Study On Forming Conditions Of The CO₂ Gas Reservoirs In Changling Fault Depression, Songliao Basin

The paper, combined with natural gas exploration practice of Changling fault depression in Songliao basin, is under the guidance of theory of volcanic rock petrology, reservoir geology, basin analysis and gas reservoir formation system. It makes full use of the seismic data, drilling cores data, logging data and test data, adopts determination means of petrochemistry and rock physics, starts with formation and evolution of the fault depression,types of the volcanic rocks and cycles and periods of the volcanic activity, discusses the genetic types and characteristics of the study area, and the control effect on the CO₂ gas reservoirs of the faults and volcanic activities, and establishes forming conditions and patterns of the CO₂ gas reservoirs.CO₂ in Changling fault depression may have two kinds of genesis: The mixing of organic and inorganic genesis,and the inorganic genesis. In Changling fault depression, high content CO₂ origin is the inorganic origin, the low content CO₂ origin is complex, inorganic, organic, mix origin coexisting.In Changling fault depression, the CO₂ gas reservoirs are mainly controlled by controlling-depression fault, also does the volcanic upwelling channel. Basement faults in different active regularity control enrichment stratum CO₂ gas. CO₂ gas controlled byâ… -â…¡-â…¢-BF mainly enrich in 3 and 4 Member, Quantou Formation. CO₂ gas controlled byâ… -BF andâ… -â…¡-â…¢-BF mainly enrich in volcanic rock body in deep Yingcheng Formation. Analysised on the degree of CO₂ gas's enrichment controlled by basement faults in different active regularity, CO₂ gas controlled byâ… -BF andâ… -â…¡-â…¢-BF are more enriched.In Changling fault depression, types of the volcanic rock are very complex, including basalt, andesite, liparite and pyroclastic rocks. The characteristic of volcanic rock in this area is advantageous to store up CO₂ gas, and the CO₂ gas reservoirs has formed. In the area the volcanic activity divides into two big cycles and seven times, and the Qingshankou- Cainozoicvolcano magmatic activity controlled source of CO₂ in Songliao basin, CO₂ had many phase of infill injection from Qingshankou to Cainozoicvolcano, but became to be stored by the later period primarily.There are four types of CO₂ gas reservoirs in Changling fault depression, including structure conglomerate gas reservoir, it may be divided into the fornix gas reservoirs, fault uplift (nose) gas reservoirs and the anticline gas reservoirs. Lithologic gas reservoir, the structure control is not obvious, this kind of gas reservoirs may be the volcanic rock gas reservoirs, may be the clastic rock class gas reservoirs,and is also common type of the including CO₂ gas reservoirs. Volcanic rock lithologic gas reservoir isn't controlled by structure. Sandstone lithological, Wanjinta Quantou group gas reservoirs and so on, are controlled by sand body.Structure lithologic gas reservoir, one kind is controlled by positive landform, one kind is controlled by faults and sand body. Formation overlap gas reservoir, mainly seen in gravel overlap gas reservoir in 1 Member, Denglouku Formation - 4 Member, Yingcheng Formation.Reservoir-forming pattern of CO₂ gas comprise volcanic eruption channel infill injection pattern, controlling-depression fault infill injection and reverse fractures infill injection . Generally volcanic eruption channel infill injection pattern relatively far from basement faults. The volcano rock body has the mode of eruption of central type or eruption of fracture- fissure(moniliforme) type. Mantle-derived CO₂ finally migrates to the volcano rock body and stored through volcanic eruption channel phase. CO₂ mainly distributes in deep combination in Yingcheng Formation; Controlling-depression fault infill injection mainly locate in basement faults root. The basement faults growed in faulted period and actived for a long time, Mantle-derived CO₂ takes basement faults as migration pathway, and may distributes in different horizon according to degree of fracture extension. but mainly aggregate in Yingcheng and Denglouku Formation. Reverse fractures infill injection also mainly locate in basement faults root. Basement faults actived for a long time, have inversion features and big extension strength in Neogene. Mantle-derived CO₂ takes reverse faults as migration pathway, and may locate in mid-shallow reservoir combination.CO₂ gas accumulation process can be divided into three stages, including Huoshiling-Yingcheng period, Quantou-Nenjiang period and Himalayan period.â‘ Huoshiling-Yingcheng period: Volcanic rock body of Yingcheng and Huoshiling Formation was formed. Because fault hydrocarbon source rock thermal evolution is low,and there is not cap rock and trap, so the oil and gas didn't aggregate.â‘¡Quantou-Nenjiang period: Fault depression hydrocarbon source rock enters the peak of hydrocarbon generation in succession. Part of organic gas aggregate, because the fault's long-term activity, preserved condition of CO₂ gas reservoirs may be affected, hydrocarbon gas is easy to dissipate, the accumulation scale is not large, then after the regional volcanic activity in late Cretaceous, also parts of CO₂ migrate and aggregate along the deep fault.â‘¢Himalayan period: In this period volcanic range of motion is relatively wide, and asement faults developing in fault depressionstage have long-term activity. Same reverse in inversion period, so it is near the mantle-derived magma of this period, and gas reservoirs at basement faults root have conditions of strong infill injection. Magma degasification produce lots of CO₂, infill inject in horizon of faults along the basement faults, reservoirs formed under the preservation of cap rock of the Qingshankou Formation area. CO₂ may locate in deep and mid-shallow reservoir combination. CO₂ gas reservoirs far from the basement faults migrate into gas reservoirs through volcanic eruption channel phase. CO₂ may locate in deep reservoir combination.