四川盆地及其周缘上奥陶统五峰组—下志留统龙马溪组页岩气“源-盖控藏”富集

根据页岩气地质理论研究进展和勘探发现,基于"源-盖控烃"论分析了四川盆地及其周缘上奥陶统五峰组—下志留统龙马溪组页岩气富集的主控因素。1"源控"主要表现在优质页岩厚度、面积、有机碳含量等物质基础参数控制了页岩的生烃能力和储集能力,上奥陶统五峰组—下志留统龙马溪组一段下部页岩(WF2—LM4笔石页岩段)具有沉积速率低、有机质类型好、有机碳含量高和生烃能力强等特点,具备良好的页岩气发育生烃物质基础;浮游藻类来源的成烃生物有利于大量生烃和有机质孔大量发育,且高有机碳含量保证了发育大量有机质孔并形成三维连通的有机质孔孔隙网络,为天然气提供良好的赋存空间和渗流通道。2"盖控"的盖层包括直接盖层和间接盖层,直接盖层指龙马溪组一段中上部(LM5笔石页岩段及其以上)页岩对五峰组和龙马溪组一段下部页岩(WF2—LM4笔石页岩段)的封闭;中下三叠统膏盐岩、泥岩间接盖层(区域盖层)保证了一定的压力封闭,使得其下的五峰组和龙马溪组页岩气能有效富集,形成高压富集的页岩气藏(焦石坝、富顺—永川、长宁等地区);在中下三叠统缺失的地区,先存的高压封闭体系被破坏,页岩气保存失去有效性,形成低压—常压富集的页岩气藏(渝东南彭水、云南昭通等地区)。源-盖空间匹配关系的数量(静态匹配)和质量(动态匹配)控制着页岩气富集位置和富集程度。四川盆地及其周缘上奥陶统五峰组—下志留统龙马溪组一段下部页岩(WF2—LM4笔石页岩段)发育厚度大,中下三叠统膏盐岩、泥岩盖层发育,主成藏期在J3—K1以新的地区,是四川盆地及其周缘上奥陶统五峰组—下志留统龙马溪组页岩气富集的有利地区。

The “source-cap hydrocarbon-controlling” enrichment of shale gas in Upper Ordovician Wufeng Formation-Lower Silurian Longmaxi Formation of Sichuan Basin and its periphery

Based on the exploration discoveries of shale gas and research progresses of relevant geological theory as well as "source-cap controlling hydrocarbon" theory, the main controlling factors of shale gas enrichment in Upper Ordovician Wufeng Formation-Lower Silurian Longmaxi Formation of Sichuan Basin and its periphery were analyzed:(1) "Source control" means that the shale hydrocarbon generation and reserving capacity can be controlled by the thickness, area, total organic carbon content of high-quality shale and other basic factors. The shale in Upper Ordovician Wufeng Formation-Lower Member 1 of Lower Silurian Longmaxi Formation (WF2-LM4 graptolite shale member) is characterized by slow sedimentary rate, good type of organic matter, high total organic carbon content and great hydrocarbon generation capacity, thus providing a favorable hydrocarbon-generation material basis for shale gas development; the hydrocarbon-generation materials such as planktonic algae are beneficial for massive hydrocarbon generation and organic matter development, and high organic carbon content is able to ensure the development of massive organic-matter pores, so as to form a three-dimensional connected network of organic-matter pores and provide favorable occurrence space and flow channel for natural gas. (2) In terms of "cap control" theory, the cap can be divided into direct cap and indirect cap. Direct cap refers to the confining bed between the shale in mid-upper Member 1 of Longmaxi Formation (LM5 graptolite shale member and above) and that in Wufeng Formation and the lower Member 1 of Longmaxi Formation (WF2-LM4 graptolite shale member). Indirect cap (regional cap) in the Mid-Lower Triassic gypsum rocks and mudstones can guarantee the pressure sealing to a certain degree. As a result, the shale in Wufeng Formation and Longmaxi Formation can be effectively enriched to form high-pressure enriched shale gas reservoirs (Jiaoshiba, Fushun-Yongchuan and Changning, etc.). In the regions with absent Mid-Lower Triassic, the pre-existing high-pressure closed system was destroyed and the shale gas preservation availability was lost, thus forming the shale gas reservoirs enriched under low-normal pressure (Pengshui in southeastern Chongqing, Zhaotong in Yunnan, etc.) The enrichment location and degree of shale gas is controlled by the quantity (static match) and quality (dynamic match) of source-cap spatial match relationship. Research results indicate that the shale in Upper Ordovician Wufeng Formation-lower Member 1 of Lower Silurian Longmaxi Formation of Sichuan Basin and its periphery (WF2-LM4 graptolite shale member) is developed with a large thickness, together with Mid-Lower Triassic gypsum rocks and mudstones. The regions with the main reservoir accumulation period of J₃-K₁ or after J₃-K₁ are favorable areas for shale gas enrichment in Upper Ordovician Wufeng Formation-Lower Silurian Longmaxi Formation of Sichuan Basin and its periphery.