鄂尔多斯盆地马家沟组四段、五段储层地球化学特征及成因分析

Geochemical characteristics and genesis analysis of the 4^(th) and 5^(th) memberreservoirs in Majiagou Formation of Ordos Basin

鄂尔多斯盆地马家沟组是奥陶系海相碳酸盐岩油气勘探开发的重点层系,马家沟组可分为6个段,其中马家沟组四段(以下简称马四段)、五段(以下简称马五段)为主要含油气层。为确定马四段、马五段碳酸盐岩储层的地球化学特征,开展了C、O和Sr同位素及多种微量元素的实验分析,研究其物质来源和古沉积环境,探讨其形成因素。研究结果表明,鄂尔多斯盆地马四段+马五段的δ^(13)C范围在-3.37‰~1.17‰,δ^(18)O范围在-11.43‰~-5.02‰;^(87)Sr/^(86)Sr范围为0.7044~0.7128,平均值为0.7092,地层在埋藏期间受到了成岩作用影响;奥陶系原始碳酸盐岩地层经历了后期风化淋滤、古岩溶作用以及流体化学的差异改造,古陆地流体中的富^(87)Sr对碳酸盐岩及其填充物发生化学反应后,^(87)Sr/^(86)Sr大幅度提高;马四段、马五段碳酸盐岩主要来源于典型的咸化海相沉积,气候炎热干旱,马五段沉积环境比马四段沉积环境水体深。研究结果为该区奥陶系海相碳酸盐岩古环境和古气候研究提供了借鉴。

Majiagou Formation in Ordos Basin is a key strata for oil and gas exploration and development of Ordovician marine carbonate rocks.Majiagou Formation can be divided into six members,of which member 4(hereinafter referred to as Ma 4)and Member 5(hereinafter referred to as Ma 5)of Majiagou Formation are the main oil-bearing reservoirs.In order to determine the geochemical characteristics of carbonate reservoirs in Ma 4 and Ma 5,the experimental analysis of C,O and Sr isotopes and a variety of trace elements was carried out to study the material source of reservoirs andpaleosedimentary environment and to explore their formation factors.The results show that theδ^(13)C andδ^(18)O values of Ma 4 and Ma 5 in Ordos Basin range from-3.37‰to 1.17‰and-11.43‰to-5.02‰respectively,and the strata are affected by diagenesis during burial;The ^(87)Sr/^(86)Sr values range from 0.7044 to 0.7128,with an average of 0.7092.The Ordovician original carbonate strata have undergone weathering leaching,paleokarst and fluid chemical transformation in the later stage.The ^(87)Sr/^(86)Sr ratio of carbonate rocks and their fillings increase greatly after the chemical reaction of ^(87)Sr rich in ancient continental fluids.The carbonate rocks of Ma 4 and Ma 5 are mainly derived from typical saline marine sediments,and the climate is hot and dry.The sedimentary environment of Ma 5 is relatively deeper than that of Ma 4.The results can be used as reference for the study of paleoenvironment and paleoclimate of Ordovician marine carbonate rocks in this area.