Hydrocarbon Geology Characteristics and Oil & Gas Resource Potential. Afghan-Tajik Basin. Wei Yin 1, a

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1 Advanced Materials Research Online: ISSN: , Vols , pp doi: / Trans Tech Publications, Switzerland Hydrocarbon Geology Characteristics and Oil & Gas Resource Potential in the Afghan-Tajik Basin Wei Yin 1, a 1 CNPC Research Institute of Petroleum Exploration & Development, Beijing ,China a yinwei600@petrochina.com.cn Keywords: Hydrocarbon Geology Characteristics, Oil & Gas Resource Potential, the Afghan-Tajik Basin Abstract. The Afghan-Tajik Basin is an intermontane depression between the mountain ranges of Gissar and Pamirs, and Jurassic system and Tertiary system are rich in large oil & gas resources. In order to assure sustainable supply of oil & gas from Central Asia, we deeply researched hydrocarbon geology characteristics and resource potentials. The basin belongs to paralic sedimentary environment, and develops 3 sedimentary strata: Jurassic, Cretaceous, and Tertiary. Afghan-Tajik Basin develops 3 main source rocks including clastic rocks of Jurassic, carbonate rocks of Cretaceous and mudstone rocks of Eocene. The basin develops 2 plays: Jurassic-Cretaceous play is gas containing one, and Tertiary play is oil containing one. Plaster stone and salt rock of upper Jurassic are regional cap rocks of Jurassic-Cretaceous gas pool, and creaming mudstone and muddy limestone of Cretaceous and Tertiary are regional or partial cap rocks. Migration and accumulation of hydrocarbon occur in the late Cretaceous and early Pliocene epoch. Afghan-Tajik Basin has larger exploration potentials, and residual resources are t. The potential zones are as follows, south part of basin, oil-gas structures of post-salt, reef limestone of pre-salt, and litho-stratigraphic traps. Introduction The Afghan-Tajik Basin is an intermontane depression, whose area is km 2, between the mountain ranges of Gissar and Pamirs. The Afghan-Tajik Basin is usually subdivided into two parts roughly equal in their territories: the Tajik-Uzbek part, located in the north and the Afghan part located in the south. These parts of the basin are separated by the Amu-Darya. A small part of the basin, the southern margin of the South-West Gissar foothills falls within the territory of Turkmenistan. The basin boundaries are formed by mountain structures composed of Paleozoic metamorphic and magmatic rocks. To the north, it is limited by the Gissar Range up to 5.0 km high. To the east, by the Darvaza Range (the Pamir) up to km high. To the south-east, by Hindu-Kush up to km high. In the west, the basin boundaries are formed by the south-western offshoots of the Gissar Range [1-3] (Fig. 1). The Afghan-Tajik Basin is rich in large oil & gas resources. Petroleum prospecting began in 1933, and in 1934 found the first oilfield-haudak oilfield, whose pay zone Uzbekistan The Amu Darya Basin Turkm enistan Kazakhstan SYR Darya Basin Gissar r ange Afghanistan Dushanbe F ergana Basin Tadzhikistan Afghan-Tajik Basin Hindu-Kush kirghzitan Pamirs nationary boundaries oilfield Legend basin boundaries gasfield China Fig. 1 Location map of Afghan-Tajik Basin mountain range All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (# , Pennsylvania State University, University Park, USA-19/09/16,01:19:32)

Advanced Materials Research Vols. 734-737 367 is Paleocene, and in 1935 put into development. Jurassic and Cretaceous oil and gas fields were found in 1960s.

