四川盆地天然气特征及气源

The characteristics of natural gas in Sichuan basin and its sources.

四川盆地是一个大型复合含气为主、含油为辅的叠合盆地。多旋回的沉积演化过程,孕育了多套海相、陆相烃源岩,且不同区域发育不同成因类型的烃源岩。目前下寒武统、志留系、下二叠统、上二叠统和上三叠统五套主要烃源岩均已进入高演化阶段,并以成气为主。由于多阶成烃、混源聚集和后期遭受TSR次生蚀变等成藏过程的复杂性使得天然气组分较干、碳同位素组成复杂,常规方法进行气源对比较困难。文中在对四川盆地沉积演化背景分析的基础上,通过对有效烃源岩发育特征和分布规律的探讨,分区域进行了气藏的分析,特别是对天然气组分、非烃组成(H2S、CO2、N2等)和碳同位素等资料综合研究的基础上,基本确定了各区块各含气层系的主力源岩。认为川东主力产层石炭系、三叠系和二叠系的气源分别为志留系、上二叠统龙潭组和下二叠统;川南气区震旦系灯影组、寒武系、二叠系和三叠系产层的气源分别主要来自下寒武统,上、下二叠系源岩;川西气区侏罗系和三叠系须家河组主产层的气源主要来自三叠系须家河组煤系烃源岩,下二叠统和嘉陵江组产层气源则可能主要来自二叠系;川中主要为产油区,下侏罗统自流井群原油应来自侏罗系源岩,浅部层系气源为上三叠统须家河组的陆相烃源岩,深部气藏则为寒武系烃源岩。由于川东北部烃源岩发育层数最多,且质量都较好,因此川东北部是烃类最富集的地区,也是勘探潜力最大的地区。

Sichuan basin is a large-scale superimposed basin, which is mainly a gas-bearing basin with subordinate oil. Multi-stage sedimentary processes developed many sets of different types of hydrocarbon source rocks in different areas. At present, five hydrocarbon source rocks, the Lower Cambrian, the Lower Silurian, the Lower Permian, the Upper Permian and the Upper Triassic, have entered into the high-matured phase, mainly generating gas. The complexity of multi-phase hydrocarbon generation and accumulation of mixed sources, and subsequent secondary alterations make the gas drier, carbon isotopes more complicated, and gas to source correlation more difficult. Based on an analysis of sedimentary evolution, we have analyzed the gas reservoir in different districts by probing into the developmental characteristics and distribution pattern of active hydrocarbon source rocks. We have primarily determined the main source rocks for different districts by the synthetic research of natural gas, non-hydrocarbon components, carbon isotopes, etc. The Carboniferous, Permian and Triassic natural gas in the eastern gas district of Sichuan basin are originated from the Silurian, the Upper Permian Longtan Formation and the Lower Permian hydrocarbon source rocks, respectively. The Sinian Dengying Formation, Permian and Triassic gas in the northern gas district are derived from the Lower Cambrian, the Upper Permian and Lower Permian hydrocarbon source rocks, respectively. In the west gas district, the Jurassic and Triassic Xujiahe Formation gas are from the coal hydrocarbon source rocks of Xujiahe Formation; the Lower Permian and Lower Triassic Jialingjiang Formation gas are generated from Permian source rocks. In the central oil district of Sichuan basin, the oil of Lower Jurassic Ziliujing Formation is generated from Jurassic source rocks; the shallow gas comes from continental source rocks of the Upper Triassic Xujiahe Formation; and the gas in deep reservoirs comes from Cambrian source rocks. Because the northeast Sichuan basin developed multilayered, good-quality hydrocarbon source rocks, it is the most favorable enrichment area for hydrocarbons, and for future prospecting.