Joseph C. Fiduk

The Perdido Fold Belt, Northwestern Deep Gulf of Mexico, Part 2: Seismic Stratigraphy and Petroleum Systems

Analysis of 12,000 km of two-dimensional multifold seismic data shows a thick succession of Mesozoic and Cenozoic deep-water strata in the Perdido fold belt, northwestern deep Gulf of Mexico. These strata differ in seismic facies, areal distribution, and reservoir/petroleum potential. Mesozoic strata are interpreted as dominantly fine-grained carbonates and show minor thickness changes. Cenozoic strata are largely mud-dominated siliciclastic turbidite deposits and vary considerably in thickness across the fold belt. These changes reflect the shifting position of Cenozoic marginal-marine depocenters. Mesozoic reservoir potential consists of fractured Upper Jurassic and Cretaceous deep-water carbonates. Cenozoic reservoir potential consists of siliciclastic deep-water turbidites. Portions of the Paleocene to lower Eocene strata are sand-prone and are the downdip equivalents of the lower and upper Wilcox shallow-marine depocenters. These strata are all incorporated within the folds. Lower to middle Oligocene strata coincide with the main growth phase of the fold belt. Potentially sand-prone middle Oligocene to lower Miocene strata are the downdip equivalents of the Vicksburg (early Oligocene), Frio (Oligocene), and Oakville (early Miocene) shallow-water depocenters. These strata form potential stratigraphic traps against the folds. Mesozoic source potential was modeled assuming Oxfordian, Tithonian, Barremian, and Turonian source beds. One-dimensional thermal maturation modeling showed these sources reached peak oil generation between 51 and 39 Ma, 39 and 8 Ma, 32 and 2 Ma, and 26 and 8 Ma, respectively. Cenozoic source potential was modeled using an Eocene source. Modeling showed this source reached only early oil generation in the basinward half of the fold belt. Thermal maturation was reached by source beds at different times in different locations due to changes in burial depth, amount of structural uplift, and underlying thickness of autochthonous salt. All of these factors indicate that seal and reservoir carry significant risk, but that the potential exists for large petroleum accumulations.

Comparing Two Models of Source Rock Potential in the Perdido Fold Belt Area, Northwestern Deep Gulf of Mexico

ABSTRACT Five intervals (Oxfordian, Tithonian, Barremian, Turonian, and middle Eocene) were modeled for source potential within the Perdido fold belt, Alaminos Canyon OCS protraction area. One-dimensional (1-D), thermal maturation models were run at sixteen locations along two regional profiles. Steady state and rifting heat flow model results are compared to evaluate potential source rocks. Maturation timing was primarily affected by the depth of burial, and to a lesser extent, by structural uplift and the thickness of underlying autochthonous salt. The rifting heat flow model at every location predicts source maturity earlier for all potential source intervals than the steady state model. Direct comparison of arrival at peak oil generation (%Ro=0.9) for the steady state to rifting model yields these ranges, respectively: Oxfordian--3.9 - 29.2 vs. 28.2 - 50.5 Ma, Tithonian--2.6 - 14.2 vs. 6.4 - 35.8 Ma, Barremian--3.2 - 10.0 vs. 4.2 - 30.2 Ma, and Turonian--1.4 - 4.2 vs. 2.7 - 22.4 Ma. Separating the 1-D model locations into upthrown and downthrown groups produced more narrow ranges (3 - 14 my differences) for both models. Calculating the difference in arrival at peak oil generation for each location, the rifting model produced peak oil generation earlier by 13.5 - 37.3 my in the Oxfordian, 11.7 - 22.0 my in the Tithonian, 11.5 - 24.9 my in the Barremian, and 10.3 - 18.0 my in the Turonian. The middle Eocene source did not reach peak oil generation in either model. In the steady state model, only Oxfordian and Tithonian source rocks reached peak oil generation at one-half or more locations. Source rocks reached peak oil generation after fold belt formation (36 - 30 Ma). In the rifting model, the Oxfordian, Tithonian, Barremian, and Turonian strata have all reached peak oil generation at one-half or more locations. However, the Oxfordian reached peak oil generation prior to fold belt formation (37.1 - 50.5 Ma) when structural traps were not present. With the number of assumptions required and our level of uncertainty, the two models probably serve as realistic end-members with the true maturation picture lying somewhere in-between.