  For a 2-dimensional map representing an expanding geometric Lorenz at-
tractor we prove that the attractor is the closure of a union of as long as
possible unstable leaves with ending points. This allows to define the notion
of good measures, those giving full measure to the union of these open leaves.
Then, for any H\"older continuous potential we prove that there exists at most
one relative equilibrium state among the set of good measures. Condition
yielding existence are given.
