Paleoenvironment of marine-continental transitional shales in the lower Permian Shanxi formation,southeastern Ordos Basin,China

The paleoenvironment of shales can be reconstructed to some extent using the combinations or concentrations of elements that correlate strongly with environmental conditions.In this study,we analyzed rare earth elements(REEs),major elements,and trace elements in the marine-continental transitional shales(transitional shales for short)of the Shan 2^(3)submember of the Shanxi Formation in the southeastern Ordos Basin.The purpose is to deduce the paleoenvironmental conditions of the shales,encompassing paleoredox,paleoclimate,paleoproductivity,and paleo-provenance.The Shan 2^(3)submember comprises four sections,namely Shan 2^(3)-1,Shan 2^(3)-2,Shan 2^(3)-3,and Shan 2^(3)-4.The Ba/Al,P/Al,and Cu/Al ratios,along with biogenic barium(Babio),indicate that the paleoproductivity of the submember peaked during the Shan 2^(3)-1 deposition and exhibited a downtrend upward in other sections.Trends in the Uau and the Ni/Co,V/Cr,U/Th,and V/Sc ratios suggest that suboxic conditions prevailed during the Shan 2^(3)-1 deposition,with the oxidation level gradually increasing from Shan 2^(3)-1 to Shan 2^(3)-4.C-value and the Sr/Cu vs.Ga/Rb cross-plot indicate a warm and arid paleoclimate during the Shan 2^(3)-1 deposition,which transitioned to cooler,drier conditions during the deposition of other sections.Indicators sensitive to paleoclimate,such as the K/Rb and Th/U ratios,along with the ICV,PIA,and Chemical Index of Alteration(CIA),highlight elevated weathering from Shan 2^(3)-2 to Shan 2^(3)-4,with Shan 2^(3)-1 exhibiting the weakest weathering during its deposition.As suggested by the REE data,the Zr/Sc vs.Th/Sr cross-plot,provenance discriminant functions,and the cross-plots of Hf vs.La/Th,Th vs.Hf-Co,and∑REE vs.La/Yb,the sedimentary provenance for the transitional shales of the Shan 2^(3)submember is of multiple origins,with significant contributions from the Upper Continental Crust(UCC).Discriminant diagrams,including those of Th-Co-Zr/10,Th-Sc-Zr/10,La-Th-Sc,and K_(2)O/Na_(2)O vs.SiO_(2),suggest that the transitional shales of the Shan 2^(3)submember were primarily deposited under tectonic settings such as continental island arcs(CIAs)and passive continental margins(PCMs).

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.

Influence of structural damage on evaluation of microscopic pore structure in marine continental transitional shale of the Southern North China Basin:A method based on the low-temperature N2 adsorption experiment

Structural damage from sample preparation processes such as cutting and polishing may change the pore structure of rocks.However,changes in pore structure caused by this structural damage from crushing and its effect on marine continental transitional shale have not been well documented.The changes of microscopic pore structure in marine continental transitional shale during the sample preparation have important research value for subsequent exploration and development of shale gas.In this study,the pore structures of transitional shale samples from the Shanxi-Taiyuan Formation of the Southern North China Basin under different degrees of damage were analyzed through low-temperature N;adsorption experiments,combined with X-ray diffraction,total organic carbon,vitrinite reflectance analysis,and scanning electron microscopy.The results showed that(1)With increasing structural damage,the specific surface area(SSA)changed within relatively tight bounds,while the pore volume(PV)varied significantly,and the growth rate(maximum)exhibited a certain critical value with the crushing mesh number increasing from 20 to 200.(2)The ratio of SSA to PV can be used as a potential proxy for evaluating the influence of changes in the pore structure.(3)Correlation analysis revealed that the microscopic pore structure of marine continental transitional shale from the Shanxi-Taiyuan Formations is mainly controlled by organic matter and clay minerals.Clay minerals play a leading role in the development of microscopic pores and changes in pore structure.

Lithofacies and pore features of marine-continental transitional shale and gas enrichment conditions of favorable lithofacies:A case study of Permian Longtan Formation in the Lintanchang area,southeast of Sichuan Basin,SW China

In this work,the Permian Longtan marine-continental transitional shale in the southeast of Sichuan Basin was taken as study object.Through petrology and geochemical analysis,lithofacies types of the marine-continental transitional shale were classified,key controlling factors of physical properties and gas content of the different shale lithofacies were analyzed.The research results show that the Longtan Formation marine-continental transitional shale in the study area has four types of lithofacies,namely,organic-lean calcareous shale,organic-lean mixed shale,organic-lean argillaceous shale,and organic-rich argillaceous shale,among which the organic-rich argillaceous shale is the most favorable lithofacies of the study area.The pore types of different lithofacies vary significantly and the clay mineral-related pore is the dominant type of the pore system in the study area.The main controlling factor of the physical properties is clay mineral content,and the most important factor affecting gas content is TOC content.Compared with marine shale,the marine-continental transitional shale has low average values,wide distribution range,and strong heterogeneity in TOC content,porosity,and pore structure parameters,but still contains some favorable layers with high physical properties and gas contents.The organic-rich clay shale deposited in tidal flat-lagoon system is most likely to form shale gas sweet spots,so it should be paid more attention in shale gas exploration.

