It is generally true that the deeper one drills in the Eagle Ford Shale, the more natural gas one is likely to find. Updates to Energy Information Administration (EIA) geologic maps spell out the subtleties.
EIA announced improved data to better characterize the formation's structure, thickness and surface area, as well as the gas-to-oil ratio (GOR) of its producing wells from January 2000 to June 2014.
GORs are expressed as cubic feet per barrel (cf/bbl), with a high GOR describing a gas-rich well or area.
"The distribution of initial GORs from Eagle Ford play wells generally corresponds to the depth of the hydrocarbons being accessed," EIA said on its website Wednesday. Deeper wells (up to 15,000 feet) in the southeast have higher initial GORs, or a relatively greater share of natural gas, while the shallower wells in the northwest (below 6,000 feet) have lower initial GORs, or a relatively greater share of oil."
In the eastern portion of the play, wells intersecting the formation between 5,000 and 12,000 feet have initial GORs of 6,000 cf/bbl or less, which means they produce more oil relative to gas. The western portion of the play includes a wider distribution of wells by depth. Here, wells intersect the formation from as deep as 14,000 feet and as shallow as 2,000 feet. Initial GORs of 6,000 cf/bbl indicate higher gas production relative to oil, EIA said.
EIA uses four major boundaries to define what it considers to be the Eagle Ford. The western boundary is the Rio Grande River, which forms the international border with Mexico. EIA doesn't have well-level data from the Mexican Eagle Ford.
The northern boundary is defined according to the thermal maturity of the hydrocarbons within the play. The oil window ends and the immature area begins above a minimum subsea depth (i.e., depth below sea level) of 3,650 feet in Frio County, TX, and counties east, and above minimum depths in Maverick and Zavala counties, ranging from 650 to 2,900 feet, according to EIA.
The Eagle Ford's southern boundary traces the edge of the hydrocarbon-bearing part of the shale along the Sligo Reef Margin.
The northeastern boundary roughly corresponds with where the carbonate-rich lower Eagle Ford (the primary target for drilling and completion) tapers off and transitions into silica-rich rock units of the Pepper Shale of the East Texas Basin, which, while hydrocarbon-bearing, is characterized as a different play than the Eagle Ford.