Abstract
The Munda? sub-basin composes the western segment of the Cear?a Basin,
which is one of a series of basins of the Brazilian Equatorial Margin
that resulted from the breakup of Western Gondwana. This study
investigates the origin and maturation of the organic matter preserved
in the rocks of the middle and outer shelf of the Munda? sub-basin, in
order to assess the source-rock potential for oil and gas generation.
The samples selected for this study are representative of the Munda?,
Paracuru, and Ubarana formations, which correlate with the rift,
transition, and drift phases of the Western Gondwana breakup. The
results indicate moderate to good potential for oil generation and good
potential for gas generation for the Munda? Formation. Fault activity
during the rift phase favored sediment supply. Kerogen type III
predominates in shallower portions and probably reflects the
contribution from the continent. Good to excellent potential for oil and
gas generation is indicated for the Paracuru Formation. Kerogen types I
and II predominate, having the marine organic matter been deposited in
reducing conditions. The most promising interval for hydrocarbon
generation is limited by the top of the Trair? Member and the top of the
Paracuru Formation. Moderate to good potential for oil and gas
generation is suggested for the Uruburetama Member of the Ubarana
Formation. The source rocks located in the middle shelf were deposited
under reducing conditions in a transgressive marine environment. Kerogen
varies from type II, to a mixture of types II and III, and to
subordinate type III. Poor to fair potential for oil generation is
indicated for the Itapaje? Member of the Ubarana Formation. Kerogen type
IV (inert or degraded) is typical in the middle shelf, and kerogen type
IV and subordinate type III are typical on the outer shelf. The presence
of inert kerogen is related to high-energy, oxidizing depositional
environments. The hydrocarbon expulsion threshold estimated for the
outer shelf is at 2106-m, for the middle shelf at 1635-m, and for the
deep waters at 3632-m depths. Source rocks of the middle and outer
shelves occur between 561 m and 4080 m, whereas those in deep waters
occur between 2754 m and 5007 m of depth. The Active Source Rock Depth
Limits (ASDL) estimated for the outer shelf is below 4500 m, whereas for
deep waters, this limit is close to 6000 m. These organic geochemical
constraints are important guidelines for oil and gas exploration in the
Cear?a and adjacent basins.
Users
Please
log in to take part in the discussion (add own reviews or comments).