Favorable lithofacies types and genesis of marine- continental transitional black shale: A case study of Permian Shanxi Formation in the eastern margin of Ordos Basin, NW China

Based on core description,thin section identification,X-ray diffraction analysis,scanning electron microscopy,low-temperature gas adsorption and high-pressure mercury intrusion porosimetry,the shale lithofacies of Shan23 sub-member of Permian Shanxi Formation in the east margin of Ordos Basin was systematically analyzed in this study.The Shan23 sub-member has six lithofacies,namely,low TOC clay shale(C-L),low TOC siliceous shale(S-L),medium TOC siliceous shale(S-M),medium TOC hybrid shale(M-M),high TOC siliceous shale(S-H),and high TOC clay shale(C-H).Among them,S-H is the best lithofacies,S-M and M-M are the second best.The C-L and C-H lithofacies,mainly found in the upper part of Shan23 sub-member,generally developed in tide-dominated delta facies;the S-L,S-M,S-H and M-M shales occurring in the lower part of Shan23 sub-member developed in tide-dominated estuarine bay facies.The S-H,S-M and M-M shales have good pore struc-ture and largely organic matter pores and mineral interparticle pores,including interlayer pore in clay minerals,pyrite inter-crystalline pore,and mineral dissolution pore.C-L and S-L shales have mainly mineral interparticle pores and clay mineral in-terlayer pores,and a small amount of organic matter pores,showing poorer pore structure.The C-H shale has organic mi-cro-pores and a small number of interlayer fissures of clay minerals,showing good micro-pore structure,and poor meso-pore and macro-pore structure.The formation of favorable lithofacies is jointly controlled by depositional environment and diagen-esis.Shallow bay-lagoon depositional environment is conducive to the formation of type II2 kerogen which can produce a large number of organic cellular pores.Besides,the rich biogenic silica is conducive to the preservation of primary pores and en-hances the fracability of the shale reservoir.

Sequence stratigraphic framework and sedimentary model of Shanxi Formation in northeast Zhoukou Depression of the North China Plate

The sequence stratigraphic framework of Shanxi Formation in the northeast Zhoukou Depression was established based on detailed sequence stratigraphical and sedimentological analysis by utilizing the logging and core data of six wells drilled in the eastern tectonic unit of Zhoukou Depression.It was divided into three third-order sequences,namely SQs1,SQs2,and SQs3 from bottom to top.Each sequence can be divided into a transgressive system tract(TST)and a highstand system tract(HST).Furthermore,four sequence boundaries and three maximum flooding surfaces were identified,and they are the bottom interface SBs and maximum flooding surface mfss1 of SQs1,the bottom interface SBs1 and maximum flooding surface mfss2 of SQs2,the bottom interface SBs3 and maximum flooding surface mfss3 of SQs3,and the top interface SBx from bottom to top.Carbonate tidal flat–clastic tidal flat sedimentary system developed in Shanxi Formation in the northeast Zhoukou Depression(also referred to as the study area)under the control of regression.Meanwhile,four sedimentary microfacies were identified in the sedimentary system,which are lime-mud flats,sand flats,mixed flats,and mud flats.The transgression in the study area occurred from the southeast to the northwest.Therefore,the northwestern part is the seaward side,and the southeastern part is the landward side.As revealed by relevant drilling data,SQs1 of the Shanxi Formation is characterized by the development of limestone and carbonaceous mudstone,with limestone,dark mudstone,and carbonaceous mudstone mainly developing.Meanwhile,lime-mud flats were mainly deposited in it.During the periods of SQs2 and SQs3,the sedimentary environment of the study area changed from the carbonate tidal flats to clastic tidal flats as the coastline migrated towards the sea.In these periods,sand flats mainly developed near the maximum flooding surfaces and were relatively continuous in the eastern and southern parts of the transgressive system tract;mixed flats were relatively continuous in the western and northern parts of the transgressive system tract as well as the eastern and southern parts of the highstand system tract;mud flats widely developed in the highstand system tract.Peat flats mainly developed in the period of HSTs2,with coal seams relatively developing in the southeastern part of the study area as revealed by drilling data.The peat flats in SQs2 are favorable hydrocarbon source layers,the lime-mud flats in SQs1 and sand flats formed in the transgression periods of SQs2 and SQs3 constitute favorable hydrocarbon reservoirs,and the mud flats form in the transgressions periods serve as favorable cap rocks.Therefore,the study area features a reservoir-cap assemblage for self-generating and self-storing of hydrocarbon,and the southeastern part of the study area can be taken as a favorable exploration area.

