Gadirov&Eppelbaum_REV_Layout 6 ANNALS OF GEOPHYSICS, 58, Fast Track 3, 2015 1 Density-thermal dependence of sedimentary associations calls to reinterpreting detailed gravity surveys Vagif G. Gadirov1, Lev V. Eppelbaum2,* 1 OilGas Scientific Research Project Institute, Baku, Azerbaijan 2 Tel Aviv University, Faculty of Exact Sciences, Department of Geosciences, Tel Aviv, Israel ABSTRACT The modern gravimetric equipment allows to register very small effects of gravity field changes and can be applied solving different geological, geophysical and environmental problems. However, sometimes insuffi- cient calculation of various kinds of geological noise complicates effec- tive application of detailed gravity field analysis. One of such factors is the temperature regime over and outside different buried objects of inves- tigation. In this paper temperature changes in a subvertical zone over a hy- drocarbon deposit and outside its contour are analyzed. The integrated density change and corresponding gravity effects are calculated for the Muradkhanly oil deposit situated within the south-east part of the Mid- dle Kura Depression (central Azerbaijan). Calculation of these effects on the basis of density- temperature data correlation analysis could signifi- cantly improve the microgravity field examination over the hydrocarbon deposits. The similar correction procedure may be applied also over un- derground water horizons and some environmental targets. 1. Introduction Development of a new modern gravimetric and variometric (gradientometric) equipment (permitting registering earlier inaccessible small anomalies and im- proving the observation methodology) and creation of new methodologies for gravity data processing and in- terpretation have triggered an increase of microgravity methodology application in environmental and eco- nomic minerals geophysics [Eppelbaum 2011]. How- ever many kinds of noise complicate the precise gravity field analysis (the detailed block-scheme of distur- bances arising in detailed gravity surveys is presented in Figure 1). One of such disturbances is the density- thermal dependence (see “Dependence on other phys- ical parameters” in Figure 1). Let us consider the density-thermal effect on ex- ample of gravity field examination over a hydrocarbon deposit. The rocks composing sections of hydrocarbon deposits (HD) are characterized as a rule by inhomo- geneity and variety of petrophysical characteristics. It is known that in HD different physical-chemical processes exist which frequently generate subvertical zones with distinct physical characteristics over HD [Berezkin et al. 1978, Berezkin et al. 1982, Gadirov 2009, Gadirov and Eppelbaum 2012]. Such zones usually outline vertical migration of fluid flows from the deposits. Registered geophysical anomalies (including grav- ity and magnetic effects) over hydrocarbon structures are explained by physical properties (for instance, den- sity and magnetization) change and specific distribution of petrophysical properties which were traced for many hundreds of meters over HD occurrences. Analysis of numerous geological-geophysical data testifies to relation of some geophysical anomalies with HD; these anomalies did not observe over the “empty” structures. Various researchers note those inhomoge- neous zones over HD distinct from the surrounding media by their magnetic, density, thermal and other physical characteristics [Berezkin et al. 1978, Seifullin 1980, Foote 1996, Maksymchuk et al. 2012]. A presence of specific columnar (subvertical) zone with lowered density over HD is a reason of majority of observed negative gravity effects. We must note that existence of subvertical zone over HD has been dis- cussing from the 70-s of the 20th century till the pres- ent, and morphology of these zones is explained from different points of view [e.g., Berezkin et al. 1982, Roslov et al. 2009, Meger’ya 