Methane-dominated TSR has been proposed to occur in Lower Triasaic Feixianguan Formation oolitic dolomite reservoirs on the northeast side to the Kaijiang-Liangping basin (Cai et at. , 2004), but not in oolitic limestone on the southwest side where there is no significant H2S and low porosity. TSR-calehe is shown to have g13C values mainly from - 10%o to - I9%o and coexists with solid bitumen, elemental sulfur, pyrite and quartz, and has replaced dolomite, anhydrite and celestite. However, present CO, gas has513C values mainly from -2%o to +2%o, suggestive of dissolution of carbonates (likely dolomite) after TSR-calcite precipitatioa Natural gases from Changxing Fm. , Feixianguan Fm. and Jialingjiang Fm. from the East to Puguang gasfield show a logarithmic relationship between methane SDC, and residual hydrocarbons (HCs). Based on thiB relationship, it can be calculated that about 15% methane of well Du4 gas from the Feixianguan Fm. may have been oxidized as the result of TSR.%川东北开江-梁平陆棚东北侧飞仙关组多孔鲕粒白云岩中发生了以甲烷为主的热化学硫酸盐还原作用(TSR),产生高达20％的H2S；而西南侧鲕粒灰岩以低孔、低H2S天然气为特征.东北侧白云岩主要发育白云石粒间溶孔或粒间扩大溶孔,这些溶孔可与方解石(δ13C=- 10‰～- 19‰)、储层沥青、元素硫、黄铁矿和石英紧密共生,可分布于片状储层沥青与白云石晶体之间,说明白云石溶解作用发生在沥青形成以后.白云石的溶解作用导致现今天然气以无机CO2为主,δ13CCO2主要介于-2‰-+2‰之间.这种溶解作用是在酸性条件下,硬石膏或天青石参与下发生的,可能先产生MgSO4配对离子,而后MgSO4又与甲烷反应产生H2S,净增大了孔隙.研究还发现,普光气田及以东天然气的来源不同于河坝和元坝天然气；对普光气田及以东天然气分析显示,甲烷δ13C值与残余烃含量[∑HCs/(H2S+ ∑HCs)]之间存在对数相关关系.这表明TSR过程中,甲烷同位素分馏作用遵从封闭体系下瑞利分馏原理.据此计算显示,渡4井约有15％甲烷被氧化了.
展开▼
