The depositional environment was isolated restricted platform and a lot of depositional cycles of shallow upward oolite shoal existed at the stage from Member 1 to 2 of Feixianguan Formation of Lower Triassic in the northeast Sichuan basin,especially gypsum lagoon at the Member 2 of Feixianguan Formation in the isolated restricted platform.On the isolated restricted platform geologic setting,the main lithology of reservoir includes remnant oolitic dolomite and finely crystalline dolomite.On the shallow upward oolite shoal geologic setting,the atmospheric water eluviation results in the lithology sequence from top to bottom is tight micrite dolomite,intragranular pore or moldic pore residual oolith micritic or powder crystalline dolomite,intragranular pore residual oolith powder or finely crystalline dolomite,and intercrystalline pore dolomite.Petrology and geochemistry of reservoir show that the reservoir dolomite underwent two stages of dolomitization.The first stage is seepage-reflux dolomitization and the second is burial dolomitization.The seepage-reflux dolomitization make an important role in preservation of early pores and increase rock strength,inhibit pressure solution and cementation,but the buried dolomitization partially fill pore space.It can be recognized two types of buried dissolution.The first stage happened after seepage reflux dolomitization and before crude oil filling and it is correlated with dissolution by organic acid.The second occurred in the deep buried environment,it is correlated with acid generated by TSR.The buried dissolution can not generate reservoir porosity,but increases permeability.Based on comprehensive studies of geologic settings,reservoir petrology and geochemistry,it is concluded that the atmospheric water eluviation is prerequisite condition for reservoir porosity generation and the seepage-reflux dolomitization is necessary condition for reservoir porosity preservation.
Barrier island is not only important for human residence,but also an important oil and gas accumulation unit.By the shore seaward,barrier dam and marshes,tidal flats,lagoons and shallow continental shelf accompanied each other,some areas of the coastal developed small delta.From the bottom,the shallow continental shelf,tidal channel,swamp and lagoon sedimentary facies constitute a complete sedimentary cycle in vertical.The barrier dam formation was related to the sea level rise,the small slope of the terrain,the development of terrain slope break belt and stability sandy source supply.Stable sea level rise and wave transformation formed terrain slope break belt near the body of a long strip of sand in the subtidal area.Small terrain slope is conducive to the gradually weaken of wave and tidal energy,and the sandy sediments gradual accumulated near the twists.With the continue arise of sea level,stable supply of sandy material source can ensure,the barrier dam to grow and not be submerged.
Based on drilling information and seismic data,and through the restoration of deposit thickness since the second member of Xujiahe Formation(T3x2 ) in the middle western Sichuan depression,the uplift-depression framework evolutionary process is reconstructed.Combined with drilling results and fluid inclusion analysis technique,the paper illustrates the gas accumulation pattern and process of T3x2 .The results show that the palaeotectonic physiognomy underwent apparent changes of configuration of the uplift-depression.The present Xiaoquan-Fenggu structural zone in the middle Chuanxi depression went through the evolution from wide gentle slope to depression,and eventually to uplift.While the evolution of Yazihe zone was from deep depression to high uplift.The palaeo-uplift of T3x2 did not play the controlling role for the conventional reservoir.The gas,sealed in the Yazihe palaeo-depression because of early densification of reservoir,was charactered by deep basin gas.The later tectonic uplift and uplift-depression evolution as well as induced fractures break the steady state of the deep basin gas,and caused gas lateral migration,eventually forming the present pattern,that is “continuous distribution & sweet point enrichment”.
