Microfracturing in the Upper Triassic Sichuan Basin tight-gas sandstones: Tectonic, overpressure, and diagenetic origins
Microfracturing in the Upper Triassic Sichuan Basin tight-gas sandstones: Tectonic, overpressure, and diagenetic origins
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DOI：
10.1306/06301009191
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发表时间：
2010-12
期刊：
影响因子：
3.5
通讯作者：
Lianbo Zeng
中科院分区：
文献类型：
--
作者：
Lianbo Zeng
四川盆地西部位于中国四川省西部龙门山前，是一个形成于晚三叠世的前陆盆地。盆地上三叠统须家河组为特低渗、低孔致密砂岩气藏。致密砂岩中存在大量的微裂缝，如粒内微裂缝、粒边微裂缝和穿粒微裂缝。微裂缝改善了天然气的储集性能和渗透性能，影响天然气的分布。微裂缝反映了构造、超压和成岩成因。利用石英和方解石流体包裹体和埋藏史，构造微裂缝被确定为形成于三叠纪末，白垩纪末，晚第三纪-早更新世末。与超压有关的微裂缝为张性微裂缝，通常充填有沥青，形成于中晚白垩世，最大压力系数为1.6 ~ 2.1。新近纪中晚期，由于川西南地区断裂活动和构造隆升，压力系数降低到1.3以下，这些张性微裂缝闭合并被方解石充填。超压使应力状态由压向拉的转变是在压缩环境中形成张性微裂缝的原因。成岩裂缝形成于晚三叠世末至侏罗纪。在强烈的压实作用下，石英中的颗粒破碎裂隙和长石中的解理裂隙形成粒内微裂隙。一些成岩作用产生的穿晶微裂隙沿着层面，平行于定向矿物颗粒。
The western Sichuan Basin located at the front of the Longmen Mountain in the western Sichuan Province of China is a foreland basin formed in the Late Triassic. The Upper Triassic Xujiahe Formation in the basin is an ultralow-permeability and low-porosity tight-gas sandstone reservoir. Microfractures, such as intragranular microfractures, grain-edge microfractures, and transgranular microfractures, are abundant in the tight sandstones. Microfractures improve storage and permeability and impact distribution of natural gas. Microfractures reflect tectonic, overpressure, and diagenetic origins. Using quartz and calcite fluid inclusions and burial history, tectonic microfractures were determined to be formed at the end of the Triassic, the end of the Cretaceous, and the end of the Neogene–early Pleistocene. Microfractures related to overpressure, being tension microfractures, are commonly filled with bitumen and were formed at the middle to Late Cretaceous when the maximum pressure coefficients were 1.6 to 2.1. In the middle to late Neogene, the pressure coefficient was reduced to less than 1.3 because of fault activity and tectonic uplift in the southwestern Sichuan Basin, and these tension microfractures closed and were filled with calcite. The transition of stress state from compression to tension by overpressure is the reason that tension microfractures were formed in the compression setting. Diagenetic fractures were formed at the end of the Late Triassic to Jurassic. Under intense compaction, grain-crushing crackle fractures in quartz and cleavage fractures in feldspar formed intragranular microfractures. Some transgranular microfractures caused by diagenesis are along the bedding plane and parallel to the directional mineral grains.