Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Transformation of Tethys T he origins of oilfields in the Turkey to Oman mountain belt started long before the Gondwana supercontinent began to break up 50 million years ago. To capture a true understanding of how these oilfields were created we must go back 250 M years, to the break-up of Pangea - the earth’s sole land mass at that time. Pangea was splitting apart, forming Laurasia in the north and Gondwana in the south. In the late Permian and Triassic, rifting occurred in Iran, Turkey and Oman which eventually led to the formation of very small fragments of crust, microplates, and the creation of two seaways, the northern Palaeo-Tethys and the southern Neo-Tethys (figure 2.2). These early rifts produced ideal depositional sites for carbonates, clastic reservoirs and sometimes organic-rich sediments in intracratonic basins, marginal basins, shelf and platform margins. The last remnants of these Tethys seaways had virtually disappeared when subsequent crustal movements led to the ocean areas being over-ridden by continental crust. In addition, intense deformation occurred in collision or crush zones where tectonic forces thrust the plates together. It is extremely difficult to reconstruct how Laurasia was interlocked with Gondwana some 250 M years ago. In southern Laurasia, a number of small crustal plates joined to form a complex region running from northern Turkey along northern Iran to Afghanistan, including the Moslem states of the former Soviet Union. Piecing these together is a headache and the problem is compounded by subduction of crust and the additional intense deformation which created the main mountain belts from the Black Sea to the Indian Ocean. Fortunately, the deformation in these frontal edges of the mountain belts has been less extreme. However, there is a major incentive to understand what has happened along this folded belt as some of the world’s richest oil and gas provinces lie under the foothills. The micro-plates of Turkey, northern Iran and the Moslem states of the former Soviet Union lay along the northern edge of the Tethys 12 Europe North America Tethys Sea South America Africa Arabia India Fig. 2.1: THE LOST OCEAN: The vast Tethys Sea was formed when the giant Pangea supercontinent broke up 250 million years ago, forming Gondwana to the south and Laurasia in the north. The Black Sea and Caspian Sea are the only remnants of this sea. Sea. And hydrocarbon reservoirs in these areas have similarities with those along other margins of the Tethys Sea where compressional subsidence occurred. The oil and gas in the Caucasian-Dag and Zagros-Bitlis-Oman regions are found in gently folded rocks which formed on both the northern and southern sides of the Tethys. The southern mountain belt (Zagros), which resulted from the closing of the Neo-Tethys, extends from south east Turkey through north east Syria, northern Iraq and southern Iran and reaches as far as the eastern UAE and northern Oman. Oil and gas is mainly of Cretaceous age in the northern (Turkey and Syria) and southern (UAE and Oman) ends of the Arabian/Iranian foreland folded zone. In contrast, both Cretaceous and younger reservoirs are found in Iraq and Iran. The northern Caucasian-Dag mountain belt extends from the Black Sea to Afghanistan and includes the states of Georgia, Armenia and Azerbaijan to the west and Turkmenistan, Uzbekistan and Tadzhikistan to the east. This fold belt was also created when the PalaeoTethys Sea closed. To the east of the Caspian Sea, in the Dag fold belt, the reservoirs are primarily gas prone. The great oilfields which made Russia pre-eminent as an oil producer in the last century were found in the foothills of the Caucasus Mountains, generally on the northern flank. The southern states of the former Soviet Union can be considered geologically as part of the Middle East. There are over 180 gas and gas condensate fields, including six giants with reserves more than 3 tcf. Close to the north east Iranian border (east of Serakhs, Iran) is the giant Dauletabad-Donmez (Sovetabad) Field which could be Turkmenistan’s largest gas field with 38 tcf of initial reserves. Here, the main reservoirs are Jurassic carbonates making them similar in age and lithology to the major reservoirs found on the Arabian platform. As with the Arab reservoirs, these are sealed by overlying Jurassic evaporites. However, some deltaic sandstone reservoirs are also present in the Jurassic. Older and more deeply buried Triassic and Middle Jurassic gas-prone, organic-rich rocks are the likely source rocks in this region. Middle East Well Evaluation Review Arabia Africa Pangea