Closed-system pyrolysis-based hydrocarbon generation simulation and gas potential evaluation of the Shanxi Formation shales from the Ordos Basin, China

The Shanxi Formation(Shan 1 and Shan 2 Members)shales show good prospects in shale gas development in the Yan'an area of Ordos Basin.Based on the simulation experiment of hydrocarbon generation of low maturity shale samples,the hydrocarbon generation characteristics of shale samples was studied systematically.Then,combined with the geochemical analysis of shale and gas generation simulation,shale gas potential was evaluated.The results reveal that Shan 1 and Shan 2 shale samples are favorable for shale gas enrichment by and large,with C_(1)-C_(5) maximum yields of 146.96-160.83 mg/g TOC and 148.48-148.67 mg/g TOC respectively at a heat rate of 20℃/h and 2℃/h.The Shan 1 and Shan 2 shales are basically the same in terms of organic carbon production potential of each unit.The carbon isotopic composition of alkane gas reveals that heteroatomic compounds(NSOs)cracking is an important mechanism for shale gas generation of Shanxi Formation shales,and conducive to gas generation at highto over-mature stages.Given thermal history and kinetic parameters of hydrocarbon generation,the shales of Shanxi Formation reached the maximum gas production potential in the Late Cretaceous,with a maximum yield of 160.3 mg/g TOC under present geological conditions.During geological history,the Shanxi Formation shales went through high-to over-maturity evolution,mainly producing dry gas,and their gas generation capacity was controlled by the organic matter abundance and cracking capacity.The gas generation potential of Shan 2 shale is higher than that of Shan 1,due to its higher TOC.

New material of Cordaites baodeensis Sun from Lower Permian Shanxi Formation of Baode,Shanxi

Based on the anatomical study of leaf cuticles of Cordaites from the Lower Permian Shanxi Formation in Baode of Shanxi, the author analysed the epidermal characters of Cordaites in this area. On the base of the abundant compressions and laboratory studies, the author complemented some new data of the cuticular characters of the speeies Cordaites baodeensis Sun for its study of taxonomy. Meanwhile, the author newly discovered a number of sporopollen fossils associated with the mega-plants of Cordctitales, described 9 species of 6 genera. The palynological data can provide a supplement evidence for defining the age of C. baodeensis and its associated plants to be Early Permian.

Reconstruction of paleo river systems and distribution of sedimentary facies of Shanxi and lower Shihezi formations in southern Ordos basin

The study area is located in the south of Huanxian county,in Yan'an and Puxian counties and to the north of Xi'an.The Shanxi and lower Shihezi formations are important gas-bearing formations.Given our analysis of the direction of rivers,the contents of stable heavy minerals and of feldspar of palaeo river systems,the study area is divided into six palaeodrainage patterns corresponding to six feldspar regions and six sedimentary facies regions.On this basis,the distribution of sedimentary facies was also analyzed.During the Shanxi stage,a delta front was deposited in the Huanxian region and delta plains and fronts were deposited in the Pingliang,Chunhua-Yaoxian and Hancheng-Chengcheng regions.In the Yan'an-Daning region,only a delta front was developed.The distribution of sedimentary facies in the earlier Shihezi stage originated from the Shanxi stage.A delta front was developed in the Huanxian region while a delta plain and front developed in the Pingliang-Zhenyuan region during the same time.Lakes originated only in the Zhenyuan-Huanxian-Huachi-Zhengning and Daning-Jixian zones.This analytical method shows that different palaeodrainage patterns can be effectively distinguished in order to forecast sedimentary facies.

Porosity model and pore evolution of transitional shales:an example from the Southern North China Basin

The evolution of shale reservoirs is mainly related to two functions:mechanical compaction controlled by ground stress and chemical compaction controlled by thermal effect.Thermal simulation experiments were conducted to simulate the chemical compaction of marine-continental transitional shale,and X-ray diffraction(XRD),CO2 adsorption,N2 adsorption and high-pressure mercury injection(MIP)were then used to characterize shale diagenesis and porosity.Moreover,simulations of mechanical compaction adhering to mathematical models were performed,and a shale compaction model was proposed considering clay content and kaolinite proportions.The advantage of this model is that the change in shale compressibility,which is caused by the transformation of clay minerals during thermal evolution,may be considered.The combination of the thermal simulation and compaction model may depict the interactions between chemical and mechanical compaction.Such interactions may then express the pore evolution of shale in actual conditions of formation.Accordingly,the obtained results demonstrated that shales having low kaolinite possess higher porosity at the same burial depth and clay mineral content,proving that other clay minerals such as illite-smectite mixed layers(I/S)and illite are conducive to the development of pores.Shales possessing a high clay mineral content have a higher porosity in shallow layers(<3500 m)and a lower porosity in deep layers(>3500 m).Both the amount and location of the increase in porosity differ at different geothermal gradients.High geothermal gradients favor the preservation of high porosity in shale at an appropriate Ro.The pore evolution of the marine-continental transitional shale is divided into five stages.Stage 2 possesses an Ro of 1.0%-1.6%and has high porosity along with a high specific surface area.Stage 3 has an Ro of 1.6%-2.0%and contains a higher porosity with a low specific surface area.Finally,Stage 4 has an Ro of 2.0%-2.9%with a low porosity and high specific surface area.

