On the background of incised paleochannel of large-scale valley filling,4 types of paleogeomorphology units,including incised-valley,clinoform,mound and lower monadnock,are developed in Wuqi area of Ordos Basin.Among the 4 types,mound,clinoform and lower monadnock are the main distribution units of the reservoir.The main accumulation locations of the oil are highs of the lower monadnock on the clinoform,source of the secondary river and the two-level mound mouth.Bottom water system in the incised paleochannel is well developed,which drove the oil-gas across the one-level mound mouth generated by the cutting of the two-level and three-level paleochannels,then reached the two-level mound mouth generated by the cutting of three-level and four-level paleochannels,and accumulated.The resultant force difference formed by the superimposition of the pressure generated by both sides of the paleochannel slope and the pressure generated by the regional monocline formation,causes the differences in the occurrence position of reservoirs at both sides of the paleochannel.The main occurrence positions in northeast bank of the Mengshan paleochannel are Yan 9,Yan 8 and Yan 7 oil-bearing formations of the Upper Jurassic.And the main occurrence positions in southwest bank are Fuxian and Yan 10 oil-bearing formations of the Lower Jurassic.
The level of exploration and geological understanding of Dongping-Niudong slope area,which is in front of Aerjin Mountain of Qaidam Basin,was poor in early period.Recently,under the guidance of the new exploration theory of looking for large lithologic reservoirs,which are outside the sources,in slope areas,we strengthened comprehensive analysis and evaluation of the lithologic gas accumulation conditions in the slope areas.The result showed that the accumulation conditions in Dongping-Niudong slope were favorable,such as large distribution area of source rocks,multiple sets of target layers department,the favorable reservoir-cap combination,the continuing development of palaeohigh(ancient slopes),regional unconformity,divert fractures for oil and gas and so on.Now,we have achieved great exploration breakthrough in the weathering crust and Lulehe Formation(E1+2) of Well Dongping 1,which located in low faulted uplift part,industrial gas in lower Ganchaigou Formation(E3) of the Well Dongping 3,which located in high fault terrace part,high-yield oil and gas flow in the Jurassic of well Niu 1,which located in Niudong nose uplift.These findings show that the natural gas accumulation condition in the whole Dongping-Niudong slope is advantageous and this area has broad prospects of exploration.
The middle member of Toutunhe Formation in southern margin of Junggar Basin is an important hydrocarbon reservoir,while the type of depositional system is still controversial.Based on detailed analysis of outcrop lithofacies and lithofacies associations and fine interpretation of outcrop section,nine lithofacies and four lithofacies associations were identified.Typical lithofacies association was characterized by dualistic structure,and obvious lateral accretion was identified in outcrop sections,which can be confirmed as point bar sequence of meandering river.It proved the existence of meandering river in the middle member of Toutunhe Formation in southern margin of Junggar Basin.According to measured results,thickness of lateral accretion sandbody in the middle member of Toutunhe Formation was about 3-5m,about 90-120m wide,with an angle about 5°.Laterally accreted mud was about 50cm thick,with a dip of 3°-6°.Based on the results of outcrop study,correlation method and steps of “identified in single wells first and predicted among wells second” for lateral accretion sandbodies in meandering river have been discussed,providing a quantitative geological knowledge base for the correlation of underground sandbodies developed in meandering river.
The Carboniferous volcanic rocks are an important new field for oil/gas exploration in the deep strata of western China.Great prospects have been proved in the volcanic rocks of Junggar Basin.This paper focused on the characteristics and controlling factors of volcanic rock reservoirs in Ludong area of the eastern Junggar Basin,based on microscopic observation,scanning electron microscopy.Additional samples were collected from the outcrop in eastern basin for geochemical analysis and temperature test of inclusions to establish the process of reservoir spaces.During the Later Carboniferous,the study area developed a suit of low potassium sodium alkali-rich and alkaline volcanic rocks,which mainly consisted of eruption (volcanic breccia and tuff) and overflow (basalt and andesite) facies volcanic rocks.Volcanic rock reservoir spaces are mainly secondary dissolution pores and fractured pores,and the reservoirs with best property are eruption facies volcanic breccia and overflow facies basalt,andesite physical.The favorable reservoir zone consisted mainly of weathered eluvial horizon,followed by collapse zone.The reservoirs in each zone were developed by weathering and leaching,tectonic fracture and hydrothermal alteration,especially the hydrothermal alteration which formed zeolite or calcite filling has important significance to the reservoir.The Carboniferous volcanic rock reservoirs in the study region are mainly controlled by the fracture distribution,and the weathering structure and lithology for the second and third controlling factors respectively.
