SHAKHOVA I.A. FSUE "Arktikmomeftegazrazvedka", Russia

The biggest gas and gas-condensate fields in the Barents Sea region (Shtokmanovskoe, Ludlovskoe, Ledovoe) have been discovered inside Shtokman-Lunin mega-saddle (SLMS).

It can be explained by favorable lithofacies conditions as well as hypsometric uplift of this mega-structure with respect to abutting deeply sunken South- and North-Barents depressions. It is opinion that during creation of oil and gas deposits. Shtokman-Lunin mega-saddle acted as structural barrier for hydrocarbons migrated from abutting depressions and accumulated in its interior unique resources. Most of researchers suppose that SLMS is the main potential object in the Barents Sea region and they think it is possible to build up there a large Russian gas-producing center based on explored resources as well as to discover new fields in this area.

Due to "structures of Shtokman-Lunin mega-saddle type are very common in the Barents-Kara plate" and can be regarded as the objects of primary exploration activities on the West Arctic shelf /I.S.Gramberg, O.I.Suprunenko, U.V.Shipilkevitch, 2000/ it is very important to know geology, history of evolution and details of origin for located on SLMS fields and this knowledge can be very useful for future oil and gas prospecting on similar geological structures.

Geological structure of SLMS and confined fields is given in many publications /A.V.Borisov, I.A.Tanygin, V.S.Vinikovsky, I.A.Borisova, 1995; I.S.Gramberg, O.I.Suprunenko, U.V.Shipilkevitch, 2000/ and well known to specialist. We only want to mention that Quaternary, Neogene, Cretaceous, Jurassic, Late Triassic and partly Middle Triassic geological sections were studied by drilling (depth 3-4 km) and all geological sections from the surface to the basement are studied by seismic exploration (depth 13-15 km). Gas and condensate fields have been discovered in Jurassic sediments, and big shows of gas were noticed in some areas in Cretaceous sediments. No hydrocarbons were found in Triassic sediments.

Although the structure of upper part of section, including Middle Triassic sediments raises no doubts and proved by well correlated seismic and drilling data, the lower (early) part of sediments and accordingly Shtokman-Lunin mega-saddle forming time is still a subject of discussions.
Some researchers suppose it is an old structure, which "at least since early (?) Palaeozoic kept its relative hypsometric position and its location on the shelf though it had some definite changes in the configuration. It is proved by comparison between structure-charts of a sedimentary cover bottom and a top of Jurassic sediments' /I.S.Gramberg, O.I.Suprunenko, U.V.Shipilkevitch, 2000; I.S.Gramberg, O.I.Suprunenko, I.A.Tanygin, U.F.Fedorovsky, U.V.Shipilkevitch 2003/.

Other researchers believe that SLMS is a quite new structure, formed within Jurassic-Cretaceous periods /A.V.Borisov, I.A.Tanygin, V.S.Vinikovsky, I.A.Borisova, 1995/.

To know an answer to this question is not only curiosity, it has also practical use. If SLMS old age is proved, we must estimate hydrocarbon potential of this mega-structure much higher because of longer period of stable configuration and favorable conditions for hydrocarbon migration from abutting depressions.

Unfortunately, the proofs of old SLMS age is based on not very reliable geophysical data, caused by a big depth of seismic reflectors in interval between a top of basement and the bottom of point out sediments, partial traceability of reflectors in this area and unreliable affixment to geological profile. Apparently all these reasons can explain similarity between structure charts of a sedimentary cover bottom (depth about 15 km) and a top of Jurassic sediments (depth L5-2 km). This similarity has been obtained by seismic data interpretation and used as a proof of SLMS old age/I.S.Gramberg. O.I.Suprunenko. U.V.Shipilkevitch 2000/. The published conclusion about SLMS lifetime from early Palaeozoic to present days surprised the authors of this idea as well. They said: "This stability is strange, because the area had big changes in its geological history like tectonic conditions, the main sources of fragmental materials, etc. " /LS.Gramberg, O.I.Supmnenko, U.V.Shipilkevitch 2000/.

According to our opinion, initially published in 1995 the concept of Jurassic-Cretaceous age of SLMS is more founded by the facts. Paleotectonic reconstructions, based on confirming each other seismic and drilling data, point out that in the beginning of Late Triassic period there was not any uplift in the Shtokman-Lunin mega-saddle's area. There was only single whole East-Barents mega-depression /E.V.Shipilov, G.A.Tarasov. 1998/ filled up by terrigenous Permian-Triassic formations, having an ancient structure differing from the present one. South- and Noth-Barents depressions began their separation after forming of the rugosity on Novoya Zemlya in Late Triassic-Early Jurassic time period /LS.Gramberg, V.LBondarev. N.N.Sobolev, L.A.Daragan-Suscheva, 2003/ and creation of Shtokman-Lunin mega-saddle between them in the Early Jurassic.

According to world experience, 200 million years history of structural development (since early Jurassic to now-days) is enough time to accumulate fields of any sizes, as well as lithological features of Shtokman-Lunin mega-saddle allow to predict its high hydrocarbon potential.

ARCTIC SHELF OIL AND GAS CONFERENCE 2004