1st Research Conference - Gulf Coast Section SEPM

Original primary porosity in False River reservoir sandstones, estimated at
42%, has been reduced by early mechanical compaction, followed by pressure
solution, cementation, and finally by late mechanical compaction, followed by
pressure solution, cementation, and finally by late mechanical compaction.
Present porosities of 20-25% in many of the fine- to medium-grained
sandstones are much higher than would normally be anticipated, in view of the
porosity reducing processes that can be identified and the great depth of
burial. Examination of thin sections and scanning electron photomicrographs
reveal extensive partial and total leaching of 15% to 20% of the framework
grains. Most of the leached grains probably had an original silicate mineralogy.
Point count studies indicate that 20% of the total porosity may be attributable
to this process. Porosity created in this manner is easily identified by the
remnant shells of authigenic chlorite which remains after dissolution of the
detrital grains. Subsequent to this grain leaching, or most likely concomitant
with it, continued compaction of the reservoir sandstones occurred. This
interpretation is supported by remnant chlorite shells that were compacted
until the opposing shell walls are nearly in contact. The occurrence of euhedral
quartz crystals and ferroan calcite within many of the collapsed or crushed
chlorite shells represent a closing stage of diagenesis. Continued porosity
reduction through pressure solution and cementation would have destroyed
reservoir capacity if geopressuring of the Tuscaloosa interval had not occurred
soon after grain dissolution.
False River Field is an example of production from reservoir sandstones with a
capacity greatly enhanced by secondary porosity. Mineralogical immaturity of
the original Tuscaloosa sediment and geopressuring of sandstones that
experienced a complex history of compaction, cementation, and dissolution are
believed to be significant factors that have combined to produce this deep
occurrence of unusually high porosity.