Article,

Geochemical constraints on the origin and distribution of Cretaceous source rocks in the Cear?a basin, Brazilian Equatorial margin

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JOURNAL OF SOUTH AMERICAN EARTH SCIENCES, (2021)
DOI: 10.1016/j.jsames.2020.103092

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.

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