Depositional Environments and Sedimentary Tectonics of Subsurface Cotton Valley Group (Upper Jurassic), West-Central Mississippi: ABSTRACT

Abstract

ABSTRACT Study of data from 65 selected wells in a 6-county area (appx. 60 60 miles) north and west of Jackson, Mississippi, discloses that Cotton Valley strata, now within the axial trough of the Mississippi Embayment, display thickness variations which demonstrate that Upper Jurassic sedimentation was strongly controlled by maximum subsidence along the same trough axis. Examination of well logs and other records and cuttings sets from 38 wells has resulted in the preparation of dip and strike cross-sections that permit an informal definition of lower, middle, and upper portions of the Cotton Valley Group throughout the area evaluated. Within these lithostratigraphic divisions, lithofacies are discriminable which represent alluvial, upper delta plain, lower delta plain, and prodeltaic environments. These facies display a general variation from coarse, often red, oxidized sediments on the north and east to mudrocks, sometimes calcareous and carbonaceous, on the southwest. Within the Cotton Valley Group here examined, two persistent clastic lobes demonstrate relative environmental stability while deposits ranging from 1500 ft (northwestern corner of study area) to 4500 ft (axial depocenter on the south) in thickness. During Cotton Valley time, west-central Mississippi, therefore, was the site of a two-toed birdfoot delta within whose deposits, locally, lignites formed. Major sediment supply was from the east and north and a minor supply source from the northwest (Ouachita-Ozarks). Irregularities in supply rates and shelf subsidence rates permitted occasional shallow, clearwater, marine incursions from the south to intercalate thin carbonate beds. Potential source and host rocks for hydrocarbon traps are thus intimately associated. Thick, organic-rich, interlobate mudrocks pass laterally and vertically into fluvial sands of the delta lobes.