鄂尔多斯盆地三叠系延长组长8-长9油层组泥页岩储层孔隙和微裂缝发育特征与页岩油勘探前景

doi:10.11743/ogg20250511

Abstract

Abstract:

Shales are well-developed in the 8th to 9th oil groups of the Triassic Yanchang Formation （also referred to as the Chang 8‒9 oil groups） in the Ordos Basin. However， their mineral composition and pore and microfracture characteristics， as well as the impacts of these factors on shale oil enrichment and mobility， remain insufficiently understood. In this study， we comprehensively investigate the shale reservoir characteristics and their controlling effects on shale oil in the Chang 8‒9 oil groups using data from a series of analyses and tests， including X-ray diffraction （XRD）， field emission scanning electron microscopy （FE-SEM）， CO₂ adsorption， low-temperature N₂ adsorption， high-pressure mercury injection （HPMI）， and geochemical analyses. The results indicate that the shales in the two oil groups exhibit three lithofacies： felsic clayey shales， clayey felsic shales， and clayey-felsic mixed shales. The Chang 8 oil group is dominated by felsic clayey shales and clayey felsic shales， while all the three lithofacies types are relatively well-developed in the Chang 9 oil group. A comprehensive assessment of pore and microfracture characteristics， fracability， and shale oil mobility reveals that clayey felsic shales are the most favorable lithofacies， followed sequentially by clayey-felsic mixed shales and felsic clayey shales. The shale reservoirs in the two oil groups contain organic pores， intercrystalline pores， intergranular pores， dissolution pores， and microfractures. There are distinct positive correlations between total organic carbon （TOC） content and the volumes of micropores and mesopores. Compared to the Chang 7 oil group， organic pores are less developed in the Chang 8‒9 oil groups. Microfractures are well-developed in both oil groups， primarily including bedding-parallel fractures induced by abnormally high-pressure from hydrocarbon generation， followed by pressure dissolution-induced bedding-parallel fractures and shrinkage fractures in organic matter. Key factors contributing to the development of pores and microfractures in the study area include high TOC content， high organic matter maturity， and high brittle mineral content. Specifically， high TOC content and high organic matter maturity promote the development of organic pores and the formation of bedding-parallel fractures induced by abnormally high-pressure from hydrocarbon generation. In contrast， carbonate cementation inhibits pore development and fills fractures with cements. A high clay mineral content favors micropore development but adversely affects mesopore and macropore growth. Primary factors controlling movable oil enrichment include TOC content （< 4%）， organic matter maturity （vitrinite reflectance （R_o） > 1.2%）， brittle mineral content （> 60%）， and fracture density （> 1.6 × 10⁴/m）. As the assessment criteria for shale oil sweet spots in the study area shown， type Ⅰ sweet spots should have oil saturation index （OSI） values of greater than 100 mg/g， possess the highest shale oil mobility and are primarily distributed in the Fuxian， Ganquan， and eastern Zhidan areas. These areas represent the most favorable shale oil enrichment regions， holding promising exploration prospects. In contrast， type Ⅱ sweet spots are characterized by OSI values ranging from 70 mg/g to 100 mg/g， and exhibit a moderate mobility， while type Ⅲ sweet spots， with OSI values below 70 mg/g， show the lowest mobility. The type Ⅲ shale oil predominates across the study area.

Key words: mobility, pore, fracture, reservoir features, shale oil, Chang 8 oil group, Chang 9 oil group, Ordos Basin

CLC Number:

TE122.2

Jiaqi ZHANG, Jingzhou ZHAO, lei CAO, Qingyuan YE. Pore and microfracture characteristics and shale oil exploration prospects of shale reservoirs in the 8th to 9th oil groups of the Triassic Yanchang Formation， Ordos Basin[J]. Oil & Gas Geology, 2025, 46(5): 1554-1581.

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地区	TOC/%				R_o/%
地区	最大值	最小值	平均值	样品数/个	最大值	最小值	平均值	样品数/个
吴起	5.16	0.38	3.13	6	1.08	0.89	0.96	4
志丹	4.96	0.96	3.28	8	1.32	1.28	1.30	5
甘泉	7.09	0.95	4.28	6	1.56	0.74	1.13	5
富县	8.75	0.51	4.60	33	1.29	1.49	1.40	9

地区	样品数/个	矿物组分平均含量/%
地区	样品数/个	石英	钾长石	钠长石	碳酸盐矿物	黄铁矿	黏土矿物	伊利石	高岭石	绿泥石	伊/蒙混层
吴起	6	28.6	5.1	15.7	3.3	1.5	45.5	35.8	14.8	19.4	30.0
志丹	5	31.3	3.8	10.2	4.0	0.8	50.0	31.4	7.0	20.2	41.4
甘泉	6	26.1	6.2	12.1	4.6	2.0	50.0	47.7	5.9	15.5	30.9
富县	22	25.2	6.4	11.8	6.4	2.9	47.2	64.0	1.7	11.8	22.5

