页岩气形成机制的生烃热模拟研究

选择Green River，Woodford及珠江口等3种不同有机质类型的低成熟页岩，采用密封金管-高压釜体系，在24.1 MPa围压下，以20 ℃/h和2 ℃/h的升温速率进行生烃热模拟实验，测定产物气态烃和液态烃的产量及气态烃的碳同位素组成，探讨页岩气生成机制。随温度的升高，页岩有机质热演化成烃可分为3个阶段：① 热解初期的生油阶段，液态烃产量增加迅速，在Ro=0.9%～1.1%时达到峰值，气态烃产量较低；② 油裂解生气阶段，液态烃产量降低，气态烃产量迅速增加；③ 气裂解阶段，气态烃产量略微增加，气体明显变干。气态烃的δ 13C先变轻后变重，I型页岩气态烃的δ 13C在热解初期阶段呈现局部倒转，δ 13C1 <δ 13C2 >δ 13C3 <δ 13nC4。气态烃来源于有机质的初次裂解和大量液态烃的二次裂解，气态烃的产量主要受有机质类型的控制，I型泥页岩气态烃产量最高。页岩气中碳同位素局部反序可能是液态烃二次裂解成气的标志，或由不同演化阶段来源气态烃混合造成的。

Pyrolysis constraints on the generation mechanism of shale gas

The three types of low mature organic-rich samples from US Green River shale(Type I kerogen)，Woodford shale(Type II kerogen)，and China Pearl River Mouth Basin(Type III kerogen) were chosen as the initial samples for pyrolysis simulation on hydrocarbon generation.A pyrolysis system of sealed gold tubes in high pressure vessels was employed to conduct pyrolysis experiments at two heating rates of 20 ℃/h and 2 ℃/h under a constant confining pressure of 24.1 MPa.The yields of gaseous hydrocarbons(C1-5) and liquid hydrocarbons(C6-14 and C+14)，and the carbon isotopic compositions of gaseous hydrocarbons(C1-C4) were measured to reveal the generation mechanism of shale gas.The results show that the organic matter conversion from organic-rich shales to oil and gas can be divided into three stages：oil generation，gas generation from oil secondary cracking and gaseous hydrocarbons cracking.At the stage of oil generation by the thermal decomposition of primary kerogen，the yields of liquid hydrocarbons increase rapidly with the increase of temperature，and reach the peak values at approximately 0.9%-1.1% Ro，whereas gaseous hydrocarbons show low yields.At the stage of oil secondary cracking，gaseous hydrocarbons increase rapidly in the yields associated with the decreases in the yield of liquid hydrocarbons.At the stage of gaseous hydrocarbons cracking，the gas yields increase slightly，whereas gas is getting drier and enriches in CH4.The carbon isotopes of gaseous hydrocarbons vary from heavier to lighter with the increase of temperature，and reach the lightest values before the end of oil cracking，and then become heavier.The partial reversals of carbon isotopic compositions of gaseous hydrocarbons at the stage of oil cracking is observed for Type I shale，i.e.，δ 13C1 <δ 13C2 >δ 13C3 <δ 13nC4.The results also suggest that shale gas is mainly derived from kerogen primary cracking and oil and gas secondary cracking，and shale gas content is constrained by the organic matter type and thermal maturity，and the Type I shale has the highest gas yield.The partial reversal distributions of carbon isotopic compositions of gaseous hydrocarbons in shale gas could be caused by oil secondary cracking，or the mixing of gaseous hydrocarbons from different stages of thermal maturation.