东海西湖凹陷天然气成藏时期的关键证据:气烃包裹体

Key evidence of gas accumulation period in Xihu Sag of the East China Sea Shelf Basin:Gas hydrocarbon inclusion

东海陆架盆地西湖凹陷油气勘探已证实深部的古近系平湖组和花港组天然气资源丰富,但油气成藏机理复杂,其中对气烃包裹体的研究薄弱,缺乏确定天然气成藏时期的关键证据。为了揭示西湖凹陷天然气的成藏时期,通过对西湖凹陷古近系砂岩储层包裹体岩相观察、激光拉曼光谱分析、显微测温等系统实验测试,结合盆地构造演化史、埋藏史、古地温演化史、烃源岩热演化史等分析,开展了该凹陷天然气成藏期次研究。研究结果表明:①该区储层发育高密度单相纯烃包裹体、高密度单相含烃包裹体、常规气液两相纯烃包裹体和常规气液两相含烃包裹体共4种气烃包裹体;②储层包裹体的气体成分主要为CH_(4),部分含有C_(2)H_(6)和CO_(2),其同期盐水包裹体均一温度集中分布于140~150℃;③该区有2期天然气充注和成藏,并且以晚期为主,第一期发生在中新世晚期,成因为原油裂解,储层在高温高压条件下捕获超临界态流体形成单相气烃包裹体,天然气成藏规模较小,第二期发生在上新世—更新世,以CH4为主的烃气自烃源岩中大量排出,向圈闭中大规模充注并成藏,是西湖凹陷天然气的主要成藏时期。结论认为,天然气成藏时期的确定为东海西湖凹陷天然气的深化勘探指明了方向,有助于发现更多新的油气藏。

Exploration in Xihu Sag of the East China Sea Shelf Basin proves that the deep Paleogene Pinghu Formation and Huagang Formation are rich in natural gas resources,but their hydrocarbon accumulation mechanisms are complicated.The study on gas hydrocarbon inclusions is weak,and there is a lack of key evidence to determine the gas accumulation period.In order to reveal the gas accumulation period in Xihu Sag,this paper carries out systematical experimental tests on the inclusions of Paleogene sandstone reservoirs in Xihu Sag,including lithofacies observation,Laser Raman spectrum analysis and microthermometry.Then,the gas accumulation period is studied systematically by analyzing the basin's tectonic evolution history,burial history and paleogeothermal evolution history and source rocks'thermal evolution history.And the following research results are obtained.First,four kinds of gas hydrocarbon inclusions are developed in the reservoirs of this area,namely,high-density single-phase pure hydrocarbon inclusion,high-density single-phase hydrocarbon-bearing inclusion,conventional gas-liquid two-phase pure hydrocarbon inclusion and conventional gas-liquid two-phase hydrocarbon-bearing inclusion.Second,the gas composition of reservoir inclusions is mainly CH_(4),and some contain C_(2)H_(6) and CO_(2).The homogenization temperature of brine inclusions of the same period is mainly between 140℃and 150℃.Third,there are two periods of gas charging and accumulation in this area,among which,the later one is dominant.The first period occurred in the late Miocene,when oil cracking was the origin,the supercritical fluid was captured in the reservoir to form single-phase gas hydrocarbon inclusions under high temperature and pressure,and hydrocarbon accumulation scale was small.The second period occurred in the Pliocene-Pleistocene,when a great amount of hydrocarbon gas dominated by CH4 was expelled from source rocks and charged into traps to accumulate in a large scale,so it is the main gas accumulation period in Xihu Sag.In conclusion,the determination of gas accumulation period points out the direction of further natural gas exploration in Xihu Sag and is conducive to the discovery of more oil and gas.