Iran Shallow seas Crust Mantle Africa Gondwana Permian Eurasia Iran Arabia Neo-Tethys Sea Palaeo-Tethys Sea Crust Marine deposits Clastics Triassic Ocean crust Mantle Rifting Foreland folding African Plate Arabian Plate Iranian micro-plates Zagros Crush Mts zone Eurasia Arabian Plate The Gulf Caspian Sea Red Sea TurkmeniaKazakhstan Ocean crust Today Fig. 2.2: (Top): The giant supercontinent of Pangea began to split up during the Permian. By the Triassic, Gondwana was separated from Laurasia by the Tethys seaways (centre). The subsequent collision of the Arabian and Asian crustal plates destroyed, or eliminated most of, the Tethys Sea and produced a highly deformed mountain belt from Turkey to Oman. Indian sub-plate Bitlis suture id yr n m rsio l Panve i Levant fault Za gro s an df old ing Fo rel Makran ns tio c s os Red Sea ne Li of se Oman mountains cr an bi e at Pl Number 13, 1992. Eurasia a Ar The Asmari carbonates which were deposited in the Zagros basinal trough are the last open marine deposits of the Neo-Tethys seaway - a body of water that extended from the Indian Ocean across the Middle East into southern Europe. The larger foraminifera found in the Asmari carbonates indicate to paleontologists that the Tethys became divided about 4.5 M years ago. After this, organisms in the Indian Ocean and the Mediterranean Sea were isolated from one another and evolved separately. The disconnection of the Mediterranean resulted in drastically reduced diversity and character in the marine faunas. Later, the Mediterranean Sea became connected to the Atlantic Ocean which resulted in the separation of the African and European land masses. However, the connection of Eurasia and Africa through the Middle East allowed African elephants, bovids, pigs and eventually homids to migrate to Europe (See Middle East Well Evaluation Review, Number 11, 1991). With a lowering of world sea level (from 40 m to 70 m) during the Miocene, these last seaways which covered the new Eurasian continent became Fig. 2.3: FROM TURKEY TO OMAN: Oil and gas reservoirs are found along the length of the intensely deformed crush zone which runs from Turkey, through Iran and the UAE into Oman. The most prolific fields are found along the edge of Iran's Zagros Mountains. Owen fz The end of the Tethys Gulf of Aden restricted and isolated, and eventually dessicated after the deposition of vast evaporite deposits. In Iran, these Miocene evaporites serve as an effective seal for oil and gas generated in more deeply buried Cretaceous rocks. The oil and gas migrates upwards through faults and interconnected fracture systems into the prolific Miocene Asmari reservoirs. 13 Ophiolites outcrops in Oman S Hawasina Ocean Oman margin Continental crust S Semail Ophiolite Hawasina sediments Oman margin Continental crust Volcanic belt Mantle N Oc Water an e i c cr u st Volcanic belt O c e nic c a r t us Many of the oilfields along the Oman fold belt produce from fractured Cretaceous carbonates - from the Bukha Field in the north to Natih Field in the south 1. The growth of the Natih structural high was triggered by movements of the deeply buried Eo-Cambrian Salt. Compressional tectonics subsequently affected the region in the Late Cretaceous (figure 2.4). The thrusting or obduction of the ocean floor slab and ophiolites onto North Oman’s continental margin occurred during this period (figure 2.5). During Tertiary times, further compressive movements generated localised structures such as the Salakh Arch jebels. Production from the Natih Formation is almost entirely fracture-related and Petroleum Devlopment Oman (PDO) is developing the field using Gas-Oil Gravity Drainage (GOGD). During the next 10 years, the fracture oil rim (ie the reservoir interval with oil-filled fractures) will be lowered by 70 m using continuous gas injection at the field crest, and additional down-dip water production. The gas rapidly invades the fracture system at the top of the field, completely surrounding the oil-filled, less permeable matrix blocks (Middle East Well Evaluation Review, Number 12, 1992). During the drainage process, gravity causes gas to be drawn (or imbibed) from the fractures into the oil-filled matrix. The matrix oil displaced by this gas moves into the fracture system, where it is partially reabsorbed into adjacent matrix blocks. Eventually it drains down to contribute to the fracture oil rim, where it is produced. By lowering the fracture oil rim, the amount of STOIIP exposed to GOGD is greatly increased, resulting in improved oil recovery - an approach which has been successfully applied in the nearby Fahud Field 2. Hawasina sediments st Oce anic cru Oce