The research indicated a distinct anomalous high porosity zone of Ess4 in 3 000-4 000m depth range in Bonan Sag. Combined with core physical property data,the paper identified normal and abnormal porosity distribution,utilizing the distribution of reservoir porosity in continuous interval depth and curves of porosity changing with depth. Integrated with thin section data,scanning electron microscopy analysis and formation pressure data,the paper explained reasons for high porosity and permeability zone by aspects of diagenetic erosion,abnormal high pressure and hydrocarbon charging at early stage. Combined with microfacies,lithofacies,development fracture,the paper discussed the genetic mechanism of high quality reservoir. The result suggests that anomalous high porosity zone in reservoir was benefited by protection of overpressure,organic acid dissolution and hydrocarbon emplacement at early stage. The main conditions of high quality reservoir were as followings:development of anomalous high porosity zone|original deposition factors such as advantage facies(underwater distribution channel and mouth bar in fan delta front),relatively high proportion of mid-coarse rigid particles(quartz,feldspar and metamorphic fragments) and fracture development.
According to the organic geochemical analysis of the ultra deep-seated oil and gas of Paleogene in Baxian Sag,the crude oil density is in the range of 0.77 to 0.83g/cm3.The oil shows wet gas feature,and methane content of natural gas is under 90%,belonging to light-oil -volatile oil.The analysis of gas chromatographic of the saturated hydrocarbon,GC-MS and the carbon isotopes indicate that oil-gas are derived from the 4th member of Shahejie Formation source rocks.The crude oil in the 4th member of Shahejie Formation obviously contains oleanane,sterane C29>C27,low gas isotope(δ13C2value lower than -32‰ is oil type gas).However,buried-hill oils contain sterane C27>C29 but no oleanane.The gas isotope is high,which is a typical feature of coal-formed gas,i.e.,δ13C2value is more than -25‰.The crude oil of the 4th member of Shahejie Formation and the gas in buried-hill were rather similar to the humic source rock,while natural gas in the 4th Member of Shahejie Formation and crude oil in the buried-hill are more similar to the sapropelic source rock.Oil-source correlation results are corresponding to the research on source rock heterogeneity of the 4th member of Shahejie Formation in the Baxian Sag in recent years.The C1/C2 of natural gas in Niudong 1 ultra-deep buried hill was 13, C1/C3 is 37,dryness coefficient reaches 94.9%,and the concentration of diamondoid of 3MD and 4MD is in the range of 35.4 to 37.9μg/g.It is indicated that the gas derived from high maturation phase and high GOR fluid charged directly,rather than genetic of oil-cracking in oil-generating window phase.Accounting for petroleum source and cause of ultra deep oil simultaneously,it is concluded that ultra deep formation still has tremendous exploration prospects.
It is difficult to recognize complex lithology of the Ordovician carbonate in Tazhong area by conventional logging analysis with too little core data of Yingshan Formation.To solve this problem,seven wells with complete core slice,borehole and logging data of the Yingshan Formation were selected as standard wells.We chose five logging curves of GR,CNL,DEN,RXO,AC which have sensitive reaction to lithology change.Using the principal component analysis,we constructed five comprehensive variables including Y1-Y5.Among these variables,the variance proportion of Y1 and Y2 accounts for 85.83% of the total contribution ratio,so they can replace the original five variables efficiently as two principal components.Through the analysis of crossplot of Y1 and Y2 ,we can identify the grain stone,dolomite and algal limestone effectively.We depicted the carbonate lithology of non-complete wells from the Ordovician Yingshan Formation in Tazhong area precisely.Compared with the conventional logging analysis,the principal component analysis can integrate more parameters and adapt to the quantitative recognition of carbonate lithology.
