川西坳陷中段上三叠统须家河组二段原油裂解成因天然气发现及成藏模式初探

川西坳陷中段具备多套烃源层、多个烃源区、多期油气生成、多期油气成藏同时又经历了多个构造期调整和破坏，纵向上发育的多套生储盖组合孕育了多层含气层系。天然气碳同位素分析表明，该区上三叠统须家河组产出的部分天然气乙烷碳同位素具有明显偏负的特征，相关图版的判别显示，这部分天然气具有原油裂解气的性质。同时在研究区上三叠统须家河组储层中发现了大量的储层沥青充填物，通过岩芯观察、显微照片、元素、热解等分析，发现该储层沥青充填物具有多种类型、多种分布形态、边界较为清楚等特征，且这些沥青充填物的反射率较高，尤其是须二段的储层沥青，这一特征与焦沥青的基本特征类似，表现为油藏裂解后残余物的特征，原油裂解气和焦沥青的发现证实了川西坳陷中段须家河组古油藏的存在。在此基础上，通过综合分析生油及裂解条件等，对研究区须家河组古油藏裂解气成藏模式进行了初步探讨，将这一过程分为印支晚期古油藏形成、燕山中期古油藏裂解、燕山晚期-喜山期气藏调整三个连续的阶段，这对深化认识川西坳陷中段天然气的成因和分布具有一定的意义。

Discovery and Forming Pattern of the Upper Triassic Xujiahe Formation Oil-cracking Gas Reservoirs in Western Sichuan Depression,China

There are a number of sets of source rocks,a number of hydrocarbon source area,multi-stage hydrocarbon generation and multi-stage hydrocarbon accumulation in the middle part of the West Sichuan Depression and it also experienced a number of tectonic stages adjustment and havoc.On it’s vertical,multiple sets of source-reservoir-cap assemblages gave birth to multi-layer gas-bearing series.According to the results of gas carbon isotopic analyses,part of upper Triassicseries Xujiahe Formation’s gas ethane carbon isotopic have obvious negative skewness characteristics in this area.Relative photos’ discrimination show that the gas has the feature of oil cracking gas.Meanwhile,a large amount of reservoir asphalt fillings have been discovered in upper Triassicseries Xujiahe formation in this area.Through the analysis of core observation,micrograph,elements,pyrolysis and so on,we can see reservoir asphalt fillings have many characteristics such as multitude types,several shapes of distribution,a more clear boundary and so on.Reservoir asphalt fillings also have a characteristic that these reservoir asphalt fillings have a higher reflecting especially in the second member of Xujiahe Formation’s reservoir bitumen.This characteristic shows residue’s characteristic which is similar to the coal tar pitch’s basic characteristic after oil reservoir cracking.The discovery of oil cracking gas and the coal tar pitch confirms the existence of paleo-oil pools of Xujiahe Formation in middle part of the West Sichuan Depression.On this basis,through the generalized analysis of genetic oil and cracking condition and so on,we carried out the preliminary discussion on the forming pattern of paleo-oil pools cracking gas of the Xujiahe Formation.We can divide this process into three consecutive phases: Late Indosinian paleo-oil pools’s formation,Middle Yanshan paleo-oil pools’s cracking and Late Yanshan-Himalayan period’s gas reservoir adjustment.It is of certain significance to deepening recognition on the formation mechanism and distribution of gas of the middle part of the West Sichuan Depression.