渤海湾盆地渤中凹陷古近系烃源岩生物标志物特征及其地质意义

Characteristics and geological significance of biomarkers from the Paleogene source rocks in Bozhong Sag,Bohai Bay Basin

渤中凹陷位于渤海湾盆地中东部,主要发育沙三、沙一和东三段烃源岩层,具有良好的油气勘探前景,前期对渤中凹陷有机地球化学特征的研究主要基于岩屑样品,对岩心样品生物标志物特征的研究比较薄弱。本次研究综合利用岩心和岩屑样品,通过有机碳、岩石热解、氯仿沥青“A”、族组分饱和烃色谱-质谱等有机地球化学分析测试,重点对渤中凹陷古近系东营组三段和沙河街组一段、三段烃源岩的有机质丰度、类型、成熟度及母质来源与沉积环境进行研究,明确了渤中凹陷三套泥页岩的地球化学特征及发育控制因素,并建立了区分3套烃源岩的典型生物标志化合物组合特征,以期望为进一步厘清该区域优质烃源岩发育模式及时空分布提供一定的理论依据,进而有利于该区油气的勘探。研究结果表明:3套烃源岩的生烃潜力具有显著差异,沙三段>沙一段>东三段。沙三段烃源岩有机质丰度最高,为好-极好烃源岩,有机质类型为Ⅰ-Ⅱ_(1)型,处于成熟阶段;沙一段次之,为中等-好烃源岩,有机质类型为Ⅱ_(1)-Ⅱ_(2)型,整体处于成熟阶段;东三段稍差,为中等烃源岩,有机质类型以Ⅱ_(1)-Ⅱ_(2)型为主,处于低熟-成熟阶段。岩心样品的生物标志化合物中姥植比(Pr/Ph)、伽马蜡烷指数(Ga/C_(30)H)、C_(27)-C_(28)-C_(29)规则甾烷含量、4-甲基甾烷指数(4-MS/C_(29)甾烷)等参数表明母质输入和沉积环境是控制3套湖相烃源岩差异的主控因素,淡水藻源输入和良好的保存条件是沙三段优质烃源岩发育的关键,低等水生生物输入和强还原的保存条件是沙一段优质烃源岩发育的关键,混源输入和良好的保存条件是东三段优质烃源岩发育的关键。

The Bozhong Sag is located in the central and eastern part of the Bohai Bay Basin,and source rocks of the first and third Member of the Paleogene Shahejie Formation(E_(2)s^(1) and E_(2)s^(3))and the third Member of the Donying Formation(E_(3)d^(3))are mainly developed,indicative of good oil and gas exploration prospects.Previous studies on organic geochemical characteristics of the Bozhong Sag are mainly based on cutting samples,and study on biomarker characteristics using core samples is insufficient.By utilizing both core and cuttings samples,this study integrates organic carbon,rock pyrolysis,chloroform bitumen"A",saturated hydrocarbon chromatography-mass spectrometry and other organic geochemical analysis.Organic matter abundance,type,maturity,source materials and depositional environment of the E_(3)d^(3),E_(2)s^(1) and E_(2)s^(3) source rocks in the Bozhong Sag were studied,and the geochemical characteristics and depositional factors of the three source rocks were studied.The combination characteristics of typical biomarker compounds to distinguish the three source rocks are established,in order to provide a theoretical basis for further clarifying the development pattern and spatial distribution of high-quality source rocks in this area,which is beneficial to the exploration of oil and gas in this area.The results show that the hydrocarbon generation potential of the three source rocks is different,where the generation potential is as E_(2)s^(3)>E_(2)s^(1)>E_(3)d^(3).The E_(2)s^(3) source rock has the highest organic matter abundance,type Ⅰ and Ⅱ_(1) kerogen,and is currently in the mature stage.The E_(2)s^(1) is medium to good source rock,with organic matter type Ⅱ_(1) and Ⅱ_(2),and is also inmature stage.The E_(3)d^(3) is a medium source rock,with organic matter type Ⅱ_(1) and Ⅱ_(2),and is in the stage of low maturity to medium maturity.The biomarker parameters,including Pr/Ph value,gammacerane index,C_(27)-C_(28)-C_(29) regular sterane content,4-methylsterane index and other parameters indicate that the source material input and depositional environment are the main factors controlling the different development of three lacustrine source rocks.The input of freshwater algae sources and good preservation conditions are the keys to the development of high-quality source rocks in the E_(2)s^(3).The input of low-level aquatic organisms and strong reducing preservation conditions are the keys to the development of high-quality source rocks in the E_(2)s^(1).Mixed source input and good preservation conditions are the key to the development of high-quality source rocks in the E_(3)d^(3).