2011]. Some similar effects (but more weak) are observed in hydrogeological and geoenvironmental investigations [e.g., Somerton 1992, Singhal and Gupta 2010, Eppelbaum et al. 2014]. Porosity and density of HD collectors do not change, but hydrocarbons extrude water from the Article history Received September 27, 2014; accepted November 19, 2014. Subject classification: Temperature, Density, Subvertical zone, Gravity anomaly. porous space that leads to density decrease in the sub- vertical zone over HD. At the same time, quantitative estimation of corresponding physical properties changes in this zone has not been conducted. Some pa- rameters of magnetic susceptibility and temperature changing in subvertical zone over HD were analyzed in [Gadirov 2009, Gadirov and Eppelbaum 2012]. In this investigation we suggest to utilize temperature data for estimation of rock density changing in subvertical zone over HD. This investigation was carried out for the example of the Muradkhanly deposit (description of this deposit was given in Eppelbaum and Khesin [2012]) situated within the south-east part of the Middle Kura Depres- sion, on the western slope of Sabirabad-Kurdamir zone of highs (Azerbaijan). This deposit was discovered in 1971 when firstly in Azerbaijan oil reserves were found in volcanogenic associations of Upper Jurassic (in in- terval of 3798-3761 m). This discovering served a foun- dation for carrying out wide-ranging drilling in this area. In these boreholes numerous measurements of temperature have been performed. The temperature data were carefully studied and the results are em- ployed in this investigation. 2. Analysis of temperature effects over hydrocarbon deposit For studying thermal characteristics of geological associations in the Mutadkhanly and adjacent areas are constructed graphs of temperature changing versus depth both within deposits and without their contours (Figures 2a and 2b, respectively). Table 1 shows some GADIROV AND EPPELBAUM 2 Figure 1. Noise affecting microgravity investigations (modified and enlarged after Eppelbaum, [2010]). 3 key lithological, density, porosity and thermal data for the Muradkhanly area. Thermal values as a whole in- crease with depth due to geothermal gradient, but some quasi-chaotic distribution was also revealed in the area. By other words, in some boreholes at one depth various temperatures were observed, and the same tempera- tures were observed at different depths. Nevertheless linear relationships of temperature versus depth with sufficiently high coefficients of correlation R were ob- tained (R for the graph presented Figure 2a is computed as 0.951 and for the graph shown in Figure 2b - as 0.927). Comparison of the obtained regressions indicates that temperature values versus depth over HD increase more rapidly than without the deposit contour. With depth increase the difference between temperatures ob- served within the deposit contour and without it is also increase (Figure 3). It is known that temperature increase causes vol- ume expansion of physical bodies (including geologi- cal associations) that leads to decreasing their density [Eppelbaum et al. 2014]. However, values of rock ther- mal expansion is so small that to detect it in HD areas practically is not possible. Deep well sections in the Mu- radkhanly area indicate that oil deposit was discovered at the depth about 3000 m in the arched uplift, and at the depth of 4000 m in the western continuation of the GRAVITY ANOMALIES DEPEND ON TEMPERATURE Geological age Lithology Depth, m Vertical thickness, m Average density, kg/m3 Porosity n, % Temperature increment in contour zone of deposit Dt, °C Coefficient of volume temperature expansion (b ·10−5, 1/°C) * Decreasing rock density over deposit calculated using Eq. (5), Dv, kg/m3 Q sandstones, clays, sand 0-400 400 2050 22 5.03 2.9 0.72 Q1, N 22 sandstones, sands, siltstones, limestone 400-2000 