鄂尔多斯盆地奥陶系马家沟组沉积环境恢复与源储配置关系

Reconstruction of Depositional Environment and Source-Reservoir Configuration Relationship of Ordovician Majiagou Formation in Ordos Basin

碳酸盐岩-膏盐岩组合蕴含了全球46%的碳酸盐岩油气储量,是重要的油气富集层位.鄂尔多斯盆地奥陶系马家沟组发育大套厚层碳酸盐岩-膏盐岩混合沉积,目前在盐上已发现了近万亿方探明储量.盐下和盐间有无勘探潜力?对该沉积体系的古环境恢复是关键.通过对盆内T112井400 m系统取芯进行描述,结合薄片鉴定、微量元素、稀土元素、碳、氧、锶同位素、岩石组分等分析,对马家沟组碳酸盐岩-膏盐岩沉积体系的沉积环境进行了恢复.根据海平面升降和水体氧化程度的变化,该体系可以划分为4个亚环境,分别是缺氧段、次氧化段、氧化段和硫化段.随着氧化程度加剧,U、Mo等微量元素含量降低,碳氧锶同位素逐渐正偏,但由于硫化段导致含氧量急剧降低,U、Mo和碳氧锶同位素急剧升高,并呈现随海平面频繁振荡分布.基于古环境与TOC和孔隙度的对比分析,硫化段和缺氧段是最有利的烃源岩发育层段,强烈蒸发作用导致水体分层及硫化,使有机质得到有效保存.氧化段和次氧化段是储层发育的有利层段,台内局部隆起沉积的丘滩体频繁暴露遭遇准同生溶蚀作用,发育大量溶蚀孔隙,经白云石化以后得到有效保存.硫化段的烃源岩与氧化段的白云岩储层纵向上呈层状频繁交互,平面上呈指状交叉,构成良好的源储配置关系.乌审旗隆起带上发育的立体型储集层网络是鄂尔多斯盆地奥陶系盐下勘探的有利区带.

The carbonate gypsum salt rock combination contains 46% of the global carbonate oil and gas reserves and is an important oil and gas enrichment horizon. The Majiagou Formation of Ordovician in Ordos basin develops a large set of thick carbonate gypsum salt rock mixed deposits. At present, nearly trillion cubic meters of proved reserves have been found on salt. Is there exploration potential under salt and between salt? The restoration of the Paleoenvironment of the sedimentary system is the key. Based on the description of 400 m systematic coring of well T112 in the basin, combined with thin section identification,supporting analysis of trace elements, rare earth elements, carbon, oxygen, strontium isotopes and rock components, this paper restores the depositional environment of carbonate gypsum salt rock sedimentary system of Majiagou formation. According to the rise and fall of sea level and the change of water oxidation degree, the system can be divided into four sub environments: anoxic section, secondary oxidation section, oxidation section and sulfide section. With the aggravation of oxidation degree, the contents of trace elements such as U and Mo decrease, and the carbon oxygen strontium isotope is gradually positive. However, the oxygen content decreases sharply due to the sulfide section, and the isotopes of U, Mo and carbon oxygen strontium rise sharply, showing a frequent oscillation distribution with sea level. Based on the comparative analysis of paleoenvironment, TOC and porosity, the sulfide section and anoxic section are the most favorable hydrocarbon source rock development sections. Strong evaporation leads to water stratification and vulcanization, so that organic matter can be effectively preserved. Oxidation section and secondary oxidation section are favorable sections for reservoir development. The mound and beach bodies deposited by local uplift in the platform were frequently exposed, encountered quasi syngenetic dissolution and developed a large number of dissolution pores,which were effectively preserved after dolomitization. The source rock in the sulfide section and the dolomite reservoir in the oxidation section are interbedded in the longitudinal direction, and finger crossing in the plane, forming a good source-reservoir matching relationship. The three-dimensional reservoir network developed on Wushenqi uplift belt is a favorable area for Ordovician subsalt exploration in Ordos Basin.