Modern Thermobaric Conditions of the Barents and Kara Seas Region
Vasily Bogoyavlensky1, Tatiana Budagova1, Andrey Bezhentsev2
1OGRI RAS (Moscow) ([email protected])
In world practice, a serious increase in exploration drilling led to the discovery of many large onshore and offshore oil and gas fields, and diligent work in the search for oil and gas fields in "unconventional" complexes of rocks and in conditions of high pressure and high temperature (HPHT) was conducted. The formation of "closed systems” with abnormally high reservoir pressure (AHRP) is possible if good regional seals are present. AHRP protects liquid hydrocarbons from destruction despite extreme high temperature that falls outside the limits of the traditional “oil window”. The presence of AHRP preserves the reservoir properties of rocks even in deep horizons (e.g. at 7-8 km with porosity of 15-20 % in wells En-Yakhinskaya-7 and Tyumenskaya-6).
The Arctic region will no doubt play a big role in the expansion of the resource base of Russia. Recoverable oil and gas resources of the Russian Arctic shelf comprise 90% of the resources of all of the offshore, estimated to be 100 billion tones of oil equivalent. Oil and gas deposits over a wide stratigraphic range (including Silurian and Ordovician) have been discovered in the Barents and Kara onshore and offshore region. Many of them are AHRP, exceeding hydrostatic gradient by 1.5-2 times.
OGRI RAS has engaged in wide-ranging research of the Arctic region’s oil and gas bearing complex using three-dimensional modeling (Tigress software) of various characteristics of environmental conditions, including thermobaric. The main objectives of this work are the following: increase in the reliability of predicting oil and gas bearing zones; determining undiscovered hydrocarbon accumulations as a result of the use of heavy drilling mud; predicting accumulations below the TD of wells; optimization of the drilling process and reduction in the number of well accidents.
Modeling was conducted at local and regional levels. During modeling for the Southern Kara Region abnormal zones of formation temperatures and pressures at different levels were determined. The unique, closely situated Kharasaveyskoye, Kruzenshternskoye and Bovanenkovskoye gas condensate fields lie in the strongest abnormal zone on area 60x100 km, with total reserves of over 13 billion tn OE. Here AHRP is present in the Lower Cretaceous deposits (1800-2000 m), while it is present regionally in Yamal-Gydan essentially in the Jurassic. Lower Cretaceous deposits of Leningradskoye and Rusanovskoye gas condensate fields have AHRP at depths more than 2000 m, abnormal factors Ka reache 1.23-1.25 at TD of wells (2454-2550 m).
Modeling of thermobaric conditions at the local level was conducted for the Murmanskoye Gas Field, which has the greatest number of wells (9) drilled of any offshore field in the Russian Arctic Regions. In the Murmanskoye field area, formation pressure gradients from sea-bottom to the Upper Triassic at depths of 2200-2300 m are close to hydrostatic (1-5 %). Since there was a smooth, almost linear increase of formation pressure with depth, a maximum measured value of 80.9 MPa at a depth of 4210 m in well 24 (abnormal factor Ka=1.96) was observed. Such distribution of formation pressures testifies to possible migration of gas from deeper deposits through systems of sub-vertical fractures and faults to the Upper Triassic - Jurassic clay seal. A positive closed temperature anomaly was determined along horizontal trends of formation temperatures at a level of 2500 m and deeper around the structure’s arch (the centre of the gas accumulations). The closed pressure anomaly starts to form at depths over 3000 m (below the location of gas accumulations) and shows a strong presence at a depth of 3500 m. Results of modeling demand further analysis through a combination of seismic and well log data. However, even at the present time there is a basis to assume the presence of additional undiscovered oil and gas accumulations in the Triassic and Upper Paleozoic complexes.
AAPG Search and Discover Article #90096©2009 AAPG 3-P Arctic Conference and Exhibition, Moscow, Russia