GEOL . CROAT. 49/2 283 - 288 5 Figs. I I I ZAGREB 1996 I Scientific paper Dipmeter Interpretation - More Accurate Structural Definition of the Ponti an Sandstones of Lipovljani and Jamarice Jurica NOV AK ', .Josipa VELIC', Bozidar JANCIKOVIC' and Bruno SAFTIC' PROCEEDINGS Key words: Dipmclcr inte rpre tation, Descriptive stat is­ ti cs, Orientation of fauil s, Fault characterisation, Pontian sandstones, Lipovljani, Jamarice, Sava depression, Croatia. Abstract Two pairs of wel ls have been chosen from the L ipovljani and Jamaricc oil fields in lhe Sava depression il1111e south-west part of the Pannonian basin. Both we ll s from each field have a measu red E-log, and one al so has a high-quality diplog. T he reservo ir rocks afC repre­ sent ed by folded and faulted Pontian sandstones . A combination of grap hical int erpre tation and SCAT (Sta ti stical Curvature Analy sis Tec hniqu cs) has allowcd identification of both structural and fault plane dips . Defining the position of fault plancs cnabled thc charac­ terisation of normal and reverse faults, incli ncd in thc direc lion of stmctural dip (or against il) , and positioned either longi tudinal , trans­ verse or d iagonal to the st ruclllre. The dcscribcd tcchnique can bc ful­ ly utilised in the crea tion of a gco logical modcl aimcd to finding the bypassed oil. I, INTRODUCTION From th e late seven ties to th e early eighti es INA , the Croatian national oi l-company purchased logging equ ipment from "Western Atlas", including the four­ pad high -reso lution dipmeter. In addition to giving an ana logue output recorded on film , this equipment is capable of digital recordin g of data, and thus enables computer data-process ing. Dipilleter measurements were taken in several well s of Li povljan i and Jamarice fie ld s. The physical and lithological characte ri sti cs of the Pontian sandstones, reservoir rocks in the aforementioned fi elds, resulted in good quality data with very c losely spaced dip mea­ surements. Thi s suggested a mcthod for thc recognition and more accurate location of faults , and the ir orienta­ tion and influence on the gcomctry of rescrvoir rocks. Two Iypical we ll s from each field have bcen chosen to illustratc the mc thod. The results of various statistical curvature analysis techniques arc given, and show the success ful defin ition of structural dips (dips of bedding I lNA -Naftap lin. Subiceva 29, HR- t 0000 Zagrcb, Cro;Jl ia. Kljucne rijeci: interpre tacija dipmetra, deskriptivna statistika, orijentacija rasjeda, karakterizacija rasje­ cia, pontski pjescenjaci, Lipovljani, Jamarice, savska depresija, f-Irvatska. Sazetak S naftnih potja Lipov lj ani i Jam aric e iz savs ke depresije u jugozapadnom dijclu panonskog bazena odabranc s u po dvij c bU SOlinc s c leklrokarOlazn im dijagramima od kojih po jedna ima jos i vrlo kva litetan diplog. Ovdje su kolektori, pontsk i pjescenjaci, bor-ani i rasjedani. Kompteksnom interpre tac ijom - grafickom i statistickom (SCAT) , razlu ce ni Sll nag ibi sloj eva od nagiba paraklaza. Ulv rd eni cleme nti orijentacije rasjeda omogucili su njihovu karakt e rizacij u na normalne iii reverznc, na prolusmjernc ii i iS losmjernc, te na uzcluznc, poprecne iii dijagonalne. Tako dobivena !j esenja nezaob ilazna su pri kreiranju geolos