Discussion On The Origin Of The Reversal Stable Carbon Isotope Distributions Between Kerogen And Its Derivatives, And The Geochemical Significances To The Marine Crude Oil Reservoirs In Tarim Basin, NW China

The stable carbon isotope reversals of organics found in the earth history can be sorted into two types. One is the carbon isotope reversal among different soluble constituents of source rocks or group compositions of crude oils, and the other one is the reversal between the soluble constituents of source rocks / crude oils and kerogens. The former one has been much discussed and some common viewpoints attained concerning its origin, while the latter one has a lot of disputes about its origin up to now.In this study two types of shale samples with low thermal maturity(RO ≤ 0.8 %) were used for thermal simulation pyrolysis, one taken from Xiahuayuan region Neoproterozoic Xiamaling formation, northern China and the other from Santanghu Basin Permian Lucaogou formation, northwestern China. The kerogens prepared from the shale rocks were thermal pyrolyzed in the confined gold tubes under different temperatures, while the rock samples were done in the closed glass tubes. The results indicated that the reversal carbon isotope distribution pattern was observed between pyrolyzed oils and the residual kerogens. Both primary biomass origin and thermal maturity level could stress influence on the stable carbon isotope distribution of the residues, pyrolyzed oils and hydrocarbon molecules, as found for the Xiamaling formation calcareous shale samples. This suggests that the primary biomass origin is the key factor controlling the reversal carbon isotope pattern. Meanwhile, the δ13C17-δ13C18- δ13CPr- δ13CPh distribution sequence from pyrolyzed oils was found inherited from their source rocks. And thus this kind distribution pattern has been used for the oil-source correlation of the marine crude oil reservoirs in the Tarim Basin. We found that the majority of crude oils from Tabei and Tazhong district have the same sequence of δ13C17- δ13C18- δ13CPr- δ13CPh as compared with TD2?, TZ62 S and YM2 O. This result is consistent with the mainstream opinions ―Most crude oils from Tabei and Tazhong district are contributed from end oil members as represented by TD2?, TZ62 S and YM2O‖.Two well-exposed outcrop sectionprofiles from the Cambrian strata at the southern Kuluketage area in the Tarim basin were comprehensively studied concerning their basic geochemical properties. The results indicated that δ13Ckerogen excursion along the Cambrian strata may be resulted from changes in planktonic algae and benthic algae in response to sea level decrease, and crude oils enriched in 13 C such as those from wells TD2? and TZ62 S in the Tarim Basin may be derived from some regional upper Cambrian source rocks. In comparison with those data from all nine profiles in Kuluketage region and TD2 well, we built up a three-box geochemical stratification model which was influenced by Dissolved Organic Carbon(DOC) and provided an innovative explanation for the well-known fractionated δ13Ccarb and δ13Corg between the platform and deep seawater. In this case, the role of high partial pressure of carbon dioxide was critical.Comparing with the carbon isotope reversal signatures existing globally in Proterozoic sediment and lower Palaeozoic black shale sediment from southern China, the conclusion was arrived at that the Ckerogen from Cambrian-lower Ordovician in Tarim Basin were probably related to surface strati?cation of the oceans formed from higher atmospheric pCO2 and frequent sea level changes.