ani cc rus t N Fahud/Natih area S Hamrat Duru Range Mantle Jebel Akhdar Semail Ophiolite N Water Continental crust Volcanic belt Oceanic crust Mantle Fig. 2.4: SUFFERING FROM COMPRESSION: During the Late Cretaceous, ocean floor slab and ophiolites (see also figure 2.5 below) were thrust onto North Oman's continental margin. During Tertiary times, further compressive movements generated localised structures such as the Salakh Arch jebels. (From Oman's Geological Heritage, by PDO). References: 1 Mercadier, C.G.L. and Makel, G.H., 1991. Fracture patterns of Natih Formation Outcrops and their implications for Reservoir Modelling of the Natih Field, North Oman. Proc. 7th SPE Middle East Oil Show Tech. Conf. & Exhib., 16-19 November 1991, Manama, Bahrain, SPE Paper 21377, p. 357-368). 2 O’Neill, N., 1987. Fahud Field Review: a switch from Water to Gas Injection. Proc. 5th SPE Middle East Oil Tech. Conf. & Exhibit., 7-10 March 1987, Manama, Bahrain, SPE Paper 15691, p. 51-66. 14 CYAN MAGENTA YELLOW BLACK Fig. 2.5: AUTHORITIVE EVIDENCE: The end of the Tethys Sea occurred when crustal movements led to the ocean areas being over-ridden by continental crust. Various deposits were left in these crush zones, including these ophiolites which are remnants of the earth's mantle. Here, author Roy Nurmi examines a very fractured ophiolite overlying the top of the Mesozoic (Jurassic) carbonates in Oman. Middle East Well Evaluation Review Zendan 3 0 00 Bat Oma inah n 2000 tem fault sys 4000 M Prousand mo a m nto ry Zagros fold belt Mak ran Accr et prisiomnary Gulf of Oman pas sive M mar gin Saih ain Hatat s Jebel Qusaybah Jebel Salakh nt ou 500 100 0 Fig. 2.6: LANDSAT photograph of North Oman showing the main structures formed by the compressive plate movements and the main jebels (mountains) in the region. Number 13, 1992. 15 FRACTIOUS RELATIONSHIPS Glass 0.001m Crack Fig. 2.7: The geometry of overlapping cracks is similar for different materials, independent of scale (Pollard and Aydin, 1984). Granitic crust rock 0.1m Quartz - feldspar vein Mancos shale 100m Minette dike Ocean basalt Ridge 1000m Fig. 2.8: Log-log plot of normalized length frequency in the Gulf of Suez. (Heffer and Bevan, 1990, SPE Paper No 20981). Increasing frequency If recent studies are to be believed there is much more order to reservoir faulting and fracturing than people first realised. The wealth of structural information gained through 3-D seismic surveys and borehole electrical imagery is helping scientists piece together what seem to be fractal relationships between faults and fractures. Fractal relationships can be seen in many features, independently of scale. For example, if you look at a rocky coastline from the air, it will appear to have a jagged edge. Even if you land and take a walk along the sea shore, it will still have a jagged edge. And, if you put the sea shore under a magnifying glass, what do you see? - you’ve guessed it - a jagged edge. Some of the first indications of fractal relationships in faults and fractures were discovered by comparing laboratory-scale rock cracking experiments with studies of earthquake characteristics. More recently a group in England has found a relationship between fault length and vertical displacement which varies with the material properties of the rock. Such studies inspired geologists in California and Japan to look at the characteristics of other faults. This proved fruitful. It showed the widthto-length ratio of wrench faults on either side of pull-apart basins (grabens) is independent of scale. The geometrical arrangements of overlapping faults are also similar at various orders of magnitude. Statistical comparisons have been made to identify relationships between characteristics such as fault frequency and vertical displacement. More recently, geologists have begun to realise that the relationship between fault length (ie lateral extent in the strike direction) and vertical displacement (or throw) differs according to the material properties of the rock. An intriguing comparison between the lateral extent and frequency of fractures and faults has been drawn up Increasing fracture/fault length (m) 16 CYAN MAGENTA YELLOW BLACK Middle East Well Evaluation Review Increasing frequency 3 Boso Peninsula, Japan 2 Onjoko, Japan Gulf of Mexico, USA Kodels Canyon, UK 1 0 -4 -3 -2 -1 0 1 2 3 4 Increasing fault displacement (m) Number 13, 1992. Fig 2.10: Log length versus log width for 70 pull-apart basins around the world. (Aydin and Nur, 1986). 