Abstract: Gravity Definition Of Chappell Formation Pinnacle Reefs, Fort Worth Basin - A Case Study

CATHEY, WILLIAM, Earthfield Technology, Inc., Houston, Texas, andDAVID WILLIAMS, Tidelands Geophysical Co., Plano, Texas

An analysis of more than 14,000 commercial gravity stations covering Montague, Wise, Cooke, and Denton counties in the Fort Worth Basin, has revealed that numerous previously unidentified pinnacle reefs may exist and be delineated using gravity data.

Several pinnacle reefs, primarily in Montague County, have been successfully sampled and have produced both oil and natural gas in the Eanes NW, and Osage Fields. These pinnacle reefs, located in the Mississippian age Chappel Formation, are extremely attractive exploration targets due to location relative to existing production facilities, and because of the shallow drilling depths required to test them (<6500 feet).

The identification of these reefs is accomplished by the detailed frequency domain filtering of the gravity data to enhance positive gravity anomalies produced by these features. The pinnacles are composed of dense carbonates completely encased in the much lower density Barnett Shale unit thus creating a strong density contrast between these formations. The gravimeter is extremely sensitive to these strong contrasts, measuring positive local gravity anomalies associated with these reefs and their adjacent flanking material.

It is therefore possible to quickly and inexpensively identify new lead areas using this technique and to help determine more efficiently where new 3-D seismic acquisition programs should be positioned.

AAPG Search and Discovery Article #90936©1998 AAPG Southwest Section Meeting, Wichita Falls, Texas