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Borehole breakout stress analysis at the Crnac field (Drava depression, Croatia)


Marković, Zoran; Saftić, Bruno; Novak, Jurica; Velić, Josipa
Borehole breakout stress analysis at the Crnac field (Drava depression, Croatia) // Pancardi 2000 - Abstracts / Tomljenović, Bruno ; Balen, Dražen ; Saftić, Bruno (ur.).
Zagreb: Hrvatsko geološko društvo, 2000. str. 78-79 (poster, sažetak, znanstveni)


Naslov
Borehole breakout stress analysis at the Crnac field (Drava depression, Croatia)

Autori
Marković, Zoran ; Saftić, Bruno ; Novak, Jurica ; Velić, Josipa

Vrsta, podvrsta i kategorija rada
Sažeci sa skupova, sažetak, znanstveni

Izvornik
Pancardi 2000 - Abstracts / Tomljenović, Bruno ; Balen, Dražen ; Saftić, Bruno - Zagreb : Hrvatsko geološko društvo, 2000, 78-79

Skup
Pancardi 2000, Pannonian Basin, Carpathian and Dinaride System, Geological Meeting on Dynamics of Ongoing Orogeny

Mjesto i datum
Dubrovnik, Hrvatska, 1-3.10.2000.

Vrsta sudjelovanja
Poster

Vrsta recenzije
Neobjavljeni rad

Sažetak
Crnac oil-field lies in the southern part of the Pannonian basin system, more precisely in the eastern part of the Drava depression in Croatia. The most important structure is an E-W trending asymmetric anticline that plunges in western direction and has a steep northern flank dipping into a reverse fault. This structure is best observed on the Rs7 marker level (Base Pannonian) that is located within the thick section of deep-water marls and lies at depth range of 2400-2550 m. Overlying basin-fill succession of Miocene to Quaternary sediments consists firstly of deep-water marls and limestones that grade up into the turbidite sandstone and shale sequence and finally into deltaic sandstones and various fluviatile deposits. Rocks older than Pannonian are the most important for HC accumulations in this field. There are two separate productive areas, in the western and eastern part of the field that lie along the axis of the anticline mentioned above but their precise location is independent of structural control. This is due to the very complex stratigraphy of the pre-Rs7 sequence, where the 70 metres thick marl unit is underlain by various volcanic and sedimentary rocks – andesitic lavas and tuffs (“Upper Volcanics Unit” – in the western part of the field), pyroclastics mixed with breccias (“Lower Volcanics Unit” – in the eastern part), quartz sandstones mixed with pyroclastics and breccias (“Sandstone Unit”) and of the predominantly dolomitic breccias (“Carbonate Breccias” – in the eastern part of the field). The western oil-pool of the Crnac field lies within the “Upper Volcanics Unit”, while the eastern one produces from the “Carbonate Breccias” that are equivalent to the regionally well developed reservoir rock facies of rockfall breccias. Out of the 24 deep wells in the Crnac field, four reached the basement rocks, composed of mica schist and gneiss. This paper is focused to reconstruct the S_Hmax in the Crnac field area by means of the wellbore breakout analysis. As observed in selected papers (DART & ZOBACK, 1989; JAROSIŃSKI, 1998) the methodology is well-known, but was not applied in Croatia yet. The data were acquired by the DIPLOG or the Oriented 4-Arm Caliper sond in 5 wells and by the CBIL (Circumferential Borehole Imaging Log) in 2 wells. Since the CBIL measurements are still very rare, the emphasis was put on the breakout analysis through the differences in the perpendicular caliper measurements of the DIPLOG. Two computer programs were developed in the INA Naftaplin Co. in order to facilitate this procedure – STRESS that calculates the breakouts from calipers and controls the quality of the calculated breakout and Windows-based BOVision that enables visualization of the breakouts (MARKOVIĆ et al., 2000). The analysis yielded the following results. S_Hmax in the “Upper Volcanics Unit” (main reservoir) is generally NNE-SSW oriented (24-204 degrees to 32-312 degrees) while the marl sections are characterised by dissipation of the stress field which made the stress interpretation unreliable. It was also observed that there seem to exist two main SHmax orientations in the sediments above the Rs7 marker level (16-196 degrees and 150-330 derees). Although this measurements represent the local stress field, obtained orientation is found to be generally conformable with the regional orientation of SHmax (PRELOGOVIĆ et al., 1998). Conclusion is drawn that such analyses can be extended to numerous other wells that have 4-arm dipmeter logs in the southern part of the Pannonian basin system in order to attempt the reconstruction of the contemporary stress field distortion. DART, R.L. & ZOBACK, M.L. (1989): Wellbore breakout stress analysis within the Central and Eastern continental United states.- The Log Analyst, Jan.-Feb. 1989, 12-25. JAROSIŃSKI, M. (1998): Contemporary stress field distortion in the Polish part of the Western Outer Carpathians and their basement.- Tectonophysics, 297, 91-119. MARKOVIĆ, Z., NOVAK, J. & JUMIĆ, Z. (2000): Stress evaluating by breakout analysis at Crnac field.- Petrol. Engin. Summer School, Workshop 6 – Rational Development of Marginal and Mature Oil and Gas Reservoirs (Syllabus), Dubrovnik, June 5-9, 2000. PRELOGOVIĆ, E., SAFTIĆ, B., KUK, V., VELIĆ, J., DRAGAŠ, M. & LUČIĆ, D. (1998): Tectonic activity in the Croatian part of the Pannonian basin. Tectonophysics, 297, 283-293.

Izvorni jezik
Engleski

Znanstvena područja
Geologija



POVEZANOST RADA


Projekt / tema
195018

Ustanove
Rudarsko-geološko-naftni fakultet, Zagreb