A method for producing alkylated hydrocarbons is disclosed. Formation
fluid is produced from a subsurface in situ heat treatment process. The
formation fluid is separated to produce a liquid stream and a first gas
stream. The first gas stream includes olefins. The liquid stream is
fractionated to produce at least a second gas stream including
hydrocarbons having a carbon number of at least 3. The first gas stream
and the second gas stream are introduced into an alkylation unit to
produce alkylated hydrocarbons. At least a portion of the olefins in the
first gas stream enhance alkylation.