Combining with the mathematical statistics method,we describe of tight sandstone reservoir characteristics and simulate the burial history and diagenetic history of the Member 8 of Yanchang Formation in the Baibao-Huachi area,Ordos basin.According to present porosity as boundary constraint conditions,geological times and burial depths,the whole porosity evolution from initial burial until present is divided into two independent processes of porosity decrease and porosity increase and establish two mathematical models,respectively.The quantitative simulation result indicates that the whole porosity-evolution quantitative simulation is a piecewise function with four sections.That is,the porosity decrease model of mechanical compaction stage is a continuous function with burial depth as the independent variable function,the compaction-cementation comprehensive stages is a continuous function with geological time and burial depth as the variables;the secondary porosity increase is caused by organic acid dissolution within a temperature window of 80-100℃.Therefore,the dissolution phase is the porosity increase model,which is a composite function of depth and burial time.After the dissolution phase,the porosity is in the phase of compaction and maintenance.The model of porosity decrease is a superimposed composite function of depth,burial time,and the porosity increase.Finally,by a case study,it is verified that the established porosity-evolution quantitative simulation is consistent with actual geological conditions and can be applied to the porosity calculation of similar lithology,burial history and sedimentary type strata in the study area.Meanwhile,this approach provides quantitative calculation method for porosity prediction and it will be helpful for oil-gas exploration of this area.
Interval Chang 81 was regarded as shallow lake sedimentary environment in Ordos basin,and the basin had a more gentle bottom morphology and more shallow water body in Shanbei area.The Chang81reservoir is investigated by means of well-logging,core properties,thin sections for reservoir types,temporal-spacial distribution,lithology and reservoir physical property.It′s showed that the area main developed underwater distributary channel sandstone and mouth bar sandstone with thin monolayer,high shale content,significant multiply cyclicity,poor continuity in vertical and longitudinal direction and wide distribution.The sandstones is lithic feldspar and feldspar lithic sandstone,which are regarded as packsand and hone.The reservoir is regarded as low porosity and low-extra-low permeability reservoir as to the poor physical property.Analysis shows that there are 3 reasons why this kind of sand body formed.First,the study area was far away from the provenance,which led to the insufficient supplication.Second,water was shallow,the bottom morphology is gentle and the lake shoreline spread widely.In addition,the water power of lake and river was weak.Therefore,in small times of lake level lifting process,see-saw type sedimentation model was formed in the Chang 81 sandstone.
By means of microscope,ESEM,constant-pressure mercury penetration and constant-speed mercury penetration，the physical property of Taiyuan and Lower Shihezi Formation reservoirs in the Shenmu area of Ordos basin is studied in this paper.The lithology of reservoirs is mainly composed of coarse and huge-coarse quartz sandstone and debris sandstone,and secondarily of grain quartz sandstone and boulder-coarse debris quartz sandstone.Owing to the intense diagenesis，most residual origin porosity had been lost.The abundant organic acid and resoluble component from coal measure improve the physical property of Taiyuan Formation reservoir.But,the Shihezi Formation reservoir is characterized by kaolinite intercrystalline micropore.The results of constant-speed mercury penetration show that the pore structure of Taiyuan and Shihezi reservoir has great differences.Compared with the Lower Shihezi Formation，Taiyuan Formation reservoir has larger bore diameter and higher ratio of pore throat,building up a favorable pore throat pattern.
Tight sandstone reservoirs of Upper Palaeozoic have characteristics with low permeability,various pore types and strong heterogeneity in Sulige gas field which lead to complex rock physical response characteristics.All the above causes the difficulty in identifying gas and water layers and quantitatively evaluating accurate gas saturation.Combining with the characteristics of tight sandstone reservoir in the study area and the logging response characteristics of gas layers，four kinds of applicable methods to identify gas layers were optimized and through comprehensive utilization of the above methods,gas layers were identified effectively.According to the characteristics of reservoir pore structure and rock electric response,the applicability of dual-porosity saturation model in the research area was determined and rock-electricity parameters of the saturation model which was applicable to the research areas were also established.Through applying these methods to a large number of wells，it shows that the four methods have good application effect in identifying gas layers in tight sandstone gas reservoirs.Compared with the Archie model,the gas saturation calculated by dual-porosity saturation model and corresponding rock-electricity parameters have higher accuracy,which have higher accordance to the actual law of gas reservoir and physical characteristics of rocks.The above methods provide effective way for logging identification and evaluation of tight sandstone gas reservoirs.