Forty five Lower Cretaceous reservoir samples collected from 8 oil-rich sags in Erlian Basin were used to study the characteristics of fluid inclusions and hydrocarbon charging features of reservoirs in different structural belts.Fluid inclusion petrography and microthermometry analysis showed the reservoirs were mainly suffered one term fluid flow except some in NTGS(Near Trough Gentle Slope) which suffered two terms fluid flows.Integrating burial and thermal history studies,the source rocks in deep trough were mature earlier than those in NTGS.In order to restore the charging pressure of the reservoirs,the PVTsim simulation of organic inclusions and the salinity-temperature method for brine inclusions were conducted to testify the results which indicated the reservoirs in deep trough were charged with overpressure,but the reservoirs in gentle slope were charged with normal pressure.Besides,the abnormal values of interval transit time logging data also verified it.The charging intensity are examined by researching on the relationship between GOI index and oil-saturation,which reveals that in the deep trough and NTGS region,the GOI is proportional to the charging intensity,whereas in the gentle slope of remote trough,the GOI can't represent the charging intensity.
A series of big-medium sized oil fields have been found in the Bohai Bay area,but the proved reserve of gas is very small.It developed multiple sets of mature source rocks,which are abundant enough for the formation of big-medium sized gas fields,thus preservation condition is a vital factor in the gas accumulation.Whether the gas could be preserved or not depends mainly on two key elements.They are(1)faulting intensity during the Neotectonism and density of the secondary faults in cap-rocks;(2)thickness of the cap-rocks and drops of the displacement pressures and overpressures between cap-rocks and reservoirs(DPCR).In the Liaodong Bay province,density and activity intensity of the faults developed during the Neotectonism are very little and DPCRs of the Paleogene reservoirs are very high,so the gas pools are well preserved in the Paleogene.Today,the two biggest gas fields in the Bohai Bay area just occur in the Liaodong Bay province.In contrast,in the Bozhong province,plenty of petroleum has migrated into the Neogene reservoirs through the intense activity faults.However,because of poor sealing property of the Neogene cap-rocks,most of the gas pools have suffered damage,whereas some of the pools in the Paleogene are preserved owing to the excellent Paleogene cap-rocks.So far,6 gas fields with reserves about several billion cubic meters have been found in the Paleogene of the Bozhong province.
Melut Basin is a rift basin formed under the control of central Africa shear zone,with the passive rift basin character in Cretaceous and active rift basin character in Tertiary.In the long exploration activity,braided river and braided delta are the main exploration targets.By integrating geological and geophysical methods,the study identified a series of fans in North Melut sub-basin,including Abyat fan delta in the east of the sub-basin and Ruman subaqueous fans in west of the sub-basin.The fans mainly formed in the initial passive rift stage of Melut Basin and developed in the Cretaceous source rock of Al Renk Formation and formations close to the source rocks,such as Galhak Formation and Al Gayger Formation,with good oil source and reservoir-caprock assemblage.The discovery of fans in Melut Basin proved that central Africa Rift Basins also can develop fan deltas and subaqueous fans with good accumulation conditions and potential in the initial passive rift stage just like active rift basins.The research further expands the exploration potential and space for central Africa Rift Basins.
This paper analyzed the petroleum gelogical conditions of Jurassic play in Ogaden Basin,and the research shows that the conditions are favorable for hydrocarbon accmulation.Jurassic play could have many trap types,such as low amplitude anticline,fault block,oolitic limestone,dolomite lens,reef,local corrosin belt,and so on.In different part of the basin,the trap type may have some differences,but most of them will be lithology traps.The Uarandab Formation is the most important oil generation source rock in Ogaden Basin,and it is overlied on the Hamanlei Formation.Latteral migration will play a very important role in the oil accmulation,and regional palaeo uplift is the most favourable zone for oil accmulation.The vertical distribution of hydrocarbon is controlled by lithologic combination of Hamanlei Formation.The Middle Hamanlei Formation could be sourced by several sets of source rocks,so oil and gas pool all could be found.The most important source rock of Upper Hamanlei Formation is the Uarandab mud,and for it has poor interlayer seal,so oil pools are likely to be found near the top of the Upper Hamanlei Formation.The Calub uplift is a palaeo-uplift,reservoirs are best developed,fault and folds are also well developed in this area,and it is also surrounded by mature source rocks.The hydrocarbon accumulation conditions are good around Calub uplift,and it should be the most realistic exploration target.