kog modet a s ciljem pronalazenja 7.aostalc !lafte. planes), and their separation from differently spaced planes caused by tectonic deformation - fa ult planes presented by e ither simple surfaces or fault zones. The oil fields of Lipovljani and Jamaricc arc located some 90 km east of Zagreb (Fig. Ib), in the south­ western part of the Pannonian basin, in the Sava depres­ sion. In J 932 the first oil and gas ex plorati ons took place there . Uplifted structures were defined by seismic reflection lines in 1959. The drilling on Lipovljani com­ mcnced in 1960, and on .Tamaricc in 1964. In accor­ dance with regional lithostratigraphic subdivi sion the productive hori7.on s were named Janj a Lipa, Kozarica and Bujavica sandstones (Figs. la, 4a), which chronos­ tratigraphi ca lly fairly closcly rcspond to the Ponti an deposits. The observable signs of fauliing in the menti oned fie lds had prcviollsly drawn thc attention o f explo­ rationists during the earl y 1970 's. This was then docll­ mented by a detailed study of conventional E-Iogs, and the cases where the same layers were be ing repeatedly drilled th rough in one wcll, or an apparent hiatus in a 2 University of Zagreb, Facul!y of Mining , Geology & Petroleum Engi neering , Pi erotlijeva 6, P.O. Box 186, HR -I 0000 Zagrcb, Croatia. 284 Lip-63 lip-123 well nearby were explained by tectonics (TUFEKCIC, 1970; HERNITZ & SIMON, 1975; JANCIK OYIC, 1977)_ 2. RESULTS OF DIPMETER ANALYSIS The dip log of well Li p-123 has been chosen from the field Li pov ljani together with E-log of Lip-63 fo r correlalion purposes . The depth interval from 1000 to 1350 m is shown (Fig. La). Correlation of log-measure­ ments (NI6, N64, SP) from these closely spaced wells depicts substantial differences in the lithological COtn- LOCAT ION MAP l . b . seA l E ~oom. LlPOVLJANI Up.-123 R, POLAR PLOT AZIMUTH DIP 270 1-----------­ . 1000-1063 . ' 1064-1080 . 1081-115~ l.c. DEPTH INTERVAL m LlPOVLJANI Lip-123 R2 POLAR PLOT AZIMUTH DIP 270 1_ 1230-1281 . 1282-1312. 1312-1360 I 1.d. DEPTI-IINTERVAL (m) LEGEND BUJAVICA SANDSTONE - KOZARICA SANDSTONE - JANJA LlPA SANDSTONE - REPEAT SECTION R-- FAULT Geologia Croalica 49/2 Fig. I a) Correl31ion scheme Lip- 63 Wel! - Lip-123 Well; b) location map: c) Lip- 123 Well R, Azimuth Dip Plol: d) Lip- 123 Well R2 Azimuth Dip Plot. position and thickness of individual sandstone bodies. It is, [or example, not iceable that the Bujavica sandstone drilled by Lip-63 well was totally absent in Lip-123_ In contras t, the Janja Lipa sandstone is some 20 m thi nner in Lip-63 than Li p-123. The dip plot of Lip-123 clearly shows a change of dip angle and azimuth in two pos i­ tions (Fig. I a) . On the polar plots o[ az imuth and dip o[ planar eiemems from chosen intervals (Figs. 1 c and I d) a concentrati on of patterns is apparent, illus trating the structural dip in the hanging wall and in the footwall, together wi th the orientation of the fau lt plane itself. This way of dip plotting doesn '1 show the depth of indi­ vi dual di ps, which is possibly confusing to less experi- Novak. Velie. Jaocikovic & Saftic : Dipmeler Inlerprel;llion - More Aeeur:lIe SlruClllr;t1 Definition ... 