| One third oil resources occurrence in the carbonate reservoir in the world. Most of large-giant oilfields are closely related to carbonate reservoirs. There obtain significant breakthrough in oil and gas exploration of carbonate reservoir and many large-scale Gasfields, have been discovered including the Luojiazhai Gasfields, the Dukouhe Gasfields, the Tieshanpo Gasfields and the Jiannan Gasfields besides of the Puguang Gasfields, which is characterized by carbonate reservoir gas field with large scale at present in China. Besides, there are new discoveries continuously at the margin area of Sichuan Basin. It was showed that the carbonate reservoir of Permian-Triassic in the northeastern part of Sichuan basin has great oil and gas exploration potential. The predecessors have carried out extensive research on the Permian-Triassic carbonate reservoir in the northeastern part of Sichuan basin, suggesting that the formation of Permian-Triassic carbonate reservoir with high quality is closely related to burial dissolution.Both the Puguang and the Jiannan Gasfields are located at the northeast of Sichuan basin. The Changxing and Feixianguan formation are characterized by carbonate reef, shoal facies, and the development of porous in hydrocarbon reservoir mainly depends on burial dissolution of carbonate. However, the reservoir scale and properties of Changxing and Feixianguan Formation in Puguang Gasfield have much better than those in the Jiannan Gasfield.According to the research condition of the carbonate reservoir and the main problems of Pugu ang and Jiannan Gasfield, combined with the present research situation of the buried corrosion me chanism at home and abroad, this paper studies the contributions of the burial corrosion to the for mation and the evolution of the carbonate rock pores through analyzing the burial history and the d iagenesis of the typical sections along the road of the "regional background analysis-typical secti on anatomy-experiment simulation-burial dissolution control factor-buried dissolution model e stablishment", and by means of analyzing the space formation and the evolution of the carbonate r eservoirs, from the macro-and micro-aspects, as well as combining the geological and the experi mental methods. And at the same time, it studies the major controlling factors and the main differe nces between the Changxin and Feixianguan carbonate rocks in Puguang and Jiannan Gas Field Pu guang, clarifies the main controlling factors of the high quality reservoirs, establishes the genetic modelof the burial dissolution, and provides a theoretical basis for the carbonate rock reservoir pre diction and further exploration based on the dynamic simulation experiments of the dissolution of t he carbonate sequence, the fluid-rock reaction and the corrosion under the deep buried conditions. The main conclusions are listed as follows.1 Successions of reservoir diagenesis, the Puguang and Jiannan GasfieldsThe successions of reservoir diagenesis of Changxing and Feixianguan Formation in Puguang Gasfield:the algae cementation of undersea environment, micritizationâ†'first generation fibrous columnar cementationâ†'selective dissolution of atmospheric fresh waterâ†'second generation granular cementationâ†'dolomitization, neomorphismâ†'pressure solution, filling during burial stageâ†'tectonic compression, recrystallizationâ†'I stage burial dissolution and fillingâ†'oil intakeâ†'the evolution from hydrocarbon to asphaltâ†'II stage burial dissolution and later infilling â†'infilling gas. The dolomitization of Changxing and Feixianguan Formation occurred before burial dissolution and tectonic compression, so the successions of reservoir diagenesis in Jiannan oilfield have the same as those in Puguang Gasfield.2 Dissolution of carbonate reservoir, the Puguang and Jiannan GasfieldsIt was suggested that the reservoirs of Changxing and Feixianguan Formation had been subjected to dissolution for three times during the formation of structure in Puguang and Jiannan Gasfields:the dissolution of atmosphere fresh water, the early(I stage)burial dissolution, the later (II stage) burial dissolution. The pores formed during dissolution of early atmosphere fresh water and the primary pores were mostly filled by cements during earl diagenesis and the earlier stage of middle diagenesis phase.The fluids generated by burial dissolution during early diagenesis are related to organic acids by thermal evolution of organic matter and CO2, the corresponding temperature 90℃-120℃. The fluids generated by burial dissolution during â…¡ diagenesis are relevant to the reaction of TSR and H2S products,the corresponding temperature 165℃-180℃. The burial dissolution pores of I stage in Puguang Gasfield formed in Jurassic period, and II stage pores formed Cretaceous period. The burial dissolution pores of â… and â…¡ stages in Jiannan oilfield formed during Jurassic period.3 Identification of the burial dissolution pores, the Puguang and Jiannan Gasfields to identify the burial dissolution pores according to diagenesis sequence of carbonate reservoir.Where tectonic compaction is widely developed in the reservoir, the pores are integrity and undeformed. According to research, it was occurred to tectonic compaction followed by pores formation. This kind of pores are burial dissolution pores.The fine-coarse dolomites are dissolved. The fine-coarse dolomites are recrystal and formed due to early dolomitization during burial stage.The pores concerned by small particles, extrusion pores and fragment pores are dissolved. It was occurred to dissolution after compaction.There is no asphalt-filling in preserved -well and undeformed pores, and the asphalt presents at around of pores. It was showed that this kind of pores are formed after evolution from hydrocarbon to asphalt, called