10000 1000 Length (m) 100 10 1 1 10 100 1000 10000 100000 Width (m) 100km 10km s ock rd r ft ro Ha 10m cks 1km 100m So after studies of Egypt’s Ras Budran Field in the Gulf of Suez and outcrops in Sinai. Similar work in other parts of the world appears to support this work but not all geologists are convinced. Some believe that extensional fractures do not exhibit the same kind of fractal nature that appear to be common in shear fracture studies. Further work on fractal relationships may pave the way for modelling reservoir faults and fractures from small-scale borehole imagery combined with 3-D seismic data. This may prove to be a particularly useful combination of data recent work in the North Sea has shown that faults which are too small to be detected by the highest resolution 3-D seismic surveys may still have a major influence on reservoir behaviour. Already in Egypt, fractal relationships between fracture apertures at the large, medium and small scales have been seen in borehole electrical imagery. 100000 Displacement Fig. 2.9: Log of fault displacement versus fault frequency for various basins around the world. Note the similar ratios in each of the plots. 1m Fig. 2.11: Summary of observations showing the relationship between fault strike-length and displacement (after Walsh and Watterson, 1988). 10cm 1cm 1m 10m100m1km10km100km1000km Mapped fault length (width) 17 Head for the hills Fig.2.12: (Right): Typical Dip Trend output. The original dip recording, made using the SHDT Tool, is shown in the left track. The dip tadpoles have been coloured according to their initial structural classification. In this processing stage, the Dip Trend analysis defines the structural dips of intervals which have consistent magnitude and azimuth. The right-hand track shows the final processing stage. This time, groups of tadpoles which belong to the same structural trend and fold patterns have been linked by coloured bands. These dip clusters can be projected on a polar plot as shown in figure 2.13 (far right). Each of the structural trends and folds is assigned an identification number. Dip trends and structural analysis 100 ft Recent hydrocarbon discoveries in areas such as Abu Dhabi and Sharjah, UAE are encouraging explorationists to search for other targets in the mountain fold belt which runs along the Musandam Peninsula. Amoco’s new discovery in Sharjah’s Lower Cretaceous Thamama carbonates was found after reinterpretation of 3-D seismic data originally acquired in 1984. The discovery well (Amoco, Kahaif-2), located 18 km south of the Sajaa and Moveyeid fields, flowed 73 million cubic feet per day of gas and 1,615 barrels per day of condensate from a 700ft-thick pay zone. The seismic re-interpretation had additional spin-offs as it led to a better definition of both the Sajaa and Moveyeid fields, which in turn guided the drilling of additional producers. Success in this region has prompted Amoco to increase its acreage by 50% and carry out new seismic surveys during 1993. The structures of the fields along the northern end of the Emirates/Oman fold belt are more complicated than those being drilled in Oman. The dip patterns seen in figure 2.12 were created by drag deformation along two thrust faults which pass through the flank of a major field in the UAE. Computer analysis of dip data, using the recently introduced Dip Trend* software, enables the geometry of the thrust faults to be defined precisely (Middle East Well Evaluation Review, Number 8, 1990). The first dip track in the Dip Trend output shows the original dip recordings made and processed using a Stratigraphic High Resolution Dipmeter Tool (SHDT*). The Dip Trend software groups and colours the dips according to their initial structural classification. During this early stage, the program defines the structural dips of Original dips and classification Structural dip 401 Dips indicate folding 323 Dips indicate folding 303 312 303 Structural dip 301 18 CYAN MAGENTA YELLOW BLACK Middle East Well Evaluation Review The Gulf W Fateh Field Oman mountains E Margham Field 0 ft Semail ophiolite 20 000 Pre-Permian Salt 40 000 Fig 2.14: Cross section through the UAE showing the structural complexity of the major exploration target, Cretaceous Thamama carbonates. intervals which have consistent magnitude and azimuth. In the final Dip Trend analysis, the program produces a second output track with coloured bands linking the dips of each structural trend and each fold pattern. An identification number is also assigned to each of these trends and is printed at the base of the dip pattern on a log plot and near the cluster (or circle) of dips when projected onto a polar plot. In this example, the structural dip zones fall in the centre of the polar plot, as they have low dip angles, whereas the dips of the folds fall