in Jurassic and Cretaceous Stratigraphy

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Earth Science Frontiers, Vol. 17, Special Issue, Aug. 2010 ISSN 1005-2321 Facies Analysis, Reservoir Characterization and Hydrocarbon Habitat of the Upper Jurassic Arab “C”, Qatar, Arabian Gulf 388 Hamad Al-Saad Department of Chemistry and Earth Sciences, Qatar University, P. O. Box 2713m Doha, Qatar The Arab C is a member of the Arab Formation, the main host of the huge Jurassic reserve in the Arabian Basin. The Arab “C” member in Qatar has similar lithologic characteristics throughout the country, and to that of the type locality in Saudi Arabia. The Arab “C” member consists of a sequence of limestone and dolomite limestone with numerous stylolites. Anhydrite occurs as nodules and in thin streaks as well as forming the reservoir cap rock. Detailed petrographic analysis of core materials and thin sections from selected wells from the onshore Dukhan Oilfield indicates that the unit represents a transgressive system tract and made up of oolitic peloidal grainstone, algal boundstone, bioclastic grainstone and packstone with dolomite of different textures and sizes. Petrographic analysis of the Arab “C” suggests that its microfacies are similar to that of Upper Jurassic sediments in other parts of the Arabian Gulf. The Arab “C” member consists mainly of repeated small cycles of micritised grainstone- packstone with some dolomites at the base, which gradually pass upwards to wackestone-packstone with local coral- algal boundstones at the middle part and ended with fossiliferous grainstones-packstone that gradually change to anhydritic dolomites and anhydrites which capped the Arab “C” member. The upper boundary of the Arab “C” was picked from positive gamma-ray deflection. At the base, the gamma-ray change across the boundary is very gradational. During the Early Jurassic, the Arabian Gulf was a vast carbonate-evaporite platform (Murris,1980). In this time, evaporitic conditions were limited to the north-western of this platform. By the end of the Early Jurassic, humid conditions predominated and evaporite deposits gave the way to carbonates. During the Middle Jurassic, a carbonate deposits covered most of the Arabian Gulf region (Murris, 1980). Mixed clastic- carbonte was deposited toward north and north-west of the region. By the end of the Middle Jurassic, major flooding of the platform led to deposit relatively deep carbonates facies under euxinic conditions. These facies are believed to be the source rocks of much of the oil reservoirs in the Arabian Gulf. In the Late Jurassic, an intra-shelf basin extended over most of the platform and siliciclastics were totally absent. Most of the studied rocks had undergone extensive diagenetic processes that modified their original features and their petrophysical properties. Dissolution created a considerable amount of moldic porosity and with the intercrystalline porosity resulted from dolomitization are the main types of porosity. Petro- physical analysis of well logs and core data indicate that high porosity (up to 30%) and permeability (up to 450 kmd) were found in the lower part of the unit. The widespread distribution of the unit, its lithologic features and reservoir characteristics make it an important exploration target. From the nature of the recognized facies and the values of porosity and permeability, the Arab C is an important reservoir rocks and more attention should be directed to explore for it. Exploration should be directed to areas where there is extensive leaching by meteoric water or where dolomite forms the larger part of the succession. Key words: Qatar; Arab C; Upper Jurassic; Hydrocarbon References: Alsharhan A., Nairn A. Sedimentary basins and petroleum geology of the Middle East. Elsevier, Amsterdam, New York, Oxford, 1997, 843. Ai-Siddiqi A., Dawe R. Qatar’s oil and gas fields: a review. Journal of Petroleum Geology, 1999, 22: 417-436. Beydoun Z. The Middle East: Regional Geology and petroleum Resources. London: Scientific Press Ltd., 1988: 293. Murris R. Middle East: Stratigraphic evolution and oil habitat. American Association of Petroleum Geologist, 1981, 64: 597-618.
