Cotton Valley Lime (Oolitic Shoal) Play

Introduction

The Cotton Valley Lime Trend of east Texas is famous for a recent 3D defined reef play, but the vast majority of production is derived from the more widespread oolitic shoal trend. The oolitic facies produces on the both the west and east limbs of the basin; however, the east side of the basin has the best volumetric statistics and best initial rates, largely due in part to recent improvements in hydraulic fracture stimulation. Since the advent of shifting fracs from acid base to sand-gel base, the economics have vastly improved, allowing for the development of titer rock on the flanks of shoals.

"Since the advent of shifting fracs from acid base to sand-gel base, the economics have vastly improved, allowing for the development of titer rock on the flanks of shoals."

H2S has always been a problem, averaging about 2%, and requiring some kind of treatment facility. Recent frac treatments help to create better yields, and recent price increases have also helped overall economic returns. Exxon dominated the play’s early development, but they targeted wells which would yield over 4 BCFGE and locations which did not require fracture stimulation. With that in mind, Exxon also kept well densities on their gas units spaced at 160 acres, or 4 wells per unit. Other operators, such as Goldston, proved that the Cotton Valley Lime could be drilled at 80 acre densities, with 40 acre offsets not experiencing any well interference. Today, 2 BCFG wells are economic, which opens up a huge area of tite gas to exploit and develop. Many wells have been abandoned in the past that, today, could be economically completed.

Early players also avoided low resistivity rock, believing it to be wet. Over time, the threshold of what is considered “low” has changed. Often, wet rock is perforated and stimulated with success, because the higher Sw rock is essentially immovable.

Mapping the fairway of the trend is not difficult, and requires no seismic. It does require an understanding of the petrophysical properties governing production, and what parameters to map. Good mapping begins with production EURs (Estimated Ultimate Recoveries) tied to simple log parameters.

To summarize, the key features to keep in mind about this play are:

  • Pushing the resistivities to lower and lower frontiers.
  • Advent of sand propped fracs and increased productivity of both old and new wells.
  • H2S, and Exxon’s massive position, have kept many players at bay.
  • The Cotton Valley oolitic facies is best developed on 80 acre densities, or eight wells per gas unit.
  • The average Cotton Valley Lime well, on the east side of the basin, will ultimately yield about 4 BCFGE, making the play economic under almost any price scenario. A modest 2.5 BCFGE well yields better than a 50% IROR with $4.25 gas.