STUDY OF THE CAPABILITIES OF AVO-METHODS FOR THE DETECTION OF HYDROCARBON ACCUMULATIONS Martin Toshev University of Mining and Geology St. Ivan Rilski, 1700 Sofia, E-mail: martin.toshev86@gmail.om ABSTARCT. The AVO-methodology is ased on the study and analysis of the wave field features related to the differing refletane of the individual surfaes in the geologial setion. Several physial parameters of geologial media are used: veloity of ompressional wave, veloity of shear wave, density, aousti impedane. The software is ased on the equation of Zoeppritz. This equation, re-proessed y Aki-Rihards, uses the linear relationship etween the amplitudes of the refleted waves and sin 2, where is the angle of inidene of the seismi wave. The amplitude's hange of refleted seismi waves, depending on the distane from the soure point of seismi energy, has een investigated on real seismi data. The presented study of the AVO-methodology possiilities for the identifiation of oil and gas deposits is ased on the seismi, lithologial and stratigraphi, and geologial and geophysial data from the Galata gas field. In tetoni terms, the area falls on the southern oard of the Varna Monoline, north of the Bliznatsi Fault. The data from a seismi line with south-north diretion and materials from two wells were used. The AVO-anomalies studied are well orrelated with the proven presene of hydroaron aumulations. Keywords: AVO, inidene angle, aousti impedane, anomalies. - - ". ", 1700, E-mail: martin.toshev86@gmail.om. -,. : (Vp), (Vs), (), (.Vp.Vs). Zoeppritz, Aki-Rihards, sin 2,.,. -, - -.,. - -. -. :,,,. Introdution The AVO-methodology is ased on the study and analysis of the wave field harateristis related to the differing refletane of the individual surfaes in the geologial setion. Several physial parameters of geologial media are used: veloity of ompressional wave (Vp), veloity of shear wave (Vs), density (), aousti impedane (p.vp and.vs). The presented study uses professional speialized software. The development of all programs that are used to extrat the listed parameters of seismi data is ased on Zoeppritz (1919) equation, revised y Aki & Rihards (1980), and later simplified y Shuey (1985), wherein is set a linear relationship etween the amplitudes of the refleted waves and sin 2 (where is is the angle of inidene of the seismi wave). The aim of the present study with data from proven hydroaron aumulations is to present the apailities of the AVO-methodology for the estalishment of oil-gas deposits. For this purpose, seismi, lithologial and stratigraphi, and geologial and geophysial materials were used from the area of the Galata gas field. In tetoni terms, the area falls on the southern oard of the Varna Monoline, north of the Bliznatsi Fault (Fig. 1).Data from seismi line TX92-21 in diretion south-north and well log materials of P-1 and P-2 Galata were used. It is interesting to note that the seismi line data used were reorded prior to the first well, disovered the deposits (late 1993 - early 1994) and efore the start of its exploitation. The Galata gas deposit is inserted in the platform aronate sediments of the Paleoene, uilt y shallow algal limestones and in the Upper Cretaeous and Middle Eoene io-lasti limestones. 94
Fig. 1. Strutural map of a seismi horizon attahed to the top surfae of the Paleoene Figure 2 shows a fragment of the seismi setion of line TX92-21 whih rosses sequentially the Bliznatsi Fault from south to north, the area of the gas deposit and the F1 fault, limiting the Galata deposit from the north. Fig. 3. Seismi line TX92-21 / Correlation etween soni log from P-1 Galata well and trae from the line; / Fragment of seismi setion with syntheti trae generated from orrelated P-wave urve of P-1 Galata; / Distriution of orrelation oeffiients for different frequenies of seismi impulse Basi urve P-wave_orr_G2 (Fig. 4). This dependene of veloities of ompression waves from the depth in P-2 Galata well was orrelated with real seismi data from line TX92-21/CDP-417 with orrelation oeffiient 0.784 and frequeny of impulse, with whih is generated 25 Hz syntheti trae; Fig. 2. Seismi setion of a line TX92-21 with