There is more and more attention being paid to the tight sandstone gas pools which have been the major force of the unconventional petroleum resources.As the tight sandstone gas pools are different from the conventional ones,whose reservoirs are tight,the rock property is bad,and the electrical response is unclear,thus it is necessary to clarify the lower limit of effective thickness for the tight sandstone gas pools.Based on the study of “the four property relationship”,this article takes the Upper Paleozoic tight sandstone gas reservoir of some gas field in Ordos basin as example,and systematically describes the method of determining effective thickness lower limit of tight sandstone gas pools,and the test method,the experience statistics method,the mercury injection parameter method and the logging curves method are respectively discussed.Compared with the actual gas testing data,the effective thickness lower limit determined by these methods is reasonable and meets up with the actual geological characters,of which the operation is simple and applies well.
The seismites is one of the research hotspots in recent years.With the deepening of the study,more and more seismite structures have been identified in ancient strata.Based on the study of typical seismites in different base,the paper analyzes formation mechanism of different seismite.Many continental clastic seismites' structures related to sand liquefaction.Micro normal fractures were formed t during stress adjustment which was occured after liquefaction of sands and micro corrugating.Vertical fault throw of micro normal fractures is equal to the differential subsidence of sands that have different particle size.Continental clastic seismites have no general sediment sequence because its distribution was mainly controlled by sedimentary facies or lithology.But similar sequence to vibration liquefaction in carbonate rock can be found in silt and clay size deposit.Seismites research is significant to structural-sedimentary evolvement analysis,earthquake forecast,exploration and development of oil & gas and regional stratigraphic correlation.
In this paper, we discuss the influence of structural figuration and structural dynamics on the transporting and reservoiring system in the Qikou sag.The results show that the Paleogene graben faults generated complex half graben-like framework on NW-SE orientation and complex duplex fault graben on N-S orientation in the Qikou sag, constrained by the Cangdong extensional fault system and Qikou extensional fault system.On a plane, the structural palaeogeomorphology pattern with the multi-convex and multi-concave, convex and concave separation formed a favorable space for lithologic reservoirs.The tectonic style of Qikou sag controlled the distribution of hydrocarbon accumulation.The main faults control the oil and gas migration and accumulation and secondary faults rework the oil and gas distribution and form the advantages conduit system.Both favorable reservoir facie and structural slope are prone to the formation of favorable trap.In the Qikou sag, we conclude four types of oil-gas accumulation, i.e.steep slope, gentle slope, broken joint associated with near-source, intrabasinal knick zone.The selected exploration targets have been verified by drilling instance.
Sandy braided river of Yungang Formation which is dominated by sands is composed of three typical lithofacies associations of channel,channel-bar and point bar.It as good oil/gas reservoirs has good reservoir properties,but there is muddy interlayer inside the sandy braided river deposits.According to measurement of typical outcrops of Jurassic sandy braided river in the Datong city,Shanxi province,we identify four genetic types of mudstone in the sandy braided rive deposits,i.e.mud plug,channel lag mud,lateral accretion mud,and flood plain mud.According to genesis types,we take the channels of single stage as basic unit to setup the distribution model of mudstone in the sandy braided river,limit the distribution range of mudstone of each genesis type.The distribution of mud plug,channel lag mud and lateral accretion mud is constrained by channels of single stage,while the distribution of flood plain mud is controlled by active channel zones.The establishment of distribution pattern can provide a concept model for the effective prediction of the geometric shape,rules and scales of spatial distribution for mudstone in reservoirs of sandy braided river.
The data of rock element and organic geochemistry confirm that a set of organic rich siliceous marine formation is distributed at the top of Permian in the northeast of Sichuan basin, whose thickness is 30 meters, and the sedimentary facies belongs to platform depression.The formation had type Ⅱ organic matter feature and the mean TOC and potential hydrocarbon ratio is 4.08% and 12.55mg/g, respectively, and the RO is 1.2%-2.0%.Thus, this formation can be cognized as excellent source sediment formation with oil yields（17.6-22.3）×104t/km2 and gas yields （1.65-1.98）×108m3/km2.Much more exploration attention should be paid to it.
