塔里木盆地玉北地区奥陶系储层成因研究

Formation of the Ordovician Reservoir in Yubei Area,Tarim Basin

关键词:  储层成因  白云岩  同位素地球化学  奥陶系  塔里木盆地

Key words:  reservoir origin  dolostone  isotope geochemistry  Ordovician  Tarim basin

摘要:

玉北地区奥陶系碳酸盐岩经历了加里东期和海西期构造作用的改造，东部断褶带和西部斜坡区断层发育，中部平台区断层不发育，导致勘探目的层鹰山组上部与鹰山组下部和蓬莱坝组储层发育特征的不同。鹰山组上部的储层主要受控于加里东期构造活动所造成的地层抬升暴露剥蚀，形成了以大气淡水溶蚀为主的储集空间类型；而鹰山组下部和蓬莱坝组储层则主要受到埋藏白云石化作用、硅化作用以及伴随着断层的形成而发育的裂缝和热液的改造作用的控制。氧碳同位素和⁸⁷Sr/⁸⁶Sr比值的分布特征也进一步证明了两种不同储层类型的主控因素。鹰山组上部洞穴中方解石的氧碳同位素值（δ¹⁸O_VPDB：-12～-4‰VPDB，δ¹³C：-3～1‰VPDB）比海水沉淀的方解石（δ¹⁸O_VPDB：-9.5～-7.5‰VPDB，δ¹³C：-1.5～0.5‰VPDB）在数值上明显偏负，说明洞穴中充填的方解石属于淡水成因。在鹰山组下部和蓬莱坝组白云岩储层中发育的鞍形白云石具有较低的氧同位素值（δ¹⁸O_VPDB：-10.7～-5.9‰VPDB）和高于海水的⁸⁷Sr/⁸⁶Sr值（均大于0.7094），因而被解释为热液成因，进一步说明这些地层中的溶蚀孔洞可能与热液改造有关。

Abstract:

The Ordovician carbonate rocks in Yubei area have undergone the Caledonian and Hercynian tectonic movements. Faults mainly developed in the eastern fault-folded belts and the western slope, and rarely developed in the central platform. This resulted in differences of the reservoir characteristics of the upper member of the Yingshan Formation, and the lower member of the Yingshan Formation to the Penglaiba Formation. The reservoirs of the upper Yingshan Formation are mainly controlled by the exposure and leaching of meteoric water during the Caledonian period, forming a karst reservoir. The lower member of the Yingshan Formation and the Penglaiba Formation are mainly controlled by the burial dolomitization, silicification, and fault-related hydrothermal alteration. The distribution characteristics of oxygen isotopes, carbon isotopes and the ⁸⁷Sr/⁸⁶Sr ratios further illustrate the main controlling factors of the two different reservoir types. The cave-filling calcite in the upper Yingshan Formation has δ¹⁸O values of -12——4 ‰VPDB and δ¹³C values of -3-+1 ‰VPDB, which are lower than those of the coeval marine calcites (δ¹⁸O_VPDBvalues of -9.5——7.5‰VPDB,δ¹³C values of -1.5-+0.5‰VPDB). This indicates that the cave-filling calcite is precipitated from meteoric fluids. The saddle dolomite cements in the lower member of the Yingshan Formation and the Penglaiba Formation have δ¹⁸O values of -10.7——5.9‰ VPDB, and the ⁸⁷Sr/⁸⁶Sr ratios are higher than seawater (>0.7094). This further support that the saddle dolomites are of hydrothermal origin, and that the dissolution vugs may be related to the hydrothermal events.