崖城地区渐新统不同沉积相带烃源岩生物标志物组合特征及其烃源意义

基于40余个不同岩性烃源岩及原油饱和烃组分的GC/MS和GC/MS/MS分析资料,揭示了琼东南盆地崖城地区渐新统不同沉积相带烃源岩的生物标志物组合特征,并与崖城油气田原油进行油源精细对比,提出了新的认识。研究结果表明,海岸平原沼泽相煤及碳质泥岩呈姥鲛烷优势(Pr/Ph>5.0),三环萜烷以低碳数化合物为主(C₁₉/C₂₃>3.0),甾烷丰度低(藿烷/甾烷>5.0), C₂₉甾烷优势显著(>60%),指示高等植物为主要的有机源,且沉积于氧化环境。浅海相泥岩中Pr/Ph值大多在1.0~2.5,表征弱还原-弱氧化环境;其三环萜烷以C₂₁或C₂₃为主峰,甾烷呈"V"字型分布,藿烷/甾烷比值小于3.0,具陆源和水生双重有机质生源。煤及碳质泥岩中奥利烷和双杜松烷等陆源标志物均较少,而泥岩中却普遍富含这些化合物,反映了近岸海相沉积有机质的特殊性。因而,在近海盆地地层中奥利烷和双杜松烷丰度并不能代表总体有机质中陆源的输入比例,更可能是指相标志。通过与烃源岩的分子地球化学对比表明,崖城油气田的主体原油兼具煤成烃和泥岩所生油的生物标志物组成,为混源油,来自渐新统不同沉积相带烃源岩。

Biomarker compositions and hydrocarbon source significances of different facies Oligocene source rocks in Yacheng region,Qiongdongnan Basin

Based on the analytical data of GC/MS and GC/MS/MS for saturated fractions of over 40 different lithofacies source rocks and oils, the biomarker compositions in various depositional facies of Oligocene source rocks in Yacheng region, Qiongdongnan Basin, were characterized, and the oil-source correlation with a new viewpoint for Yacheng oil & gas field was intensively made. The results indicated that the coals and carbonaceous mudstones in coastal plain-swamp facies show a pronounced pristane predominance in isoprenoids(Pr/Ph>5.0), low molecular weight compound dominance in tricyclic terpanes(C₁₉/C₂₃>3.0), low amount of steranes(hopanes/steranes >5.0)and dominant C₂₉ in steranes(>60%), indicating a predominantly terrigenous organic source and deposition in an oxic environment. The shallow marine mudstones display Pr/Ph ratios mostly in 1.0~2.5, indicative of subanoxic-oxic condition, and have a maximum peak at C₂₁ or C₂₃ in tricyclic terpanes and a "V" shaped modal in C₂₇~C₂₉ sterane distribution, suggesting a mixing input of terrigenous and aquatic organic matter. Oleanane and bicadinane used usually as terrigenous indicators are in low content in all the coaly rock samples, but commonly enriched in the mudstones, reflecting a special character for depositional organic matter in the nearshore marine environment. Therefore, the abundance of oleanane and bicadinane in the sediment of offshore basin could not be used as proportion for land plant input to overall organic matter, rather may be act as a facies-related indicator. According to the geochemical correlation by biomarkers with source rocks, the major oils in Yacheng oil and gas fields are believed to have a multiplex biomarker composition with coal and mudstone source characteristics, and be the mixed oils derived from various depositional facies of Oligocene source rocks.