New Mexico Geological Society Annual Spring Meeting — Abstracts

In Fall of 2023 Tap Rock Resources engaged with Advanced Hydrocarbon Stratigraphy (AHS) to use Rock Volatiles Stratigraphy (RVS) on drill cuttings from a Delaware basin well drilled in Lea county NM to profile the distribution of H2S from above the Lamar in the Delaware Mountain Group through the Bone Spring and into the Wolfcamp, among other goals. RVS, developed by AHS directly measures 40+ volatile subsurface fluids entrained in rock samples, typically drill cuttings and core, including sulfides, organic acids, water, C1-10 HCs and others using a novel cryo-trap mass spectrometry system developed by AHS. In addition to profiling the distribution of H2S, other volatile subsurface fluids measured by RVS were used to gain insights into the other subsurface volatile fluids that may control or influence the distribution of the H2S, several key dependencies/correlations in the RVS data were identified. These dependencies in the RVS data combined with experience relating to the distribution of iron and H2S in production allow for the development of a model that relates likely mechanisms of H2S generation in the subsurface, its distribution, and implications for its presence in production. The model consists of the following elements:

1. From the top of the Avalon up in the analyzed section measured H2S has strong oppositional relationships with carbon disulfide (CS2) and carbonyl sulfide (OCS) – these and other data suggest that in this section H2S is being generated by a hydrolysis process where CS2 and OCS are converted to H2S. Such mechanisms have previously been reported in environmental and industrial literature among others, but it is believed this is the first observation of potentially such a process in the subsurface. Other data from RVS support this such as an oppositional relationship to the presence of organic acids and H2S; increasing acidity is known to retard such hydrolysis reactions

2. In the deeper portion of the well, consisting mostly of the Bone Spring, H2S is most present in the carbonates, with the highest concentration being in the upper half of the 3rd Bone Spring Lime

3. The presence of the H2S in the majority of cuttings samples analyzed in this section correlates linearly to the presence of sulfate measured by RVS; this does strongly suggest a mechanism of local generation using the sulfate as a feedstock-this relationship most frequently breaks down in the Bone Spring sands and the presence of sulfate may not correlate to TDS as there are existing brine measurements from across North America which would suggest on the whole a potentially oppositional relationship between sulfate and TDS. Temperature regimes are thought to preclude thermochemical sulfate reduction and on balance other volatiles relationships suggest the process may be from biological sulfate reduction.

4. Past experience has demonstrated a strong oppositional relationship between the presence of iron in the sands that producing laterals are landed in and produced H2S; this is supported and augmented by literature that iron is frequently more present in sand than carbonate, potentially vice versa with sulfate, and that carbonates typically host more H2S than sands

5. Combining points 2-4, it appears that produced H2S comes not from the landing zones in the sands, but is likely locally migrated from stimulation fractures that access the carbonates; the dependency on the iron in the sand laterals despite the lack of H2S detected in the sands suggests that the locally available iron may be acting as an in place H2S scavenger during production given the very thermodynamically favorable process of reacting H2S with iron to form pyrite and is likely why the relationship between sulfate and H2S breaks down commonly in the sands

More recently RVS was applied to the unpreserved cuttings samples from New Mexico Tech’s CarbonSAFE Strat Test 1 well drilled in 2022 and analyzed in late 2024. While there are gaps in cuttings coverage, the shallow section of the well where cuttings were available, the Upper Mancos through to the base of the Dakota, contains multiple depths with notable H2S response, particularly in the Upper Mancos, Gallup, and upper half of the Lower Mancos (there is virtually no cuttings coverage of the Dakota). Interestingly, in this case neither of the previously described mechanisms appear to be at play. There is no significant correlation between the presence of H2S and sulfate or other sulfides – the only meaningful correlation appears to be with CO2. Additionally, meaningful quantities of mercaptans (for example methanethiol) are detected which is unique compared to the Delaware basin case. These observations may be indicative of a very different reaction mechanism in the production of the H2S in the San Juan basin than those described in the Delaware basin.