Page 209 - Carbonate Facies in Geologic History

196 Pennsylvanian-Lower Permian Shelf Margin Facies

subaerial exposure. The cross section of Townsend field (Fig. VI-22) demonstrates
this geologic history. Along the axis of the Townsend-Kemnitz trend at the begin-
ning of Permian time, sedimentation of black, cherty spiculitic mud gave way to
normal marine, bioclastic lime wackestone. These sediments accumulated along
with some of the algal plate mud-mound wackestone and built up a few tens of
meters of section. Apparently, no sedimentation downslope was taking place;
here a dark, thin, lag conglomerate formed, bearing eroded carbonate blocks
derived from upslope shallower water sediments which were already cemented.
Argillaceous and dolomitic beds in the lower part of the buildup represent breaks
in reef sedimentation during the times that the conglomerate was forming down-
slope and perhaps an early period of subaerial exposure. During an ensuing
transgression both the axis of the trend and its basinal slope developed the
characteristic micritic facies with algal plates and Tubiphytes, the latter best devel-
oped with cornuspirid foraminifera over the crest. The more northerly areas
behind the crest received the bioclastic debris. Finally, as sea level lowered, or as
the mound accumulated into wave base, a cap of grainstone and tubular forami-
niferal boundstone developed. The cornuspirid boundstone, with Tubiphytes, is
particularly thick and well-developed on the foreslope area. The relief of the
Townsend-Kemnitz trend at this time was at least 30 m because about 15 m of
compacted shale filled in the basin in front of it and lapped unconformably
against the foreslope facies. This unconformity, also developed over the top of the
mound, records another period of subaerial exposure and diagenesis. A second
conglomerate is developed basinward at this stratigraphic interval. A higher cap-
ping unit of pure carbonate debris and boundstone occurs above the main Town-
send-Kemnitz reef and is separated from it by a "shale break" in immediate
backreef areas. This upper unit represents a transgression which continued into
Wolfcampian time as evidenced by the overlying blanket of euxinic and open
circulation lime sediment which filled in the basin south of the reef trend and
evenly overlaps the area above the reef crest.
According to Dunham (1969b), leaching in the vadose zone is the major
diagenetic factor responsible for the porosity, although early collapse brecciation
which aided the leaching and dolomitization is also important. Dolomite is found
mainly in the lower part of the mound and is very fine-grained and pervasive
through both grains and matrix. Some very coarse-grained void-filling dolomite
also occurs. Sucrose texture and dolomitization preferentially in the original lime
mud matrix does not occur. In general, dolomitization is not important in reser-
voir development. The vadose processes are related chiefly to the lowering of
relative sea level, exposure of the mound, and development of a fresh water lens at
the end of reef growth. The presence of the "fairway" with its thick section of more
or less continuous porosity probably results from the leaching of the maiQ mass of
poorly bedded organic carbonate which was the topographically highest part of
the mound and which, because of its high content of boundstone, developed the
best pore-connected network following diagenesis (Fig. VI-23). Thin layers of po-
rosity are developed in the northern backreef area and result from the bedded
character of the strata deposited behind the reef front. About 80% of the esti-
mated 14.5 million barrels of ultimate recoverable oil came from the fairway
demonstrating excellent reservoir connection and drainage in this narrow belt.