莺歌海油气苗调查及其成因探讨

INVESTIGATION AND ORIGIN OF OILGAS SEEPAGES IN THE YINGGEHAI SEA

莺歌海的油气苗已有近百年的发现历史。首次采用声纳及卫星一导航定位调查在该区发现油气苗近百处。产状以连续型产出为主，亦见间歇型。气苗未经明显的氧化作用，主要成分为烃类气体，其中以甲烷占优势（＞97％），也见C7组分，C1／ΣCn高，δ^13C1o -33．91％－-38．34‰（PDB），属深成高熟热解气（Ro=1.0%-2.0%)。来自油气苗区海底岩样抽提物（油苗）的甾萜烷特征与莺歌海盆地莺歌海组－黄流组成熟烃源岩有良好的对比关系。换言之。油气苗来自本区上第三系成熟烃源区。

More than 100 seepages have been discovered by a sonar and a satellite navigation system in The Yinggehai Sea.The composition of them is mostly hydrocarbon gases (50%-70%) with a small content of no-hydrocarbon gases (CO_2:11.02%,N_2:12.17%,O_2:0.57%).The hydrocarbon gases are characterized by high C_1/ΣC_n ratios of 0.970-0.999,higher carbon number hydrocarbons (up to C_7),heavy isotopic compositions,with δ^(13)C_1 value of —33.91‰——38.34‰ PDB,δD value of—162.2‰ SMOW.The features reveal that these seepages are thermogenic gases from the same source,similar to gases of The Yacheng 13-1 gas field.It is suggested that the oil-gas seepages come from source rocks under the depth of 3500m in the region.