徐家围子断陷深层天然天的形成

通过对松辽盆地徐家围子断陷深层天然气的地质和地球化学分析，揭示了天然气的成因特征和形成过程。该区天然气的分子组成和同位素组成均显示出较大的变化，甲烷是天敢中的主要组分，质量分数在５７．４％－９８．２％之间，平均为９０．１％，主要非烃气体是ＣＯ２和Ｎ２，平均质量分数分别为４．９％和３．２％，且ＣＯ２质量分数变化范围较大，气体同位素分析结果表明，甲烷同位素显示煤型气特征，而乙烷和丙烷的同位素显示油型 的特征，表明徐家子深部除煤和ＩＩＩ型干酪根作为主要气源外，仍有一定含量趋于生油的ＩＩ型干酪根作为次要气源，实测和计算结果证实天然气主要是有机质在高成熟阶段形成的产物，运移过程中和成藏后的次生变化使天然气的组成和同位素面目变得非常复杂，尤其是由盖层微渗漏造成的蒸发分馏的作用使同位素出现例转，徐家围子断陷深层煤型气的发现为

GENERATION AND HABITAT OF NATURAL GASES IN DEEP STRATA IN XUJIAWEIZI DEPRESSION, SONGLIAO BASIN,NORTH EAST CHINA

Chemical composition and stable isotopic studies were undertaken for 27 gas samples in deep strata in Xujiaweizi depression in the Songliao Basin to investigate their origin and generation. Gas molecular and isotopic compositions show great variety. Methane is the main component of all studied samples which range from 57.4 to 98.2% with an average of 90.1%. The main non hydrocarbon gases are carbon dioxide and nitrogen with an average of 4.0% and 3.2% respectively, and the former show even larger variety. Methane isotope data indicates that these deep gases are mainly coal type gases, while ethane and propane isotopic indicates oil type gas characteristics. These results suggest that deep gas in study area mainly derived from coals interbedded shales with type III kerogen and minor proportion of oil type gases were generated from shales with type II kerogen. Both of them are formed during the postmature stage of hydrocarbon generation. Secondary alteration during migration and/or in reservoirs makes chemical and isotopic composition rather complicated, especially the evaporative fractionation via micro leakage through cap rocks resulting in isotopic inversion. The discovery of coal type gases in Xujiaweizi depression provides new prospects for the further exploration in this region.