The influence factors of gas-bearing and geological characteristics of Niutitang Formation shale in the southern margin of Xuefeng Mountain ancient uplift: A case of Well Huangdi 1

In order to evaluate the geological characteristics and gas-bearing factors of Niutitang Formation within the Lower Cambrian of northern Guizhou,the Huangping area located at the southern edge of the ancient uplift belt of Xuefeng Mountain was selected as the target area,and Well Huangdi 1 was drilled for the geological survey of shale gas.Through geological background analysis and well logging and laboratory analysis such as organic geochemical test,gas content analysis,isothermal adsorption,and specific surface area experiments on Well Huangdi 1,the results show that the Niutitang Formation is a deep-water shelf,trough-like folds and thrust fault.The thickness of black shale is 119.95 m,of which carbonaceous shale is 89.6 m.The average value of organic carbon content is 3.55%,kerogen vitrinite reflectance value is 2.37% and kerogen type is sapropel-type.The brittle mineral content is 51%(quartz 38%),clay mineral content is 38.3%.The value of porosity and permeability are 0.5%and 0.0014 mD,which the reservoir of the Niutitang Formation belongs to low permeability with characteristics of ultra-low porosity.The gas content is 0.09‒1.31 m^3/t with a high-value area and a second high-value area.By comparing with the geological parameters of adjacent wells in the adjacent area,the accumulation model of“sediment control zone,Ro control zone,structure controlling reservoir”in the study area is proposed.Therefore,deep-water shelf-slope facies,Ro is between high maturity-early stage of overmaturity and well-preserved zones in the Niutitang Formation in this area are favorable direction for the next step of shale gas exploration.

New Re-Os Isotopic Constrains on the Formation of the Metalliferous Deposits of the Lower Cambrian Niutitang Formation

The Terreneuvian Epoch（541-521 Ma）is also an important period for metallogenesis in South China, as was represented by the widespread occurrences of Ni-Mo polymetallic layers on the antecedent shallow platform margin and the V-enriched black shales in deeper slope-basin settings. In this study, we have measured Re-Os isochron ages of Ni-Mo polymetallic layers（Songlin, Niuchang, Sancha, Chuanpengwan）, V-rich black shales（Bahuang）, and non-metalliferous black shales（Shuidong） in the basal Niutitang Formation in Guizhou and Hunan province, South China. The Ni-Mo polymetallic layers and V-enriched black shales have similar Re-Os isochron ages, suggesting concurrent deposition of these two types of metalliferous ores. This suggestion is consistent with the traditional stratigraphic correlation by using the nodular phosphorite bed directly underlying these metalliferous layers as a stratigraphic marker. Furthermore, the metalliferous ores and non-metalliferous black shales have similar initial ^（187）Os/^（188） Os ratios of 0.8-0.9, arguing for a dominant seawater origin with minor contribution of hydrothermal activity. Furthermore, Re-Os isotopic data also imply that Ni-Mo and V ore might have derived from the same source. We suggest that the spatial distribution of metalliferous ores can be explained by the development of non-sulfidic anoxic-suboxic wedge（NSASW） in the slope-basin and sulfidic wedge in the previous platform margin. Upwelling of deep water first transects the mildly reduced, organic rich NSASW, in which V（V） is reduced to V（IV）, and is preferentially removed from seawater by organometallic complex formation. As a result, V-rich black shale deposits in the slope-basin of Yangtze Platform. Further movement of deep water into the sulfidic platform margin results in Ni-Mo polymetallic layer formation.

Insights into Pore Structure and Fractal Characteristics of the Lower Cambrian Niutitang Formation Shale on the Yangtze Platform,South China

Shales from the Lower Cambrian Niutitang Formation of Yangtze Platform have been widely investigated due to its shale gas potential.To better illustrate the pore structure and fractal characteristics of shale,a series of experiments were conducted on outcrop samples from the Lower Cambrian Niutitang Formation on Yangtze Platform,including X-ray diffraction(XRD),field-emission scanning electron microscopy(FE-SEM) and low-temperature nitrogen adsorption.Frenkel-Halsey-Hill(FHH) model was adopted to calculate the fractal dimensions.Furthermore,the relationships between fractal dimensions and pore structure parameters and mineral composition are discussed.FE-SEM observation results show that interparticle pores are most developed in shale,followed by intraparticle pores.This study identified the fractal dimensions D1(ranging from 2.558 0 to 2.710 2) and D2(ranging from 2.541 5 to 2.765 2).The pore structure of the Niutitang Formation shale is primarily controlled by quartz and clay content.Fractal dimensions are able to characterize the pore structure complexity of Niutitang Formation shale because D1 and D2 correlate well with average pore diameter and quartz content.

