AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Algal Cysts in the Marcellus Shale (Middle Devonian), Appalachian Basin: The Impact of Algal Blooms on Gas Shale Reservoirs
(1) Core Lab, Houston, TX.
To confirm the existence of algal blooms during deposition of Marcellus shale and its impact on gas shale reservoir properties, more than 40 Marcellus cores have been investigated over a large area of the Appalachian Basin in western Pennsylvania and West Virginia.
Mineralized algal cysts (Tasmanites) are widespread throughout the Marcellus basin, and particularly abundant in the high gamma ray zone in the lower portion of the Union Springs Member. These cysts are mainly preserved as siliceous or calcareous grains often with pyritic rims; dolomite- and barite-cemented cysts are rare. The morphology of the cysts ranges from spherical to oval and elongate or even irregular, depending upon the timing of mineralization and degree of compaction. Generally, the cysts are better preserved in early cemented carbonate concretions compared to those found in organic-rich mudstone. Well-preserved, spherical algal cysts range in diameter from 50 to 250 µm and mineralized cysts tend to appear as aggregates of sand- to silt-sized (30 to 120 µm) quartz or calcite crystals. Both forms of the algal cysts exhibit a strong contrast in grain size to the detrital silt grains in the shale which tend to range from 4 to 25 µm.
The extremely high abundance of algal cysts in the lowermost part of the Marcellus shale is correlative with the highest total organic carbon concentration and represents an anomalous period of intense phytoplankton growth across the basin. The explosive algal growth is attributed to volcanic eruptions, which brought additional nutrients, especially iron, via ashes into the relatively closed Marcellus marine basin. The outburst of algal growth greatly increased the organic input and also enhanced the preservation of organic matter, resulting in mudrocks with superior organic content. This high organic content is not only responsible for ample gas in place values, but also optimizes petrophysical reservoir properties, specifically porosity and permeability.