西湖凹陷平湖斜坡构造带油藏成藏期次厘定

The oil accumulation period in the Pinghu slope tectonic belt of the Xihu sag

西湖凹陷位于东海陆架盆地东北部,是我国近海油气勘探前景最好的含油气构造之一,以天然气资源为主。近年来随着勘探程度的逐渐深入,发现西部斜坡构造带具备油藏勘探的巨大前景。该地区地质条件复杂,并非连续沉降盆地,经历了多次抬升-埋藏过程。复杂的成藏过程导致准确厘定油藏成藏时代和期次存在困难。本文通过流体包裹体测温-埋藏史结合方法以及地质类比法,综合分析了西湖凹陷西部平湖斜坡构造带油藏的成藏历史。研究结果表明,西部斜坡带平湖组储层发育两期颜色不同的烃类包裹体,一期为发淡黄—黄绿色荧光的烃类包裹体,捕获温度在157~173℃之间;另一期为发蓝白色荧光的烃类包裹体,捕获温度在169~191℃之间。结合埋藏史模拟结果,厘定平湖斜坡带存在两期成藏,第一期成藏时间约为15~13 Ma,第二期成藏时间约为2~0 Ma。地质类比法研究表明,自中新世(17~15 Ma)起,平湖斜坡带平湖组烃源岩即进入生烃门限,开始供烃,龙井运动虽然造成了区域性的地层抬升,但烃源岩埋深一直处于生烃门限以下,可以持续供烃,与流体包裹体研究结果一致。研究表明,西湖凹陷平湖斜坡带的生排烃期和构造运动共同决定了储层矿物中保留了两期不同荧光颜色的油包裹体,流体包裹体所反映的两期成藏时代,分别对应于原油进入储层的最早时代和大规模充注的时代。龙井运动为油藏晚期聚集创造了有利条件,早期原油充注强度低,晚期的原油充注是该区油藏成藏的关键期。

The Xihu sag is located in the northeastern part of the East China Sea Shelf Basin. It is one of the petroliferous units with good prospects for offshore oil and gas exploration in China, and is dominated by natural gas resources. The geological conditions of the Xihu sag are complex because it is not a continuous subsidence basin, and has experienced multiple uplift-burial processes. Its complicated petroleum accumulation process makes it difficult to accurately determine the age and stage of oil charging. This paper comprehensively analyzed the oil accumulation history of the Pinghu slope tectonic belt in the west of the Xihu sag by combining fluid inclusion temperature measurement with burial history information and geological analogy. The research results show that there are two stages of hydrocarbon inclusions with different colors in the Pinghu Formation reservoir of the western slope belt. The fluorescence color of the first stage hydrocarbon inclusions is pale yellow to green-yellow, and the trapping temperatures are between 157℃ and 173℃. The other stage is blue-white fluorescent hydrocarbon inclusions with trapping temperatures of between 169℃ and 191℃. Combined with the simulation results of the burial history, it was determined that there were two stages of oil accumulation in the Pinghu slope belt. The first stage of oil accumulation is about 15 Ma to 13 Ma, and the second stage of oil accumulation is about 2 Ma to 0 Ma. The geological analogy method shows that the source rocks of the Pinghu Formation have entered the hydrocarbon generation threshold and began to supply hydrocarbons since the Miocene(17 Ma to 15 Ma). Although the Longjing movement caused a regional uplift, the burial depth of source rocks has always been below the hydrocarbon generation threshold, and hydrocarbons can be continuously supplied, which is consistent with the results of fluid inclusion studies. The study shows that the periods of hydrocarbons generation and expulsion and tectonic movements of the Pinghu slope belt in the Xihu sag jointly determined that these two stages of oil inclusions with different fluorescence color were preserved in reservoir minerals. The two periods of oil accumulation inferred by the fluid inclusions in the reservoir minerals correspond to the earliest era of crude oil entering the reservoir and the era of large-scale oil charging into the reservoir, respectively. The Longjing movement created favorable conditions for the late oil accumulation. The intensity of the early period oil charging was weak, and the late period was the critical period for the oil accumulation in this area.