Impact of volcanism on the formation and hydrocarbon generation of organic-rich shale in the Jiyang Depression, Bohai Bay Basin, China

Globally,most organic-rich shales are deposited with volcanic ash layers.Volcanic ash,a source for many sedimentary basins,can affect the sedimentary water environment,alter the primary productivity,and preserve the organic matter(OM)through physical,chemical,and biological reactions.With an increasing number of breakthroughs in shale oil exploration in the Bohai Bay Basin in recent years,less attention has been paid to the crucial role of volcanic impact especially its influence on the OM enrichment and hydrocarbon formation.Here,we studied the petrology,mineralogy,and geochemical characteristics of the organic-rich shale in the upper submember of the fourth member(Es_(4)^(1))and the lower submember of the third member(Es_(3)^(3))of the Shahejie Formation,aiming to better understand the volcanic impact on organic-rich shale formation.Our results show that total organic carbon is higher in the upper shale intervals rich in volcanic ash with enriched light rare earth elements and moderate Eu anomalies.This indicates that volcanism promoted OM formation before or after the eruption.The positive correlation between Eu/Eu*and Post-Archean Australian Shale indicates hydrothermal activity before the volcanic eruption.The plane graph of the hydrocarbon-generating intensity(S1+S2)suggests that the heat released by volcanism promoted hydrocarbon generation.Meanwhile,the nutrients carried by volcanic ash promoted biological blooms during Es_(4)^(1 )and Es_(3)^(3) deposition,yielding a high primary productivity.Biological blooms consume large amounts of oxygen and form anoxic environments conducive to the burial and preservation of OM.Therefore,this study helps to further understand the organic-inorganic interactions caused by typical geological events and provides a guide for the next step of shale oil exploration and development in other lacustrine basins in China.

Shale oil development techniques and application based on ternary-element storage and flow concept in Jiyang Depression,Bohai Bay Basin,East China

The ternary-element storage and flow concept for shale oil reservoirs in Jiyang Depression of Bohai Bay Basin,East China,was proposed based on the data of more than 10000 m cores and the production of more than 60 horizontal wells.The synergy of three elements(storage,fracture and pressure)contributes to the enrichment and high production of shale oil in Jiyang Depression.The storage element controls the enrichment of shale oil;specifically,the presence of inorganic pores and fractures,as well as laminae of lime-mud rocks,in the saline lake basin,is conducive to the storage of shale oil,and the high hydrocarbon generating capacity and free hydrocarbon content are the material basis for high production.The fracture element controls the shale oil flow;specifically,natural fractures act as flow channels for shale oil to migrate and accumulate,and induced fractures communicate natural fractures to form complex fracture network,which is fundamental to high production.The pressure element controls the high and stable production of shale oil;specifically,the high formation pressure provides the drive force for the migration and accumulation of hydrocarbons,and fracturing stimulation significantly increases the elastic energy of rock and fluid,improves the imbibition replacement of oil in the pores/fractures,and reduces the stress sensitivity,guaranteeing the stable production of shale oil for a long time.Based on the ternary-element storage and flow concept,a 3D development technology was formed,with the core techniques of 3D well pattern optimization,3D balanced fracturing,and full-cycle optimization of adjustment and control.This technology effectively guides the production and provides a support to the large-scale beneficial development of shale oil in Jiyang Depression.

Mapping and resource evaluation of deep high-temperature geothermal resources in the Jiyang Depression,China

