Geomechanical model of rootless gas-filled structures genesis: a case study from Medvezh’e field

Kozhemyakin I.D.

A.A. Trofimuk Institute of Petroleum Geology and Geophysics SB RAS, Novosibirsk, Russia

Ikozhemyakin@gmail.Com

The purpose of this study is to build geomechanical model of Medvezh’e field using seismic and well data.

The study field is related to Medvezh’e tectonical structure which is located in the Nadumskiy petroleum bearing district in Western Siberia. Background material for this study was provided by 121 seismic lines and 81 wells data. Basic reflecting horizons were traced using Petrel software to develop structural maps, supplied by integrated interpretation of geological, seismic and well data. These reflecting horizons are related to tops of basement and Jurassic, Neocomian, Aptian-Albian-Senomanian deposition associations.

According to field of study tectonic history evaluation Medvezh’e structure is rootless. In other words, it means that structure amplitude increases towards section up, although a basement of the structure is not coincided to pre-Mesozoic tectonic offset. During Jurassic time there was monocline submerging in East direction on the studying field. Current upfold development began in the end of Senomanian stage, and had been formed in Cenozoic. Such phase of tectonic activation associates with formation of large amount of faults cutting whole Mesozoic-Cenozoic cover, as can be concluded from seismic sections analysis. Taking into account, that in Western Siberia hydrocarbons generating was most intensive in Cenozoic, several actions coinciding in time, as hydrocarbons generating, anticlinal trap development, and faults which works like chimneys for hydrocarbon migration origin, provided development of Medvezh'e giant gas field.

Many giant oil and gas fields from north of Western Siberia and from Barents and Kara sea shelves are related to rootless structures. But those structures genesis mechanism is not clear yet, as it can’t be produced by subvertical movements of different Earth crust blocks. Archimedes flotation of relatively light gas-filled rock volumes is one of possible genesis mechanisms. Flotation process is self-intensifying, as once initialized by some way gas-filled structure will lift up, increasing gas-filled pore space and becoming lighter to float further. This flotation model was built in Lunev approach [1] describing ambient deformation as Newtonian liquid creeping flow. Model building operation runs reconstructing initial structure state, using backward in time approach [2]. However, since ambient deformation is non-linear process, several iterations of fitting was undertaken to decrease discrepancy. Thus, process of fitting is comparison of structure derived from backward in time and following forward in time modeling with real one, produced from seismic data. Such flotation process activation may be induced by many agents. It can be an impact of fault deformation in basement or depositional cover. Development of Western Siberia depositional cover folds is by far influenced by fault deformations as outlined by Gogonenkov [3]. Physical model experiments integrated with rated ones produced by Stephanov [4] shows enlargement in pore space and expansion in the number of gas chimneys as a result of dilatansy zones appearance caused by ambient deformations, described above.

Mathematical modeling results in similarity between shapes derived by flotation and Medvezh’e structure shapes, thus it backs up accuracy of proposed genesis mechanism.

Study was supported by cross-disciplinary Siberian Branch of Russian Academy of Sciences grant №127.

References:

1. Lunev B. V. (1986) Isostasy as the dynamic equilibrium of a viscous fluid, Akademiia Nauk SSSR, Doklady (ISSN 0002-3264), vol. 290, no. 1, 1986, pp. 72-76. In Russian

2. Ismail-Zadeh, A.T., Korotkii, A.I., and Tsepelev, I.A., (2003) Numerical approach to solving problems of slow viscous flow backwards in time, in M.I.T. Computational Fluid and Solid Mechanics 2003, K.J. Bathe (Ed.), Elsevier Science, Oxford, pp. 938-941,.

3. Gogonenkov G.N., Nekrasova L.A., Timursiev A.I. (2011) Plastic deformations of sedimentary cover of West Siberia in zones of horizontal shifts of the basement, in Oil and gas geology, 3/2011, pp.55-62. In Russian

4. Stefanov Yu. P. (2010) Dilatation and compaction modes of deformation in localized zones, Physical Mesomechanics, 13, Elsevier. pp.44-52. In Russian