negative OIIP result from MBE

[sharpjiy]

dear Guys

I've question about estimating OIIP with MBE.
I already knew there is a gascap with no aquifer,
but I'v no idea OIIP, GIIP(so I need to assume m value)
So, I used gas cap drive solution but two different results make me confused.

first, when I assume m values (adjust m value ~ 0.3), OIIP was 7MMstb
(used havlena & odeh m ratio specified)
but, when I used N intercept, G slope method,
OIIP got negative value -3MMstb with OGIP 3MMscf
Can I ask the reason of two different results?
Or, any Idea to deal this problems ?
what input data should be re-checked??