Origins of High H2S-bearing Natural Gas in China

Vo.79No.5pp.697-708ACTA GEOLOGICA SINICAoct.2005Origins of High H2s-bearing Natural Gas in ChinaZHU Guangyou, ZHANG Shuichang, LIANG Yingbo, DAI Jinxing and Li JianResearch institute of Petroleum Exploration and Development, China National petroleumorporation,Beijing100083;E-mail:zhuguangyou@petrochina.com.cnAbstract Natural gas containing hydrogen sulphide(H2S)has been found in several petroliferous basins in China, suchas the Sichuan Basin, Bohai Bay Basin, Ordos Basin, Tarim Basin, etc. Natural gas with higher H2s contents(H2S >5%mol )is mostly distributed in both the gas reservoirs of Dukouhe, Luojiazhai, Puguang and Tieshanpo, which belong to theTriassic Feixianguan Formation in the northeastern Sichuan Basin and those of the Kongdian-Shahejie formations in thenortheastern Jinxian Sag of the Jizhong Depression, Bohai Bay Basin. In the Sichuan Basin, the H]S contents of naturalaverage over 9% and some can be 17 % while those of the Bohai Bay Basin range from 40 to 92 %, being then one ofthe gas reservoirs with the highest H]S contents in the world. Based on detailed observation and sample analysis results ofa total 5000 m of core from over 70 wells in the above-mentioned two basins, especially sulfur isotopic analysis of gypsumbrimstone, pyrite and natural gas, also with integrated study of the geochemical characteristics of hydrocarbons, it isthought that the natural gas with high H2S contents resulted from thermochemical sulfate reduction (TSR) reactions.Among them, the natural gas in the Feixianguan Formation resulted from TSR reactions participated by hydrocarbon gas,while that in the Zhaolanzhuang of the Jinxian Sag being the product of Tsr participated by crude oil. During theonsumption process of hydrocarbons due to TSR, the heavy hydrocarbons were apt to react with sulfate, whichaccordingly resulted in the dry coefficient of natural gas increasing and the carbon isotopes becoming heavierKey words: natural gas, hydrogen sulphide, gypsum, thermochemical sulfate reduction (TSR), northeastern SichuanBasin, Zhaolanzhuang1 Introduction(1999)separately conducted earlier studies on geologicalaspects of H]s and carried out pertinent research as well asAs one of the hazardous components of natural gas, the experimental analysis on its formation, reactionexistence of H2s not only decreases the percentage of temperature, reaction condition, reaction kinetics andhydrocarbon gas within the natural gas and thus its genetic features, etc. Many scholars thought that the highlyindustrial value, but also threatens each aspect of drilling concentrated H2s gas within petroliferous basins mainlyand exploitation. In addition, H2S may cause serious originated from either biogenic agency or thermochemicalaccidents because of its strong toxicity and causticity that reduction(Machel et al., 1995)). Among these, the h2saffects safety, which accordingly leads to higher resulting from biogenic sulfate reduction (BSr)wasexploration and production costs and the risk of natural gas generated by the dissimilation and reducing metabolism ofwith a higher H2S content. The contents of H2s within sulfate by sulfate-reducing bacteria. The reaction formula isnatural gas can be from 0 to 99%. Usually, natural gas with as followsits HS content being greater than 5% can be specified as Organic Matter +CaSO4 sulfate reducing bactena action,natural gas with a high H]S content(Dai, 1985 )and, withinCaCO3+h2S+H,Ocarbonate reservoirs, the H2s contents are comparativelyMoreover, the thermochemical origin of H2s mainlyhigher. Because the natural gas containing H2s is the refers to the generation of H2s due to the thermochemicalimportant part of the natural gas resources, and that the reduction of sulfate, i. e, the sulfate reacts with the organicnatural gas with high H2S content also being one of the key matter or hydrocarbons, and deoxidizes the sulfate mineralssources of sulfur, to a certain extent, the study of H2s has into H中国煤化工 The reaction formuldeoxidized while thebeen emphasized globally. Thereunto, scholars, such as Orr gase(1974), Stark and Wallace(1984), Ohmoto and Felder is asCNMHG( 1987), Anderson and Garven(1987), Krouse et al. ( 1988), Hydrocarbons+ CaSO4- CacO3+ H2s+Sassen(1988), Schoonen and Barnes(1991), Goldhaber CO+ S+ H20and Orr(1995), Machel et al. (1995), worden et al. ( 1995,Due to the actions of BsR and