原油及源内残余沥青裂解成气差异及地质意义

为了研究不同类型原油和源内残余沥青在高演化阶段的甲烷产率，明确天然气成因类型，系统整理了不同类型原油及源岩的金管模拟实验结果，统计了甲烷产率随模拟温度的变化，发现原油性质对生气过程和生气量都有明显控制，重质油起始生气温度低于轻质油和正常油，在原油裂解成气初期，甲烷产率变化为重质油>正常原油>轻质油，重质油对天然气成藏贡献较大；在原油大量裂解过程中，轻质油的甲烷产率很快超过正常原油和重质油，最终成为天然气成藏的主力。重质油产气早是因为其富含非烃和沥青质，裂解活化能低，产气率低与H/C值（原子比）低有关，轻质油产气晚是因为其富含饱和烃，裂解活化能高，产气率高与H/C值（原子比）高有关。轻质油开始裂解对应成熟度约为Easy% R_O=1.5%。干酪根及源内分散沥青生气与原油裂解受相同的因素控制，H/C值高低控制了不同类型干酪根的生气量，在各成熟阶段上甲烷产率始终是Ⅰ>Ⅱ>Ⅲ型有机质。源内分散沥青在化学组成上接近重质油，但比重质油更容易裂解，除活化能低外，还受到黏土矿物催化的影响，其起始裂解成熟度大体为Easy% R_O=1.0%。这种差异对热演化程度极高的四川盆地天然气成因类型确定和潜力评价有非常重要的地质意义。

Difference in gas generation from thermal cracking of oil within reservoir and from residual bitumen within source rock and its geological significance

In order to study methane generation yields from different types of crude oil within reservoirs and from residual bitumen within source rocks during thermal cracking process and to determine natural gas genetic types，pyrolysis experimental results in closed gold tube system have been systematically evaluated in the present study.Correlation between methane yields and heating temperatures has been established.Physical properties of crude oil exert critical influence on gas generation temperature and potential.Initial temperatures of gas generation from heavy oil and bitumen are lower than these from light oil and normal oil.In the early stage of oil cracking，methane yield from heavy oil is higher than that from normal oil and light oil.In the main oil cracking stage，methane yield from light oil exceeds normal oil and heavy oil rapidly and becomes the major contributor to gas inventory.Such difference is caused by their chemical compositions.Heavy oil is rich in resins and asphaltenes which have low activation energies so that its cracking to gas is easy，while its low methane yield is controlled by low H/C ratio.Light oil is rich in saturated hydrocarbons which have high activation energies so that its cracking to gas is difficult and starts atEasy% R_O=1.5%，whereas its high methane yield is governed by high H/C ratio.The cracking of kerogen and residual bitumen within source rocks share some similarity as crude oils.Methane yield is controlled by H/C ratios with an order of Ⅰ>Ⅱ>Ⅲ in all situations.The chemical compositions of residual bitumen are similar to heavy oil，which is prone to crack at relatively low temperature due to lower activation energy.Meanwhile，clay minerals in source rocks can take a rule as catalyst，which can further facilitate thermal cracking process.Simulation results suggest that thermal cracking of residual bitumen starts at Easy% R_O=1.0%.Such difference has significant impact on gas genetic type determination and resource potential evaluation in over matured petroleum systems like Sichuan Basin.