Detailed gravity, magnetics successful in exploring Azerbaijan onshore areas

The Kura megasynclinorium, having complex heterogeneous geological structure, includes the Lower Kura depression and the large part of the Middle Kura depression (Fig. 1) which join at the zone of the Saatly-Kurdamir gravity maximum.⁸ The thickness of the sedimentary complex in the Middle Kura depression is estimated as 12-14 km.

The Mesozoic complex surface occurs in the axis zone of the Evlakh-Agdzhabedy depression at a depth of 7-8 km, in the side zones of this depression at a depth of 3-4 km, and in Pre-Lesser Caucasian zone at a depth of 500-700 m. Deep wells discovered deposits in the Middle Jurassic (in the area of the Soviet Saatly SD-1 superdeep borehole to Quaternary, with falling separate stratigraphic units from the section.

The thickness of sedimentary deposits in the Lower Kura depression reaches 20 km. In the central part of this depression the surface of Mesozoic deposits occurs at a depth of 10-12 km and in the depression sides arises at 3 km. Because of the large depth of occurrence the Mesozoic complex is not studied in detail. The geological section of the depression composes deposits from the PS to Quaternary.

In the Middle Kura depression, buried magmatic volcanoes, and in the Lower Kura Depression and in the Absheron buried and active mud volcanoes, are developed.

Seismic and gravity investigations in the Kura depression and Precaspian-Kuba hydrocarbon regions identified numerous highs and fringes; in some of these structures the presence of oil and gas was detected. In the Precaspian-Kuba zone, deep drilling identified steeply dipping (75-85°) horizons of Jurassic, Cretaceous, and Tertiary deposits.

The Kura megasynclinorium, Absheron Peninsula, and adjacent water area, as well the Precaspian-Kuba region of Azerbaijan mainly are characterized by a negative gravity field of the first order.¹⁶

Recognized maxima and minima of the second order are associated with large buried structures and independent tectonic blocks of the earth's crust.

In this region are selected Yalamin, Gusar-Hachmas, Dibrar, Yavandag-Sangachaly, Navaga, and Baky archipelagos, Saatly-Kurdamir, Pre-Talysh, Geokchai-Mingechavir and Eldar gravity maxima, and Hudat, North-Absheron, Lower Kura, Alazan-Agrichai, Evlakh-Agdzhabedy, and Chatma minima. These anomalies are mainly associated with uplift and lowering-in of the crystalline basement.¹⁷

Within the identified second order gravity anomalies dozens of higher-order local anomalies were detected. These local anomalies are caused by local structural elements and oil and gas factors.¹⁸

Gravity anomalies of the hydrocarbon areas of onshore Azerbaijan are characterized by intensive and less expressed gravity gradients that bound positive and negative difference gravity field Δg^B8-20. Between the gravity gradients zones of positive and negative field Δg_B8-20 are located where relative positive maxima and minima were revealed.₁₃

In the Precaspian-Kuba region, Absheron and Lower Kura depression low-intensive magnetic minima, and sometimes magnetic maxima, are observed. The magnetic field pattern in the Middle Kura depression is manifold. In its southeastern part a large and intensive Zardob-Muradkhanly magnetic maximum with intensity of 700-750 nanoTesla is observed. In the periphery of this maximum less intensive local magnetic maxima and sometimes minima were delineated.¹³

In this region is established a relationship of hydrocarbon deposits occurring with sedimentary, volcanogenic, and volcanogenic-sedimentary associations: Upper Jurassic-Eocene-Miocene in the Middle Kura and Precaspian-Kuba regions; PS and Akchagyl-Absheron in the Lower Kura depression and Absheron zone.

Upper Jurassic deposits in the Middle Kura depression have the most excess density: 300-350 kg/cu m and sometimes greater.⁸ Volcanogenic rocks are characterized here by high magnetization: (800 ÷ 4,000)∙10^–5 SI, whereas host sedimentary rocks are practically nonmagnetic. The volcanogenic rocks are characterized also by a high density: 2,550-2,840 kg/cu m. Their density is decreased only in the upper part of effusives in (100-150 m) up to 2,200-2,240 kg/cu m.¹⁹ At the same time their density contrast consists of +(250-300 kg/cu m).

