四川盆地及其周缘奥陶系五峰组志留系龙马溪组页岩气富集主控因素分析

四川盆地及周缘地区奥陶系五峰组志留系龙马溪组页岩气富集主控因素包括：(1)特殊的古环境所形成的优质页岩是富集的基础。四川盆地及其周缘在五峰组和龙马溪组沉积时期，在弱挤压背景下发生了陆内拗陷沉降，奥陶纪末期冰期后全球性海平面快速上升，形成了较大规模的深水陆棚环境，为低等生物的大规模繁殖提供了有利条件，闭塞的海湾背景，为有机质的保存提供了有利的地球化学环境，欠补偿状态导致了地层中的高有机质含量。特殊生物类型所形成的大量有机硅，使成岩早期就形成了具有较强抗压实能力的岩石骨架，为早期生成原油的滞留提供了良好的空间，也为后期油向气转化时形成并保持大量的有机质孔奠定了物质基础。(2)有机质孔和特殊裂缝是页岩气富集的重要保障。有机碳含量高的优质页岩，有机质孔也发育较多，页岩的含气量大，游离气所占的比例更高。特殊裂缝主要指页岩层理缝和小尺度裂缝。在层理面堆积的笔石和藻类等成烃生物促进了这类层理缝的发育。小尺度裂缝提高了页岩自身的储集空间和渗流能力，有利于页岩气的聚集和成藏。(3)适度抬升状态下的有效保存是富集的关键。构造变形和抬升剥蚀是一把双刃剑。如果变形与抬升剥蚀作用太强，地层的封闭保存系统会破坏，导致不论是常规还是非常规的油气系统完全或者部分失效。因抬升产生微裂缝但没有出现大的穿层裂缝或断裂，即表现为一种“裂而不破”的状态是最理想的。远离断裂尤其是通天断裂、具有一定埋深且地层平缓的地区页岩气保存条件良好。四川盆地内与周缘地区保存条件的差异，主要是由于燕山中期运动所控制的构造变形、沉降沉积与抬升剥蚀作用的差异性所决定的。四川盆地及周缘地区奥陶系五峰组志留系龙马溪组页岩气有利富集区是位于富有机质黑色页岩厚度较大的深水陆棚沉积中心区、有机孔和微裂缝或页理发育的层段以及燕山期抬升时间晚且喜山期抬升幅度小区带所叠合的区域。

The main factors of shale gas enrichment of Ordovician Wufeng Formation-Silurian Longmaxi Formation in the Sichuan Basin and its adjacent areas

The main controlling factors of shale gas enrichment in the Ordovician Wufeng FormationSilurian Longmaxi Formation in the Sichuan Basin and its adjacent areas include: (1) High quality shale formed in favorable palaeoenvironment is the base for shale gas enrichment. Sichuan Basin and its adjacent areas experienced intracontinental depression in a weak compressional setting during the deposition of Wufeng and Longmaxi Fms. A spacious deep water shelf was formed during this period due to rapid global sea level rising after the glacial epoch, providing a favorable setting for massive breeding of simple organisms. In the mean time, the obliterative bay provided a favorable geochemical environment for the preservation of organic matter, resulting in the high organic content in the strata. A large number of organic silicon, generated by special types of biology, formed rock framework with high compress strength in the early stage of diagenesis, which provided favorable space for crude oil generated in the early phase, as well as a material base for the formation and preservation of abundant organic pores in the stage of oil to gas. (2) Organic pores and special fractures can guarantee the enrichment of shale gas. High quality shale with high organic carbon content contains more organic pores and larger gas content, in which the proportion of free gas is higher. Special fractures include bedding parallel fractures and small scale fractures. Hydrocarbon forming organisms accumulating on bedding surface, such as graptolites and algae, promoted the development of bedding parallel fractures. Small scale fractures enlarged reservoir space and improved filtration capacity of shale, which was in favor of shale gas accumulation. (3) Effective preservation under moderate uplifting is the key factor for shale gas enrichment. However, if deformation and denudation of strata were too strong, the closed preservation system would be destroyed, both conventional and unconventional petroleum system might become invalid totally or partially. It would be the best if the microfractures caused by uplifting did not form large crossing bed fractures. The subdued strata with certain burial depth and without tectonic faults, especially opening faults, have favorable conditions for preservation of shale gas. The difference in preservation conditions between Sichuan Basin and its adjacent areas was mainly due to different tectonic deformation, sedimentation, uplifting and denudation, controlled by the middle Yanshanian tectonic movement. Shale gas play fairways of the Ordovician Wufeng Fm Silurian Longmaxi Fm in Sichuan Basin and its adjacent areas are located in the composite region where thick black shale enriched in organic matter were deposited in the central area of deep water shelf, the shale layers with prosperous organic pores, microfractures and bedding parallel fractures were well developed and the timing of uplifting in the Yanshanian was late and the amplitude of uplifting in the Himalayanian was relatively small.