There are two main natural gas pay zones in the Zhongba gas field,that is,Member 2 of Xujiahe Formation,Upper Triassic (T3x2) and Member 3 of Leikoupo Formation,Middle Triassic (T2l3) in the northwest Sichuan basin.The gas source for T2l3 remains controversial.The paper analyzes the components as well as carbon and hydrogen isotopic composition characteristics of gases fromT2l3.To compare with the gases from the Leikoupo Formation,the paper also selects the gases from Member 2 of the Upper Triassic Xujiahe Formation in Zhongba gas field,the Permian Maokou Formation and the Changxing Formation as well as the Lower Triassic Feixianguan Formation in the Hewanchang gas field and the Feixianguan Formation in the northeastern Sichuan basin.Gases from the Leikoupo Formation display high gas dryness index around 0.97,high H2S and CO2 contents.Gases from the Leikoupo Formation and the Xujiahe Formation of the Zhongba field,the Hewanchang field and the Feixianguan Formation in the northeastern Sichuan basin display great differences in carbon and hydrogen isotopic composition characteristics.The ethane carbon isotopic values of the Leikoupo Formation are heavier than those of the Hewanchang gas field and the Feixianguan Formation in the northeastern Sichuan basin,but the methane carbon isotopic values are lighter than those of the Feixianguan Formation.The methane hydrogen isotope of the Leikoupo Formation are about -140‰,20‰ lower than those of the oil-associated gases of the Feixianguan Formation,but 30‰ greater than those of the coal-derived gases of the Xujiahe Formation.Comprehensive analysis about gas components,carbon and hydrogen isotopic composition characteristics,light hydrocarbon compositions,Mercury contentssuggested that gases from the Leikoupo Foramation of the Zhongba field are mainly Permian oil-associated gases mixed with a proportion of coal-derived gases from the Xujiahe Formation,and experienced thermochemical sulfate reduction transformation.
Based on GC and GC-MS analyses of bulk crude oil and source rock samples in different regions and layers of Lishu Fault Depression,the geochemical characteristics of crude oil and source rocks were systematically studied.According to different composition characteristics of the biomarkers,the crude oils in the Lishu Fault Depression were divided into three genetic types.The GroupⅠ,mainly distributed in the southeastern slope belt and Siwujiazi oilfield,were characterized by high C24-tetracyclic/C26-tricyclic terpanes ratios,and low tricyclene/pentacyclictriterpane,gammacerane/C30-hopane,Ts/Tm,C29Ts/C29norhopane,C30 diahopane/C30-hopane and C29ββ/（ββ+αα）ratios;The GroupⅡ,mainly found in the northern slope belt,have low C24-tetracyclic/C26-tricyclic terpanes value,and high tricyclene/ pentacyclictriterpane,gammacerane/C30-hopane,Ts/Tm,C29Ts/C29 norhopane,C30 diahopane/C30-hopane and C29ββ/(ββ+αα） ratios.The Group Ⅲ,mainly located in the central structural zone and seven tree synclinore zone,display the intermediate chemical compositions between the GroupⅠand Group Ⅱ oils.Combined with the physical properties of crude oils,geochemical indicators to different types of crude oil are established,and the genesis and sources of crude oil were defined.
In this paper,we measure the chemical composition of light hydrocarbons,the Mango parameters,and thermal maturity for 40 oil samples from the Jinhu sag.The methylclohexane index reveals that the oils are derived from type I and type II kerogen.The C5 light hydrocarbons indicate that the organic matter is composed of both sapropelic and humic.The Mango parameters K1 is consistent with the chemically stable structure model of light hydrocarbons,suggesting that the oils would be formed in the similar depositional environment of organic matter.Predominance of C5 to C7 tricyclic light hydrocarbon abundance is more than that of pentacyclic and hexacyclic light hydrocarbon,showing that oils are mainly originated from lacustrine source rocks.Both heptane value and isoheptane index are little,indicating the thermal maturity of oils is low.The formation temperature of oils is in range of 120-128℃.The results provide the geochemical information for further understanding of oils and source rock in the Jinhu sag.