In China,geothermal resource utilization has mainly focused on resources at shallow and medium depths.Yet,the exploration of deep,high-temperature geothermal resources holds significant importance for achieving the“dual carbon”goals and the transition of energy structure.The Jiyang Depression in the Bohai Bay Basin has vast potential for deep,high-temperature geothermal resources.By analyzing data from 2187 wells with temperature logs and 270 locations for temperature measurement in deep strata,we mapped the geothermal field of shallow to medium-deep layers in the Jiyang Depression using ArcGIS and predicted the temperatures of deep layers with a burial depth of 4000 m.Through stochastic modeling and numerical simulation,a reservoir attribute parameter database for favorable deep,high-temperature geothermal areas was developed,systematically characterizing the spatial distribution of geothermal resources within a play fairway of 139.5 km2 and estimating the exploitable deep geothermal resource potential by using the heat storage method and Monte Carlo data analysis.The study reveals that the Fan 54 well block in the Boxing-Jijia region is of prime significance to develop deep,high-temperature geothermal resources in the Jiyang Depression.Strata from the Cenozoic to the Upper Paleozoic are identified as effective cap layers for these deep geothermal resources.The Lower Paleozoic capable of effectively storing thermal energy and possessing an exploitable resource volume up to 127 million tons of standard coal,is identified as a target system for the development of deep high-temperature geothermal resources,providing significant insights for the efficient development of geothermal resources in the Jiyang Depression.

Distribution and Geochemical Characteristics of Hydrogen in Natural Gas from the Jiyang Depression, Eastern China

Hydrogen gas accelerates hydrocarbon generation, but little is known about its distribution and origin in petroliferous basins, which has hindered the further exploration.Taken the Jiyang Depression in eastern China as an example, this study collected natural gas from different tectonic units, and analyzed various geochemical characters including gas contents, and carbon and hydrogen isotopic composition.The result shows that：（1） hydrogen gas is widespread distributed, but its content is very low, which typically ranges from 0.01% to 0.1% in this region;（2） the ratios of H2/3He, indicative of the origins of hydrogen gas, suggest that mantle-derived hydrogen is dominant.Even in tectonically stable areas absent with deep fluid activities, there is also mantle-derived;（3） the isotopic composition of hydrogen falls in the range of –798‰ to –628‰（relative to VSMOW standard）.In areas with deep-derived fluids, the hydrogen gas has a similar isotopic composition with the previously documented deep-sourced gas, with lighter isotopic composition.In contrast, hydrogen gas has a heavier isotopic composition in relatively stable areas.The isotopic signatures suggest that there is a mixture of mantle- and crust-derived hydrogen gas in the relatively stable area, which is consistent with the H2/3He ratios.Therefore, it is clear that the hydrogen gas has a much wider distribution than found in the deep-derived fluid area, resulting in a much broader area with hydrogenating effect for resource rock.This understanding will provide new insights for hydrocarbon generation research and resource assessment in petroliferous basins.

Exploration of Subtle Trap in Jiyang Depression

This article analyses the procedure of exploration of the Tertiary subtle trap in Jiyang depression and divides the Tertiary subtle trap into 3 types (lithologic reservoir, stratigraphic reservoir and fractured reservoir) and 8 groups, then summarizes the common feature and founding discipline of the subtle trap and finds 4 accumulating modes including steep slope mode, depression mode, center anticline mode and gentle slope mode. Its main exploration methods are explicated from the viewpoint of reservoir geological modeling, description of recognizing traps and comprehensive evaluation of reservoir and so on.

Enrichment rules and exploration practices of Paleogene shale oil in Jiyang Depression, Bohai Bay Basin, China