1200 500 2130 25 13.03 2.9 2.53 pay section N 12 clays, sandstones, siltstones 2000-2200 200 2235 18 14.03 2.7 2.18 N1, N 21 sandstones, clays 2200-2750 500 50 2210 15.9 17.03 2.9 2.52 P–3+N 1 1 limestone, clays 2750-3400 650 2300 16.2 20.03 2.7 2.94 P–2 clays, calcareous sandstones 3400-3900 500 2340 11.2 23.03 2.9 2.89 Dvaver = 2.45 kg/m3 Table 1. Some geological-petrophysical characteristics of rocks composing section of the Muradkhanly deposit. * bw = 59 ·10 −5, 1/°C (bw is the volume thermal expansion of water). Figure 2. Graphs of the temperature dependence on the depth in oil deposit Muradkhanly: (a) inside the deposit contour, (b) outside the deposit contour. H is depth, and R is the correlation coefficient. structure. The temperature difference at the depth of 4000 m between the arched uplift and western part con- sists of about 23° (Figure 3). Applying Equation (1) (see below) and accepting value of volume thermal ex- pansion b as 2.9 · 10−5 1/°C, and density as 2340 kg/m3 (see Table 1) we obtain Dv ≈ 2 kg/m3. It will be very hard to detect such rock density changes by any geo- physical method in the central part of the deposit con- cerning to the peripheral part (even in the case of their identical stratigraphy and lithology). However, presence of fluids in geological sections sharply changes the physical pattern. For instance, vol- ume thermal expansion of water under temperature of 50-80°C (bw = 59 · 10 −5 1/°C) almost in 20 times ex- ceeds the volume thermal expansion of sandstone. It is known that water in geological section fills porous space in the majority of geological rocks. Consequently we cannot consider volume thermal expansion of water and mineral skeleton separately since water ex- pansion in the porous rock space will create corre- sponding pressure to the solid shell of the rocks. In this case porous pressure will increase and probably a zone of abnormally high layer pressure will be generated. Analysis of pressures in boreholes at Muradkhanly area has shown that there is not a clear relationship between the pressure distribution and HD location in this area. The abovementioned facts indicate that temperature change in the studied areas is accompanied by volume and density change by constant mass. A relationship between the density and tempera- ture is well-known [e.g., Somerton 1992]: where v0 and vt are the targets densities at tempera- tures 0 and t°C, respectively, and b is the coefficient of volume thermal expansion of targets. Bodies with density vt1 at temperature t2 will have density vt2 : Based on Equation (2) we can determine the dif- ference in body densities depending on temperature changes: or where Dv Dt is target density change by corresponding temperature change for Dt interval, and vt1 is the tar- get density at temperature t1. Equation (4) can be applied for a homogenous body. Geological rocks occurring in in the depth inter- val of 0–10 km as a rule consist of solid part, liquid (water) and gaseous phase filling the rock porous space. Taking into account that contribution of gaseous phase is usually negligible, let us present changes of solid phase density and water, respectively as: and where Dvr Dt and Dv w Dt are the changings (increments) of rock and water density, respectively depending on temperature changing Dt; br and bw are the coefficients of volume thermal expansion of rock and water, re- spectively. The volume occupied by liquid (water) may be es- timated from use coefficient of porosity n. Then solid phase will occupy (1 − n) part of the volume unit. Total density change of solid skeleton and water may be pre- sented as: (7) After substituting Equations (5) and (6) to Equa- tion (7) we receive: (8) Equation (8) enables us to calculate density de- crease over HD relative to it peripheral part (outside the deposit contour). We must note that denominator in Equation (8) is close to one and we