Page 265 - Petroleum Geology
P. 265

fore a source at higher energy than the accumulation  is required in the area
            of the permeable reservoir.
              The  importance  of  resolving this conflict of  evidence is obvious, because
            either  the  geochemical  approach is  right  or the physical approach is right:
            they cannot both  be right. By far the more serious of the two possibilities  of
            error is the first, because exploration takes geochemistry more seriously into
            account  than  fluid  mechanics.  If  the  geochemical  concept  of  maturity of
            source rock is in error, as seems possible from the Ekofisk data, areas may be
            dismissed  as  unprospective  when  they  should  not be.  Some questions that
            need answers for Ekofisk are:
              (1) If the analytical techniques that discriminated against a Paleocene source
            for the accumulated  oil are valid, why do they not also discriminate against
            migration  “through the section”?  Is this migration  through the section seen
            as leakage from the reservoir, or by some other path?
              (2) Where is the oil in the Paleocene mudstone going?
              (3) If  faults are not conduits for oil migration, do physical arguments similar
            to those above preclude a Jurassic source? Does the postulated Jurassic source
            satisfy  the  requirement  of  possessing greater energy than the oil at the oil/
            water  contact, with a path of  continuously decreasing energy between them?
              (4) Are there two sources for this oil, above and below?
              The  most  important  contemporary  problem  of  petroleum  geology is, in
            my  view,  the  reconciliation  of  geochemistry  and  fluid  mechanics because,
            once  petroleum  has  been  generated,  its  movement  is governed by physical
            laws  within  geological constraints.  The  most  fundamental  of  these  is that
            fluids lose energy while in motion. If  the source postulated for an accumula-
            tion has less energy than the accumulation, then it must be established that
            it could  have had  higher energy at the time of  migration, or that there was
            a source of energy. No real progress can be made in understanding petroleum
            generation and migration until this central problem is understood.
              Ekofisk  is  not an isolated  example.  About 400 km north  of  Ekofisk lies
            the  Frigg  gas field, on the Norwegian and United Kingdom boundary. This
            field, one of  the largest offshore gas fields in the world, was found in what
            has been interpreted as a submarine fan (Hkritier et al., 1979,1980) of Paleo-
            cene age, sealed by  middle Eocene mudstones. Recoverable reserves are esti-
            mated at 7 Tcf  (200 X  lo9 m3), and the gas is 95.5% methane. There is a 10
            m naphthenic oil zone that cannot be economically produced on account of
            its density (23--24”API,  s.g. 0.91-0.92).  The pressures are normal hydrostatic.
              The crude oil of  the main Frigg accumulation is described as “anomalous,
            suggesting biodegradation caused by bacteria”  (Hkritier et al., 1979, p. 2018;
            1980, p.  78), compounds with  carbon numbers less than C,7 being a minor
            fraction of  the crude oil, and n-alkanes are almost absent. This quality is not
            shared with other accumulations in the area: the East Frigg pool, for example,
            also has an oil leg, but it contains a significant proportion of n-alkanes. If we
            are correct in our conclusion  that such alterations took place during second-
   260   261   262   263   264   265   266   267   268   269   270