In the Lower Kura depression and in Absheron by core data one petrodensity boundary with excess density of 100-130 kg/cu m (relating to the PS-Balakhan floor) was identified. In the section of meganticlinorium of the Greater Caucasus, together with the Kusary-Divichi depression in the Precaspian-Kuba region, several petrodensity floors were delineated. The most density contrasts on Paleogene-Neogene (±280 kg/cu m), and Cretaceous (±250 kg/cu m) deposits were determined. This zone is mainly characterized by low values of magnetization (≈70∙10^–5 SI).³

It was believed formerly that in the Kura depression (megasynclinorium) separating meganticlinoriums of the Greater and the Lesser Caucasus (Fig. 1), thick sedimentary rocks deposited on the crystalline Pre-Alpine basement, and these megastructures were separated by subvertical deep faults.

On the buried uplift of the basement (as assumed earlier on the basis of high velocities of elastic waves and rock densities) a Saatly superdeep borehole (SD-1) was designed in 1965.⁸ However, analysis of magnetic properties of rocks led to the conclusion that the basement was not magnetized while a large part of a geological section of the Middle Kura depression consisted of the Mesozoic magmatic associations of basic and intermediate composition with high magnetization.

The above-mentioned mainly Jurassic associations are widely distributed in the northeastern part of the Lesser Caucasus. The associations have a deep-seated underthrust of a gentle slope under thick sand-shale series of the Jurassic of the Greater Caucasus. These results were obtained by a 3D combined gravity-magnetic modeling in the region.^{8 13} The validity of the interpretation was fully confirmed by the results of SD-1 drilling. The borehole exposed Mesozoic volcanogenic rocks at the depth of 3.6 km and remained within these rocks at least down to 8.2 km (the bottom of the well).

Numerous deep wells have been drilled in this area. They confirm the developed model of a deep structure of the Kura depression and occurrence of hidden stocks of the Mesozoic magmatic rocks (Fig. 2). Seismic and gravimetric surveys revealed Mesozoic magmatic and carbonaceous rocks with high velocities of elastic waves and densities under the Cenozoic terrigenous cover.

At the same time, magnetic survey enable the Mesozoic section to be classified into magmatic (magnetic) and carbonaceous (nonmagnetic) complexes according to their composition (Fig. 2). As a result, oil and gas traps of previously unknown type have been revealed in zones of carbonaceous rock pinchout near the stocks of igneous rocks and in eroded roofs of these stocks.

Application of gravity-magnetic methods

Accomplished numerous petrophysical and geophysical materials testify that oil-and-gas deposits are distinct by physical parameters from the host media (including water-bearing reservoirs).

Within an oil and gas deposit is observed a decrease of density on 100-250 kg/cu m, decrease of magnetization on 2-8 times, and temperature increase on 10-18%.^{2 3 7 20}

We believe that the main physical-geological precursors for application of gravity and magnetic methods for hydrocarbon deposit prognosis are foremost decreasing of density and magnetization around the deposit.

Practical analysis performed in the countries of the former USSR^{2 5} indicate that density contrast within the HD occurring may consist –(70 ÷ 200) kg/cu m. This density contrast may provide appearing negative anomalies in the integral gravity field, the value of which is estimated as 0.1-2.0 mGal (0.1-2.0∙10^–5 m/s²), depending on the deposit's thickness and consistency.²

In a hydrocarbon deposit section by magnetic properties, four elements characterized by different magnetization are selected: hydrocarbon deposit and host reservoir, reduction zone, zone of subvertical inhomogeneties, and oxidation zone.^{6 21} Distribution of the main iron-containing minerals (magnetite, maghemite, hematite, etc.) in the hydrocarbon deposit sections results in a reflection in the observed magnetic field. It is known that the presence of secondary magnetite produces appearing saw-shaped anomalies.^{4 6 21} Besides this, over hydrocarbon deposits might be observed relative magnetic minima with amplitude up to several tens of nanoTesla.²²

Analysis of density characteristics in the Kura depression displays decreasing of rock density within HD up to 150-170 kg/cu m. Gravity field modeling indicates that gravity anomaly from the oil deposit may consist 0.15-0.25 mGal depending on its vertical thickness (mainly Δh < 50 m). Magnetization decreases in several times not only with hydrocarbon deposits but in overlying deposits above them.²⁰

Gravity-magnetic surveys performed in the Kura depression testify that over the known deposits (Muradkhanly, Jafarly, Tarsdallyar, Gazanbulag, Babazanan, Byandovan, etc.) gravity and magnetic minima of –(0.2 + 0.8) mGal and 20-30 nT were observed.^{20 23} Therefore, integrating these methods increases an effectiveness of oil and gas deposit prognosis (theoretical basis of the integration preferences on example of Azerbaijan were given in Khesin and Eppelbaum⁹).

