The differences in reservoiring conditions of natural gas and oil, as well as th e multicycle developments and evolution histories of major gas bearing basins in China, and the geological environments characterized by relatively active tecto nic movements in later period have resulted in the forming conditions of gas fie lds in China to be more complicated than those in many basins all over the world . Summarizing the reservoiring courses of large and medium gas fields in China, they can be divided into 4 types, being bounded by Neogene, Paleogene and Mesozo ic, i.e. ①super-late (Neogene to Quaternary) hydrocarbon generating and reserv oiring type;②late(Paleogene to Neogene)hydrocarbon generating and reservoiring type;③early(Mainly in Mesozoic)hydrocarbon generating and accumulating and late (Paleogene to Quaternary)finalizing and reservoiring type; and④early(mainly in Mesozoic) hydrocarbon generating and reservoiring type. In China, those mainly o f late and super-late reservoiring and finalizing gas fields have the most exte nsive distribution and the best prospects; while those of early hydrocarbon gene rating and reservoiring type have also relatively good prospects in basins with good preservation conditions. The three major gas exploration areas in China are : the foreland basins in west-central China, the three large cratonic composite basins (Sichuan, Ordos and Tarim), and the offshore rift and pericontinental ba sins. These indicate that the natural gas has mainly been finalized and reservoi red in late and super-late stages, which constitute the major enriched areas an d domains of natural gas in China.
Pertoleum accumulation of low porosity (is smaller than 12% in general)and perme ability (in mostly is 1×10-3μm2 or so)of fragmental rock which become s reservoir is the most primary types of pertoleum accumulations at stabilize cr atonic basin or superposed basin in center and west areas of our country. Howeve r it find that there are many prolific oil and gas accumulations which have high -quality reservoir sandstones in low porosity and permeability oil and gas zone s by the recently exploration. The authors discuss the forming condition of the high-quality reservoirs from the structure, sediment and diagensis of high-qua l ity reservoirs formation and enumerate much element. Expound the principle and m ethod basis that describe and discriminate them. Authors believe that original c omposition and structure of reservoirs are the most crucial element which effect the feature of physical property, congenital provenance condition is ultimate s ource of high-quality reservoir. It is important to explain their genetic from tectonic and deposit environmental condition.
In this paper different mathematic models are discussed, including the relations hip models connected the stable carbon isotope composition with source rock matu ration, distillation models based on Rayleigh expression and kinetic models, and a relationship diagram between instantaneous and accumulation carbon isotope co mposition linked to source bed maturity is proposed on the basis of other resear chers, which can be used to roughly estimate the accumulation/loss history of co al-derived gas pool. Applicable extents of different models are illustrated. The gas accumulation/loss history of Kela 2 gas pool in the Kuqua depression of Tar im Basin, the biggest gas field of China, is restructured as an example by using isotope kinetic model and others. The results suggest that the natural gas of K ela 2 pool, with the carbon isotopic value of -27.78‰~-27.73‰, cumulated fro m early stage of gas generation to about 2.0 Ma, which was hardly lost during th e source rock maturation and gas generation because of coalbeds with strong abso rbability. However, the late generated gas was not filled into this pool due to the overpressure development starting at about 2.0 Ma, which is supported by non -hydrocarbon components in this gas pool.
The research and practice of exploration show that overpressure systems in sedim entary basins have close relationship with hydrocarbon occurrence and are signif icant for exploration of deep reservoirs.Overpressure inhibits or retards the generation and maturation of hydrocarbon and improves the porosity and permeabil ity of the reservoirs. It also strengthens the sealing for hydrocarbon and broa dens the range of“liquid state window”which determines the accumulation and po oling of natural gas. So it creates favorable conditions for the exploration an d development of reservoirs in the deep of basin. Overpressure systems formes i ndependent hydrocarbon accumulation systems as a form of compartment from which fluid releases repetitively through a cycle of healed state to fracturing state .There are still many problems to resolve, especially for the relationship bet ween overpressure and hydrocarbon generation, which has been argued, and the mec hanism of hydrocarbon accumulation in overpressure systems. The issues of hydro carbon accumulation in overpressure systems will be important tasks for oil or g as exploration in future.