层位	岩相	脆性指数/%			黏土矿物含量/%	样品数/个
层位	岩相	最大值	最小值	平均值	黏土矿物含量/%	样品数/个
长8油层组	长英质黏土页岩	48.6	38.9	45.2	52.7	8
长8油层组	黏土质长英页岩	65.8	62.7	64.5	34.1	3
长9油层组	长英质黏土页岩	65.3	46.5	57.0	53.9	8
	黏土质长英页岩	85.0	65.2	75.1	40.8	6
	黏土-长英混合页岩	77.1	62.4	69.2	44.7	7

样品编号	层位	孔隙类型	孔隙度/%	面孔率/%	总面孔率/%	配位数
B-9	长9油层组	有机质孔	0.100	80.70	0.12	0.04
		无机质孔	0.020	18.70
		黄铁矿晶间孔	0.001	0.60
6-13	长9油层组	有机质孔	0.400	36.80	1.11	0.13
		无机质孔	0.700	62.50
		黄铁矿晶间孔	0.010	0.70
6-3	长8油层组	有机质孔	0.200	58.10	0.31	0.03
		无机质孔	0.100	41.80
		黄铁矿晶间孔	—	0.08
2-11	长8油层组	有机质孔	0.400	98.90	0.48	0.16
		无机质孔	0.080	1.10
		黄铁矿晶间孔	—	0.02

样品编号	地区	层位	埋深/m	孔隙度/%	渗透率/（10^-3 μm²）	排驱压力/MPa	中值压力/MPa	最大汞饱和度/%	最大孔喉半径/μm	中值孔喉半径/μm	平均孔喉半径/μm	退汞效率/%	曲线类型
7-4	吴起	长8油层组	2 145	0.5	0.003 0	13.8	13.8	86.7	0.05	0.007	0.011	71.5	Ⅰ类
B-8	富县	长9油层组	1 482	1.0	0.000 3	13.8	13.8	90.9	0.05	0.007	0.011	80.6	Ⅰ类
B-10	富县	长9油层组	1 484	1.3	0.010 2	8.3	114.6	89.9	0.09	0.006	0.013	81.6	Ⅰ类
5-1	志丹	长9油层组	2 011	2.2	0.001 2	23.5	—	44.0	0.05	—	0.100	79.4	Ⅱ类
5-3	志丹	长9油层组	1 801	2.3	0.001 1	20.6	—	45.0	0.04	—	0.009	51.0	Ⅱ类

Pore and microfracture characteristics and shale oil exploration prospects of shale reservoirs in the 8th to 9th oil groups of the Triassic Yanchang Formation， Ordos Basin

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Metrics

样品编号	孔体积/（mL/g）			孔隙比表面积/（m²/g）
样品编号	介孔	宏孔	总孔	介孔	宏孔	总孔
B-8	0.003 2	0.000 6	0.003 8	0.380 0	0.030 0	0.450 0
B-10	0.007 7	0.001 3	0.009 0	0.385 0	0.068 0	0.453 0
7-4	0.001 4	0.000 3	0.001 7	0.382 0	0.070 0	0.452 0
5-1	0.003 2	0.000 8	0.004 0	0.720 0	0.090 0	0.810 0
5-3	0.003 8	0.000 8	0.004 5	0.498 0	0.022 0	0.520 0

甜点类型	Ⅰ类	Ⅱ类	Ⅲ类
石英+长石含量/%	> 55	50 ~ 55	< 50
黏土矿物含量/%	< 40	40 ~ 45	> 45
缝密度/（10⁴条/m）	> 2.4	1.6 ~ 2.4	< 1.6
孔隙度/%	> 4.5	3.0 ~ 4.5	< 3.0
OSI/mg/g	> 100	70 ~ 100	< 70
TOC/%	< 3	3 ~ 4	> 4
S₁/（mg/g）	> 4	3 ~ 4	< 3
R_o/%	> 1.4	1.4 ~ 1.2	< 1.2
T_max/℃	> 460	460 ~ 455	< 455
岩相	黏土质长英页岩	长英质黏土页岩、黏土-长英混合页岩	长英质黏土页岩、黏土-长英混合页岩

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层位	缝密度/（条/m）	平均缝长/μm	缝宽/μm	样品数/个
长8油层组	14 808	239	0.88	14
长9油层组	11 889	238	0.60	15