Based on formation testing data of more than 40 wells with industrial oil flow, systematic observation of 1 010.26 m long cores taken from 4 wells and test data of over 10 000 core samples combining with drilling and pilot fracturing data of multiple wells, the geological characteristics of the upper submember of the Sha 4 Member to the lower submember of the Sha 3 Member of Paleogene(Es4s-Es3x) in the Jiyang Depression were investigated to find out factors controlling the enrichment of shale oil and the accumulation model of shale oil, and a comprehensive evaluation method for shale oil sweet spots was established. It is found through the study that the target shale layer is characterized by strong heterogeneity, weak diagenesis, low thermal evolution and high content of clay and carbonate minerals. Shale lithofacies, microcrack, thin interlayer and abnormal pressure are the main factors affecting enrichment and stable production of shale oil, the organic rich laminar shale has the best storage and oil-bearing capacity, microcrack network system improve the storage capacity and permeability of the shale, the thin interlayer is the main flow channel for stable shale oil production, and the abnormal high pressure layer is rich in free state shale oil and high in oil content. The shale oil layers in the target section were divided into three types: matrix, interlayer and fracture ones. According to the occurrence state and exploration practice of shale oil at home and abroad, it is concluded that the interlayer shale oil is the most profitable type at present. The selection parameters for the different types of shale oil were determined, and accordingly the favorable areas were pointed out by comprehensive evaluation of multiple factors. Vertical wells in the interlayer shale oil reservoir, such as Fan 159, Fan 143 and GX 26, were stimulated by volume fracturing and high conductivity channel fracturing jointly. After fracturing, they had a daily oil production of over 6 t, up to 44 t, and stable productivity. Shale oil is expected to become an important replacement energy resource in the Jiyang Depression.

Origin and diagenetic evolution of dolomites in Paleogene Shahejie Formation lacustrine organic shale of Jiyang Depression,Bohai Bay Basin,East China

The origin of dolomite in Shahejie Formation shale of Jiyang Depression in eastern China were studied by means of petrologic identification, compositional analysis by X-ray diffraction, stable carbon and oxygen isotopic composition, and trace element and rare earth element analyses. The results show that the development of dolomite is limited in the lacustrine organic rich shale of Shahejie Formation in the study area. Three kinds of dolomite minerals can be identified: primary dolomite(D1), penecontemporaneous dolomite(D2), and ankerite(Ak). D1 has the structure of primary spherical dolomite, high magnesium and high calcium, with order degree of 0.3-0.5, and is characterized by the intracrystalline corrosion and coexistence of secondary enlargement along the outer edge. D2 has the characteristics of secondary enlargement, order degree of 0.5-0.7, high magnesium, high calcium and containing a little iron and manganese elements. Ak is characterized by high order degree of 0.7-0.9, rhombic crystal, high magnesium, high calcium and high iron. The micritic calcite belongs to primary origin on the basis of the carbon and oxygen isotopic compositions and the fractionation characteristics of rare earth elements. According to the oxygen isotopic fractionation equation between paragenetic dolomite and calcite, it is calculated that the formation temperature of dolomite in the shale is between 36.76-45.83 ℃, belonging to lacustrine low-temperature dolomite. Based on the maturation and growth mechanism of primary and penecontemporaneous dolomite crystals, a dolomite diagenetic sequence and the dolomitization process are proposed, which is corresponding to the diagenetic environment of Shahejie Formation shale in the study area.

Permeability damage micro-mechanisms and stimulation of low-permeability sandstone reservoirs: A case study from Jiyang Depression, Bohai Bay Basin, China

According to the characteristics of"structural elements"(framework grain,interstitial material and pore throat structure)of low-permeability sandstone reservoir,the"step by step dissolution and separation"acidification and acid fracturing technology has been developed and tested in field.There are three main mechanisms affecting permeability of low-permeability sandstone reservoir:(1)The mud fillings between the framework grains block the seepage channels.(2)In the process of burial,the products from crystallization caused by changes in salinity and solubility and uneven migration and variation of the syn-sedimentary formation water occupy the pores and throat between grains.(3)Under the action of gradual increase of overburden pressure,the framework grains of the rock is compacted tighter,making the seepage channels turn narrower.The"step by step dissolution and separation"acidification(acid fracturing)technology uses sustained release acid as main acidizing fluid,supramolecular solvent instead of hydrochloric acid to dissolve carbonate,and a composite system of ammonium hydrogen fluoride,fluoroboric acid,and fluorophosphoric acid to dissolve silicate,and dissolving and implementing step by step,finally reaching the goal of increasing porosity and permeability.By using the technology,the main blocking interstitial material can be dissolved effectively and the dissolution residual can be removed from the rock frame,thus expanding the effective drainage radius and increasing production and injection of single well.This technology has been proved effective by field test.

