碳酸盐岩储层中原油裂解及碳同位素演化模拟实验

Simulation experiments on crude oil cracking and carbon isotopic evolution in carbonate reservoirs

为了研究碳酸盐岩储层中原油裂解产物及其同位素演化特征,采集塔里木盆地塔河油田TK772井奥陶系鹰山组产层的原油,利用半开放实验体系"地层孔隙热压生排烃模拟仪"开展仿真碳酸盐岩储层条件的原油裂解成气模拟实验,并对实验气体产物地球化学特征进行分析。实验结果表明,碳酸盐岩中原油裂解成气过程主要受温度的影响,实验中硫酸镁的加入导致了TSR(硫酸盐热化学还原反应)的发生,影响了原油裂解过程及其产物特征,导致重烃规模裂解的温度降低,消耗大量重烃的同时导致大量非烃气体生成。原油裂解烷烃气碳同位素变化主要受热演化程度控制,表现为随温度的升高逐渐变重,但有硫酸盐存在的温度条件下烷烃气碳同位素序列发生部分倒转(δ^(13)C_1<δ^(13)C_2>δ^(13)C_3)。研究表明,碳酸盐岩储层中TSR反应会改变天然气的化学组成,是高含硫气藏中普遍存在的碳同位素序列倒转的重要原因。

To study the characteristics of crude oil cracking and isotopic evolution in carbonate reservoirs,crude oil samples were collected from Well TK772 in the Ordovician Yingshan Formation,Tahe oilfield,Tarim Basin.We carried out the simulation experiment of oil cracking into gas process under simulated carbonate reservoir conditions by using the semi-open experiment system ＂hydrocarbon generation and expulsion simulation instrument under the conditions of stratum-porosity and thermo-compression＂,and then analyzed the geochemical characteristics of gaseous products.The experiment results showed that oil cracking process in a carbonate reservoir was mainly influenced by temperature,and the occurrence of the TSR was caused by the adding of magnesium sulfate.TSR affected the oil cracking process and the characteristics of its products,resulting in the drop of temperature for the large scale cracking of heavy hydrocarbons,and the generation of large amount of non-hydrocarbon gases in addition to the consuming of large amount of heavy hydrocarbons.The alkane gas carbon isotopic composition is mainly controlled by thermal evolution degree.The carbon isotopes get heavier progressively along with the increasing of temperature,but the alkane gas carbon isotopic sequence partially reverses （δ^13C1〈 δ^13C2 〉δ^13C3） at certain temperature intervals when sulfates exist.The study shows that TSR not only could alter the chemical composition of natural gas,but also might be an important factor for alkane gas carbon isotopic sequence reversion that is common in the carbonate gas reservoirs with high content of sulfur.