A field discovered in $1953$ and developed by $1956$ had an initial producing gas$-$oil ratio of $2,300$ scf$/$STB$.$ The stock$-$tank liquid had a gravity of $56\frac{A}{P}$I and the producing gas$-$oil ratio increased to a maximum of $31,000sc\frac{f}{?}$ STB$.$ What type of fluid is in this reservoir? Why?