Geological controls of shale gas accumulation and enrichment mechanism in Lower Cambrian Niutitang Formation of western Hubei, Middle Yangtze, China

The lower Cambrian Niutitang Formation is of crucial importance for shale gas target reservoirs in western Hubei,China;however,little work has been done in this field,and its shale gas accumulation and enrichment mechanism are still unclear.Based on survey wells,outcrop data,and large numbers of tests,the geological conditions of shale gas accumulation were studied;moreover,the factors that influence the gas content were thoroughly discussed.The results show that the Niutitang Formation(Є1n)can be divided into three sections:the first section(Є_(1)n^(1)),the second section(Є_(1)n^(2)),and the third section(Є1n3).TheЄ_(1)n^(2) is the main shale gas reservoir.The deep shelf facies is the main sedimentary facies and can be divided into three main lithofacies:argillaceous siltstone,carbonaceous shale and carbonaceous siliceous rock.The total organic carbon(TOC)content shows gentle growth trends until bottom of theЄ_(1)n^(2) and then decreases rapidly within theЄ_(1)n^(1),and the TOC content mainly ranges from 2%to 4%horizontally.The calcite and dolomite dissolution pores,clay intergranular pores and organic pores are the main pore types and the micropore types are clearly related to the mineral compositions and the TOC content.Vertically,the gas content is mainly affected by the TOC content.Horizontally,wells with high gas contents are distributed only southeast of the Huangling anticline,and the combination of structural styles,fault and fracture development,and the distribution of the regional unconformity boundary between the upper Sinian Dengying Formation(Z2d)and theЄ_(1)n^(2) are the three most important factors affecting the gas content.The favorable areas must meet the following conditions:a deep shelf environment,the presence of theЄ_(1)n^(1),wide and gentle folds,far from large normal faults that are more than 5 km,moderate thermal evolution,and greater than 500 m burial depth;this includes the block with the YD2–ZD2 wells,and the block with the Y1 and YD4 wells,which are distributed in the southern portion of the Huangling anticline and northern portion of the Xiannvshan fault.

Shale gas accumulation conditions and gas-bearing characteristics of the Lower Cambrian Niutitang Formation in Well Changye-1 in northwestern Hunan Province

To evaluate the accumulation conditions and gas-bearing properties of shale gas in the Lower Cambrian Niutitang Formation,northwestern Hunan Province,the first shale gas parameter well(Well Changye-1)that takes the Niutitang Formation as the target horizon in the Hunan Province was selected preferably and drilled,cumulatively revealing the thickest dark shale horizon of the Niutitang Formation among the single-well drillings in China,with a true vertical thickness of 674.5m.Through analyzing the core samples in terms of organic geochemistry,rock mineralogy and reservoir properties,the black shale horizons in the Niutitang Formation of Well Changye-1 have high organic carbon content(average 3.9%),moderate maturity(equivalent Ro average 2.6%),high brittle mineral content(quartz content average 50.1%),low clay mineral content(average 32.4%),low porosity(1.7%)and low permeability(1.32×10^(-3)mD),and well-developed meso to micro-pores and fractures,indicating good conditions for shale gas accumulation.Field desorption and laboratory isothermal adsorption measurements on core samples show that Well Changye-1 has good gas-bearing properties,and gas content generally increases with depth.The black shale horizons at the depth of 1100-1250m have an average organic carbon content up to 10.1%,total gas content of 0.5-2.1m^(3)/t and 3.7-6.4m^(3)/t,and this is the most favorable depth for shale gas development,indicating that the Niutitang Formation has good a prospect for shale gas exploration.Due to huge sedimentary thickness,the black shale in the middle-lower part of the Niutitang Formation should be given priority for exploration.