Wells with high gas oil ratio of Jurassic Da′anzhai Formation are widely distributed in the western part of Central Sichuan Basin,where oil wells are also mainly distributed.Study on natural gas origin is helpful to understand the hydrocarbon enrichment regularity in Jurassic Da′anzhai Formation,central Sichuan Basin.Based on the comparison of natural gas compositions and carbon isotopes between Da′anzhai Formation and Upper Triassic Xujiahe Formation,combined with the characteristics of condensate light hydrocarbons of Da′anzhai Formation,it suggests that gas of high oil gas ratio wells is characterized by humic or near-humic origin which is similar to the gas from Xujiahe Formation.The mixture of sapropelic gas from Da′anzhai Formation with humic gas from Xujiahe Formation leads to high gas oil ratio wells in Da′anzhai Formation,western part of Central Sichuan Basin.On one hand,when the faults crossed the Da′anzhai Formation,light hydrocarbons in the primitive reservoir of Da′anzhai dissolved in the gas injection from underlying Xujiahe Formation and migrated to the upper formation.On the other hand the injected gas caused deasphalting to fill the reservoir′s pores and fractures,which leads to poor exploration effects near faults.
Diamondoids,as a kind of compounds with high thermal stability,have been used to determine maturity widely.However,the practical applicability of diamondoid parameters remains controversial.Based on this problem,the pyrolysis simulation experiment of the coal-measure mudstone from Xujiahe Formation,Sichuan Basin was carried out.The results indicate that,with the increase of equivalent vitrinite reflectance(EasyRO),diamondoids experience generation(1.0%
Simulation of source rock for hydrocarbon generation and expulsion in a semi-open system has been carried out on the Type Ⅲ carbonaceous mudstone from Tao 10 Well in the Liaohe Basin.The procedure of slow heating thermal simulation experiments was carried out by the way of temperature-time control(T-t) and pressure-temperature-time control(P-T-t),which all included six temperature points:350℃,400℃,450℃,500℃,520℃ and 540℃ to explore how temperature and pressure affected the rate of pyrolytic products.The result showed that the lowest products appear at 500℃ and 450℃,respectively in T-t and P-T-t series.The maximum gaseous hydrocarbon products appeared at 520℃(T-t,39.452mg/g) and 500℃(P-T-t,27.697mg/g).At the same temperatures,the total hydrocarbon production rate also reaches the peak points,which respectively were 51.788 and 52.057mg/g.After this thermostage,the hydrocarbon yields all had the tendency of reduction.The comparison of expelled oil,residual oil,total oil and total hydrocarbon indicated that pressure promoted the generation of hydrocarbon of type Ⅲ kerogens.As the pyrolysis products evolution had the same evolution with traditional sedimentary organic matter,so a relatively accurate experiment has been done and it placed an important part in the understanding of actual geological process,which also provide theoretical guidance for the study of hydrocarbon potential of basins or hollows.
The Yanji Basin is a Mesozoic fault basin.Based on cores and well logs of 20 boreholes,facies of the Tongfosi Formation are mainly fan-delta and lacustrine.Thickness of dark-colored mudstone ranges from 20 to 500m.Analyses of organic carbon,chloroform bitumen “A”,hydrocarbon generation potential,and organic matter type on more than 700 dark-colored mudstone samples show that,source rocks of the First,Second,and Third members of the Tongfosi Formation have relatively high organic matter concentration and are characterized by type-Ⅱ kerogen.Results of organic maturity degree（RO）analysis on 90 dark-colored mudstone samples show that,apart from part of the third member in the Dexin Sag,most of the dark-colored mudstone of the Tongfosi Formation in the Yanji Basin is slightly mature or mature,and is interpreted as effective source rock.Thus,the Yanji Basin is a potentially productive petroliferous basin.Based on five evaluation indices,such as hydrocarbon production,facies,and trap types,a series of well blocks(i.e.,Y 13 to Y 20,Yan 10 to Yan 12,and Y 1 to Y 3) are selected as priority prospects.
