Класс Месторождения: Уникальное
Тип Месторождения: Нефтегазоконденсатное
Местоположение:
Местность:
Стадия разработки: Добыча
Год открытия: 1959
Источник информации:
Метод открытия:
Площадь: 180.1 км²
The Souedih Oilfield
The Souedih (also spelled Suwaidiyah) Oilfield is located in the extreme northeast of Syria in part of the Mesopotamian Basin. The principal reservoir of most fields in this region is the Upper Cretaceous, carbonate-rich Massive Formation.
Syria occupies a key tectonic position at the northern tip of the Arabian Plate. It forms the apex of a structural triangle between the Zagros Fold Thrust Belt (ZFTB) and the Dead Sea Fault-Bitlis Fault system. The extreme northeast of the country lies within the northwestern Mesopotamian Basin, in which are located most of the main oil fields of Syria. The traps are mainly structural in nature and the oilfields located at the crests of the principal anticlinal structures; for example: Souedih, Rumailan, Ulayyan, and Mashouk anticlines (Figure 1). These are separated from the Sinjar-Abed el Aziz area by deep reverse faults. This whole region is part of the ZFTB that extends southeastwards into the very prolific oil province of Iraq and Iran. The structural and stratigraphic setting has close similarities along the length of the ZFTB.
Figure 1: Generalised tectonic map of the Arabian Platform (Konert et al., 2001). The Souedih Oilfield is located at 37˚N and 42˚E.
The Souedih Anticline contains Syria’s largest oil field, known as Souedih Field. It extends approximately 25 km west-northwest to east-southeast, reaching a maximum width of about 12 km.
The field extends eastwards across the Iraqi border, where it becomes known as the Saffia Field, but the main part of the structure (ca. 18 x 12 km) is located within Syria. The field is situated about 15 km to the south of the Karatchok Field, and extends westward towards the Rumailan Field, separated from both by important faults.
The principal reservoir of most fields in the Syrian Mesopotamian Basin is the Upper Cretaceous, carbonate-rich, Massive Formation. However, although these are amongst the largest oilfields and some of the earliest discoveries in Syria, there has been very little detailed work on the region in general, and little published on petrophysical and reservoir characteristics of the Massive reservoirs.
The region remains highly prospective, with ongoing exploration and discoveries successfully
targeting the Massive Formation.
Stratigraphic context
The surface topography in northeast Syria is relatively flat-lying with a mean elevation of around 450 m. Recent to Pliocene continental sediments occur at the surface overlying a mainly shallowmarine Tertiary succession. The total thickness of this cover above the top of the reservoir formations encountered in wells is about 900 to 1,100 m, with a maximum of just over 1,300 m in places. The estimated depth to economic basement is approximately 6,000 m (Figure 2).
Figure 2: General stratigraphy, lithology and formation thicknesses for the Souedih Oilfield, northeast Syria. Depth is measured depth, as taken from Well 1100 (not shown).
The thick Mesozoic series are dominated by carbonate rocks – limestone and dolomite, which are more-or-less argillaceous. Anhydrite and shale become more common in the Triassic. The Massive Formation (Albian–Lower Campanian) consists entirely of carbonate rocks and is considered the main productive formation in the region in general, and in the Souedih Field in particular. It is unconformably overlain by the Shiranish Formation (Upper Campanian–Maastrichtian), and unconformably overlies the Rutbah (Barremian–Aptian) or Qamchouqa (Middle Jurassic) formations.
Massive Formation
The Massive Formation can be divided informally into three members from top to base as follows (Figure 2).
Massive A (Coniacian–Lower Campanian) is, as its name implies, a relatively massive (i.e. thick to very thick-bedded and structureless in aspect) limestone unit, which is only slightly argillaceous in places.
Massive B (Upper Cenomanian–Turonian) is a limestone unit with thin beds of dolomite, increasing in frequency downwards, and less commonly with thin beds of anhydrite.
Massive C (Albian–Lower Cenomanian) is mainly dolomite, with thin beds of limestone and anhydrite. With reference to Sharland et al.’s (2001) Arabian Plate sequence stratigraphic framework, the Massive Formation straddles their AP8 and AP9 mega-sequences. It is believed that their K160 and K170 maximum flooding surfaces sit within the Massive A Formation, the K140 and K150 within the Massive B Formation and the K90, K100, K110, K120 and K130 within the Massive C Formation.