Overpressures in sedimentary basins are usually controlled by many factors,which make recognition and evaluation of each overpressure mechanism be difficult.The paper divided the causes of overpressure into four general categories: disequilibrium compaction,fluid expansion,overpressure transfer,and lateral tectonic stress.The comprehensive recognition model of these overpressure mechanism types was established.Disequilibrium compaction and lateral tectonic stress overpressures are typically associated with abnormally high porosities and overpressures generated by fluid expansion and overpressure transfer mechanisms are not associated with a porosity anomaly.Sonic velocity-vertical effective stress and sonic velocity-density plots are used to distinguish between overpressures generated by the four types.On a sonic velocity-vertical effective stress plot,sediments that have undergone disequilibrium compaction remain on the exponential function curve of normal compaction loading.However,overpressures generated by fluid expansion or overpressure transfer or lateral tectonic stress follows a sonic velocity-vertical effective stress path away from the loading curve.On a sonic velocity-density plot,sediments that have undergone disequilibrium compaction or lateral tectonic stress remain on the loading curve.However,overpressures generated by fluid expansion or overpressure transfer follow a sonic velocity-density path away from the loading curve.On the basis of the hypothesis that the decreasing magnitudes of vertical effective stress are equal to the overpressures generated by fluid expansion or overpressure transfer or lateral tectonic stress,the overpressure mechanisms are assured and the magnitudes of overpressure and the relative contribution generated by these overpressures were evaluated combined with the regional geological conditions.Although the evaluation results will underestimate the overpressure generated by these mechanisms,these can provide a good method for recognition and evaluation of overpressure in complex regions of sedimentary basins.
In the mid-shallow formations of Ledong area of Yinggehai Basin,forced regression system tracts are widely developed.According to the comprehensive analyses of sediments,seismic,logging,core etc.,it is proposed that forced regression system tracts are developed in shallow depositional environment.The depositional wedges prograde toward basin center along downdip direction of the continental slope in section and shorelines migrate toward basin center in turn with method of approximately paralleling in plane.Fine reservoirs include longshore bar,channel sandstone,turbidite sandstone.Forced regression system tracts in Ledong depression area possess the basic condition of forming large-scale oil and gas reservoirs and is strategically important to rolling exploration in the research area.
Based on simulation experiments with and without sulfate,the authors comparatively analyze the gas components obtained from experiment,and then investigate the rule of alteration in TSR-altered gases.The result shows: TSR advanced the total gas production rate of samples and the main portions are non-hydrocarbon gases,which refers to that high-pressure gas reservoir can be generated easily because of TSR alteration in the consistent condition of porosity and temperature; production rate of methane is low,and the influence of TSR on production rate of methane is relatively great.Because organic matter may be under joint action of pyrolysis and TSR,it is not sure whether TSR-altered methane can cause of decease of methane production rate or not; the impaction of TSR on production rate of ethane may be related to organic matter type.Ethane production rate of typeⅢorganic matter increases,whereas that of typeⅠ and typeⅡ organic matter deceases due to TSR.The possible cause is that typeⅠand typeⅡ are more prone to TSR than type Ⅲ.In the consequence,the formers′ capability to form ethane is relatively weaker; the content of hydrogen sulfide is less than that of carbon dioxide in experiment,but it is generally converse in geological field.The formation of bicarbonate due to the combination of carbon dioxide and calcium ions may be the main reason for the reduction of carbon dioxide.Fraction process of carbon dioxide probably advances the property of reservoir a lot.
In recent years,analytical techniques of the petroleum geochemical exploration in China have made great progresses,especially hydrocarbon analytical techniques and methods are constantly updated and improved.There have been many improvements based on the traditional analyses,some new analytical techniques emerged,and sample collection and field analyses also develop continually.However,some problems still exist,such as the low refinement of analytical techniques and less high-resolution analytical items.Development direction of the analysis of petroleum geochemical exploration is as follows:to enrich the theoretical basis of technology;to improve traditional technologies;to learn advanced analytical techniques in the other fields;to develop fine analysis;to promote standardization and field analytical technology;to combine analytical technology effectively based on oil and gas source types;to focus on the detection technology of C6-C18 hydrocarbons,polycyclic aromatic hydrocarbons and benzene;to integrate analytical technology of the petroleum geochemical exploration,the remote sensing exploration,the geophysical exploration and the seismic exploration.
