistent with the accumulated asphalt being from earlier generation from the source rock, when isotopically lighter products are generated. Multi-phase hydrocarbon filling Generation and migration of hydrocarbons have continued over geologic time. As the source rock gets more deeply buried in a basin, there is ever increasing thermal stress of and as result the composition of fluid expelled from source rocks becomes increasingly lighter with higher GORs. Asphaltene solubility in oil reaches a minimum as lighter oil or gas is mixed into an existing oil column and it starts to precipitate. Such asphaltenes in tar mat have formed at a later stages of oil/gas filling, As the migration pathway into a reservoir is commonly along the higher permeability zones, the tar occurs in zones of better quality rock with the best permeability and higher porosity as seen in thicker channel sands of Upper Zubair. High-asphalt content in conjunction with an undegraded paraffin profile, as observed in geochemical data, is characteristic of heavy oils or tars where undegraded oil has entered the reservoir at a later date. Gravitational segregation Gravitational segregation of fluids takes place in reservoirs when there is a variation of components which differ in gravity, such as asphaltenes in a lighter oil. In such instances, lighter (low asphaltene) oil accumulates near the top of the oil column, whereas denser (high asphaltene) material settles at the base of an accumulation, at the oil-water contact. Once the asphaltene content of oil becomes higher than its solubility, asphaltene precipitation occurs, and asphaltene drops out of solution forming a tar layer. Frequent occurrences of tar mats above permeability baffles in Uper Zubair Sand, often within a relatively thick oil column, indicate that the tarmats have most likely formed by the gravitational settling of asphaltenes. Hydrocarbon migration along leaking faults The equivalent thick tar observed in wells RA-E and RAF-F were mapped from PNL and dynamic data. Distribution of the tar is restricted around few more wells near the crest of the structure. Occurrence of such thick tar in form a plug filling right to the top of major channels can not be explained from gravity segregation. Continuous leakage of hydrocarbon along faults leading to drop in pressure which accelerates asphaltene drop out could be the possible reason. Occurrence of a thick heavy oil zone in a single well of Lower Burgan reservoir is another example where leakage along a fault could be invoked for its origin. Distribution of Tarmats Geochemical, logging and production data have been used to map the distribution of tarmat in the field for Upper Zubair Sand Reservoir. As discussed above, two distinct type tar zones have been mapped: depth related tar zone and thick tar zone above it.

Depth related tar zone is observed to be restricted to depth window -9345 to -9446 ft, SS. Bottom of tar mat is observed as deep as -9505 ft if the channel is vertically continuous over the tar window. The occurrence is also restricted to high permeable layers of Upper Zubair Sand. Low porosity thin sands lying in the tar window do not contain tarmat. The depth control on presence of tarmat is imposed by precipitation of asphaltene at the base of oil column. This tar zone show higher water saturation than the overlying hydrocarbon zone mainly because most part of the tar zone lies below the mapped oil water contact (at -9375 ft, SS). Average water saturation in tar zone is 55-65% in most of the wells. Only 13 wells have water saturation in the range of 30 to 50% and they lie closer to crest. In recently drilled wells, the water saturation in the water swept zones above tarmat (which were filled with light oil earlier) is in the range of 65 to 75%. The observation would suggest that the depth related tarmat had a lot of mobile hydrocarbon in it. Further support to the theory is from the fact that the reservoir is under active water drive: getting full pressure support from the aquifer. In some of the wells, there is a 10-20ft thick zone immediately below oil water contact showing low residual oil saturation and this oil has remain unmoved over time as seen from time lapsed PNL logs. The depth related tar zones continue to show high oil saturation with time. The tar zone observed above depth related tar zones of RA-E and RA-F has been mapped in the surrounding wells mainly from PNL and dynamic information. A recent well drilled 600m west of RA-F show a water-swept interval at the level of tar occurrence in the latter well. The oil water contact is 120 ft above that seen in RA-A. Well lying in between these two show water fingering within the zone of immobile oil. Some of the nearby wells do not show any change on fluid content in time lapsed PNL logs till date while others show water encroachment. This criteria of immovable oil combined with open hole log signatures was used to map this tar zone (Figure 18). This tar zone typically shows very low water saturation with mostly immobile oil. As observed in geochemical data of RA-F, there are thin zones of mobile oil trapped within the thick tar channels. Production logs have been planned to be recorded for confirmation of tar plugs. Testing results would also be helpful in further delineation of tarzone. Conclusions The Upper Zubair Sand reservoir has a 100 ft thick tar mat near the oil water contact restricted to high permeable clean sand zones. The tar mat has asphaltene content in excess of 80%. In most of the wells the zone contained movable hydrocarbon: high water saturation is observed in swept tar zones and in tar zones occurring below oil water contact. Early asphaltene from immature charge, bio degradation at low trap temperature, asphaltene precipitation from subsequent more mature gaseous charges and gravity