later burial pores.There is pore-lining of asphalt at the edge of dolomite crystal filling in the early pores. It was happened to burial stage after the evolution stage from oil to asphalt.4 Genesis of dolomite reservoir, the Puguang and Jiannan GasfieldsThe genesis of dolomite reservoirs from Feixianguan and Changxing formation in Puguang Gasfield is different from those in Jiannan oilfield. The dolomite reservoirs in Puguang Gasfield are mainly products of dolomitization during penecontemporaneous stage. Quantities of early dolomite were formed. However, the dolomite reservoirs in Jiannan Gasfield are mainly products of dolomitization during burial stage, formed small amount of dolomite. It was showed that these are mainly reasons why there are so great differences about scale of dolomite reservoir between the two oilfields. Also, these are mainly reasons why the dolomite in Puguang Gasfield has good reservoir property, but bad reservoir property in Jiannan Gasfield.The differences of dolomitization genesis between the two Gasfields are closely related to sedimentary environments. The Changxing and Feixianguan formation in Puguang Gasfield are characterized by reef facies developed at margin platform and restricted and evaporate platform. This provides sufficient brine with high ratio of Mg/Ca, which is favorable to form large-scale dolomite reservoir (bodies). However, the reef shoal sedimentary facies in Jiannan Gasfield are adjacent to open platform on condition that the seawater is not easy to evaporate. It was difficult to dolomitization of reservoir during penecontemporaneous stage. Therefore, there was developed small dolomite reservoir during burial stage.The burial dissolution degree and reservoir development are directly controlled by the genesis and degree of dolomitization.5 Tectonic compactionThe Tectonic compaction has important roles on development of burial pores from Changxing and Jiannan formation in Puguang and Jiannan Gasfields, which is important factor to form quality reservoirs. Especially in the carbonate reservoir, the role of Compressive tectonic stress is very important to form good reservoir. On condition that the rocks were easy to compressed to crack/rupture and formed a mass of micropores and pores, which provided significant channels for acid fluids to reservoir. These conditions are favorable to develop of dissolution during burial stage.6 Simulation experiments on burial dissolution mechanismThe simulation experiment on interaction among fluids and rocks showed that the different aid fluids had an effect on dissolution of carbonate, having different temperature with most dissolution degree.The simulation experiment on carbonate dissolution at action of acetic acid showed that the carbonate dissolution has following variation trend, weakâ†'strongâ†'weak at 50MPa,with a temperature of normal to 200℃. The temperature distribution with the strongest dissolution is about 90℃, when the temperature exceeded 90℃, the dissolutin will decline, and approaching to 200℃, dissolution with different type samples have small difference.The simulation experiment on carbonate dissolution at action of CO2 showed that the carbonate dissolution has following variation trend, strongâ†'strongâ†'weak at 50MPa,with a temperature of normal to 200℃. The temperature distribution with the strongest dissolution is about 60℃-90℃, when the temperature get 90℃, the dissolutin is the strongest, when the temperature exceeded 90℃, the dissolutin will declineand approaching to 200℃, dissolution with different type samples have no difference.The simulation experiment on carbonate dissolution at action of H2S showed that the temperature distribution has large variable ranges,60℃-150℃. The temperature distribution with the strongest dissolution is about 120℃-150℃. when the temperature exceeded 150℃, the dissolutin will declineand, dissolution with different type samples have no difference.The simulation experiment on sequence of carbonate dissolution showed that the mineral components, acid fluids and surface area of mineral contact are main factors influencing burial dissolution of carbonate.The simulation experiment on burial dissolution of carbonate verse different acid fluids showed that the degree of dissolution are controlled by fluids and surface area of mineral contact according to dissolution experiments of variable granularity and pressures. Under the same conditions, comparative to limestone, the dolomites are easy to dissolute, become quality reservoirs.7 Favorable conditions of carbonate reserviorsAccording to comparative analysis of reservoir diagenesis, tectonic, deposit setting between the Puguang Gasfield and Jiannan Gasfield, combined with simulation experiment on burial dissolution mechanism showed that the development of reservoir with high quality is controlled by reef and shoal sedimentary facies, dolomitization, tectonic compression and burial dissolution. The favorable conditions are listed below.The dolomitization environments are beneficial to develop for quality reservoirs.The reef and shoal faices including adjacent to restricted platform, evaporate platform or tidal flat facies are favorable zones, which provide sufficient fluids with high ratio of Mg/Ca.Tectonic compression zones are favorable. On condition that the rocks are easy to compressed to crack/rupture and formed a mass of micropores and pores, which provided significant channels for acid fluids to reservoir. These conditions are favorable to development of dissolution during burial stage.The paleostructure high point close to hydrocarbon geo-temperature of 90℃-120℃ and 165 ℃-180℃ is favorable to occurrence of burial dissolution during I and II stage, and this maybe effective zone for quality reservoir. |
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