285 ,",~"'H ,., ,~" ,"~ 23 SCAT and Stick Plot IIIP ,'~ Ill""',. enced analysts. The "azimuth vs. dip" cross-plot allows singular dips to be traced to their precise depth so that their "origin" can be controlled. Generall y speaking, in th is way patterns analysed with the help of" a cross-plol will show a Gauss normal distribution , whereby dips from one side of the [aull or another are put into one or more stochastic groups. In the case of Lip- 123, not only a ve rt ical but al so a horizontal di splacement can be deduced from the differences in the dip azimuth (Fig. 2). Furthe011ore, it is only possible by cross-plot to pre­ cisely define the position of the transverse and longitu­ di nal plane, marked by st ruclUral d ip and well axis. A series of related c roSS-pIOIS is shown on Fig. 2. These arc: Mercator projection of azimuth vs. depth, "dip vs. dept h", the transverse and longitudinal dip component vs. depth and the two most characteristic perpendicular sti ck plot s vs. depth . Thorough analysis of the cross­ plot s allowed ca lculation of the thi ckness of the deformed inlervals and, more importantly, the dip and " I~ I'I'I VI l W ..... , ~'s I.':;" '" va" I fIQn "oS Fig. 2 Lip - 123 Well R, SCAT and Slick Plm. azi muth of fault planes. ]n Lip ~ 123 two faults are docu­ mented - R, with a fault plane 125/50 on 1070 m and R, positioned 190/60 on 1298 m. The first fault is younger, nonnal , inclined against the structu ral dip and diagonal to the structu re (Fig. 3), the o ther is o lde r, probably reverse, inclined in the same direction as the st ructural dip but steeper than bedding dip values. Well -log correlation be tween 1am-74 and Jam- 160 showed marked differences in sandstone thickness (Fig. 4a). The dip plot of Jam - 160 shows well-marked di s­ continuities of both dip and azimuth al deplhs corre­ sponding 10 the pos iti ons of reduced sandstone thick­ ness as defi ned by E-Iog correlation with Jam-74. These discontinuities are also observable on the Mercator pro­ jection of azimuth vs. depth, as well as on the "d ip vs. depth" plot (Fig. 5). It is al so noticeable that both the transverse and longitudinal dip component have similar di scontinuities at the same depths. The patterns at J203 and 1250 m have an inclination opposite to the disconti - CROSS SECTION Lip-63 - Lip-123 lip-63 lip-1 23 D SUJAVICA SANDSTONE I ?1NORMAl »" fAULT IiiiiiiiiiiKOlARICA _ SANDSTONE t .:-.: IREVERSE -'::-. FAULT Fig_ 3 Cross sec Lion Lip-63 Well - Lip-123 Well. ~JANJALIPA !L..±2:lISANDSTONE ~REPEAT _ SECTION nuities located at 1280. 1308 and 1355 m respectively. This is due to the spatial position of faults that arc appropriately shown in the chosen sections . Figures 4b, c, d, e and f show azimuth vs. dip cross-plots from Jam- 160. The positions of fault planes are as fol1ows: R] with 220/55 at 1203111, R2 with 290/45 at 1250 m, R3 with 145/50 at 1280 m, R4 with 75/45 at 1308 m and R, with 90/65 at 1355 m. Based on the relationship between the structural dip and orientat ion of fault planes, faults R] and R2 arc diagonal with fault planes inclined in the same direction as the structural dip, R3 and R" are diagonal with inclination again st the struc­ tural dip, while rault Rs is longitudinal to the structure and inclined against the structural dip. All the faulls have 110rmal displacement. These solutions are essential in finding the so-called "bypassed oil", especially in the phase of building a complex geological model (BASSIOUNI & YELIC, 1996; PRELOGOYIC et aI., 1995) in order to explore for oil and gas morc economi­ cally. 