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Trenton Limestone Reservoirs in Northern New York: Where does the gas come from?

Richard Nyahay, Rick Bray, Rose Schulze and Langhorne Smith, Reservoir Characterization Group, New York State Museum, Albany, NY 12230, [email protected]

The Trenton Limestone has produced gas in New York for almost 120 years in numerous shallow (<2500 ft) fields near Lake Ontario, NY. Wells encountered intense pressure kicks that typically blew down to very low rates in a few hours or days. Most of the production occurred in the late 1800s and early 1900s and there is little modern data. A study was undertaken to answer the questions: Does hydrothermal dolomite have anything to do with this production? Is there any porosity in the limestones that might work as a reservoir for natural gas? Does the gas come from fractured limestone or fractured shale or both? To answer these questions we acquired 160 sidewall cores along with a full suite of logs in a recently drilled well that had strong gas shows from several intervals in the Trenton Limestone.

The cores from the productive intervals have no matrix porosity in the limestone and no significant dolomite. SEM photos showed what may be minor microporosity in the shales and shaley limestones. The FMI from the well showed no significant vertical fractures in the intervals with shows. Based on this data and other observations, it is our conclusion that most of the gas in this well came from horizontal bedding plane partings between interbedded limestone and shale. The bedding-plane partings are interpreted to have been propped open by the high pressure of the gas. When they were penetrated, the overpressured gas flowed back at high rates but quickly blew down to almost nothing as the partings closed due to the lithostatic pressure. Wells with longer lasting production may penetrate open vertical fractures that connect larger pore volumes. Targeting areas with vertical fractures around faults and/or developing new drilling and completion techniques may help make this an economic play.

AAPG Search and Discovery Article #90059©2006 AAPG Eastern Section Meeting, Buffalo, New York