Carboniferous–Permian marine transgressions in the Jiyang Depression

In recent years, great attention has been paid to oil and gas exploration in the Carboniferous-Permian strata of the Bohaiwan basin, especially the Carboniferous-Permian marine transgression, using data from drilling, outcrops and carbonate acid-insoluble residue experiments together with the tectonic evo- lutionary history of the peripheral orogenic zones of the North China plate and the Tan-Lu fault zones. The .＇;tudy concludes that marine transgressions took place on six occasions during Carboniferous-Perm- ian time in the Jiyang Depression. The marine transgressions were concentrated in the Late Carbonifer- ous： two marine transgressions occurred in the early Late Carboniferous, and the scale of the first was smaller and the time was shorter than those of the second. The other four marine transgressions hap- pened in the late Late Carboniferous, the first and the fourth of which were larger in scale and longer in time than the second and the third. The seawater came from the Jiaobei area, the eastward part of the Qinling-Dabie residual sea basin, and invaded progressively as a planar flow from south to north and from east to west. These findings have great significance for thorough analysis of the sedimentary characteristics and evolution of the Carboniferous-Permian strata in the livang Denression.

Genetic types and geochemicai characteristics of natural gases in the Jiyang Depression,China

Natural gases were widely distributed in the Jiyang Depression with complicated component composition, and it is difficult to identify their genesis. Based on investigation of gas composition, carbon isotope ratios, light hydrocarbon properties, as well as geological analysis, natural gases in the Jiyang Depression are classified into two types, one is organic gas and the other is abiogenic gas. Abiogenic gas is mainly magmatogenic or mantlederived CO2. Organic gases are further divided into coaltype gas, oil-type gas, and biogas according to their kero- gen types and formation mechanisms. The oil-type gases are divided into mature oil-type gas （oil-associated gas） and highly mature oil-type gas. The highly mature oil-type gases can be subdivided into oil-cracking gas and kerogen thermal degradation gas. Identification factors for each kind of hydrocarbon gas were summarized. Based on genesis analysis results, the genetic types of gases buried in different depths were discussed. Results showed that shallow gases （〈1,500 m） are mainly mature oil-type gases, biogas, or secondary gases. Secondary gases are rich in methane because of chromatographic separation during migration and secondary biodegradation. Secondary biodegradation leads to richness of heavy carbon isotope ratios in methane and propane. Genesis of middle depth gases （1,500-3,500 m） is dominated by mature oil-type gases.Deep gases （3,500-5,500 m） are mainly kerogen thermal degradation gas, oil-cracking gas, and coal-type gas.

Structural characteristics of continental carbonate-rich shale and shale oil movability: A case study of the Paleogene Shahejie Formation shale in Jiyang Depression, Bohai Bay Basin, China

Based on rock mineral and geochemical analysis, microscopic observation, physical property measurement, and thin laminae separation test, etc., the characteristics of typical laminae of the Paleogene Shahejie Formation carbonate-rich shale in the Jiyang Depression were analyzed, and the organic matter abundance, reservoir properties, and oil-bearing properties of different laminae were compared. Typical shale storage-seepage structures were classified, and the mobility of oil in different types of shale storage-seepage structure was compared. The results show that the repeated superposition of mud laminae and calcite laminae are the main layer structure of carbonate-rich shales. The calcite laminae are divided into micritic calcite laminae, sparry calcite laminae and fibrous calcite vein. The mud-rich laminae are the main contributor to the organic matter abundance and porosity of shale, with the best hydrocarbon generation potential, reservoir capacity, and oil-bearing property. The micritic calcite laminae also have relatively good hydrocarbon generation potential, reservoir capacity and oil-bearing property. The sparry calcite laminae and fibrous calcite vein have good permeability and conductivity. Four types of shale storage-seepage structure are developed in the carbonate-rich shale, and the mobility of oil in each type of storage-seepage structure is in descending order: sparry calcite laminae enriched shale storage-seepage structure, mixed calcite laminae enriched shale storage-seepage structure, fibrous calcite vein enriched shale storage-seepage structure, and micritic calcite laminae enriched shale storage-seepage structure. The exploration targets of carbonate-rich shale in the Jiyang Depression Shahejie Formation are different in terms of storage-seepage structure at different thermal evolution stages.