Physical characteristics of subvertical zone

Many investigators note that over hydrocarbon deposits exist some distinctive zones.^{2 3 6 10 22}

Drilling results also discovered areoles of hydrocarbon invasion over the oil and gas deposits. Interestingly that geochemical analyses indicate that such zones are traced up to the earth's surface.

Obviously that by physical characteristics this zone must differ from the host media. However, quantitative estimations of physical parameters of a subvertical zone over hydrocarbon deposits were not performed.

Quantitative estimation of changing of accomplished data^{7 20} of magnetization, density, and temperature was applied for development of detailed physical-geological model of the Muradkhanly deposit (Fig. 3). Geometry, stratigraphy, lithology, and location¹⁷ in the model were taken from the deep well cores.¹⁵ Data about density, porosity, and magnetization were utilized from the works.^{19 24} Density contrast within the subvertical zone over hydrocarbon deposits was calculated by the use of developed approach.⁷

As it was shown in Fig. 3, physical parameters of the zone over hydrocarbon deposits¹⁶ strongly differ from the surrounding medium.^{18 20} With increasing of depth increase contrast parameters of magnetization (up to 200∙10^–5 SI) and temperature (up to 17° C.), and decrease density contrast (up to 3.6 kg/cu m) in sedimentary deposits in the hydrocarbon deposit zone. Analysis of selected cores also shows essential decreasing of magnetization in effusive associations in the structure arch in the vicinity of the deposit.

Computed gravity and magnetic anomalies (curves 6 and 11, Fig. 3) from subvertical zone (16) consist of ≈0.35 mGal ∏∙ ≈35 nT. Comparison of theoretical curves of gravity and magnetic fields (graphs 1 and 9) with observed curves (graphs 2 and 10) indicates that they have a similar form. Both in observed and theoretical graphs relative decreasing of gravity and magnetic fields over the HD is shown. Retrieved maxima on gradient zone changing (4) clearly trace local gravity and magnetic minima.

We suggest that just migration of hydrocarbon light fraction to overlying deposits (layers) influences the physical properties of rocks occurring over hydrocarbon deposits. At the same time it is necessary to note that secondary factors are studied insufficiently.

Integrated effect of the aforementioned factors (both from HD pool and from zone overlying it) stipulates the anomalous changing of gravity and magnetic fields observed over HD. The observed precise gravity and magnetic data contain local anomalies reflecting presence of HD in the geological section.

Hydrocarbon signatures

Experimental investigations performed in different regions of the world have detected some peculiarities of hydrocarbon deposit reflection in gravity and magnetic fields.^{5 7 10-12 22 25}

For revealing weak local gravity and magnetic anomalies associated with hydrocarbon deposit various potential geophysical field transformations are applied: second and third derivatives of gravity and magnetic potential, downward analytical continuation, total normalized gradient, etc.^{1-3 8} However, despite the great available arsenal of transformation methodologies, many authors testify to ambiguity and inapplicability of the conventional transformation methods for revealing local gravity minima associated with hydrocarbon deposits.^{2 5}

Third derivatives of gravity potential Wzzz were computed along a few dozen profiles crossing deep boreholes in the Muradkhanly area of the Middle Kura depression. It was detected that minima of W^zzz were registered both in intervals where oil and gas factors are known and in intervals where hydrocarbons were not detected. These data testify to the ambiguity of W_zzz data analysis. Some indeterminacy of employment of the gravity downward analytical continuation and total normalized gradient for searching oil and gas signatures was noted in Berezkin et al.²

Results of gravity-magnetic field analysis carried out in different Azerbaijani onshore oil and gas areas indicate that local gravity and magnetic anomalies over hydrocarbon deposits could be delineated by application of a special approach.^{7 20} The main characteristic peculiarity of this approach is utilization of sharp potential field gradient changing and extracting local anomalies.