TSR, which both belong1996), Vester and Ingvorsen(1998)and Saunders et al. to the sulfate reduction of hydrocarbons and can both698Origins of High HS-bearing Natural Gas in Chinagenerate sulfide and bitumen, and as well as their northeastern Sichuan Basin, while their H2s content variestemperature scopes overlap, meanwhile the sulfate separately from 40% to 92% and 9% to 17%(Zhu et al(gypsum) must exist within the stratum, therefore, their 2004a). In particular, the H2s content within the Zhao-2origin types are not easy to distinguish. Earlier, scholars well, drilled in the Zhaolanzhuang gas reservoir, which isthought that TSR reactions normally occur within a high- one of the gas reservoirs in the world with maximum H2stemperature diagenetic environment(80-1000C<<<150- content, can be 92%. Based on previous studies, the H2S180 C, with the temperature for BSR reactions being lower accumulated within the above-mentioned two areas was(0CC2H6, the sulfur can produce CO2 and H2s normal paraffin hydrocarbons. Especially, the contents ofthrough further reaction with the hydrocarbons. Therefore, those with lower carbon numbers are much lower and itthe CO2 is generated as a consequence of the reactions seemed that they had been subjected to biologicalparticipated in by sulfurdegradation. Neverthelof biologicwithin the continental clastic strata, a high concentration degradation nor spectrum peaks of norhopane, etc, haveof H]S gas can also be accumulated, and the residual H2s been observed on a GC-MS spectrogram, which indicatescan be preserved after the heavy metal ions have been that the heavy crude did not result from biologicaldepleted by H2sdegradation, and also, just proves that it is easier for normalThe formation of natural gas with high H2s content paraffin hydrocarbons that have lower carborexhibits a close relationship with the distribution of participate in TSRgypsolith within strata. The gypsolith is a-reactant of TSR,In addition, it has also been discovered that, in theand at the same time, its tight physical properties provide Zhaolanzhuang area, the carbon isotope of saturatedsuperior sealing conditions for the preservation of natural hydrocarbons among higher H2S-bearing crude was.8gas with high H2S content(o)PDB, while that of crude, having the same source andIn addition, the natural gas bearing higher H2s usuallymaturity but not participating in TSR reactions, is-29.2%0experienced a higher burial temperature. Within this It can be calculated that the weight of carbon isotope hasprocess, along with the evolution of hydrocarbons, the increased by 1.4%c, and is the key identification symbol forsulfur-bearing organic matter also was cracked while a TSR, i. e, during the process of TSR, due to the bondcertain amount of H2S was generated. Therefore, the Hs energy of lc being lower than that of C, the Cwithin such natural gas was inevitably mixed with a little preferentially participated in TSR reactions and mostly wasH2S resulting from the thermal cracking of sulfur-bearing consumed, which led to the carbon isotopes of theorganic matter, although its proportion can be rather low. remaining hydrocarbons becoming heavier. The aboveFrom this point of view, there exists a certain relationship analysis further proves our conclusion that in thebetween H2S and the source rocks. In particular, in the Zhaolanzhuang area, the crude did participate in TSRZhaolanzhuang area, the source rocks were deposited in reactions. By this token, the evidence for a TSR origin ofsaline and reducing settings with higher sulfur content natural gas bearing higher H2S in the Zhaolanzhuang area isamong the organic matter. During the thermal evolution even more convincing.period of organic matter with rich sulfur, only a certainamount of H2S can possibly be generated. However, Acknowledgementscause its amount is rather low, it cannot have supplied theconsumption by the heavy metal ions located within the The research was jointly funded by projects of"Thereservoirs.Therefore, most of the H2S within the higher Reso中国煤化工 pment Strategy of theH2S-bearing natural gas has no direct relationship with the GasCNMHG in China(040502-10-source rocks. The majority of H]S was generated within01)”la anu uic ivauunal Key Basic Researchreservoirs by TSR reactions between hydrocarbons andand Development Planning Project(2001CB209100). 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