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Study On The Overpressure Development, Evolvement And Origin Mechanism In The Carbonate Reservoir Of The Northeast Area, Sichuan Basin

Posted on:2009-01-01Degree:DoctorType:Dissertation
Country:ChinaCandidate:C W WangFull Text:PDF
GTID:1100360242497807Subject:Energy Geological Engineering
Abstract/Summary:PDF Full Text Request
Lots of sedimentary basins develop overpressure commonly, which not only initiate flowing of well, but also has intimate correlations with petroleum accumulation. So far lots of reserches about overpressure have been done in classic rock layer system, but little has been done about carbonate rock layer system. Although the overpressure caused by undercompaction was not developed in carbonate reservoir because the cemention of diagenesis was strong during early stage, the oil cracking to gas, organic or inorganic water-rock reactions during lat period were of important function to the development and evolvement of overpressure. The northeast eara of the Sichuan Basin was important area to natural gas exploration focused on the carbonate reservoir. The distribution of overpressure was very complex, it was not clearly in the mechanism of overpressue development and evolvement. Based on the paccavi, numerical simulation and theoretic calculation, the mechanism of overpressue development and evolvement in the Permian and Triassic carbonate reservoir of northeat area of Sichuan Basin were studied focused on two core science problems including oil cracking to gas and thermochemical sulfate reduction reaction. This dissertation were wished to offer some academic references to the reserches of petroleum accumulation mechanism in carbonate rock layer system in this area.This was directed by structural geology, sedimentology and petroleum accumulation dynamic theories. Based on the basic petroleum geology background in the carbonate rock layer system in the northeast area of the Sichuan Basin, lots of numerical simulations were carried out and each affecting factor in carbonate rock layer system of northeast area in the Sichuan Basin was studied. The research cognitions were as follows.1) There were three obvious distinguishing features of formation pressure among structures bearing natural gases in the northeast area of the Sichuan Basin. Firstly, the distribution of formation pressure was very complex; there was different formation pressure not only among different structures but also different formation in one structure. Secondly, the distribution of overpressure in the carbonate rock layer system was closely correlated with the lithology and the type of reservoir. The overpressure did not exist in the Porous type or fracture-porous type reservoir, but was strongly in the fracture type or porous-fracture type reservoir. Thirdly, the natural gas in the fracture or porous-fracture type reservoir with overpressure did not include hydrogen sulfide gas; however, the natural gas in the porous or fracture-porous type reservoir with normal formation pressure included high hydrogen sulfide gas.2) The technique of fluid inclusion thermodynamics modeling, a scientific and accurate method, can reconstruct the formation palaeopressure. Combined with laser scanning confocal microscope and the PVTsim modeling software, the palaecomponet and formation palaeopressure were acquired in this researching area. The results showed that the overpressure was developed in the Feixianguan and Changxing formation in the Puguang and Maoba structures during early Yanshan movement period, and declined from 130Ma. Today, the carbonate reservoir in the Puguang structure bearing normal formation pressure and the Maoba 1, 2, 3 well block in the Maoba structure were overpressure. These were two different formation pressure development models.3) The main affecting factors of the carbonate reservoir pressure in the northeast area of the Sichuan Basin included oil cracking to gas, TSR, tectonic compression and denudation caused by uplift. The first three factors were the main mechanisms of the overpressure in this area.Oil cracking to gas could cause the formation pressure increasing enormously. The numerical simulations were carried out; the results showed that oil cracking to gas in the closed system in geological situation could cause powerful overpressure, the pressure coefficient reached to 2.56. However, in open system, the system pressure were not enough high. If the system volume expanded 20 percent, the pressure coefficient was only 1.36. The porous type reservoirs with oolite sands facies were experienced the changes of palaeo-oil pool to gas pool and there were lots of bitumen in the reservoir. The main mechanism of overpressure in the carbonate reservoir of the Puguang and Maoba structure was oil cracking to gas. The thermochemical sulfate reduction (TSR) could cause the formation pressure change. The numerical modeling results showed that the TSR participant by methane whatever happened in closed system or opened system could cause the formation pressure decline; the TSR participant by ethane happed in opened system could cause the formation pressure decline but if it happened in closed system, the formation pressure could increase. The TSR participant by propane and butane whatever happened in closed system or opened system could cause the formation pressure increase. The tectonic uplift could cause the formation pressure decline obviously.4) The tectonic compaction during late Yanshan movement period and Himalayan movement period maybe was the main reason to cause the overpressure in the fracture type carbonate reservoir in the northeast of the Sichuan Basin. Different rock types had different stress and strain character and different sensibility degree to stress. The carbonate rock pertained to ductile deformation league. The fracture was closed by the stress firstly when the effective pressure increased and the pore began to deform when the fracture closed completely. However, the ooide dolomite rock pertained to elastic deformation like sand stone. The pore was compress firstly when the effective pressure increased and this could cause pore space decline and fluid pressure increase. At the equal stress condition, the carbonate rock or dolomite rock with fracture type were developed overpressures firstly, and because of high self closure property, the overpressure could release hardly. So, the tectonic compaction could be the main overpressure mechanism in the carbonate or dolomite rock reservoir with fracture type. The different lithology types, its petrophysical property and water-rock interaction during geology history maybe was the main reason for different formation pressure among each structure in the carbonate rock layer system.The main innovations in this were as bellows:1. The study on the overpressure developed in carbonate reservoir belongs to international frontier research field. The overpressure development and evolvement and its mechanism of carbonate rock layer system in the northeast area of the Sichuan Basin were studied in this thesis, this topic was of academic frontier.2. The main affecting factors in overpressure development and evolvement in the northeast area of the Sichuan Basin included oil cracking to gas, thermochemical sulfate reduction, reservoir uplift and overburden rock ablation caused by tectonic movement. The total formation pressure evolvement process affected by multi-factors were studied in this thesis, this belongs to part of exploratory investigation.3. It was very difficult to acquire the palaeopressure in carbonate rock layer system by now., especially to the northeast area of Sichuan Basin, which were expired multiphase reconversion by multi movements, complicated hydrocarbon-water-rock interactions and multiple diagenisis. Focusing on this problem, the fluid inclusion thermodynamics modeling technique was adopted, combined laser scanning confocal microscope and the PVTsim modeling software, the formation palaeopressure were calculated. The method in palaeopressure of carbonate rock layer system was of some innovation.
Keywords/Search Tags:Carbonate reservoir in the northeast area of the Sichuan Basin, Formation palaeopressure, Fluid inclusion thermodynamics modeling, Oil cracking to gas, Thermochemical sulfate reduction (TSR)
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