orrelated P-wave urves from wells P-1 and P-2 in the Galata area Aross the line are orrelated three seismi horizons attahed to: the upper surfae of Lower Cretaeous sediments (horizon 1), the upper surfaes of the Middle and Upper Eoene (horizons Pg2e2 and Pg2e3). Within the interval CDP- 370 to CDP-515 of seismi line TX92-21 a loal high amplitude anomaly was oserved a right spot. Anomalies of this type are so well-orrelated with the existene of hydroaron aumulations that they are known as diret hydroaron identifiers. Methodology and preparation of data neessary for AVO study The speialized profisional software used inludes a set of programs for analysis of pre-stak seismi data for the purposes of evaluation and modeling of amplitude anomalies depending on offset. For AVO-proessing of the Galata area hosen line TX92-21, the following seismi and well log information was used prepared in terms to e disoverale and mathing the demands of AVO pakage: CDP Gathers reords from seismi line TX92-21; Basi urve P-wave_orr_G1 (Fig. 3). This dependene of veloities of ompression waves from the depth in P-1 Galata well was orrelated with real seismi data from line TX92-21/CDP-505 with orrelation oeffiient 0.795 and frequeny of impulse, with whih is generated 28Hz syntheti trae; Fig. 4. Seismi line TX92-21 / Correlation etween soni log from P-2 Galata well and trae from the line; / Fragment of seismi setion with syntheti trae generated from orrelated P-wave urve of P-2 Galata; / Distriution of orrelation oeffiients for different frequenies of seismi impulse P-1 Galata well - original urve of natural Gamma ray (Fig. 5, - Trak1); P-1 Galata well - original urve of natural Gamma ray (Fig. 5, - Trak1) 95
a AVO-tehnis ased on refletion apaility of seismi oarders in geologial setion; AVO-analysis of rok properties in the setion. Pg 3 Pg 2 e 3 K 2 K 1 -ap-al K 1 r-v N 1 m Pg 3 AVO-modeling of gas saturation AVO-modeling preedes possile AVO area studies aiming at predition of hanges whih would our in the harateristis of the wave field with hange of the fluid filling olletors in the geologial setion. Figure 7 presents AVO-modeling on well data P-1 Galata. The orrelated P-wave_orr_G1 is used at onstant density = 2.22 g/m 3 and porosity = 24% for water saturation 5%, 30% 100%. J 3 J 1-2 Pg 1 Pg 2 e 3 K 2 K 1 Fig. 5. Well log information, used for the urrent study; a/ -1 Galata well; / -2 Galata well) Density urve (Fig. 6,), otained from asi urve through Gardner s transformation; S-wave urve of the veloities of shear waves aording to depth (Fig. 6,), otained through - Krief's - transformation of asi urves; Curve of Poisson's ratio oeffiients (Fig. 6,d); Distriution of refletion oeffiients, alulated from asi urve (Fig. 6,); On the urve otained at the lowest water saturation (5%), there was a signifiant derease of the veloities in the target area (interval with Middle Eoene aged) y aout 17% ompared to the 100% water saturation veloity. This would reate anomalous impedane values approximate 1100 m/s. g/m 3. For eah AVO model in Figure 7 were presented also syntheti reords with a maximum offset of 1996m, as is the maximum offset in line TX92-21, whih signifiantly exeeds inidene angle 30º. The nature of the resulting syntheti reords largely explains the deep Mute applied to standard seismi data proessing in the line. RA 5% Water 30% Water 100% Water RAW 5% Water 30% Water 100% Water a d e Fig. 6. -1 Galata well urves reated y the orrelated P-wave for the purposes of AVO developed methodologies / asi urve P-wave_orr; / density urve from Gardner s transformation of asi urve; / urve of veloities of S-wave from Krief s transformation of asi urve; d/ urve of Poisson s ratio oeffiient; / Distriution of refletion oeffiients alulated of asi urve The last four desried urves are transformations of P- wave_orr_g-1 urve used for P-1 Galata. Analogial set of urves (not presented here) was prepared and used for P-2 Galata well. AVO-tehnology gives the opportunity to perform three group of researhes: AVO-modeling of gas or oil saturation; Fig. 7. AVO-modeling at water saturation 5, 30, 100%, onstant density 2.22g/m³ for target area and