on a circle. The Dip Trend software adds identification numbers to dip populations that define individual folds on both polar and log plots. A cross section showing the precise geometry of the folding is drawn by the StrucView* software program which is currently under test in the Middle East. In this case, the angle of the thrust fault was estimated since the survey was undertaken using a Dipmeter. Had borehole imagery been used, it would have been possible to measure the exact dip and strike angles of the fault planes. Well-to-well correlations in this area show that parts of the formation are repeated which confirms the interpretation of thrust faulting. (See Charismatique, Issue 1 and WER Structural Geology Supplement, 1990). Northwest Depth m 0 6450 6500 6550 6600 6650 6700 6750 6800 6850 6900 6950 StrucView Duadip Southeast 90 0 Duadip 90 Fig. 2.15: This cross section shows the precise structural geometry of the example shown in figure 2.12, based on Dip Trend analysis of dipmeter data. However, the angle of the thrust fault must be estimated. Had borehole imagery been used, it would have been possible to measure the exact dip and strike angles of the fault plane. The orientation and vertical scales can be changed to match other cross sections, such as a seismic section. 7000 7050 7100 7150 7200 340 350 0 330 10 20 30 320 40 310 50 300 60 Small circle defining a fold (#303) 290 70 Structural dip trends (301 & 401) 80 401 280 270 Small circle defining a fold (#323) 260 90 301 100 110 250 120 240 230 130 220 140 210 200 Number 13, 1992. 190 180 170 160 150 19 Peering into Turkey’s fractures Borehole imagery, Dip Trend analysis and land 3-D seismic surveys are giving a new insight into the complexity of the reservoirs housed in the fold belt which runs from Oman to Turkey. Structural and fracture analysis of borehole images have revealed that Turkey’s reservoirs have been put through a complex mixture of normal, thrust, reverse and wrench faulting. Rift faulting first affected southeastern Turkey along the northern edge of the Arabian Plate. Later, the closure of the Tethys Sea caused thrust faulting which reactivated some of the normal faults within these rift blocks. Subsequent interaction of the Arabian and Eurasian plates during the Miocene produced widespread wrench faulting and reactivated both normal and thrust faults along the Zagros-Bitlis Mountain belt forming the East Anatolian fault zone. The dominant structural influence on the Turkish Petroleum Company's (TPAO) fields in south east Turkey is the NE-SW Adiyaman wrench-fault system. This has a left-lateral displacement and has formed structures with imbricated and faulted anticlines which are overthrust from north to south. The region has also suffered at least two phases of major tectonic deformation, one in the Late Cretaceous and the other during Miocene times. Studies by TPAO in three fields, Ozan Sungurlu, Karakus and Cendere, give some idea of the complicated geological history of south east Turkey and the structural geometry of its reservoirs. Analysis of borehole imagery from three wells in Ozan Sungurlu Field has revealed faults, unconformities and fractures. A Dip Trend structural inter- Fig. 2.16: STRETCHING THE IMAGINATION: The dominant structural influence in the TPAO fields has been the Adiyaman wrench-fault system. This has produced a left-lateral displacement and has formed complex structures, with faulted anticlines, which are overthrust from north to south. Dip Trend structural analysis of dipmeter and FMS data has helped to unravel some of the complexities of these structures. Fig. 2.17: How the Adiyaman wrenchfault system has displaced many of the imbricated and faulted structures in TPAO's fields. pretation of one of these wells is shown in figure 2.19. A 3-D structural model of the field has since been made and incorporates the various fault movements over time. Similar reservoir complexities and fault types have been highlighted by 3D seismic surveys of N V Turkse Shell’s Mardin reservoirs. The seismic data shows that the thrust faults later developed a lateral shear component. It also indicated that there was a previously undiscovered reservoir fault block to the south which is separated from Kastel Field by a normal fault. (Details of the complexity of Shell’s Beykan Field can be found in Middle East Well Evaluation Review Structural Geology Supple- Bitlis extension of the Zagros fold belt S N Melange Nappe Ophiolite Nappe +2 Tertiary +1 0 -1 -2 Upper Cretaceous Paleozoic Mardin Limestone km Fig. 2.18: Typical cross section through TPAO's fields showing the complex faulting that has taken place. (WER Structural Geology Supplement, 1990). 