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segregation are the possible reasons for formation of the tarmat. Occurrence tar zone above historical tar is confirmed from geochemical studies. However, this tar is restricted to few crestal wells. Leakage of hydrocarbon along faults is expected to be the reason for formation this tar. Tar zones have been characterized and mapped over the field. Lateral extent of tar above historical tarmat needs to be confirmed with more dynamic data. NMR is promising in identifying tar zones and would be used in future wells. Acknowledgements We are grateful to management of Kuwait Oil Company and Ministry of Oil, state of Kuwait for permission to publish this work. We appreciate the contribution of many KOC and BP colleagues during the progress of the modeling work. References 1. Al-Ajmi, Hamad, Gaur R.S., Brayshaw A.C., “The Minagish

Field Tarmat: Formation, Distribution and Impact on Waterflood”, presented at GEO1998 International Conference and Exhibition, Bahrain, 1998.

2. Yousif, S and Nouman G.: “Jurassic Geology of Kuwait,” Geo Arabia, Vol. 2, No. 1, 1997

3. Nemcsok, S., Morrison, N. H., Carruthers, A., Abdullah, Sh., “Sedimentary Interpretation of a Multilayered Clastic Oil Reservoir: Impact on Development plans for the Zubair Reservoir, Raudhatain Field,” Paper SPE 48972 presented at the 1998 SPE Annual Technical Conference and Exhibition, New Orleans, Sept. 23-26.

4. Al-Dashti H, Nemcsok S., Morrison N., Al-Matar B., “The

Development Process; Zubair Reservoir, Raudhatain Field,” Paper SPE 53171 Presented at the 1999 SPE Middle East Oil Show & Conference, Baharain, Feb. 20-23.

5. Adamson, K., Coy, G, Cross N, Imelda, G.J., Whear, E., “Revised Reservoir Geology of the Zubair, Burgan and Mauddud Formations of the Raudhatain and Sabiriyah fields, North Kuwait”, November 2000

6. Davis Roger, Payne Dorothy and Taylor Katy.: “Reservoir Geology of the Zubair Formation in Raudhatain and Sabiriyah fields, North Kuwait”, Unpublished KOC-BP Report, Dec. 1997.

7. Brennan, P., 1990, Raudhatain Field - Kuwait. Arabian Basin. In Atlas of Oil and Gas Fields, Structural Traps.

8. Burger, Jon, Elrod, L., Gupta, D., “Geochemical Analysis of Tar

mat in Raudhatain Field, Upper Zubair Formation, North Kuwait”, Unpublished KOC Report, March, 2004.

