Differential hydrocarbon enrichment in deep Paleogene tight sandstones of the Dongpu Depression in Eastern China

To clarify the characteristics and enrichment rules of Paleogene tight sandstone reservoirs inside the rifted-basin of Eastern China, the third member of Shahejie Formation (abbreviated as Es3) in Wendong area of Dongpu Depression is selected as the research object. It not only clarified the geochemical characteristics of oil and natural gas in the Es3 of Wendong area through testing and analysis of crude oil biomarkers, natural gas components and carbon isotopes, etc.; but also compared and explained the types and geneses of oil and gas reservoirs in slope zone and sub-sag zone by matching relationship between the porosity evolution of tight reservoirs and the charging process of hydrocarbons. Significant differences have been found between the properties and the enrichment rules of hydrocarbon reservoirs in different structural areas in Wendong area. The study shows that the Paleogene hydrocarbon resources are quasi-continuous distribution in Wendong area. The late kerogen pyrolysis gas, light crude oil, medium crude oil, oil-cracked gas and the early kerogen pyrolysis gas are distributed in a semicircle successively, from the center of sub-sag zone to the uplift belt, that is the result of two discontinuous hydrocarbon charging. Among them, the slope zone is dominated by early conventional filling of oil-gas mixture (at the late deposition period of Dongying Formation, about 31–27 Ma ago), while the reservoirs are gradually densified in the late stage without large-scale hydrocarbon charging (since the deposition stage of Minghuazhen Formation, about 6–0 Ma). In contrast, the sub-sag zone is lack of oil reservoirs, but a lot of late kerogen pyrolysis gas reservoirs are enriched, and the reservoir densification and hydrocarbon filling occur in both early and late stages.

Kerogen amount calculation required for the formation of hydrocarbon deposits in the Western oil and gas region of Ukraine

This paper considers the issue of determining the maximum hydrocarbons amount that can be generated by kerogen using thermodynamic methods. It is shown that the chemical composition of natural gas or gas condensate contains information about the generative capacity of kerogen from which it was formed. Based on experiments of type II and I kerogen pyrolysis and thermodynamic calculations by entropy maximization method, we propose a new method for determining the amount of kerogen from which gas was formed, which contains 1 dm3 of methane at a given ratio of butane isomers. The obtained data are interpreted as an indicator of kerogen maturity in the context of the depth of its destruction. This method is applied to theWestern oil and gas region of Ukraine hydrocarbon deposits. The analysis of kerogen transformations in the region sedimentary strata, using criteria of the GASTAR diagram, is carried out. We assessed the trends of kerogen conversion in the region in the areas of “maturity” and “biodegradation” in the ratio of ethane/propane (C2/C3) to ethane/isobutane (C2/i-C4). It is shown that the majority of deposits in the Western oil and gas region developed in the direction of maturation and only a small group of gas deposits – biodegradation. To establish the gases genesis in the region, we built a graph of the two geochemical indicators dependence – the methane/ethane ratio (C1/C2) and ethane/propane ratio (C2/C3). It is shown that some of the gas fields is formed due to the conversion of organic material of oil deposits. At the same time, gas condensate fields in the region, with few exceptions, are formed due to the primary destruction of kerogen. Based on the results of the calculations, maps of the methane (generated by type II kerogen) amount distribution were constructed. It is established that kerogen, which was the source material for hydrocarbon deposits of Boryslav-Pokuts oil and gas region, has practically exhausted its gas generation potential. Instead, kerogen from gas and gas condensate fields in the Bilche-Volytska oil and gas district still retains the potential to generate hydrocarbons.

Type I kerogen-rich oil shale from the Democratic Republic of the Congo: mineralogical description and pyrolysis kinetics

The Democratic Republic of the Congo holds important reserves of oil shale which is still under geological status. Herein, the characterization and pyrolysis kinetics of type I kerogen-rich oil shale of the western Central Kongo (CK) were investigated. X-ray diffraction, Fourier-transform infrared spectroscopy and thermal analysis (TG/DTA) showed that CK oil shale exhibits a siliceous mineral matrix with a consistent organic matter rich in aliphatic chains. The pyrolysis behavior of kerogen revealed the presence of a single mass loss between 300 and 550 °C, estimated at 12.5% and attributed to the oil production stage. Non-isothermal kinetics was performed by determining the activation energy using the iterative isoconversional model-free methods and exhibits a constant value with E = 211.5 ± 4.7 kJ mol−1. The most probable kinetic model describing the kerogen pyrolysis mechanism was obtained using the Coats–Redfern and Arrhenius plot methods. The results showed a unique kinetic triplet confirming the nature of kerogen, predominantly type I and reinforcing the previously reported geochemical characteristics of the CK oil shale. Besides, the calculation of thermodynamic parameters (ΔH*, ΔS* and ΔG*) corresponding to the pyrolysis of type I kerogen revealed that the process is non-spontaneous, in agreement with DTA experiments.