www.saltworkconsultants.com Salty MattersJohn Warren - Wed., November 30, 2016 Gases in Evaporites: Part 2 of 3: Nature, distribution and sources This, the second of three articles on gases held within salt de- older and younger sets of gas analyses conducted over the years posits, focuses on the types of gases found in salt and their ori- in various salt deposits are not necessarily directly comparable. gins. The first article (Salty Matters October 31, 2016) dealt with Raman micro-spectroscopy is a modern, non-destructive meth- the impacts of intersecting gassy salt pockets during mining or od for investigating the unique content of a single inclusion in drilling operations. The third will discuss the distribution of the a salt crystal. There is a significant difference in terms of what is various gases with respect to broad patterns of salt mass shape measured in analysing gas content seeping from a fissure in a salt and structure (bedded, halokinetic and fractured) mass or if comparisons are made with conventional wet-chem- ical methods which were the pre Raman-microscopy method What’s the gas? that is sometimes still used. Wet chemical methods require sam- Gases held in evaporites are typically mixtures of varying propor- ple destruction, via crushing and subsequent dissolution, prior to tions of nitrogen, methane, carbon dioxide, hydrogen, hydrogen analysis. This can lead to the escape of a variable proportion of sulphide, as well as brines and minor amounts of Units dominated by Units dominated by Units with more or other gases such as argon inclusions from the N-O inclusions from less equal numbers CH -H -H S groups of both groups and various short chain and N2 groups 4 2 2 hydrocarbons (Table 2). CH A There is no single dom- 4 inant gas stored in salt across all evaporite de- N2-CH4 posits, although a particu- lar gas type may dominate N -CH -H S or be more common in a 2 4 2 particular region. For ex- N -CH -H ample, CO2 is common- 2 4 2 place in the Zechstein salts of the Wessen region N -CH -H -H S of Germany (Knipping, 2 4 2 2 1989), methane is com- mon in a number of salt N2-O2-CH4 dome mines in central Germany and the Five-Is- N -O lands region in Louisiana, 2 2 USA (Kupfer, 1990), ni- trogen is dominant in N 2 A B C other salt mines in Ger- many and New Mexico, 0 20 40 60 80 100 0 20 40 60 80 100 0 20 40 60 80 100 while hydrogen can occur Relative distribution (%) of gaseous components measured in individual stratigraphic layers in elevated proportions in z3BT (colored salt) z3AM (anhydrite/salt) z3OS/ssure (orange salt) the Verkhnekamskoe salt z3BK/BD (banded salt) z3HA (main anhydrite) z3BS/ssure (base rock salt) deposits of the Ural fore- z3OS (orange salt) z2SF (Stassfurt - potash salt) deep (Savchenko, 1958). z3LS (lineated salt) z2HS (main rock salt) Before considering the distribution of the various Figure 1. The relative distribution % of gaseous components in inclusions in salt as measured in indi- gases, we should note that vidual stratigraphic layers (replotted from Siemann and Elendor, 2001). Page 1 www.saltworkconsultants.com the volatile compounds during the crush stage, such as methane, hydrogen, ethane and aromatic hydrocarbons, especially of those components held in fissures and more open intercrystalline posi- tions. Any wet chemical technique gives values that represent the average of all the inclusion residues and intercrystalline gases left in the studied sample, post preparation. In contrast, Raman Mi- Gases crospectroscopy indicates content and proportion within a single N inclusion in a salt crystal. So, free gas results and wet chemical 2 CH compositions, when compared to Raman microscopy determi- 4 C H nations from inclusions, are not necessarily directly comparable. 2 6 C3H8 With this limitation in mind, let us now look at major gas phases i-C H occluded in salt. 4 10 n-C4H10 i-C5H12 Nitrogen n-C5H Gassy accumulations in salt with elevated levels of N2 occur in n-C6H14 many salt basins in regions not influenced by magmatic intru- sions (Table 1). In an interesting study of spectroscopic gases held in inclusions in the Zechstein salt of Germany, Siemann and Elendorff (2001) document a bipartite distribution of in- clusion gases. With rare exceptions, the first group, made up of N and N -O inclusions reveals N /O ratios close to that of Figure 2. Composite of free gas compositions in Sylvinite Layer IV of 2 2 2 2 2 Potash Horizon 3 in the Krasnoslobodsky Mine (Permian salt, Urals, after modern atmosphere, which they interpret as indicating trapped Andreyko et al., 2013) paleoatmosphere (Figure 1). Similar conclusions are reached in earlier studies of nitrogen gas held in Zechstein salts, us- gas-rich inclusions are mostly of the N -O grouping, the gases ing wet chemical techniques (Freyer and Wagener, 1975). The 2 2 from the brine-rich inclusions are mostly of the N -CH group, second group documented by Sieman and Elendoorff (2001) 2 4d emphasizing different origins for the gas-rich and brine-rich in- is represented by inclusions that contain mixtures of N , CH 2 