Geochemical and Geological Characterization of Marine-Continental Transitional Shale: A Case Study in the Ordos Basin, NW China

The organic-rich shale of the Shanxi and Taiyuan Formation of the Lower Permian deposited in a marinecontinental transitional environment are well developed in the Ordos Basin,NW China,which is considered to contain a large amount of shale hydrocarbon resources.This study takes the Lower Permian Shanxi and Taiyuan shale collected from well SL~# in the Ordos Basin,NW China as an example to characterize the transitional shale reservoir.Based on organic geochemistry data,X-ray diffraction(XRD)analysis,field-emission scanning electron microscopy(FE-SEM)observations,the desorbed gas contents of this transitional shale were systematically studied and the shale gas potential was investigated.The results indicate that the Lower Permian Shanxi and Taiyuan shale has a relatively high total organic carbon(TOC)(average TOC of 4.9%)and contains type III kerogen with a high mature to over mature status.XRD analyses show that an important characteristic of the shale is that clay and brittle minerals of detrital origin comprise the major mineral composition of the marine-continental transitional shale samples,while the percentages of carbonate minerals,pyrite and siderite are relatively small.FE-SEM observations reveal that the mineral matrix pores are the most abundant in the Lower Permian shale samples,while organic matter(OM)pores are rarely developed.Experimental analysis suggests that the mineral compositions mainly govern the macropore development in the marine-continental transitional shale,and mineral matrix pores and microfractures are considered to provide space for gas storage and migration.In addition,the desorption experiments demonstrated that the marine-continental transitional shale in the Ordos Basin has a significantly potential for shale gas exploration,ranging from 0.53 to 2.86 m^3/t with an average value of 1.25m^3/t,which is in close proximity to those of terrestrial shale(1.29 m^3/t)and marine shale(1.28 m^3/t).In summary,these results demonstrated that the Lower Permian marine-continental transitional shale in the Ordos Basin has a significantly potential for shale gas exploration.

Investigations of methane adsorption characteristics on marine-continental transitional shales and gas storage capacity models considering pore evolution

Methane adsorption is a critical assessment of the gas storage capacity(GSC)of shales with geological conditions.Although the related research of marine shales has been well-illustrated,the methane adsorption of marine-continental transitional(MCT)shales is still ambiguous.In this study,a method of combining experimental data with analytical models was used to investigate the methane adsorption characteristics and GSC of MCT shales collected from the Qinshui Basin,China.The Ono-Kondo model was used to fit the adsorption data to obtain the adsorption parameters.Subsequently,the geological model of GSC based on pore evolution was constructed using a representative shale sample with a total organic carbon(TOC)content of 1.71%,and the effects of reservoir pressure coefficient and water saturation on GSC were explored.In experimental results,compared to the composition of the MCT shale,the pore structure dominates the methane adsorption,and meanwhile,the maturity mainly governs the pore structure.Besides,maturity in the middle-eastern region of the Qinshui Basin shows a strong positive correlation with burial depth.The two parameters,micropore pore volume and non-micropore surface area,induce a good fit for the adsorption capacity data of the shale.In simulation results,the depth,pressure coefficient,and water saturation of the shale all affect the GSC.It demonstrates a promising shale gas potential of the MCT shale in a deeper block,especially with low water saturation.Specifically,the economic feasibility of shale gas could be a major consideration for the shale with a depth of<800 m and/or water saturation>60%in the Yushe-Wuxiang area.This study provides a valuable reference for the reservoir evaluation and favorable block search of MCT shale gas.

