Thermal Effects on Composition of Rearranged Hopanes in Hydrocarbon Source Rocks

Objective The distribution characteristics and formation mechanism of rearranged hopanes in hydrocarbon source rocks are affected by various geological conditions.Among these geological conditions,thermal action has an important influence on the formation of rearranged hopanes,which has been however little documented previously.

Characteristics of Rearranged Hopanes of Hydrocarbon Source Rocks in Saline Sedimentary Environment： A Case Study of the Songliao Basin

The Songliao Basin, one of the biggest continental petroliferous basins in eastern China, is a Mesozoic- Cenozoic fault-depressed and fault-subsided basin developed on the Hercynian fold basement. Generally, rearranged hopanes are considered to be formed by clay- mediated acidic catalysis under oxic or suboxic environment, whereas high abundance of rearranged hopanes were found in hydrocarbon source rocks and crude oils that are derived from salty environment in the Songliao Basin. This phenomenon rarely happens all over the world.

New Discoveries of the Influence of Sedimentary Environment on Rearranged Hopanes in Source Rocks

Objective The distribution of rearranged hopanes in hydrocarbon source rocks is influenced by thermal maturity and original source input, and is also controlled by depositional conditions. Through comparison of lacustrine and coal-bearing source rocks, this work attempted to analyze the composition and origin of rearranged hopanes in hydrocarbon source rocks. Taken the source rocks from the Songliao Basin, Ordos Basin and Kuqa Depression as examples, we aimed to investigate the effect of the redox conditions, water salinity and oxygen content of the source-rock depositional environment on the formation of rearranged hopanes to provide theoretical basis for the genesis of rearranged hopanes.

Genesis of Abnormally High Abundant Rearranged Hopanes in Condensates from the Kuqa Depression, Tarim Basin

Objective The Tarim Basin is China＇s largest ore-bearing interior basin, and contains mainly marine oils. The Kuqa depression, a secondary structural unit within the northem Tarim Basin, is composed of Mesozoic and Cenozoic clastic sedimentary rocks dominated by continental oil. Previous research suggests that the crude oils especially condensates in the Kuqa depression are rich in abnormally high abundant rearranged hopanes. On the basis of 41 condensate samples and five oil samples from the Kuqa depression, this work systematically discussed the relationship between biomarker parameters and rearranged hopanes and deeply investigated the influence of depositional environment, original source,

The Biomarkers in the Mesoproterozoic Organic-rich Rocks of North China Craton:Implication for the Precursor and Preservation of Organism in the Prokaryotic Realm

Mesoproterozoic marine organic-rich rocks are widely distributed in the North China Craton,include the Gaoyuzhuang(GYZ),the Hongshuizhuang(HSZ),and the Xiamaling(XML)formations.According to the T;value and isomerisation ratio of C;homohopanes,the XML,HSZ,and GYZ samples were in low mature,mature and high mature stage,respectively.Biomarker distribution in extractable organic matter(EOM)of three Mesoproterozoic organic-rock samples in different maturity were analysed to reveal the organic precursor and preservation pathway of in the Mesoproterozoic Combined with gold-tube pyrolysates of three Mesoproterozoic samples,it could further illuminate the chemical composition of Mesoproterozoic kerogen,given excluding.The results indicated that the three formations were all deposited under reducing condition and their organic precursors mainly were some aquatic organisms.High content of rearranged hopanes was detected in EOM of XML and HSZ samples,whereas they were relatively low in the high mature GYZ sample.Contrast to that in EOM,the relative concentration of rearranged hopanes sharply decreased in the gold-tube pyrolysates of the XML kerogen,then slightly increased but was still significantly lower than the EOM of XML sample,which indicated that catalysis of clay minerals in the early diagenesis only changed the chemical composition of the unstable functional groups of the kerogen during the preservation.Due to the thriving heterotrophic microbes and low sink rate of particulate organic matter during the Mesoproterozoic,primary producers suffered extensive degradation during sinking process,only some resistant biopolymers lacking of lipid compounds survived from heterotrophic degradation,while heterotrophic microbes contained more proportion of organic precursors.Abundant pristane(Pr)and phytane(Ph)were only released in high mature stage because of the protection of the macromolecular structure of resistant biopolymers which prevented biomarkers from being altered by the thermal stress.The absence of 13α(n-alkyl)-tricyclic terpanes in the high matured hydrocarbon products also indicated the different precursors between different parts of Mesoproterozoic kerogen.The evolution of the biomarker composition and content of Mesoproterozoic kerogen showed some special characteristics differing from those of Phanerozoic kerogen.The total concentrations of hopanes displayed with an order of low mature stage>high mature stage>mature stage.Relative content of rearranged hopanes in the hydrocarbon generated in high mature stage was significantly lower than that in the low maturity stage.The ratios of Pr/n-C_(17) and Ph/n-C_(18) increased with thermal maturity,and the ratio of nC_(21-)/nC_(22+) decreased in the high maturity stage,thus displaying another order of mature stage>high maturity stage>low maturity stage.The unique preservation pathway of Mesoproterozoic organisms was attributed to the special evolution characteristics of biomarker distributions,which should be considered in the Mesoproterozoic marine environment and biological studies.