Figs. 4 and 5 display a segregation of local anomalies from the observed gravity-magnetic data along profiles (curves 1 and 2) on examples of the Jafarly and Arabkubaly areas. For this aim regional trend (3) is constructed by such a manner that to connect to the anomalous curves from the side of the lowest values of the employed fields. Such an approach enables to delineate local maxima (4) caused by oil and gas structures. After this, by zones of changing gradients, local maxima (5) are reestablished, and on their background distinctive local maxima (6) caused by presence of anomalous objects in the geological section, are extracted.

Results of combined gravity-magnetic application for HD prognosis were positively estimated in the areas of Jafarly and Bozgoby of the Middle Kura depression where drilled wells confirmed the presence of hydrocarbon deposits on zone of local gravity and magnetic minima. New gravity-magnetic predictable areas were delineated in the vicinity of Naftalan-Gedakboz, Arabkubaly, Byandovan, and Geutepe (Middle and Lower Kura depressions) and Gala and Govsany (Absheron) where well drilling is planned shortly.

Conclusions

It was shown that detailed gravity-magnetic investigations could be effectively used for unmasking local effects from the hydrocarbon pools onshore in Azerbaijan.

The significant role of geophysical effects from the subvertical zone associated with the hydrocarbon pool is demonstrated. It was detected by factual data decreasing of magnetization and increasing of temperature in a subvertical zone over and below a hydrocarbon pool. The theoretical computations demonstrate decreasing density in the subvertical zone above and below a hydrocarbon pool.

References

1. Nettleton, L.L., "Gravity and magnetics in oil prospecting," McGraw-Hill, New York, 1976.

2. Berezkin, V.M., Kirichek, M.A., and Kunarev, A.A., "Application of geophysical methods for direct searching oil and gas deposits," Nedra, Moscow, 1978 (in Russian).

3. Berezkin, B.M., Loschakov, A.I., and Nikolaev, M.I., "Magnetic prospecting employment for searching oil and gas deposits," Applied Geophysics (Prikladnaya Geofizika), No. 103, 1982, pp. 128-36 (in Russian).

4. Donovan, T., Forgey, R., and Roberts, A., "Aeromagnetic detection of diagenetic magnetite over oil fields," AAPG Bull., Vol. 63, 1979, pp. 245-48.

5. Agulnik, I.M., Zvyagin, E.M., Kolchin, S.A., Mikhalov, I.N., and Yakovenko, A.A., "Experience and results of high-precise gravity prospecting application by direct searching of oil on example of Verkh-Tarskoe Maloichskoe deposit," in "Increasing geological effectiveness and practical methods of gravity surveys," VNIIGeofizika, Moscow, 1982, pp. 58-65 (in Russian).

6. Saunders, D.F., Burson, K.R., and Thompson, C.K., "Model for hydrocarbon microseepage and related near-surface alterations", AAPG Bull., Vol. 83, No. 1, 1999, pp. 170-85.

7. Gadirov, V.G., "Results of application of gravity-magnetic surveys for prognosis oil and gas deposits in the Kura depression of Azerbaijan," Geophysics (Geofizika), No. 2, 2009, pp. 51-56 (in Russian).

8. Eppelbaum, L.V., and Khesin, B.E., "Geophysical Studies in the Caucasus," Springer, 2012.

9. Khesin, B.E., and Eppelbaum, L.V., "The number of geophysical methods required for target classification: quantitative estimation," Geoinformatics, Vol. 8, No. 1, 1997, pp. 31-39.

10. Karshenbaum, N.A., Veselov, K.E., Gladchenko, L.G., and Mikhailov, I.N., "Application of high-precise gravity surveys for direct searching hydrocarbons in the Kerch Peninsula," Applied Geophysics (Prikladnaya Geofizika), Vol. 94, 1997, pp. 91-96 (in Russian).

