渤海海域莱州湾凹陷油气超晚期成藏与油气运移滞后效应

Study on the Ultra-Late Hydrocarbon Accumulation and Migration Lagging Effect in Laizhouwan Sag,Bohai Sea

油气运移滞后效应是指油气从烃源岩中生成并排出的时间早,而聚集成藏的时间晚,油气经历了长时间长距离运移,已聚集成藏的油气相态与成熟度低于现今烃源岩成熟演化的现象.这种油气运移滞后效应在东部新生代陆相断陷盆地中普遍存在,在斜坡带表现最为明显.通过利用地球化学分析,平衡剖面技术、盆地模拟、包裹体分析等手段,对渤海海域莱州湾凹陷各成藏要素的时空耦合关系进行研究,认为研究区存在明显的油气运移滞后效应及超晚期成藏,主要表现在:莱州湾凹陷沙四段烃源岩在东营组中晚期已进入排烃期,而浅层圈闭形成于新构造运动时期,油气主要充注期则在2 Ma至今;且油气成藏呈现出靠近北洼的构造成藏时间早,远离北洼的构造成藏时间晚,深部层系成藏时间早,浅部层系成藏时间晚的规律.此外,还对油气运移滞后效应的成因进行探讨,认为构造运动对烃源岩生排烃进程与油气垂向运移控制作用、油气差异聚集原理控制油气充注次序以及烃源岩层段发育的超压阻滞流体的运移是油气运移滞后效应产生的主要因素.

The lagging effect of oil and gas migration refers to the phenomenon that the oil and gas generated and discharged from the source rock too early but accumulated too late,so the oil and gas experienced long-term and long-distance migration,resulting in the phase state and maturity of the accumulated reservoir being lower than that from the current source rock.This lagging effect of hydrocarbon migration is common in Cenozoic continental rift basins in eastern China,especially in slope zone.By means of geochemical analysis,balanced section technique,basin simulation and inclusion analysis,this paper studies the spatiotemporal coupling relationship of various reservoir forming factors in Laizhouwan Sag,Bohai Sea,It is show that there are obvious oil and gas migration lagging effect and ultra-late hydrocarbon accumulation in the study area.The source rocks of the Sha4 formation have entered the hydrocarbon expulsion period during the middle-late Dongying Formation,while the shallow traps were formed in the Neotectonic movement period,and the main hydrocarbon filling period is from 2 Ma to now;the oil and gas accumulation time that near northern depression is early,while the structures that far away from northern depression is late,the deep strata is early,while the shallow strata is late.In addition,this paper also discusses the causes of the oil and gas migration’s lagging effect.It is considered that tectonic movement controls the process of hydrocarbon generation and expulsion and the vertical migration of oil and gas,the principle of oil and gas differential accumulation controls the sequence of oil and gas charging,and the movement of overpressure blocking fluid developed in the source rock section is the main factors for the lagging effect of oil and gas migration.