AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Three Cenozoic Megasequences in the Northwestern Gulf of Mexico: Depocenters, Sandstone Composition, and Hinterland Tectonic Phases
(1) Shell Exploration & Production Co., Houston, TX.
(2) Shell International EP, Houston, TX.
Cenozoic siliciclastic deposits of the NW Gulf of Mexico comprise the objective section for the vast majority of U.S. offshore wells. These deposits can be subdivided into three megasequences (MS), each characterized by (1) a downlapping or wedging seismic sequence on the abyssal plain; (2) related paleogeography of deltas and basin-floor fans; (3) a distinctive distribution of sandstone compositions (especially abundance and composition of lithic grains); and (4) a corresponding phase of hinterland tectonism.
MS1, Late Laramide Phase (Paleocene-early Eocene: Wilcox Group), is dominated from NE Mexico to central Louisiana by sandstones with abundant variably ductile metamorphic lithic grains. Wilcox paleogeography was dominated by three major deltas and four basin-floor fan depocenters of different ages, all sourced primarily from the active Laramide orogenic belt following emergence of the Cretaceous Interior Seaway. On the abyssal plain, MS1 overlies Cretaceous carbonates and wedges toward the SE.
MS2, Southwestern Volcanic Phase (middle Eocene to Oligocene: Claiborne, Jackson, Vicksburg, Frio, Anahuac), has two distinct sandstone provinces: (1) S Texas and NE Mexico, rich in volcanic and carbonate lithics; and (2) E Texas and SW Louisiana, with lower, carbonate-free lithic content. In contrast to MS1, most deltas (except for the Oligocene Norias delta in S Texas) were relatively small and short-lived, and lacked corresponding basin-floor fans. The abyssal plain was dominated by the immense “Golden Lane” fan, apparently sourced from the Tampico-Tuxpan region of Mexico; it downlaps MS1 toward the N, NE, and E.
MS3, Plateau Uplift Phase (Neogene) is marked by the inception in Louisiana of the ancestral Mississippi delta which, together with the corresponding south-wedging MCAVLU basin-floor fan, was the dominant depocenter in the NW GoM. Overall, sandstones are more mature and polycyclic than in MS1 and MS2, constituting two distinct provinces: (1) Louisiana, which is quartz-rich in the Miocene but decreases in maturity up-section, and (2) Texas, with mixed volcanic and sedimentary lithics including an unusually high content of detrital carbonates.
In this paper we describe the known distribution of deep-water sands within each megasequence and province, speculate about sand distribution in poorly calibrated regions, and explore how differences in clastic composition impact reservoir quality changes during burial.