porosity 24% (P-1 Galata well) AVO-tehniques ased on the refletivity of seismi oundaries in the geologial setion All theoretial developments (the equations of Zoeppritz, 1919; Aki & Rihards, 1980 et.) whih serve as the asis for applied AVO-tehniques are proven to e valid for inidene angle up to 30º. In order to determine the range of offsets to meet the limitation ondition desried, a gradient analysis was made on a real reord (CDP-470) y seismi line TX92-21 (Fig. 8.a). The same proedure is repeated on a syntheti reord y data from the P-wave_orr_G1 urve from the P-1 Galata well (Fig. 8.). In oth ases it an e seen that the offset satisfying the ondition of the inidene angle does not exeed 30º, is small - no more than 1150 m. This does not allow us to use suh well-funtioning AVO-tehniques ased 96
on the refletane of refletive surfaes in the geologial setion, suh as near and far setions, and angle staks for different inidene angles. porous roks strongly influenes the Vp / Vs ratio, the gradient setion is a good indiator of suh reservoirs. a Fig. 9. Setion of AVO-property Interept (A) Fig. 8. Gradient analysis a/ on real data from CDP-470 on line TX92-21; / on syntheti data from P-wave urve P-1 Galata well AVO-analysis of the properties of the roks in the setion In areas with favorale rok features, it is possile to detet hydroarons diretly - using standard seismi data. However, not all areas have so favorale physial properties on the roks that the seismi amplitudes in the Post-Stak Migrating data provide information on the reservoir quality and the presene of hydroarons. In suh ases, it is useful to apply omplex AVO-tehniques to see how the amplitudes of pre-stak data vary depending on the offset. Here are shown the setions of four AVO-properties of the roks in Galata area: Interept (A) (Fig. 9), Gradient (B) (Fig. 10), Poisson's Ratio (Fig. 12) and Fluid Fator (Fig. 13). AVO-Property Interept (A) and Gradient (B) The interept is the intersetion of the est math line with axis at zero distane soure-reeiver (zero offset) or the zero inidene angle. This is the amplitude of the zero offset diretly related to the refletion oeffiient (Ostrander, 1984). In this sense, the setion of the property Interept (A) represents the variation of the refletion oeffiient of the P-waves on the line indued y the lateral variation of the physial parameters of the geologial media ontating the refleting surfae. For a simple umulative trae, the amplitude value of a ertain time is averaged for the amplitudes of all offsets. Thus, this averaging eliminates the amplitude of information that is arried y eah offset. On the other hand, however, information aout the hange of offset amplitude is used in the alulation of the "interept-trae". In this sense, the intereptsetion an e onsidered as a more informative setion than the onventional one in the sense of amplitude anomalies. The gradient is the slope of the est math line. A setion of AVO-property Gradient (B) desries from CDP-trae to the next CDP-trae, along the line, the refletion oeffiient is hanged aording to the offset. The anomalous values are related to the loal variation of the wave veloities aove and elow the refleting surfae. The large hange in the Vp / Vs ratio auses high gradient values. Sine the presene of gas in Fig. 10. Setion of AVO-property Gradient (B) AVO-Analysis - Crossplot (A / B) Figure 11 present the result of the AVO-analysis Crossplot that implements the interonnetion etween AVO-properties Interept (A) and Gradient (B). Refletions from the upper surfae of water-saturated sediments are plotted around linear dependene (wet trend), passing through the start of the oordinate system. Its slope depends on the average Poisson's oeffiient. Refletions from the upper surfae of hydroaron aumulations are grouped under the wet trend, and refletions from the lower surfae of the reservoir - aove it (Castagna, & Swan, 1997). The two marked areas lue (F) and yellow (E) - orrelate very well with the gas-saturated intervals on the ross-setion shown on Fig. 11,d, where the two wells P-1 and P-2 Galata are loated. Fig. 11. AVO - analysis rossplot A/B a/ Fragment of AVO-property Interept (A) in target zone (line TX92-21); / Fragment of AVO-property Gradient (B) in target zone (line TX92-21); / VO - Crossplot in target zone (line TX92-21); d/ AVO Cross-setion in target zone (line TX92-21) AVO-Property Poisson's Ratio In Figure 12, the AVO-indiator Poisson s Ratio Coeffiients are onsidered for the "right spot" anomaly only. Generally, the veloities of lays and sandstones are not signifiantly different. The presene of fluid in a olletor an ause a differene in the veloities of the P- and S- waves, and hene F E d 97