Pore throat structure and classification of Paleogene tight reservoirs in Jiyang depression,Bohai Bay Basin,China

The pore throat structure characteristics of Paleogene tight sandstone and sandy conglomerate in the Jiyang depression are studied using cast thin section,conventional mercury injection,constant rate mercury injection and micro CT scanning data,and a reservoir classification scheme based on pore throat structure parameters is established.The material composition and structural characteristics of tight reservoirs are analyzed by casting thin section data.The pore throat structure characteristics of tight reservoirs are studied by conventional mercury injection,constant rate mercury injection and micro CT scanning.Ten pore throat structure parameters are analyzed by cluster analysis.Based on the classification results and oil test results,the classification scheme of Paleogene tight reservoirs is established.The Paleogene tight reservoirs in the Jiyang depression have the characteristics of macropores and microthroats,with pores in micron scale,throats in nano-submicron scale,and wide variation of ratio of pore radius to throat radius.The permeability of the tight reservoir is controlled by throat radius,the smaller the difference between pore radius and throat radius,and the more uniform the pore throat size,the higher the permeability will be.The lower limits of average pore throat radius for the tight sandstone and tight sandy conglomerate to produce industrial oil flow without fracturing are 0.6μm and 0.8μm,respectively.Reservoirs that can produce industrial oil flow only after fracturing have an average pore-throat radius between 0.2-0.6μm,and reservoirs with average pore throat radius less than 0.2μm are ineffective reservoirs under the current fracturing techniques.Different types of tight sandstone and sandy conglomerate reservoirs are classified and evaluated,which are well applied in exploratory evaluation.

Hydrocarbon Generation Potential Evaluation of Coal Shale Gas of Permo-Carboniferous in Jiyang Depression

In order to study the hydrocarbon-generating potential of Carboniferous-Permian coal shale in Jiyang Depression,geochemistry,petroleum geology and coal geology were applied to study the residual strata distribution of Carboniferous-Permian in Jiyang Depression,organic matter abundance,organic matter types and organic matter maturity of mudstone.The results show that the thickness of the Carboniferous-Permian residual strata in Jiyang Depression is generally 200-800 m,with a maximum thickness of 900 m;the organic matter abundance of coal-bearing shale is good,and it is type III kerogen,which is conducive to gas generation,and the maturity of organic matter reaches a mature-higher maturity stage;the hydrocarbon generation potential of Benxi Formation and Taiyuan Formation is better;Medium to good hydrocarbon source rocks is able to be found in every sag of Shanxi Formation hydrocarbon source rocks,but the scope is limited,and the overall evaluation is still medium.Generally speaking,the Permo-Carboniferous coal-bearing shale in Jiyang Depression has certain shale gas exploration potential.

The Coupling Relationship between the West Shandong Rise and the Jiyang Depression, China