The accumulation of natural gas is a dynamic equilibrium process of gas accumula ting and dissipating. The late accumulation is more favorable for the gas preser vation. By analyzing the characteristics of natural gas pools in Midwest China, it is indicated that the modern structure movement significantly controls the la te accumulation of natural gas, which is showed in four points: ① the late accu mulation is favorable for the late generating gas of gas source rocks and the la te quick subsidence is also advantageous to the late quick curing of gas source rocks; ② The modern structure movement provides the new driving force for the l ate accumulation of gas; ③ The late formation of traps controls the late accumu lation of natural gas; ④ The late fractures act as the migration pathway for th e late accumulation of shallow gas. This article will work in the four points.
There is a very low exploration and study extent to Miocene stratum in Qiongdon gnan basin, and the result of the study is not very good. Though there were good oil and gas show generally to be seen in the Miocene stratums, but the logg ing interpretation hasn't found hydrocarbon beds. Because the varied reasons, only Sanya formation of Miocene in Ya 13-1, Ya13-4, Ya 13-6 and Meishan for mation in BD 19-2 structures have been found to have high production commercial gas or better discovery. But the other areas haven't got a good finding. So t he Miocene oil and gas exploration potential is still be very big. So we think, to wholly and systematacially analyze and summarize Miocene exploration result s and the oil and gas transferring and collecting law, and to strengthen the exploration and study to Miocene in this area, we believe that we must make an improvement.
Based on the system division of the primary and secondary gas accumulation， the paper depicts the different characteristics between primary and secondary gas a ccumulation of Pingluoba gas field in western Sichuan. Secondary gas accumulatio n can be divided into redistributed secondary gas accumulation， non-redistribu ted secondary gas accumulation and mixed secondary gas accumulation， according to the relationship between gas accumulation and generating rock. Primary gas ac cumulation refers to these that lie above or under the generating rock and conta ct with or are near to it, or,lie inside the generating rock,The secondary gas in Pingluoba has high fixed hydrocarbon content and relative density， low metha ne content and little non-hydrocarbon gas. The ratio of iso-pentane to normal-pentane of primary gas is similar to or a little higher than that of secondary gas，however, the ratio of iso-butane to normal-butane of secondary gas is hi gher than that of primary gas. In Jurassic in the region, the carbon-isotope is heavier in primary gas than in secondary gas,at the same time, primary gas has more content of heavy residual hydrocarbon.
Triassic black mudstones are extensively distribut ed in Tarim basin with great thickness and higher content of total organic matte r (TOC) and extractable organic matter (EOM). Abundance pregnanes and higher con centrations of tricyclic terpanes are also observed in extractable organic matte r (EOM) of Triassic source rocks. All of above characteristics show that organic matters in Triassic source rocks of Tarim basin are rich in aquatic-life deriv ed organic material and Triassic source rocks are favorable for oil and gas gene ration. Triassic source rock organic matters are also mature in thermal maturity in most parts of Tarim basin. The oil/gas and source rocks correlation studying have proved that considerable amount of oil and natural gas produced in Tarim b asin are derived from Triassic source rocks. Triassic source rocks are in good c ondition with relatively higher potential for oil and gas generation.
YA 13-1 gas field, the biggest offshore gas field in China, is located in the w est of Yanan depression.In the southeast, there lies YA 21-1 structure. In g eneral, YA 21-1 structure have “three poor” for gas accumulation, that is, so urce rock, reservoir and energy field. The natural gas of YA 13-1 gas field co mes from Yachen and Lingshui formation of Yinggehai and Qiongdongnan basin. How ever, the source rock of YA 21-1 comes from Ledong depression, that is the firs t poor.The reservoirs of YA 21-1 is worse than YA 13-1, that is the second poor. Regarding the energy fields, it shows that the caps rock and reservoirs ha ve over-pressure, that is the third poor.So, the source rock, reservoirs and energy fields controlled the gas accumulation of YA 21-1 structure.