9. Najia, W etal, “Nuclear Magnetic Resonance (NMR), a Valuable

tool for Tar Detection in a carbonate Formation of Abudhabi, UAE, Paper SPE78485, presented at 2002 SPE Abudhabi International Petroleum Conference and Exhibition

Appendix-A Reservoir Zonation scheme in Zubair Z10. A thin basal package composed of stacked fluvially- dominated mouthbars interpreted as resulting within a phase of falling base level. This is largely made up of thin generally high-quality sandbodies (8-20 ft thick) which can be traced with confidence from well-to-well on a km-scale. Vertical amalgamation of sandbodies is seen in core and lateral amalgamation is suggested by correlations. The interbedded mudrock packages (typically <10 ft) are likely to form local scale (<1-2km) vertical and potentially lateral transmissibility barriers or baffles. The top of Z10 is marked by a field wide flooding surface which forms an intrafield seal. Z20. It comprises of a number of high frequency regressive- transgressive parasequence sets that occur during a background (low frequency) relative sea-level cycle bounded by flooding surfaces. Z20 is complex package which contains three main episodes of channel incision. Channel incisions occur during higher frequency sea level falls as the background sea-level conditions pass from transgressive to regressive. Incisions are the maximum during zones Z24 and Z26 where sequence boundaries form relatively deep incised valleys (up to 80 ft). These estuarine channel-fills can be mapped at different levels across the field. The top of Z20 is marked by a major flooding surface which forms a key intrafield seal. This package is defined as lying between the Z22FS and Z32FS, and is divisible into four principal layers (Z22-Z28, in ascending stratigraphical order). Z30. This zone comprises a large-scale, coarsening-upward package (typically 200’ thick) which is largely mudrock-prone but it does contain good-quality reservoir sands. On the western flank of the field, the overlying channelised interval Z40 is considered to incise into and replace the upper part of the interval. Z30 comprises of several high frequency relative sea-level fluctuations that occur during a low frequency HST. The higher frequency relative sea-level fluctuations result in the progradational and retrogradational stacking of parasequence sets, as well as forming Z36 flooding surface. The zone is dominated by marine mudrocks and subordinate sandstones. Z40. Forms the main reservoir interval of the Zubair Formation. It comprises vertically-stacked and laterally- coalesced estuarine channel-fills. Towards the top of the section individual channel packages become discrete and are bound by flooding surfaces. As one moves up the section there is a progressive reduction in the number and thickness of channel-fills and an increase in the volume of mudrocks. Mapping indicates that Z40 is more deeply incised in to Z30 on the western flanks of the field. Z50. Comprises a predominantly mudrock-prone section which forms a fieldwide seal. The section passes upwards into a composite shoreface sandstone package towards the top. Z60. Comprises a mixed marine mudrock/carbonate package which contains a discrete episode of estuarine channel incision and fill.

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Appendix-B Genetic depositional units for Reservoir Facies Estuarine Channel-Fill Sandstones Estuarine channel-fills represent the most important reservoir facies in the Zubair Formation. The are sharply-based, fining upward units with vertical thickness of 10 to 70 ft. Cross bedded sandstones dominate the unit with clean, fine grained, well sorted sands; rippled and carbonaceous sandstones locally dominate. Coals, rooted horizons and abundant carbonaceous material occur frequently. Major channels can be traced on Km scale. The limits of the channel systems are easily mapped across the field in areas of dense well control. Generally they show an east to west trend. The distribution of the channel-fill trends within the field are expected to continue beyond honoring the characteristics seen in areas of dense well control. Mouth Bar Sandstones They comprise relatively sharply-based (non-erosional) sandbodies (8-20 ft thick) which form the upper parts of larger-scale (<30 ft) coarsening upward units, coarsening up rapidly from mudrocks and wavy laminated heterolithics which may show synaeresis cracks. The sandbodies comprise flat to low-angle cross-stratified, fine-grained sandstones. Locally mud-draped cross-sets are present. Bioturbation is frequently observed. The presence of thin coals (<.5ft) with associated rooted horizons suggests the thicker units are composite. These deposits are interpreted as tidally-influenced mouthbars, the relatively sharp transition and dominance of low angle stratification reflecting relatively unconfined channel mouth deposition. Three flow units (Z10, Z06, and Z04) in the Lower Zubair are interpreted to have been deposited as mouth bar sandstones. The resulting gross isopachs of the flow units are mapped to be relatively isopachous. Reservoir quality was determined to be decreasing rapidly from the west to the east. Delta Top Sandstones These are sharply-based, fining-upward (<10 ft) packages of fine-grained sandstones, displaying cross-stratification or lamination. The upper parts are commonly rooted and pass upwards in to coals (typically <2 ft thick). Carbonaceous material and amber grains are common. These packages comprise highly heterogeneous deposits made up of small scale sandbodies and interbedded mudrocks and coals. Only the Z26 flow unit is interpreted to be mainly composed of delta top sandstones. The depositional model predicts thinning and poorer reservoir quality should occur to the east. However, this unit is heavily incised by the overlying Z26CH channel system that made it very difficult to correlate and map effectively.