The shale gas potential of oil shales of the Lucaogou Formation in SE Junggar Basin was investigated by geochemical analyses,such as Rock-Eval pyrolysis,total organic carbon(TOC),organic matter composition and biomarkers.The average TOC from 45 outcrop samples is 9.71%.The average hydrogen index(IH) value is 471.56mgHC/gTOC,indicative of TypeⅡ kerogen.Based on the Tmax and biomarker analyses,all the samples are classified as low mature and mature.The analytical results and the comparison with geochemical and geological parameters from five United States shale formations suggest that the study area is qualified to have themogenic shale gas generation potential and that parts of the study area may have the presence of microbial shale gas.Sangonghe area,one of the three locations of this study is supposed to be the most potential shale gas generation area in the study area,because of its best organic matter richness,the highest maturity and the biggest reservoir thickness.
Based on the collection of a great number of data,according to the chemical composition and carbon isotope composition of 10 natural gas samples from the central zone of Zengmu Basin,the geochemical characteristics and origin of natural gas in this area were analyzed systematically.At the same time,the biomarker features of condensate oil associated with gas were analyzed,and its oil source was discussed.Research shows that,the alkane gases in the central zone of the basin are mainly composed of methane,and the dryness coefficient ranges from 0.68 to 0.97,indicating the existence of both dry and wet gases.Carbon isotope values have positive sequence among the C1-C3 alkanes.δ13C1 varies from -45.6‰ to -31.5‰,δ13C2 varies from -32.7‰ to -24‰,and δ13C3 varies from -30.1‰ to -23.4‰.All these indicate that the hydrocarbon gases are of biogenic origin and belong to the primary kerogen cracking gas.while the non-hydrocarbon gas are mainly composed of CO2 and N2 and δ13CCO2 varies from -10.8‰ to -2.4‰,which shows that CO2 is of abiogenic origin condensate oil associated with natural gas have high Pr/Ph ratio,high content of oleanane and bicadinane,which have the genetic relationship with coal-measure source rocks.
Water phase trapping of tight gas reservoir caused by its high capillary pressure prevents gas reservoirs from being detected,precisely evaluated and economically developed.The present physical and chemical methods for removal of water phase trapping is seldom applied because that they might cause other damages.The experiments of N2 displacement to stimulate formation water tight sandstone core samples with the permeability of less than 0.1×10-3μm，(0.1-0.3)×10-3μm，and more than 0.3×10-3μm2 are conducted under the conditions of in situ stress and constant flow pressure difference or increased pressure differences to recognize the water flowback behavior in tight sandstone.The results indict that water saturation of core samples decreases gradually,and the lower the permeability of cores is,the lower the decrease rate of water saturation will be,and that the decrease rate of water saturation of core with the permeability of (0.1-0.3)×10-3μm2 displaced by increasing pressure difference is lower than that displaced by constant pressure difference within 150 hours,but the water flowback rate of core displaced by increasing pressure difference is higher.The analysis of results show that water phase trapping is severe.Pore throat structure,permeability and pressure gradient are the most important factors.The process of water flowback from cores includes two effects,water displaced by gas and water evaporation due to gas flow.If the pressure difference of water flowback increases properly,it will accelerate water evaporation effect when gas can flow through different sizes of pore throats.
Dina 2 Gasfield is an ultra-high pressure,low-permeability fractured condensate gas reservoir with high heterogeneity.The relationship among single well production, fractures and physical properties of the matrix is analyzed by core, logging and test data.On the basis of the results,development wells are divided into 3 classes by using Q-type cluster analysis, synthesizing thickness of fractured intervals,fracture density, fracture aperture and KH value of matrix. Taking the classified gas wells as samples, classification discriminant function is established based on linear regression, and its results are in agreement with the ones of clustering method.Therefore, the results provide a reasonable basis for productivity classification evaluation and policy making of development technology in Dina 2 Gasfield.
As the effectiveness of conventional stimulation treatment is under-expected for tight reservoir and unconventional resources are urged to exploit,based on thermal properties of rock,formation heat treatment (FHT),is developed as an innovative well stimulation technique,which is focused on the permeability of rock.Experimental evaluation methods of FHT are shown in this paper.Comprehensive analysis indicates that the essence of the physical-chemical changes for tight rock under high temperature is a set of multi-scale processes involving evaporation of water in pore,change of mineral structure and generation of fracture network.Under the action of thermal stress,the main mechanism of enhancing permeability is the fracture network developed from micro-fracture and fracture propagation.Successful field cases of FHT are presented.On the basis of multi-scale effect in tight reservoir and its production mechanism,the advantages of FHT are reflected in preventing water phase trapping,decreasing fracture pressure,forming fracture network,enhancing-desorption and diffusion of gas in pore.In view of multi-scale mass-transfer theory,a new well stimulation technique,integrated the superiority of FHT,fracturing and acidizing is expected.