2 Advanced Materials Research Vols is Paleocene, and in 1935 put into development. Jurassic and Cretaceous oil and gas fields were found in 1960s. The south part of the basin is buried deep, and exploration degree is lower. The north part of the basin is buried shallowly, exploration degree is higher, and the obtained oil and gas discoveries mainly come from the north part of the basin. Jurassic and Cretaceous systems are mainly rich in natural gas, and Tertiary system is mainly rich in oil. 25 gas fields of Jurassic and Cretaceous systems have been found, and obtained m 3 gas recoverable resources. 28 oilfields of Tertiary systems have been found, and obtained t oil recoverable resources. The economic and political crisis caused by the breakup of the Soviet Union brought the exploration activities in the Afghan-Tajik Basin to several standstills. In order to assure sustainable supply of oil & gas from Central Asia,we deeply researched hydrocarbon geology characteristics and resource potentials [4-5]. Basin evolution and structural framework The structural evolution of the Afghan-Tajik Basin is related to south Paleotethys Ocean and surrounding orogenic belts, and the thrust and compression of orogenic belts developed large scale thrust faults and related folds. Thrust faults not only control the structural framework of basin, but also petroleum distribution. The Afghan-Tajik Basin experienced 3 structural evolution stages: (1) back-arc foreland stage of late Permian-Triassic, at that time, Paleotethys Ocean closed, the Afghan-Tajik Basin experienced compressed, formed thrust faults and folds, and created the unconformity contact relation between Jurassic and Triassic system. Strata are red continental clastic rocks and volcanic, and the strata thickness is m. (2) faulted-depression stage of Jurassic-Eocene. The Afghan-Tajik Basin experienced extension after orogenesis, and piled a set of continental molasse formation, at that time, The Afghan-Tajik Basin was subjected to 3 large scale collisions, 3 transgressions from west to east, and formed 3 continent-marine sedimentary cycles. (3) the regeneration foreland basin stage of Oligocene to now. From Neogene, India plate and Eurasia plate started suffering collision. In Miocene, The Afghan-Tajik Basin sustained strong compression, and developed so many thrust faults and related folds [6] (Fig. 2). Fig. 2 Cross section of The Afghan-Tajik Basin Structurally, the basin consists of a series of north-south trending depressions and separating uplift zones. They include, from east to west: the Kulyab Depression; the Obigarm Uplift; the Vahsh Depression; the Kafirnigan Uplift; the Surhandarya Depression; and the South-West Gissar Foothills. The Dushanbe trough in the north of the basin is an exception in that it has a generally west-eastern strike. The Kulyab Depression is the deepest in the basin and represents a foreland basin to the Pamir Fold Belt [7] (Fig. 3). 38 o N 36 o N 66 o E The Amu Darya Basin South-West Gissar Foothills Afghan-Tajik Basin Surhanda rya Depression 68 o E Dushanbe t ro ugh A according to IHS, 2006 Ka firnig an Uplift Vahsh Depression Fig. 3 Structural units and petroleum distribution of The Afghan-Tajik Basin Obigarm Uplift 70 o E Fergana Basin A' K ulyab Depression Location of Fig. 2 Legend oilfield gasfield basin boundary 72 o E condensate field prospect struture unit boundary cross section km

