鄂尔多斯盆地吴起地区三叠系延长组长6 —长9油层组高自然伽马值砂岩特征及成因

doi:10.11743/ogg20250519

Abstract

Abstract:

Sandstones with high natural gamma-ray （GR） values （also referred to as high-gamma sandstones） occur in the 6th to 9th oil groups of the Yanchang Formation （collectively referred to as the Chang 6‒9 oil groups） in the Wuqi area， Ordos Basin. By conventional log interpretations， these sandstones are often mistakenly identified as siltstones or even mudstones， leading to an underestimation of the effective reservoir thickness. In this study， we conduct a fine-scale lithological division by combining core observations with grain size data. Accordingly， the logging responses， petrological characteristics， physical properties， and oil-bearing properties of high-gamma sandstones in the study area are summarized， followed by an analysis and exploration of their origin. The results indicate that the high-gamma sandstones exhibit significant conventional logging responses， including high GR values， elevated sonic interval transit time， and pronounced negative spontaneous potential （SP） anomalies. In the spectral GR logs， these sandstones show the characteristics of high uranium （U） and thorium （Th） concentrations， along with low potassium （K） concentration. Compared to common sandstones， the high-gamma sandstones generally contain higher contents of feldspar， mica， and clay minerals. Despite comparable porosity， the high-gamma sandstones show slightly lower permeability and oil saturation than common sandstones. Furthermore， their oil-bearing properties tend to decrease with an increase in the concentrations of radioactive elements such as U， Th， and K. The primary factors controlling the formation of the high-gamma sandstones include volcanic activity， sedimentary environment， and clay mineral type. The debris produced by volcanic eruptions supplies abundant radioactive materials for the study area. The northeastern and southwestern parts of the study area show differences in the migration， accumulation， and preservation conditions of radioactive elements， especially U and Th， which exhibit distinct degrees of enrichment under different sedimentary environments. Clay minerals exhibit varying adsorption capacities for radioactive elements. Specifically， illite and mixed-layered illite-montmorillonite demonstrate higher adsorption capacities， whereas kaolinite and chlorite exhibit limited adsorption capacities. Fine-scale investigations of the lithological and developmental characteristics of high-gamma sandstones in the Wuqi area hold great practical significance for increasing hydrocarbon reserves and enhancing productivity in the Ordos Basin， while also providing a key geological basis and theoretical guidance for the exploration and exploitation of similar reservoirs in other basins of China.

Key words: reservoir characterization, high-gamma sandstone, Chang 6?9 oil groups, Yanchang Formation, Triassic, Ordos Basin

CLC Number:

TE122.2

Hao DIAO, Xinzhi YAN, Jingzhou ZHAO, Rong MA. Characteristics and genesis of high-gamma sandstones in the 6th to 9th oil groups of the Triassic Yanchang Formation， Wuqi area， Ordos Basin[J]. Oil & Gas Geology, 2025, 46(5): 1700-1716.

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类别	井号	深度/m	自然伽马值/API	矿物组分含量/%
类别	井号	深度/m	自然伽马值/API	石英	斜长石	钾长石	碳酸盐矿物	云母	黄铁矿	黏土矿物
常规砂岩	Ti70	2 439.8	65.7	35.4	42.7	12.4	2.0	—	—	7.5
	S5	2 142.2	75.7	42.6	35.4	9.0	0.8	—	—	12.2
	W79	2 435.5	82.3	41.5	33.5	8.7	1.2	—	—	15.1
	ZE88	2 320.8	83.2	41.7	50.2	—	0.7	—	—	7.4
	W71	2 120.2	66.2	49.4	25.6	16.0	3.9	—	—	5.1
		2 125.7	81.9	22.1	39.3	10.5	21.2	—	—	7.0
		2 126.6	69.5	27.3	23.7	7.7	32.8	—	—	8.4
		2 127.5	75.3	38.8	39.5	7.6	2.4	—	—	11.7
		2 345.2	54.6	21.5	24.4	15.2	30.0	—	—	9.0
		2 346.1	53.6	20.0	42.5	7.2	25.2	—	—	5.2
	W94	2 331.5	79.4	37.8	40.1	9.5	0.9	—	—	11.7
		2 333.4	77.5	29.9	26.1	6.7	27.5	—	0.5	9.3
		2 334.3	76.0	32.2	42.3	12.7	2.1	—	0.5	10.2
		2 337.3	75.3	36.6	40.5	7.6	3.2	—	0.6	11.5
		2 337.8	74.1	34.8	39.5	8.8	6.3	—	—	10.6
		2 338.4	56.2	38.8	39.0	9.6	0.4	—	0.5	11.7
	平均值	—	71.7	34.4	36.5	9.9	10.0	—	0.5	9.6
高伽马砂岩	Ti70	2 439.3	103.1	33.1	44.8	6.1	7.3	—	—	8.7
	T22	2 042.4	93.2	30.6	19.8	6.3	4.9	—	0.5	37.9
	ZE95	2 047.2	89.5	40.9	47.8	—	1.2	—	—	10.1
	ZE88	2 322.6	87.4	25.3	41.2	—	29.9	—	—	6.2
	ZE88	2 326.7	89.2	32.4	53.4	—	—	4.2	—	10.0
	W71	2 128.4	88.5	32.6	34.6	15.5	3.0	—	—	14.3
		2 343.3	85.8	20.6	36.9	9.5	21.3	—	—	11.8
		2 343.6	85.6	20.8	29.7	9.3	31.7	—	—	8.5
	W94	2 332.9	93.5	38.7	43.0	6.3	3.1	—	0.4	8.6
		2 335.0	92.4	35.1	38.4	7.8	7.3	—	—	11.4
		2 335.4	93.1	35.2	38.6	8.1	2.2	—	—	15.9
		2 336.6	87.0	26.0	28.6	8.2	28.8	—	—	8.5
	ZE96	2 098.4	97.6	34.7	49.9	—	2.9	1.7	—	10.8
		2 098.9	86.7	37.4	49.6	—	3.9	—	—	9.1
		2 099.8	89.3	29.7	57.3	—	4.2	—	—	8.9
		2 101.2	102.5	28.6	55.8	—	1.6	3.7	—	10.4
		2 104.9	99.4	24.0	62.5	—	2.2	7.3	—	5.1
		2 105.0	102.2	33.1	59.9	—	—	—	—	7.0
	平均值	—	92.6	31.0	44.0	8.6	9.7	4.2	0.45	11.0

