Calcite Cements in Burrows and Their Influence on Reservoir Property of the Donghe Sandstone, Tarim Basin, China

Abundant burrows of Skolithos linearis, Palaeophycus tubularis, and Taenidium barretti are preserved in the Upper Devonian Donghe sandstone of Tarim basin, China. They are commonly highlighted in core by the color of the burrow fill in contrast to the surrounding matrix and have dif-ferent textures and fabrics from the matrix. There are three kinds of calcite cements in burrows, microcrystalline-crystalline calcite cement, mosaic calcite cement, and ferroan calcite cement. Microcrystalline-crystalline calcite cement is widely distributed in the lower part of the Donghe sand-stone, but it is rare in burrows filling. It displays a globule structure under burial cementation and sel-dom replaces the quartz grains in shallow burial depth stage. Mosaiccalcite cement is widespread in the Donghe sansdtone reservoir of North Tarim basin. It shows chrysoidine, orange and bright orange lu-minescence, and intensely replaces the quartz grains, forming in early diagenetic and shallow burial depth stage. Ferroan calcite is asymmetrically distributed in the Donghe sandstone reservoir of Central Tarim basin. It fills the remanent pores in the shape of mosaic and replaces the quartz grains, matrix, and early calcite cement, forming in late diagenetic and deep burial depth stage. The burrows filled with white calcite cements have low oil saturation or may be oil-stained. In contrast, there is high oil saturation in the sandstone reservoir where the bioturbation is sparse or not present. With increased bioturbation, the porosity, permeability, and oil saturation decrease; thus, bioturbation intensity and reservoir property appear to be negatively correlated.

Physical characteristics of high concentrated gas hydrate reservoir in the Shenhu production test area,South China Sea

High concentrated and heterogeneous distribution of gas hydrates have been identified in the gas hydrate production test region in the Shenhu area,South China Sea.The gas hydrate-bearing sediments with high saturation locate at two ridges of submarine canyon with different thickness and saturations just above the bottom simulating reflection.The crossplots of gamma ray,acoustic impedance(P-impedance)and porosity at four sites show that the sediments can be divided into the upper and lower layers at different depths,indicating different geotechnical reservoir properties.Therefore,the depositional environments and physical properties at two ridges are analyzed and compared to show the different characteristics of hydrate reservoir.High porosity,high P-wave velocity,and coarse grain size indicate better reservoir quality and higher energy depositional environment for gas hydrate at Sites W18 and W19 than those at Sites W11 and W17.Our interpretation is that the base of canyon deposits at Sites W18 and W19 characterized by upward-coarsening units may be turbidity sand layers,thus significantly improving the reservoir quality with increasing gas hydrate saturation.The shelf and slope sliding deposits compose of the fine-grained sediments at Sites W11 and W17.The gas hydrate production test sites were conducted at the ridge of W11 and W17,mainly because of the thicker and larger area of gas hydrate-bearing reservoirs than those at Sites W18 and W19.All the results provide useful insights for assessing reservoir quality in the Shenhu area.

The influence of bioturbation on sandy reservoirs: the delta front sand of the lower Zhujiang Formation, Baiyun Depression, Zhujiang River Mouth Basin

Ichnofossils are well developed in clastic rock reservoirs in marine and transitional facies, which can considerably change the physical properties of the reservoir. However, this influence is not well understood, raising an important problem in the effective development of petroleum reservoirs. This paper analyzes continental shelf margin delta reservoirs through core observation, cast thin section observation and reservoir physical property test. Some important scientific insights are obtained:(1) The presence of Cruziana ichnofacies, including Asterosoma, Ophiomorpha, Planolites, Skolithos, Thalassinoides, and other ichnofossils can be used to identify in subaqueous distributary channels, subaqueous levee, frontal sheet sand, abandoned river channels, crevasse channels, main channels and channel mouth bars. Considerable differences in the types of ichnofossils and the degree of bioturbation can be observed in the different petrofacies.(2) Ichnofossils and bioturbation play a complex role in controlling reservoir properties. The reservoir physical properties have the characteristics of a decrease–increase–decrease curve with increasing bioturbation degree. This complex change is controlled by the sediment mixing and packing of bioturbation and the diagenetic environment controlled by the ichnofossils.(3) Sea-level cycle changes affect the modification of the reservoir through sediment packing. Bioturbation weakens the reservoir's physical property when sea level slowly rises and improves the reservoir's physical property when base level slowly falls.

