| 鄂西宜昌地区晚奥陶世—早志留世页岩气藏的主控地质因素与富集模式 |
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| 引用本文：陈孝红,张保民,陈林,张国涛,李培军,张淼.2018.鄂西宜昌地区晚奥陶世—早志留世页岩气藏的主控地质因素与富集模式[J].地球学报,39(3):257-268. |
| DOI：10.3975/cagsb.2018.050901 |
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| 基金项目:中国地质调查局项目(编号: DD20179615);国家重大科技专项(编号: 2016ZX05034-001-2) |
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| 中文摘要:湖北宜昌地区奥陶系五峰组—志留系龙马溪组年代和岩石地层格架显示, 宜昌地区奥陶系—志留系界线附近地层连续, 但自南而北在早志留世鲁丹阶上部至埃隆阶下部有不同程度的笔石带缺失, 并由此造成了富有机质页岩厚度的减薄。页岩微量元素V/Cr、Ni/Co、V/(V+Ni)的比值和化学蚀变指数(CIA)指示, 宜昌地区在奥陶纪凯迪期晚期—志留纪埃隆期早期总体处于缺氧-贫氧海洋环境和寒冷干燥的气候条件, 富有机质页岩的厚度变化受气候变冷所引起的海平下降和宜昌上升作用的双重影响。页岩中总有机碳含量(TOC), 受海底氧化还原环境和海底有机碳通量(Mo-xs)的双重制约。由于不同古地理部位TOC与Ni-xs的相关性不同, 在盆地靠近隆起一侧, TOC与Mo-xs的相关性明显高于TOC与Ni-xs的相关性, 据此, 并结合同期地层碳酸盐碳同位素具有负偏特征, 与全球其他地区同期碳同位素组成特点相反, 推测奥陶纪凯迪期晚期—志留纪埃隆期早期可能因海平面下降或宜昌上升作用导致海底发生了天然气水合物的溶解和甲烷释放, 并由此引起海底有机碳通量的升高, 促进有机质的聚集保存。页岩中总有机碳含量与页岩的含气性和孔隙度具有明显的相关性, 但孔隙度主要受储层的渗透率和岩石类型制约, 与储层的含气性关系不明显, 证明孔隙不是页岩气富集的最主要因素。由于储层的含气性主要受控于优质储层的厚度、总有机碳含量和压力系数, 因此, 有利的沉积相带和良好的保存条件是志留系页岩气藏的主控地质因素。具体到宜昌地区奥陶系五峰组—志留系龙马溪页岩气保存富集上, 奥陶纪—志留纪过渡时期中, 扬子台地内部坳陷盆地的形成和其中广泛发育的富有机质页岩是页岩气形成富集的基础。而宜昌地区古老基底的发育及其从三叠纪晚期之后开始的缓慢抬升, 导致志留系页岩气储层埋深浅, 生气高峰时间晚, 是志留系页岩气富集保存的关键。 |
| 中文关键词:页岩气 地质条件 富集模式 黄陵隆起 五峰组 龙马溪组 奥陶系 志留系 宜昌 |
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| Main Geological Controlling Factors and Enrichment Pattern of Shale Gas Reservoirs in the Late Ordovician–Early Silurian Strata of Yichang, Western Hubei Province |
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| Abstract:The chronostratigraphic and lithostratigraphic framework of Ordovician Wufeng Formation–Silurian Longmaxi Formation in Yichang of Hubei Province indicates that there exists a conformable Ordovician–Silurian boundary in Yichang area. However, from south to north, varying degrees of missing deposits in the graptolite zone and thickness reduction of organic-rich shales were found from the uppermost Rhuddanian Stage to the lower part of Aeronian Stage. V/Cr, Ni/Co, V/(V+Ni) and chemical index of alteration (CIA) show an anoxic-suboxic marine environment and a cold and dry climate from the late Ordovician Kaitian Stage to the early Aeronian Stage of the Silurian period in the Yichang area. Thus, the variation in thickness of the organic-rich shales was affected by both sea-level falling caused by climate cooling and Yichang elevation, and the total organic carbon (TOC) in the shales was controlled by both redox environment and organic carbon flux (Mo-xs). According to the different correlations between TOC and biogenic Ni (Ni-xs) at different paleogeographic locations, the higher correlation between TOC and Mo-xs compared with TOC and Ni-xs along the sections close to the uplift and the carbon isotopic negative shift in the carbonates that is inconsistent with global trend, the authors infer that there should exist methane release from dissolution of gas hydrate due to sea-level falling or Yichang elevation, which led to increasing of organic carbon flux and further promotion of the reservation of organic matters. There are distinct positive correlations between TOC with gas-bearing property and porosity. However, the porosity should be mainly controlled by permeability and lithology and had an ambiguous relationship with gas-bearing property. This could be an evidence proving that the porosity was not the most important factor controlling the shale gas reservoir. Because the gas-bearing property was mainly controlled by TOC, thickness and pressure coefficient of the shale gas reservoir, the sedimentary facies and preservation condition constituted the key to the Silurian shale gas reservoirs. As for the preservation and enrichment of the Silurian shale gas reservoirs in Yichang area, the development of depression basin during the late Late Ordovician–early Early Silurian period and its rapid filling after the Silurian mid Llandovery Stage seem to have been the basic factors, and the development of the old basement and its slow uplift since the late Triassic may be the key. |
| keywords:shale gas geological conditions enrichment pattern Huangling uplift Wufeng Formation Longmaxi Formation Ordovician Silurian Yichang |
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