20 CYAN MAGENTA YELLOW BLACK Middle East Well Evaluation Review ment, 1990). Borehole electrical imagery provides more detailed structural information on a smaller scale than 3-D seismic surveys. Using these images, it has been possible to investigate the fracture systems and unconformities which separate the main units in the Cretaceous Mardin Group (the Areban, Sabunsuyu, Derdere and Karababa formations). The porosity of the Mardin Group rocks is associated with unconformities and fracture-related dolomitization. In fact, fracturing is often critical to the viability of these reservoirs which normally have a matrix porosity of less than five percent. In these fields, Dip Trend analysis is proving to be invaluable to both structural and fracture analysis as it highlights changes in dip associated with faults and folds. It also reveals the slight dip changes which are related to the Mardin reservoir unconformities which house different fracture types. In Mardin reservoirs, FMS data has given a clear indication of fracture orientation. It has shown that there are numerous changes in direction and density of fracturing intersected by the wells. There is also a wide variation in fracture porosity and permeability with depth. Fig. 2.19: (Left): This structural interpretation of one of the wells in the Ozan Sungurlu Field have revealed a complex mixture of faults, unconformities and fractures. The analysis was made using Dip Trend computer software and dip data derived from borehole electrical imagery (above). Polar projections (below) of this data have helped in the identification of structural trends and this information is now being used to make a 3-D structural model of the field. This will also incorporate the various fault movements over time. Number 13, 1992. 21 The Dip Trend analysis revealed that the various fracture orientations seen in each well can be related to specific structural events. In addition, plots derived from Dip Trend analysis were correlated with well logs which proved that the highest fracture densities were linked to some major faults, certain lithologic units and unconformities. Changes in fracture orientations and fracture porosity seem to go hand-inhand with increased fracture density and proximity to unconformities. The fracture widths increase in zones below the unconformities in the Mardin reservoirs and this could be due to stress relief and leaching along cracks. In general, the tectonic fractures are indicative of tensional stresses and show that the rocks probably failed during uplift or due to deformation close to faults. The widths of these fractures normally decrease with increased fracture density - the reverse of what is seen in the cracking associated with unconformities. A close look at drilling-induced fractures, coupled with an investigation of borehole shape, has given a good indication of the principal horizontal stress direction across the field. Variations in the stress direction often occurred near major faults. The Ozan Sungurlu 1 well to the north east of the study area contains the greatest density and widest range of fracture orientations. The strike histograms in figure 2.22 show that the orientations vary from NW to ENE but they remain sub-parallel to the major faults. The predominant NW fracture set is influenced by the principal regional stress - and this is also reflected in the drilling-induced fractures. The fracture orientation shows a sharp change across the fault zone at 2,680 m suggesting that a different, more localized, stress regime exits on either side of some of the main faults. 22 CYAN MAGENTA YELLOW BLACK Fig. 2.20: These FMS images clearly reveal the fractured nature of the Mardin reservoirs. There are numerous changes in direction and a wide variety of fracture types and size with depth. The various fracture orientations can be related to specific structural events. Fig. 2.21: A study of these drillinginduced fractures coupled with an investigation of borehole shape has given a good indication of the principal horizontal stress across the Ozan Sungurlu Field. Middle East Well Evaluation Review Fig 2.22: (Immediate left): Strike histograms show that the orientations vary from NW to ENE but they remain sub-parallel to the major faults. The predominant NW fracture set is influenced by the main regional stress. The fracture orientation shows a sharp change across the fault zone at 2,680 m (far left) and this indicates that a more localised stress regime exists on either side of the main faults. Fig 2.23: This FMS shows an unconformity where the Karababa 'B' meets the Karababa 'A' Formation. A potential unconformity can be seen at 3,192 m where the Areban meets the Sabunsuyu Formation, producing a clear change in bedding orientation. The FMS images reveal a lack of faults at this depth but show a high density of fractures beneath the unconformity. The wide apertures of the fractures indicate that they may have been leached. Number 13, 1992. 