Characteristics and genesis of fractures in Middle Ordovician Majiagou Formation, Daniudi Gas Field, Ordos Basin
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摘要: 鄂尔多斯盆地大牛地气田天然气潜力巨大,但中奥陶统马家沟组风化壳岩溶储层发育多种裂缝,成因复杂。为探明该储层裂缝特征,应用岩石学和测井技术,从裂缝发育频率、有效性、期次、赋存层位、形态、倾角等方面,比较了陡坎成因裂缝与构造裂缝的差异性;并通过弹性理论和断裂力学理论分析了裂缝端部区域的应力分量,探讨了陡坎裂缝的成因。陡坎成因裂缝以垂直缝、高角度斜交缝为主;裂缝延展方向和裂缝形态不规则,裂缝面不平整,具有上宽下窄、尾端无分叉特征;缝间有溶蚀现象,裂缝呈未—半充填状态,裂缝有效性高。陡坎成因裂缝受控于古地貌陡坡地形的坡度、岩层倾角、初始裂缝密度及岩石力学性质等因素;与构造裂缝相比,陡坎成因裂缝更有利于油气的运移。研究陡坎成因裂缝特征及成因,有助于进一步认识风化壳岩溶储层的展布规律,落实大牛地气田下一步勘探开发有利区。Abstract: The Daniudi Gas Field in the Ordos Basin has great natural gas potential, but a variety of fractures with complex origins was developed in the weathering crust karst reservoirs in the Middle Ordovician Majiagou Formation. In order to study the characteristics of fractures in the reservoirs, petrology and logging techniques were used to compare the differences between the fractures caused by steep ridges and structural fractures in terms of fracture development frequency, effectiveness, period, occurrence layer, shape, dip angle, etc. With elastic theory and fracture mechanics theory, the stress component at the end of fractures was analyzed, and the cause of the steep ridge fractures was discussed. Results show that the fractures caused by steep ridges are mainly vertical fractures and high-angle fractures. The fracture extension direction and fracture shape are irregular, and the fracture surface is uneven, with the characteristics of wide top and narrow bottom, and no bifurcation at the tail end. There is dissolution between the fractures, which is in a non- to semi-filled state, and the effectiveness of the fractures is high. The steep ridge fractures were controlled by factors such as ancient steep slope angle, rock layer inclination, initial fracture density and rock mechanical properties. Compared to structural fractures, the steep ridge fractures are more favorable for hydrocarbon migration. Studying the characteristics and causes of steep ridge fractures will help to further understand the distribution of weathered crust karst reservoirs, and identify favorable areas for the next exploration and development of the Daniudi Gas Field.
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Key words:
- steep ridge fracture /
- fracture origin /
- Majiagou Formation /
- Middle Ordovician /
- Daniudi Gas Field /
- Ordos Basin
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图 1 鄂尔多斯盆地大牛地气田中奥陶统马家沟组构造裂缝特征
a.区域构造应力缝(D39井,2 899.46~2 899.63 m),岩石发生剪切破裂形成的高角度或近垂直的裂缝,缝面形态规则;b.层面滑移缝(D53井,3 013.80 m),沿着软弱面顺层滑动产生,具有擦痕现象;c.基底断层复活派生裂缝(D92井,2 967.24~2 967.30 m),地层底部早期存在基底断层,后期基底断层差异复活,形成与低幅褶皱或断层派生的高角度张性缝或张剪性缝
Figure 1. Characteristics of structural fractures in Middle Ordovician Majiagou Formation, Daniudi Gas Field, Ordos Basin
表 1 鄂尔多斯盆地大牛地气田中奥陶统马家沟组陡坎成因裂缝与构造裂缝特征对比
Table 1. Characteristics of steep ridge and structural fractures in Middle Ordovician Majiagou Formation, Daniudi Gas Field, Ordos Basin
项目 构造裂缝 陡坎成因裂缝 发育频率 24.1% 24.6% 成因期次 印支、燕山和喜马拉雅构造运动 继承早期裂隙,后期派生拉张应力等诱发 赋存层位 马五1、马五2及马五5上部地层 马五3、马五4、马五5下部和马五6 裂缝形态 形态规则,延伸规模一致(图 1a) 形态不规则,延伸规模大小不等 缝面特征 见擦痕、阶步等现象(图 1b) 上宽下窄,不平整、局部弯曲,尾端无分叉 岩心上裂缝延伸长度/cm 0~10 0~40 岩心上裂缝张开度/cm 0~0.4 0~0.8 组系 平行组系(图 1c),倾角分布广泛 主要为垂直缝,高角度斜交缝次之 充填性 充填程度高 未充填和半充填状态为主 充填物 主要为方解石 方解石、白云石、砂泥质及石英等矿物 有效性 低 高 表 2 鄂尔多斯盆地大牛地气田中奥陶统马家沟组不同裂缝长度(2a)下的临界拉张应力值(σc)
Table 2. Critical tensile stress values (σc) for different values of crack length (2a), Middle Ordovician Majiagou Formation, Daniudi Gas Field, Ordos Basin
项目 参数 裂隙长(2a)/m 0.25 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00 两条裂隙的中点距离(ω)/m 4.937 5 4.875 0 4.750 0 4.625 0 4.500 0 4.375 0 4.250 0 4.125 0 4.000 0 连通率(k)/% 6.25 12.50 25.00 37.50 50.00 62.50 75.00 87.50 100.00 临界拉张应力(σc)/MPa 8.65 8.56 8.44 8.21 7.84 7.27 6.39 4.89 0.30 表 3 鄂尔多斯盆地大牛地气田中奥陶统马家沟组地层陡坎成因裂缝判别标准
Table 3. Classification standards of fractures caused by steep ridges in Middle Ordovician Majiagou Formation, Daniudi Gas Field, Ordos Basin
分析方法 判别标准 岩心观察法 形态不规则,延伸规模大小不等;上宽下窄,裂缝面曲折、不平整、局部弯曲,尾端无分叉;缝间具有明显的溶蚀现象;延伸长度大,分布范围为0~40 cm;张开度大,分布范围为0~0.8 cm;裂缝有效性高 岩心薄片法 裂缝宽度较大,大于400 μm的占比为52.4%;裂缝面曲折,成网状,裂缝延展方向和裂缝形态不规则;常表现为一束主要破裂叠加多条树枝状分枝;缝间溶蚀现象明显 成像测井法 裂缝形态为混乱的、不连续的暗色—亮色影像;具有与溶蚀孔洞伴生的特点;裂缝张开度变化大,裂缝形态不规则;具有单周期的正弦曲线等构造裂缝的特征;走向分布杂乱,走向分布没有明显特征 表 4 鄂尔多斯盆地大牛地气田二级古地貌单元划分标准
Table 4. Classification standards of secondary paleomor-phological units in Daniudi Gas Field, Ordos Basin
地貌单元 开壳层位 马五1-4厚度范围/m 岩溶高地 马五14 >100 残丘(剥蚀区) 马五31 75~100 斜坡 马五22 50~75 沟槽 马五31 <50 注:表中数据据雷涛等[22]修改。 -
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