Based on principles of natural gas diffusion

mass balance and hydrocarbon concentration sealing

a model was established to assess natural gas diffusion loss from multiple source rocks.The model was applied to the shallow multiple source rocks in the north of the Songliao Basin.There are four shallow gas source rock intervals with a total accumulative gas volume generated of 387.5TCM

in which the 2nd member of the Cretaceous Nenjiang Formation (K

1

n

2

) is the largest in gas volume generated.Peak gas generation occurred at the end of the Nenjiang age.Calculation of the actual gas diffusion loss shows that K

1

n

2

is the highest in diffusion loss

followed by the 2nd and 3rd member of the Cretaceous Qingshankou Formation (K

1

qn

2+3

)

K

1

n

1

and K

1

qn

1

.The actual cumulative gas diffusion loss with and without hydrocarbon concentration sealing effect considered are 199.42TCM and 105.44TCM

respectively.The ratio of diffusion loss to generation with hydrocarbon concentration sealing effect considered is lower than that without hydrocarbon concentration sealing effect considered.The latter is around 2-3 times as the former.