桂中坳陷天然气组分特征及其非烃组分成因

Characteristics of Natural Gas Components and Origin of Non-hydrocarbon Gas in Guizhong Depression

桂中坳陷浅层泥盆系-石炭系、二叠系-三叠系天然气显示丰富,其组分主要包括CH4、CO2和N2三种.自北向南,天然气的组分呈现明显分异,石炭系广泛出露区多见富N2气聚集,体积分数达80 %90 %,忻城-合山-带二叠系-三叠系残存区,烃类相对富集;中部桂中1井CH4与CO2共存,体积分数相当;向南至D907 井见到CH4体积分数达80%.区内CO2为深部热作用下碳酸盐岩分解成因;N2来源于大气,与地面淡水下渗、岩溶作用发育有关.认为坳陷内天然气的较好显示及其组分在空间分布上的差异系由该区保存条件存在差异性所致.

Natural gas is rich in shallow-buried Devonian-Carboniferous and Permian-Triassic reservoirs in GuizhongDepression. Natural gas generally consists of hydrocarbon gas, CH4, and non-hydrocarbon gas, CO2 and N2. Gaspresents obvious differentiation of components from north to south and as a result N2 is getting rich and reaches up to80% - 90% of volume fraction in the area of wide-exposed Carboniferous reservoirs. Hydrocarbon gas is relatively richin the area of residual Permian-Triassic reservoirs. For instance, CH4 is coexistent with CO2 with equivalent volumefractions in Well GZ1 in the central of depression but southwards the CH4 may accumulate to 80% of volume fractionin Well D907. It is shown that the CO2 is derived from carbonate rock by decomposition of deep hydrotherm and the N2commonly comes from atmosphere, which is relative with karstification by infiltration of surface fresh water. It issuggested that the differentiation of gas components in spacial distribution are the result from different preservationconditions in this depression. Different gas components are the evidence of gas accumulation and preservation.