  We prove that a partially hyperbolic attractor for a $C^1$ vector field with
two dimensional center supports an SRB measure. In addition, we show that if
the vector field is $C^2$, and the center bundle admits the sectional expanding
condition w.r.t. any Gibbs $u$-state, then the attractor can only support
finitely many SRB/physical measures whose basins cover Lebesgue almost all
points of the topological basin. The proof of these results has to deal with
the difficulties which do not occur in the case of diffeomorphisms.
