Research On Characteristics Of Natural Gas Reservoiring In The Upper Paleozoic In The Northern Ordos Basin

With an emphasis on Upper Paleozoic in the Ordos Basin which has low porosity, low permeability, low pressure, low abundance, and large-scale distribution of lithologic and tight gas reservoir features. On the basis of summing up and analyzing the results of previous studies, this paper applied the latest theories which include petroleum geology, Pool-forming dynamic, pressure compartment, petroleum system, etc. and analysis-test tools. Studied Daniudi gas field in north of Ordos Basin and Hangjinqi area, the study include pressure distribution, pressure causes, accumulation in combination, inside structure of pressure compartment, formation water characteristics, the effectiveness of accumulation process with lithologic trap, and mechanism of gas accumulation. Finally, the main results obtained are as follows:(1) Through research on formation pressure distribution in the Daniudi gas field and Hangjinqi area, we thought that gas reservoirs in the Upper Paleozoic of Daniudi gas field were mainly atmospheric pressure - low pressure gas reservoir, and Upper Paleozoic of Hangjinqi were belongs to low-pressure gas reservoir. The cause of subnormal pressure in the Upper Paleozoic of Daniudi gas field mainly was tectonic uplift, which caused temperature drop, porosity rebound and gas loss. The temperature drop can cause the pressure drop of about 33% after calculated. Due to the lateral closed condition as well, the cause of subnormal pressure was the spread of gas leakage in the Upper Paleozoic of Hangjinqi area.(2) First discovered the existence of hybrid origin formation water in the Upper Paleozoic of Hangjinqi area. Through analyzed the source of Mg2+ enrichment, PH was low abnormal, salinity was high abnormal, geothermal gradient was high abnormal and Caexcess relationship with Nadeficit in fault zone proved that hybrid origin formation water was formed by the mixture of Nepal Jiang Haizi basement fault activity to promote deep rich Mg2+ , acidic fluid upwelling of hot water and Upper Paleozoic formation water. Such hybrid process was conducive to the maturity of source rocks, and the formation of secondary pores. This area was the favorable oil and gas accumulation zone.(3) According to the form of excess pressure in the Upper Paleozoic of Daniudi gas field, we classified the excess pressure into three types. TypeⅠhad one gradual change peak, typeⅡhad two gradual change peak, typeⅢhad one or two abrupt change peak. Then we analyzed the relationship between different types and accumulation of natural gas.(4) According to the contrast of hydrocarbon source rock, tracer of gas accumulation, transporting system , migration agent and the relationship of configuration about hydrocarbon source rocks etc, then we established model map of gas migration and accumulation in the Upper Paleozoic in Daniudi gas field. Proximal plays through the fracture and the sand's joint transportings, had accumulaited natural gas in He2 and He3. Within the source plays through the sand's transpotings formed gas accumulation areas. Within the source plays the development of high-angle fracture in sandstone cause the gas vertical disperse and form the gas dispersion area, which leads to decrease the content of methane in traps, the text of reservoir where gas disperse showed water gas formation, gas water formation and gas-water-bearing formation. We established different filling mode of natural gas plays in the Upper Paleozoic in Ordos Basin. Distal plays developed in Hangjinqi area, its main migration agent was buoyance, and the charging mode of distal plays is buoyant flow. Proximal plays and within the source plays'main migration agent were the difference of overpressure, there means of natural accumulation were"piston type", in the high-quality reservoirs of dense background buoyancy plays a important role, even the high-quality reservoir reaches a certain continuous distribution where could form local edge and bottom water in the low construction position. The context of tight gas reservoirs where buoyancy plays important role called as"dessert".(5) This paper evaluated the validity of the process of natural gas accumulation in the Upper Paleozoic by evaluating the effectiveness of dynamics, transporting condition and closed condition. By evaluate the process of accumulation of lithologic traps in the Upper Paleozoic in Daniudi area, we thought that: In the dual closing conditions of regional physical properties and pressure seal, natural gas only accumulated under it, the controlling factors of the validity of accumulation process in lithologic traps in He2+3 which were proximal play was the validity of dynamics and transporting conditions; the controlling factors of the validity of accumulation process in lithologic traps which were within norce plays was the validity of part sealing gland condition and transporting condition. In Hangjinqi area, the validity of accumulation process ctrolled by closed conditions because of the development of fault, unconformable surface, sand's development, bad conservation. We established evaluste system for the validity of natural gas accumulation process in the Upper Paleozoic for further evaluation and search for natural gas rich region conducive to providing optimal conditions, has important theoretical and practical significance.(6) Gas accumulation of Upper Paleozoic in Daniudi area is mainly dominated by the gas generation intensity, conditions of regional sealing glands, pressure compartment inside the structure, migration agent and transporting system. The gas generation intensity control the distribution range of the main producing formation, pressure compartment inside the structure affected the gas producing formation and gas bearing, migration agent influenced the enrichment intensity of natural gas, transporting system controlled accumulation formations and the enrichment intensity of natural gas, the tight gas reservoirs where buoyancy plays a important role are high-yield rich regions. Then we established the model of compartment Pool-forming dynamics in the Upper Paleozoic in Daniudi area.(7) The model of the southern fault of Hangjinqi is low abundance lithologic proximal accumulation, trap was the most important factor in controlling accumulation, and the favorable reservoirs were He1 and Shanxi formations. The model of the north of the fault was distal tectonic—lithologic accumulation, He1 was"zone of discharge", only in the low potential zone it could accumulate. The development of fracture and small fault improved the accumulation conditions of He2 and He3, the favorable reservoir was lower Shihezi formation, especially He2 and He3, the scale of them were bigger than He1, the favorable area was Shiguhao area.