Hydrogen Sulfide (H2S) is a hazardous gas commonly found during drilling and production of crude oil and gas. H2S can be naturally generated in reservoirs through microbial sulfate reduction (MSR), thermochemical sulfate reduction (TSR), and kerogen cracking. H2S can also be generated during production by modern microbial sulfate reduction (MSR) and/or contributed from other formations outside of the hydrocarbon producing intervals of interest. H2S concentrations in the Delaware Basin vary from zero up to >100,000 ppm in the gas phase. To accurately predict H2S distribution and delineate the high H2S risk areas, understanding H2S source origins and the primary controlling factors is essential. This understanding directly impacts planning of safe operations and facility design to mitigate elevated H2S concentrations in produced fluids.
ConocoPhillips applies an integrated workflow to improve understanding of in-situ H2S distribution and concentration in the Delaware Basin. In this workflow, geochemistry, geology, geophysics, petrophysics, petrography, basin modeling, and reactive transport modeling (RTM) are integrated to understand H2S occurrence, origins, and the primary controlling factors for in-situ H2S in the reservoirs.
This paper will share the current understanding of in-situ H2S origin for a shallow producing formation (Formation A) from one asset (Area T) in the Delaware Basin. Very high H2S in produced gas is encountered in Formation A where many fractures or possible faults are present in the overlying Lamar Formation. In Formation A, MSR is the key generative mechanism of high H2S in the produced gas. During early diagenesis, sulfate-bearing water percolated from overlying anhydrite into the reservoir and sulfate was reduced microbially by using organic matter as electron donor to generate H2S. Carbonate-rich reservoirs have limited scavenging capacity to remove H2S; therefore, the excess H2S either remained in the reservoir or reacted with kerogen to form Type IIS kerogen.
The distribution of H2S in the Delaware Basin is complex and related to multiple generative mechanisms. Geochemical data indicates that in-situ H2S risk does exist prior to drilling in certain formations/areas. Therefore, geologic in-situ souring processes by RTM were stimulated with data input from petrography and geochemistry. With further integration of geology, geophysics and petrophysics, ConocoPhillips has developed an advanced workflow to identify sources of H2S in the subsurface and thus help better predict in-situ H2S distribution and concentration for the Delaware Basin.
