Abstract

Phylloid algal limestone are economic reservoirs in many parts of the Permian Basin and elsewhere in the world, yet Pennsylvanian-aged phylloid algal limestones cropping out on the Eastern Shelf rarely show porosity development. Why are the Eastern Shelf outcropping phylloid algae so devoid of reservoir potential? Does this give us insight to help predict porosity development in phylloid algal limestones elsewhere in the Permian Basin or Midcontinent?

Bedding and mesoscale fabric patterns provide insight to the processes that help control porosity development. Environmental changes during deposition form bedding planes. Bedding planes are surfaces of heterogeneity at which stylolites nucleate during burial. Stylolites are the main source of late-diagenetic cement, and late calcite cementation ultimately controls porosity distribution. Most major phylloid algal reservoirs and their outcrop analogs show massive or thick bedding. In contrast, the Eastern Shelf phylloid algal limestones are predominantly thinly bedded. Bed thickness, in addition to other effects related to bedding, may ultimately control the distribution of preserved porosity in buried phylloid algal limestone reservoirs.

The relationship between bedding planes and environmental change is demonstrated first in argillaceous limestones and then in clean limestones. Where the Winchell Limestone interacts with the Perrin Delta system, limestone is thinly interbedded with shale. Limestone beds are “two faced.” The centers of limestone beds contain predominantly phylloid algae with clean carbonate mud, indicating deposition in a clear-water benthonic photosynthetic community. Limestone near the bedding plane, shale beds, and shale-filled cavities contain predominantly bryozoa, crinoids, brachiopods, etc. This represents deposition by a filter-feeding community active during siliciclastic mud deposition. Filter feeding depends on suspended plankton and organic detritus. This suspended material blocks light and kills the benthonic photosynthetic community. The two communities repeatedly and rapidly replace each other to form limestone with shaly bedding planes. The benthonic photosynthetic community and filter-feeding community alternate with each other in response to changing influence of the delta on water quality. Limestone deposition is maintained close to the delta because both communities form hard substrates that the other community can use to initiate growth.

Bedding surfaces in pure limestones are also controlled by environmental changes. The phylloid algal community (represented by larger phylloid algal plates) occurs near the center of beds, whereas bedding planes are surrounded by limestone with few whole phylloid algae and more carbonate mud. Fossils of filter feeding and other organisms are concentrated near the bedding planes, especially where there are traces of siliciclastic mud. It appears that the bedding planes mark a break in phylloid algae accumulation during which a more diverse fauna is established and carbonate mud is generated by breakdown of the phylloid algal plates. Something periodically kills the phylloid algal community. A sparse filter-feeding community may or may not develop. Shallow burrowers churn the relict sediment, comminuting the algal plates to a coarse carbonate mud. This mud remains in the area because the Eastern Shelf is an attached shelf without strong wave or current action that will wash the carbonate off the shelf.

Such environmental changes have two effects. First, burrowing produces sediment with more carbonate mud. Carbonate mud reduces the potential for shelter voids. It also decreases permeability of the unlithified carbonate, thus reducing its potential for moldic porosity formation during meteoric diagenesis. The second effect is more subtle but possibly more important. The environmental change creates a lithological heterogeneity at which stylolites can form. Stylolites generate late cement that will occlude porosity that formed in the phylloid algal grains. Where beds are thin, stylolites have closer spacing, and they generate more cement. The cement will occlude any moldic porosity that developed during early diagenesis.

East Shelf limestones are typically thinly bedded, with environmental changes every few inches in most outcrops. Many of these surfaces are enhanced by stylolites. The thin-bedded character of Eastern Shelf limestones may be the result of deposition on the interior part of a mixed carbonate-clastic shelf. Most phylloid algal grains, especially the fossil codiaceans, develop moldic porosity during early diagenesis. The molds are occluded by coarsely crystalline, blocky calcite, which is a late cement. Only one relatively fresh Winchell limestone location had phylloid algal moldic porosity development. Porosity occurs in an anomalously thick bed. The bed is close to a flooding surface, which created sufficient accommodation space to allow rapid accretion of phylloid algae in near growth position.

The importance of bedding and environmental changes at bed breaks for reservoir quality development in phylloid algal limestone has not been widely recognized. To use this concept for exploration, locate settings likely to develop thick bedding in phylloid algal facies. In theory, flooding surfaces in positions on the middle to outer shelf should have sufficient accommodation space for rapid, uninterrupted accretion of thicker beds. Look for pre existing topography to facilitate rapid stabilization of the sea floor and initiation of carbonate sedimentation.