Reservoir characteristics and controlling factor of shale gas in Lower Cambrian Niutitang Formation, South China

Large-scale exploration and development of shale gas in Lower Cambrian Niutitang Formation of South China has been carried out in recent years,but the result is not good,only except some drilling wells in Jingyan-Qianwei area of Sichuan Basin and Yichang of Hubei obtain some commercial gas flows.In order to clarify reasons for failure of shale gas exploration and development in Niutitang Formation around Sichuan Basin and to provide reservoir geological parameters for subsequent efficient exploration and development,taking a case of shale gas reservoirs in Lower Cambrian Niutitang Formation in northeast Chongqing around Sichuan Basin,some experimental methods,such as analysis of organic carbon pyrolysis,determination of equivalent vitrinite reflectance,focused ion beam scanning electron microscope(FIB-SEM)and other are adopted in this study.The results show that the average TOC of shale samples in Niutitang Formation in northeast Chongqing is 3.1%,the equivalent vitrinite reflectance ranges from 3.0%to 4.0%,and the degree of thermal evolution reaches the post-matureemetamorphic stage.Due to excessively high degree of the thermal evolution,organic pores in shale samples are not developed in Lower Cambrian Niutitang Formation in the study area,instead,micro-nano pores dominated by intergranular pores and intragranular pores are developed in the shale.The degree of the thermal evolution controls the sustaining gas generation of kerogen and retained liquid hydrocarbons in the shale,it also controls the development of organic pores of the shale.The evolution of organic pores and hydrocarbon generation in the shale of Niutitang Formation in northeast Chongqing around Sichuan Basin do not match best with each other,that is,during development period of a large number of organic pores,thermal evolution degree of reservoirs is further enhanced because the strata are not uplifted in time,therefore,the quantity of organic pores is decreased sharply,the shale gas would be escaped due to the absence of organic pores as effective storage space after the shale gas generation.Therefore,the efficient exploration and development of the shale gas in Lower Cambrian Niutitang Formation in South China should be focused on the shale development area where the shale is characterized by moderate thermal evolution degree(2.0%<R_(o)<3.0%)and shallow buried depth,that is,the shale distribution area with paleo-uplift or paleo-buried hill.

Characteristics and Sources of Organic Matter from the Early Cambrian Niutitang Formtion and Its Preservation Environment in Guizhou

The Early Cambrian Niutitang Formation on the Yangtze Block is a set of high-quality marine source rock.However,hydrocarbon-forming organisms of these organic-rich shales was poorly understood.In this paper,the results of palynofacies analysis and hydrocarbon-forming organism characteristics of the Niutitang Formation from the Yangtze Block are reported for the first time,and the sedimentary environment is discussed in combination with geochemical data.Palynofacies analysis show that the organic matter(OM)of the Niutitang Formation is mainly composed of amorphous organic matter(AOM),with a small amount of structural organic matter(STOM)and palynomorphs(PL).The results of CONISS(stratigraphically constrained incremental sum of squares)cluster analysis show that the PL can be divided into three zones in the ascending order:Leiosphaeridia-Micrhystridiumfungal zone,algal bloom zone and fungal development zone,corresponding to three stages:transgression,high-stand and regression,respectively.The principal component analysis(PCA)shows that the main hydrocarbon-forming organisms include planktonic green microalgae or dinoflagellates that are related to Leiosphaeridia and Microcystidium.Palynofacies analysis and geochemical data show that the Niutitang Formation was deposited under anoxic or dysoxic condition which was favorable to the enrichment of organic matter.

