Carbon And Hydrogen Isotopic Characteristics Of N-alkanes In The Cambrian-Ordovician Source Rocks And Deep Oils From The Tarim Basin

The Cambrian-Ordovician source rocks from the Tarim Basin are generally in the high to over maturity stage,and the content of soluble organic matter of the source rocks is very low and thus easily affected by later migrated hydrocarbons.Deep marine oil from the Tarim basin is highly mature and has been affected by some secondary processes,the biomarkers are therefore very difficult to be detected.In recent years,the carbon isotopic composition of n-alkanes has become an important method for the study of the main hydrocarbon sources and mixing processes of marine oil from the Tarim Basin.The thermal evolution trend of crude oil can be further constrained by combining with the hydrogen isotopic composition of n-alkanes.Previous studies have mainly used traditional extraction method to study the carbon isotopes in free,soluble organic matter in the Cambrian-Ordovician source rocks from the Tarim Basin,and comparative analysis of carbon isotopic compositions bewtween free and kerogen-bonded n-alkanes is not well performed.In addition,there is also lack of both carbon and hydrogen isotope analysis of n-alkanes in source rocks.Four sets of source rocks from the Tarim Basin,i.e.the Lower Cambrian,the Middle-Upper Cambrian,the Lower Ordovician and the Middle-Upper Ordovician,have been selected as the main targets in this study.Both carbon and hydrogen isotopic compositions of n-alkanes,obtained by traditional extraction method,have been determined.Furthermore,kerogen-bound n-alkanes were extracted by high-pressure catalytic hydropyrolysis and their carbon isotopic compositions were determined.Both carbon and hydrogen isotopes in n-alkanes of the Ordovician marine oil from the Tazhong area were measured for a comparative reason.The significances of carbon-hydrogen isotopic composition of n-alkanes for the oil-source correlation and main hydrocarbon sources in the high to over maturity stage,have been discussed.As a result of these efforts,the following conclusions have been achieved:?1?The distributions of n-alkanes,either in the light oil from the Tazhong area,or in the samples of the Lower Cambrian and the Middle-Upper Ordovician outcrops from the Tarim Basin,generally exhibit a unimodal pattern.However,those of the extraction and hydropyrolysis products from the Cambrian-Ordovician drilling core samples show an evident preference for even carbon numbers between nC₁₄ and nC₂₀,which is obviously different from the cases of the Lower Cambrian and Middle-Upper Ordovician outcrop samples and the TaZhong light oil.The Pr/nC₁₇7 and Ph/nC₁₈ ratios of partial drilling core samples are not consistent with relatively high maturity of the samples,their significances for indicating the characteristics of hydrocarbon sources accordingly needs to be further studied.?2?The carbon isotope ratios of kerogens from the Tarim Basin generally follow the order of the Ordovician sourece rocks>the Cambrian drilling core samples>the Lower Cambrian outcrop samples.In addition,the carbon isotope ratios of the kerogens from the Lower Cambrian source rocks show obvious variations for different areas,which reflect the controlling effects by both the type of inputted orgainisms and the sedimentary environment.?3?The n-alkanes in the extracts from the Lower Cambrian outcrops have very light?¹³C values,and the?¹³C values of those in the hydropyrolysis products are within this range from-34‰to-31‰.The?¹³C values of n-alkanes in the extracts from the Cambrian-Ordovician drilling core rocks range from-32‰to-29‰,and those from the hydropyrolysis products show relatively large variations,ranging from-31‰to-28‰which is heavier than those from the extraction products.The?¹³C values of n-alkanes from the extracts of the Salgan and Yingan formations vary from-33‰to-30‰,and those from the corresponding hydropyrolysis products are similar.?¹³C values of n-alkanes from the Tazhong light oils are between-35‰and-31‰.?4??D values of n-alkanes extracted from the Lower Cambrian outcrops of the Tarim Basin are relatively heavy,ranging from-90‰to-60‰,and?D values of those extracted from the Salgan and Yingan formations are between-110‰and-80‰,which are slightly lighter than those from the Lower Cambrian outcrops.The n-alkanes extracted from the drilling core samples in the basin have the lightest?D value,ranging from-125‰to-95‰,and those from the Cambrian rocks of well Yingdong2 are obviously relatively low.?D values of n-alkanes in the Tazhong light oil vary from-120‰to-60‰.?5?Based on the comparison of the average carbon and hydrogen isotope ratios of n-alkanes between source rocks and the Tazhong oil,it is found that n-alkanes from the Tazhong light oils have more light carbon isotope(¹²C)and more heavy hydrogen isotope?D?.These characteristics are similar to those exhibited by the samples collected from the Lower Cambrian and Middle-Upper Ordovician outcrops.Combined with the evaluation of the effects of maturity on both carbon and hydrocarbon isotopic compositions,it is tentatively determined that the Tazhong light oil mainly originated from the Lower Cambrian source rocks of slope facies,and the Middle-Upper Ordovician source rocks of basin facies may also have contributions.