ABSTRACT: Sequence Stratigraphy and Seismic Stratigraphy of the Lower Cretaceous Section in the Northeastern Gulf of Mexico

Abstract

ABSTRACT The Lower Cretaceous (LK) section in the offshore Alabama and Mississippi area has been a petroleum target since the 1970's in the Main Pass and the Viosca Knoll areas. To date, no comprehensive stratigraphic analysis of this section in these locations has been published. This work focuses on the characterization of the sequence stratigraphy and seismic stratigraphy of the LK strata offshore Alabama and Mississippi. Approximately 3,500 kilometers of 2D multi-channel seismic reflection data are interpreted and integrated with gamma ray, lithologic well log descriptions, and well log lithostratigraphic picks from more than 50 wells, and with 527 meters of core from wells in the study area and surrounding areas. Seven check-shot surveys are used to integrate the seismic and well log data, and software for PC is used to integrate and interpret the various data sets. In the study area, the LK section is composed mostly of carbonates with minor siliciclastic beds that were deposited in inner-middle shelf, shelf margin, slope, and basin environments. Diagnostic well log signature patterns were associated with different paleoenvironments. Associations of seismic facies were utilized to recognize the various depositional settings. Eight major third-order depositional sequences were recognized. This section overlies a Lower Cretaceous (Valanginian) unconformity associated with a significant hiatus and is overlain by an Upper Cretaceous (mid-Cenomanian) unconformity associated with a deposition hiatus of more than 10 million years in the Main Pass 253#6 well. Highstand systems tract and lowstand systems tract deposits are generally the only strata seismically recognizable. Transgressive systems tract deposits consist of a single seismic cycle or are not resolvable. The shelf margin, characterized by discontinuous skeletal reefs in the carbonate sequences, shifted basinward during the Early Cretaceous. The shelf edge experienced greater subsidence with respect to the back-reef and middle shelf areas. Tectonic activity, probably associated with salt movement, was restricted to the shelf margin and slope settings.