37

Fig.  30:  GCs  of  four  Venezuelan  crude  oils  showing  the  general  variation  from  n-paraffin
(upper  left)  to  naphthene  based  crudes  (lower  right).

2.5.1.2  GC-MS  analysis
Biomarkers  and  their  relations  generally  represent  source  and  maturity  of  crude  oils.  While
source  parameters  are  derived  from  the  specific  biomass  from  which  an  oil  has  been  formed,
maturity  parameters  represent  chemical  changes  occurring  during  oil  maturation.
Some  of  them  can  be  highly  characteristic,  i.e.  specific  of  a  particular  production  area,  or  even
of  an  oil  field.  But  others  may  have  only  a  very  low  meaning,  when  an  oil  spill  sample  is
compared  with  samples  from  suspected  sources.
The  relation  of  the  C31+  enantiomers  in  the  hopane  series,  for  example,  indicates  maturity  of
organic  matter  in  samples  of  potential  source  rocks.  With  increasing  maturity  this  value
reaches  about  60:40  although  it  may  slightly  vary  depending  on  the  carbon  number  of  the
homologues  being  measured.  The  value  of  the  second  pair,  expressed  as  100*32abS/32abR
(%32abS),  of  all  260  samples  of  the  collection  of  crude  oils  from  all  over  the  world  has  found
to  be  58  with  a  standard  deviation  of  only  2.2%  (Figure  31).