In the Tarim Basin,black shale series at the bottom of Cambrian is one of the important marine facies hydrocarbon source rocks. This research focuses on the analysis of the isotope of noble gas of 11 cherts. The R/Ra ratio of chert in the Keping area is 0.032―0.319,and 40Ar/36Ar is 338―430. In Quruqtagh the R/Ra ratio is 0.44―10.21,and 40Ar/36Ar is 360―765. The R/Ra ratio of chert increases with 40Ar/36Ar from the west to the east accordingly. They have evolved from the crust source area to the mantle source area in a direct proportion. Surplus argon 40ArE in chert is in direct proportion to the R/Ra ratio,indicating that it has the same origin of excess argon as in fluid and mantle source helium. Comparison of the R/Ra ratios between the west and the east shows that the chert in the eastern part formed from the activity system of the bottom hydrothermal venting driven by the mantle source,where the material and energy of crust and mantle had a strong interaction in exchange;whereas in the western part,chert deposited from the floating of hydrothermal plume undersea bottom,which is far away from the centre of activities of the hydrothermal fluid of ocean bottom. In addition,from noble gas isotope composition of chert,it is suggested that the ocean anoxia incident happened at the black shale of the Cambrian bottom probably because of the large-scaled ocean volcanoes and the followinghydrothermal activities.
SUN ShengLi1,CHEN JianFa2,3,ZHENG JianJing4 & LIU WenHui4 1 Monitoring Center for Marine Resource and Environment,Guangdong Ocean University,Zhanjiang 524088,China
This study reversed the developing environments of hydrocarbon-source rocks in the Ordos Basin and evaluated carbonate rocks as hydrocarbon-source rocks and their distributions on account of the fact that China’s marine carbonate rocks as hydrocarbon-source rocks are character- ized by intensive thermal evolution and relatively low abundance of organic matter, by taking the Lower Paleozoic of the Ordos Basin for example and in light of the calculated enrichment coefficients of trace elements, the REE distribution patterns, the mathematical statistics analysis of elements and carbon isotopes and their three-dimensional diagrammatization in combination with the necessary organic parameters. As for the Ordos Basin, TOC=0.2% is an important boundary value. Studies have shown that in the strata where TOC is greater than 0.2%, Ba is highly enriched with positive δ 13Ccarb shifts and δ 13Corg less than ?28‰, reflecting a thigh paleo-productivity, high burial amounts of organic matter, relatively good hydrocarbon-generating potentiality and intensive REE fractionation. All these indicated that the settlement rates are low and the geological conditions are good for the preservation of organic matter, hence favoring the development of hydrocarbon-source rocks. The Klimory and Wulalik formations show certain regularities in those aspects and, therefore, they can be regarded as the potential effective hydrocarbon-source rocks. In the strata where total TOC is less than 0.2%, the contents of Ba are low, δ 13Ccarb values are mostly negative, and δ 13Corg values range from ?24‰― ?28‰, demonstrating low burial amounts of organic matter, poor potentialities of hydrocarbon gen- eration, weak REE fractionation and rapid settlement rates. These facts showed that most of the hy- drocarbon-source rocks were formed in shallow-water, high-energy oxidizing environments, thus un- favoring the development of hydrocarbon-source rocks. It is feasible to make use of the geochemical method to comprehensively assess the highly evo
The organic matter-rich strata of the Yurtusi Formation of Lower Cambrian are wildly spread and steady in the Tarim Basin, in which cherts are developed, companied with phos-phorite. Al/(Al+Fe+Mn) ratio and Si/(Si+Al+Fe) ratio of cherts range from 0.0023 to 0.0046 and 0.965 to 0.98, respectively, suggesting that cherts are formed in submarine hydrothermal activity and far away from terrestrial. Trace elements, such as As, Hg, Pb, Zn, Cu, Co, P, V, Ba, etc., are markedly rich in organic matter-rich sedimentary rocks of which Ba/Sr ratios are between 21.2 and 158.1. Compared with modern hydrothermal sediments, their Ba/Sr ratios are similar, it in-dicates that the organic matter-rich strata have the geochemical feature of submarine hydro-thermal sediments. The total content of rare earth elements (SREE) in cherts is from 8.81 to 56.682 mg/g, on average 31.41 mg/g and the SREE of cherts is between continental margin cherts and abyssal cherts. The LREE/HREE ratio of cherts varies from 1.01 to 3.56, which re-veals the characteristics of hydrothermal sediments. There is positive correlation between the total organic content (TOC) and Ba/Sr ratio which is geochemical index of submarine hydro-thermal activity, which indicates that the submarine hydrothermal activity obviously influences the richness of organic matter in the Yurtusi Formation.
CHEN Jianfa1, SUN Shengli2, LIU Wenhui3 & ZHENG Jianjing3 1. Petroleum University, Beijing 102249, China