4 clusions Siemann and Elendorff (2001) conclude that the latter occasionally H or H S. The most abundant subgroups in this 2 2 gas group is a product of thermally evolved anhydrite-rich parts second group are N -CH and N -CH -H mixtures, that is, the 2 4 2 4 2 or potash seams that have generated hydrocarbons catagenically, methane association (Figure 1). Siemann and Elendorff (2001) with these products migrating into the overlying and deforming argue that these methanogenic and hydrogenic gas mixtures of Main Rock Salt 3. the second group are the product of decomposition of organic material under anoxic subsurface conditions. They note that the Work on the free gas released during mining of the Permian methane and hydrogenic compounds, as well as some portion Starobinsky potash salt deposit in the Krasnoslobodsky Mine, of the nitrogen, are not necessarily derived from decomposing Soligorsk mining region, Russia shows that the dominant free organics held within the salt. They could have been generated by gas is nitrogen, along with a range of hydrocarbons, including degassing of underlying Early Permian (Rotliegendes) or Car- methane (Figure 2; Andreyko et al., 2013). The compositional boniferous organic-rich sedimentary rocks with subsequent en- plot is based on free gases released from the main pay horizon trapment during early stages of fluid migration, possibly driven of the Krasnoslobodsky Mine, which it the Potash Salt Horizon by Zechstein halokinesis. 3. The exploited stratigraphy is 16 to 18 m thick in the centre of the minefield and thins to 1 m thick at the edges of the ore Different origins and timings of both main nitrogen gas group- deposit. Depth to the potash horizon varies from 477 to 848 m ings in inclusions in the salt host is supported by stratigraphic below the landsurface. It consists of three units: 1) top sylvinite correlations (Siemann and Elendorff, 2001). In the stratigraphic unit, which is classified as non-commercial due to high insoluble layers which contain mainly mixtures of N and O or pure N 2 2 2 residue content; 2) mid clay–carnallite unit, which is composed inclusions of the Nv-CH -H -H S-group are rare (A in Figure 4 2 2 of alternating rock salt, clay and carnallite; and 3) bottom syl- 1) and vice versa: layers which are rich in N -4-H -H S do not 2 2 2 vinite unit, which is the main ore target and is composed of six contain many pure N -O inclusions (B in Figure 1). The major- 2 2 sylvinite layers (I-VI), alternating with rock salt bands (Figure ity of layers investigated in the salt mostly contain inclusions of 3). The distribution of gas across the stratigraphy of units I-VI the N -O group, sans methane. Only two anhydrite-rich lay- 2 2 shows that the free gas yields are consistently higher in the syl- ers of Zechstein 3 (Main Anhydrite and Anhydrite-intercalat- vinite bands (Figure 3). ed Salt) contain mainly inclusions of the second group (i.e. with abundant methane) as seen in B in Figure 1. The Zechstein Oxygen levels in salt are not studied in as much detail as the oth- 3 potash seams, as well as secondary halites, contain more or er gas phases due to their more benign nature when released in less the same population of inclusions from every main group the subsurface. Work by Freyer and Wagener (1975) focusing on (C in Figure 1). A comparison of the gas-rich inclusions and the relative proportions of oxygen to nitrogen held in Zechstein the gases in the brine-rich inclusions of the Zechstein 2 layer, salts was consistent with the inclusions retaining the same rela- Main Rock Salt 3, also shows distinct differences. Whereas, the Page 2 www.saltworkconsultants.com Well Depth (m) CO2 N2 CH4 C2H4 C2H6 C3H8 SCO H2 H2S C-like NH4Cl Oil Halite zones Siberia (average - 99.0 traces traces 1.0 + no no from 15 wells) Sylvinite zones in the vicinity of magmatic intrusions Nepa-7 927 99.0 n.d. traces 1.0 0.0 yes Nepa-13 698 99.7 0.1 0.2 0.0 no 99.5 0.4 0.1 742 99.9 0.1 no Nepa-116 937 96.8 1.7 1.5 + no 92.9 3.6 3.5 + no 97.7 1.3 1.0 + no 95.7 3.5 0.8 + no Nepa-185 1183 100.0 + no Nepa-194 839 54.4 43.4 0.9 1.3 no 843 95.1 1.9 3.0 traces traces no 23.0 6.5 89.3 0.6 0.6 0.6 + no 882 5.8 94.2 traces traces traces aliphatics 886 97.9 traces 0.5 1.6 no Nepa-111 852 89.7 8.4 1.9 + 0.0 no 91.5 6.4 2.1 + 92.8 5.6 1.6 + Sylvinite zones with no magmatic intrusions in the drilling profile Zverevsk-1 832 6.3 93.7 1.7 no 8.0 88.6 0.9 0.6 0.1 0.3 7.1 89.3 1.0 0.5 0.1 no Nepa-154 843 22.6 73.7 2.3 0.9 0.5 0.0 no 4.6 95.4 no 15.5 85.5 no Korshunovsk-3 917 98.2 1.1 0.3 0.4 aliphatics 86.3 9.3 1.5 2.9 aliphatics 74.5 11.2 3.8 10.6 aliphatics 924 96.7 3.3 0.3 94.2 5.8 0.0 Well Depth (m) CO2 N2 CH4 C2H4 C2H6 C3H8 H2 H2S C-like NH4Cl Oil Other sylvinite intervals without magmatic intrusions (minor CO2) Permian salts at Verhnekamsk 85.6 11.5 2.0 0.9 (Russia) 55.0 45.0 0.04 see also Anderyko et al., 2013 66.5 33.5 — 4.4 77.8 17.8 + — 1.8 82.1 16.1 + — traces 100 0.4 aliphatics Upper Eocene potassic salts (Spain) 2.5 74.2 23.0 0.3 + (Pironon et al., 1995) 3.9 77.0 18.8 0.3 + 87.0 13.0 0.3 Table 1.