Sulfur and Oxygen Isotopes of Sulfate Extracted from Early Cambrian Phosphorite Nodules： Implications for Marine Redox Evolution in the Yangtze Platform

Phosphorite nodule beds are discovered in the black shale of basal Niutitang Formation throughout the Yangtze Platform in South China, recording an important phosphorite-generation event. Platform-wide phosphorite precipitation requires special oceanographic and geochemical conditions, thus the origin of the Niutitang phosphorite nodules may provide valuable information about the ocean chemistry in the Early Cambrian. In this study, we measured sulfur and oxygen isotopic compositions of sulfate extracted from phosphorite nodules collected from the basal Niutitang Formation. Phosphorite associated sulfate（PAS） is a trace amount of sulfate that incorporates into crystal lattice during phosphorite precipitation, accordingly PAS records the geochemical signals during phosphorite nodule formation. Sulfur isotopic composition of PAS（δ^（34）S_（PAS）） ranges from-1.16‰ to ＋24.48‰（mean=＋8.19‰, n=11）, and oxygen isotopic value（δ^（18）O_（PAS）） varies between-5.3‰ and ＋26.3‰（mean=＋7.0‰, n=8）. Most phosphorite nodules have low δ^（34）SPAS and low δ^（18）O_（PAS） values, suggesting PAS mainly derived from anaerobic oxidation of H_2S within suboxic sediment porewater. We propose that phosphate was delivered to the Yangtze Platform by a series of upwelling events, and was scavenged from seawater with the precipitation of FeOOH. The absorbed phosphate was released into suboxic porewater by the reduction of FeOOH at the oxic-suboxic redox boundary in sediments, and phosphorite nodule precipitated by the reaction of phosphate with Ca^（2＋） diffused from the overlying seawater. The platform-wide deposition of phosphorite nodules in the basal Niutitang Formation implies the bottom water might be suboxic or even oxic, at least sporadically, in Early Cambrian. We speculate that the intensified ocean circulation as evident with frequent occurrences of upwelling events might be the primary reason for the episodic oxidation of the Yangtze Platform in Early Cambrian.

Geological Characteristics of Shale Gas in Different Strata of Marine Facies in South China

In the Middle and Upper Yangtze region of South China,there are well developed sets of marine shale strata.Currently only Wufeng Longmaxi shale gas has been developed in scale,while the Sinian and Cambrian shale gas are still under exploration.The various shale strata show different characteristics in lithological features,such as lithofacies types and reservoir physical properties,which are due to the influence of tectonic pattern,sedimentary environment,and diagenesis caused by tectonic subsidence.This will affect the later fracturing technology and fracturing effect.The shale reservoirs of Sinian doushantuo,Cambrian Niutitang,Upper Ordovician Wufeng,and the Lower Silurian Longmaxi Formation shale were evaluated and compared with each other with respect to their sedimentary environments,lithofacies,minerals compositions,and micro pore characteristics.The reservoir characteristics of the shale and the main control factors of shale gas enrichment were summarized in this study.

Reservoir characteristics and well-logging evaluation of the Lower Cambrian shales in southeast Chongqing, China

To accurately identify the reservoir characteristics and main controlling factors of the Cambrian Niutitang black shale in the southeast Chongqing,a series of systematic measurements were conducted on core samples from Well Yuke-1 and Well Youke-1.The measurements include clay mineral analysis,XRD analysis,petrophysical properties,specific surface area,pore diameter and TOC,as well as R_(o) and maceral compositions.The Niutitang shale reservoir contains detrital minerals and clay minerals averagely of 51.34%and 32.74%,respectively.The average effective porosity and permeability are 1.2%and 8.0×10^(-3)mD,respectively,typically of ultra-low porosity and permeability.Mesopores are dominant,accounting for approximately 73%of the total pore volume.The average BET specific surface area is 7.75m^(2)/g and the average pore diameter is 5.3nm.The average TOC is 2.29%and the average R_(o) is 3.12%,indicating that the organic matter is in over-maturity stage.Statistical analyses of the measurements show that the micropore volume is positively correlated with TOC but negatively correlated with the total content of clay minerals;the mesopore volume is positively correlated with TOC,the total content of clay minerals and the con­tent of carbonate minerals;the macropore volume is positively correlated with TOC and the con­tent of carbonate minerals,weakly positively correlated with the content of detrital minerals but negatively correlated with the total content of clay minerals.The porosity and TOC of the Niuti­tang shale are computed using the conventional well-logging method for the Yuke-1 and Youke-1 wells,and the porosities are quite consistent with the core analysis resultsr,but in the shallow res­ervoir that contains no oil and gas,the TOC exhibits no correlation with the core analysis results.