Geological conditions and reservoir characteristics of various shales in major shalehosted regions of China

China is home to shales of three facies:Marine shale,continental shale,and marine-continental transitional shale.Different types of shale gas are associated with significantly different formation conditions and major controlling factors.This study compared the geological characteristics of various shales and analyzed the influences of different parameters on the formation and accumulation of shale gas.In general,shales in China’s several regions exhibit high total organic carbon(TOC)contents,which lays a sound material basis for shale gas generation.Marine strata generally show high degrees of thermal evolution.In contrast,continental shales manifest low degrees of thermal evolution,necessitating focusing on areas with relatively high degrees of thermal evolution in the process of shale gas surveys for these shales.The shales of the Wufeng and Silurian formations constitute the most favorable shale gas reservoirs since they exhibit the highest porosity among the three types of shales.These shales are followed by those in the Niutitang and Longtan formations.In contrast,the shales of the Doushantuo,Yanchang,and Qingshankou formations manifest low porosities.Furthermore,the shales of the Wufeng and Longmaxi formations exhibit high brittle mineral contents.Despite a low siliceous mineral content,the shales of the Doushantuo Formation feature a high carbonate mineral content,which can increase the shales’brittleness to some extent.For marine-continental transitional shales,where thin interbeds of tight sandstone with unequal thicknesses are generally found,it is recommended that fracturing combined with drainage of multiple sets of lithologic strata should be employed to enhance their shale gas production.

Favorable lithofacies and pore characteristics of the Permian Longtan Formation shale in the southern Sichuan Basin

The marine–continental transitional shale (MCTS) reservoirs of the Longtan Formation (LTF) are widely distributed in the Sichuan Basin. However, the LTF shale exhibits considerable variations in mineral composition and pore characteristics, which makes identifying the 'sweet spot'a challenging task. To address this issue, 10 samples from four typical shale gas wells in the LTF in the southern Sichuan Basin were selected and analyzed for total organic carbon (TOC) content, whole-rock composition using X-ray diffraction (XRD), low-pressure gas adsorption, and high-pressure mercury intrusion. The lithofacies distribution and pore structure of the MCTS were studied to determine the pore structural characteristics and the primary factors influencing pore formation in different types of shale lithofacies in the LTF. The lithofacies of the LTF shale in the study area can be classified into three categories: siliceous clay shale, clay shale and mixed shale. Mineral content has a significant impact on the pore characteristics, while TOC content has a minor effect on the pore volume and specific surface area of micropores and mesopores. It can be inferred that the mesopores in the MCTS are mainly related to clay mineral pores, and mineral dissolution and TOC content are not the primary factors contributing to pore formation.

Discovery of Marine-terrestrial Transitional Facies Shale Gas in Taiyuan Formation of Zhenjia 1 Well,Northern Shaanxi,Ordos basin

Objective At present, there is controversy regarding the existence of marine-terrestrial transitional facies that can act as a source of shale gas. This detailed study of Carboniferous-Permian age geological data from the northern Shaanxi area （China） provides new insight for this type of shale gas. In addition, a new deposition and accumulation pattern for this type of shale gas is established.

Quantitative evaluation of organic richness from correlation of well logs and geochemical data: a case study of the Lower Permian Taiyuan shales in the southern North China Basin

Marine-continental transitional shale is a potential energy component in China and is expected to be a realistic field in terms of increasing reserves and enhancing the natural gas production.However,the complex lithology,constantly changing depositional environment and lithofacies make the quantitative determination of the total organic carbon(TOC)suitable for marine shales not necessarily applicable to transitional shales.Thus,the identification of marine-continental transitional organic-rich shales and the mechanism of organic matter enrichment need to be further studied.As a typical representative of transitional shale,samples from Well MY-1 in the Taiyuan Formation in the southern North China Basin,were selected for TOC prediction using a combination of experimental organic geochemical data and well logging data including natural gamma-ray(GR),density(DEN),acoustic(AC),neutron(CNL)and U spectral gamma-ray(U),and TH spectral gamma-ray(TH).The correlation coefficient,coefficient of determination,standard deviation,mean squared error(MSE)and root mean squared error(RMSE)were selected to conduct the error analysis of the evaluation of different well log-based prediction methods,involving U spectral gamma logging,ΔlogR,and multivariate fitting methods to obtain the optimal TOC prediction method for the Taiyuan transitional shale.The plots of TOC versus the remaining volatile hydrocarbon content and the generation potential from Rock Eval show good to excellent potentials for hydrocarbon generation.The integrated results obtained from the various log-based TOC estimation methods indicate that,the multivariate fitting method of GR-U-DEN-CNL combination is preferable,with the correlation coefficients of 0.78 and 0.97 for the entire and objective interval of the Taiyuan Formation respectively,and with the minimum MSE and RMSE values.Specifically,the U spectral gamma logging method based on single logging parameter is also a better choice for TOC prediction of the high-quality intervals.This study provides a reference for the exploration and development of unconventional shale gas such as transitional shale gas.