Geological characteristics and development potential of transitional shale gas in the east margin of the Ordos Basin,NW China

The shales in the 2nd Member of Shanxi formation in the east margin of the Ordos Basin were deposited in a marine-nonmarine transitional environment during the Permian.Based on the recent breakthroughs in the shale gas exploration and theoretical understandings on the shale gas of the study area,with a comparison to marine shale gas in the Sichuan Basin and marine-nonmarine transitional shale gas in the U.S.,this study presents the geological characteristics and development potential of marine-nonmarine transitional gas in the study area.Four geological features are identified in the 2nd Member of the Shanxi Formation in the study area has:(1)stable sedimentary environment is conductive to deposition of widely distributed organic shale;(2)well-developed micro-and nanoscale pore and fracture systems,providing good storage capacity;(3)high content of brittle minerals such as quartz,leading to effectively reservoir fracturing;and(4)moderate reservoir pressure and relatively high gas content,allowing efficient development of shale gas.The 2nd Member of Shanxi Formation in the east margin of Ordos Basin is rich in shale gas resource.Three favorable zones,Yulin-Linxian,Shiloubei-Daning-Jixian,and Hancheng-Huangling are developed,with a total area of 1.28×104 km2 and resources between 1.8×1012 and 2.9×1012m3,indicating a huge exploration potential.Tests of the 2nd Member of Shanxi Formation in vertical wells show that the favorable intervals have stable gas production and high reserves controlled by single well,good recoverability and fracability.This shale interval has sufficient energy,stable production capacity,and good development prospects,as evidenced by systematic well testing.The east margin of the Ordos Basin has several shale intervals in the Shanxi and Taiyuan formations,and several coal seams interbedded,so collaborative production of different types of natural gas in different intervals can be considered.The study results can provide reference for shale gas exploration and development and promote the rapid exploitation of shale gas in China.

Lithofacies palaeogeography of the Carboniferous and Permian in the Qinshui Basin, Shanxi Province, China

The Qinshui Basin in the southeastern Shanxi Province is an important area for coalbed methane（CBM） exploration and production in China, and recent exploration has revealed the presence of other unconventional types of gas such as shale gas and tight sandstone gas. The reservoirs for these unconventional types of gas in this basin are mainly the coals, mudstones, and sandstones of the Carboniferous and Permian; the reservoir thicknesses are controlled by the depositional environments and palaeogeography. This paper presents the results of sedimentological investigations based on data from outcrop and borehole sections, and basin-wide palaeogeographical maps of each formation were reconstructed on the basis of the contours of a variety of lithological parameters. The palaeogeographic units include the depositional environments of the fluvial channel, flood basin（lake）, upper delta plain, lower delta plain, delta front, lagoon, tidal flat, barrier bar, and carbonate platform.The Benxi and Taiyuan Formations are composed mainly of limestones, bauxitic mudstones,siltstones, silty mudstones, sandstones, and economically exploitable coal seams, which were formed in delta, tidal flat, lagoon, and carbonate platform environments. The Shanxi Formation consists of sandstones, siltstones, mudstones, and coals; during the deposition of the formation, the northern part of the Qinshui Basin was occupied mainly by an upper delta plain environment, while the central and southern parts were mainly occupied by a lower delta plain environment and the southeastern part by a delta front environment. Thick coal zones occur in the central and southern parts, where the main depositional environment was a lower delta plain. The thick coal zones of the Taiyuan Formation evidently occur in the sandstone-rich belts, located mainly in the lower delta plain environment in the northern part of the basin and the barrier bar environments in the southeastern part of the basin. In contrast, the thick coal zones of the Shanxi Formation extend over the mudstone-rich belts, located in the areas of the lower delta plain environments of the central and southern parts of the Basin.The Xiashihezi, Shangshihezi, and Shiqianfeng Formations consist mainly of red mudstones with thick-interbedded sandstones. During the deposition of these formations, most areas of the basin were occupied by a fluvial channel, resulting in palaeogeographic units that include fluvial channel zones and flood basins. The fluvial channel deposits consist mainly of relatively-thick sandstones, which could have potential for exploration of tight sandstone gas.

Early-Middle Permian palynoflora of Shandong Province,eastern North China

The Permian Taiyuan and Shanxi formations exposed in Shandong Province,eastern North China,contain abundant spores and pollen.In this study,a total of 42 genera and 146 species of spores and pollen from these Permian formations,native to northern China,are identified and related to the three epochs of the Permian Period(Cisuralian,Guadalupian,and Lopingian Epochs)as two assemblages:Assemblage I—the Laevigatosporites-Granulatisporites assemblage,inferred as the Cisuralian(~298.9-272.9 Ma);and,Assemblage II—the Gulisporites-Sinulatisporites assemblage,inferred as the Guadalupian(~272.9-259.1 Ma).Assemblage I represents growing ferns,whereas Assemblage II represents gymnosperms.The assemblage division and analysis indicated that the palaeoclimate of the study area during Early-Middle Permian time was dominated by warm and humid conditions,and later in the Middle Permian changed into moderately dry conditions.