Heat flow and presence of oil and gas (the Yamal peninsula, Tomsk region)

The possibilities of Geothermy as a geophysical method are studied to solve forecast and prospecting problems of Petroleum Geology of the Arctic regions and the Paleozoic of Western Siberia. Deep heat flow of Yamal fields, whose oil and gas potential is associated with the Jurassic-Cretaceous formations, and the fields of Tomsk Region, whose geological section contents deposits in the Paleozoic, is studied. The method of paleotemperature modeling was used to calculate the heat flow density from the base of a sedimentary section (by solving the inverse problem of Geothermy). The schematization and mapping of the heat flow were performed, taking into account experimental determinations of the parameter. Besides, the correlation of heat flow features with the localization of deposits was revealed. The conceptual and factual basis of research includes the tectonosedimentary history of sedimentary cover, the Mesozoic-Cenozoic climatic temperature course and the history of cryogenic processes, as well as lithologic and stratigraphic description of the section, results of well testing, thermometry and vitrinite reflectivity data of 20 deep wells of Yamal and 37 wells of Ostanino group of fields of Tomsk region. It was stated that 80 % of known Yamal deposits correlate with anomalous features of the heat flow. Bovanenkovskoe and Arkticheskoe fields are located in positive anomaly zones. 75 % of fields of Ostanino group relate to anomalous features of the heat flow. It is shown that the fields, which are characterized by existence of commercial deposits in the Paleozoic, are associated with the bright gradient zone of the heat flow. The forecast of commercial inflows in the Paleozoic for Pindzhinskoe, Mirnoe and Rybalnoe fields is given. The correlation between the intensity of naftidogenesis and the lateral inhomogeneity of the deep heat flow is characterized as a probable fundamental pattern for Western Siberia.

. This is the main parameter, determining the thermal history of potentially oil source sediments, the realization degree of generation potential of organic matter, the syngenesis of foci of hydrocarbon generation and accumulating reservoirs (Isaev, 2004). To the point, the following is a quotation from monograph (Kurchikov, 1992): "Thus, according to the new obtained data about deep heat flow, a widespread viewpoint concerning the omnipresent confinedness of oil and gas deposits to geotemperature anomalies zones is not confirmed. Nevertheless, it is found out that the majority of hydrocarbon accumulations are located in zones of substantial lateral inhomogeneity of the deep heat flow" (italics by article authors). A relevant contribution to the formation of Geothermy as an oil prospecting method, especially for the Arctic regions, of A.R. Kurchikov (Kurchikov, 2001) and M.D. Khutorskoy (Khutorskoy et al., 2013;Nikitin et al., 2015) must be mentioned.

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Objects for present research are territories of hydrocarbon fields localization in the northern part of Yamalo-Nenets Autonomous Okrug, where oil and gas potential is associated, predominantly, with the Lower Cretaceous deposits of the Akhskaya suite, and in Tomsk Region, which geological section contents deposits in the Upper Jurassic as well as in the contact zone of the Paleozoic and the Mesozoic and in the Inner Paleozoic.
Research purpose is to investigate a consistent pattern of the deep heat flow changes, to evaluate the correlation between the heat flow peculiarities and localization of hydrocarbon fields and to define possible prospecting geothermal criteria of oil and gas potential through the example of the territories, which were mentioned earlier.
Investigation and evaluation have been carried out, foremost, based on calculated values of the deep heat flow density from the base of a sedimentary section, besides the experimental determinations of the deep heat flow have been also analysed.

