珠江口盆地文昌凹陷烃源岩发育时期湖盆古生产力构成及分布规律

Composition and distribution of lacustrine paleoproductivity during the deposition of hydrocarbon source rocks in Wenchang Sag,Pearl River Mouth Basin

沉积盆地中有机质的富集是烃源岩发育的本质过程,而古生产力是控制该过程最重要的因素。文昌凹陷是珠江口盆地主要的生烃凹陷之一,前人对其进行了包括烃源岩分布、有机质丰度、有机质类型、成熟度和生烃潜力等方面的研究,但是对古生产力和有机质富集机制方面的研究还有待加强。笔者等以文昌凹陷古新统文昌组二段(文昌组)和恩平组一段(恩一段)两套烃源岩为研究对象,利用有机地化数据和元素分析数据,对有机质来源、古生产力构成及分布规律进行了研究。结果表明:文昌凹陷文二段和恩一段烃源岩的有机质均为混合来源,其中文二段有机质中水生生物的贡献较大,具有更高的原地有机质比例,而恩一段陆源高等植物的贡献较大;恩一段具有较高的总生产力指数,其分布具有较强的非均质性,北部缓坡带的总生产力指数较南部陡坡带低,总生产力指数的高值主要分布在物源输入处和盆地中心地区。本次研究可以为后期有机质富集机理的研究提供支撑。

The enrichment of organic matter in sedimentary basins is the essential process of hydrocarbon source rock deposition,and the paleoproductivity is the most important factor to control this process.The Wenchang Sag is one of the main hydrocarbon-generating sags in the Pearl River Mouth Basin.A lot of researches have been carried out in the Wenchang Sag,including the distribution of hydrocarbon source rocks,the abundance,type,and maturity of organic matter,and the hydrocarbon-generating potential,etc.However,the researches on paleoproductivity and organic matter enrichment mechanism need to be strengthened sequentially.Therefore,in this paper the composition and distribution of paleoproductivity during the sedimentary period of hydrocarbon source rock in the Wenchang Sag were analyzed to provide support for the followed study of organic matter enrichment mechanism.Methods:The analysis of TOC content and rock pyrolysis was conducted using a CS-230 carbon—sulfur analyzer in The Key Laboratory of the Ministry of Education of Terrestrial Shale Oil and Gas Formation and Efficient Development,Northeast Petroleum University.The analysis of major and trace element contents was carried out in Suzhou Sike Geological Exploration Technology Service Co.,LTD.The samples were analyzed for major element concentrations by a Axios mAX X-ray fluorescence(XRF)spectrometer.The samples were analyzed for trace element concentrations by a NexION 300D inductively coupled plasma mass spectrometry.Further details are described by Ma Pengfei et al.(2015)and Gao Ping et al.(2015).The analysis of kerogen macerals and saturated hydrocarbon GC-MS were conducted at The Analysis and Testing Center of CNOOC Zhanjiang Branch Company.The instrument used for the saturated hydrocarbon GC-MS analysis was an Agilent 5977B chromatography mass spectrometer.Results:The macerals of kerogen in the source rocks from the 2nd Member of Wenchang Formation(E_(1)w^(2))are mainly composed of sapropelic group.While the macerals of kerogen in the source rocks from the 1st Member of Enping Formation(E_(1)e^(1))are mainly composed of vitrinite and inertinite.The distribution of n-alkanes is mainly front unimodal or bimodal in the E_(1)w^(2) source rock,and it is mainly post unimodal or bimodal in the E1e1 source rock.The distribution of C_(27)—C_(29) regular steranes shows a“V”shape in the Wenchang Sag.The average ratio of(W+T)/C_(30)H in E_(1)w^(2) and E_(1)e^(1) source rocks is 0.38 and 2.03,respectively.Conclusion:Organic matter in the E_(1)w^(2) and E_(1)e^(1) source rocks of the Wenchang Sag are both mixed sources.The aquatic organism in the E_(1)w^(2) source rock has a greater contribution,with a higher proportion of in-situ organic matter.While the terrestrial higher plant in E1e1 source rock has a greater contribution.The E_(1)e^(1) source rock has a higher total productivity index whose distribution has strong heterogeneity.During the sedimentary period of the E1e1 source rock,the total productivity index in the northern gentle slope zone is lower than that in the southern steep slope zone,and the high value of the total productivity index is mainly distributed in the places of provenance input and the central areas of the basin.