The Kashirskian-Podolskian carbonate deposits are the overlying return horizon for many fields in the Republic of Bashkortostan. The rocks are characterized by a significant degree of dolomitization, sulfation and silicification. The void space is expressed by pores, micropores, caverns, and cracks of subordinate significance. The complex structure and geological heterogeneity make it difficult to assess the value of porosity and saturation. In order to increase the reliability of quantitative interpretation of neutron logging in sediments of a complex reservoir, additional core studies were required. The main goal of the additional research program was to assess the neutron-absorbing properties of the mineral components of the rock. Mineral component model taking into account anomalous microelements will allow to set up neutron methods for studying wells and estimate the magnitude of the error in determining the current saturation coefficient. To obtain additional information on the properties of the Kashirskian-Podolskian deposits, studies of the elemental composition of rocks were carried out using the X-ray fluorescence, inductively coupled plasma mass spectrometry, thermogravimetric method and pyrolysis. As a result of complex processing of the data array obtained by various methods of core research, mineral-component models were built to calculate the lifetime values and macroscopic absorption cross section of thermal neutrons. The resulting lithotype models and neutron constants will improve the accuracy of oil saturation assessment based on pulsed neutron methods. Square deviation of the random error of the result of an indirect measurement inaccuracy in the definition of “mineralogy” can lead to an error in saturation determination of up to 14%. The lower is the porosity coefficient, the higher is the error in calculating the saturation coefficient. For limestone the oil saturation will be overestimated, for dolomite rock it will be underestimated. When assessing the nature of saturation, one should not use a single boundary value of the neutron lifetime or capture cross section for all lithotypes, but a relationship between the neutron lifetime and the porosity and salinity of formation water adjusted on one’s own core.
References
1. Privalova O.R., Burikova T.V., Akchurin A.A. et al., Refinement of current oil saturation calculation technique by evaluation of neutron characteristics of the rock (In Russ.), Karotazhnik, 2018, no. 3(285), pp. 38-49.
2. Burikova T.V., Savel’eva E.N., Khusainova A.M. et al., Lithological and petrophysical characterization of Middle Carboniferous carbonates (a case study from north-western oil fields of Bashkortostan) (In Russ.), Neftyanoe khozyaystvo = Oil Industry, 2017, no. 10, pp. 18–21, DOI: https://doi.org/10.24887/0028-2448-2017-10-18-21
3. Mirnov R.V., Alekseeva T.V., Paleosols in the Kashira deposits in the south of the East European Craton (Republic of Bashkortostan): characteristics, paleoecological and stratigraphic significance (In Russ.), Litosfera, 2022, V. 22, no. 5, pp. 694-704, DOI: https://doi.org/10.24930/1681-9004-2022-22-5-694-704
4. Kozhevnikov D.A., Neytronnye kharakteristiki gornykh porod i ikh ispol’zovanie v neftegazopromyslovoy geologii (Neutron characteristics of rocks and their use in oil and gas field geology), Moscow: Nedra Publ., 1982, 221 p.
5. Lawson C. L., Hanson R. J. Solving least squares problems, Prentice-Hall, 1974, 340 p.
6. Rezvanov R.A., Radioaktivnye i drugie neelektricheskie metody issledovaniya skvazhin (Radioactive and other non-electric methods of well survey), Moscow: Nedra Publ., 1982, 368 p.
7. MI 2083-90.Rekomendatsiya. Gosudarstvennaya sistema obespecheniya edinstva izmereniy. Izmereniya kosvennye. Opredelenie rezul’tatov izmereniy i otsenivanie ikh pogreshnostey (Recommendation. State system for ensuring the uniformity of measurements. Measurements are indirect. Determination of measurement results and estimation of their errors), Moscow: Izdatel’stvo standartov Publ., 1991, 11 p.
