The Turonian-Cenomanian marine source rock
(Figure 17) is presently considered
the most prolific one. Gross thickness up to 550 m have been
observed with Total Organic Carbon (TOC) levels between 4 and 7%;
DSDP/ODP wells penetrated the source rock with TOC up to 14% over
the outer north-eastern part of Demerara Plateau.
Modelling of the maturity indicates that the source rock is mature
for oil over large parts of the basin off the main flexure zone
(non-active transfer zone) and west of the Demerara High. Oil
expulsion started during Eocene times.
A biomarker study on the oils from Tambaredjo and Calcutta
indicated that they are sourced from a Late Cretaceous fully
mature (Vre .85-.9%) marine source rock in line with the estimated
maturities in the active kitchen area.
Towards the kitchen area API gravity increases. Migration routes
exceed 100 km.
In addition to the Late Cretaceous Petroleum System, a Jurassic
(rift valley) and Tertiary (deep basin) Petroleum System has been
inferred in the offshore area.
In the Galibi-1 well located in the southern part of the Demerara
high indications of a poor quality Aptian source rock (TOC about
1%) have been observed.