Proximal Shoreface Sandstones and Distal Shoreface Sandstones These are sharply-based, fine-grained and bio-turbated sandstones from 10-30 ft thick. The depositional model for the flow units comprised of predominantly proximal and distal shoreface sandstones indicates the resulting gross isopachs of these units should be relatively consistent. This consistent behavior is shown in all of the proximal and distal shoreface flow units that are not incised by an overlying channel system. Strong sand development trends paralleling depositional strike exist in these environments. Marine mudrocks These comprise dark grey to black, typically non-calcareous mudstones to muddy siltstones (Mb/l) which may contain thin (<1 ft) argillaceous bioturbated sandstones. Bioturbation is characterised by a mm-scale textural mottling. A relict flat lamination is locally apparent. Thin bioclast-rich lags are present and poorly preserved ammonites have been recovered. Marine mudrocks form fieldwide to subfield scale vertical transmissibility barriers. The thicker packages form intra- Zubair seals. Limestones Bioclast-rich wackestones and packstones, locally rich in Orbitolids are present in the lower and uppermost part of the Zubair Formation (Z10 and in Z64). They represent a shut- down in the clastic system forming condensed horizons possibly associated with flooding events.

8 SPE 101303

Figure 1 Location map of North Kuwait and Geological setting of Zubair Formation. Figure 2 Structure on top Zubair and fault pattern mapped from 3D seismic. Main orientations of faults are NW-SE in the northern part of the field and SW-NE in the southern part. Subordinate E-W trending faults frequently act as major barriers as observed in southern part.

IRA IRA

SAUD ARABI

WAFR

BAHRA

Ruge

Mutrib

ABDAL

Kuwait

MEDIN

0 K 4

MINAGIS

GREATE BURGA

UM GUDAI

SABIRIYA

RAUDHATAI

RUMAIL

Kr Mar

North

West

South KuwaiDHARI

ABDULIYA

KHASHMA

Neutra Zon

bn 1 Heavy

) ( ..

