We study the generalised 2D surface quasi-geostrophic (SQG) equation, where the active scalar is given by a fractional power α of Laplacian applied to the stream function. This includes the 2D SQG and Euler equations as special cases.Using Poincaré's successive approximation to higher α-derivatives of the active scalar, we derive a variational equation for describing perturbations in the generalized SQG equation. In particular, in the limit α → 0, an asymptotic equation is derived on a stretched time variable τ = αt, which unifies equations in the family near α = 0. The successive approximation is also discussed at the other extreme of the 2D Euler limit α = 2-0. Numerical experiments are presented for both limits.We consider whether the solution becomes more singular or regular with increasing α. Two competing effects are identified: the regularizing effect of inverse "Laplacian" and cancellation by symmetry (nonlinearity depletion). Near α = 0 (complete depletion) as α increases the solution becomes more singular. Near α = 2 (maximal smoothing effect of inverse Laplacian) as α decreases the solution becomes more singular, suggesting that there may be some α in [0,1] at which the solution is most singular.We also present some numerical results of the family for α = 0.5, 1 and 1.5. On the original time t, the H 1 norm of θ generally grows more rapidly with increasing α. However, on the new time τ , this order is reversed. On the other hand, contour patterns for different α appear to be similar at fixed τ , even though the norms are markedly different in magnitude. Finally, point-vortex systems for the generalized SQG family are discussed to shed light on the above problems of time scale.