古盐度对陆相页岩发育的控制作用:以东营凹陷古近系沙三下页岩为例^*

Paleosalinity control on development of lacustrine shale: an example of shale from the lower submember of Member 3 of Paleogene Shahejie Formation,Dongying sag

摘要

陆相优质烃源岩发育大多与湖泊咸化相关,但湖泊咸化对页岩发育的影响仍不清晰。以东营凹陷古近系页岩重点取心井樊42井为例,通过岩石薄片观察、XRD、岩石热解测试、XRF二维元素扫描、微量元素测试等分析,结合有机地球化学测试数据、微量元素含量的垂向变化等,分析不同盐度演化阶段页岩成分、纹层结构、有机质丰度的变化。结果表明,研究区目的层段页岩可划分为5种岩相类型。目的层段环境演化可以划分为5个阶段,受控于气候、陆源输入等因素。古气候在沙三下亚段沉积时期呈现暖湿—相对湿冷—暖湿—相对干冷—相对暖湿的变化特征。陆源输入量呈多期旋回变化。不同类型岩石的沉积环境特征各异,高盐度的湖泊水体营养盐浓度高,具有较高的初级生产力,水体分层强烈,水体循环较弱。氧/盐度跃层位置相对较高,富氧带水深较浅,使得有机质更快脱离氧化环境沉降进入还原性水体,有利于有机质的保存。碳酸盐矿物含量高,且呈纹层状发育于滞水层中。

Abstract

The development of lacustrine high-quality hydrocarbon source rocks is often associated with lake salinization,but the impact of salinization on shale development remains unclear. In this paper,taking the Paleocene shale of the Dongying sag as an example,we analyze the changes in shale composition,laminae structure,and organic matter abundance across different salinity evolution stages. This is done by means of thin-section observation,XRD,pyrolysis test of rocks,2D elemental scanning of XRF,and trace elemental test,combined with organic geochemistry data and variations in elemental content. The results show that the shale within the depth range of 3120-3160 m in the lower submember of Member 3 of the Shahejie Formation of Well F42 can be divided into five lithofacies. The environmental evolution of the target section can be divided into five stages,which are controlled by factors such as climate and terrestrial input. During the deposition of the lower submember of Member 3 of the Shahejie Formation,the paleoclimate transitioned from warm and humid to relatively wet and cold,then to warm and humid,relatively dry and cold,and finally to relatively warm and humid. Terrestrial input showed multi-period cyclic changes. Characteristics of sedimentary environments varied among different lithofacies. High-salinity lakes had high nutrient concentration,high primary productivity,strong water stratification,and weak water circulation. The position of oxycline/halocline was relatively high,and the oxygen-rich zone was shallow,facilitating the rapid deposition of organic matter into reducing environments,which favored organic matter preservation. The carbonate mineral content was high and developed as laminae in stagnant water layers.

关键词

咸化湖盆 / 湖相页岩 / 纹层发育 / 有机质富集 / 古近系沙河街组 / 东营凹陷

Key words

salinized lake / lacustrine shale / laminae development / organic matter enrichment / Paleogene Shahejie Formation / Dongying sag

基金资助

*国家自然科学基金(编号: 42272119,42072164)和山东省重点研发计划(编号: 2020ZLYS08)联合资助