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Abstract

The central Gulf of Mexico Basin formed by Callovian rifting followed by oceanic spreading during anticlockwise rotation of Yucatan Block between Oxfordian (160Ma) and Valanginian (140Ma). The rotation pole was in the Straits of Florida, resulting in a wiper-blade motion having greatest sweep along a transform paralleling the coast offshore Tampico-Misantla Basin. Oceanic crust progressively separated the 160-140Ma stratigraphy, creating bands of ocean crust lacking Oxfordian, Kimmeridgian, Tithonian and, near the ridge, even Berriasian deposition: present day, these stages appear progressively absent towards the spreading center.

After a basin-wide 140Ma (Valanginian) unconformity on the rimming shelves at the end of spreading, thermal subsidence resumed and water deepened further, in the basin. However, an arch separating the Gulf of Mexico and paleo-Caribbean basins across the Florida Straits remained a bathymetric sill to deep ocean circulation post-140Ma as before.

The impact of this tectonic/bathymetric evolution on the presence and ultimate expellable potential (UEP) of source rocks in the basin center is profound. Using a scheme of basin-wide correlated organic matter depositional acmes (recently developed by Petroleum Systems LLC, where the acme age is expressed in Ma), candidate source rocks in the Gulf of Mexico are: Acme A157 (late Oxfordian); A154 (Kimmeridgian); A148 (Tithonian); A144 (Berriasian/Portlandian). All four acmes have zones of nondeposition due to oceanic spreading, and older acmes are absent over a greater area. Some workers have proposed the main deep-water basin source rock in the United States is Tithonian; however, new work (so far limited to eastern deep water Mississippi Canyon, Atwater Valley, and Desoto Canyon) indicates that A157 (i.e., the oldest, most susceptible to nondeposition) is the most effective source rock.

Risk on Acme 157-144 non-deposition can be mitigated if source rocks younger than 140Ma, having significant ultimate expellable potential, were deposited in the deep basin. Based on recent reappraisal of data in DSDP Leg 77, Cuba and rafted sediment blocks in U.S. waters, significant ultimate expellable potential does exist in five younger Early to mid-Cretaceous acmes: A138 (Valanginian); A120 (early Aptian); A110 (early Albian); A101 (late Albian); and A98 (early Cenomanian). A138 may result from restricted circulation, reinforced during the 140Ma uplift of rimming shelves, while A120, A110, and A101 can be correlated with oceanic anoxic events OAE1a, b, and d.

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