川东北地区热化学硫酸盐还原反应机理及成藏效应研究

MECHANISM AND ACCUMULATION EFFECT OF THERMOCHEMICAL SULFATE REDUCTION IN NORTHEAST AREA OF SICHUAN BASIN

在碳酸盐岩层系热化学硫酸盐还原反应(TSR)是普遍发生的地质现象,一定厚度的硬石膏、充足的烃类和较高的古地温是TSR反应发生的必备条件,但TSR反应是十分复杂的化学反应,应用简单的化学平衡方程并不能完全表达该反应的过程。通过烃类与硬石膏反应的活化能分析可知,TSR反应是一个优先消耗重烃的过程。通过对川东北地区飞仙关组钻井岩心和薄片观察、地球化学分析表明,TSR反应是形成高含H2S气藏的重要原因。指出TSR反应的成藏效应在于:①TSR反应本身是个储层矿物体积减小的过程,有利于深部储层产生次生孔隙;②TSR反应生成的流体对储层具有一定的溶蚀作用,可促进次生孔隙的生成;③TSR反应优先消耗重烃,使天然气藏干燥系数增加,烃类碳同位素变重,同时引起了硫同位素分馏。认为研究TSR反应机理及成藏效应,对川东北地区碳酸盐岩层系飞仙关组高含H2S气藏的勘探具有重要的意义。

Thermochemical sulfate reduction （TSR） was normal geological phenomena in sedimentary basins. The anhydrite, hydrocarbon and high strata temperature were three absolutely necessary conditions. It is hard to express total reaction process through some simple chemical equilibrium equations since the TSR were of very complicated chemical reactions. Based on the analysis on activation energy between anhydrite and hydrocarbon, the heavy hydrocarbons will be firstly consumed in the TSR process. After observation of abundant well cores and slices and geochemical analysis in the Feixianguan Formation of northeast area of Sichuan Basin, the results show that TSR is the key factor for the high H2S concentration in this area. The accumulation effect of TSR is as follows： ①TSR could generate secondary porosity in deep reservoir because it is a shrinking process; ② The resultant of TSR is corrosive to reservoir and could accelerate secondary pore formation; ③The heavy hydrocarbon is firstly consumed during the TSR process, so the dryness coefficient of natural gas increases, the carbon isotopic compositions of hydrocarbon gas become enriched in ^13C, and the sulfur isotopic fractionation is also happened. Thus, it is important to elucidate the mechanism and accumulation effect of thermochemical sulfate reduction in the northeast area of Sichuan basin.