The article analyses the oil & gas distribution and migration character in Biyang depression,and the denuded influence to the oil & gas distribution,and statistics the stratum’s datum and oiliness proportion protracts the oil distributing figures of each phase,and can see the changing of oil and gas distribution in different periods,simultaneity,applies the stratum denudation principle to illustrate the migration direction of the oil and gas.
With pyrolysis experiment, we extracted and analyzed the soluble organic matter from original samples of hydrocarbon source rock of Jurassic coal measures in Zh ungeer basin and residual of different temperature, treated residual kerogen, me asured Ro and TOC, and gave chromatograph and mass spectrograph test to so me sample series. The results indicate that there is some geological and geoch emical difference of hydrocarbon source rock of coal measures at different therm al evolution stage, natural gas generated at low evolution stage and thermal deg radation stage is the result of different thermodynamic conditions and chemical reaction of mother matter of gas generation. At low evolution stage, the vitrin ite reflectance of source rock does not increase much, usually Ro<0.7%.The analytical results of soluble organic matter of original samples and residue in pyrolysis experiment indicate that hydrocarbon gas is mainly from releasing of radicals of original soluble organic matter and thermal degradation of margin al hydrocarbon (polar component) of large-molecule bitumen and kerogen. At t hermal degradation-thermal cracking stage, the temperature of hydrocarbon sourc e rock increases following the increase of buried depth, and thermal evolution r elatively accelerates, the vitrinite reflectance increases from 0.55%~0.70% to 1.40%~1.70% quickly, at the same time, the thermodynamic effect is big enough to destroy the constitutional hydrocarbon of kerogen to generate oil and gas, it is the time of kerogen to release a lot of hydrocarbon Geochemical parameter s at this stage show properties of thermal degraded constitutional hydrocarbon of kerogen.
Because Kela 2 gas field is gas reservoir that has normal temperature and abnorm al high pressure, it's better to use depletion production in gas recovery. By us ing perforation Completion, recommended design can product 10 billion cubic meter natural gas per year, with 4.25% velocity of gas recovery. Oil pipeline erosion, pipeline diameter optimization, completion pipeline design, production casing and liner structure, wellhead tree and safe control system were discussed in this paper. The better designs of well completion and gas recovery were given in the paper, also.
In the stage of exploration, to remedy good evaluation of the reservoir characteristics, to decrease exploration risk and investment, it is necessary for us to erect a geological knowledge base in order to remedy disadvantage of the lack of data. This is because, in this stage, there may has bad ground conditions, sophisticated underground geological situations and rare wells. To do this, suitable outcrops should be selected as the experimental area of the field reser voirs. By the analysis of the outcrop profile and the sedimentary face correlati on, we can make sedimentary models so that the distribution of faces in different time can be defined. At the same time, the tracing of the sandstone outcrop in the profiles and the analysis of the compositional factors can make the models of the original sandstone framework. Subsequently, with the combination of loggi ng and drilling data, the geological prediction model of reservoirs can be made through the random modeling method. On doing this, prediction of the sandstone s uperimposition between different wells in space and the interbed distribution ru les can be realized on account of certain geological laws. Kuche Depression is l ocated in front of Tianshan Mountain. The tectonic and ground conditions are very complicated; seismic imaging in the mountain areas and signal-to-noise ratio are low; drilling in these areas is of unexceptionally difficult. Due to all of these reasons, it is very important to make geological models of reservoirs. Th is method has been used in the reservoir description in Tuziluoke gas field succ essfully. For the evaluation of the reservoir quality and potentiality in the ga s field, it is a reliable method.
Oil security is the key of energy security. Energy security is focused on oil security. From the end of 20 century, especially post "9?11 event", global main developed countries continue adjusting their strategy of oil security, including using multiple source for oil supply, adding strategy reserve of petroleum, expanding use ratio of national petroleum reserve, establishing oil resources base in foreign country, spreading policies of save oil and limit oil.
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