The methodology of deep heat flow calculation
The deep heat flow is determined by solving inverse problem of Geothermy with the aid of software package tools for 1D basin modelling (Isaev et al., 2018а;Isaev et al., 2018b). Solution is performed within the parametric description of the sedimentation history and the history of thermophysical properties of the sedimentary layer only, beginning from the Jurassic, without invoking information about the nature of heat flow and geodynamics below the base of the sedimentary section. For the conditions of Western Siberia, characterized, starting from the Jurassic time, by the quasistationarity of the deep heat flow (Duchkov et al., 1990;Kurchikov et al., 2001), the solution of the inverse Geothermy problem -determining the density of heat flow -is carried out uniquely.
To solve the inverse problem of Geothermy, we use as "observed" both measurements of reservoir temperatures obtained during well tests and temperature logs of steadystate boreholes (Method of definite geotemperature gradient, DGG) and also geotemperatures recalculated (Isaev, Fomin, 2006) from the values of vitrinite reflectance (VR). The geotemperatures from VR are included immediately in a rigorous mathematical form. No separate "calibrations" for VR temperatures are required.
The first boundary condition of the model is determined by the temperature of the surface of sedimentation in the Mesozoic-Cenozoic, i.e. paleoclimate factor, and is given as a piecewise linear function of the "local" secular temperature pattern on the Earth's surface. The "local" secular temperature patterns of the Arctic zone and the south-eastern part of Western Siberia were built on the basis of synthesis of experimental definitions and paleoclimatic reconstructions (Iskorkina et al., 2015;Isaev et al., 2016;Isaev et al., 2017).
Parametrization of sedimentary section, exposed by a well, which defines parameters of sedimentation and thermophysical model, accepts in compliance with the lithologic and stratigraphic dedicated breakdown.
The upper boarder of sedimentation massdepositional surface, daylight surface. Therefore, paleotemperature reconstructions immediately are conjugated with paleostructural reconstructions. The sedimentation rate in the model may be set to zero and negative values, that allows to take into account nondepositional hiatus and denudation.
The main criterion for the accuracy of the results of modeling is the optimal consistency ("discrepancy") of the maximum calculated geotemperatures with the "observed" temperatures of the "maximum paleothermometer" -temperatures determined by VR. The optimality of the "discrepancy" of calculated geotemperatures with "observed" reservoir temperatures is equally important. The optimal "discrepancy" is the mean square difference between the calculated and "observed" values, which is equal to the error of observations (Strakhov et al., 2000). In our case, the error of observations is of the order of ± 2ºС (Isaev et al., 2018b). To be noted that not infrequently "discrepancy" of calculated geotemperatures with "observed" reservoir temperatures at depths less than 1,000 m noticeably goes beyond the optimal.
The important criterion for the accuracy of the results of modelling is the consistency of the calculated values of the heat flow density with the data of its experimental determination in the study area.