Oil and gas is mainly produced from the Massive A Member, which has an average thickness of about 100–120 m, varying from over 200 m in some wells to less than 15 m in others. In general, it increases in thickness towards the east. The average depth to the top of the Massive A is about 1,750 m, but ranges from 1,582 m to over 1,900 m subsurface. We have therefore restricted this initial study of porosity and lithology to the Massive A Member.
Figure 3: Structural model developed from Petrel software showing depth to top of the Massive A reservoir, Souedih Field. Solid circles show the locations of wells used for this study; specific wells referred to in the text and/or other figures are labelled. The field of view is 22 km (left to right) and 10 km (top to bottom). Depth (m) is below sea level (TVDSS) (true vertical depth sub sea. Depths shown as (TVDSS).
Porosity Distribution
Both regional and vertical variations in average porosity values in the Massive A are evident from an examination of the porosity logs from the wells in the study area. On a regional basis, there is a tendency for porosity values to decrease in an eastwardly direction. Porosity would normally be expected to decrease with depth, but in this case appears to show a marked increase (see Figure 3). In general, average porosity values are higher in the deeper western wells, low in the shallower eastern wells, with a transition region shown in the more central parts of the field.
With reference to three sets of three closely spaced representative wells from each of the western, central transition and eastern areas, mean porosity values were derived. The values of average porosity in the western wells are over 14% (with few exceptions), exceeding 20% in some wells. The equivalent values are less than 5% in the eastern wells (with very few exceptions), and they are more variable in the transition region, exceeding 20% in some wells, decreasing to less than 6% in others, and in general they average greater than 10%.
With regard to vertical variation in porosity, we recognise the existence of five zones, which themselves show regional variations in thickness (Figure 4). The regional model for the distribution of porosity is constructed for each zone and shown in Figure 6.
Figure 4: Comparison between the distribution of the vertical zones (A1 to A5) in the Massive A Member in selected wells across the Souedih Field (depth below sea level). Well locations shown in Figure 3.
The Massive A porosity zones are, from top to base:
Figure 5: Petrel-derived models showing the distribution of the porosity in zones A1 to A5 of the Massive A Member, Souedih Field. Note the changing of the thickness in each zone across the area. Field of view is 22 km across (left to right). The thickness of the cross section for A5 (second image up from the bottom) is 500 m (from lowest point to highest point of cross-section). Note, the vertical exaggeration here is x5. The images alternate between cross section at the top to plan view beneath for each of the 5 zones.
Zone A1 extends between the top measured horizon (which generally corresponds with the top of the Massive A, or a few meters beneath it) and horizon 1. This zone is characterized by porosity values generally equal to or more than 15%. It mainly exists in the western area (where it may extend downward to the base horizon) and in the transitional region. It generally thins towards the east and is totally absent in the extreme east of the field. In some wells, Zone A1 contains thin intervals in which the porosity values are less than 8%; these have their minimum thickness and frequency in the western parts of the field.
Zone A2 extends between horizon 1 and 2. It has an average porosity value of about 10–12% but, like Zone A1, it contains thin intervals in which porosity values may be less than 6%. Zone A2 extends in some western wells from horizon 1 to the base horizon, but it has its maximum thickness in the transition region, where it exceeds 60 m in Well 411.
Zone A3 extends between horizons 2 and 3, with average porosity less than 6%. Its thickness varies widely from minimum values in the western part, where it may be absent in some wells, to maximum values in the eastern part, where it may occupy the whole Massive A in some of the most easterly wells.
Zone A4 extends between horizons 3 and 4, with average porosity values mostly in excess of 10%. However, it has some thin intervals (2–4 m thick), in which porosity values exceed 20%, and other intervals with values less than 5%. This zone is absent in most of the western and extreme eastern wells, but its thickness may exceed 40 m in some of the transition region wells (e.g. Well 4).
Zone A5 extends between horizon 4 and the base of the Massive A, with average porosity values less than 5%. It is absent, or very thin (less than 2 m), from the entire western and most of the transition area wells, while its thickness may reach about 100 m in the eastern wells.
Sediment Facies and Microfacies
A total of 36 rock specimens and thin sections were selected from the available cores in two wells (Figures 3 and 6): (1) Well S691 located in the western part of the field (450 m kelly bushing elevation, and measured depth interval of Massive A is 1,886–1,980 m). (2) Well S7VH located in the transition region (475 m kelly bushing elevation, and measured depth interval of Massive A is 1,775–1,950 m).