) () (

RATQ Quarternary Holocene Surface

Pleistocene Dibdibba Tertiary Pliocene Lower Fars

Miocene Ghar Oligocene Dammam

Eocene Rus Paleocene Radhuma

Cretaceous Maastrichian Tayarat Quarna

Campanian Harta Sadi

Santon Khasib Coniac

Turonian Cenomanian Mishrif

Upper Rumaila Ahmadi

Lower Albian Wara Mauddud Burgan

Aptian Shuaiba Zubair

Barremian Hauterivian Valangian

Ratawi sh & ls Beriassian Minagish

Makhul Jurassic Tithonian Hith

Gothnia Kimmerian Nahma

Upper Oxfordian Callovian Bajocian Sargelu

Bathonian Middle Aalen Dharuma

Toarcian Marrat Pliensbachium

Sinemurium Lower Hettangium

Triassic Rhaetian Minjur Norian Carnian Ladinian Jilh Anisian Sudair

Scythian

Lower

Hiatus

Zubair

Quarternary Holocene Surface Pleistocene Dibdibba

Tertiary Pliocene Lower Fars Miocene Ghar

Oligocene Dammam Eocene Rus

Paleocene Radhuma Cretaceous Maastrichian Tayarat

Quarna Campanian Harta

Sadi Santon Khasib Coniac

Turonian Cenomanian Mishrif

Upper Rumaila Ahmadi

Lower Albian Wara Mauddud Burgan

Aptian Shuaiba Zubair

Barremian Hauterivian Valangian

Ratawi sh & ls Beriassian Minagish

Makhul Jurassic Tithonian Hith

Gothnia Kimmerian Nahma

Upper Oxfordian Callovian Bajocian Sargelu

Bathonian Middle Aalen Dharuma

Toarcian Marrat Pliensbachium

Sinemurium Lower Hettangium

Triassic Rhaetian Minjur Norian Carnian Ladinian Jilh Anisian Sudair

Scythian

Lower

Hiatus

Zubair

SPE 101303 9

Figure 3 Type log of Zubair Formation showing main subdivisions Upper, Middle and Lower (Sand and Shale).

Figure 4 Cross section showing main subdivisions and the fluid distribution in Zubair Formation. The reservoirs within Zubair have separate oil water contacts. Upper Zubair Sand is the main producing reservoir with OOWC at -9375 ft TVDSS.

1,400 Feet

UPPER

MIDDLE

LOWER

“ SAND ”

“ SHALE ”

GR +

“ SHALE ”

“ SAND ”

“ SHALE ”

“ SAND ”

1,400 Feet

UPPER

MIDDLE

LOWER

“ SAND ”

“ SHALE ”

GR +

“ SHALE ”

“ SAND ”

“ SHALE ”

“ SAND ”

10 SPE 101303

Figure 5 New Zonation scheme based on sequence staratigraphy: Major subdivisions are correlated with old scheme.

Figure.6 Location map of Zubair: Iatroscan measurements were carried out in wells RA-A, B, C and D showing presence of well-defined tar mats in Upper Zubair and Middle Zubair Sands. Geo-chemical analysis in Well RA-F was used to understand heavy oil/tar observed during production logging and testng of the well RA-E.