Partial completion in horizontal well is the exclusive methodology to effectively exploit bottom water reservoir,and it is widespread used around the world.However,the relevant researches about the effect of partial completion parameters on water coning are extremely poor.Utilizing potential superposition principle and mirror-reaction principle,the wellbore/reservoir coupling calculation model was obtained,and further the bottom water breakthrough model of partial completion was gained.The comparison between forecasted results with real value certified the accuracy of this model.Then,the study on the effect of partial completion parameters revealed the following results by calculation.The breakthrough height of bottom water in blank pipe and the breakthrough time of bottom water in production sections gradually increase with the increase of open degree.Following the movement of blank pipe,the breakthrough height and time of bottom water in blank pipe remove correspondingly.And when blank pipe moves from heel to toe,the breakthrough time of bottom water in production sections close to heel slightly increases,while the breakthrough time close to toe marginally drops.Furthermore,with the increase of open segments,both the breakthrough height in blank pipe and the breakthrough time in production sections slightly increase.
Based on the analyses of sedimentary sequence,lithology,particle size,and logging data from more than 20 drilling wells in Maxian region on the northern margin of the Qaidam Basin,the sedimentary characteristics and its evolution of Paleogene Lower Xiaganchaigou Formation were studied.The results showed that braided fluvial facies were the main sediment types of this formation,including channel bar,point bar,and flood plain,and the environment with water withdrawal was the main depositional environment in this period.According to the fine structural interpretation of 3D seismic data,seismic attributes can also well illustrate the changes of depositional environment of the Lower Xiaganchaigou Formation:In the early period,the sediments were mainly from northeast of the Maxian region,and a little from the north,and sandbodies were mainly dominated by the northeastern provenance;Along with the basin development,the northeastern provenance gradually migrated westward,and its supplies also decreased,in contrast,the supplies from the northern provenance increased;In the later period,the above two provenances got closed,and then developed large and contiguous sandbodies.Preliminary conclusions about the environmental evolution of Lower Xiaganchaigou Formation in Maxian region and three sandbodies distribution can provide reliable geological data for the next oil and gas exploration.
During the shallow lithologic gas reservoir exploration in the western Sichuan Depression,seismic amplitude “bright spot” anomaly is an important basis for the reservoir identification.However,more and more drilling wells confirmed that the “bright spot” anomaly may be a trap resulted from the non-reservoir such as mudstone,thin-interbedded layer,dense sandbody.Based on the typical drilled well seismic analysis in the Deyang syncline,western Sichuan Depression,this paper summarized the seismic response characteristics of sandbody with different physical properties,and established a high quality reservoir identification model with features of “strong trough top,strong peak bottom” and “weak top and strong bottom” of adjacent peak reflection energy.At the mean time,seismic amplitude “bright spot” trap can be excluded by using the characteristics such as “strong top and weak bottom,double strong” of adjacent peak reflection energy,improve the reservoir prediction accuracy which is based on seismic energy reflection.This method is simple,intuitive,easy to realize,moreover,it is accurate and reliable which has been proved by the drilling results.
In order to analyze the distribution of different levels of tectonic fractures in carbonate buried hills,based on core observation and statistics of carbonate reservoirs of Ordovician in Futai Oilfield in Bohai Bay Basin,tectonic fractures are divided into three levels according to aperture and fluid phase behavior in fractures.The distribution of different levels tectonic fractures in each formation is numerically simulated subsequently.The results indicate as follows,tectonic fractures in carbonate reservoirs of Ordovician in Futai buried hill can be divided into large aperture fracture (aperture>1.0mm),medium aperture fracture (aperture:0.5-1.0mm) and small aperture fracture (aperture<0.5mm).Distribution of different levels of tectonic fractures is affected by tectonic position,faults and lithological characters.The numerical simulation results show that large aperture fractures mainly distribute in anticline core and near faults in O2bd,O2smand O2xm.Medium aperture fractures mainly distribute in anticline flank and O1y.Small aperture fractures scatter in O2bd and O2sm.
