鄂尔多斯盆地西南地区蓟县系古地温演化历史及岩浆热事件的影响

Paleo-temperature evolution history of the Jixian System and theinfluence of magma thermal event of the southwestern Ordos basin

深层油气成藏机理研究的首要前提是要明确烃源岩的热演化历史,这对区域油气勘探潜力的评价有着重要的指导意义。鄂尔多斯盆地西南缘蓟县系烃源岩在中生代以来的热演化史有何特征,是否受早白垩世岩浆热作用的影响,影响程度如何等问题的不明确,制约着人们对该地区中元古界烃源岩的生烃潜力的认识及进一步勘探开发的思路。通过对安口—铜城地区出露的三叠系、侏罗系及铜城岩体进行磷灰石裂变径迹、磷灰石/锆石(U-Th)/He测试,结合镜质组反射率数据,分别恢复了该地区沉积岩在中三叠世以来和侵入岩在早白垩世以来的冷却历史,结合岩浆岩体的空间分布特征和泥页岩镜质组反射率,估算蓟县系烃源岩古地温。热年代学模拟表明,蓟县系烃源岩自中生代以来先后经历了三叠纪—侏罗纪的正常埋深增温,达到了生烃温度门限,自早白垩世约130~110 Ma开始冷却,其中个别样品表现为自始新世中期至45 Ma微弱加速冷却,中新世晚期至8 Ma以来快速冷却。研究表明,中晚侏罗世是鄂尔多斯盆地西南缘蓟县系生烃的关键时期,烃源岩处于主生油温度范围,之后的早白垩世晚期岩浆侵入事件的热作用范围有限,对蓟县系烃源岩古地温的影响仅发生在局部地区。鄂尔多斯盆地西南缘如安口地区等未受早白垩世晚期岩浆热事件影响的广大地区,中元古界蓟县系具有一定的勘探潜力,值得进一步深入研究。

The primary task of research into oil and gas exploration and accumulation mechanisms is to recover the thermal evolution history of source rocks,which has a significant impact on the evaluation of regional petroleum resources.This study aims to understand the characteristics of the thermal evolution history of the Jixian System source rocks in the Tongcheng-Ankou area of the southwest margin of the Ordos basin since the Mesozoic,and whether they were affected by the Early Cretaceous magmatic activity and to what extent.These questions restrict the understanding of the hydrocarbon generation potential and the hydrocarbon formation period of the Middle Palaeozoic source rocks of the southwestern Ordos basin.Apatite fission track,apatite/zircon(U-Th)/He of the Triassic System,the Jurassic System and the Tongcheng rocks exposed in the Ankou-Tongcheng area,as well as vitrinite reflectance data,were used to restore the tectonothermal evolutionary history of the sedimentary rocks since the Middle Triassic and the intrusive rocks since the Early Cretaceous in this area.Since the Mesozoic,the Jixian System source rocks have experienced a normal burial temperature increase from the Triassic to the Jurassic,reaching the hydrocarbon generation temperature threshold.They began cooling from 130~110 Ma in the Early Cretaceous.Modeling results showed a slight acceleration in cooling from the Mid-Eocene to~45 Ma and rapid cooling from the Late Miocene to~8 Ma.The study reveals that the Middle-Late Jurassic was the key period of hydrocarbon generation for the Jixian System in the southwest margin of the Ordos basin,and the hydrocarbon source rocks were in the main oil-generating temperature range.The thermal effect of the Late Early Cretaceous magmatic intrusion event was limited in scope,and its influence on the paleogeothermal temperature of the Jixian System source rocks only occurred locally.The Jixian System in the Middle Palaeozoic has some hydrocarbon generation potential in vast areas that were not affected by the Late Early Cretaceous magmatic thermal event,such as the Ankou area on the southwest margin of the Ordos basin which merit further research.