Earth Science Frontiers, Vol. 17, Special Issue, Aug. 2010 ISSN 1005-2321 Oil Shales from Timan-Northern Ural Region (Russia): New Results of Investigations and Prospects of Development I.N. Burtsev, V.A. Saldin, L.A. Anishchenko, S.S. Klimenko, V.V.Udoratin, O.S. Protsko, I.V. Popov, D.O. Mashin, D.N. Shebolkin, N.S. Suvorova, A.Yu. Utova Institute of Geology Komi SC UB RAS, Syktyvkar, Russia ( E-mail: burtsev@geo.komisc.ru; litgeo@geo.komisc.ru ) In Russia oil shales are widely developed in the Timan-Northern Ural Region (Oil shales, They are included in the vast Volga-Pechora shale-bearing pro- vince and confined to the Upper Jurassic, namely to Upper Jurassic Volgian stage Middle Volgian substage (J3v2 zone Dorsoplanites panderi) and correspond to the Upper Tithonian (J3t3 1 ) (Lyurov, 1996). In 2008-2009 field works were carried out at Aiyuva and Chim-Loptyugskoe deposits of the Timan- Northern Ural region. The authors took a task to study the deposits and to evaluate prospects of their development taking new data into consideration. The Upper Jurassic section is composed of clays and oil shales. Between these rocks there are gradual transitions to carbonaceous clays and argillaceous oil shales. The section is predominated by dark grey, grey and green-grey clays. Their composition includes illite, smectite, kaolinite, chlorite and mixed-layered minerals. Most samples show heulandite, sometimes its content reaches 20%. Generally clays are calcareous with inclusions of quartz grains and feldspars of silt sizes. Oil shales are isolated in three strata with thickness from 0.4 to 3.2 m. The organic matter of the oil shales includes highly hydrolyzed compounds in- cluding amino acids, humic acid, chloroform A bitumen, alcohol-benzol mixture and insoluble organic matter (OM) – kerogen. These data testify to multicomponent composition of OM of Jurassic oil shales, as well as considerable participation of humic matter. On the basis of pyrolitic and elemental analyses, microscopic study and semicoking methods, four OM types are determined: 1) predominantly algal (alginate) (type I, Tissot and Welte, 1982; 2) alginate-inertinite (types I-IV); 3) resinite-alginite (types I-II) and 4) predominantly humic (types III-IV). Total thickness of shale-bearing interval varies from 1.2 to 17 m. The Upper Jurassic deposits include remains of various groups of marine fauna and flora from protozoa to highly organized organisms. Macrobenthos includes remains of bivalves, mud-eaters and urchins. Inarti- culate brachiopods are found. Microbenthos population is represented by rare remains of ostracodes and foraminifers, pelagic nekton – by cephalopodae (ammonites, belemnites) and perhaps by fish (phos- phate biogenic remains are found). Zooplankton is represented by rare shells of radiolarians, and phytoplankton – by green and golden algae. Dominating remains testify to normal marine salt content of waters and boreal basin. New lithological markers are determined, and new strata correlation of oil shales is done. The lower part of Upper Jurassic shale-bearing section, corresponding to the lower bed (III) is characterized by small cyclitic structure. New data on mineralogy and geochemistry of oil shales and enclosing strata are obtained. Maximal Мо – 0.001%-0.002% and V – 0.060%-0.080% con- tents are correlated to oil shales development in the section and distinctly concentrated in semicoking products. It is determined that increased content of Ni – 0.050%-0.060% and Co – 0.001%-0.003% in underlying clays are related to authigenic vaesite microcrystals on the surfaces of various mineral grains. The technological characteristics of oil shales depend on the composition and type of organic matter. The conducted technological studies prove high quality of oil shales: combustion heat 7.5–30 MJ/kg; resin yield –8 %–30 %; sulphur content in resin –2%-7%. Taking into account the considerable contrast of enclosing clays and oil shales on radioactivity and specific atomic density it is recommended to apply the preliminary sorting and separation of mineable rock mass – to separate shales from clays and to increase raw quality for processing. This technology allows to mine both standard shales and lower thin layers. Mining works may be conducted by Wirtgen Surface Miner. It will allow working out thin layers with minimal loss of oil shales. The processing of oil shales results in four basic industrial products: shale oil, energy, fuel gas, shale ashes. The further prospects are connected to both enhancement of processing in each listed product, and creation of new branches of industrial production. Key words: Upper Jurassic deposits; Tithonian; oil shales; Timan-Northern Ural; Clays; Organic matter; Technology References: Dedeev V.A., Аminov L.Z. (ed). Fuel and energy base of the European northeast of the U.S.S.R. Syktyvkar: Komi SC UB AS USSR, 1991: 304 . Lyurov S.V. Jurassic deposits NorthRussian platform. Ekaterinburg: UB RAS, 1996: 139. 389
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