Molecular Composition Of Petroleum In Unconventional Reservoirs And Its Formation Mechanism

The discovery of unconventional reservoirs,such as high temperature petroleum and high pressure petroleum,has broken the understanding of traditional petroleum geology theory.The formation mechanism of unconventional petroleum is more complicated than conventional petroleum,however,there were few reports about the composition of organic compounds in unconventional petroleum,especially those with high molecular weight,high polarity and low volatility.In this paper,ultra-deep petroleum from Tarim Basin,ultra-high sulfur petroleum from Bohai Bay Basin,outcrop source rock from Gonghe Basin were collected as the main research objects.Selective separation methods combined with high resolution mass spectrometry were applied to characterize the molecular composition of the special organic compounds in unconventional petroleum,the source and formation mechanism of these compounds was also speculated.The phenomenon of the methylation reaction of small molecule aromatic hydrocarbons in petroleum was observed.The complexation of terminal methylation products of alkylbenzenes with Ag⁺led to the formation of Ag-cationized monomer and homo-and hetero-Ag-cationized dimer complexes in the positive-ion electrospray ionization source?ESI?mode,which could lead to incorrect assignment of sulfur compounds.The possible reaction pathways for the heptamethylbenzenonium and silver adduct ions was proposed.Feasible suggestions are made to eliminate the interference by alkylbenzenes methylation and silver adduct ions,which could improve the method for mass spectrometry analysis of molecular composition of petroleum sulfur compounds based on methylation reaction.Sulfur and nitrogen heteroatom compounds in 57 ultra-deep petroleums from different blocks of Tarim Basin,China were systematically characterized by gas chromatographic sulfur chemiluminescence detector?GC-SCD?,gas chromatographic nitrogen chemiluminescence detector?GC-NCD?,electrospray ionization mass spectrometry?ESI MS?,Atmospheric pressure photoionization mass spectrometry?APPI MS?.Selective Michael addition derivatization followed by Fourier transform ion cyclotron resonance mass spectrometry?FT-ICR MS?and comprehensive two-dimensional gas chromatography-mass spectrometry?GC×GC MS?analysis was applied to molecular characterization of thiols in the selected deep strata crude oils?condensates?.Abundant diamondoidthiols found in condensates were from high-temperature reservoirs.The pronounced ³⁴S enrichment in H₂S and high-abundance diamondoids indicated that the oils suffered severe TSR alteration.Various new thiol compounds with multi heteroatoms?S₂,S₃,S₁O₁,S₁O₂ and S₂O₁ class species?were detected in condensates from low-temperature reservoirs.The low reservoir temperature made it possible to preserve the thiol compounds with multi heteroatoms.The pathway of thiols formation was speculated as follows:the TSR-generated H₂S reacted with petroleum hydrocarbons to produce thiols.The results provided new insights into the TSR reaction which may assist regional oil exploration and facilitate to evaluate the extent of TSR alteration.The thiophenic and sulfidic fractions were isolated from the ultra-high sulfur crude oil from Jinxian Sag of Bohai Bay Basin,China by methylation/demethylation and characterized by FT-ICR MS and GC-MS.Abnormal high abundance of sulfur compounds with 27-29 carbon atoms were assigned to sulfurated steranes and sterenes with tetrahydrothiophene moieties.Other structural sulfur compound types were also detected in thiophenic and sulfide fractions:isoprenoid thiophenes,isoprenoid benzothiophenes,long-chain 2,5-di-n-alkylthiolanes,2,5-di-n-alkylthianes,bicyclic terpenoid sulfides,etc.Acidic sulfur compounds in crude oils from Jinxian Sag and Jiyang Sag were characterized by Orbitrap mass spectrometry?Orbitrap MS?.New sulfate species were detected,which have not been reported in fossil fuels.The high abundant sulfur compounds in crude oils from Jiyang Sag were composed dominantly of those S₁with DBE=1 series,which was possibly the results of light sulfurization.The complex distribution for the sulfur compounds and the coincidence for carbon number distribution on biomarkers in crude oil from Jinxian Sag was possibly the results of severe sulfurization.Ketones in source rock from Gonghe Basin,China were characterized by GC-MS,FT-ICR MS,Orbitrap MS.Abundant ketones with different structure types were detected,which were believed to be derived from the oxidation of hydrocarbons.Ketones were derivatized with the Girard T reagent under weakly acidic conditions to enhance their detectability by ESI MS analysis.APPI MS was also used for the molecular characterization of ketones without chemical derivatization.The chemical derivatization followed by positive-ion ESI MS analysis is suitable for the analysis of ketones in a trace amount,but it had discrimination on high double bond equivalent?DBE?species?DBE?9?.Direct analysis with positive-ion APPI MS showed comparable ionization efficiency throughout the DBE range.Based on the analysis results,an analytical method for distinguishing aliphatic ketones and aromatic ketones in fossil fuels according to different ionization pathways was proposed.