 | UNCONVENTIONAL COMMERCIAL HYDROCARBON-BEARING POOLS WITHIN OBOLON ASTROBLEMESouthern Margin of Dnieper-Donets Depression,
Ukraine, 2011 | |  Figure 1.Localization of new hydrocarbon prospects within Obolon astrobleme, ranked by the results of joint analysis of geophysical and geochemical data |
GEOLOGICAL PROBLEM In recent years, in Dnieper-Donets depression there is a tendency of decreasing sizes and number of anticlinal oil and gas traps prepared for deep drilling. To increase hydrocarbon potential in Ukraine, it is necessary to explore not only for anticlines but also for all other
potential types of hydrocarbon-bearing pools. Basing on world experience, astroblemes which are meteorite-impact structures, should be considered as one of the new types of oil and gas bearing objects. Commercial oil and gas production is proven today in ten out of twenty astroblemes in North America. According to the "Earth Impakt Database", Obolon astrobleme is a large circular depression almost 20 km in diameter.
| Two exploration wells #5301 and #5302 were drilled within the central part of the depression in 1965-1966 after regional exploration activities. The drilling results showed no signs of hydrocarbon presence and oil shales in particular. However, taking into account the large size of the depression and theoretically predicted hydrocarbon resources, in 2010-2011 an additional detailed geological and geophysical surveys were conducted, including 3D seismic survey (1:25 000), high-precision gravimetric and magnetometric survey (1:10 000), as well as regional geochemical observations (1:200 000).
| 
Figure 2. Rock density distribution within the sedimentary cover (A) and crystaline basement (B). Section line crosses the firs priority
prospect and the recommended well #1
|
GEOLOGICAL RESULTS
Overall seven prospects associated with low-density anomalies have been mapped within Obolon astrobleme (fig. 2). All of them were ranked by priority into three groups - perspective, high-promising and first priority (fig. 1). To estimate the priority of new prospects and to select the top priority one for exploration drilling additional data of magnetic, geochemical, and thermometric surveys were involved into study. The last ones were used as indicators of integrity/tectonic fragmentation of predicted pools and as indicators of areas with potential deposit destruction, characterized by higher activities of hydrocarbon migration.
The first priority prospect for exploration drilling is the object around recommended well #1 (fig 1, 2, 3). The prospect is presented by a tectonically screened block within the crater rim in the South-Western part of the structure. Perspective hydrocarbon reservoirs were mapped within the upper part of the crystalline basement, breccia complex and Jurassic basal sandstones of overlaying complex.
| Other six prospects were recommended for further drilling, including one deep target in the crystalline basement within the North-Western close to the central part of the structure and also several prospects within and outside the rim and breccia rocks. The potential hydrocarbon reserves of new prospects were estimated by means of the volumetric Monte Carlo method. New prospects were ranged by the value of predicted pore space volume within the entire geological section, as well as for each lithological / stratigraphical unit. By the value of predicted pore space volume, the most promissing sediments belong to breccia complex - 2.08 km3, less promising are Jurassic basal sandstones - 0.87 km3 and basement rocks - 0.14 km3. It should be noted, that the most promising pools of the basement are concentrated in the central part of the astrobleme, while in a sedimentary cover new prospects are localized outside of the astrobleme's crater. | 
Figure 2.Spatial distribution of rock density in the upper part of the basement (A), breccia complex (B) and Jurassic basal sandstones of an overlaying complex (C)
| 3D GTAVITY INVERSION WORKFLOW
Structural framework for the initial 3D density model of Obolon astrobleme was built using the results of 3D seismic survey - structural maps of five reflecting horizons in Jurassic (IIб, ІІв1, ІІв2), Carboniferous (Vb) and upper part of the crystalline basement (VII). Basing on the dependence of rock's density on its porosity, 3D density model of the sedimentary cover was calculated from the 3D cube of porosity by seismic data. In addition, to construct 3D density model with 5 m vertical resolution, well log data from boreholes #5301 and #5302 within Obolon area were used, as well as core data interpretation from borehole #232 of the neighboring field.
|
The constructed 3D density model’s dimensions were 2 km x 2 km laterally and 8 km vertically. The individual cell size of 100 m x 100 m x 5 m resulted in the total of over
6 878 480 cells.
To refine the parameters of 3D property model of Obolon area, 3D joint linear inversion of gravity, seismic and borehole data was performed. Standard deviation between the observed gravity and gravity calculated from the initial 3D density model was 3.61 mGal; between observed gravity and gravity calculated from the final 3D density model was 0.066 mGal. Relative to the gravity field, the initial 3D density model was improved by 55 times. | PUBLICATIONS 1. Ганженко Н.С., Петровський О.П., Анищенко Ю.В., Федченко Т.О. Оболонська астроблема як новий об'єкт для пошуків нафтогазо-перспективних структур. Геоінформатика - Теоретичні та прикладні аспекти XI міжнародна конференція. Електронні тези доповідей – EAGE, 2011. – С. 4. DOI: 10.3997/2214-4609.201402988 2. Fedchenko T.O., Petrovskyy O.P., Anischenko J.V., Zeikan O.J. Obolon Astrobleme - An Application of 3D Geomodelling and Inversion to Hydrocarbon Exploration. 75th EAGE Conference & Exhibition incorporating SPE EUROPEC 2013 Deep Water Sedimentary Systems 10 June 2013 DOI: 10.3997/2214-4609.20130779 3. Fedchenko T.,Petrovskyy O., Anischenko J. 3D geomodel of Obolon Astroblem, Ukraine, as a key for revealing new exploration plays. SEG Technical Program Expanded Abstracts 2013: pp. 1288-1292. doi: 10.1190/segam2013-1272.1 4. Аніщенко Ю.В., Федченко Т.О., Ганженко Н.С., Петровський О.П. Оболонська астроблема: застосування 3D геомоделювання та інверсії з метою вирішення питання нафтогазоносності. Геодинаміка. – В-во Львівської політехніки, 2013. - №2 (15) – С. 81-83.
| 5. Петровский А.П., Анищенко Ю.В., Федченко Т.А., Ганженко Н.С. Оболонская астроблема – интегральная геолого-геофизическая модель нетрадиционных резервуаров как новый объект нефтегазо-поисковых работ. Геофизика, 2015. - № 2. - C. 61-68. 6. Федченко Т.А., Анищенко Ю.В., Ганженко Н.С., Петровский А.П. Глубинное строение Оболонской аст-роблемы по результатам пространственного интегрального сейсмограви-тационного моделирования. Геофизический журнал. – 2015. - №3, Т.37. – С.153 – 162. 7. Ганженко Н.С., Федченко Т.О., Петровський Д.О., Аніщенко Ю.В. Застосування методу Монте-Карло для оцінки ємності порового простору прогнозних нафтогазо-перспективних ділянок Оболонської астроблеми. Нафтогазова галузь України. 2015. №2. С.9-13. ISSN 2409-7500. |
|