3 368 Resources and Sustainable Development Petroleum geological characteristics The Hercynian basement of the Afghan-Tajik Basin comprises Pre-Cambrian and Paleozoic highly disturbed and metamorphosed rocks. Basement rocks are exposed in the surrounding Tien Shan and Pamir ranges.there are 3 units on the basement: Back-arc Unit, Platform Unit and Collisional Unit. At the end Late Paleozoic, a number of micro-continents which included the North and the Central Pamir split off Gondwana and started drifting northward. The so-called "Transitional Complex" of the Afghan-Tajik Basin overlies the basement and comprises thick (2.5-4 km) Upper Permian-Lower Triassic Series represented by marine clastics and carbonates with interbedded volcaniclastics. It is probably related to a back-arc sequence deposited during the Paleotethys ocean closure. An inversion took place in the study area at the end Early Triassic. The Platform Genetic Unit corresponds to the so-called "Platform Complex" (megasequence) of the Afghan-Tajik Basin. The "Platform Complex" overlies the "Transitional Complex" with a sharp angular and stratigraphic unconformity. Its stratigraphic age is Rhaetian to Oligocene, and it begins with fluvial sediments, breccias, coal and weathering crust filling the Pre-Jurassic topography. The thrusting of the Pamir fold system onto the Afghan-Tajik Basin's sedimentary cover had caused a significant compressional stress and a structural reorganisation in the study area. A thick (up to 5 km) sequence of red- and greycolour molasse formations, known as "the Orogenic Complex", was deposited during the latest Oligocene-Quaternary. The platform unit is closely related to petroleum generation, migration and accumulation, therefore, the platform unit is the emphasis [7]. 2.1 Source rock The lack of modern geochemical studies limits the understanding of the Afghan-Tajik Basin's source rocks/petroleum systems. It is believed that Lower-Middle Jurassic clastics represent the main source rock for gas accumulations, while Eocene and possibly Lower Cretaceous rocks are the sources of oil. Jurassic Source Rocks: The basin's principal source rocks are Lower to Middle Jurassic continental to shallow marine clastics with coal. The organic matter (OM) content in Bathonian-Bajocian sediments varies from hundredths of percent to 15-25%, depending on the coal content. Thermally mature Jurassic source rocks charged predominantly pre-salt carbonate reservoirs with natural gas. They are believed to have contributed gas to the post-salt clastic Cretaceous reservoirs where the salt is missing [8-9] Eocene Source Rocks: The Eocene Suzak Formation is represented by distal greyish green mudstones and proximal sandstones and siltstones. The Suzak mudstones are rich in illite and montmorillonite and range in thickness from 10 to 180 m. They thin out to the east and northeast, where the mudstones become more calcareous, and beds of sandstones and carbonates appear. Total Organic Carbon content (TOC) in thin (0.5 to 2 m) organic-rich Suzak beds in Tajikistan is typically 11-19%, but can be as high as 48%. These beds are characterised by high gamma ray well-log responses. The organic matter is Type II (marine) kerogen. The organic-rich mudstones have a hydrogen content of 7.65% to 8.08%. They contain no terrestrial organic matter. Resin content is 10% and sulphur content is 4.5%. Lower Cretaceous Source Rocks: Possible Lower Cretaceous source rocks are represented by Aptian to Albian shales and argillaceous carbonates. The Aptian to Albian section has TOC content of 0.3%-3.46%, but can locally be as high as 18% (Klett at al., 2006). Organic matter is mainly of continental origin (Type III kerogen). 2.2 Reservoir Proven reservoirs in the Afghan-Tajik Basin occur in the Jurassic, Cretaceous and Paleogene sections. Jurassic and Cretaceous system produce gas, and Paleogene produces oil, therefore, most gas fields distribute in the Jurassic and Cretaceous system and most oil fields distribute in the Paleogene.

$Lithologic types are dolomite, clastic limestone and reef limestone. Reservoir type is fracture. Strata depth is 1.4-3.$ 3km, sedimentary thickness is 130-250m,reservoir porosity is 8-16%,permeability is 100-500mmd. Cretaceous reservoir: sedimentary environment is paralic.

3km, sedimentary thickness is 130-250m,reservoir porosity is 8-16%,permeability is 100-500mmd. Cretaceous reservoir: sedimentary environment is paralic.

