Formation Process And Main Influencing Factors Of Thermogenic Solid Bitumen

Highly mature solid bitumen,as a residue of oil cracking,is widespread in the lower Paleozoic paleo-oil reservoirs in China.Solid bitumen is not a simple,pure component,but an organic matter with variable composition and structure.Therefore,it is of a great significance for source identification and resource evaluation of the deep oil and gas to strengthen the understanding of formation and evolution of solid bitumen.In this study,thermal simulation experiments were used to investigate the formation process of thermogenic solid bitumen and the main controlling factors,such as,oil composition?whole oil and different group components?,hydrocarbon source rock extract?expellable hydrocarbons and residual hydrocarbons?and reservoir environment influencing the formation of solid bitumen.Ten series of pyrolysis experiments were carried out in a closed gold tube system.Three of which were used to evaluate the influence of the group composition of crude oil on the formation of solid bitumen during the oil cracking process by thermal simulation experiments of main group components of crude oil?i.e.,saturated,aromatic,and resin+asphaltene fractions?.The five series were used to reveal the effects of reservoir pressure and water on the oil cracking by simulating the oil cracking under different reservoir conditions.The other two series dealt with the cracking of extracts?including expellable and residual hydrocarbons?from source rocks.Through quantitative analysis and carbon isotope analysis of pyrolysis products(including methane,C₂–C₅gaseous hydrocarbons,C₆–C₁₂ light hydrocarbons,C₁₃₊heavy hydrocarbons,and solid bitumen),this study tries to reveal the formation process of solid bitumen under geological conditions and the main factors that may affect the formation of solid bitumen.Based on the experimental results,a set of techniques and methods related to solid bitumen were established for gas source correlation and natural gas resource evaluation.Combined with the case study of gas reservoirs in Tadong Gucheng area,the experimental results and understanding were corrected and improved.To summarize the above work,main conclusions are obtained as follows:?1?The pyrolysis process of crude oil under anhydrous conditions can be divided into four stages:I—liquid hydrocarbon generation stage?EasyRo<1.0%?,II—condensate generation stage?corresponding EasyRo value 1.0¹.5%?,III—wet gas generation stage?corresponding EasyRo value 1.5².1%?and IV—dry gas generation stage?corresponding EasyRo value>2.1%?.The formation of solid bitumen begins at the condensate and wet gas generation stage,and mainly occurs in the dry gas stage,indicating that solid bitumen is the product of crude oil cracking at the higher mature stage.?2?The yield of solid bitumen produced by cracking of different groups of crude oil gradually increases with the increase of maturity.In the liquid hydrocarbon generation stage?EasyRo=0.5-1.0%?and condensate oil generation stage?EasyRo=1.0-1.5%?,a large amount of solid bitumen are rapidly generated from resin+asphaltene fractions.solid bitumen is generated in large quantities from saturated hydrocarbons and aromatic fractions in the wet gas generation stage?EasyRo=1.5-2.1%?and dry gas generation stage?EasyRo=2.1-4.5%?.When EasyRo=4.5%,the yield of solid bitumen generated by saturated hydrocarbon,aromatic hydrocarbon and resin+asphaltene fractions reached the maximum,41.91%,62.91%and 64.22%respectively.According to the content of different groups of components in crude oil,the contribution of saturated hydrocarbon components,aromatics and resin+asphaltene fractions in the process of cracking solid bitumen is up to 53.7%,20.3%and 26%respectively.Therefore,the corresponding solid bitumen yield of crude oil can be calculated according to the yield of solid bitumen in the cracking process of different groups of components obtained from the experiment,and the content of each component of different crude oil,without measuring the yield of solid bitumen of each crude oil.?3?The carbon isotope value of methane generated from the expulsion hydrocarbons is slightly lighter than that of the residual hydrocarbons.The?¹³C values of solid bitumen generated by different group fractions in crude oil at different stages of maturity have relatively constant?¹³C values?i.e.,-31.5‰±0.5‰?,and are more positive than the carbon isotope values of initial reactants.Thus,the?¹³C value of solid bitumen preserved in paleo-oil reservoirs can indicate the?¹³C value of their source rocks.In addition,the carbon isotope values of methane,ethane and propane generated by different groups of crude oil components gradually become heavier with the increasing cracking temperature,and there is a positive sequence distribution feature of?¹³C₁<?¹³C₂<?¹³C₃.?4?The experimental results show that the presence of water has little effect on the formation of solid bitumen.Under different pressure conditions?25MPa,50MPa,75MPa and 100MPa?,the experimental results of crude oil cracking also show that reservoir pressure has no significant influence on the solid bitumen yield,gaseous hydrocarbon yield and carbon isotope value of gaseous hydrocarbon generated in the process of crude oil cracking?5?The pyrolysis experiment of source rock asphalt?expellable and residual hydrocarbons?shows that the maximum yield of solid bitumen generated during the pyrolysis of residual hydrocarbons is obviously higher than that of solid bitumen generated by expellable hydrocarbons,while the gaseous hydrocarbon generated by residual hydrocarbon is lower than that generated by corresponding expellable hydrocarbons.On the one hand,this result can help us to evaluate the gas generation of residual hydrocarbons?equivalent to shale?and expellable hydrocarbons?equivalent to crude oil?.On the other hand,it can deepen the understanding of the composition and content of the solid bitumen formed by residual hydrocarbon in shale and the solid bitumen formed by crude oil cracking in reservoir.?6?There is a certain difference in the carbon isotope value of kerogen between the lower Cambrian Xidashan formation and the middle Ordovician Heituao formation in the two sets of source rocks in the Gucheng area of the eastern part of the tarim basin.The Heituao formation is heavier than-30.0‰,and the xidashan formation is lighter than-30.0‰in most cases.The carbon isotope value of the solid bitumen in the reservoir is about-32.0‰^-30.3‰.Combining the relationship between the carbon isotope value of the solid asphalt and the carbon isotope value of kerogen,it can be preliminarily concluded that the hydrocarbon source rock of the solid bitumen in the reservoir comes from the Xidashan formation.Most of the lower Paleozoic reservoirs in the Gucheng area of the eastern part of the tarim basin experienced a high maturity,and the cracking of crude oil mainly occurred in the late ordovician to silurian.Solid bitumen and methane were also generated in this stage,and their conversion rate was controlled by the maximum maturity.