岩性	井号	深度/m	物性参数			测井值
岩性	井号	深度/m	渗透率/（10^-3 µm²）	孔隙度/%	含油饱和度/%	GR/API	U含量/10^-6	Th含量/10^-6	K含量/%
常规砂岩	ZE84	2 456.42	0.11	4.90	64.78	76.89	1.97	9.71	2.02
	ZE84	2 458.05	0.08	4.06	—	84.94	1.87	10.75	2.61
	Ti15	2 323.08	0.09	7.40	73.80	84.97	2.97	11.67	2.29
	Ti27	2 095.98	1.48	15.90	58.21	81.74	3.48	8.48	1.95
		2 096.13	1.10	14.90	58.22	78.42	1.52	5.46	1.91
		2 384.69	0.31	9.40	68.62	78.24	2.14	7.04	1.65
	X2	2 086.69	0.71	6.25	69.38	67.61	2.16	5.50	2.12
		2 087.60	0.73	9.24	—	79.42	1.52	6.87	2.47
		2 088.26	0.73	5.91	79.65	75.66	2.30	6.05	2.38
		2 089.14	0.77	8.14	—	79.20	2.57	4.95	2.57
	X53	1 963.03	0.32	8.60	74.54	84.43	3.48	4.84	2.68
	L27	2 067.26	0.22	9.51	78.27	72.73	2.38	7.35	2.27
	平均值	—	0.55	8.68	69.50	78.69	2.36	7.39	2.24
高伽马砂岩	ZE84	2 459.68	0.07	5.10	63.25	87.86	3.04	8.66	2.52
	ZE84	2 460.39	0.11	4.50	57.51	94.21	2.78	7.08	2.88
	ZE86	1 974.59	0.54	13.53	57.34	90.70	3.19	10.05	2.47
		1 976.61	0.55	12.48	51.14	94.12	3.64	9.35	2.17
		1 979.44	0.89	15.32	56.33	94.08	3.74	11.71	2.06
	Ti15	2 318.22	0.42	4.40	51.73	89.45	3.17	14.52	1.84
		2 319.09	0.07	6.00	64.10	91.66	4.67	10.21	2.16
		2 323.08	0.09	7.40	73.80	85.97	2.97	11.67	2.29
		2 323.94	0.18	4.80	53.56	91.08	2.76	13.69	2.18
	Ti23	2 250.20	0.06	5.90	45.94	95.01	2.24	10.86	3.13
	Ti31	2 131.75	0.15	3.83	—	90.13	2.38	11.69	1.66
	Ti31	2 132.11	0.24	10.60	62.10	89.98	2.58	11.07	1.42
	ZE48	2 346.34	0.37	5.15	—	98.55	2.51	13.67	2.47
	Ti27	2 095.83	1.08	14.6	61.78	85.7	10.6	8.46	1.96
	Ti27	2 097.17	0.27	8.60	59.25	89.47	9.79	8.37	2.68
	ZE55	2 288.63	0.06	6.73	—	94.70	2.25	13.79	2.29
	X56	1 914.70	0.26	12.90	44.90	95.42	2.15	12.73	2.32
	X56	1 915.39	0.53	12.10	—	90.05	2.51	11.57	2.51
	W118	1 904.59	0.64	13.22	62.22	86.80	2.45	7.49	2.67
	平均值	—	0.35	8.80	57.66	91.31	3.65	10.88	2.30

类别	井号	深度/m	自然伽马值/API	黏土矿物组分相对含量/%
类别	井号	深度/m	自然伽马值/API	蒙皂石	伊利石	高岭石	绿泥石	伊/蒙混层	绿/蒙混层
常规砂岩	S5	2 142.2	75.7	—	38	7	52	3	—
	ZC129	2 126.1	82.6	—	9	10	76	5	—
	W79	2 435.5	82.3	—	31	6	50	13	—
	W91	2 345.8	71.3	—	17	4	54	25	—
高伽马砂岩	Z12	2 065.7	88.8	—	31	6	51	12	—
高伽马砂岩	T22	2 042.4	94.3	—	50	2	34	14	—

Characteristics and genesis of high-gamma sandstones in the 6th to 9th oil groups of the Triassic Yanchang Formation， Wuqi area， Ordos Basin

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