Pore Characteristics of the Fine-Grained Tight Reservoirs in the Yabulai Basin, Northwestern China

This work investigated the pore structure characteristics and reservoir features of the finegrained tight reservoirs in the lower member of the Xinhe Formation(J2x1) in the Xiaohu subsag,Yabulai Basin based on core samples through various techniques. Interbedded silt/fine sandstones and mudstones are developed in the study area. Scanning electron microscopy(SEM) images were used to delineate different types of pores, including primary intergranular pores, secondary intergranular and intragranular pores, organic pores and fractures. The pore types were distinguished by pore size, pore area, location and formation process. The pore radii of the fine-grained rocks range from 1 nm to 1.55μm, mainly concentrated between 5 and 300 nm by low pressure N2adsorption and MICP analyses. The pore structure parameters of pore throat size and pore throat sorting coefficient are both positively correlated with porosity, while pore throat sorting coefficient has a negative correlation with permeability. The pore structures of the studied samples are much related to the mineral type and content and grain size, followed by TOC content. In these rocks with relatively low TOC and low maturity, the rigid minerals protect pores with pressure shadow from collapse, and dissolution-related pores contribute a lot to inorganic porosity. In contrast, these rocks with abundant TOC contain a large number of organic pores. The permeability of the fine-grained tight reservoir is mainly dominated by larger pore throats, while a large number of small pores(mostly <0.1 μm) contribute considerably to porosity. These results have deepened our understanding of the interbedded fine-grained tight reservoirs and can be applicable to fine-grained reservoirs in a similar setting.

Shale Gas Reservoir Evaluation by Geophysical Measurements:A Case Study of the Upper Ordovician–Lower Silurian in the Fenggang Block,Northern Guizhou Province

With the aid of geophysical measurements,including seventeen two-dimensional(2 D)seismic lines and the well logging curves of well FGY1,the structure and reservoir characteristics of the Upper Ordovician–Lower Silurian strata in the Fenggang block,northern Guizhou Province,were analyzed thoroughly to identify desert areas and favorable intervals.The results show that Longmaxi-Wufeng is the most prospect-rich formation,consisting of a thick succession of overmature black shale,this formation remaining partially in the Suiyang,Fenggang and Jianchaxi synclines.The Longmaxi-Wufeng shale,especially the lower member,was deposited in a reducing low-energy environment with relatively high U content and a low Th/U value.In this shale,the organic matter type(sapropelic and humic-sapropelic),total organic carbon(TOC)content,gas content,gas adsorption capacity,vitrinite reflectance and brittle mineral content are profitable for shale gas preservation and development.The fractures of this shale were closed because of its high overburden pressure.The gas adsorption capacity of this shale increases with increasing TOC content and Ro.In the Longmaxi-Wufeng Formation at well FGY1,the most favorable intervals are in the depth ranges of 2312.4–2325.1 m and 2325.8–2331.1 m.

