Controls on Hydrogen Sulfide Formation in a Jurassic Carbonate Play, Turkmenistan
Gary H. Isaksen, Mukhammetnur Khalylov, 2007. "Controls on Hydrogen Sulfide Formation in a Jurassic Carbonate Play, Turkmenistan", Oil and Gas of the Greater Caspian Area, Pinar O. Yilmaz, Gary H. Isaksen
Download citation file:
An integrated geological and geochemical evaluation of the Charjou terrace area of the Amu Darya Basin in Turkmenistan and Uzbekistan shows that the main controls on hydrogen sulfide (H2S) distribution are the lithofacies of the reservoir and seal rocks and reservoir temperature. Thermo-chemical sulfate reduction (TSR) has likely occurred where reservoir temperatures are greater than approximately 100°C, and the reservoired gas is trapped in dolomitic reservoirs in contact with anhydrite. These chemical reactions occur both in carbonate platform and reefal buildups capped by anhydrite, as well as in structural traps that have undergone thrust-induced juxtapositioning of anhydrite with the dolomite gas reservoir. The reservoirs can have an H2S concentration as much as 4 vol.%. This, together with temperatures of 100–110°C and sulfate-rich formation waters, suggests that these reactions represent the earliest stage of TSR. Carbonate buildups, including reefs, capped by marine shales have very low to trace levels of H2S because the thermochemical sulfate reaction is not initiated, presumably caused by a lack of sulfate. Furthermore, H2S concentrations can vary greatly within a single field with stacked pay zones. Reservoir compartments sealed by marine shales have no H2S gas. A covariance between increased gas wetness and lower initial temperatures for TSR is also suggested.
Figures & Tables
Throughout time, the greater Caspian area has maintained its position as one of the major petroleum provinces in the world. Its early history as a prolific producer of oil is well known. Caspian region exploration dates to the seventh century B.C., during the time of the Median Kingdom in today's southern Azerbaijan. Oil played an important role in the everyday lives of these ancient tribes of the region, and it is still a very important commodity today. The past two decades have seen many important advances in our knowledge of the geological evolution of hydrocarbon-bearing sedimentary basins. The success of modern exploration is, to a large extent, based on new advances in both deep and 3-D seismic imaging, as well as improved pressure-prediction and pre-drill oil and gas quality predictive methodologies, to mention just a few. Nevertheless, large areas of the greater Caspian region have remained unexplored due to a variety of factors such as deep-water conditions and zones with high pore-pressures in the South Caspian Sea and The Black Sea, and vast shallow-water regions with harsh winter ice conditions in the North Caspian Sea. This publication contains 12 extended abstracts and 6 full-length papers that discuss technology development, challenges in estimating proven and potential reserves, outcrop-based studies of potential reservoirs, regional tectonics and geodynamic evolution, and source rock and stratigraphic analyses of the greater Caspian area.