中下寒武统海相地层地球化学特征及油气地质意义——以塔里木盆地柯坪地区为例

Geochemical characteristics and geological significance for petroleum of the Middle-Lower Cambrian marine strata:A case study of Keping area in the Tarim basin

为了进一步研究塔里木盆地寒武系海相地层垂相上的沉积环境演化特征和油气地质条件,本文以塔西北柯坪地区肖尔布拉克剖面中、下寒武统为研究对象,通过镜下观察、主量元素、微量元素、稀土元素以及碳氧同位素测试分析,探讨了该套地层的地球化学特征、沉积环境以及油气地质意义,并为地层对比提供依据以及在全球背景下碳同位素负漂移的原因进行分析,认为塔里木盆地寒武系油气生、储、盖层的发育受控于沉积环境的演化.研究结果表明:1)氧化还原指标Th/U,Ni/Co,δU等指示寒武系玉尔吐斯组、肖尔布拉克组、沙依里克组为缺氧的沉积环境,吾松格尔组为氧化环境,阿瓦塔格组指示强烈的氧化环境,其氧化还原环境受海平面变化的控制;2)玉尔吐斯组存在明显δEu正异常,δEu值高达61.60,并且大量富集Ba和Zn元素,结合主量元素协变图,认为受到热液作用的影响,热液作用携带的大量营养元素促进生物勃发,有利于有机质生成;而缺氧的沉积环境有利于有机质的保存,玉尔吐斯组有机质的富集主要受控于氧化还原条件;3)中-下寒武统分界线处存在着明显的δ^(13)C负漂移("ROECE"事件),δ^(13)C的漂移幅度为9.21‰,结合微量元素分析,认为是发生全球性海平面上升过程中,底层δ^(13)C亏损的缺氧水体上涌导致,研究区的δ^(13)C负漂移可与全球其他地区的"ROECE"事件进行对比,可作为地层对比的依据;4)玉尔吐斯组形成于缺氧的盆地-斜坡环境,富集大量有机质,是优质的烃源岩层;肖尔布拉克组发育于高能的台地边缘斜坡环境,其中-上段的微生物碳酸盐岩是良好的储集层;阿瓦塔格组形成于强烈氧化的蒸发环境,膏岩层发育,可作为有效的盖层;玉尔吐斯组的优质烃源岩、肖尔布拉克组的微生物碳酸盐岩以及阿瓦塔格组的膏岩层构成一套良好的生储盖组合,具有优越的成藏条件,是深层油气勘探的有利区带.

To study the evolution characteristics of the vertical depositional environment and petroleum geological condition of Cambrian marine strata in Tarim basin in further,this paper focused on the Middle-Lower Cambrian strata in the Xiaoerbulake section of the Keping area in the northwest of Tarim basin.Based on the analysis of microscopic observation,major elements,trace elements,rare earth elements,carbon and oxygen isotopes,the geochemical characteristics,sedimentary environment and the geological significance of the studied strata were discussed,moreover,the analyzing basis of stratigraphic correlation and the negative drift of carbon isotopes under the global background were provided.It is concluded that the development of hydrocarbon source rocks,reservoir and cap rocks is controlled by sedimentary environment evolution.The results show that:1)Redox indexes such as Th/U,Ni/Co,U indicate that the Yuertusi,Xiaoerbulake and Shayilike formations in the Cambrian are rich in redox sensitive elements,which deposit under anoxic sedimentary environment.While Wusonggeer formation is formed in oxidation environment,and Awatage formation is deposited in a strong oxidizing environment,and its redox environment is controlled by eustatic sea level change.2)An obvious positive Eu anomaly,with the value of up to 61.60,can be found in Yuertusi formation,meanwhile,some elements such as Ba,Zn are enriched.Combined with major elements covariation diagram,it is suggested that hydrothermal activity might influence the deposition in this stage.A large number of nutrient elements carried by hydrothermal activities promote biological prosperity and are beneficial to the generation of organic matter.The anoxic sedimentary environment in Yuertusi formation is favorable for the preservation of organic matter.The enrichment of organic matter is mainly controlled by redox conditions.3)A significantδ13C negative drift of 9.21‰is observed at the boundary of the Middle-Lower Cambrian(“ROECE”event).The trace elements and carbon isotopic data suggest that the reason leading to the negativeδ13C drift might be the upwelling of anoxic water withδ13 C deficit in the bottom,during the global sea level rising.Theδ13 C negative drift in the study area can be compared with the“ROECE”in other areas of the world,which can be used as an evidence for stratigraphic correlation.4)The Yuertusi formation is composed of high-quality source rocks with abundant organic matter and formed in an anoxic basin-slope environment which is conducive to the preservation of organic matter.The Xiaoerbulake formation is developed in a platform marginal slope environment with high energy,in which the bio-carbonates in the upper section is favorable porous reservoir.The uppermost strata Awatage formation is deposited in strongly oxidized evaporation situation,where gypsum layers are developed and can be regarded as effective cap rocks.The high-quality source rocks in the Yuertusi formation,the reservoir of bio-carbonate rocks in the Xiaoerbulake formation and the gypsum rocks in the Awatage formation constitute a set of effective source-reservoir-cap assemblage,which forms the favorable zone for deep oil and gas exploration.