Reservoir Characteristics And Genesis Of Natural Gas Of The Volcanic Rock Gas Pool Of Yingcheng Formation In The Songnan Gas Field

Through the researches for Volcanic rocks lithology, lithofacies, property, natural gas geochemistry and genesis of natural gas of the Yingcheng formation in the Songnan gas field, we conclude the following points:(1) This Volcanic rocks area mainly developed 8 lithologies: stomatal rhyolite tuff lava, crystal glass dust tuff, welded tuff, rhyolitic tuff, tuff breccia, volcanic breccia, in-situ volcanic breccia. Of these, rhyolite is the major volcanic rocks in the area, followed by trachyte and andesite, the overall performance of mainly acidic extrusive rock.(2) The volcanic rocks phase are characteristics of Changes in the volcanic rocks with a vertical speed, horizontal distribution of instability and the main outbreak phase and effusive phase mainly. Based on Characteristics of volcanic rocks, combined with previous researches for the volcanic phase, we identified five major phases of the volcanic vent facies, intrusive facies, outbreak phase, effusive phase, volcanic sedimentary facies in the area, The eruption sequence is complete, at the center of the eruption. Because of its volcanic eruption in the form of fissure eruptions, each phase and subfacies development is not complete, this area mainly develop the pyroclastic flow facies, Nokia-phase heat and air off phase 3 sub-phases of the outbreak, facies upper, middle facies and lower subfacies three facies of effusive facies, the development features fully.(3) The main types of development of stomatal pore area, solution holes, gravel between the (internal) pore, porous, structural fractures, micro-cracks, the main types of reservoir are fractured-porous type and porous type, its average porosity was 6.6 % and permeability 0.299 MD. pyroclastic flow facies of outbreak phase average of only 5.5% porosity, permeability 0.068MD, the average porosity of the upper facies in effusive phase up to 9.1%. Comprehensive consideration of volcanic rocks lithology, lithofacies and reservoir space on the contribution of crack cases that effusive facies with upper rhyolite stomatal development, fragmentation of the dissolution breccia is better reservoir site, and The dense ignimbrite, rhyolite is the major layers of fracture development.(4) In Yingcheng formation, the CH4 gas content is about 70%, C₂H₆ heavy hydrocarbon content of 0.51%¹.81%, C₃ hydrocarbon content is extremely little, natural gas dry coefficient is 0.97 to 0.99, is characteristic with the dry gas features; non-hydrocarbon gas content in the natural gas is high, which mainly contain CO₂ and N₂, in which CO₂ content 20.74%²5.70%, N₂ content was 3.73%⁶.98%. In the Yingcheng formation, methane carbon isotopeδ¹³C₁ value are greater than -30‰, shows carbon isotope negative sequenceδ¹³C₁>δ¹³C₂>δ¹³C₃>δ¹³C₄ or part of the reverse phenomenon.(5) Through analysis the Volcanic gas geochemistry and genesis in the area ,we have made the conclusions: the value ofδ¹³C₁ is greater than -20‰, methane isotope emphasis on natural gas withδ¹³C₁>δ¹³C₂>δ¹³C₃ negative sequence or isotopes reverse, performance has inorganic mantle methane input features, the source of methane through the end of the isotope mixture of different proportion that Yaoyingtai Volcanic gas in a larger proportion of mantle-derived methane. CO₂, N₂ and other inorganic gases are derived from the mantle source gas, therefore, the cause of the natural gas in the area has a "crust - mantle" dual characteristics of coal - mantle mixture.