Raudhatain Field

OLD NEW General CLASSIFICATION ZONATION Lithology

Group Subgroup Layer Zone Subzone SHUAIBA CARBONATES

UPPER SHALE Z60 Z64 UPPER Offshore SHALE 1 Z62 SAND Shoreface

Z56

Z50 UPPER Marine SHALE Mudrock Z54

UPPER Z52

ZUBAIR 2 Z48

Z40

UPPER 3 Z46 SAND Stacked

4 Estuarine Z44 Channel

5 Fills Z42

MIDDLE 6 Z30 Z36 SHALE Shoreface -

Offshore Z32 MIDDLE 7

ZUBAIR MIDDLE Z28

SAND 8 Z20 Z26 Delta Top

9 Shoreface - Estuarine Fill Offshore Z24

LOWER 10 Cycles SHALE

LOWER Z22

ZUBAIR LOWER 11 Z10 SAND Fluvial Z10

Mouthbars RATAWI SHALE

Significant Erosional Contact

Max. Flood

Raudhatain Field

OLD NEW General CLASSIFICATION ZONATION Lithology

Group Subgroup Layer Zone Subzone SHUAIBA CARBONATES

UPPER SHALE Z60 Z64 UPPER Offshore SHALE 1 Z62 SAND Shoreface

Z56

Z50 UPPER Marine SHALE Mudrock Z54

UPPER Z52

ZUBAIR 2 Z48

Z40

UPPER 3 Z46 SAND Stacked

4 Estuarine Z44 Channel

5 Fills

Raudhatain Field

OLD NEW General CLASSIFICATION ZONATION Lithology

Group Subgroup Layer Zone Subzone SHUAIBA CARBONATES

UPPER SHALE Z60 Z64 UPPER Offshore SHALE 1 Z62 SAND Shoreface

Z56

Z50 UPPER Marine SHALE Mudrock Z54

UPPER Z52

ZUBAIR 2 Z48

Z40

UPPER 3 Z46 SAND Stacked

4 Estuarine Z44 Channel

5 Fills Z42

MIDDLE 6 Z30 Z36 SHALE Shoreface -

Offshore Z32 MIDDLE 7

ZUBAIR MIDDLE Z28

SAND 8 Z20 Z26 Delta Top

9 Shoreface - Estuarine Fill Offshore Z24

LOWER 10 Cycles SHALE

LOWER Z22

ZUBAIR LOWER 11 Z10 SAND Fluvial Z10

Mouthbars RATAWI SHALE

Significant Erosional Contact

Max. Flood

SPE 101303 11

Figure 7 Tarmat in Zubair from Iatroscan Analysis of four wells. Dark staining in core correlates with high Ashphaltene content. Two major tarmats are apparent: Upper Zubair Sand (101 ft) and Middle Zubair Sand (37 ft). Tars are randomly distributed in Lower Zubair Sand.

Figure 8 Production logging in this crestal well RA-E indicated no contribution from zone marked by immobile zone. Subsequent zonal testing re-confirmed presence of heavy oil/tar.

Figure 9 SARA Stacked and Extract vs. Depth for the well RA-F. The tar zone (shown in black) is identified from high extract yield and asphaltene but low saturates, aromatics and polars. The study confirmed extention tarmat seen at well RA- E.

Figure 10 Asphaltene content vs. Carbon Number Isotope value. The asphalt-rich samples are about 0.5 ‰ less enriched (lighter or more negative) than the samples with lower asphalt contents.

ORIGINAL TAR

IMMOBILE ZONE

GR + RE S

ORIGINAL TAR

IMMOBILE ZONEIMMOBILE ZONE

GR + RE S

12 SPE 101303

Figure 11 Polar fraction Chromatography showing possible contamination Peaks.

Figure 12 Biomarker distribution showing presence of Homomoretanes. The samples have low Ts/Tm ratios and significant quantities of homomoretanes: consistent with lower maturity or early generation oil.

SPE 101303 13

Figure 13 Resistivity and SP logs give indication of tar: Clear separation among resistivities at -9100 ft sand is light oil. Lesser separation at 9300 ft and below 9250 sand indicates Tar. SP value at light oil is -145 mv compared to 100 mv at tar interval.

Figure 14 NMR T2 distribution Light oil and Tar zones indicating loss of porosity in Tar zones.

Light Oil

Tar

Light Oil

Tar

Z 4

8 C

H Z

4 8

Z 4

6 C

9150

9200

9250

9300

9350

9400

9750

9800

9850

Light Oil

Tar

Z 4

8 C

H Z

4 8

Z 4

6 C

9150

9200

9250

9300

9350

9400

9750

9800

9850

Light Oil

Tar

14 SPE 101303

Figure 15 Comparison of porosity from NMR with conventional Neutron-Density measurement NMR indicates deficit in Tar zone and can be used as the indicator of tar.

Figure 16 Tar zone has high residual oil saturation and doesn’t move in time lapse PNL: Such high Sor zones can be used as the indicator of tar.

Tar

Mobile Oil

Tar

Mobile Oil

TarTar

SPE 101303 15

Figure 17 Burial History and Trap Temperature by Barwise, LGC Labs, 1998

16 SPE 101303

A. Map View

Figure 18 Distribution of Tar mat in Tar window (-9345 to -9446 ft) Upper and Tar plug above Tar mat as observed in RA-E and RA- F: Map View (Above) and Cross Sectional View (Below).

Outer limit of Main Tarmat in Tar Wndow

Limit of Tarplug Above Tar window

Outer limit of Main Tarmat in Tar Wndow

Limit of Tarplug Above Tar window

WELL: RA-F

OWC

Tar zone in RA-F Wells producing oil from these intervals and no indication of Tar

Oil leaking along faults

Possible Tar Zone

Light oil

Tar Window

WELL: RA-F

OWC

Tar zone in RA-F Wells producing oil from these intervals and no indication of Tar

Oil leaking along faults

Possible Tar Zone

Light oil

Tar Window