鄂尔多斯盆地奥陶系原生天然气地球化学特征及其对靖边气田气源的意义

Geochemical characteristics of primary gas in the Ordovician and their significance for the gas source of Jingbian gas field,Ordos Basin

通过对鄂尔多斯盆地余探1井奥陶系天然气气源分析,来重新认识靖边气田风化壳气藏气源。余探1井奥陶系烃源岩地球化学特征及天然气碳同位素对比分析发现,中奥陶统乌拉力克组有机碳含量(TOC)在0.30%~1.16%,平均为0.51%,暗色泥岩厚度52.59 m,可以成为有效烃源岩;天然气甲烷的碳同位素组成明显偏轻,δ13C1值在-39.11‰^-38.92‰,乙烷的碳同位素较偏重,δ13C2在-27.26‰^-27.17‰,如果根据乙烷碳同位素来判别,应具有煤成气特征。然而,烃源岩热模拟实验计算的天然气成熟度(R o=1.86%~1.89%)与烃源岩实测的热成熟度(R o=1.83%~1.92%)基本一致,都具高热演化特征。从气藏储、盖配置关系上看,气藏上覆奥陶系泥岩厚度大,上古生界煤成气难以混入;天然气偏轻的甲烷碳同位素特征与碳酸盐岩生油岩的甲烷热解气碳同位素组成相似。这些证据表明,余探1井奥陶系天然气应具有油型气的特征。以余探1井奥陶系天然气作为鄂尔多斯盆地油型气端元,对靖边气田中-北部及南部地区天然气碳同位素组成对比分析,结果表明:(1)甲烷碳同位素应作为判识鄂尔多斯盆地奥陶系天然气气源的主要指标,δ13C1小于-38‰是靖边气田风化壳气藏油型气的判别标志;(2)靖边气田整体仍以高成熟混合型煤成气为主,但油型气混入比例南部地区大于中部及北部地区;(3)乙烷次生裂解作用可能是造成奥陶系油型气乙烷碳同位素偏重的主要原因。

Gas source of the Ordovician weathering crust gas reservoirs in Jingbian gas field is restudied by analyzing gas source of Well Yutan1 in Ordos Basin.An analysis of the geochemical features of source rock and a comparative analysis on carbon isotopes of natural gases are performed .The total organic carbon （ TOC） of the Middle Ordovician Wulalike Formation is 0.30%-1.16%（ averaging at 0.51%） and the thickness of dark mudstone is 52.59m.These two parame-ters indicate that this formation can be effective source rock .The carbon isotopes of methane in natural gas from Yutan-1 well are significantly lighter with δ13 C1 value in the range of -39.11‰--38.92‰,which is similar with the pyrolytic methane from carbonate source rocks .In contrast,the carbon isotopes of ethane are relatively heavier with δ13 C1 value in the range of -27.26‰and -27.17‰,which seems to be the characteristics of coal-derived gas.However,the calculat-ed gas maturity （Ro =1.86%-1.89%） from source rock thermal simulation experiment is coincided with the measured gas maturity （Ro =1.83%-1.92%） from source rock,both indicating high thermal maturity .In addition,the overlying thick Ordovician mudstone of the gas reservoir makes it difficult for the coal-derived gas from the Upper Paleozoic to mi-grate into the reservoir.All these evidences show that the Ordovician gas of Yutan-1 well should be oil-generated gas. Using the Ordovician gas of Yutan-1 well as the end member of oil-generated gas in Ordos basin ,a comparative analysis is performed on the carbon isotopes of gas from the central-northern and southern Jingbian gas field .The following results are obtained.（1） Carbon isotope of methane should be considered as the major gas source index for the Ordovician gas ,and δ13 C1 〈-38‰is a index for identifying oil-generated gas in Jingbian weathering crust gas field;（2） The gas source in the entire Jingbian gas field is still dominated by mixed coal-derived gas of high maturity ,but the proportion of oil-generated gases is higher in the southern Jingbian gas field than that in the central-northern Jingbian gas field;（3） Secondary pyroly-sis of ethane may be the major factor causing heavier ethane carbon isotope of the oil -generated gas in the Ordovician .