The Smackover Formation is an important gas-producing interval in the Mississippi Salt Basin, but inorganic components frequently reduce the economic value of the gas. Fluid inclusions in overgrowths and fracture-filling calcite cements provide samples of gases that were present in the system at various times in the past. We have analyzed the composition of gases in fluid inclusions from 27 different depths in the Conoco Mary Higgins #2 well. This well is located on the eastern shoulder of the Wiggins Arch and penetrated the entire Smackover Formation over the depth range from 19,177 ft to 19,400 ft. The carbonates of the Smackover Formation directly overlie basement, and this is intruded at 19,456 ft by a 3 inch rhyolite dike that contains calcite fracture-filling cements. The reservoir interval extends from 19,100 ft to 19,335 ft.

The compositions of gases in individual fluid inclusions were determined mass spectrometrically. Inclusions in calcite cements were ruptured by heating in vacuum, where each inclusion produces a 25 msec burst of gas that is analyzed by a high-speed, computer-controlled mass spectrometer to give the gas composition for the individual inclusion. Typically several hundred individual inclusions were analyzed for 10 mg samples. The mass spectrometer was calibrated with pure gases and known synthetic gas mixtures. Supporting information for calcite growth temperatures was obtained from fluid inclusion homogenization temperatures (T_h). Preliminary microthermometric analysis gave fluid inclusion T_h values that are all in the 158 - 202°C range (with most between 174 and 194°C), so that they all exceed the ~140°C required for thermochemical sulfate reduction (“TSR”). In addition, freezing point depression studies indicate that in many cases the aqueous phase was close to saturation (for sodium chloride equivalent), and in some cases a cube of (presumed) halite was present at room temperature.

The compositions of the gases in fluid inclusions show wide variations over short vertical distances. Gases at both the top and bottom of the Smackover Formation are water dominated with little methane or non-hydrocarbons (Figure 1). In other samples, such as the calcite from 19,195 ft, the gases contain greater than 50% methane with carbon dioxide and hydrogen sulfide. No ethane or higher hydrocarbons were observed. The calcite from 19,218 ft (shown in Figure 1) has high carbon dioxide contents (even compared with water) together with significant concentrations of hydrogen sulfide. The wide variations in hydrogen sulfide concentrations probably reflect the availability of both anhydrite and methane. Free-energy minimization thermodynamic calculations were used to estimate equilibrium gas composition and showed that when anhydrite is absent from a particular depth range no hydrogen sulfide is generated. While carbon dioxide is frequently present in small concentrations, nitrogen seems to be absent from the gases in the fluid inclusions analyzed, even though the reservoir has produced gases with high nitrogen contents (in some cases up to 90%).

The depths of samples analyzed and selected gas compositions are shown in Figure 1.

We are pleased to acknowledge support from the Petroleum Research Fund of the American Chemical Society.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah