The Cotton Valley Limestone Pinnacle Reef Trend, 20 Years Later, East Texas Basin

Abstract

In 1993, Marathon Oil Corp. opened the Jurassic pinnacle reef play with their discovery of the #1 Poth well in Leon County, Texas. The well flow tested 40.6 million cubic feet of gas per day unstimulated from an open hole completion in a pinnacle reef. That well and its neighbor, the #2 Poth S.T. drilled 960 feet away into the same reef, have accumulated 43 billion cubic feet of gas and 61 billion cubic feet of gas, respectively. These wells remain two of the most prolific wells in the pinnacle reef trend and were considered “the crown jewels” of the trend within Marathon Oil. Since then, more than 50 Cotton Valley reef wells along an 80 mile linear trend have produced more than 0.5 trillion cubic feet of gas. 3D seismic and the unique seismic signature of the pinnacle reefs made it possible to identify pinnacles as small as 5 acres in area. The pinnacles were estimated to be 300 feet to 600 feet tall from the drilling results in the late 1990s. However, ongoing drilling by companies (such as Red Willow Production Co. and Encana) found two older reef cycles buried within the carbonate shelf totaling up to 1500 feet thick and composed of three 500-foot cycles. 90% of the pinnacle reef drilling was done when it was believed that only one pinnacle reef cycle existed. The sparsely-drilled middle reef cycle should contain significantly more gas than the upper cycle, as the reefs widen at their base. In general, no water has been produced from the upper two pinnacle reef cycles, but there are indications the 3rd oldest cycle could contain high water saturation. Two of the largest pinnacle reef wells discovered in the Jurassic pinnacle reef trend were drilled by Encana in 2010 and 2012: the #1 Embra well (18.7 billion cubic feet of gas) currently producing 6500 million cubic feet of gas per day, and the #1 James McDonald well (26.7 billion cubic feet of gas) currently producing 30 million cubic feet of gas per day.

AAPG Datapages/Search and Discovery Article #90219 © 2015 GCAGS, Houston, Texas, September 20-22, 2015