The heat flow of Yamal
The Mesozoic-Cenozoic sedimentary cover in the study area began to form in the Lower Jurassic. In this time Kiterbyutskaya argillaceous suite J 1 t, which had an oil-source potential, was accumulating. Till the www.geors.ru GEORESURSY 127 GEORESURSY = GEORESOURcES 2019. V. 21. Is 3. Pp. 125-135 end of the Volgian Age the marine transgression has expanded, Bazhenov formation (J 3 tt+K 1 b), which is the most enriched in organic matter, was accumulating.
According to the average VR value -R 0 vt = 0,96 %the Bazhenov formation is in the end of the main oil generation zone within the limits of the Arctic area.
Marine regime has prevailed since the Aptiancenomanian until the beginning of the Eocene. Thickness analysis of Paleogene-Neogene testified that sedimentation had gone to the middle of the Miocene for 32 million years (Nyurolskaya, Tavdinskaya, Atlymskaya, Novomikhaylovskaya, Turtasskaya, Abrosimovskaya formations) and amounted to 335-535 m, then these sediments have been degraded in the Early Bicheul time.
In the Middle Miocene-Early Pliocene, since the end of the Bicheul time and till the end of the Novoportov accumulation had gone of the order of 100 m thick sediments, which in the subsequent stage of positive tectonic movements over 1.3 million years were denudated. With the onset of the Late Miocene, Pliocene-Quaternary lacustrine-alluvial sediments accumulated.
The Middle Jurassic oil and gas complex (OGC) includes reservoirs in Vimskaya suite (J 2 b 1 1 ) with YuYa 7-9 formations and Malyshevskaya suite (J 2 b 2 2 -bt 1-2 ) with YuYa 2-4 formations in lower subsuite. The Upper Jurassic OGC combines Nurminskaya suite (J 2 bt 3 -k+J 3 ok-tt 1 ), while the Cretaceous -Akhskaya, Tanopchinskaya and Yarongskaya the Lower Cretaceous suites. Achimovskaya sequence is identified in the bottom of the Cretaceous with a group of Ach formations.
Stratigraphic breakdowns, well testing results and VR data of deep wells (Database of the Trofimuk Institute of Petroleum Geology and Geophysics of Siberian Branch of Russian Academy of Sciences, 2018) are invoked for purpose of building sedimentation and thermophysical 1D-models. The breakdowns are accepted considering the dynamic of tectonic events during the formation of sedimentary section in the territory of Yamal. Account the permafrost is performed beginning from 0.52 Ma, while ice mass -beginning from 0.182 Ma (Isaev et al., 2017). The glacier has completely degraded by the time of 15 thousand years ago (the end of the Sartan time).
The scheme of heat flow density of Yamal ( Fig. 1A) was built using calculated values of heat flow density from the base of the sedimentary section in 8 wells and data of experimental determinations of deep heat flow density in 12 wells (Table 1).
To be noted, that the pattern concerning increasing the deep heat flow density towards the north-west direction (Khutorskoy et al., 2013), which was earlier established, is observed on the obtained calculated scheme as well (Fig. 1).
It is interesting to match calculated values of the heat flow, obtained with the authors' methodology (Isaev et al., 2018а;Isaev et al., 2018b), with calculated values of heat flow, obtained earlier via use of the physicalmathematical model of A.R. Kurchikov (Kurchikov, 1992). Thus, cataloged values of the heat flow density (Kurchikov, 1992) in wells of Kharasaveyskoe, Kruzenshternskoe and Bovanenkovskoe fields are in the range of 56-62 mW/m 2 , whereas the scheme (Fig. 1) gives the range of 57-62 mW/m 2 . And then cataloged values of the heat flow density (Kurchikov, 1992) in wells of Arkticheskoe and Sredne-Yamal'skoe fields are in the range of 54-56 mW/m 2 , whereas the scheme (Fig. 1) gives the range of 51-57 mW/m 2 .
Presumed, Table 1 and also Table 3 (calculated heat flow, "discrepancies" of modelling) are of self-consistent interest. Along with that, this information is an argument for reliability of modelling results, values of the heat flow in tables may become a basis for following 2D and 3D basin modelling. One of the most problemplagued and quite difficult stage of the basin modelling is receiving the heat flow appraisals. The complicacy and ambiguousness of heat flow determination from the base of sedimentary section, which is based in systems GALA, Temis, PetroMod on the models of the lithosphere rifting ("defined rift phases") (Hantschel et al., 2009;Kontorovich et al., 2013;Galushkin, 2016), is known, but far from always mentioned.
Analysis of correlation between the heat flow and localization of 13 well-known fields shows the following. 6 fields are in the zones of positive anomalies of heat flow (46 % of the total number), the biggest among them -Bovanenkovskoe and Arkticheskoe. 1 field is in the zone of negative anomaly of the heat flow (8 %) -Yuzhno-Tambeyskoe. 4 fields are in the zones of bay-shaped configuration of isolines (31 %): Kruzenshternskoe, Neytinskoe, Rostovtsevskoe and Novoportovskoe.
Consequently, in the order of 80-85 % of known hydrocarbon fields of Yamal are associated with anomalous peculiarities of the deep heat flow.

