Geochemical Characterization Of Crude Oil In Shuntuoguole Low-uplift Of Tarim Basin

Eleven?11?crude oil samples from Shuntuoguole low-uplift of the Tarim Basin have been analyzed by routine GC-MS technique to elucidate their molecular compositions and genetic origins.Globally,carbon isotopic value of the Cambrian originated oil is lighter than that from the Ordovician,while heavy carbon isotopic value was assigned to the Cambrian sourced oils,which caused numerous confusions in oil-source correlation partially due to the complicated geological history and various secondary alterations.Oil maturity has been assessed by biomarker ratios,such as terpane index,sterane index and diamantine index.While some difference has been illustrated,all oils show very high maturity in nature.Secondary alterations includingthermalcracking,biodegradation,evaporativefractionation,thermochemical sulfate reduction?TSR?and abnormal thermal event influence have been examined.Oils are suffered varying degree of thermal cracking.Only one oil has suffered from biodegradation in history and two oils have experienced fractionation.However,no obvious TSR and unusual thermal event occur in the studied area.Based on the different n-alkane distribution modes,relative abundance of isoprenoid alkanes,sterane and terpane component distributions,the studied crude oil samples can be divided into two groups.New geochemical indexes proposed in the present study include isoprenoid to n-alkane ratios,2-methyl to 3-methyl alkane ratios,C₁₉ tricyclic terpane to C₂₀ tricyclic terpane,C₂₄ tetracyclic terpane to C₂₆ tricyclic terpane ratios.Oil samples from wells of S7 SX1 SX101 have low content of isoprenoid,high 2-/3-methyl alkane ratios and high C₁₉ tricyclic terpane abundance are possibly derived from the Upper Ordovician source rocks which are enriched in macroalgae fragment and deposited under dysoxic environment,while oil samples from Shunbei and Yuejin area showing the opposite geochemical characteristics can be classified into another group,which is similar to most oils distributed in the Tazhong and Tabei uplifts and possibly derived from the Cambrian-lower Ordovician algal enriched source rocks.