陆相湖盆富有机质泥页岩中方解石脉成因及油气地质意义——以松辽盆地白垩系青山口组为例

Genetic mechanism and petroleum geological significance of calcite veins in organic-rich shales of lacustrine basin:A case study of Cretaceous Qingshankou Formation in Songliao Basin,China

基于岩心、岩石薄片观察分析,结合阴极发光、激光拉曼、流体包裹体、LA-ICP-MS原位U-Pb测年等技术手段对松辽盆地白垩系青山口组页岩储层中的方解石脉成因机制及油气地质意义进行研究。结果表明:①在宏观上方解石脉以顺层为主,呈透镜状、“S”形、叠锥状、羽状,在微观上分为向生式块状或柱状晶体结构和背生式纤维状晶体结构脉体。②块状方解石脉中的盐水包裹体均一温度为132.5~145.1℃,原位U-Pb测年绝对年龄为(69.9±5.2)Ma,表明青山口组烃源岩中成熟期和常规油的形成时期为晚白垩世明水组沉积期。纤维状方解石脉中的盐水包裹体均一温度为141.2~157.4℃,对应于烃源岩晚成熟期,U-Pb测年绝对年龄为(44.7±6.9)Ma,指示青山口组烃源岩的中高成熟期和古龙油页岩的形成时间为古近纪依安组沉积期。③向生式块状或柱状晶体结构方解石脉的形成与成岩成烃作用有关,脉体的形成经过了裂缝的开启、成脉流体充填和脉体生长3个阶段,构造挤压活动和流体超压是裂缝形成的诱导因素,成脉流体以短距离的扩散流为主,是一种有竞争的结晶生长模式。背生式纤维状方解石脉主要是在一种无竞争生长环境下由结晶力驱动形成的。研究认为,研究区青山口组富有机质泥页岩中的方解石脉对于松辽盆地构造活动、流体超压、泥页岩生排烃及成岩-成藏年龄具有一定指示意义。

Based on the observation and analysis of cores and thin sections,and combined with cathodoluminescence,laser Raman,fluid inclusions,and in-situ LA-ICP-MS U-Pb dating,the genetic mechanism and petroleum geological significance of calcite veins in shales of the Cretaceous Qingshankou Formation in the Songliao Basin were investigated.Macroscopically,the calcite veins are bedding parallel,and show lenticular,S-shaped,cone-in-cone and pinnate structures.Microscopically,they can be divided into syntaxial blocky or columnar calcite veins and antitaxial fibrous calcite veins.The aqueous fluid inclusions in blocky calcite veins have a homogenization temperature of 132.5–145.1℃,the in-situ U-Pb dating age of blocky calcite veins is(69.9±5.2)Ma,suggesting that the middle maturity period of source rocks and the conventional oil formation period in the Qingshankou Formation are the sedimentary period of Mingshui Formation in Late Cretaceous.The aqueous fluid inclusions in fibrous calcite veins with the homogenization temperature of 141.2–157.4℃,yields the U-Pb age of(44.7±6.9)Ma,indicating that the middle-high maturity period of source rocks and the Gulong shale oil formation period in the Qingshankou Formation are the sedimentary period of Paleocene Yi’an Formaiton.The syntaxial blocky or columnar calcite veins were formed sensitively to the diagenetic evolution and hydrocarbon generation, mainly in three stages (fracture opening, vein-forming fluid filling, and vein growth). Tectonic extrusion activities and fluid overpressure are induction factors for the formation of fractures, and vein-forming fluid flows mainly as diffusion in a short distance. These veins generally follow a competitive growth mode. The antitaxial fibrous calcite veins were formed under the driving of the force of crystallization in a non-competitive growth environment. It is considered that the calcite veins in organic-rich shale of the Qingshankou Formation in the study area has important implications for local tectonic activities, fluid overpressure, hydrocarbon generation and expulsion, and diagenesis-hydrocarbon accumulation dating of the Songliao Basin.