Marcellus Shale

According to the USGS assessment, the Marcellus Shale contains about 84 trillion cubic feet of undiscovered, technically recoverable natural gas and 3.4 billion barrels of undiscovered, technically recoverable natural gas liquids.
The USGS uses a statistically-based process to calculate the likely range of its estimate. The range of values extends from a 5% or greater likelihood of occurrence (the F5 value, or largest estimated value) to less than 95% likelihood of occ
RESERVES are quantities of oil and gas that are already discovered, recoverable, and commercial.The USGS assesses UNDISCOVERED RESOURCES, which are those that are estimated to exist based on geologic knowledge and theory.
The USGS publishes its oil and gas resource assessments on the Energy Resources Program web site as soon as they are completed, peer reviewed by scientists knowledgeable about the subject, and formatted
This question cannot be answered clearly because the methods used by those organizations to estimate resource volumes are not publically available to compare step-by-step. The USGS does not include previously discovered reserves in its estimates. It al