塔里木盆地哈拉哈塘凹陷天然气地球化学特征

哈拉哈塘凹陷位于塔里木盆地塔北隆起中部，具有良好的石油地质条件，是近期油气勘探的重点区带。天然气地球化学特征研究表明，该区天然气干燥系数较低，表现出典型湿气的特征，普遍含有微量的H2S；烷烃气δ13C1和δ13C2值分别为-50.5‰~-42.6‰和-40.2‰~-35.5‰，δD1值介于-262‰~-156‰之间，碳氢同位素系列表现出典型正序特征； C7轻烃组成具有正庚烷优势分布， C5~7轻烃组成以正构和异构烷烃为主。哈拉哈塘凹陷及周缘奥陶系天然气均为海相油型气，既有干酪根裂解气，也有原油裂解气，其中哈拉哈塘天然气中混入了相当比例的原油裂解初期形成的湿气，主要来自于南部阿满过渡带地区的中上奥陶统烃源岩，天然气中具有高δ13C值特征的CO2主要来自碳酸盐岩储层在酸性地层水作用下发生的溶蚀， H2S主要源自含硫化合物的热裂解。其中天然气发生的同位素部分倒转主要源自原油伴生气与原油裂解气的混合。

Geochemical characteristics of natural gas from Halahatang Sag in the Tarim Basin

Halahatang Sag is located at the central part of the Tabei Uplift, Tarim Basin with favorable petroleum geological condition. It is an important exploration area. Geochemical characteristics of natural gas indicate that the natural gas displays low dryness indexes with trace-amount of H2S. Theδ13C1,δ13C2 andδD1 values of natural gas are in the ranges of -50.5‰--42.6‰, -40.2‰--35.5‰ and -262‰--156‰, respectively, with positive carbon and hydrogen isotopic series. The C7 light hydrocarbons are dominated by n-heptane with C5~7 dominated by normal alkanes and isoalkanes. The Ordovician-reservoired gases in Halahatang Sag and the adjacent area are marine oil-type gas from both kerogen cracking and oil cracking. The gases from Halahatang Sag include a significant proportion of wet gas formed in the early stage of oil cracking. They were mainly derived from the Middle-Upper Ordovician source rocks in the Awati-Manjiaer transitional zone. CO2 with highδ13C values in the Ordovician-reservoired gas was mainly derived from the dissolution of carbonate reservoirs under acidic formation water, with the H2S from thermal decomposition of sulfides. The partial isotopic reversal of gases originated mainly from the mixing of oil-associated gas and oil-cracking gas.