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低渗透烃源岩中的裂缝作用是由有机质向较低密度的流体(油和气)转化过程中的孔隙压力变化造成的。这些裂缝提高了渗透率并为烃类的运移提供了通道。根据四个主要假设,推导出一个压力变化方程,这四个假设是:(1)烃源岩的渗透率非常小(0.01μD;10~(-20)m~2),因此由有机质转化产生的孔隙压力积聚要比由孔隙流体流动引起的孔隙压力损失快得多;(2)应力状态为各向同性,所以水平应力和垂直应力相等。当孔隙压力等于上覆压力时,烃源岩被破坏;(3)在石油生成过程中,岩层、有机质及流体的性质保持不变。当随深度的变化(即压力和温度)不大时这个假设是成立的;(4)有机质的转化仅需要两个反应速率,一个是干酪根向石油转化的低温反应速率(E≈24kCal/mol,A≈10~(14)/Ma);另一个是油向气转化的高温度应速率(E≈52kCal/mol,A≈5.5×10~(26)/Ma)。将生烃率方程和压力变化方程应用于奥斯汀烃源岩,通过调整几个变量,使之与该区的地化数据、岩心饱和度及实测的产油气能力相一致。本次应用说明,这些方程易于应用于低渗透烃源岩中的初次运移深度的计算。
Fractures in low-permeability source rocks are caused by changes in pore pressure during the conversion of organic matter to lower-density fluids (oil and gas). These fractures increase permeability and provide access to hydrocarbon migration. Based on four main assumptions, a pressure change equation is derived. These four hypotheses are: (1) the permeability of source rocks is very small (0.01μD; 10 ~ (-20) m ~ 2) Of the pore pressure accumulation is much faster than the pore pressure loss caused by the pore fluid flow; (2) The stress state is isotropic, so the horizontal stress and the vertical stress are equal. When the pore pressure is equal to the overburden pressure, the source rocks are destroyed. (3) The properties of rock, organic matter and fluid remain unchanged during oil formation. This assumption holds true when the variation with depth (ie, pressure and temperature) is not significant; (4) the conversion of organic matter requires only two reaction rates, one at low reaction rate from kerogen to petroleum (E≈24 kCal / mol , A≈10 ~ (14) / Ma); the other is the high temperature rate of oil to gas conversion (E≈52kCal / mol, A≈5 × 10 ~ (26) / Ma). The hydrocarbon generation equation and pressure change equation were applied to the Austin source rock by adjusting several variables to be consistent with the geochemical data, core saturation, and measured oil and gas production in the area. This application note shows that these equations are easily applied to the calculation of initial migration depth in low-permeability source rocks.