柴达木盆地一里坪地区新近系沉积环境及碳酸盐岩碳氧同位素特征

柴达木盆地一里坪地区新近系是一个潜在的油气勘探领域,前人研究表明上油砂山组及上干柴沟组内部分层段发育有优质的烃源岩层段。为明确其沉积时期古湖泊的环境特征与优质烃源岩层段发育的控制因素,选取了研究区2口取心井:里3井的上油砂山组和博1井的下干柴沟组开展了有机地球化学、碳氧同位素分析等测试工作。分析结果表明:一里坪凹陷里3井上油砂山组沉积碳酸盐δ^13C值介于-6.4‰~-1.7‰之间,平均值为-3.2‰,δ^18O值介于-11.8‰~-6.4‰之间,平均值为-9.2‰;茫崖凹陷博1井,上干柴沟组沉积碳酸盐岩δ^13C值介于-2.1‰~-1.5‰之间,平均值为-1.84‰,δ^18O值介于-12.4‰~-4.9‰之间,平均值为-9.66‰。通过碳氧同位素、岩性及周缘大地构造背景综合判别表明:一里坪地区整体为处于半开放—开放、微咸水湖泊环境,湖平面震荡频繁,平均气温相对较低,气候变化频率较快。通过沉积环境与烃源岩分布关系研究,研究区相对优质烃源岩主要发育于咸化环境内,与柴西烃源岩发育特征一致;同时,湖水平静期、气候相对温暖湿润时期也发育较好的烃源岩。一里坪地区咸化中心是下一步天然气勘探的重要方向。

Carbon and oxygen isotope characteristics of carbonate and Neogene depositional environment in the Yiliping area of Qaidam Basin

The Neogene in the Yiliping area of Qaidam Basin is a potential area of oil and gas exploration. In order to make clear its environmental characteristics of the ancient lakes sedimentary period and discusses the controlling factors of high quality hydrocarbon source rock development,we select the core samples from two wells—Well Li3 and Well Bo1 to analyze their organic geochemistry,carbon and oxygen isotope etc. The analysis and test results suggest that δ^13 C in the N22 of Well Li3 is between-6.4‰ and-1.7‰,with an average value of-3.2‰,and δ^18 O is between-11.8‰ and-6.4‰,with an average value of-9.2‰. In Well Bo1 of Man’ai Depression,the13 C of sedimentary carbonate rock of Shangganchaigou Formation is between-2.1 ‰ and-1.5‰,with an average value of-1.84‰,18 O is between-12.4‰ and-4.9‰,with an average value of-9.66‰. The Yiliping area is in a semi-open-open,brackish lake environment,with frequent oscillations in the lake plane,relatively low average temperature,and rapid climate change. Through the study of the relationship between sedimentary environment and source rock distribution,the relatively high quality source rock in study area mainly develops in the saline environment,which is consistent with the development characteristics of the source rock in the western Qaidam Basin. Meanwhile,the lake calm period,in which the climate is relatively warm and humid,also developed good source rocks. The salinization center of Yiliping area is an important direction of gas exploration.