GAS BEARING CAPACITY OF WESTERN SIBERIA: THE PROSPECTS OF DEVELOPMENT IN XXI CENTURY S.M. Karnaukhov, V.S. Kovalenko (Gazprom), V.A. Skorobogatov (VNIIGAZ ) The history of study of geological structure, oil & gas bearing capacity of the West Siberian sedimentary megabasin (WSMB), prospecting and exploration of HC accumulations (HCA) in Jurassic-Cretaceous section of the sedimentary cover embraces over six decades (1948 - 2008). During this period more than 700 HC fields, including 240 gas -containing fields (gas, gas -oil, oil-gas, gas condensate, gas condensate-oil, oil-gas condensate, i.e. with phase isolated pools of free gas) have been discovered within WSMB. Among them the largest, by initial HC in place (considering production and preliminary estimated res erves) are as follows, tcm: the Bolshoi Urengoy (12.6), Yambur g (7.4), Bovanenkovskoye (4.9), Zapolyarnoye (3.7) and Medvezhy e (2.3). Initial gas in place exceeded 47 trn m3, over 90% of which are accumulated in the northern part of the megaprovince, including the fields of Nadym -Pur-Taz region (NPTR), Yamal and Gydan peninsulas (and similar gas and oil bearing districts), the Gulfs of Ob and Taz and the Kara Sea shelf. By 2008, 13.6 tcm of gas had been produced in Western Siberia. The main gas -bearing complexes are Albian -Cenomanian and Neocomaptian. Sufficiently lesser reserves have been discovered and partially explored in Achimovian thickness of Berrias -Low er Valanzhinian and upper horizons of the Jurassic productive complex (Yu 1, Yu 2 -4). The further development of mineral resource base (MRB) of gas production in WSMB and planning and exploration, including exploration carried out by Gazprom, are determined, firstly, by a value, structure and reliability of probable (undiscovered) gas resources. It is known that there are official estimates of initial potential and undiscovered HC resources, expert evaluations of independent researchers and corporate estimates of scientific teams of producing companies. As of 01.01.1993, gas potential of West Siberian region amounted to 97.7 tcm (including 92.8 tcm for Yamalo-Nenets Autonomous Area) and gas potential of the Kara Sea region amounted to 36.2 tcm. Thus, the total gas potential accounted for 134.0 tcm. In spite the fact that no principally new HC fields and poo ls within WSMB were discovered for the period between 1988 -1993, the total value of initial potential gas resources was increased due to NPTR (estimate of reserves for Yamal did not change, for Gydan this estimate even decreased by 1.8 tcm). In 2001-2004, the state reevaluation of initial potential resources of HC for the land and shelf of Russia was performed as of 01.01.2002, including Western Siberia and the Kara Sea. The initial potential gas resources for the megaprovince were increased up to 134.6 tcm. The major part of these resources is accumulated in the north of the megaprovince: in Yamalo-Nenets Autonomous Area (96.7 tcm) and in Khanty-Mansi Autonomous Area (6.0 TCM). Based on real knowledge (by drilling) of the most promising and productive Cretaceous horizons in particular oil and gas -bearing regions and districts, a value of initial potential gas resources seems to be somewhat overestimated for the north and strongly overestimated for some central and western regions of the megaprovince, especia lly for Frolovskaya, Krasnoleninskaya, Sredneobskaya and Kaimysovskaya regions. As for the productive complexes, the official estimate for the Albian-Cenomanian horizon seems to be the least accurate one (48.6 tcm) (land of WSMB together with gulfs). It should be noted that for a long time (1979-1988) the initial potential gas resources of the complex were estimated at 35.6 tcm (only the land) and the majority of experts considered this estimate acceptable (i.e. reasonable for confirmation). According to V NIIGAZ data (2002 -2005), the initial potential gas resources of Western Siberia, including the sea, were estimated at 100-105 tcm, including 3.0 tcm for central and southern regions. This is a conservative estimation. The probability of its final confirmation in future reserves is estimated at 75-80%. Thus, according to calculations of 1988-2002, gas potential of the megaprovince was evaluated by different researchers in the interval from 102±3 to 130±5 tcm. Maximum estimates here are official, while minima l estimates (conservative, actually confirmed) are given and substantiated by VNIIGAZ experts. The last estimation of gas resources and their structure for the north of the megaprovince is show n in the table. The table – Estimates of initial potential gas resources of the north of Western Siberia (VNIIGAZ, 2008) tcm Oil and gas Total Land Shelf complex Gas-salinskaya 2.0 2.0 - formation (turon) Alb-Cenomanian 39.5 33.5 6.0 Aptian 17.5 10.2 7.3 Neocomian 25.0 16.9 8.1 AT 7.0 7.0 - Jurassic 7.5 6.3 1.2 Oil-gas contact 2.0 1.5 0.5 Pr e -Jurassic 1.5 1.5 - Grand total 102.0 79.9 23.1 By regions: NPTR – 51.0; Yamal – 20.8; Gydan – 7.1; shelf – 23.1. Relative to a trade-off evaluation of HC potential of WSMB, undiscovered (promising and probable) gas resources in Western Siberia are estimated at 45 -46 TCM (18-19 TCM in the shelf). The main undiscovered gas resources are accumulated in Apt, Neocom and Middle Jurassic of the arctic regions of WSMB, including the Kara Sea. Among gas fields the discovery of 3-4 super giant (over 1 tcm, open shelf), 22-25 large and giant (0.1-1.0 tcm), 70 -80 large (30-100 bcm) and many hundreds of average and small fields (with one or several pools) are forecasted. By 2030, under active exploration the increase of explored (proved) gas reserves is expected to reach 8.4 tcm on the WSMB land (350-400 bcm for central and southeastern regions), including 5.2 tcm by GAZPROM and no less than 8.5-9.0 on shelf (the Gulfs of Ob and Taz and West-Preyamal shelf). After 2030 the exploration and searching for HC fields (including gas fields) in Western Siberia will continue. These works will be mainly carried out at shelf and at depth of 4 -6 km. It is expected that newly discovered fields will be mainly average and small in size (less than 20 -15 mln of reference fuel). The development of HC resources, i.e. their transfer to commercial reserves, and the development of reserves being under preparation will extend operational life of West Siberian oil and gas -produc ing region till the end of the XXI century. .