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天然气地球科学 2004
DISTRIBUTION OF ACID BEARING-OXYGEN COMPOUNDS AND ITS GEOCHEMICAL SIGNIFICANCE IN SOURCE ROCKS OF BIOGAS, BINBEI AREA, SONGLIAO BASIN
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Abstract:
The immature or lower-mature source rock was collected from the Cretaceous of Binbei area in Songliao Basin, with low vitrinite reflectance of 0.33%~0.70%. Acid bearing-oxygen compounds in non-hydrocarbon consisted of normal-monocarboxylic acid, normal-binarycarboxylic acid, pristane acid, phytane acid, hopanoid acid and steroid acid. Normal monoacid had a higher content and occurred two modeles of peak distribtuion, the one was single peak group with C_(16) dominant position and C_(18), C_(14) higher aboundance, which indicated the input of algae, the other had two peak groups, the fromer with C_(16) main peak characterized by aquatic organisms source matirials, the latter group was secondary, with C_(24) higher peak, this suggests the input of terrestrial higher plants transformed by bacteria. Those acid non-hydrocarbon was dominated by even carbon number, due to the fatty acid synzyme of organisms synthetized acetyl unit with two carbon atom. Normal binaryacid had a lower content and the amount of hopanoid acid was more than steroid acid. In the most of samples, those ratios of Pr/Ph acid and ster/hopan acid decreased with increasing of depth, this suggests that depositional envirenment changes to reducing conditon. As de-carboxyl reaction taked place, acid bearing-oxygen compounds, in particular nonmal alkyl fatty acid was formed into hydrocarbon and CO_2, not only supplied for the large amount of nutritive energy-carbon origin, also supplied for the largest substrate of respiratory metabolism to methanogenic bacteria During Qingshankou and Nenjiang periods of Cretaceous, water system changed to deep with expanding of lake area, shallow and mid-deep lacustrine facies deposit occurred at parts of Binbei area, leading to biogas field was formed.
