Abstract

The Middle Jurassic-Lower Cretaceous stratigraphy of the onshore region of the Gulf of Mexico in Texas consists of five major second-order (approx. 15 myr duration) supersequences, defined as large regionally-correlative, retrogradational - aggradational/progradational accommodation packages. Each exhibits systematic vertical/lateral stacking patterns of subordinate third-order sequences (1-3 myr. duration) and component lateral/vertical facies and systems tracts. The 5 supersequences are: Supersequence 1 (SS1) - Upper Bathonian to Lower Kimmeridgian (158.5 - 144 mya), SS2 - Lower Kimmeridgian to Berriasian (144 - 128.5 mya), SS3 - Upper Valanginian to Lower Aptian (128.5 - 112 mya), SS4 - Lower Aptian to Upper Albian (112 - 98 mya), SS 5 - Upper Albian to Santonian (98 - 84 mya). Supersequences 2 and 4 are the focus of this paper.

An analysis of Gulf of Mexico (GOM) regional Mesozoic sequence stratigraphy and paleogeography has direct application to enhancing our understanding of carbonate reservoirs within the Upper Jurassic stratigraphy of the East Texas Salt Basin (ETSB) and the Lower Cretaceous stratigraphy of south Texas. In both systems, carbonate-buildup reservoir facies (pinnacle reefs, grain-support shoals, and biostromal banks) form as the terminal phase of carbonate deposition on top of regionally backstepped ramps (Upper Jurassic) and low-relief rimmed shelves (Lower Cretaceous). Both systems are similar in that reservoir facies occur within the transgressive systems tract (TST) of GOM-wide second-order supersequences. Near the top of the TST, the retrogradational carbonate systems are draped with regionally widespread marine shale facies (which represent deposits of the second-order maximum flooding). The marine shale facies provide both hydrocarbon source and reservoir seal. The top of the Cotton Valley Lime/Haynesville carbonate in SS2 and the top of the Sligo (in SS4) are both diachronous surfaces characterized by original depositional topography and are onlapped by marine shales along a submarine condensed section. Little evidence exists for a significant relative sea-level drop at this surface in either system. The reservoir-bearing, second-order TST’s carbonates are separated from second-order progradational highstand systems tract carbonates by GOM-wide second-order supersequence boundaries which mark points of accommodation space minima.

The Upper Jurassic supersequence boundary (the “144” myr boundary; base of SS2) in the ETSB and the Lower Cretaceous (the “112” myr boundary; base of SS2) in south Texas are marked by change in architecture from progradation beneath the boundary to retrogradation above the boundary. In both examples, identification of this critical boundary is based on analysis of the vertical stacking architecture of subordinate, higher-frequency sequences (e.g., third-order) in well-logs and cores. Boundary identification is augmented by seismic patterns of toplap and onlap. In south Texas, a lowstand wedge of authochthonous carbonate onlaps the terminal Sligo highstand margin (top of SS2) in a downdip position, further highlighting this supersequence boundary. A detailed, high-resolution sequence strati-graphic model is presented for the Upper Jurassic (SS2) of the ETSB based on unpublished subsurface data. The general theme of this model can be applied to the internal architecture of the Lower Cretaceous Sligo based on additional (but as yet unpublished) subsurface date.

This model can be applied to well-log correlations and seismic interpretations in analogous accommodation settings. Analagous “pinnacle reefs “ from around the world typically are linked to the terminal phase of carbonate deposition near the top of regional second-order TST’s. Pinnacle geometries are promoted by increasing accommodation space during a retrogradational stacking of carbonate facies belts. Differential compaction of shaley onlapping facies about pre-existing rigid carbonate buildups enhances their seismic recognition. Hydrocarbon-productive examples include the Devonian of Canada, the Miocene of southeast Asia and the Mississippian Lodgepole of the Williston Basin. Integration of key lessons learned from the Upper Jurassic of the ETSB and the Lower Cretaceous from south Texas should fuel the search for other, as of yet, unrecognized carbonate buildups and “pinnacle reefs” within similar accommodation windows in other areas.