鄂西宜昌地区秭地1井陡山沱组古沉积环境演化特征

Characteristics of Paleo-sedimentary Environment Evolution ofDoushantuo Formation in Well Zidi 1 of Yichang Area in West Hubei of China

陡山沱组为鄂西地区重要的页岩气勘探目的层之一,针对该地层古环境研究较为薄弱的现状,通过采集岩心样品进行TOC、主量元素和微量元素测试,分析了鄂西宜昌地区陡山沱组古环境演化规律,明确了陡山沱组页岩气富集有利层段。研究表明:陡一段和陡三段为贫有机质地层,陡二段和陡四段为富有机质地层,其中,陡二段显著富集Ca、Mg、P、Ba、Sr、Zn、U、Mo、Cu,陡四段显著富集Ca、Si、P、Mo、U、Ba、V。陡一段和陡三段气候干燥,盐度较高,主要为氧化环境,古生产力较低;陡二段为温湿与干热气候交替,盐度较高,氧化与贫氧频繁交替;陡四段为温湿气候,盐度较低,为缺氧环境。陡山沱组古水动力较弱,沉积于强滞留盆地,陡二段和陡四段古生产力较高。有利区块位于宜昌市以西的深水台间盆地,陡二段中部为页岩气富集有利层段,其中,TOC>2.00%的富有机质地层厚度超过20 m,沉积于贫氧-缺氧环境,古生产力相对最高,为陡山沱组页岩气勘探地质“甜点”。研究成果可为鄂西地区陡山沱组页岩气勘探提供地球化学依据。

Doushantuo Formation is one of the important shale gas exploration target formations in the west Hubei of China.In view of the weak research on the paleo-environment of this formation,the paleo-environmental evolution law of the Doushantuo Formation in the Yichang Area of the west Hubei was analyzed through the collection of core samples for the testing of TOC,major elements and trace elements,and the favourable formations for the enrichment of the Doushantuo Formation were clarified.The study shows that the Member 1 and Member 3 of Doushantuo Formation are of organic-lean strata,and the Member 2 and Member 4 are of organic-rich strata,in which the Member 2 is significantly enriched in Ca,Mg,P,Ba,Sr,Zn,U,Mo,and Cu,and the Member 4 is significantly enriched in Ca,Si,P,Mo,U,Ba,and V.Member 1 and Member 3 have dry climate,high salinity,mainly oxidizing environment,and low paleoproductivity;the Member 2 has alternating warm-wet and dry-hot climate,high salinity,and frequent alternation between oxidizing and oxygen-poor environment;and the Member 4 has warm-wet climate,low salinity,and oxygen-deficient environment.Doushantuo Formation has weak paleohydraulic forces which was deposited in a strong euxinic basin,and Member 2 and Member 4 have higher paleoproductivity.Favorable block is located in the deep-water inter-platform basin at the west of Yichang City,the middle of Member 2 is a shale gas-rich favorable formation,in which the thickness of organic-rich strata with TOC>2.00%is more than 20 m,deposited in an oxygen-poor and oxygen-deficient environment,with the highest paleoproductivity,and it is the geological"sweet spot"of Doushantuo Formation shale gas exploration.The study results provide geochemical basis for the shale gas exploration of Doushantuo Formation in west Hubei of China.