Ray E. Ferrell

Clay Mineral Variations in Four-Point Bayou, Terrebonne Parish, Louisiana: ABSTRACT

Cores of unconsolidated sediments from the Four-Point Bayou area, representing an interdistributary basin deposit less than 1000 years old, were examined for variations in clay mineralogy. Two specific environments of deposition, interpreted from grain-size analyses and sedimentary structures, were compared: a sand-rich natural levee deposit and an overlying clay-rich interdistributary bay. Analyses of the < 2 ..mu..m clay fraction using Schultzs method show smectite to be the predominant clay in both environments. The relative abundance of kaolinite:illite:smectite in the natural levee, near-channel deposits is 1:3:6. In bay deposits, it is 2:3:5. Hydraulic fractionation of clays during deposition should result in a high K/I ratio in natural levees and a low ratio in bay deposits, the reverse of what was observed. Kaolinite should have been enriched in natural levees at the expense of illite. Other processes must have been operative either at the time of deposition or during early diagenesis of these deposits. Possible mechanisms include localized redistribution and sorting or chemical equilibration with early pore fluids associated with saltwater intrusion. The mixing of clays from different sources could also account for the observed differences.

Clay Mineralogy of Cubits Gap Crevasse Splay, Mississippi Delta: ABSTRACT

ABSTRACT Relative abundances of clay species in the <2 µm fractions were determined for the three lithofacies identified in a 5 m core from the proximal portion of Cubits Gap crevasse-splay, Mississippi Delta, in order to determine the control of depositional mechanics on clay-mineral composition. Proximal splay lithofacies, defined by sedimentary structures and the relative abundances of cohesive (mud) versus non-cohesive (silt and sand) sediments, are: (1) suspension-deposited mud, (2) interlaminated current-deposited silt and suspension-deposited mud, and (3) current-deposited sand. The weight percentages of sand, silt, and clay (s:slt:cly) in these lithofacies, defining discrete fields in a ternary grain-size plot, are: (1) suspension-deposited mud (6 samples), 0-9 wt% : 40-68 wt% : 28-60 wt%; (2) interlaminated silt and mud (6 samples), 1-30 wt% : 60-98 wt% : 0-31 wt%, and (3) current-deposited sand (2 samples), 79-100 wt% : 0-17 wt% : 0-4 wt%. The range in relative peak-height percentages of illite, smectite, and kaolinite plus cholrite (I:S:K + C), calculated using modified methods of Griffin (1971), are: (1) suspension-deposited mud, 33-41% : 35-50% : 18-26%, (2) interlaminated silt and mud, 31-39% : 43-52% : 17-20%, and (3) current-deposited sand, 43% : 38% : 19%. These results overlap with the observed range of illite, smectite, and kaolinite in Mississippi River suspended sediment reported by Johnson and Kelley (1984) but are richer in smectite and depleted in kaolinite relative to clay abundances in sediment on the Louisiana continental shelf (Brooks et al, 1976). No systematic variation in the relative abundance of illite, smectite, and kaolinite plus chlorite is present in the <2 µm size fractions of the three lithofacies at Cubits Gap. In the crevasse-splay studied, physical processes that operated during sedimentation were incapable of fractionating significantly the clay species in the <2 µm fraction. These results suggest that differing clay compositions in comparable, ancient, deltaic, depositional systems reported elsewhere are the result of diagenetic processes and are not due to physical segregation in the depositional environment.

Fluid Movement and Diagenesis in Fine-Grained Geopressured Sediments of Frio Formation (Oligocene), Kaplan Field, Southwestern Louisiana: ABSTRACT

Investigation of structure, temperature, pressure, salinity, and core samples at Kaplan field yields information on diagenesis of fine-grained sandstones deposited in an outer shelf/upper slope depositional environment. Cross sections and structural maps reveal a domal structure at 15,000 ft (4,572 m) of depth and a northeast-striking growth fault. Post depositional faults occur at shallower depths (11,500 ft; 3,505 m). A large growth fault forms the northern border of the study area. The shallow occurrence of End_Page 446------------------------------ geopressure is related to structure and a high shale/sand ratio. Low isothermal surfaces in the down fault blocks accompanied by anomalous high temperatures in the upthrown blocks indicate vertical leakage of fluids along growth faults from underlying geopressured aquifers. The association of low salinity fluids (less than 60,000 ppm) with leakage zones affirms structural control of fluid movement through the Anahuac and Frio formations (Oligocene) at Kaplan field. The Frio Formation core samples from 16,700 to 19,600 ft (5,090 to 5,974 m) of depth, representing channel and channel-edge turbidite sandstones, were examined petrographically and by SEM. The arkosic composition of late stage diagenesis sandstones at Kaplan field suggests an original arkose or lithic arkose composition (classification of McBride). Nonferroan calcite cementation, chlorite rims and cement, and quartz overgrowths characterize early diagenesis. At a middle stage of diagenesis secondary porosity is developed by dissolution of unstable grains and calcite cement. Samples flushed by geopressured waters from greater depth show kaolinite pore-fill and quartz overgrowths, chlorite (polytype IIb) and illite cement, and feldspar overgrowths in the late diagenetic stage. Premetamo phic textures are apparent in the deepest section at 338°F (170°C). The low permeability of sandstones with extensive early chlorite cement (channel-edge sandstones) precludes development of extensive secondary porosity. In contrast, sandstones with little early chlorite cement develop and maintain secondary porosity through the late diagenetic stage. Restriction of fluid movement by early chlorite cement has ramifications for migration of hydrocarbons or geothermal waters, and for gas production at Kaplan field. End_of_Article - Last_Page 447------------