The Study Of The CO₂ Forming Mechanism And Mode In Jiyang Depression

The found CO2 reservoirs in Jiyang depression, such as Balibo, Yang25, Pingfangwang, Pingnan, Gaoqing and Hua17, are mainly in Ordovician, Mesozoic, Es₄, Es₃, Es₁, Ng and Nm. Lithologies are mostly sand and carbonate rocks, caps are mud rock, shale and magmatite. Entrap types are anticline, fault block rebuilt by breaking and the screened entrap diapered rock mass. The composition is CO₂, N₂, CH₄, C₂H₆ and He etc. It is magma air source body and its geologic fashions are intrusion and buried volcanic conduit. The relations between gas reservoirs and gas source bodies have three types: magma intrusion-breaking-communicated gas reservoir, magma intrusion-contacted CO₂ reservoir and buried volcanic conduit-contacted gas reservoir. The CO₂ migration in magma intrusion is consisted of fusing and crystallizing phases;it in volcanic conduit is consisted of near-surface effusion and crystallizing phases.The buoyancy of CO₂ in water far more than migration resistance in breaking or chink, CO₂ is easy float upward, the floating can results in differentiation of different density gases and concentration of sealed gas. The gas in sand reservoir firstly migrate into the higher porosity and coefficient of permeability sand, and along with the pressure going up it migrate into the lower. In magma intrusion-breaking-reservoir migration, CO₂ firstly migrate into watered breaking, began gravity differentiation and concentrate, the gas pressure time and again go up, CO₂ migrate into reservoir and concentrate under expansibility as the pressure reach upward a given extend. The CO₂ in reservoir experience four breaking modes: chemistry deposition, dissolution, diffusion and mechanic breaking, the pressure balance can be broken by faulting and the CO₂ will further migrate and form new reservoir.The reservoirs based on magma intrusion in Jiyang depression experience three phase: cap formation-forming before magma invasion, magma invasion and early stage of CO₂ reservoir-forming, magma crystallizing degasification and late CO₂ reservoir-forming, the three phases of different reservoir have different geologic epoch. The reservoir based on magma volcanic conduit also has three phases: before magma effusion, effusion and volcanic conduit bury and reservoir-forming. CO₂ reservoir was formed during magma crystallizing from magma invasion or effusion to Alluvium period, but the breaking process basically is in step with reservoir-forming process, the factor of the reservoirs-exist is continuance gas supply.The CO2 reservoir forming in Jiyang depression has three geologic reservoir-forming modes: intrusion-breaking-reservoir turnabout, intrusion-reservoir direct forming and buried volcanic conduit-reservoir direct forming. The profitable reservoir-forming area for CO2 in Jiyang depression is the low of deep U geologic unit in Yangxin sag, the area around Yang25-Sha4 volcanic conduit and the upper of crater, the southern area of Bingul and the area around fault belt intersected intrusion, the area around volcanic conduit in south of Hual5 well, the Gao 10 and Gao 101 gas well Early Tertiary system, Hua501 and Gao53 gas well Early Tertiary system, the Later Tertiary system and Mesozoic between Hua2 and Hual7 gas well.