The heat flow of Ostanino group of fields (Tomsk Region)
The study area is located in Parabel District of Tomsk Region between rivers the Chuzik and the Chizhapka (Fig. 2). As to sediments of the platform cover the research territory is in the articulation zone of two tectonic structures of the first order: Nyurol'skaya megadepression and the south part of Srednevasyuganskiy megaswell (Kontorovich et al., 2006).  (Khutorskoy et al., 2013) 9 Rusanovskoe 2 --   Table 2). The deposits of the uppermost part of basement rocks are relating to metasomatically altered bioaccumulated limestones, which represent as reservoir rocks of porous-fissured type.
commonly spread in the study territory the Upper Jurassic Bazhenov Formation is an oil source rock for the Middle and Upper Jurassic OGC (Fomin, 2011). Potentially oil-source the Lower Jurassic Togurskaya suite is accepted as a traditional hydrocarbon source for the pre-Jurassic OGC (Kostyreva, 2005). However, this formation has quite limited spread -the south-western edge of the study area ( Fig. 2A). In this connection it is impossible to except the version, which suggests accounting the Devonian strata as a hydrocarbon generating source for the pre-Jurassic reservoirs . The hypothesis of "Bazhenov source of origin" of the pre-Jurassic hydrocarbon deposits is more disputable and it has only indirect arguments (Galieva, Krutenko, 2019).
The calculations of the heat flow density performed for geological sections of 35 exploratory and 2 parametric wells on the study area (Fig. 2). Data of deep wells testing (reservoir temperatures) and temperature logs DGG were investigated and summed up from primary "well historical data", reserve calculation reports, operational analysis reports and generalization of geological and geophysical database of Tomsk Region The comparison of modelling temperatures with reservoir temperatures and with temperatures determined by DGG and by VR is provided in the Table 3. As is evident, the calculated model of heat distribution in the sedimentary section coincides with "observed" values in the optimal way at a level of ±2°С.
The reliability of the results of paleotemperature modelling is confirmed consistency of the obtained calculated values of the heat flow density with experimental determinations of A.D. Duchkov (Western Siberia. Geology and mineral resources of Russia, 2000). Experimental data is surrounded with isoline of 60 mW/ m 2 , calculated values are in the range of 41-65 mW/m 2 .
The following anomalous peculiarities are observed in the map of the deep heat flow distribution (Fig. 2B): "positive anomaly", "gradient zone", "negative anomaly", "bay-shaped configuration of isolines".
Hydrocarbon fields in the eastern part of the territory are associated with the striking gradient zone, which surrounds a large positive anomaly. There 6 fields are located: Rybal'noe, Selimkhanovskoe, Pindzhinskoe, Mirnoe, Ostaninskoe, Severo-Ostaninskoe, that accounts 50 % of the total number of the fields in the study area. 3 fields (25 %) are in the zone of bay-shaped configuration of isolines -Pel'ginskoe, Gerasimovskoe, Zapadno-Ostaninskoe. Remarkably, hydrocarbon fields are absent in the zone of negative anomaly of the heat flow in the north-western part of the territory. Therefore, 9 fields (75 %), which are located in the study area, are associated with the anomalous peculiarities of the deep heat flow distribution. 3 fields -Shirotnoe, Yuzhno-Tambaevskoe, Tambaevskoe, do not correlate with the anomalous peculiarities of the heat flow.  Independently important to note, that Selimkhanovskoe, Ostaninskoe, Severo-Ostaninskoe and also Gerasimovskoe fields, within which deposits are exposed with commercial hydrocarbon inflows, are associated with the substantial lateral inhomogeneity of the heat flow density (gradient zones) (Table 2). Pindzhinskoe, Mirnoe and Rybal'noe fields are also included in the striking gradient zone, within them it is possible to forecast deposits with commercial hydrocarbon inflows.
Yuzhno-Tambaevskoe and Tambaevskoe fields, within which deposits are exposed with commercial hydrocarbon inflows, are separated from anomalous peculiarities of the heat flow. Interesting to point, that particular these fields are located within the boarders of spread of potentially oil-source the Lower Jurassic Togurskaya suite.

conclusion
The scheme and the map of the deep heat flow were built for the territory of localization of the Arctic hydrocarbon fields of Yamal and Tomsk Region, where the Paleozoic oil deposits were exposed in geological section.

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It was stated that 75-80 % of hydrocarbon fields, which were located in the study areas, correlated with the anomalous peculiarities of the deep heat flow distribution.
The reliability of calculated values of Yamal heat flow was reasoned with performing of the classical geophysical criterion -criterion of "discrepancy", consistency with earlier revealed tendency of the heat flow density increase towards the north-western direction, comparability with the catalogue of calculated values of the heat flow, which was presented in earlier published monographic work (Kurchikov, 1992). The reliability of calculated values of the heat flow of Ostanino group of fields of Tomsk Region was substantiated with performing the criterion of "discrepancy", consistency with the map of the heat flow determinations of Western Siberian Plate.
The confinedness of fields with commercial hydrocarbon inflows from the deposits of pre-Jurassic OGC to the strongly marked gradient zone of the heat flow density values was placed emphasis through the example of Ostanino group of fields. This allows to forecast getting commercial inflows from the pre-Jurassic OGC in Pindzhinskoe, Mirnoe and Rybal'noe fields.
Afore-named characterize lateral inhomogeneity of the heat flow (gradient zones), probably, not as forecast criterion of oil and gas potential, but rather as an existence of the fundamental correlation between the intensity of naftidogenesis and lateral inhomogeneity of the deep heat flow of Western Siberia.