11. Land, J.P., "Nonseismic methods can provide many views of a drillsite," OGJ, Vol. 94, No. 9, 1996, pp. 69-73.

12. Yunxiang, L., Xiaofang, X., and Hualu, S., "The study of 3D gravitational and magnetic data acquisition, processing, interpretation and its application," Trans. of the CPS/SEG Intern. Geoph. Conf., Beijing, No. 1097, 2009, pp. 1-4.

13. Ismail-Zade, T.A., and Khesin, B.E. (eds.), Alexeyev, V.V., Gadjiev, T.G., Karkoshkin, A.I., and Khesin, B.E., "Gravity and magnetic anomalies of Azerbaijan and their geological interpretation," Printing Map Factory, Leningrad, 1989 (in Russian).

14. Rakhmanov, P.P., "Hydrocarbon potential of the Caspian Sea shelf and adjacent areas of Azerbaijanian Land," Teknur, Baku, 2009 (in Russian).

15. Ali-Zadeh, A.A., Akhmedov, G.A., Akhmedov, A.M., Aliyev, A.K., and Zeinalov, M.M., "Geology of oil and gas deposits in Azerbaijanm," Nedra, Moscow, 1966 (in Russian).

16. Tzimelzon, I.O., "Deep structure of Earth's crust and tectonics of Azerbaijan according to geophysical data analysis," Soviet Geology, No. 4, 1965, pp. 103-11 (in Russian).

17. Gadjiev, R.M., "Deep geological structure of Azerbaijan," Azerb. Govern. Publ., Baku, 1965 (in Russian).

18. Amiraslanov, T.C., "Geological interpretation of gravity and magnetic anomalies in the central part of the Middle Kura depression," Azerbaijan Oil Industry (Azerbaidzhanskoe Neftyanoe Khozyaistvo), No. 7, 1990, pp. 13-17 (in Russian).

19. Gadjiev, T.G., Karkoshkin, A.I., Khesin, B.E., Alexeyev, V.V., Potapova, E.I., and Salekhli, T.M., "Petrodensity characteristics of geological associations in Azerbaijan," Azerneshr, Baku, 1984 (in Russian).

20. Gadirov, V.G., "The physical-geological principles of application of gravity and magnetic prospecting in searching oil and gas deposits," Proceed. of 10th Petroleum Congress and Exhibition of Turkey, Ankara, 1994, pp. 197-203.

21. Elmore, R.D., Engel, M.H., Crawford, K.N., Imbus, S., and Sofer, Z., "Evidence for a relationship between hydrocarbon and authigenic magnetite," Nature, Vol. 325, 1987, pp. 6,103-06.

22. Nikitsky, V.E., and Glebovsky, Yu. S., eds., "Magnetic prospecting, Reference Book," Nedra, Moscow, 1990 (in Russian).

23. Mamedov, S.G., "Results of precise gravity survey for oil and gas deposits in Azerbaijan," Azerbaijan Oil Industry (Azerbaijanskoe Neftyanoe Khozyaistvo), No. 2, 1984, pp. 30-35 (in Russian).

24. Salekhli, T.M., "Regularities of physical parameters changing in sections of Kura depression and their relationships with epigenetic processes," deposited in the VINITI USSR Acad. of Sci., No. 2,828-79 Dep., 1979, pp. 1-32 (in Russian).

25. Khesin, B.E., Alexeyev, V.V., and Eppelbaum, L.V., "Interpretation of geophysical fields in complicated environments," Kluwer Academic Publishers (Springer), Ser.: Modern Approaches in Geophysics, Boston-Dordrecht-London, 1996.

The authors

Vagif Gadirov has been working at the Geophysical Institute in Baku, Azerbaijan, since 1978. At present he is head of the gravity-magnetic laboratory. His main interests cover gravity-magnetic data interpretation in oil and gas geology. He received an MSc from the Oil-&-Gas University in Baku and a PhD from the Institute of Geology (Baku).

Lev V. Eppelbaum is an associate professor at Tel Aviv University. In 1982-90 he worked as researcher and senior researcher in the laboratory of gravity and magnetics at the Institute of Geophysics in Baku. His research interests include processing and interpretation of potential geophysical fields in complex environments and analysis of thermal/pressure data in wells. He has an MSc from the Oil-&-Gas University in Baku and a PhD from the Institute of Geophysics in Tbilisi. He completed postdoctoral studies in the Department of Geophysics at Tel Aviv University.