4 up per lo wer up per mid dle-lower Advanced Materials Research Vols Jurassic reservoir: sedimentary environment is shallow-sea platform. Lithologic types are dolomite, clastic limestone and reef limestone. Reservoir type is fracture. Strata depth is km, sedimentary thickness is m,reservoir porosity is 8-16%,permeability is mmd. Cretaceous reservoir: sedimentary environment is paralic. Lithologic types are siltstone, sandstone and limestone. Reservoir type is mainly gas-bearing. Strata depth is km, sedimentary thickness is m,reservoir porosity is 10-20%, permeability is mmd. Paleogene reservoir: Proven Paleogene reservoir intervals occur in the Bukhara/Guri carbonates (Paleocene), in the Eocene Alay limestones and sandstones, and in the Oligocene Sumsar sandstones. Bukhara reservoirs include limestone and dolomite, The gross thickness of reservoir is m, net thickness is 25-70m,porosity is 5-30%,permeability is mmd. The gross thickness of Eocene Alay reservoir is m,net thickness is 6-15m. in Obigarm depression, the max gross thickness is up to 200m, porosity is 10-18%, permeability is mmd. Sumsar reservoir is located in the upper. The most thickness is 100m. Paleogene carbonate reservoir properties from east to West has improved, and This is mainly due to increased dolomite content [6,10] (Tab.1). 2.3 Seal The Upper Jurassic Gaurdak salt and shales are the most important regional seal in this basin. Distribution range spread Kulyab depression and obigarm uplift. Thickness span of seal with high plasticity and shielding ability is between tens of meters of basin margin and 800m of basin center. Cretaceous and Tertiary gypsum, mudstone, shale and dense dolomite are also regional and local good oil gas seal [11] (Fig. 4). 2.4 Reservoir type strata Jurassic Cretaceous Paleogene Tab. 1 Reservoir characterictics of Afghan-Tajik Basin petroleum type porosity type Reservoir types are various, and they are 6750 divided into 2 types: stratified and massive Stratified reservoir can be separated into Fig. 4 Stratigraphic column of Afghan-Tajik Basin dome and closed. The common characteristic of stratified reservoirs is that there existed low permeability rock on/under the reservoir, and most reservoirs distribute in local structures that are mainly anticlines with the development of fault. There are also dome reservoirs controlled by lithology, for example controlled by lithological change zone. Massive reservoirs refer to Paleogene reservoirs, whose reservoirs are limestone and dolomite embedded with mudstone, gypsum and karstenite. Reservoirs are controlled by thrust faults, whose displacement is m. The length of reservoir is km,width is km, height is m [5,7] (Fig. 5). lithology depth(km) gross thickness(m) net thickness(m) porosity (%) permeability (mmd) gas fracture limestone/dolomite gas porosity sandstone/siltstone Bukhara oil fracture carbonate Alay oil fracture limestone/sandstone Sumsar oil porosity sandstone Erathem SystemSeries Ceno zo ic Mesozo ic Eo gene Cr etaceous Jurassic Oligocene 3250 Eocene Paleocene 3500 m lithology lithology m u d st o n e, g y p su m m u d st o n e an d san d st o n e m u d st o n e, san d st o n e an d l i m est o n e l i m est o n e an d an h y d r i t e l i m est o n e, m u d st o n e an d an h y d r i t e mudstone intercalated with limestone layer limestone, mudstone, sandstone and an h y d r i t e limestone, mudstone and sandstone sandstone mudstone argillaceous siltstone sandstone salt coal source rock reservoir seal type petroleum system discovered oil fiels III discovered gas fiels undiscovered gas fiels II I

370 Resources and Sustainable Development Fig. 5 Accumulation model of oil and gas of Afghan-Tajik Basin Analysis of accumulation and resource potential of oil and gas 3.

