Seismic Detection of Onondaga Patch Reefs

Morris, James R.
Belden & Blake Corporation, North Canton, OH

In the late 1980s, and early 1990s, the Quaker State E & P group, (acquired by Belden & Blake Corporation in 1995), discovered gas in the Devonian Onondaga Limestone while drilling Silurian Medina development wells in northwestern Pennsylvania. The source of the gas was from patch reef reservoirs, some 50 feet below the top of the Onondaga, and totally encased within regional limestone. The largest Onondaga Patch Reef found so far has an area extent of 900 acres with total pool EUR of over 7 BCF of gas.

The purpose of this paper is to show how surface seismic can be used to detect Onondaga Patch Reefs. From 1991 through 1997, Quaker State E & P and Belden & Blake Corporation acquired several 2D seismic lines over three different Onondaga patch reefs. With sonic and density logs available in the Medina Field, several patch reef models were constructed. Seismic responses were then generated to understand how seismic signatures would be affected by varying frequency and phase of model wavelets. Using this well data, wavelet extractions were performed on the 2D seismic lines with the intent of converting each line to a zero phase format. With the seismic lines in a zero phase format, several key indicators become apparent when patch reef thicknesses of at least 30 feet were present. Along with these indicators, AVO modeling and analysis was done to determine if any AVO anomalies were present. From both post-stack and pre-stack modeling of patch reef seismic responses, techniques to improve seismic exploration success can be made from spectral shaping in field acquisition, to spectral preservation in processing, to wavelet processing and attribute analysis during interpretation.

AAPG Search and Discovery Article #90031©2004 AAPG Eastern Section Meeting, Columbus, Ohio, October 3-5, 2004