The correct judgment of the hydraulic fractures′ morphology is very important part for fracturing operation design and productivity prediction.This paper studied the true tri-axial hydraulic fracture experiment of coal and obtained the judgment criterions for the conversion conditions between horizontal,vertical and complicated fracture.The result shows that the hydraulic fractures′ morphology will be changed between vertical and horizontal fracture when the stress difference (the minimum horizontal stress minus vertical stress) span from 4MPa to 6MPa.High confining pressures will make the hydraulic fractures′ morphology complicated when the stress difference is stable.The nature fractures and cleats have different effects on the initiation and extending process of hydraulic fracture.The stress states of coal bed decide the hydraulic fracture strike.When the stress difference coefficient,i.e.Kv,belongs to 0.6 to 0.7,the nature fractures and cleats have significant influence on the hydraulic fractures′ morphology.From the Kv value,we can know that it need harsh stress condition to make the hydraulic fractures horizontal at the stage of initiation without near wellbore nature fracture.
Ordos basin with abundance of tight gas is a backup area of natural gas exploration. Based on reviewing tight gas development and present situation,we discuss the characteristics of Sulige tight gas reservoirs as a typical case and compare the features of geological background and gas reservoir between American developed tight gas field and Sulige gasfield. Four types are classified. Based on development history and test results, we conclude development difficulties and challenges of tight gas reservoir, and point out the the following research direction in the future.
The gas percolation is not adherence to Darcy′s law due to existence of starting pressure gradient in the low permeability gas reservoir.Based on low permeability gas reservoir with non-Darcy low-velocity seepage differential equation,I establish the low permeability gas reservoir Trinomial equation with consideration of starting pressure gradient effect,and present the solving method.The proposed method is feasible and reliable calculation results after the crude data are calculated in a gas reservoir.
Pelagian basin in North Africa is one of Meso-Cenozoic oil-gas bearing basins,which developed reservoir form Jurassic Jerba Group to Miocene Tazerka Group,but the lower Eocene Nummulite limestone is the main reservoir. From now on the oil discoveries of nummulitic limestone in Libya have been about 80% of the total reserves. It had been trilled two failed wells in the high part of the structure because of the lack of nummulitic limestone. Based on the method of forward model,inversion and seismic attribute extraction,the limestone reservoirs of the Metlaoui Group is identified in this paper. The distribution of possible nummulitic limestone reservoir rocks is also predicted in this paper,combined with the study of geology,paleogeomorphic,drilling and logging data.
It has proved that the model simulated by seismic inversion in practice is often too much model-dependent and sometimes go beyond the seismic band limit as well as be erroneous for using inaccurate estimated wavelet.Seismic spectral blueing and colored inversion can effectively whiten the seismic wavelet to enhance seismic resolution and ensure that the blued seismic data show the true reflectivity of the subsurface without going beyond the seismic band limit.In this paper,we design the optimal workflow of seismic spectral blueing and colored inversion and use it to the 3D seismic data acquired for coalbed methane exploration.The results show that application of spectral blueing and colored inversion improves the seismic resolution of thin coal beds.Simultaneously,the colored inversion acoustic impedance is applied to aid at the interpretation of thin coal beds,which removes the erroneous interpretation occurred during previous interpretation using merely the original seismic data.
In submarine Block A,traditional post-stack wave impedance cannot effectively predict the fluid-distribution range because of low ability of P-impedance to identify gas and water. In this paper,we adopt the pre-stack extended elastic impedance (EEI) inversion technique. EEI inversion technique including more information about lithology and fluids is an effective technique for the fluid detection and recognition. On the basis of briefly presenting the concept,method and principle of EEI,we use this inversion method to the submarine Block A. We check the sensitive parameters for fluid detection,optimize the optimal theory angles for fluid detection,and calculate the EEI curves for fluid detection. The results show that the pre-stack extended elastic impedance inversion is favorable for the submarine reservoir fluid identification,especially for the gas and water detection. So the effective fluid identification is established for deep submarine fans.
