塔里木盆地顺北地区奥陶系超深层油藏蚀变作用及保存

Secondary Alteration and Preservation of Ultra-Deep Ordovician Oil Reservoirs of North Shuntuoguole Area of Tarim Basin,NW China

塔里木盆地顺北地区超深层垂深为7200~7863.6 m的奥陶系一间房-鹰山组储层中发现了挥发油藏和轻质油藏,油藏赋存深度下限不断突破传统认识.使用地球化学方法研究了顺北地区不同断裂带油气藏的地球化学特征及蚀变作用.顺北地区不同断裂带原油均具有轻碳同位素特征,C23三环萜烷/C21三环萜烷>1,C28甾烷含量低的特点,三芴系列组成中具有较高含量的二苯并噻吩含量,表明与塔河原油具有相似的母源.(C21+C22)甾烷/(C27~C29)甾烷、C27重排/C27规则甾烷、甲基菲指数和二苯并噻吩系列成熟度表明顺北地区原油成熟度呈现1号断裂带(含分支断裂)≈3号断裂带>次级断裂带>5号断裂带>7号断裂的特征,原油成熟度受控于油藏初始静温.顺北地区奥陶系天然气均为湿气,天然气甲烷碳同位素分布范围为-50.7‰^-44.7‰,不同断裂带天然气成熟度的差异与不同断裂带原油成熟度的分布规律相似.顺北地区原油(4?+3?)甲基双金刚烷含量较低,分布范围为9.25~36.44μg/g,指示原油裂解程度较低.原油中均可检测出完整系列的低聚硫代金刚烷,含量分布范围为0.76~18.88μg/g,表明原油硫酸盐热化学还原作用(TSR)弱,顺北地区天然气为湿气及甲烷碳同位素轻表明油气藏未遭受气侵作用.地温研究表明顺北地区地温梯度低,为2.12℃/100 m,埋深8000 m的地层目前仅为160~170℃,地质历史时期,奥陶系地温未超过170℃,未达到原油大量裂解温度的门限.顺北地区奥陶系长期的低地温加之油气藏蚀变作用弱,是顺北地区奥陶系保持挥发油相的关键.

Volatile reservoir and light reservoir have been discovered in the ultra-deep Ordovician Yijianfang to Yingshan Formations with vertical depths ranging from 7200 to 7863.60 m from North Shuntuoguole area in the Tarim basin.The limit depth of oil phase occurring has been continuously challenging the traditional theory.This study focuses on geochemical characteristics and secondary alterations of reservoirs from different faults in North Shuntuoguole area.Most oils of different faults from North Shuntuoguole area are characterized by light carbon isotope,C23TT/C21TT>1,lower C28sterane content,relatively high abundance of dibenzothiophene,suggesting that the source rock of oil in the North Shuntuoguole area is similar to that of Tahe oilfield.Using(C21+C22)sterane/(C27~C29)sterane,C27diasterane/C27regular steane,MPI index,dibenthiophene series maturity,the oil maturity trend in the North Shuntuoguole area is No.1 fault(including splay fault)≈No.3 fault>secondary fault>No.5 fault>No.7 fault.The oil maturity is controlled by the reservoir initial static temperature.The natural gas in North Shuntuoguole area is wet gas and has light methane carbon isotope,ranging from-50.7‰to-44.7‰.The gas maturity trend in different faults is similar to that of the oil.The abundance of(4-+3-)methyldiamantane in the oils from different faults is in the range of 9.25-36.44μg/g,indicating low degree of oil cracking.The lower thiadiamondoids can be detected in the oils,with content ranging from 0.76 to 18.88μg/g,showing low degree of thermochemical sulfate reduction(TSR).The Ordovician reservoir has suffered little gas invasion from the natural gas characteristics.The formation temperature indicates the geothermal gradient in the North Shuntuoguole area is low,only about 2.12℃/100 m.The formation temperature of strata buried at 8000 m depth at present is in the range of 160-170℃.The maximum paleo-geotemperature of Ordovician has never been higher than 170℃,without reaching the temperature threshold of great scale oil cracking.The long term of low geothermal environment combined with low degree secondary alteration is the key to the preservation of ultra-deep Ordovician volatile reservoir in the North Shuntuogule area.