3. CONCLUSION The interpretation of very dense dipmeter data requires certain skill and experience. Only by utilisation of a number of graphical and statistical techniques can satisfactory and unambiguous results be obtained. Examples of two pairs of wells from the Lipovljani and Jamarice fields, with intensively rolded and faulted sandstone reservoirs, are uscd for Ihe illustration of how structural dips can be discerned from the fault planes; how the fault plane OrieJ1lalion can be defined; and finally how the character of a fault can be deduccd. Geologia Cwatica 49(2 4. REFERENCES BASSIOUNI, Z. & YELIC, J. (1996): Prospecting for Bypassed Oil and Gas.- Geol. Croat., 49/2, Proceed­ ings of I st Internat ional Symposium of Petroleum Geology, 1.97-202. HERNlTZ, Z. & SIMON, J. (1975): Rasjedi i izrada karata debljina naslaga (Faults and Preparalion of Isopach Maps). - Zbornik radova Rud arsko-gco lo­ sko-naftnog fakulteta Sveucilista u Zagrcbu u povo­ du 35 god. rada, 77 -85, Zagreb. JANCIKOYIC , B. (1977): Principi mjerenja pad a sloje­ va u busotinama i mogucnost definiranja prostorne orijcntacije rasjeda u neogenim slojevima jugoza­ padnog dijcJa Panonskog bazena (Principles of Dip Logging and the Possibility of Determining the Spa­ tial Orientation of Faults in Neogenc Deposits in the South-western Part of the Pannonian Basin).­ Zborn ik radova Ill. godiSnjeg skupa Znanstvcnog savjeta za naftu Jugoslavenske akademije znanosli i umjetnosti, 171-207, Novi Sad . PRELOGOVIC, E., JAMICIC, D., ALJINOYIC, B., YELlC, .T., SAFTIC, B. & DRAGAS, M. (1995): Dinamika nastanka struktura juznog dijela Pan on­ skog bazena.- In: YLAHOYIC, I., YELIC, I. & SPARICA, M. (cds.): First Croatian Geological Congress, Proceedings, 2,481-486, Zagreb. TUFEKCIC, D. (1970): 0 revcrsnom rasjedanju struk­ lure Lipovljani (Compression Type of Reverse Faulting in Lipovljani Structure).- Nafta, 21/3, 99- 104, Zagreb. Nova\... Velie. J:lIlc ikovic &. Sanie: Diplllclcr Imcrprc[:l[ion - More Accurate Stnlcturnl Derini[ ion .. 287 Jom-74 ~ Jom- l bO SP CJ c::J o o '00 N[t. 20- N64 20.2 o SP '00 o o LEGEND Nl6 20 "" '" 0 90 BUJAVtCA SANDSTONE Cd KOZARICA 2 SANDSTONE R fAun KOZARICA 1 SANDSTONE G:;gJ JANJA L1PA SANDSTONE - - - ---------------------------- ' . b 4 .c . 4 . d . " . 4 ,I. · · " • • .. • • .. '. • • " • ~I .. '. JAMARICE Jam-160 AZIMUTH VII DIP CROSS PLOT ~ • O IP A1:IMtHIi • "oo- ,,~~ . 11~'W3 . ':1(><-12<01 OlP T" I'H E RVAL .,- JAMARICE Jam·160 AZIMUTH ys DIP CROSS PLOT R, I . A to '80 ,1tI DIP AI IM U TIi • ' 2, (l.'2« 1 • ,2<2· ,250 . ' 25'·'~ OEPTH INTER VAL 1tI'} JAMARICE Jam -t OO AZIMUTH V5 DIP CROSS PLOT R, 'I ,.;--~=;-~.~ D IP A7. IMIHH • 1~127e • 1271-12S , . '2B2· , :lQ2 DEPTIi INTERVAL (m· JAMARICE JAM-t60 AZIMUTH VII, DIP CROSS PLOT R, ~l .. '. J .. • • " • • .• D IP AIIIIIUTII JAMARICE Jam-160 AZIMUTH VII. DIP CROSS PLOT R. · . .. : .. -'0 .­ '. • '" DIP AZ IMUT H D EP TH INTERVAL (.,1 Fi g. 4 a) Corre lation scheme Jam-74 We ll - Jarn-160 Well: b) Jam- 160 Wel l R I azimut h vs. dip cross plol: c) Jam-l 60 Well R2 az imuth vs. dip Cross Plot: eI) Jam-160 We ll RJ