This article reports 21 AFT （apatite fission track） data from the West Shandong （山东） rise （WSR） and Jiyang （济阳） depression, and mainly studies their Cenozoic uplifting/subsidence history and the relationship between them. Furthermore, we improve our insights into the Bohai Bay Basin （BBB）. Our AFT analysis and AFT T-t modeling indicates that the WSR was uplifted at ca. 65 Ma with apparent uplift rate of 0.019 mm/a; it underwent two relatively rapid uplifting events at 43-33 and 16-0 Ma with rates of 0.097 and 0.052 mm/a, respectively. Meanwhile, the Jiyang depression subsided at rate of 0.032 mm/a at 52-43 Ma, and the rate increased to 0.13 mm/a at ca. 42-33 Ma; finally the subsidence rate increased to 0.053 mm/a in 16-0 Ma. They all underwent a uplift in time of 23-16 Ma with rate of 0.04-0.07 mm/a. A careful comparison shows that the Cenozoic uplifting of the WSR coupled well with the subsidence of the Jiyang depression. Our research also suggests that the uplift- basin coupling events are part of the couplings between the Bohai Bay Basin and its peripheral moun- tains. This intraplate mountain-basin coupling is a reflection of global tectonic events.

Yinggehai Basin Gas Exploration:Comparison with Jiyang Depression

Yinggehai（莺歌海） basin and Jiyang（济阳） depression experienced similar tectonic evo-lution,which is mainly controlled by the strike-slip faults.The strike pull-apart basins are characteris-tic by multiple deposition cycles,migration of deposition and subsidence center,and diversity deposi-tional systems.Furthermore,both basins show abnormal formation pressure.Compared with the oil and gas-rich Jiyang depression,Yinggehai basin developed the similar geological background that is favorable to the formation of funnel-shaped meshwork-carpet subtle reservoirs.Overpressure diapir body is the core of hydrocarbon accumulation in central diaper zone of Yinggehai basin.Driven by high pressure,oil and gas migrate along the funnel-shaped passage system into the overlying low-potential zone formed.The overlying sand bodies of overpressure diapirs are the favorable gas exploration zone.

Shale Oil Reservoir Characteristics and Enrichment in the Jiyang Depression, Bohai Bay Basin, East China

Based on the observation of the well cores, thin section and FESEM, combined with X-ray diffraction, physical property testing and geochemical indicators, the reservoir characteristics and the controlling factors of the shale oil enrichment of the Ess 4-Esx 3 shale in the Jiyang depression were detailed analyzed. Studies show that carbonate and clay minerals are dominated in the shale. According to the triangle chart, the TOC content（2% and 4%）, carbonate and clay minerals, nine lithofacies have been identified. The reservoir space types are rich in the shale, in which, the laminated fractures, recrystallization intracrystalline pores and organic pores are high-quality reservoir spaces. The shale oil enrichment is mainly determined by the hydrocarbon-producing potential and reservoir capacity. The hydrocarbon-producing capacity is controlled by the organic geochemistry indicators, especially the TOC content for the study area, and the thickness of the organic-rich shale. The reservoir capacity is mainly affected by the lithofacies, the TOC content and the structural activities. In addition, the shale oil production is influenced by the fracability of the shale, which is mainly controlled by the lithofacies, structural activities, formation pressure, etc. The shale oil reservoir evaluation should focus on the TOC content, the thickness of the organic-rich shale, lithofacies and structural factor.

Use of brGDGTs in surface geochemical exploration for petroleum —A case study of oil and gas fields in the Jiyang depression

Branched glycerol dialkyl glycerol tetraethers(brGDGTs),likely produced by bacteria in soil and peat,are widely distributed,easily detected,newly adopted biomarker compounds.In this study,brGDGTs were used to explore the relationship between the absolute abundance of brGDGTs and the distribution of oil and gas fields in the Duoshiqiao area of the Jiyang depression.The results showed that the concentrations at the Xiakou fault and in the oil and gas fields were obviously higher than those in the contrast areas.The clear relationship among the concentration of brGDGTs,the distribution of oil and gas fields,and the acidolysis hydrocarbon(ethane)indicates that the concentration effectively responds to hydrocarbon seeps from the oil and gas field below.brGDGTs may become some of the most important indicators in surface geochemical prospecting for oil and gas.

