Abstract The integration of microbial hydrocarbon microseepage signatures and 3-D seismic data directed explorationists to a new play concept and resulted in discovery of the Park Springs (Conglomerate) field in Montague County, Texas. A standard 3-D geophysical survey identified a prospective Ellenburger structure at a depth of approximately 2200 m (7200 ft) in the northern portion of the 5.6-km 2 (3.5-mi 2 ) survey area. A reconnaissance hydrocarbon microseepage survey of the area was conducted in December 1995, using the Microbial Oil Survey Technique (MOST). The Ellenburger structure was detected by a positive, but small, hydrocarbon microseepage signature. A much stronger and larger microbial anomaly was found above a structural trough, located 1 mi (1.6 km) south. In February 1996, additional MOST samples were collected over the area of interest. The new survey confirmed the small hydrocarbon anomaly associated with the seismic prospect, as well as the large anomaly associated with the structural trough a mile south. A review of geologic play analogs for northern Texas suggested that the trough might contain Atokan conglomerates—a risky exploration target in the Fort Worth Basin—but the primary prospect remained the Ellenburger structure and its associated smaller microseepage anomaly. The seismic prospect was drilled at the crest of the structure in March 1996. The Silver #1 well encountered 1.8 m (6 ft) of tight Salona sand in the Ellenburger section. Completion efforts recovered only marginal hydrocarbons: Approximately 340 barrels (bbl) of oil have been produced in three years. The operators intensified their investigation of a possible conglomerate reservoir. In October 1996, the J. G. Stone Sutherland Unit #1 well was drilled within the geochemical anomaly coincident with the trough; it encountered two separate conglomerate zones, each with 3 m (10 ft) of pay. Initial production in the lower zone was 500 thousand cubic feet of gas per day (mcf gas/day) and 5 bbl of oil/day (BOPD). The next well found three charged conglomerate zones, including 6.7 m (22 ft) of pay in the lower zone, and initial production near 1000 mcf gas/day. In all, 14 producing wells have been drilled in or near the conglomerate trough. In addition, four dry holes have been drilled in the trough feature, but they were located in areas outside the microbial microseepage anomalies. In October 1997, the microbial survey was repeated in order to document changes in the microseepage signature, over time, in the new field discovery. In general, areas with background microbial values remained stable after one year’s production, while the two producing areas experienced as much as a 50% decrease in their microseepage signature. This phenomenon of apparently reduced hydrocarbon microseepage over producing fields is thought to have been caused by a decline in reservoir pressure, as well as by changes in the drive mechanism controlling microseepage.

Abstract: Kelts (1988) was the first to point out that the ionic composition of lacustrine waters could exert an important effect on the preservation of organic matter in the sediments of these systems. In lakes with high sedimentation rates, the sediments become devoid of molecular oxygen at or very near the sediment-water interface. The rates of anoxic (anaerobic) decomposition of organic matter vary with the amount and "type" of organic matter being buried as well as with the electron acceptor being reduced. The amount of energy produced per organic molecule being oxidized is more for microbial sulfate reduction than for methane production. Therefore, where sulfate is abundant, organic carbon should be more readily oxidized than where other anions such as Cl – and HCO 3 – dominate the chemistry. We have tested this hypothesis by determining in-situ sulfate reduction via radiolabel techniques in sediments from Freefight Lake, a Na + -Mg 2+ -SO 4 2– saline lake in southwestern Saskatchewan. These data are then compared to sulfate reduction rate measurements from other sedimentary environments. Of particular interest is our previous work on algal flat sediments from Na + -Mg 2+ -Cl – salt lakes and pans in Bonaire, N. A. The maximum sulfate reduction rate for the nearshore, algal flat sediments of Freefight Lake was 400 nmol/ml/d. This is 2 to 2.5 times lower than rates from similar settings in Bonaire, even though the sulfate concentrations were 10–20 times higher at Freefight Lake. In addition, the S:OC ratios of the Freefight Lake sediments were extremely low, indicating the lack of sulfate reduction in these sediments. Although the total reduced sulfur concentration increased with depth at Freefight Lake, an extremely high percentage of this total S 2– is in the form of acid-volatile sulfur. This would not have been predicted from the low sulfate reduction rates. A number of important conclusions can be drawn from this work. They include the following: (1) sulfate reduction rates in salt lake sediments are not solely dependent on SO 4 2– concentrations nor on the ionic composition of the lake waters; (2) even though the organic carbon concentrations of these sediments are high, sulfate reduction is not rapid; and (3) the low sedimentation rate (0.03 cm/yr) may lead to the utilization of the most metabolizable of the carbon present in the oxic zone by aerobic bacteria, thereby slowing sulfate reduction rates.