5 370 Resources and Sustainable Development Fig. 5 Accumulation model of oil and gas of Afghan-Tajik Basin Analysis of accumulation and resource potential of oil and gas 3.1 Play Play includes a series of geologic factors: reservoir, seal, trap, accumulation and their effectiveness of the time-space matching, therefore, play can be defined a group of prospective traps they are sharing common reservoirs, regional seals and petroleum accumulation system. The Afghan-Tajik Basin can divide 2 plays: Jurassic-Cretaceous and Tertiary. The Jurassic-Cretaceous play is located on the north-west part of the Afghan-Tajik Basin, and the play is mainly rich in natural gas and condensed gas. Condensed gas fields mainly distribute in South-West Gissar anticline belts and gas fields distribute in South-West Gissar anticline belts, the Surhandarya Depression, the Dushanbe trough and the Kafirnigan Uplift. More than 80% reservoir belongs to Jurassic-Cretaceous play. The Tertiary play is located on middle part of basin, including the Surhandarya Depression, the Kafirnigan Uplift, the Vahsh Depression, the Kulyab Depression and The Dushanbe trough, and this play is mainly rich in oil(fig. 3). 3.2 Resources calculation 25 oil-gas reservoirs (mainly gas fields) are found in the Jurassic-Cretaceous play and 28 (mainly oil fields) are found in the Tertiary play. If the number of oil reservoirs or gas reservoirs exceeds 6, we can use discovery process method to calculate resources. The principle of the discovery process method is that based on discovered oil and gas fields, through probabilistic method to predict resources of evaluation units. In Tertiary play, oil undiscovered resources are 224mmb, condensed oil undiscovered resources are 1.3mmb, natural gas undiscovered resources are scf. In Jurassic-Cretaceous play, oil undiscovered resources are 16mmb, condensed oil undiscovered resources are 81mmb, natural gas undiscovered resources are scf(tab. 2). Oil undiscovered resources are mainly coming from Tertiary, accounted for 93%. Condensed oil undiscovered resources are mainly coming from Jurassic-Cretaceous, accounted for 98%. Natural gas undiscovered resources are mainly coming from Jurassic-Cretaceous, accounted for 98%. Tab. 2 Resources of Afghan-Tajik Basin resource types total resource undiscovered resource play T oil condensed oil natural gas ( 10 8 scf) oil condensed oil natural gas ( 10 8 scf) J-C total

Advanced Materials Research Vols. 734-737 371 Fig. 6 Distribution of undiscovered resources in Afghan-Tajik Basin 3.

and the Afghan part of the southern basin is low exploration degree.

6 Advanced Materials Research Vols Fig. 6 Distribution of undiscovered resources in Afghan-Tajik Basin 3.3 Analysis on resource potential (1)Low exploration degree area of southern basin The special political and economic reasons of Afghan-Tajik Basin lead to unbalanced exploration of the whole basin, and the Afghan part of the southern basin is low exploration degree. The southern part and northern part of the basin has roughly exploration area, except the Dushanbe trough and the South-West Gissar, other structural units distributes both south part and north part of the basin. Structural and sedimentary feature of the same structural unit are similar, and Kafirnigan Uplift, Vahsh Depression, Kulyab Depression have found large scale oil and gas discovery, therefore, south basin should be have certain exploration potential. The disadvantageous factor is that buried depth is deeper, and brings certain difficulty for exploration and development. (2)Oil and gas bearing structure in syncline belts of post-salt Cretaceous system and Neogene The post-salt of Afghan-Tajik Basin develops structure trap, and the main types are anticlines, faulted noses, faulted anticlines and complex faulted blocks. The discovered oils and gases of Cretaceous and Neogene are all distributed in above-mentioned traps. According to oil and gas distribution relations and regional structural cross sections, only part traps have been drilled. There are so many traps undrilled, and undrilled traps and drilled traps have similar structural and sedimentary feature, so the undrilled traps have the petroleum potential. (3)Reef limestone of pre-salt Jurassic system Afghan-Tajik Basin develops thick salt on the Jurassic reef limestone, and reef limestone and thick salt form effective vertical play. Adjacent to the west Uzbekistan, have discovered reservoirs in Jurassic reef limestone. The west Amu Darya basin have found under a huge reserves of natural gas reservoir in pre-salt, and Afghanistan - Tajik basin has proven natural gas reserves of scf, therefore, the basin pre-salt Jurassic has huge exploration potential, demand to tap the potential, and increase production. (4)Litho-stratigraphic traps There developed 3 reservoirs in Afghan-Tajik Basin: Jurassic marine carbonate rocks, Cretaceous river and lake facies sandstone and siltstone, tertiary marine carbonate rocks. The reef flat facies of marine environment and fluvial, fan body, beach-bar facies have the advantage conditions of forming lithological reservoirs. The lateral pinchout of favorable reservoir has the potential of forming stratigraphic reservoir. The Afghan-Tajik basin suffered exposure and erosion, may form unconformity reservoirs controlled by unconformity surface. The basin litho-stratigraphic traps exploration is still in its infancy, still need to do a lot of work. Conclusions (1) The Afghan-Tajik Basin is mainly a gas-bearing basin, and experienced 3 structural evolution stages: back--arc foreland stage, faulted-depression stage, the regeneration foreland basin stage. Faulted-depression stage has a close relation with petroleum generation, migration and accumulation.

7 372 Resources and Sustainable Development (2) Structurally, the basin consists of a series of north-south trending depressions and separating uplift zones, with the structural framework of 4 depressions and 3 uplifts. Each tectonic unit from north to south presents strip distribution, and the north is narrow, and the south is wide. (3) The Afghan-Tajik Basin develops 3 source rocks, 3 reservoirs, and salt of upper Jurassic system is the regional seal. Based on the reservoir, the basin is divided into 2 plays: Jurassic-Cretaceous gas-bearing play and Tertiary oil-bearing play. (4) Oil remaining recoverable resources of the whole basin is 499mmb, Condensed oil remaining recoverable resources is 218mmb, natural gas remaining recoverable resources is scf. Resource potential areas are low exploration degree area of southern basin, Oil and gas bearing structure in syncline belts of post-salt Cretaceous system and Neogene, reef limestone of pre-salt Jurassic system and litho-stratigraphic traps. Acknowledgements This project was supported by the Major National Science and Technology Projects of China (No. 2011ZX ). References [1] Xiaoguang Tong and Shubao Xu. Atlas of world petroleum exploration and development [M]. Beijing: Oil Industry Press, In Chinese. [2] Chinese Academy of Geological Sciences. Geological map of China and its adjacent areas [M]. Beijing: Geology Press, In Chinese. [3] Yixiu Zhu, Luofu Liu and Changsong Lin. Petroleum geology of Fergana Basin in Central Asia [J]. Journal of Lanzhou University(Natural Sciences (1): In Chinese. [4] Afghanistan: the oil and gas opportunities of China [J]. Chinese Petroleum and Petrochemical Processing In Chinese. [5] Zuoxiang An and Zhengqin Hu.The Central Asian oil and gas area [M]. Beijing: Oil Industry Press, 1993: In Chinese. [6] Chengzao Jia, Shufeng Yang. Tectonic geology and natural gas of basins in the North Tethys margin [M]. Beijing: Oil Industry Press, 2001: In Chinese. [7] Afghan-Tajik Basin, HIS internal data. [8] Yixiu Zhu and Luofu Liu. Petroleum Geology of South Tajikistan Basin in Central Asia [J]. Xinjiang petroleum, 2007, 28 (2): In Chinese. [9] Otto S C. Mesozoic-Cenozoic history of deformation and petroleum systems in sedimentary basins of Central Asia: implications of collisions on the Eurasian margin [J]. Petroleum Geoscience. 1997, 3(6): [10] Yongxiang Sun. Oil-bearing quality of intermountain basin of Tianshan Mountain [J]. petroleum exploration and development, 1994, 15(1): In Chinese. [11] G. I. Smith. The quaternary salt deposit in southwestern Afghanistan [J]. Foreign mineral deposits, 1989.

entered a rapid development phase. Annual increased proven reserves are above 500 billion cubic meters (bcm) from 2003, and annual natural gas product

(), entered a rapid development phase. Annual increased proven reserves are above 500 billion cubic meters (bcm) from 2003, and annual natural gas production has increased from 50bcm in 2000 to nearly