湘中南大庙口地区构造变形特征及其对区域页岩气构造保存条件的启示

近几年来对湖南省页岩气地质条件进行了大量调查研究，但对页岩气的构造保存条件研究和评价很少，因而影响了区域页岩气勘查工作的科学部署。笔者在湘中南永州凹陷西北缘的水岭-大庙口地区开展了1∶50 000页岩气地质调查，对该地区构造特征和构造演化过程进行了研究，在此基础上探讨了页岩气保存和富集的构造条件。调查区构造变形强烈，主要构造类型有：上古生界盖层中发育的NNW-近SN向（主）褶皱和EW向（次）褶皱，下古生界中发育的NNE向和NE向膝折，切割上古生界和下古生界的NNW-SN向、NE向、NW向等3组断裂（主要为逆断裂、正断裂，少量为走滑断裂），下古生界中极为发育的NNW向板劈理以及泥盆系跳马涧组中发育的SN向、NNE向破劈理等。结合区域构造背景，初步确定上述构造主要形成于早古生代以来的8期构造变形事件，其中以加里东运动、印支运动和早燕山运动的挤压事件以及白垩纪区域伸展事件等最为重要。奥陶系烟溪组和天马山组底部的黑色页岩为调查区页岩气勘探目标层，但钻孔岩心页岩气解吸实验表明含气性极差。分析认为，多期构造运动造成的抬升和剥蚀及形成的断裂、劈理，既导致页岩气的散逸，也使得目标层仅局部有保存；多期构造运动形成的复杂构造样式，使得上古生界之下目标层的空间展布特征难以探明；加里东运动和印支运动所诱发花岗质岩浆活动相关的高地热场区会显著增高有机质热演化程度，总体不利于页岩气的保存。调查区对页岩气富集和保存不利的构造条件基本适用于永州凹陷及周缘其它地区，因此认为永州凹陷及周缘页岩气成藏潜力非常有限。本研究成果对区域页岩气成藏条件的客观认识以及页岩气勘查工作的合理部署具有重要的启示意义。

Geological structure characteristics of Damiaokou area in central-southern Hunan andtheir significance to regional shale gas preservation conditions

There has been a lot of research on the shale gas geological conditions in Hunan Province in recent years, but there has been little research and estimation on the tectonic conditions of shale gas preservation, which affects scientific deployment of regional shale gas exploration. The authors carried out 1∶50 000 shale gas geological survey on the Shuiling-Damiaokou area, northwest periphery of Yongzhou depression, central-southern Hunan, studied the structure characteristics and tectonic evolution, and then probed into the tectonic conditions of shale gas preservation. In the survey area, there occur strong deformations including NNW- to SN-trending folds mainly and EW-trending folds secondly in Upper Paleozoic, NNE-trending and NE-trending kink structures in Lower Paleozoic, NNW- to SN-trending, NE-trending and NW-trending faults（mainly thrust and normal faults with minor strike-slip faults）cutting Lower and Upper Paleozoic, violent NNW-trending slaty cleavages in Lower Paleozoic, SN-trending and NNE-trending fracture cleavages in Devonian Tiaomajian Formation. Combined with regional tectonic settings, 8 deformation events related with above deformations have been determined primarily since Early Paleozoic. In the deformation events, the compressive events such as Caledonian Movement, Indosinian Movement and Early Yanshanian Movement, and Cretaceous regional extensional event were most important. The black shale of Ordovician Yanxi Formation and bottom Tianmashan Formation is shale gas target layer of the survey area. Canister desorption experiments with the samples collected from drill core indicated that the gas-bearing property of the target layer is very poor. Several tectonic movements caused rising and denudation of the survey area, which made shale gas escape and made the target layer preserved locally only. Complicated structure styles formed by several tectonic movements make it be difficult to ascertain the expanding characteristics of the garget layer under Upper Paleozoic. Caledonian Movement and Indosinian Movement caused granitic magmatism and respondent high geothermal field, which improved thermal evolution stages of organic matter considerably, and made shale gas preservation to be difficult. The unfavourable tectonic conditions of shale gas preservation in survery area is introduced in Yongzhou depression and its adjacent areas, therefore shale gas potentialities in the areas are very limited. This paper is of great importance to the study and objective knowledge of regional shale gas reservoir and to shale gas prospecting arrangement.