鄂尔多斯盆地后期改造及其对油气赋存的影响

【作者】 桂小军；

【导师】 刘池阳；

【作者基本信息】 西北大学 ， 矿产普查与勘探， 2006， 硕士

【摘要】 鄂尔多斯盆地的发育时限为中晚三叠世-早白垩世,晚白垩世以来为盆地的后期改造时期。油气作为流体矿产。流体极易随所处环境的变化而发生流动,改变其分布状态和位置。所以,油气赋存(聚散、成藏和分布等)既严格受成盆期和成盆期前构造格局的控制,又明显受成盆期后每一次构造运动和任一种改造形式的影响。晚侏罗-早白垩世为鄂尔多斯盆地生排油(气)、成藏高峰期,此后盆地又经历了多期次、多阶段、不同形式的构造变动和改造,这种在生油(气)高峰之后的构造变动势必会对油气的赋存状态产生重大影响,造就油气藏的部分破坏、调整、重新分配或运移散失以及地层能量场的改变。 本文根据各主要地质事件发生、动力学环境演变和地层接触关系及沉积环境变化等,对鄂尔多斯盆地及其邻区晚白垩世以来的后期改造进行了期次(阶段)划分,并系统、全面地讨论了盆地,尤其是盆地本部(或日鄂尔多斯地块)晚白垩世以来后期改造的主要表现形式及其强度的时空展布特征,在此基础上,从后期改造阶段油气散失留下的各类地质-地化证据入手,以抬升剥蚀导致盆地上古生界地层降压为线索,选择典型、代表性实例剖析,初步探讨了后期多期次构造变动对油气的影响。 鄂尔多斯盆地及邻区晚白垩世以来可划分为5个演化阶段:①晚白垩世-古新世;②始-渐新世;③中新世早中期;④中新世晚期-上新世;⑤第四纪。此间盆地本部经历了东强西弱、向东渐次增大的差异抬升与不均匀剥蚀,直至8Ma前后,地块东部剥蚀终止而率先接受风尘沉积,结束了晚侏罗世以来一直表现的相对东隆西降的格局,地块发生隆降反转。此后一直延续到现今,地块仍不断抬升,且抬升在时间上和空间上表现出不均匀。晚白垩世以来盆地本部的差异抬升剥蚀,对油气产生了正反两方面的影响,它的破坏性在于:(1)地层剥蚀减薄,甚或边缘高部位目的层直接出露,从而改变了地下油(气)的保存环境,加快了油气的散失:(2)地层倾向反转导致下古奥陶系顶部风化壳气藏内部物质的重新调整,然气藏内部结构完整,故天然气损失甚微。它的建设性体现在:(1)适度剥蚀,诱导封存箱内超压释放,形成次生气藏;(2)促使天然气脱溶、煤层气解吸附,可作为补充气源。更多还原

【Abstract】 Ordos basin developed from Mid-late Triassic to Early Cretaceous, then, underwent reformation. As liquid, in response to the changes of environment hydrocarbon tends to flow, changes its distribution and place, so not only the structural framework during and before basin-forming, but also any tectonic movements or any kind of reformation that occur after basin-forming affect its occurance . Late Jurassic-early Cretaceous is the rush hour of the hydrocarbon’s generation and accumulation in Ordos, then the basin experience multiple tectonic disturbances of many kinds, absolutely, they would have a significant effect, such as result in the destruction, adjustment, redistribution, leakage and the modify of energy field.In this paper, according to the geological events, evolution of dynamic setting, stratigraphic contact and change of depositional environment et al, we divide the stages of the late stage reformation after Late Crataceous in Ordos and its adjacent area, systematically discuss the main manifestation of reformation and spatio-temporal distribution feature of its intensity. Based on this, starting with many geological-geochemical evidences resulted from oil and gas leakage in reformed process, taking the depressurizing in Neopaleozoic due to erosion as clue , analyzing the typical and representative examples, primarily discussing the influnce reformatin to hydrocarbon.Since Late Crataceous, evolution of Ordos basin and its adjacent area can be divided into 5 phases : ①Late Crataceous-Palaeocene;②Eocene-Oligocene;③Early-Middle Miocene;④Late Miocene;⑤ Quaternary . Basin base has experienced differential, detailedly say, east strong and west weak, having a gradually increasing trend eastward. Uplift and erosion till about 8Ma. At about 8Ma ago, erosion action ended and aeolian deposits occurred in eastern block, the framewok that uplift east and subside west since Late Jurassic changes, Ordos block reversed . From then on, block still continuous differential uplift by this time. The effect of tectonic movements discussed above on hydrocarbon have two sides, its destruction are:(l) as erosion proceeds, the thickness of the overlying formation becomes smaller, or even it results in the objective interval coming out to the day in edge , consequently changes subsurface environment and increases velocity of hydrocarbon leakage;(2) upturning of beds causes readjustment of gas in Ordovician top weathering crust gas reservoir, because of good seal, the gas loss is very little. Its construction are (1): moderate erosion induces release of overpressured fluid in compartment, and forms secondary gas reservoir;(2): natural gas release from groundwater and crude oil, coal-bed gas desorption occurs, these gases can be a new additional gas source for accumulation.更多还原