煤岩初次和二次生烃动力学及其对沾化凹陷孤北天然气成因的解释

Kinetics of primary and secondary generation of coal-derived gases and its application to genesis of natural gases found in Gubei area, Zhanhua Depression

沾化凹陷孤北地区石炭—二叠系(C—P)潜山产出煤成气,油气勘探需要搞清楚产出的煤成气是煤岩初次埋藏、还是抬升剥蚀后再次埋藏生成的。在查明该区煤岩地质演化的基础上,采用热模拟实验方法,探讨了煤岩初次和二次生烃过程中气态烃与主要单体烃生成动力学特征,发现煤岩二次生烃的活化能比一次生烃高40～50 kJ/mol,认为孤北地区C—P煤系源岩达到4200m深度条件(Ro约0.9%)才发生二次生烃作用。与煤岩初次生气相比,二次生气具有iC4/nC4<1、n(C6+C7)/i(C6+C7)>0.8、甲苯/苯<1.2等特征,据此认为孤北煤成气是煤岩二次生烃形成的。

Coal-derived gas is discovered from the Gubei area in the Zhanhua Depression. It is important for explorers to know that the gas is a primary generating gas from Mesozoic burial history or a secondary one from Tertiary reburial history. Having investigated the geological evolution of the C - P coal sequence in the Gubei area, the authors conducted a series of pyrolysis experiments on the primary and the secondary hydrocarbon generation from the coal samples and calculated the kinetic parameters for the total hydrocarbons and some individual hydrocarbons derived from the pyrolysis. It is found that the active energy of the total hydrocarbon generated from the secondary pyrolysis from the coal samples is 40 - 50 kJ/mol greater than that from the primary pyrolysis, which means that the coal sequence generated secondly hydrocarbons at re-burial depth greater than 4200 m （corresponding to R^o =0.9% ）. Compared with the hydrocarbon derived from the primary pyrolysis, the hydrocarbons generated from the secondary pyrolysis are characteristics of iC4 /nC4 〈 1, n（C6 ＋ C7） / i（C6 ＋ C7） 〉 0. 8 and methal-benzol/benzol 〈 1.2. It is concluded that the coal-derived gas discovered from the Gubei area may be the secondary pyrolysis products from the C -P coal sequence in Late Tertiary.