Atoka Sands (Pregnant Shale or Davis Sand) Play

Introduction

The Pregnant Shale, also known as the Davis and Grant Sands, is found in a widespread region along the county lines of Parker/Palo Pinto and Parker/Wise, and the southeast corner of Jack County in Texas. The area covers over 1,400 square miles (896,000 acres). Map 1 shows the extent of the play in the Fort Worth Basin. The area may hold as much as 4 trillion cubic feet of gas (TCFG) in place, using 40 acre drainage areas for each well, a 75% completion success rate, and the typical recoverability of the average well. The fairway of production, defined in the literature, is shown on Map2.

Because the Railroad Commission of Texas lumps the producing wells into any number of generic Atokan age horizons or “County Regular” fields, no public or private database is capable of quickly isolating known producers. Producers can be identified by log ties, bracketing along common depths, and using the characteristic hyperbolic decline curves of producing histories. Over the course of a several months, a database was compiled for known and suspected Pregnant Shale producers.

The statistics for these producers were then calculated, and a body of technical literature and actual rock data, including core, was integrated into the data set. Many interviews with local producing companies and their geologists, has revealed that the Pregnant Shale is not a primary target in the Fort Worth Basin because of the marginal economics of the zone; however, it has always been considered a fair secondary target or “bail out zone” when the primary target failed to materialize. Interviews with various experts at the Bureau of Economic Geology and the authors of technical papers have also served to gain an understanding of the Pregnant Shale.

New logging analysis techniques and new hydraulic fracturing technology may help to rationalize this vast resource of tite gas into an economic zone of interest as a primary target. These new techniques and analogies to the active Barnett Shale play of the Fort Worth Basin will be outlined in this report.

This report explains the production characteristics of the Pregnant Shale/Davis Sand within a statistical framework. The geological characteristics are described to help explain these production characteristics. Finally, these features are compared to the prolific and currently very active Barnett Shale trend, and analogies are drawn to predict how new technology might be harnessed to economically extract gas from this tite gas resource. Cash flow models are derived from actual production histories of typical wells. The cash flow derived from a ten well program having the same statistical distribution as a group of 147 producers, using both dry holes and good wells in the model demonstrates how the law of averages can yield an economic investment program.