Geochemical Investigation and Prospective Evaluation of Natural Gas Hydrates in the Permafrost of Mohe Basin

doi:10.19657/j.geoscience.1000-8527.2018.05.13

Abstract

Abstract:

The Mohe basin is a major permafrost region in China.Gas hydrate reservoir system is well deve-loped in the basin, and the basin shows good accumulation conditions for gas hydrates. The 1∶50k scale regional gas hydrate geochemical survey was conducted in the Mohe basin to delineate prospecting gas hydrate target areas, and to recognize oil and gas traps and identify the hydrocarbon source. Our results show that: (1) In the forest bog region, headspace gas and fluorescence spectra are the main indices of gas hydrate exploration. Considering the AMT and geology materials, and the previous gas hydrate geochemical work in the Muli permafrost of Qinghai, we consider the Yuanbao depression as the gas hydrate target. (2) Methane carbon isotope analysis on core samples shows that the primary hydrocarbon gas is multi-genetic, and only a few adsorbed gases are from bacterial and hydrocarbon pyrolysis. (3) Multiple classification-GIS technique employed in this study can exclude the interference factors and precisely delineate the boundary of geochemical anomalies, and can thus be applied in the geochemical exploration of gas hydrates.

Key words: natural gas hydrate, geochemical investigation, permafrost area, Mohe basin, prospective evaluation

CLC Number:

P618.13

ZHANG Fugui, TANG Ruiling, YANG Zhibin, ZHU Jinghua, ZHOU Yalong, SUN Zhongjun. Geochemical Investigation and Prospective Evaluation of Natural Gas Hydrates in the Permafrost of Mohe Basin[J]. Geoscience, 2018, 32(05): 1003-1011.

Figures/Tables 9

Fig.1 Tectonic unit division of the Mohe basin and location of the study area

Table 1 Eigen values of soil geochemical indicators

指标	最大值	最小值	平均值	标准离差	变化系数
顶空气CH₄	644.50	2.80	12.48	14.95	1.20
顶空气C₂H₆	621.27	0.01	2.22	19.54	8.78
顶空气C₃H₈	405.02	0.03	2.26	12.51	5.54
顶空气iC₄H₁₀	11.24	0.01	0.09	0.39	4.36
顶空气nC₄H₁₀	172.68	0.02	1.02	5.42	5.29
顶空气iC₅H₁₂	11.11	0.02	0.46	0.56	1.22
顶空气nC₅H₁₂	80.09	0.01	3.85	5.22	1.36
酸解烃CH₄	17.80	1.10	3.84	1.26	0.33
酸解烃C₂H₆	0.81	0	0.05	0.05	0.94
酸解烃C₃H₈	0.44	0	0.04	0.03	0.77
酸解烃C₂₊	1 251.04	0.34	20.89	41.45	1.99
荧光光谱(320 nm)	211.60	0.82	10.42	8.20	0.79
荧光光谱(360 nm)	367.50	1.15	12.68	14.07	1.11
荧光光谱(405 nm)	573.48	0.35	6.78	15.05	2.22

Fig.2 Geochemical anomaly map of headspace methane in the Mohe basin

Fig.3 Geochemical anomaly map of headspace heavy hydrocarbons in the Mohe basin

Fig.4 Geochemical anomaly map of fluorescence spectrum in the Mohe basin

Fig.5 Discrimination diagram of desorbed gas components and carbon isotopes from Well MK-2 of the Mohe Basin

Fig.6 Fractal characteristics of geochemical indicators in the soil of the Mohe basin

Table 2 Fractal statistics of geochemical indicators in the soil of the Mohe basin

指标	分形层次性(n)	分维数(D_i)	含量区间	样品数	$n × D i$	∑n×D_i	结构能系数(e)
w(CH₄)	1	0.416	0.447~0.950	728	0.416	12.624	3.156
	2	3.452	0.95~1.544	1 755	6.904
	3	1.768	1.544~2.300	18	5.304
w(C₂₊)	1	0.099	-0.471~0.880	489	0.099	14.153	3.538
	2	0.995	0.881~1.400	1 401	1.990
	3	2.864	1.400~2.045	601	8.592
	4	0.868	2.045~3.097	10	3.472
F320	1	0.212	0.312~0.750	387	0.212	10.325	2.581
	2	2.631	0.751~1.510	2 088	5.262
	3	1.617	1.511~2.325	26	4.851
F360	1	0.162	0.061~0.780	381	0.162	4.388	1.097
F360	2	2.113	0.781~2.565	2 120	4.226	4.388	1.097

Table 2 Fractal statistics of geochemical indicators in the soil of the Mohe basin

指标	分形层次性(n)	分维数(D_i)	含量区间	样品数	$n × D i$	∑n×D_i	结构能系数(e)
w(CH₄)	1	0.416	0.447~0.950	728	0.416	12.624	3.156
	2	3.452	0.95~1.544	1 755	6.904
	3	1.768	1.544~2.300	18	5.304
w(C₂₊)	1	0.099	-0.471~0.880	489	0.099	14.153	3.538
	2	0.995	0.881~1.400	1 401	1.990
	3	2.864	1.400~2.045	601	8.592
	4	0.868	2.045~3.097	10	3.472
F320	1	0.212	0.312~0.750	387	0.212	10.325	2.581
	2	2.631	0.751~1.510	2 088	5.262
	3	1.617	1.511~2.325	26	4.851
F360	1	0.162	0.061~0.780	381	0.162	4.388	1.097
F360	2	2.113	0.781~2.565	2 120	4.226	4.388	1.097

Fig.7 Prospective areas for hydrates forecasted by geochemical fractal-GIS in the Mohe basin

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