Diageneses and Their Influences on Reservoir Properties of Chang 2 Oil Member in Renshan Region, Zhidan Oilfield, Ordos Basin

1 Introduction Yanchang Formation in Upper Triassic,Ordos basin contains the most abundant hydrocarbon resources in North China(Wang et al.,2014).The sandstones are the most important oil-bearing reservoirs in Yanchang

Assessment of natural gas hydrate reservoirs at Site GMGS3-W19 in the Shenhu area,South China Sea based on various well logs

To obtain the characteristics of the gas hydrate reservoirs at GMGS3-W19,extensive geophysical logging data and cores were analyzed to assess the reservoir properties.Sediment porosities were estimated from density,neutron,and nuclear magnetic resonance(NMR)logs.Both the resistivity and NMR logs were used to calculate gas hydrate saturations,the Simandoux model was employed to eliminate the effects of high clay content determined based on the ECS and core data.The density porosity was closely in agreement with the core-derived porosity,and the neutron porosity was higher while the NMR porosity was lower than the density porosity of sediments without hydrates.The resistivity log has higher vertical resolution than the NMR log and thus is more favorable for assessing gas hydrate saturation with strong heterogeneity.For the gas hydrate reservoirs at GMGS3-W19,the porosity,gas hydrate saturation and free gas saturation was 52.7%,42.7%and 10%,on average,respectively.The various logs provide different methods for the comprehensive evaluation of hydrate reservoir,which supports the selection of candidate site for gas hydrate production testing.

Mechanism of ultrasonic strengthening fluidity of low mature shale oil: A case study of the first member of Lucaogou Formation, western Jimusaer Sag, Northwest China

The low mature shale oil resources of Lucaogou Formation in Jimusar Sag have a great potential, but the heavy oil quality limits large-scale economic development significantly. Ultrasonic is a typical representative of heavy oil viscosity reduction and anhydrous fracturing technology, and how to understand the action characteristics and mechanism of ultrasonic effect on reservoir is a critical issue to enhance shale oil production in the industrialized application of power ultrasonic. Therefore, the comparative experiments with different time of power ultrasonic loading were conducted to analyze the response mechanism of reservoir characteristics and the change of fluid mobility. The results indicate that the ultrasonic treatment is ameliorative to the pore-fracture structure, and the improvement degree is controlled by the mechanical vibration and cavitation of ultrasound. Generally, the location with weak cementation strength or relatively developed microcrack is preferred to pore expansion. After the ultrasonic treatment, the shale oil quality becomes lighter, and the transformation of shale oil from adsorbed to free, is accelerated due to enhanced fluidity. Pore-expanding effect and fluid mobility enhancement are essential aspects of the power ultrasonic loading to improve the recovery of low mature shale oil. The results of this study support the feasibility analysis of ultrasonic enhanced shale oil exploitation theoretically.

Types,Petrophysical Properties and Pore Evolution of Late Ediacaran Microbial Carbonates,Tarim Basin,NW China

The Upper Ediacaran microbial carbonates of the Tarim Basin are potential reservoir geobodies for future hydrocarbon exploration with rising interest in exploration for deeply-buried reserves.However,little knowledge has been acquired on the types of microbial carbonates that are present,the properties of the reservoir and the pore evolution,hampering predictions of high-quality reservoirs in these carbonates.Integrated with petrography and in-situ U-Pb dating geochronology analyses,this study aims to clarify the types of microbial carbonates present and to reconstruct the pore evolution processes of the potential reservoir rocks.The Upper Ediacaran microbial carbonates of the Tarim Basin can be divided into four types,based on their features in terms of different scales(macro-to micro-):microbial laminite,stromatolite,spongiomicrobialite and microbial-peloidal wackestone/mudstone.Petrophysical properties show that all these microbial carbonates have low porosity and very low permeability with poor connectivity.These carbonates were subject to long-term and complex diagenetic processes,mainly consisting of dissolution,compaction,pervasive dolomitization,cementation and fracturing.The most important reservoir spaces are contributed by vugs and dissolution-enlarged pores,which are likely to have been associated with the widespread uplift of the Aksu area in the terminal Ediacaran.In contrast,the cementation of the fine-to-medium crystalline dolomite greatly reduced the pre-existing pores.Pore types are closely related to different microbial fabrics,which played an important role in the pore evolution of the microbial carbonates.