the anomalous values of the Poisson s Ratio oeffiient (Koefoed, 1955). AVO - Property Fluid Fator Figure 13 shows the AVO-indiator Fluid Fator. The AVOproperty Fluid Fator is defined y Castagna (Castagna, 1993) as a deviation from the alulated on drilling data dependene Vp/Vs for water-saturated layey roks and sandstones. Eoene, ut a signifiant improvement in the apaity harateristis of the olletor is also reorded at the lower part of the Eoene; An important result is otained from the AVO-tehnology AVO-analysis Crossplot, where the upper and lower surfae of the intervals with a sustantial gas-saturation is oserved well on the image of Figure 11,d- Cross-setion The anomalous zones in the tetoni lok, where is loated P-1 Galata well around the CDP-450, mark an area, where a future well would e in a more favorale strutural position and as olletor quality as ompared to the P-1 Galata well position. This is partiularly well illustrated y the setions from AVO-properties: Poisson's Ratio (Fig. 12) and Fluid Fator (Fig. 13). In onlusion, it an e noted that the interpretation of amplitudes of pre-stak data is extremely effetive in reduing the risk when hoosing a loation for a searh or exploitation drilling. Fig. 12. Setion of AVO-property Poisson s Ratio Aknowledgements I would like to express my gratitude to Rexim Seis Ltd. (www.reximseis.om) for the provided software produts and real seismi data with the help of whih the experimental studies were arried out. Fig. 13. Setion of AVO-property Fluid Fator Conlusion The results of all AVO-tehniques give rise to several important onlusions: At eah setion of the AVO-properties, it is notieale that the anomalies appear on a lower noisy akground than the standard setion; The desried amplitude anomaly "right spot" in the standard seismi setion TX92-21 appears to e an anomalous area of the Interept (A) and Gradient (B) setions, in view of the fat that oth are related to variations in the refletion oeffiient; The setions of the AVO-properties show a more detailed division of the gas-saturated interval and an uneven distriution of the roks olletor properties in the target zone, inluding sediments of the following periods: Upper Cretaeous, Paleoene, and Middle Eoene. The gassaturated setion interval in the tetoni lok P-2 Galata well is one attahed to the limestone of the Paleoene and Upper Cretaeous, while in the northern lok (the area of P-1 Galata), the main anomalous zone is attahed to the upper part of the Middle Referenes Aki, K. & P.G. Rihards. Quantitative Seismology. - W.H. Freeman and Co., 1980. - 202-228. Castagna, J. P. Rok physis - the link etween rok properties and AVO response. - In: Offset dependent refletivity theory and pratie of AVO analysis (eds. J. P. Castagna & M. M. Bakus). Investigations in Geophysis, 8, 1993. Soiety of Exploration Geophy-siists. Castagna, J. P. & H. W. Swan. Priniples of AVO. Crossplotting. The Leading Edge, 16, 1997. - 337-42. Koefoed, O. On the effet of Poisson s ratios of rok strata on the refletion oeffiients of plane waves. - Geophysial Prospeting, 3, 1955. - 381-387. Ostrander, W.J. Plane-wave refletion oeffiients for gas sands at nonnormal angles of inidene. - Geophysis, 49, 1984. - 1637-1648. Shuey, R. T. A simplifiation of the Zoeppritz equations. - Geophysis, 50, 1985. - 609-14. Zoeppritz, K. Uer reflexion und durhgang seismisher Wellen durh Unstetigkerlsflashen. Uer Erdeenwellen VIIB. Nahrihten der Koniglihen Gesellshaft der Wissenshaften zu Gottingen. - Math. Phys., K1, 1919. - 57-84. The artile is reviewed y Prof. Dr. Boyko Rangelov and Prof. Stefan Dimovski, DS. 98