COMPARTMENTALIZATION OF THE EAGLE FORD SHALE, SOUTH TEXAS – A NOBLE GAS INVESTIGATION

Ten gas samples from the Eagle Ford Shale were collected from five production wells in South Texas, for analysis of the complete set of stable noble gases (He, Ne, Ar, Kr, Xe) in an effort to characterize the source(s) and behavior of this unconventional natural gas reservoir. All samples are entirely dominated by the presence of crustal ⁴He. A small He mantle component is also present as evidenced by their R/R_a values (where R is the measured ³He/⁴He ratio and R_a is the atmospheric ratio), above those of typical crustal production values (~0.02 – 0.05). All samples also show the presence of non-atmospheric ²¹Ne, ⁴⁰Ar and ¹³⁶Xe with ²¹Ne/²²Ne, ⁴⁰Ar/³⁶Ar and ¹³⁶Xe/¹³⁰Xe ratios up to 0.03639, 489.4 and 2.196, respectively. A small mantle Ne component is also present. ²⁰Ne/²²Ne and ²¹Ne/²²Ne ratios reflect a three-component mixing between the atmosphere, the crust, and the mantle (Mid Ocean Ridge Basalts – MORB and Ocean Island Basalts – OIB) with MORB-type dominance over OIB.

The noble gas record suggests the presence of distinct natural gas reservoirs and thus, a shale gas reservoir that is compartmentalized, with gas compartments displaying significantly different ages. Correlation between reservoir age and depth is not observed. The presence of individual gas compartments is likely controlled by the tight permeability, complex gas histories, i.e., transport, lithologic control and maturity, and the presence of faults that may control gas migration. No spatial correlation with age can be established at this time. Additional sampling will be required to characterize these individual reservoirs. Of interest is also the presence of significant temporal variations in the noble gas composition of at least one well. Such variations, particularly significant in terms of Ar and Ne isotopic composition suggest connection with two other producing wells. If confirmed, this has the potential to affect long-term well production.