can neglect it. For estimation of the temperature effect the well- DvDt = 1 + b Dt b Dt vt1 . DvDt r = 1 +brDt brDt vt1 r ,. DvDt w = 1 +bwDt bwDt vt1 w,. DvDt =DvDt r 1 - nQ V +DvDt w n. DvDt = 1 + br +bwQ VDt vt1 r br 1 - nQ V +vt1 w bw n! $Dt . vt = v0 1 +bt 1 , vt2 . vt1 1 + b t2 - t1Q V 1 . vt1 -vt2 = vt1 -vt1 1 + b t2 - t1Q V 1 = 1 +b t2 - t1Q V b t2 - t1Q V vt1 . GADIROV AND EPPELBAUM 4 Figure 3. Graph of changing of temperature versus depth within the contour of Muradkhanly deposit. (1) (2) (3) (4) (5) (6) 5 studied Muradkhanly deposit (Middle Kura Depression, Azerbaijan) was selected. This deposit was compre- hensively investigated by numerous drilling operations and geophysical methods and may be considered as polygon for testing different methodologies. The temperature data processed for the Murad- khanly area enabled us to estimate the density change in this area. Application of Equation (8) indicates that decreasing rock density in the contour part of the de- posit relative to the peripheral part averagely consists of 2.45 kg/m3. Practically the density decrease exceeds this value since besides thermal volume expansion, extend- ing liquid phase also produces some rock expansion. 3. Calculation of gravity effect caused by thermal regime peculiarities How this nonsignificant rock density decrease could influence to amplitude and behavior of gravity anomaly? For this aim it is necessary to compute the di- rect calculation of deficit mass over the oil pool affect- ing the gravity field. We will develop a model of gravity deficit mass over HD in the subvertical zone with the example of the Muradkhanly oil deposit. As was aforementioned the oil deposit in this area was discovered at the depth of 3000 m on the arch and consisted of more than 4000 m on the wings. In the oil-bearing part of the geological structure with a dimension 3 × 4 km the total volume of the deposits consists approximately 42 · 109 m3. In this case the nonsignificant density decrease (−2.45 kg/m3) could cause to deficit of mass in this area as several tens of million tons. In this concrete example the deficit of mass is: m = ov = 42 · 109 · 2.45 kg = 102.9 · 109 kg = = 102.9 · 106 tons ≈ 108 tons. Which gravity effect may produce such a mass? We will conduct some simple calculations. Let’s assume that the “missing” mass is concentrated in the middle of the geological section as a sphere (Figure 4). Then the gravity effect from this target may be calculated using known expression: where G is the universal gravity constant, M is the mass, h is the depth of sphere occurrence, and r is the distance from the sphere center to the point of gravity effect cal- culation. Let us compute the gravity effect in points x0 and xl disposed directly over the sphere and over the bound- ary of deposit, respectively: The difference Dx between points x0 and xl consists of 0.122 milliGal (1 milliGal = 10−5 m/s2). In fact, tak- ing into account that our calculation was applied to “missing mass”, we must use this value with an inverse sign: −0.122 milliGal. The distribution of gravity effect due to the geothermal phenomenon between the x0 and xl (xl here was assumed as 2000 m) is presented in Fig- ure 5. Today when we speak about microGal (1 µGal = 10−8 m/s2) anomalies [Eppelbaum 2011], the afore- mentioned value (122 microGal) may have an impor- tant role in gravity field examination in oil and gas bearing areas. Performed 3D modeling of gravity field have shown that for real situations the estimations of grav- ity effects decrease in 1.25-1.4 times, but and for these cases the gravity effects are significant and considerably exceed the errors of the Bouguer gravity determina- tion in field conditions. Calculation of temperature ef- fect will enable to receive more exact gravity anomalies and to avoid some mistakes by geophysical data analy- sis not only in hydrocarbon geophysics, but also in hy- drogeological, environmental (e.g., karst localization) and other investigations. Dg = GM h2 1 = 0.22 mGal 1 mGal = 10-5 m/s2Q V Dgxl(xl = 1500 m) = GM r3 h = 0.098 mGal G J. Dg = GM r3 h , GRAVITY ANOMALIES DEPEND ON TEMPERATURE Figure 4. Model for computation of gravity effect of “deficient” mass in the subvertical zon over deposit. 