Thermal destruction (fragmentation) in a variety of its methods is the main technique for analytical and geochemical characterization of oil shales. The yield of the fragmentation products as well as their molecular and structural distribution and degree of conversion can be very different. For that reason there arises the inescapable question about the alterations which will occur in the composition of the products when using different thermal destruction methods and just what kind of general strategy should be used for the analytical investigation of sedimentary rocks. In this article, data for gas extraction and hydrogenation of two shales are provided. These results are from tests with variables optimized in order to achieve maximum yield of liquid product, as compared to results from Fischer Assays. It is shown that 97 % conversion of kukersite kerogen into benzene-soluble liquid product is feasible with CO2 extraction (this process achieved solid-residue free product and with only 3% by-product gas formation). The Dzham shale (Uzbekistan) does not liquefy as readily, providing a maximum yield of benzene-soluble product of only up to 55% of the feed kerogen. These tests used hydrogen as produced from HCOONa. Distribution of heteroelements (O, N, S) among destruction products depends on the method of thermal destruction. It was shown to be quantitatively different: as a result of semi-coking, 50 % of kukersite and 69 % of Dzham kerogen heteroelements were transformed into gaseous products, but using gas extraction or hydrogenation under the mild conditions (360-degrees-C, 4-6 hours) to destruct Dzham shale, 86 % of heteroelements remain in the kerogen and they do not combine with the gaseous or liquid products. Conversion rates for liquefaction of different kerogens can vary, even when they are treated under the same conditions. As a result, non-comparable products are obtained. This precludes a successful categorization of kerogens. Yield of the liquid product with gas extraction is considerably higher than with a semi-coking process. This is caused by retention of intermediate fractions - asphaltenes and highly-polar malthenes in the extract. With further thermal treatment the asphaltenes and highly-polar compounds, which are thermally unstable, form gas and coke. The asphaltene content is highest in the extract and lowest in the semicoking oil (49 and 3 % for Kukersite and 7 and 0 % for Dzham shale, respectively). A significant observation is that the produced alkenes and homologous series of n-alkanes, n-alkanones and alkylarenes have characteristics which are different than those in the primary product. As a result of continual formation and accumulation of identical primary and secondary compounds during thermal treatment, similarities in the concentrations of those compounds reflect the degree of destruction of the kerogen and its fragments. But they are not characteristic of the kerogen composition. Char produced from Dzham kerogen by gas extraction or hydrogenation, unlike semi-coke, is capable of producing liquid product in higher yield. This potential for additional liquid product can be realized under special experimental conditions. Therefore, effective liquefaction of different kerogens can be accomplished using a multi-stage thermal destruction procedure. It is shown that numerous essential differences in the composition of the liquid products from gas extraction, hydrogenation and semi-coking are mainly caused by different yields of liquid products. This is based on tests using physico-chemical, chromatographic and infrared spectroscopic methods. To reflect the original kerogen structure and to understand the changes taking place during destruction, all structural units and fragments of kerogen decomposition should be taken into account. It is shown that the higher the yield of liquid product, and the value of physico-chemical parameters (such as d20(20), n(D)20, and M), the higher the content of asphaltenes and the concentration of odd carbon-number homologues in the fractions of n-alkanes, alkanones and n-alkylarenes. The value of the H/C ratio approaches that of the source kerogen. Results of the quantitative interpretation of infrared spectra show that the concentrations of -OH and -C = O functional groups increase in conjunction with -(CH2)n- concentrations, as the yield of liquid product increases. The yield and extent of destruction of liquid product from gas extraction, hydrogenation and pyrolysis is very different and ambiguous. Ideas about structure are achieved when either a single method is used or various methods are applied in parallel. A stepwise scheme for effective thermal destruction of kerogen is suggested in order to optimize kerogen transformation into the liquid product. Hence, the liquid product contains the identity of the source kerogen within its composition. The mechanism of kerogen destruction could be controlled by using a multiple-step destruction process on the intermediate compounds. Various kerogens could be adequately categorized on a common basis: equal (and maximal) yield of liquid product.