Development characteristics and genetic mechanism of carbonate minerals in shale member of the lower Carboniferous Luzhai formation in the depression of central Guangxi
The Carboniferous was significant for the development of "black strata" in the world. The exploration and development of shale gas in North America first began with the Carboniferous marine shale,and the Carboniferous (Mississippian) Barnett Shale has become one of the most important strata of shale gas production in the world. The distribution of Carboniferous marine shale in China is relatively limited,primarily developed in the Qian-Gui rift basin at the edge of the Yangtze plate. It is roughly distributed in depressions such as central Guangxi-Nanpanjiang in Guangxi,south Guizhou-southwest Guizhou,and central Hunan. In recent years,drilling activities in the Carboniferous depressions,such as Guirongye Well 1 and Qianziye Well 1 have yielded promising shale gas displays. In 2023,Guirongye Well 2 achieved industrial gas flow from the Carboniferous after the compression fracture was completed,indicating favorable exploration prospects. The Carboniferous strata in the Guizhou-Guangxi region are anticipated to become the next significant exploration target in China,following the shale gas of Wufeng-Longmaxi formation. However,drilling in this area revealed a rapid phase transition in the early Carboniferous,unstable distribution of shale,high levels of carbonate minerals within the shale,and the presence of common carbonate rock interlayers. On-site experiments on water invasion and gas logging suggest that shale gas production is influenced by carbonate minerals in the shale. The origins of these carbonate minerals in the shale of this region and their impact on gas production remain unclear.To this end,the Luzhai formation shale of the early Carboniferous in the depression of central Guangxi was selected to analyze the origin of carbonate minerals within the shale. This study aims to provide theoretical reference for predicting distribution of various types of carbonate minerals in the subsequent shale formation,as well as for gas content analysis and the optimization of the lower Carboniferous shale gas "sweet spot" in the depression of central Guangxi. The study area is situated in the Liucheng slope in the northern depression of central Guangxi. Field profiles and core samples from Rongye Well 1 were utilized as the research objects. Following the identification of carbonate mineral types,geochemical analyses of cathodoluminescence and carbon and oxygen isotopes were conducted. This was achieved through a comprehensive approach including X-ray whole-rock diffraction,thin section identification,and other analytical tests.Research findings indicate as follows,(1) The study found that the carbonate minerals in the Luzhai formation shale are primarily composed of calcite,with a content ranging from 5％ to 80％,averaging at 30.33％. Additionally,there is a small amount of dolomite,varing from 0％ to 9％,with an average of 2.1％. Carbonate minerals are mainly developed in carbonaceous shale,calcareous mudstone,silty calcareous mudstone,bioclastic carbonaceous mudstone,mud-microcrystalline limestone,and muddy limestone. They are also present in calcite veins distributed along the layers and in structural calcite veins that traverse the layers. (2) Cathodoluminescence analysis shows that massive calcareous mudstone and horizontally laminated micrite carbonate minerals are generally non-luminescent. Some carbonate minerals in silty calcareous mudstone emit weak red light,while others do not. The luminescent carbonate minerals include calcareous cements and certain calcareous bioclastics. The "floating" carbonate bioclastics found in carbonaceous shale typically emit weak red light. Lens-shaped and vein-shaped calcite exhibits weak cathode luminescence. (3) Analyses of carbon and oxygen isotopes show that the δ13C values of native carbonate minerals range from 2.70‰ to 5.81‰,with an average of 4.68‰. The δ18O‰ values range from-8.89‰ to-5.97‰,with an average of-7.54‰. For allochthonous carbonate minerals,the δ13C values range from 2.62‰ to 4.39‰,with an average of 3.30‰,and the δ18O‰ values range from-8.89‰ to-7.67‰,with an average of-8.04‰. The δ13C‰ values of diagenetic vein-shaped carbonate minerals range from-1.18‰ to 5.73‰,with an average of 2.78‰,while the δ18O‰ values range from-12.76‰ to-5.75‰,with an average of-9.77‰.Comprehensive exploration shows that the formation of carbonate minerals primarily results from normal sedimentation in seawater,followed by long-distance transport in seawater after being fragmented by waves and storm currents,and recrystallization during diagenesis. The formation of horizontal vein-shaped calcite may be attributed to supersaturated precipitation within differential compaction and contraction fractures filled by formation fluids during burial,while the vertical vein-shaped calcite is associated with multiple tectonic activities. Based on these findings,various models of carbonate mineral genesis have been established,providing theoretical reference for the optimal selection of sweet spots for shale gas in the lower Carboniferous depression of central Guangxi.
shale gascarbonate mineralformation mechanismLuzhai formation of the lower Carboniferousdepression of central Guangxi
万方数据
维普