Methane adsorption characteristics in coal play an important role in the gas content estimation and gas productivity prediction in coal-bed,but there are differences of methane adsorption isotherms at different temperatures,so many adsorption isotherms need to test at each temperature,which will cause high cost and long time.According to the metamorphic grade of coal,anthracite coal,lean coal and gas-fat coal were chosen as test coal samples.Gas adsorption tests,which relied on high/low temperature adsorption experiment device,were carried out at 30℃,20℃,-10℃,-20℃ and -30℃ to study anatomic features of methane adsorption characteristic curves.And with adsorption heat theory the gas adsorption isotherms of different coal rank were predicted in the high/low temperature environment.The experimental results show that with the decrease of environmental temperature,the gas adsorption capacity of different coal rank all increase.The isosteric adsorption heat of different coal rank has good linear relationships with the isothermal adsorption quantity.The prediction values of gas adsorption quantity with adsorption heat theory agree well with the experimental results,whose error is less than 2%.With little measured data of isothermal adsorption tests,the method of adsorption heat theory can accurately predict the adsorption ability of coal at different temperature and pressure,which will greatly reduce the workload and provide an important basis to study the coal reservoir adsorption properties.
Finding out the changing laws of dynamic water porosity can provide the theory base for predicting the water production and making drainage working flow in drainage stage of coal bed methane wells.According to the theory of complex variable functions,elastic and plastic mechanics,the mathematical model about reservoir fracture stress distribution was established in original state under different fracture distribution.According to the damage theory,principle of effective stress,and the structural deformation theory,the stress and damage evolution model was established during the dynamic changes of production process.According to water migration equilibrium conditions and the pore and fracture distribution,coupled with the effect of the stress and deformation of the structure,the mathematical model of dynamic water porosity was established.Taking Fanzhuang block of Qinshui Basin as a calculating example,the results show that:the critical width of the crack is 0.1μm according to the changing values of the stress and structural damage,and it can be testified by related research results.The reservoir water porosity is reduced exponentially and tends to be stable finally in the drainage process.It increases linearly with the increase of porosity,reduces linearly with the increase in smaller crack ratio.
The length of the horizontal section of a horizontal well has a direct effect on controlled reserves,single well output,drilling cost and other indicators.It is a key factor to determe the development performance.For carbonate reservoir with low permeability,thin layer and strong heterogeneity in the Jingbian Gas Field,based on the hydro-electrical analogy on gas percolation mechanism,to optimize the length of horizontal section,a comprehensive analysis has been carried out,including numerical simulation,economic evaluation and field application.The evaluation results show that a horizontal well gas drainage area is approximately elliptic,the major percolation parts are on the heel end and toe end.When the horizontal length is greater than 1000m,after 5-year plateau the production allocation can reach three times more than a vertical well|when the length is 3 000m,the economic benefit is optimal|for the targets of output and efficiency,as well as drilling result,it was determined that the reasonable horizontal length of well Ma51+2 is 1 000m to 1 500m.The research results have been applied in the production capacity construction of Jingbian Gas Field,and it provides a reference for other similar horizontal gas wells design.
Kozeny-Carman(KC)equation is a semi-empirical equation,which is widely used to predict permeability of porous media in the field of flow.Since the establishment of this equation,many new methods were adopted to increase its accuracy.In this paper,an analytical expression for the permeability in porous media using the fractal theory and capillary model was derived based on Posenille law and Darcy equation,which reflects the permeability,porosity,specific surface area relation.The new proposed model is expressed as a function of three properties of porous media considering the specific surface area from the classical KC equation.Meanwhile the fractalKC constant with no empirical constant is obtained.The result shows that permeability of porous media is the function of fractal dimension of pore structure,tortuosity,macroscopic petrophysical parameters(porosity and specific surface area).The KC constant is not constant and has close relationship with tortuosity,fractal dimension and microscopic pore structure parameters.It is concluded that the permeability calculated by using new fractal model is more accurate than that by other KC equations.
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