Tectonic Controls on Near-Surface Variations in CH₄ and CO₂ Concentrations along the Northwestern Margin of the Ordos Block, China

Table 1

CH₄ and CO₂ concentrations at the surface (0 m) and a height of 1.5 m in different tectonic settings around the western margin of the Ordos Block in China.

Block (area)	No.	Location		CH4 (ppb)				CO2 (ppm)				Maximum shear strain (10^-9/a)	Tectonic setting
		Longitude (°E)	Latitude (°N)	1.5 m	Mean	0 m	Mean	1.5 m	Mean	0 m	Mean
Minqin Block (I)	1	105.73	39.87	1905	1908	1909	1911	407.6^b	406.0	406.5	406.0	[34]	Relatively stable
	2	105.40	39.56	1907		1909		404.8		405.6
	3	105.70	39.35	1912		1914		405.5		406.0
Ordos Block (II)	1	106.89	38.34	1969	1955	1973	1958	415.5	411.1	415.9	412.4	[34]	Relatively stable
	2	107.44	38.17	1965		1969		413.6		415.6
	3	107.66	38.55	1996		1997		412.4		413.3
	4	108.04	39.08	1916		1919		405.7		407.8
	5	107.50	39.25	1931		1931		408.6		409.1
Hetao Graben (III)	1	107.59	40.70	1932	1927	1934	1931	409.5	408.4	409.1	409.8		Extensional
	2	107.92	40.77	1936		1946		409.5		410.4
	3	108.36	40.75	1926		1927		410.0		410.0
	4	108.75	40.59	1927		1929		409.4		410.3
	6	108.37	41.06	1928		1928		410.0		410.0
	5	107.59	41.06	1910		1926		402.1		409.3
Southern part of Yinchuan Graben (IV)		106.18	38.11	2054		2068		427.1		431.0			Extensional
Northwestern part of Yinchuan Graben (line 1+line 2)^a (IV)	L1-1	106.03	38.50	2021	1964	2028	1982	403.8	405.7	405.9	407.3		Extensional
	L1-2	106.05	38.60	1947		2054		400.7		409.2
	L1-3	106.09	38.66	1951		1956		406.6		405.8
	L1-4	106.14	38.74	1960		1959		408.6		408.5
	L1-5	106.19	38.83	1992		1993		409.6		410.6			Extensional
	L1-6	106.25	38.90	1973		1975		409.4		410.3
	L1-7	106.32	39.00	1909		1912		401.3		400.5
	L2-7	106.44	39.00	1981	1969	1966	1966	404.6	412.8	405.9	413.5		Extensional
	L2-6	106.38	38.92	1945		1948		410.0		410.6
	L2-5	106.34	38.83	1937		1939		411.7		412.1
	L2-4	106.27	38.74	1975		1978		411.8		411.8
	L2-3	106.22	38.66	1927		1927		410.5		412.0
	L2-2	106.18	38.59	1930		1942		412.1		412.3
	L2-1	106.18	38.50	2087		2062		429.0		430.1
Middle-northern part of Yinchuan Graben (line 3+line 4) (IV)	L3-1	106.25	38.51	1971	2204	1965	2199	413.8	438.6	415.9	440.4		Extensional
	L3-2	106.29	38.58	2091		2079		427.4		432.6
	L3-3	106.34	38.67	2466		2472		454.8		456.9
	L3-4	106.36	38.73	2354		2346		439.9		440.1
	L3-5	106.42	38.84	2260		2255		454.4		458.5
	L3-6	106.51	38.93	2161		2153		446.9		446.1
	L3-7	106.59	39.00	2125		2125		433.2		433.1
	L4-7	106.69	39.00	2015	2050	2014	2043	423.7	420.3	424.1	421.1		Extensional
	L4-6	106.64	38.92	1983		1981		418.8		419.0
	L4-5	106.57	38.83	2008		2006		421.2		419.9
	L4-4	106.50	38.75	2071		2066		420.6		420.0
	L4-3	106.45	38.67	2056		2054		416.7		414.7
	L4-2	106.39	38.58	2033		1999		406.7		409.3
	L4-1	106.32	38.50	2184		2179		434.7		440.8
Northeastern part of Yinchuan Basin (line 5+line 6) (IV)	L5-1	106.41	38.51	2029	2183	2002	2174	409.4	420.7	405.7	421.7		Extensional
	L5-2	106.48	38.58	2240		2274		425.0		434.2
	L5-3	106.55	38.68	2244		2249		411.7		411.7
	L5-4	106.60	38.77	2260		2190		439.7		432.7
	L5-5	106.65	38.83	2159		2157		404.8		404.9
	L5-6	106.75	38.91	2108		2090		407.5		407.8
	L5-7	106.84	39.08	2240		2255		447.0		454.6
	L6-7	106.93	39.03	2244	2087	2243	2087	452.9	422.4	456.6	423.2		Extensional
	L6-6	106.85	38.91	2208		2219		440.2		444.1
	L6-5	106.77	38.84	2092		2104		430.2		432.7
	L6-4	106.72	38.77	1971		1960		403.9		397.8
	L6-3	106.62	38.68	2006		2006		399.7		398.1
	L6-2	106.59	38.58	2051		2034		411.0		413.2
	L6-1	106.56	38.49	2040		2042		419.1		419.5
Haiyuan Block (V)	1	105.69	36.54	1996	1987	1990	1976	404.0	408.4	404.2	409.5	[34]	Extrusion; High deformation
	2	105.89	36.31	2027		1978		426.6		430.7
	3	105.55	36.27	1971		1968		397.5		398.2
	4	105.74	36.78	1986		1990		408.8		408.9
	5	105.42	36.83	1955		1952		405.1		405.6

^aIn the northwestern part of the Yinchuan Graben, the sampling line containing samples line 1-line 6 (abbr. L1-L6) is oriented from west to east. The sampling line containing samples 1-7 is oriented from south to north. ^bConcentrations that are higher at 1.5 m than at 0 m are marked in bold.