Figure 6: Porosity zones (A1 to A5) and location of the selected samples from the wells S-691 and 7VH. For well location see Figure 3. Depth (m) is below sea level (TVDSS). Depths shown as (TVDSS).
The samples from both wells were taken from Zone A1, which has the maximum average porosity, and from Zone A3, in which the average porosity value is one of the lowest.
In hand specimen the sediments are uniformly grey brown in colour, with an apparently fine-grained groundmass and variably abundant bioclasts. Many of these appear as remnant ghosts that have been more-or-less replaced by microsparitic to micritic cement. There is no evidence of lamination or other primary sedimentary structures. Bioturbation appears common, though indistinct, while burrow traces are rare.
In thin section, the composition and classification of all the samples are very similar to each other (Figures 7 and 8): bioclastic packstone and packstone-grainstone in S691, and bioclastic packstone in S7VH.
Figure 7: Photomicrographs of thin sections to show characteristic microfacies types. Note that mouldic porosity is well-developed in some samples and absent in others.
(a) Bioclastic Grainstone: the bioclasts consist mainly of benthic foraminifera (note the Orbitoids near the centre of the photo). Well S-691; depth interval 1,450–1,451 m.
(b) Bioclastic Packstone-Grainstone: the bioclasts consist mainly of benthic foraminifera. Well S-691; depth interval 1,452–1,453 m.
(c) Bioclastic Packstone: bioclast ghosts plus clear longitudinal section through echinoid spine. Well S-691; depth 1,455 m.
(d) Bioclastic Packstone; bioclasts include ostracod (right) and bivalve fragment (bottom). Well S-691, depth 1,457 m.
(e) Bioclastic Packstone; bioclasts include rudist fragments. Well 7VH; depth 1,375 m.
(f) Bioclastic Packstone, partially dolomitised: the dominated bioclasts include benthic foraminifera (mainly Orbitoids), plates of echinoderms and fragments of bivalves. Well 7VH; depth 1,371 m. Depth (m) is below sea level (TVDSS). Depths shown as (TVDSS).
The dominant bioclasts consist mainly of bivalves (shallow water types), molluscs, echinoderm plates, benthic foraminiferans (particularly orbitoids and miliolinids), and ostracods. In some thin sections we observe planktonic foraminiferans inside benthic species. The bioclasts appear abraded, and moderately to well sorted. Most samples contain some large, dispersed, cloudy crystals of diagenetic dolomite (not exceeding 10–20%). Other mineral grains encountered in some thin sections are small scattered pyrite crystals, greenish-coloured glauconite grains, and small fragments of phosphate and chert. In some samples there are bitumen spots and impregnations.
The principal cement is microsparite or micrite and microsparite, with at least two generations evident. Larger sparite cement is found locally, partially or wholly filling mouldic pore spaces or channel spaces. Early micritisation of the walls of the bioclasts has helped them to resist the effect of compaction, but the effect of the later dissolution, cementation and partial dolomitisation, has made them difficult to distinguish clearly. They commonly appear in some thin sections as remnant ghosts.
By far the dominant and in many cases the only porosity observed is mouldic porosity, from both zones (A1 and A3) (Figure 9). In a few samples there is also some channel porosity and micro-fracture porosity. Overall the relative abundance and size of the mouldic pore spaces in the thin sections taken from Zone A1 are clearly greater than those taken from Zone A3. It is possible that the increased amount of sparitic cement in certain parts, for example Zone A3, have infilled preexisting mouldic porosity. However, the alternative interpretation that the sparite came first and has not been fully removed by secondary dissolution cannot be ruled out. Further work is required.
Figure 8: Representative photomicrographs of the microfacies and dominant porosity type (mouldic). Thin sections taken from Zone A1 and Zone A3 of wells S-691 and 7VH.
(a) Zone A1; high porosity. Thin section from Well S-691; depth 1,452 m.
(b) Zone A1; high porosity. Thin section from Well 7VH; depth 1,343 m.
(c) Zone A3; low porosity. Thin section from Well S-691; depth 1,481 m.
(d) Zone A3; low porosity. Thin section from Well 7VH; depth 1,416 m. Depth (m) is below sea level (TVDSS). Depths shown as (TVDSS).
Data source: Carbonate reservoir characteristics and porosity distribution in Souedih Oilfield, northeast Syria. Amer Ghabra, Dominic Tatum, Andy Gardiner and Dorrik Stow. GeoArabia, 2014, v. 19, no. 2, p. 177-192
Следующее Месторождение: Sfaiyeh