GEOL. CROAT. 49/2 1227 -230 I 4 Figs. I I I ZAGREB 1996 Scienfi/i'c: note Seismic Line Calibration - A Main Reference for the Rational Exploration and Development of Very Complex Oil and Gas Fields and Reservoirs (Bizovacki Cret Case Study) PROCEEDINGS Slobodan GACESA' and Ivan MESIC' Key words: Seismic line calibration, Synthetic seismo­ gram, Synthetic attributes, Key - seismic line. Abstract Comparison of WeJl - J log data, and Seismic Line - I reprocess­ ing with synthetic seismogram parameters, rcs ull s in a high level of correlation making possible Ihe precise location of the Well-Ion par­ ticular seismic trace or Seismic Line - 1. Results indicate that slich Key seismic line (a reference) may be use ful as an entry for three-dimensional (3~) seismic surveys. The application of suc h procedures in complex reservoirs as the 13izovac field area (characterised by fracture porosity) , wi ll minimise 1l1e ri sk in dri lling a horizontal, re-entry well (to determine the pro­ duction potel11ia! of the area around the We !!~!), and drilling a second well on the Nonhcasl side of Bizovacki Cret Structure. 1.lNTROOUCTlON The 13izovacki Cre! locali ty was discovered during the development of the Bizovac oil field after classical interpretat ion of two- dimens ional (2D) seismic data. Tn the first outpost well, located 2.6 km from the nearest producing well and 1.2 km from a dry well , o il was discovered under the Upper Pannonian unconfor­ mity approximately 300 m deeper than the Bizovac oil field (Fig. I ). Drillstem test (DST) carried ou t in the lap of the reservoir resulted in 6 m3jday of oil, proving the oil prospects of this area. The top of the reservoir was de termined 50 J11 deeper than predicted. In a vertical sequence some of lithological sctt ings were determined analogous to the known lithology of Bizovac field (basement gneiss breccia, basalt, diabase, tufrite, Upper Cretaceous mudstone, 13adennian siltstones, breccia - GACESA & MESIC. 1994). The reservoir rocks benc- Kljucne rijeci: kalibriranje scizmickog profila, sintets­ ki seizmogram, seizm icki atributi, kljucni seizmicki profil - etalon. Saietak Poslupcirna priblizavanja paramctara sintctskog scizmogram_I 'i-li! ~~~" ..... Com'll-u~l~d ~y s. Gd("'" , Compu te,i r)V ~L )\" ',., Fig. I Upper Panllonian unconformity map. 2. PROCESSING AND INTERPRET ATION The only possibility at our disposal was to make morc detailed analyses of lhe seismic data, applying special seismic processing methods and their combina­ tions. Applying (he velocity data from the Bizovac oil field resulted in a 50 m error when the entry point in the assumed reservoir top (target) was predicted. Therefore, new velocity surveys and Vertical Seismic Profiling (VSP) measurements were performed in Well-I. The results of VSP measurements were used to check the seismic velocity survey and to compare it with a syn­ thetic seismogram. Sonic logging was carried out from 350 m and den­ sity log [rom 1009 m down to the total depth. For the missing intervals the logs from onc well of the Bizovac field well were also used in order to create the synthetic logs. W c!J-J Iml T '"' w J '=, " " ::I M, K2'> ,en 0 !d~ -21OC m~ ~~ "'" , Geologia Croatic;] 49/2 Transfer of thc identified well data to the seismic line was performed as follows: 1. Checking the seismic velocity surveys - Only five shot points out of 12 shot points used in the velocity survey were suitable for thc time/depth conversion, while the remaining points would have caused too large distortion on the sonic log curve. 2. Synthetic seismogram - Implementation of density log of another well was proved to be belle I' than the application of the computed "Gardener's" density log. The synthetic seismogram was generated by applying various filters. Filtering with 20 Hz filters (50% matching) provided the best matching along the entire wellbore length with the existing migrated seismic line. 3. Seismic reprocessing - The seismic linc reprocessing was done with a minimum phase. Matching of the reproccssed line and the synthetic seismogram was beUer, but still not satisfactory. 4. Intervention by applying the Ricker wavelet - The 20 Hz Ricker wavelet was used [or final intervention on thc synthetic seismogram. 5. Sliding the synthetic seismogram - The well is locat­ ed approximately 50 m Cram the seismic profile trace. The best matching of the synthetic seismogram and seismic line was achieved by sliding the synthct­ ic seismogram from 132 elF on seismic line to 142 elF. The comparison with the VSP tracc also indicat­ ed that the results were very good (Fig. 3). 6. The analyses of seismic attributes - The following seismic attributes were analysed: phases, amplitudes and frequencics. Along the seismic section only amplitude changes have been observed. The ampli­ tudes are highcr (in range from 4500 - 6000) in the central part of the paleouplift, while on both flanks they arc lower (1500 0 4500) (Fig. 4). Fig. 2 Geological cross- scc(ion of (he Bizovac field and Bizovacki erc( location. Gacesa & Mesic: Seismic Line Calibration - II Main Reference for the Rational Exploration and ... 229 The amplitudes of the cap rock (shale) are the same along the pa1aeoup!ift. Higher amplitudes of the reservoir lithology cou ld be an indicator of the less fractured reservoir rocks . The decreasing velocity and density in highly fractured reservoirs results in decreasing amplitudc values. Am plitudes analyses alone still can not be used to distinguish good reservoir areas from non­ reservoir, or poor reservoir rocks. Further improve­ ment will occur when analysing the amplitude versus offset (A VO) and by analyses of acoustic impedance along Seismic Line - I. 3. CONCLUSION AND RECOMMENDATION Analysis of a "key" seismic line previously deter­ mined with syntheti c seismogram makes possible more (2T) 1000 1100 1200 1300 tiJ 1400 Cl 0:: 1500 f5 1600 1700 1600 Fig. 3 Synthetic .sei.srnogram procedure on the Bizovacki Cre! location. precise and reliable determination of the top of the reservoir and allows the transfer of the well "proper­ ties" to the seismic line. In this case it is possible to state that south-west of the Well -1 and on the north-east side of the Bizovacki Cret paJaeouplift, there are morc favourable reservoir rocks than in the central part of the paleoup!ift. 'fhe results of this procedure can be summarised in the fo llowing: better understanding the petroleum geological model of Bizovacki Cret locality; better planning and con trolling of the slim hole (re­ entry) drilling project; better matching of all the available seismic lines in the prospect area with Seismic Line - 1. This procedure alone is still insufficient to locate a new well with a high degree of certainty. Using slleh ·6000 "500 Fig. 4 In terpretation of magnified amplitude section of Seismic Line - I. 230 procedures in complex reservoirs (characte rised by fracture porosity), will be very useful in planning the three-dimens ional (3~) seismic surveys as a basis for locating other wells, and for determining the develop­ mcnt plan with optimally spaced wells. Acknowledgement The aut hors would li ke to thank to INA dd., Nana­ plin for permission to publish this paper. Thanks are also offered to colleagues Marija Vidovic and Z. Budanovic for the special seismic processing, and to M. Saler, Z. Durie , B. N iko li c and N. Filipovic for their contribution in preparing this article by using various PC-software. Geologia Croatica 49/2 4. REFERENCE GACESA. S. & MESIC, I. (1994): Usc of fundamental ambient analyses in the field - reservoir develop­ ment by mean s of optimal VSP survey geometry planing.- Nana, 45/2, 123- 127, Zagreb.