Terrigenous Clastic Facies Distribution and Sandstone Diagenesis, Subsurface Lewis and Evans Format Units (Chesterian), on Northern Shelf of Black Warrior Basin

Arthur W. Cleaves, David T. Bat

Deltaic clastic depositional systems of the Chester Series on the northern shelf of the Black Warrior basin produce significant natural gas and smaller quantity of crude oil from 12 major reservoir units. The lowest two of these units, the Lewis and Evans sandstones, can be mapped as distinct format units over most of the shelf. Regionally transgressive marine carbonate markers overlie each of the genetic sandstone assemblages and the open shelf Tuscumbia Limestone floors the entire sequence. Both of the named sandstone units document deltaic facies tracts, each of which has a source area in the Mid-Continent interior. Subsurface isolith maps, vertical wireline log signatures (SP or natural gamma-ray curves), and lithologic data from four cores are mainly used to confirm he deltaic interpretation.

Within the Lewis format unit, six dip-elongate river-dominated cratonic delta lobes constitute the dominant coarse-grained clastic units. Lewis deltaic facies prograded southeastwardly downdip over a flat, relatively featureless, shallow shelf surface and show no systematic thinning until the southern margin of the shelf (Oktibbeha and Lowndes Counties, Mississippi, and Pickens County, Alabama). Higher in the Evans format, five strike-elongate (cuspate) wave-dominated delta lobes are present along the northern rim of the basin. The Evans interval progressively thins from its subcrop beneath the Mesozoic-Cenozoic cover toward the southeast. This unit directly underlies a black shale facies (Neal shale) laterally equivalent to the sub "Millerella" limestone portion of the Bangor Limesto e in Alabama. The black shale is interpreted as a deeper water subbasin superimposed on the distal part of the northern shelf.

Limited petrographic data from the Lewis and Evans sandstone units associated with the dominant subsurface deltaic facies tract demonstrate a predominance of monocrystalline quartz and sedimentary rock fragments, as well as an absence of abundant orogenic lithologic indicators such as polycrystalline quartz grains, muscovite, and metamorphic rock fragments. Porosity development in the two sandstones is obtained through the formation of moldic secondary porosity and enlarged intergranular porosity. Primary porosity within these quartz-arenite and sublitharenite sandstones is occluded by the precipitation of quartz overgrowths and an early-stage calcite cement. Secondary, moldic porosity was generated through the dissolution of feldspars (largely albite), shale rock fragments, and very are phyllite rock fragments. Enlarged intergranular porosity was gained from the dissolution of detrital illite matrix. Secondary porosity itself was partially occluded by authigenic kaolinite and illite, as well as by the formation of late-stage pyrite and dolomite.

AAPG Search and Discovery Article #91036©1988 GCAGS and SEPM Gulf Coast Section Meeting; New Orleans, Louisiana, 19-21 October 1988.