4. Conclusions It was shown increasing temperature over hydro- carbon deposit only for several degrees causes small decreasing of this zone rocks comparting with it pe- ripheral part (about 2.45 kg/m3). However, even such nonsignificant density changing by sufficient vertical zone thickness (several thousand meters) over HD cre- ates considerable gravity effect - several tenth parts of milliGal that can play a significant role for detection and localization of hydrocarbon deposits including ineffi- cient and deep-seating ones. Thus, the detailed gravity analysis over the hydrocarbon structures, as well as in hydrogeological and environmental studies should be accompanied by thermal data examination. Acknowledgements. The authors would like to thank two anonymous reviewers who thoroughly reviewed the manuscript, and which valuable suggestions were very helpful in preparing this paper. References Berezkin, V.M., M.A. Kirichek and A.A. Kunarev (1978). Application of Geophysical Methods for Direct searching Oil and Gas Deposits, Nedra, Moscow (in Russian). Berezkin, V.M., G.A. Bannikova and I.S. Mazurova (1982). 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Results of application of gravity- magnetic surveys for prognosis oil&gas deposits in the Kura Depression of Azerbaijan, Geophysics (Moscow), 2, 51-56 (in Russian). Gadirov, V.G., and L.V. Eppelbaum (2012). Detailed gravity, magnetics successful in exploring Azerbaijan onshore areas, Oil and Gas Journal, Nov. 5, 110 (11), 60-73. Maksymchuk, V., R. Kuderavets, I. Chobotok and V. Ty- moschuk (2012). High-resulution surveys for oil and gas fields searching in NW part of the Carpathian fore deep, Trans. of 2nd Intern. Conf. Alpine-Petrol 2012, Krakow-Poland, 83-84. Meger’ya, V.M. (2011). Searching and prospecting of hydrocarbon deposits associated with geosoliton de- gassing of the Earth on the basis seismic imaging, Geophysics (Moscow), 1, 67-74 (in Russian). Roslov, Yu.V., N.N. Efimova, A.N. Kremlev and A.D. Pavlenkin (2009). Seismic features of fluid flow and associated with it deposits, Geophysics (Moscow), 2, 26-30 (in Russian). Seifullin, R.C. (1980) Relationship of thermal and mag- netic anomalies over hydrocarbon deposits with nat- ural electric fields, Oil&gas Geology and Geophysics, 8, 42-43 (in Russian). Singhal, B.B.S., and R.P. Gupta (2010). Applied Hydro- geology of Fractured Rocks, Springer. Somerton, W.H. (1992) Thermal Properties and Tem- perature-Related Behaviour of Rock-Fluid Systems, Elsevier. *Corresponding author: Lev V. Eppelbaum, Tel Aviv University, Faculty of Exact Sciences, Department of Geosciences, Tel Aviv, Israel; email: levap@post.tau.ac.il. © 2015 by the Istituto Nazionale di Geofisica e Vulcanologia. All rights reserved. GADIROV AND EPPELBAUM 6 Figure 5. A distribution of gravity effect between the x0 and xl (l = 2000 m). << /ASCII85EncodePages false /AllowTransparency false /AutoPositionEPSFiles false /AutoRotatePages /None /Binding /Left /CalGrayProfile (Dot Gain 20%) /CalRGBProfile (sRGB IEC61966-2.1) /CalCMYKProfile (U.S. Web Coated \050SWOP\051 v2) /sRGBProfile (sRGB IEC61966-2.1) /CannotEmbedFontPolicy /Warning /CompatibilityLevel 1.3 /CompressObjects /Tags /CompressPages true /ConvertImagesToIndexed true /PassThroughJPEGImages true /CreateJobTicket false /DefaultRenderingIntent /Default /DetectBlends true /DetectCurves 0.1000 /ColorConversionStrategy /LeaveColorUnchanged /DoThumbnails false /EmbedAllFonts true /EmbedOpenType false /ParseICCProfilesInComments true /EmbedJobOptions true /DSCReportingLevel 0 /EmitDSCWarnings false /EndPage -1 /ImageMemory 1048576 /LockDistillerParams true /MaxSubsetPct 100 /Optimize false /OPM 1 /ParseDSCComments true /ParseDSCCommentsForDocInfo true /PreserveCopyPage true /PreserveDICMYKValues true /PreserveEPSInfo true /PreserveFlatness true /PreserveHalftoneInfo false /PreserveOPIComments false /PreserveOverprintSettings true /StartPage 1 /SubsetFonts true /TransferFunctionInfo /Apply /UCRandBGInfo /Preserve /UsePrologue false /ColorSettingsFile (None) /AlwaysEmbed [ true /AndaleMono /Apple-Chancery /Arial-Black /Arial-BoldItalicMT /Arial-BoldMT /Arial-ItalicMT /ArialMT /CapitalsRegular /Charcoal /Chicago /ComicSansMS /ComicSansMS-Bold /Courier /Courier-Bold /CourierNewPS-BoldItalicMT /CourierNewPS-BoldMT /CourierNewPS-ItalicMT /CourierNewPSMT /GadgetRegular /Geneva /Georgia /Georgia-Bold /Georgia-BoldItalic /Georgia-Italic /Helvetica /Helvetica-Bold /HelveticaInserat-Roman /HoeflerText-Black /HoeflerText-BlackItalic /HoeflerText-Italic /HoeflerText-Ornaments /HoeflerText-Regular /Impact /Monaco /NewYork /Palatino-Bold /Palatino-BoldItalic /Palatino-Italic /Palatino-Roman /SandRegular /Skia-Regular /Symbol /TechnoRegular /TextileRegular /Times-Bold /Times-BoldItalic /Times-Italic /Times-Roman /TimesNewRomanPS-BoldItalicMT /TimesNewRomanPS-BoldMT /TimesNewRomanPS-ItalicMT /TimesNewRomanPSMT /Trebuchet-BoldItalic /TrebuchetMS /TrebuchetMS-Bold /TrebuchetMS-Italic /Verdana /Verdana-Bold /Verdana-BoldItalic /Verdana-Italic /Webdings ] /NeverEmbed [ true ] /AntiAliasColorImages false /CropColorImages true /ColorImageMinResolution 150 /ColorImageMinResolutionPolicy /OK /DownsampleColorImages true /ColorImageDownsampleType /Bicubic /ColorImageResolution 300 /ColorImageDepth -1 /ColorImageMinDownsampleDepth 1 /ColorImageDownsampleThreshold 1.10000 /EncodeColorImages true /ColorImageFilter /DCTEncode /AutoFilterColorImages true /ColorImageAutoFilterStrategy /JPEG /ColorACSImageDict << /QFactor 0.15 /HSamples [1 1 1 1] /VSamples [1 1 1 1] >> /ColorImageDict << /QFactor 0.15 /HSamples [1 1 1 1] /VSamples [1 1 1 1] >> /JPEG2000ColorACSImageDict << /TileWidth 256 /TileHeight 256 /Quality 30 >> /JPEG2000ColorImageDict << /TileWidth 256 /TileHeight 256 /Quality 30 >> /AntiAliasGrayImages false /CropGrayImages true /GrayImageMinResolution 150 /GrayImageMinResolutionPolicy /OK /DownsampleGrayImages true /GrayImageDownsampleType /Bicubic /GrayImageResolution 300 /GrayImageDepth -1 /GrayImageMinDownsampleDepth 2 /GrayImageDownsampleThreshold 1.10000 /EncodeGrayImages true /GrayImageFilter /DCTEncode /AutoFilterGrayImages true /GrayImageAutoFilterStrategy /JPEG /GrayACSImageDict << /QFactor 0.15 /HSamples [1 1 1 1] /VSamples [1 1 1 1] >> /GrayImageDict << /QFactor 0.15 /HSamples [1 1 1 1] /VSamples [1 1 1 1] >> /JPEG2000GrayACSImageDict << /TileWidth 256 /TileHeight 256 /Quality 30 >> /JPEG2000GrayImageDict << /TileWidth 256 /TileHeight 256 /Quality 30 >> /AntiAliasMonoImages false /CropMonoImages true /MonoImageMinResolution 1200 /MonoImageMinResolutionPolicy /OK /DownsampleMonoImages true /MonoImageDownsampleType /Bicubic /MonoImageResolution 1200 /MonoImageDepth -1 /MonoImageDownsampleThreshold 1.08250 /EncodeMonoImages true /MonoImageFilter /CCITTFaxEncode /MonoImageDict << /K -1 >> /AllowPSXObjects false /CheckCompliance [ /None ] /PDFX1aCheck false /PDFX3Check false /PDFXCompliantPDFOnly false /PDFXNoTrimBoxError true /PDFXTrimBoxToMediaBoxOffset [ 0.00000 0.00000 0.00000 0.00000 ] /PDFXSetBleedBoxToMediaBox true /PDFXBleedBoxToTrimBoxOffset [ 0.00000 0.00000 0.00000 0.00000 ] /PDFXOutputIntentProfile (None) /PDFXOutputConditionIdentifier () /PDFXOutputCondition () /PDFXRegistryName (http://www.color.org) /PDFXTrapped /Unknown /CreateJDFFile false /SyntheticBoldness 1.000000 /Description << /ENU (Use these settings to create PDF documents with higher image resolution for high quality pre-press printing. 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