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ROCK-EVAL METHODS AND APPLICATIONS FOR REPRESENTATIVE SEDIMENT SAMPLES FROM CENTRAL SOUTH MOESIAN PLATFORM MARGIN

N. Botoucharov

Abstract

Rock-Eval pyrolysis is an exceptionally reliable and effective method in analysis and evaluation of source rocks. The suitable core or cutting sample choices, consequence of processing and investigations are indispensable condition for successful scientific applications. This study demonstrates geochemical data of representative pyrolysed Permian, Triassic and Jurassic fine grained sediments from Central South Moesian Platform Margin. The quantity and quality of the organic matter in rocks are measured and calculated after high temperature heating with Rock-Eval apparatus and Leco analyzer. As a result there are several important indices, which characterize hydrocarbon generative potential of the studied deposits (TOC, S1, S2, Tmax). The applied methods allow also calculation of HI and PI, which can be plotted versus Tmax, \%Ro and sample depths. According to the results of this investigation hydrocarbon generative potential of the selected Permian and Lower Triassic terigenous sediments is negligible. The geochemical parameters show low TOC content (0.10 to 0.20 wt. \%) and poor quality of the organic matter (kerogen type III and IV). These sediments are deeply buried under thick Mesozoic and Cenozoic complexes and are thermally over mature towards the Alpine thrust front. The source rock analyses demonstrate poor to fair petroleum potential for Middle Triassic carbonate deposits. The Ladinian shales, clayey limestones and marls (Mitrovo Fm.) are with higher organic matter content in some cases exceeding 0.90 \% TOC. The Formation is mature and over mature containing kerogen type II/III and III. These preconditions and relatively thin sections determine a small oil-gas potential. The Upper Triassic carbonate and terigenous-carbonate complex demonstrate poor potential. TOC values are locally up to 0.80 wt. \%, but usually between 0.12 to 0.36 \%. The kerogen type varies II, II/III and III. Thermal maturity of the investigated samples is in the range of late mature to over mature in the southernmost zones of the study area. The Jurassic sediments are considered as the primary source rock in North Bulgaria. The clayey limestones, marls and shales of Bukorovo member (Ozirovo Fm.) and pure to silty shales of Stefanets member (Etrople Fm.) are with the best hydrocarbon potential. They are enriched in organic matter with TOC values usually more than 1.0 wt. \%. The kerogen type is I, II and sometimes II/III. The total thickness is from few hundreds to more than 1000 m. Maturity varies from immature and early mature in the northern part of the Moesian platform to over mature towards the Alpine thrust front.

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