Microscopic characterization and formation mechanisms of deep-water sandy-debris-flow and turbidity-current sandstones in a lacustrine basin： a case study in the Yanchang Formation of the Ordos Basin, China

Deep-water deposition is a current issue in sedimentological research. Sandy-debris-flow sandstones and turbidity-current sandstones are the main types of sandstone that are the focus of considerable disputes in this research. Previous studies mainly focused on description of the macroscopic sedimentary structure and theoretical derivation of the formation mechanisms. The microscopic petrological characteristics, reservoir properties, and formation mechanisms of deep-water sandy-debris-flow and turbidity-current sandstones have been studied in the Yanchang Formation of the Ordos Basin,China, by means of field outcrop surveys, thin-section identification, geochemical element analysis, and porosity and permeability measurements under overburden pressure. The content of detrital grains in the sandy-debris-flow sandstones is high, whereas the contents of mica sheets and matrix are low. The fine-grained matrix is distributed unevenly within the pores. A considerable number of residual intergranular pores are preserved in the middle of single sand bodies, resulting in relatively better reservoir properties. The total number of detrital grains in the turbidite sandstone is low, while it contains abundant mica sheets and matrix. The mica sheets and fine-grained matrix are distributed evenly within the pores, resulting in serious damage to pores and poor reservoir properties. The sandy-debris-flow sandstones in the center of the lake basin form a high-quality reservoir; thus, this area is suitable for oil and gas exploration.

Evaluation of clayed silt properties on the behavior of hydrate production in South China Sea

Gas hydrate is one kind of potential energy resources that is buried under deep seafloor or frozen areas.The first trial offshore production from the silty reservoir was conducted in the South China Sea by the China Geological Survey(CGS).During this test,there were many unique characteristics different from the sand reservoir,which was believed to be related to the clayed silt physical properties.In this paper,simulation experiments,facilities analysis,and theoretical calculation were used to confirm the hydrate structure,reservoir thermo-physical property,and bond water movement rule.And the behavior of how they affected production efficiency was analyzed.The results showed that:It was reasonable to use the structure I rather than structure II methane hydrate phase equilibrium data to make the production plan;the dissociation heat absorbed by hydrate was large enough to cause hydrate self-protection or reformation depend on the reservoir thermal transfer and gas supply;clayed silt got better thermal conductivity compared to coarse grain,but poor thermal convection especially with hydrate;clayed silt sediment was easy to bond water,but the irreducible water can be exchanged to free water under high production pressure,and the most obvious pressure range of water increment was 1.9–4.9 MPa.

The effect of inorganic salt precipitation on oil recovery during CO2 flooding:A case study of Chang 8 block in Changqing oilfield,NW China

Static experiments and dynamic displacement experiments were conducted to quantitatively determine the amount of precipitate generated by the CO_(2)-formation water reaction at different temperatures,pressures,and scaling ion concentrations during CO_(2) flooding in the Chang 8 block of Changqing Oilfield,the influence of precipitate on the physical properties of reservoirs was investigated,and the corresponding mathematical characterization model was established.The mathematical characterization equation was used to correct the numerical simulation model of E300 module in Eclipse software.The distribution pattern of inorganic salt precipitates during continuous CO_(2) flooding in Chang 8 block was simulated,and the influence of inorganic salt precipitates on oil recovery was predicted.The inorganic salt precipitate generated during CO_(2)-formation water reaction was mainly CaCO_(3),and the pressure difference and scaling ion concentration were proportional to the amount of precipitate generated,while the temperature was inversely proportional to the amount of precipitate.The rate of core porosity change before and after CO_(2) flooding was positively correlated with temperature and flooding pressure difference.The core porosity increase in the CO_(2)-formation water-core reaction experiment was always lower than that of CO_(2)-distilled water-core reaction experiment because of precipitation.The area around the production wells had the most precipitates generated with the injection of CO_(2).The oil field became poor in development because of the widely distributed precipitate and the recovery decreased to 33.45% from 37.64% after 20-year-CO_(2) flooding when considering of precipitation.

