Newtonia Field: Model for Mid-Dip Lower Tuscaloosa Retrograde Deltaic Sedimentation

Michael D. Hogg

Newtonia field is located in Wilkinson County, southwestern Mississippi, and produces oil and gas from the Stringer Member A sandstone of the lower Tuscaloosa Formation. Paleoenvironmental analysis suggests that the A sandstone represents low-sinuosity, active, distributary fill, retrograde deltaic deposition upon a truncated fluvial/deltaic land surface in response to rising sea level. The simple low-sinuosity channel character suggests that other fields interpreted as deltaic point bars may be more complex systems of amalgamated similar low-sinuosity distributary sands.

Retrograde deposition on a competent substrate precluded synsedimentary A sandstone gravity expansion. Substrate character may be reflected in the width/depth ratio of low-sinuosity distributary reservoirs. Retrograde deposits will tend toward moderate width/depth ratios, and progradational distributary sands will have relatively low width/depth ratios. Laterally accreted sand bodies, deposited by sinuous channels, have high width/depth ratios. Because reserves in stratigraphic accumulations are largely a function of sand geometry, understanding the paleogeography and relative sea level during the deposition of a prospective reservoir is important.

Depositional patterns and sand-surface topography within the lower Tuscaloosa section had a pronounced effect on positioning of overlying lower Tuscaloosa sandstone units. Subtle topographic highs and lows associated with differential compaction, differential subsidence, and possibly erosion surfaces resulted in either focusing or deflection of stream courses, e.g., relatively thick areas of amalgamated B and C Stringer Member sandstones (below the A sandstone) occur above lows on the basal lower Tuscaloosa sandstone surface. In contrast, areas of relatively thick A-sandstone accumulation overlie an area of relatively thick B/C sandstone. Since productive A sandstone thickness often approaches the lower limit of resolution for recognition by seismic velocity analysis, isopach and stru ture maps of deeper sandstones would assist identification of the frequently productive A sandstone occurrences.

Present-day structure at the top of lower Tuscaloosa level reflects minor effects of differential compaction between the A sandstone and adjacent fines superimposed on regional dip. Relief on the compaction-related anticline is insufficient to account for the hydrocarbon accumulation, indicating the trap to be stratigraphic. Counter-regional dip on the updip side of the A sandstone suggests potential for seismic recognition of similar compaction-related anomalies.

Diagenetic features of the A sandstone typical of lower Tuscaloosa sandstones in the area include pervasive pore-lining chlorite and chloritized lithic grains, patchy post-chlorite pore-filling carbonate, and minor quartz overgrowths. These diagenetic mineral and textural alterations have considerable impact on wireline log responses. Certain features, e.g., local early pre-chlorite poikilotopic calcite, framboidal pyrite, and uncompacted pyritized vascular plant fragments, suggest the presence of depositional or shallow burial marine pore water. A field-wide low permeability zone cemented with post-chlorite calcite and ankerite, and pre-chlorite siderite occurs at the base of the A sandstone. The low-permeability zone separates Newtonia-field fluid content from that of a truncated, j xtaposed, oil-bearing, sandstone body downdip from the field oil-water contact. Similar low-permeability zones are capable of forming diagenetic/stratigraphic traps and may also explain some anomalous dry holes and other reservoir peculiarities encountered in lower Tuscaloosa fields.

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