Based on the definition of shale gas,the high abundance of organic matter is the material basis for hydrocarbon generation as well as the important carrier of adsorbed shale gas.This method estimated the organic carbon content of strata with the high-resolution vertical logging curves in shale strata and overcame the difficulty of evaluating shale gas which was caused by the shortage of core sample and the cost limitation of experimental analysis.Based on the logging response feature of the different logging curves and the logging information such as resistivity,acoustic slowness and natural gamma-ray spectrometry,this paper regressively calculated the organic carbon content with the methods of ΔLgR and natural gamma-ray spectrometer.Taking the calculation of shale organic carbon content of two wells in Sichuan Basin for example,the core data of TOC was subjected to error analysis and tested.The results showed that the congruent of ΔLg R and U curve was more applicable for continental facies shale,while good results have been achieved by both methods for marine facies shale.
Based on coalbed methane adsorption,CBM is successfully developed.it is great effect on shale gas exploration to comparatively analysis of coal bed methane and shale gas adsorption.It is subjected to the isothermal adsorption experiment with different temperatures to describe adsorption characteristics between varioues TOC and thermal maturity shale and different rank coal.The results shows that the adsorption quantity of methane on shale and coal decrease with temperature increase,but it is more sensitive for gas shale adsorption to temperature than that for coal. PL value in coal is negatively correlated with RO and positive with organic matter content, VL value in coal is positive correlated with Ro and negative with matter content;in constrast, PL and VL of shale decrease with RO and increase with organic matter content.Methane adsorption quantity for coal is more than that for shale.It is more noticeable effect on CBM production by means of pressure step-down desorption than that on gas shale development.
The paper systematically investigated the shale outcrops and profiles of Niutitang Formation in Lower Cambrian in Northern Guizhou Province,and a series of experiments of organic geochemistry,mineral composition,capacity of gas storage,porosity and permeability were conducted. The results show that,the shale of Niutitang Formation in Lower Cambrian has a stable distribution and favorable buried depth in Cengong-Shiqian-Tongren areas,and its thickness varies from 40 to 60 meters,the TOC is higher than 2.5%. The thermal evolution of organic matter is generally high,which indicates that the shale is in the stage of maturation to over maturation. The black shale contains an average of 2.74 cubic meters adsorption gas per ton. The TOC and clay mineral obviously impacted the gas potential,which was regularly changed in the profile. The Cenye 1 well was located and the well drilling was conducted in Yangqiao of Cengong area. The completion depth of Cenye 1 well is 1 526m and the black shale of Niutitang Formation in Lower Cambrian has been revealed for a total thickness of 53 meters,which enriched in carbon. What's more,the shale gas anomalies were monitored.
The black mud shale in the second Member of Kongdian Formation is the important source rock of oil and gas fields in southern Kongdian,Huanghua depression.Oil and gas shows are extensively found in the mud shale during conventional oil and gas exploration.The results indicate that semi-deep-deep lacustrine sedimentary environment controlled the development and distribution of the organic matter-riched mud shale and interlayer in the member,the black shale is the thickest in the depocenter in the deep abyss.The black mud shale in the second member of Kongdian Formation is characterized by large thickness,wide distribution,good organic matter type，high content of organic matters and proper maturity.The reservoir is various in reservoir spaces and types and is enriched with sand,siliceous and fragile minerals,which have the characteristics of low permeability and low porosity because of its compactness.The oil reservoir was restricted by lithology rather than local structures,which have certain abnormal reservoir characteristics.In the conventional exploration,a large amount of oil and gas shows and small-scale oil and gas flow have been encountered in these areas,so they possess the beneficial geological conditions to form shale oil reservoirs.The second member of Kongdian Formation in southern Kongdian area is abundant in shale oil,especially some blocks,like Wangguantun-Zaoyuan,can be taken as the favorable target areas in recent exploration of shale oil and they were hoped to have a breakthrough by old wells re-examination and sand fracturing.
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