羌塘盆地侏罗纪黑色岩系古环境研究

Study on the Paleoenvironments of Jurassic Black Rock Series in the Qiangtang Basin

摘要: 本文以羌塘盆地侏罗纪黑色岩系为研究对象，通过沉积学、元素地球化学及同位素分析，结合区域构造演化背景，系统探讨了该套岩系的古环境特征及其对有机质富集的控制作用。研究表明，羌塘盆地侏罗纪经历了裂谷期、前陆盆地期和陆内改造期三个构造–沉积阶段，黑色岩系主要发育于中侏罗世前陆盆地期的局限蒸发环境。地球化学指标(如V/Cr > 2、U/Th > 1.25、Pr/Ph < 1)及沉积构造(纹层状灰岩、膏盐层)显示，黑色岩系形成于缺氧–强还原条件下的咸水湖盆或深水台沟环境，受古特提斯洋闭合导致的构造沉降与物源供给共同影响。有机质富集主要受控于古气候(干旱–半干旱)、古盐度(高盐度)及氧化还原条件(强还原)的协同作用，其中布曲组与夏里组黑色页岩的TOC值最高(>2%)，干酪根类型以I-II型为主，具备优质烃源岩潜力。研究建立的“构造控盆–环境耦合”沉积模式，为深化特提斯构造域演化及油气资源评价提供了理论依据。

Abstract: This paper takes the Jurassic black rock series in the Qiangtang Basin as the research object. Through sedimentological, elemental geochemical and isotopic analyses, combined with the regional tectonic evolution background, the paleoenvironmental characteristics of this set of rock series and their control on the enrichment of organic matter are systematically discussed. The research shows that the Jurassic in the Qiangtang Basin experienced three tectonic-sedimentary stages: the rift stage, the foreland basin stage and the intracontinental reformation stage. The black rock series mainly developed in the restricted evaporative environment of the foreland basin stage in the Middle Jurassic. Geochemical indicators (such as V/Cr > 2, U/Th > 1.25, Pr/Ph < 1) and sedimentary structures (laminated limestone, evaporite layer) indicate that the black rock series formed in a saline lake basin or deep-water platform trench environment under anoxic-strongly reducing conditions, and was jointly affected by tectonic subsidence and provenance supply caused by the closure of the Paleo-Tethys Ocean. The enrichment of organic matter is mainly controlled by the synergistic effect of paleoclimate (arid-semiarid), paleosalinity (high salinity) and redox conditions (strongly reducing). Among them, the black shales of the Buqu Formation and the Xiali Formation have the highest TOC values (>2%), and the kerogen is mainly of Type I-II, showing the potential of high-quality source rocks. The established “tectonic basin control-environmental coupling” sedimentary model provides a theoretical basis for deepening the understanding of the evolution of the Tethys tectonic domain and the evaluation of oil and gas resources.