Thermophysical study of the Solotvyno depression: history, methodology and potential development for solving modern applied tasks

Abstract

Background. The studied area belongs to the south-eastern part of the Transcarpathian basin. The complex of studies, conducted since the middle of 20th century, has shaped the current vision on geological structure, perspective of the exploration and exploitation of mineral deposits. However, there are still many uncertainties caused by methodological, instrumental, and systemic shortcomings in previous studies, which reduce the region's investment appeal for future research. The development of new geological exploration projects requires a comprehensive analysis and reinterpretation of accumulated geological and geophysical data, as well as the implementation of modern computer modeling technologies to minimize existing risks. This article presents the results of the analysis of laboratory studies of core samples and thermal logging data, as well as their integration into one-dimensional thermal models, using a number of wells in the Solotvyno depression as an example.
Methods. The sources of the raw data include archival geological and geophysical materials, the latest monographs, and publications in domestic and foreign scientific journals. A comparison of thermal logging data obtained after wells had been left to settle for varying periods of time was made. Methods of mathematical statistics were used to process the core data – histograms were constructed showing the distribution of thermal conductivity and temperature conductivity coefficients for lithotypes with statistically significant samples: tuffs, sandstones, siltstones, and argillites. One-dimensional thermal models were constructed taking into account the stratigraphic and lithologic subdivision of the section, heat flux data, existing reconstructions of the thermal history, and the parameters of oil and gas source rocks in adjacent areas.
Results. Most of the thermal logging data from the Solotvyno depression underestimate formation temperatures by 10–15 °C. The most studied areas in terms of temperature conditions and thermophysical parameters of rocks are the Danylovo–Tereblia and Solotvyno areas; the least studied are Nankovo and the region between the Hrushiv and Teresva areas, which have not been investigated by deep drilling. Based on 124 determinations of the thermal conductivity coefficient and 59 of the temperature conductivity coefficient, an overlap in the distributions for tuffs and shales, siltstones, and sandstones was established; the main statistical characteristics of the studied samples were determined. For one-dimensional models, the theoretical temperature changes were found to agree with the thermal logging data within the margin of error determined by the reliability of these studies, and new data on the history of oil and gas-bearing systems within the Solotvyno depression were obtained.
Conclusions. The developed models indicate a likely biogenic origin of the gas in the deposits of the Teresvynska and Solotvynska formations, and a thermogenic origin for the Novoselytsia formation and older horizons. According to the updated chronological framework for the development of the Solotvyno depression, four oil and gas-bearing systems have been identified, and the time intervals for the formation of their components and the windows for hydrocarbon generation have been refined. Future modeling efforts will focus on reinterpreting well-logging data and conducting a detailed comparison of theoretical and observed values of the rocks' thermophysical parameters.