Gas Geochemistry Signature of Leaky Formations Over Geologic Time

Abstract

The majority of the Marcellus Shale in the Appalachian Basin is a proven hydrocarbon producing reservoir with repeatable production results. These results are related to the geologic parameters of the shale in an area as well as the completion techniques applied to a given well. However, portions of the Marcellus Shale, specifically those deposits residing near the Allegheny Structural Front, have undergone folding and faulting related to Alleghenian tectonism which allowed hydrocarbons to escape through geologic time. The variable migration of hydrocarbons out of the system is manifested as inconsistent production results from wells at some proximity to mapped faults. This study compares the methane and ethane isotopic signatures of Marcellus Shale production from wells in structurally complex and structurally benign geologic settings. In a normal maturation trend, the methane and ethane isotopes show isotopic reversal at higher thermal maturities. Methane and ethane isotopes in structural complex areas trend isotopically lighter than would be expected based on the shale's thermal maturity. When these isotopic signatures are evaluated relative to thermal maturity parameters such as vitrinite reflectance, relationships indicate the gases have experienced a fractionation of the isotopes during folding and faulting events and/or the isotopes preserve the composition of the methane and ethane when the structural event occurred. In the more structurally complex areas these isotopic signatures may be attributed to increased migration out of the source rock over geologic time.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015