【摘要】 湘中坳陷泥盆系佘田桥组是中扬子地区页岩气勘探的重要层系之一,本文在对钻井岩芯描述、野外露头剖面观测及系统的地球化学采样测试分析基础上,对湘中坳陷邵阳凹陷佘田桥组沉积特征、沉积相类型及沉积环境进行了研究,建立了佘田桥组垂向沉积相序及其沉积环境演化。研究认为,邵阳凹陷内佘田桥组主要发育台缘斜坡相、台间盆地相、台地相及潮坪相沉积,其中过渡相区垂向上发育台缘斜坡相-台间盆地相—台缘斜坡相—局限台地相充填序列,而在相对浅水台地区主要发育潮坪相—台地相—潮坪相沉积序列,表明沉积相横向上变化快,指示邵阳凹陷碳酸盐台地和台间盆地横向相间的沉积格局。垂向演化序列指示水体具有先缓慢加深后逐渐变浅的发育特征,与区域内海平面先上升后下降趋势相一致。镁铝比值指示佘田桥组形成于稳定海水环境,其中在中段相对最大,表明该时期水体相对最深。利用U/Th、U~*、V/(V+Ni)、V/Cr指标对环境氧化还原条件进行分析,结果表明台缘斜坡处佘田桥组总体上处于氧化环境,但在中期发育稳定还原环境,反映该时期广泛海侵作用导致的水体变深。依据生物来源Ba及Ba/Al比值分析了佘田桥组沉积期古海洋生产力条件,结果显示生产力具有自下而上逐渐减小的发育规律。综合分析表明,佘田桥组地化指标分析结果与沉积相特征及演化具有一致性,水体氧化还原条件构成了有机质富集的主控因素;在佘田桥期中期的广泛海侵作用下,台间盆地相区发育相对最优保存环境和生烃物质条件,构成了区内页岩气勘探最有利层段。更多还原

【Abstract】 The Shetianqiao Formation is one of the most important strata for shale gas exploration in the Shaoyang Sag of the middle Yangtze region. Based on the drilling core and outcrop profiles and geochemical data, this paper analyzed the sedimentary facies and depositional environment of the Shetianqiao Formation in Shaoyang Sag. The four facies of the Shetianqiao Formation were recognized, including the platform marginal slope, intrashelf basin, shelf and tidal flat facies. The vertical filling sequence at the deep water slope area is composed of the marginal slope facies, intrashelf basin, marginal slope facies and restricted shelf facies, while the vertical filling sequence at the relatively shallow water area consists of the tidal flat facies, shelf facies and tidal flat facies. The results show a distinct lateral change of the sedimentary facies, which is characterized by a alternating pattern of the shelf and intrashelf basins. The vertical evolution sequence indicates that the water depth first slowly deepened, then gradually became shallower, which is consistent with the rising and descending trend of regional sea level. The overall ratios of MgO/Al₂O₃ indicate a stable high salinity environment during the deposition of the Shetianqiao Formation, but the relative maximum value of MgO/Al₂O₃ in the middle Shetianqiao Formation suggests the relatively deepest water condition. The U/Th, U^*, V/（V+Ni） and V/Cr data indicate an oxic seawater at the platform marginal slope with a brief anoxic condition in the middle Shetianqiao Formation, which reflects an extensive transgression event. The Ba and Ba/Al ratios indicate decreasing productivity from the bottom to top of the Shetianqiao Formation. The results suggest that the redox condition of water is the main controlling factor of organic matter enrichment. The intrashelf basin facies in the middle Shetianqiao Formation resulted from the transgression provide the relatively optimal preservation condition for hydrocarbons, constituting the most favorable interval for shale gas exploration in the area.更多还原