Transgressive-Regressive Cycles: Application to Petroleum Exploration for Hydrocarbons Associated with Cretaceous Shelf Carbonates and Coastal and Fluvial-Deltaic Siliciclastics, Northeastern Gulf of Mexico

Ernest A. Mancini and T. Markham Puckett: Center for Sedimentary Basin Studies and Department of Geological Sciences, University of Alabama, Tuscaloosa, Alabama

Abstract

The establishment of a stratigraphic framework for a region or basin is critical in developing a successful exploration strategy for that area. The sequence stratigraphic relationships of the Upper Cretaceous and Paleogene deposits in the northeastern Gulf of Mexico are useful for establishing such a stratigraphic framework for this geologic time period for this area. In this case, unconformity-bounded units can be recognized in surface exposures based on physical surfaces and stratal geometries, and associated systems tracts can be traced regionally in outcrop. However, with the Lower Cretaceous deposits in the eastern Gulf Coastal Plain, there is difficulty in distinguishing unconformity-bounded units because these strata only occur in the subsurface in this area and because core sections recovered from wells drilled rarely record the unconformities bounding the units. Also, for much of this area, sediments have accumulated in the shelfal area behind a rimmed shelf margin; therefore, the seismic profiles for these strata generally only consist of a reflection configuration of prograding clinoforms that are characterized by offlap reflection termination. Therefore, a stratigraphic analysis based on the cyclicity recorded in these strata (transgressive-regressive cycles) has greater utility in establishing a stratigraphic framework for the Lower Cretaceous strata in the northeastern Gulf of Mexico rather than a sequence stratigraphic approach.

Transgressive-regressive facies cycles and depositional cycles are of particular interest because the documentation of these cycles in the rock record provides an approach for regional correlation. The transgressive-regressive facies cycles used in this paper follow the concept of these cycles as discussed by Jacquin and de Graciansky (1998). Recognition of these transgressive-regressive facies cycles is based on the identified facies and stratal stacking patterns rather than the documentation of unconformities. That is, the cycles are described by using seismic profiles to identify large-scale stratal patterns and terminations and core and well log data to determine facies stacking patterns and the partitioning of sediments following changes in shelfal accommodation. Thus, the transgressive phase of a cycle defines stratal packages characterized by onlap geometry associated with retrogradational seismic facies exhibiting a concordant and parallel reflection configuration and onlap reflection termination. The regressive phase of a cycle defines stratal packages characterized by offlap geometry associated with progradational seismic facies exhibiting a discordant and sigmoid-prograding clinoform reflection configuration and downlap reflection termination. Individual cycles are separated by discontinuities in the sedimentary rock record as recognized by abrupt changes or interruptions as observed in seismic profiles, in well log signature patterns, and/or in core sections obtained from the drilling of wells.

Three transgressive-regressive (T-R) facies cycles of 7 to 9 million years in duration have been recognized in Lower Cretaceous strata in the northeastern Gulf of Mexico area. These are the LKEGR-TR 1 (Lower Cretaceous, Eastern Gulf, Transgressive-Regressive) (121 to 114 Ma), the LKEGR-TR 2 (114 to 105 Ma), and the LKEGR-TR 3, which include the Andrew Formation and Dantzler Formation (105 to 98 Ma) facies cycles. Two depositional cycles are recognized in the LKEGR-TR 1 facies cycle: the LKEGR-TR 1-1, which includes the Sligo Formation (121 to 118 Ma) and the LKEGR-TR 1-2, which includes the Pine Island Shale, "Donovan" sandstone and James Limestone (118 to 114 Ma) depositional cycles. Two depositional cycles are recognized in the LKEGR-TR 2 facies cycle: the LKEGR-TR 2-1, which includes the Bexar Formation and Rodessa Formation (114 to 110 Ma) and the LKEGR-TR 2-2, which includes the Ferry Lake Anhydrite, Mooringsport Formation and Paluxy Formation (110 to 105 Ma) depositional cycles. A Lower Cretaceous (Hosston Formation) transgressive-regressive depositional cycle underlies the LKEGR-TR 1 facies cycle, and this cycle probably represents an aggradational section which is related to the T-R 1 facies cycle. It has time duration of 132 to 121 Ma. A Cretaceous (Washita Group) transgressive-regressive depositional cycle overlies the LKEGR-TR 3 facies cycle in the offshore area of the northeastern Gulf of Mexico and in the onshore area of the western Gulf Coastal Plain (Texas). This cycle is probably part of the T-R 3 facies cycle.

To date, oil and natural gas production in the northeastern Gulf of Mexico is chiefly from the fluvial-deltaic siliciclastic progradational lithofacies ("Donovan," Paluxy, and Dantzler sandstone reservoirs) of the regressive phase of the T-R facies cycles. Significant hydrocarbon production also occurs from shoal and reef lithofacies (Sligo, James, and Mooringsport carbonate reservoirs) of the regressive phase of the T-R depositional cycles. Hosston fluvial-deltaic sandstone hydrocarbon reservoirs in this area are probably associated with the aggradational phase of a T-R facies cycle, and the Washita shoal and reef reservoirs are probably associated with the regressive phase of a T-R facies cycle. Hydrocarbons are also produced from Andrew and Sligo carbonate reservoirs of the transgressive phase of T-R facies cycles. The marine shelf calcareous shales and argillaceous carbonate mudstones (Pine Island Shale, Bexar Formation, and Mooringsport Formation) of the transgressive phase of the T-R facies cycles have petroleum source rock potential. Globally, these Aptian and Albian deposits have been demonstrated as source rocks. Thus, the fluvial-deltaic sandstone and carbonate shoal and reef lithofacies associated with the regressive phase of Lower Cretaceous T-R facies and depositional cycles in the northeastern Gulf of Mexico are the major producing reservoirs in the northeastern Gulf of Mexico and therefore should be continued to be targeted as oil and gas exploration objectives in the shelfal areas of this region.