The main target formation of B-area in Panyu Field is a lowstand-tract wedge-shaped delta sandstone developing on the shelf break zone in SQ21.0 sequence,which has thin layers and fast horizontal changes.Aiming at the difficulties of low resolution of seismic data and the uncertainty of sand pinchout boundary,this paper built geological and geophysical model from seismic profile of typical deposition,studied on the seismic response on sand pinchout of calcareous high-velocity overlying strata and different shale interlayer thickness by forward modeling;on the base of these,we applied seismic attribute analysis and pre-stack simultaneous inversion to finely describe the sand pinchout line;lastly,we confirmed the availability of the technology combination with the verification of real seismic and drilled data.
he middle Yangtze region is one of the most important petroleum provinces,in which the shale of Wufeng-Longmaxi Formation is widely distributed and the exploratory well in Western Hubei and Hunan provinces has good gas show.At present,research of shale gas in China is hot,while the production of shale gas in North America has achieved success.Therefore,in our country,strengthening the study of characteristics of shale reservoirs and gas content has great significance for shale gas exploration and production.In this paper,based on the observation of field outcrops and cores,integrating with analysis of rock slice,mineral content,physical properties,organic geochemistry and other methods,the features of lithofacies,mineral,reservoir space and gas content of shale in Wufeng-Longmaxi Formation are studied in detail.The results show that shale of Wufeng-Longmaxi Formation in the study area is mainly composed of mud shale,siliceous shale,graptolite shale,carbonaceous shale and silt shale,quartz and clay consisting of essential minerals.The main reservoir space types are matrix pore,organic pore,dissolved pore,microfracture.Moreover,the content of free gas and adsorbed gas,which is mainly controlled by total porosity and organic carbon content,ranges from 0.09-0.17m3/t and 0.20-0.84m3/t respectively.The total gas content is between 0.29m3/t and 1.01m3/t. According to the definition standard of shale gas,the favorable region is located at Enshi-Pengshui area,and the estimated resources range from 0.23×1012m3 to 0.81×1012m3.These features indicate that the shale of Wufeng-Longmaxi Formation in the middle Yangtze region has good shale gas exploration prospect.
Based on the geological and organic geochemical features of the Upper Triassic mudstone in Junggar Basin,mudstone core gas content of the Upper Triassic in the well Da 9 wad determined,as well as the molecular composition and carbon isotopes of the gaseous hydrocarbons.Mudstone has some content of gas,mainly 0.18-0.24m3/t.The gas is dominated by alkane gas,with low level of nonhydrocarbon such as CO2 and N2,etc.The alkane gas is dominated by methane with low content of heavy hydrocarbon,indicating dry gas.For hydrocarbons with same molecular weight,isoparaffin is more predominant than normal paraffin.Among the nonhydrocarbon,nitrogen has apparently higher content than CO2.Alkane gas has relatively low carbon isotopes,demonstrating features of low-mature sapropel-type natural gas in place which might be related to short time desorption and lower maturity of source rock etc.
Based on the measured isotherm adsorption data of marine and continental organic-rich shale,discussion has been performed on the characteristics of isotherm adsorption and its relationship with all controlling factors.The difference between them was illustrated in detail.The results are as follows.The shape of the isotherm curve of marine shale changes smoothly,while it is divided into two parts for the continental shale,i.e.shale with coals has much higher maximum adsorption than that of marine shale,and shale without coal has relatively low maximum adsorption.The maximum adsorption of marine and continental shale is positively correlated with total organic carbon (TOC) and specific surface area.In addition,the marine shale adsorption increases with vitrinite reflectance (RO) and quartz percentage in bulk composition,and decreases with clay percentage,however,these features are unclear for the continental shale.Although during the over mature stage,the maximum adsorption of marine shale is still positively correlated with TOC.with the continental shale has lower metamorphism,and the relationship between maximum adsorption and RO displays a mode of two-section type,i.e.RO<1.3%,namely before the mature stage,the maximum adsorption increases with the increase of RO;RO>1.3%,namely during the mature stage,it becomes smaller.The sedimentary environment,organic thermal evolution and type of marine and continental shale played important roles in these features.
To study the effect of the occurrences on the formation and dissociation of methane hydrate in sediment,the formation and dissociation experiments of methane hydrate in four different occurrences in sand,including massive,layered,nodular and disseminated,were conducted.The results showed that the effect of occurrences on the formation and dissociation of methane hydrate was evident and clear.The conversion rate of methane was highest in massive,lower in layered and disseminated,lowest in nodular.The methane hydrate formed fastest in the state of disseminated whose peak of methane conversion rate was highest,too.It took relatively the longest time for massive methane hydrate to form,secondly for layered,thirdly for nodular,and shortest for disseminated.It also showed that in dissociation process,the stabilization of massive methane hydrate was the best,secondly was layered,thirdly was nodular,and the stabilization of disseminated methane hydrate was the worst.
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