Quantitative characterization of shale gas reservoir properties based on BiLSTM with attention mechanism

Evaluating the potential of shale gas reservoirs is inseparable from reservoir properties prediction.Accurate characterization of total organic carbon,porosity and permeability is necessary to understand shale gas reservoirs.Seismic data can help to estimate these parameters in the area crossing-wells.We develop an improved deep learning method to achieve shale gas reservoir properties estimation.The rela-tionship between elastic attributes and reservoir properties is built up by training a deep bidirectional long short-term memory network,which is suitable for time/depth sequence prediction,on the logging and core data.Except some commonly used technologies,such as layer normalization and dropout,we also introduce attention mechanism to further enhance the prediction accuracy.Besides,we propose to carry on the normal scores transform on the input features,which aims to make the relationship between inputs and targets clear and easy to learn.During the training process,we construct quantile loss function,then use Adam algorithm to optimize the network.Not only the characterization results,but also the confidence interval can be output that is meaningful for uncertainty analysis.The well exper-iment indicates that the method is promising for reducing prediction errors when training samples are insufficient.After analyzing in wells,the established model is acted upon seismic inverted elastic attri-butes to characterize shale gas reservoirs in the whole studied area.The estimation results coincide well with the actual development results,showing the feasibility of the novel method on the characterization for shale gas reservoirs.

Classification and Diagenetic Characteristics of the Cretaceous Sandstones in the Southern Bredasdorp Basin,Offshore South Africa

A systematic petrographic and geochemical studies of 92 representative sandstone samples from exploration wells E-AH1,E-AJ1,E-BA1,E-BB1 and E-D3 in the southern part of the Bredasdorp Basin was undertaken to classify the sandstones as well as unravel the main diagenetic processes and their time relations.Petrographic study shows that the sandstones are largely subarkosic arenite and arkosic litharenite,which have underwent series of diagenetic processes as a result burial,rifting and subsequent uplift.The main diagenetic processes that have affected the reservoir properties of the sandstones are cementation by authigenic clay,carbonate and silica,growth of authigenic glauconite,dissolution of minerals and load compaction.The major diagenetic processes reducing the porosity are calcite cementation in the subarkosic arenite,and compaction and quartz cementation in arkosic litharenite.On the other hand,the formation of secondary porosity due to the partial to complete dissolution of early calcite cement,feldspars and minor grain fracturing has improved the reservoir property of the sandstone to some extent.The clay minerals in the sandstones commonly acts as pore choking cement,which reduces porosity.In general,there is no particular diagenetic process that exclusively controls the type or form of porosity evolution in the sandstones.

Petrophysical properties assessment using wireline logs data at well#3 of Srikail gas field,Bangladesh

This study focused on the quantitative analysis of the petrophysical parameters in characterizing the reservoir properties of the Srikail gas field using multi-scale wireline logs.Petrophysical parameters(shale volume,porosity,water saturation and hydrocarbon saturation)were estimated from the combination of gamma ray log,resistivity log,density log and neutron log for three hydrocarbon(gas)-bearing zones at well#3.At the first time,log records at 0.1 m and 0.2 m intervals were read for this study.Result showed the average shale volume is 21.07%,53.67%and 51.71%for zone-1,zone-2 and zone-3,respectively.For these zones,the estimated average porosity was 35.89%,29.83%and 28.76%,respectively.The average water saturation of 31.54%,16.83%and 23.39%and average hydrocarbon saturation of 68.46%,83.17%and 76.61%were calculated for zone-1,zone-2 and zone-3,respectively.Thus zone-2 is regarded the most productive zone of well#3.It was found that the values of some parameters(porosity,hydrocarbon saturation and permeability)are higher than the existing results.Therefore,this study confirmed that the log reading at minute/close interval provides better quantitive values of the reservoir’s petrophysical properties.It is expected that this result will contribute to the national gas field development program in future.