23 Some of the world’s largest reservoirs are contained in the gentle fold belt that lies on the south western side of Iran’s Zagros Mountains. The biggest reservoir is the Oligocene-Miocene Asmari Formation which is made of carbonate rocks which were deposited just before the Tethys Sea closed. An indication of the highly fractured nature of the carbonate Asmari fields came at the turn of the century when oil was first discovered in Iran. Numerous ‘gushers’ were found and all the wells had high production rates. By 1920, pressure measurements at Masjidi-Sulaiman Field indicated that reservoir communication could only be explained by fracturing - an important observation that radically changed the field development strategy. Instead of drilling many closely-spaced wells, fewer more widely spaced wells were put down. Without knowing it, the engineers were sowing the seeds of modern reservoir management. Rock fractures in the younger Asmari reservoirs also provide the route for fluid and pressure communication with the older and deeper Cretaceous (Bangestan Group) rocks. This explains why Cretaceous oils are found in the Asmari rocks. The fracture systems in many of these reservoirs are complex. However, recent studies of a supergiant field, which lies further away from the Zagros range, have revealed a surprisingly orderly fracture distribution. Most of the fractures that occur are in the crestal portions of the carbonate layers and it is thought that the interlayered and highly porous reservoir sandstones have a dampening effect on the stresses in the anticline. The distribution of fracture characteristics in the same field is neither uniform, chaotic nor random. However, the distribution can be investigated using FMS/FracView summary logs, and lithology and porosity details can Photo: BP. Iran’s fractured formations be obtained from log analysis. Most of the fractures are of tectonic origin but a few are karstic. As would be expected, the tectonic fractures are congregated along the crest and their orientation is parallel to the axis. There is a systematic decrease in fracture characteristics (aperture, width, density and length) from the crest to the flanks. The uppermost carbonate unit contains the most fractures. However, in this and lower layers, the fracture patterns do not conform a 3-D fracture block network that is traditionally assumed during reservoir simulation and production analysis. Instead, they have a preferred orientation but rarely intersect. This means that they increase the vertical permeability of these tight rocks and provide a permeability anisotropy which runs parallel to the field’s structural axis. Using today’s horizontal drilling technology, it is now possible to intersect such fractured zones and significantly improve well production. Fig. 2.24: Studies of Asmari outcrops reveal similar fracture characteristics to those seen in the studied supergiant fields. (WER Iran Special Supplement, 1991). Recent FMS imagery has provided quantitative information about the field’s fracture distribution. The flanks of the field contain between 50 % and 70 % fewer fractures than the crest and there seems to be a gradual decrease from crest to flank. This suggests that most of the fractures occurred at the same time rather than over a prolonged period. Fig. 2.25: Geological cross section across south east Iran. SW Supergiant Asmari reservoir Masjid-i-Sulaiman oilfield Agha Jari Fm Gachsaran Fm 24 CYAN MAGENTA YELLOW BLACK Asmari Fm Bangestan Group Khami Group Middle East Well Evaluation Review 0 Tectonic 180 360 Karst 180 0 360 0.2 ms 0.2 ms X Y Fig. 2.26: To the right is a text book example of simple extensional fracturing which resulted from the gentle folding of a supergiant Asmari reservoir. The fractures are only present in the limestone zones interlayered with highly-porous sandstone zones. The tectonic fracture lengths, density and apertures decrease systematically from the crest towards the flanks. Karstic fracturing is also present immediately below uncomformities (top right) formed during times of low sea level of the Neo-Tethys Sea. Crest Top carbonate Porous sandstone 155/0.96 mm Second carbonate 94/0.80 mm 60/0.49 mm Total number of fractures and their aperture 42/0.37 mm Asmari formation outcrops Intensely deformed core NW Number 13, 1992. 25