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通过分析辽河坳陷东部凹陷29个天然气样品的地质和地球化学特征,探讨了研究区天然气成因类型的新认识。研究区主要烃源岩为古近系沙河街组和东营组,母质类型以偏腐殖型为主,热演化程度主体处于0.5%~1.0%之间。天然气组分以烃类气体占绝对优势,干气、湿气并存;δ13 C1值为-46‰~-38‰、δ13C2值为-30‰~-25‰、δ2 HCH4值为-250‰~-210‰;C5-7轻烃化合物中富含异构烷烃,庚烷值和异庚烷值均落在低演化区。对比分析吐哈低熟气特征,指出研究区探明天然气的主体属于低熟气范畴,满足其2个本质属性:母质为偏腐殖型、成熟度处于低演化阶段。气源对比表明,天然气主要来源于古近系沙三段(Es3)陆相湖盆偏腐殖型源岩,它的成熟度(RO)与基于“δ13C1—RO”计算得出的天然气RO值(0.50%~0.90%)在空间分布特征上相吻合。此外,微生物活动对气藏后期的改造作用也不容忽视:大部分C2+烃类被微生物消耗而使天然气组分变干;C2、C3含量降低至一定程度时,δ13C2值、δ13C3值呈现异常高,甚至达到正值,而δ13C1值受微生物活动影响较小;正庚烷容易被降解而使研究区天然气庚烷值相比吐哈低熟气更低。
By analyzing the geologic and geochemical characteristics of 29 natural gas samples in the eastern depression of Liaohe Depression, new understanding of the genetic type of natural gas in the study area is discussed. The main hydrocarbon source rocks in the study area are Paleogene Shahejie Formation and Dongying Formation, and the parent material types are mainly humus. The main body of thermal evolution is between 0.5% and 1.0%. The natural gas component takes the absolute advantage of hydrocarbon gas, and the dry gas and moisture coexist; δ13C1 value is -46 ‰ ~ -38 ‰, δ13C2 value is -30 ‰ ~ -25 ‰, δ2HCH4 value is -250 ‰ ~ 210 ‰; C5-7 light hydrocarbon compounds are rich in isoparaffin, heptane and isoheptane values fall in the low evolution area. The characteristics of Turpan-Hami low-climatic gas are compared and analyzed. It is pointed out that the main body of proven natural gas in the study area belongs to the low-mature gas category, satisfying two essential attributes: the parent material is humic and the maturity is in a low evolution stage. Comparison of gas sources shows that natural gas is mainly derived from humus-type source rocks in the continental shelf of Es3 in the Paleogene. Its maturity (RO) and natural gas RO value calculated based on “δ13C1-RO” (0.50% ~ 0.90%) coincide with the spatial distribution characteristics. In addition, microbial activities can not ignore the remodeling effect of gas reservoirs: most of C2 + hydrocarbons are depleted by microorganisms and make the natural gas components become dry. When the contents of C2 and C3 decrease to a certain extent, δ13C2 and δ13C3 values are abnormally high, Even reached a positive value, while the δ13C1 value was less affected by microbial activity. The n-heptane was easily degraded to lower the natural gas heptane in the study area compared with that of the Turpan-Hami Low Cooked Gas.