Pore throat characteristics of tight sandstone of Yanchang Formation in eastern Gansu,Ordos Basin

An important factor to evaluate reservoir quality is the pore-throat size.However,the strong heterogeneity makes it difficult to characterize the pore-throat distribution in tight reservoirs.The field emission scanning electron microscope(FESEM),high pressure mercury injection and rate-controlled mercury injection are used to investigate the pore-throat size distribution in tight sandstone reservoirs of Member 7 of the Yanchang Formation in eastern Gansu,Ordos Basin,and studies of the pore throat size controlling on physical property of the tight sandstone reservoirs are also carried out.The result shows that the pore type is mainly dominated by the residual intergranular pore,dissolution pore,micropore and a few micro-fractures;the high-pressure mercury injection experiment indicates that the pore-throat size ranges from 0.0148 μm to 40mm,the pore throat more than 1 mm is less;the ratecontrolled mercury injection experiment reveals that for samples with different physical properties,the pore radius mainly varies from 80 μm to 350 μm;the throat radius exhibits the strong heterogeneity,and is from 0.12 μm to 30μm;the pore-throat size can be effectively characterized by combination of high-pressure and rate-controlled mercury injections,and it varies from 0.0148 μm to 350 μm.The permeability is mainly controlled by the large pore throat(>R_(50))which accounts for a small proportion;in the tight sandstone with the permeability greater than 0.1 mD,the permeability is mainly controlled by the micropore and mesopore;in the tight sandstone with the permeability smaller than 0.1 mD,the permeability is mainly controlled by the nanopore and micropore;the proportion of small pore throat increases with reduction of permeability,it is important that the small pore throat influences the reservoir storage property though its effect on permeability are small.

Research advances in the formation poredynamics of sedimentary basins

In the 21st century,the geodynamics is developing towards quantitative researches.However,due to the irreversible geological processes,it was very difficult to recover the geological process.In particular,the restoration of geological parameter evolution process at the microscopic scale has become a major scientific problem in geology presently.Thereby,a concept of the formation poredynamics is revised and proposed,and the formation poredynamics is a fundamental discipline which focus on the mechanical characteristic of porous media,the pore evolution law,the dynamic genesis and the seepage property of pore fluid during the burial process of clastic rocks.Moreover,it is a new interdiscipline of underground diagenetic dynamics and pore fluid dynamics,and also is as an important part of sedimentary basin dynamics.Research advances were made in both basic theory and applied research.The advances in the basic theory include:(1)the static equilibrium principle of the formation pore,(2)the porosity evolution mechanism and quantitative model of sandstone during the burial diagenetic process,(3)the compaction characteristic and the porosity evolution quantitative model of mudstone,(4)the theoretical relationship between the underground pore fluid temperature and the pore fluid pressure,(5)the influence of the tectonism-induced additional geostress on the pore fluid pressure,and(6)the relationship between the mudstone compaction and the vitrinite reflectance(R_(o))of organic matter.The advances in the applied research include:(1)the geotemperature-geopressure system division of the sedimentary basin and the interpretation of the hydrocarbon distribution dynamic,(2)the modification of the strata pressure prediction model,(3)the construction of the reservoir critical properties and the reservoir dynamics evaluation system,(4)the simulation of the evolution process of the formation fluid pressure,(5)the numerical simulation and physical experimental simulation on the sandstone hydrocarbon charging dynamics,and(6)the dynamic process analysis of the hydrocarbon accumulation in tight sandstone.Through the integration between the pore genesis evolution and the pore fluid dynamic evolution,the formation poredynamics is one of the representative discipline branches that the geological dynamics research had developed toward the underground microscopic scale in recently 20 years,and it also is an inevitable result from the quantitative development of the formation and distribution mechanisms of sedimentary mineral deposits.Based on the formation poredynamics research,eight important research achievements are summarized,and the geological researches are extended from the macroscopic scale to the microscopic scale,to find out the pore parameter evolution law under control of the formation pore evolution during the burial process,and update and improve exploration and production application technologies.