碳酸盐岩有效排烃动力条件与临界有机质丰度关系

在碳酸盐岩的可能排烃动力学模型中，微裂缝作为压力释放的有效通道至关重要。碳酸盐岩烃源岩生烃过程中微裂缝的形成难以直接观察，但可以通过模拟实验，根据产物计量和一定的数学模型，计算生烃增压值，并通过岩石物理性质得到的理论破裂压力阀值判断其形成的温压条件。结果表明，碳酸盐岩有效排烃动力与有机质丰度有关。当有机质类型和成熟度一定时．有机质丰度越高，所产生的生烃增压就越大。有机质丰度很低的碳酸盐岩在一般地温条件下，难以产生足够的生烃增压使岩石破裂并形成徽裂缝作为效排烃通道。在常规的油-2生成的温度压力条件下，有机碳含量0．2％～0．3％可能是满足有效排烃动力条件的l临界有机质丰度。图5表1参10

Relationships between dynamic conditions of effective hydrocarbon expulsion and critical abundances of organic matter in carbonate source rocks

In the possible dynamic models for hydrocarbon expulsion in carbonate rocks, the formation of micro fractures is very important as the increased pressure can be released through these fractures. It is difficult to observe the formation of micro fractures directly in the hydrocarbon generation process in carbonate source rocks, but the amount of the pressure increase resulting from hydrocarbon generation could be estimated by measurements of the generated products in experimental simulations and certain mathematical models. The temperature and pressure conditions for the formation of the effective hydrocarbon expulsion pathways in carbonate source rocks could be determined based on the obtained values of the theoretical pressures for rock cracking, suggesting that the driving force for effective hydrocarbon expulsion is relative to organic matter abundance in carbonate rocks. At normal subsurface temperatures, hydrocarbon generation by carbonate rocks with very low abundances of organic matter can hardly create enough additional pressures to result in the formation of cracking fractures, which constitute effective hydrocarbon expulsion pathways. The total organic carbon content of 0. 2%-0. 3% is determined to be the critical organic matter abundance for the dynamic conditions for effective hydrocarbon expulsion.