Geochemical characteristics, genetic types, and controlling factors of natural gas in Jiyang Depression, Bohai Bay Basin (eastern China)

The Jiyang Depression is an important oil and gas production zone in the Bohai Bay Basin.Through a systematic investigation of the gas components and stable carbon isotopes,the genetic types of natural gas found in the Jiyang Depression were determined,that is,biogas,oilassociated gas,coal-derived gas,high-mature oil-related gas,and mantle-derived carbon dioxide(CO_(2)).From the results,natural gas in the Jiyang Depression can be divided into four groups.Group I,which is distributed in the northwest area,is the only typical oil-associated gas.Group II,distributed in the northeast area,is dominated by oil-associated gas,and involves biogas,coal-derived gas,and high-mature oil-related gas.Group Ⅲ,distributed in the southeast area,has all genetic types of gas that are dominated by oil-associated gas and have mantle-derived CO_(2).Group IV,distributed in the southwest area,is dominated by biogas and involves coal-derived gas and oil-associated gas.The differences in each group illustrate the lateral distribution of the natural gas types is characterized by the eastern and southern areas being more complex than the western and northern areas,the vertical distribution of gas reservoirs has no obvious evolutionary law.The main controlling factor analysis of the spatiotemporal changes of the gas reservoirs revealed that the synergy of geochemical characteristics,thermal evolution of the Shahejie Formation and Carboniferous-Permian source rocks,and sealing properties of various faults are jointly responsible for determining the gas reservoir spatiotemporal changes.

Iteration and evaluation of shale oil development technology for continental rift lake basins

By benchmarking with the iteration of drilling technology,fracturing technology and well placement mode for shale oil and gas development in the United States and considering the geological characteristics and development difficulties of shale oil in the Jiyang continental rift lake basin,East China,the development technology system suitable for the geological characteristics of shale oil in continental rift lake basins has been primarily formed through innovation and iteration of the development,drilling and fracturing technologies.The technology system supports the rapid growth of shale oil production and reduces the development investment cost.By comparing it with the shale oil development technology in the United States,the prospect of the shale oil development technology iteration in continental rift lake basins is proposed.It is suggested to continuously strengthen the overall three-dimensional development,improve the precision level of engineering technology,upgrade the engineering technical indicator system,accelerate the intelligent optimization of engineering equipment,explore the application of complex structure wells,form a whole-process integrated quality management system from design to implementation,and constantly innovate the concept and technology of shale oil development,so as to promote the realization of extensive,beneficial and high-quality development of shale oil in continental rift lake basins.

Shale primary porosimetry based on 2D nuclear magnetic resonance of T_(1)-T_(2)

Porosity,a key parameter in assessing the physical properties of shale reservoirs and the reserves,is of great significance to the selection and evaluation of shale sweet spots.There are many methods at present to characterize shale porosity,most of which are aimed at post-core cleaning shale,such as those involving helium and saturated fluid(namely liquid-involved porosimetry).However,due to the low efficiency of shale core cleaning and the possible damage to pore structure during the core cleaning process,it's hard to guarantee the accuracy of porosity measurement.In this regard,we resort to the two-dimensional(2D)nuclear magnetic resonance(NMR)technology of T1-T2 in characterizing the primary shale porosity with samples taken from pressure coring in the 4th member of Shahejie Formation(Sha 4 Member)in well Fanxie 184 in the Jiyang Depression.Moreover,comparative experiments of shale porosity measurement by three methods,namely the simultaneous distillation extraction(SDE),helium and fluid measurement,are carried out simultaneously.The results show that the values obtained by SDE,gas and liquid measurement are similar,which are about 0.6 times of 2D NMR porosimetry.Core cleaning efficiency tends to seriously affect the results involving gas and liquid.In addition,the core cleaning treatment is bound to change the shale pore structure,and this is especially true in clay which tends to swell;consequently,porosity results are to be distorted by gas and liquid methods.It is thereby recommended to utilize the 2D NMR technology to characterize the primary total porosity of shale samples without core cleaning.The effective porosity of samples from pressure coring is determined by T2 cutoff value of around 0.3 ms.