KIREYEV G.I., PREOBRAZHENSKAYA, E.N., ZONN, M.S., VEVEL YA.A., ISAKOVA T.N., NIKOLAYEV A.I., KOSSOVAYA O.L. SOBOLEV X.N., USTRITSKIY V.I.

FSUE ArktikMorNetteGazRazvedka, Russia, FSUE VNHOkeanGeologiya, Russia,
FSUE IGIRGI, Russia,VNIGRI, Russia, GIN RAN, Russia, VSEGEI, Russia

The Carboniferous-Lower Permian carbonates are the principal oil and gas pay horizons in offshore discoveries made in the Pechora Sea. Recent drilling data from the Medyn-More prospect provide a possibility to refine sequence stratigraphy, paleontology, lithology, and the extent of pre-Triassic scour.

Stages and horizons (Regional stages Rs) were recognized on the basis of foraminifers, corals, brachiopods and conodonts datings with reference to lithologic and geophysical markers.

The lower boundary of the Tournaisian Stage of the Carboniferous System is detected on well log curves by a thin carbonate-mudstone marker member Rnm-t (5-10 m). In some well sections near the base of the marker the lenses of sedimentary brachia presents. The Tournaisian (62-80 m) consists of biogenic limestones with chert nodules, in some sections consisting of two parts separated by a carbonate-mudstone member. The presence of both the lower and upper substages has been identified from brachiopods and foraminifers.

The lower part of the Visean Stage (Kozhim and Tula Rs) is composed of mudstones and carbonate-mudstone sequences (16-120 m). Major unconformity at the base of Tula Rs (20-60m) accounts for the varying thicknesses of the Kozhim Rs and the frequent omission of its upper sandstone member in the sequence. The Kozhim and the lower milestone member of the Tula Rs are important markers. The upper part of the Visean, the Oka Super Rs (110-250 m) is composed of biogenic-detrital limestones with foraminifer, brachiopod, pelecypod and ostracod shells and bryozoan and echinoderm fragments. This interval is divided on four units. In whole they demonstrate the transgrassive-regrassive sedimentary cyclit. In uppermost part of this interval present argillicious unit (30 m) - marker Rv/s. The Late Visean age of the sequence was determined from foraminifer, coral and brachiopod finds in the core from well no. 2. The boundary with the Serpukhovian Stage is drawn at the base of the marker carbonate-mudstone member. In the section Medyn More prospect the Visean/Serpuchovian boundary is conforming, but on the nearest Prirazlomnay field the boundary is disconforming with some hiatus.

The Serpukhovian Stage is generally similar in composition to adjacent sequences in the northern Timan-Pechora province but distinguished by an extremely low proportion of sulfates. The lower part of the stage classified as the Tarusa and Steshevsk Rs begins with overlain by a high-resistivity dolomite sequence (105-110 m). Onshore, in the upper part of this sequence contains anhydrites. The sequence is crowned by two closely bedded marker carbonate-mudstone beds (30 m). In places, they provide seals for small oil pools. The upper part of the Serpukhovian Stage classified as the Protva Rs (75-64 m) is composed largely of limestones and dolomites; full sequences, however, contain a marker mudstone bed at the top (10 m). A foraminifer assemblage identified as Serpukhovian was found below the marker bed in well no. 3, and the Baslikirian foraminifers were identified above the marker bed in well no. 2. The Serpukhovian in well no. 3 is immediately overlain by the Triassic.

The Middle Carboniferous deposits were identified in well no. 1 and 2. The presence of the Bashkirian and Moscovian stages was identified by various guide fossils. The Middle/Upper Carboniferous boundary was drawn in the interval 1365-1366 m in well no. 2 based on foraminifers, corals, and conodonts. The total thickness of the Middle Carboniferous is 50-60 m.

The foraminifers and conodonts of the Kasimovian Stage of the Upper Carboniferous were for the first time identified in the section of well Medyn-More 2. The presence of the Gzhelian Stage is also inferred. The Upper Carboniferous deposits are composed of rhythmically interbedded bioclastic, detrital and pseudo-oolitic limestones with variegated marine fossils. Grainstone beds are always porous. The thickness of the Upper Carboniferous sequence in well no. 2 is 24 m (1366-1349 m).

The Lower Permian carbonates were identified with confidence in well no. 1 (1162-1186 m) and inferred in well no. 2 (1323-1349 m). They are composed of gray limestones with a characteristic foraminifer assemblage and other Asselian and Sakmarian forms. The deposits are represented by shallow-water, probably shoal lithofacies. The small thickness of the member remaining after the pre-Triassic scour event attests to its Asselian age.

ARCTIC SHELF OIL AND GAS CONFERENCE 2004