Pore Types and Quantitative Evaluation of Pore Volumes in the Longmaxi Formation Shale of Southeast Chongqing, China

The common microscale to nanoscale pore types were introduced and divided into organic and inorganic pores to estimate their contributions to porosity in the Lower Silurian Longmaxi Formation shale of southeast Chongqing. Following the material balance principle, the organic porosity values, which changed with formation subsidence and thermal evolution, were calculated using chemical kinetics methods and corrected via the organic porosity correction coefficient, which was obtained from field emission scanning electron microscopy. Grain density values were determined using the contents and true densities of compositions in the shale samples. The total porosity was calculated based on the grain and bulk densities. The inorganic porosity was determined from the difference between the total porosity and organic porosity at the same depth. The results show that inorganic pores mainly contain microfractures, microchannels, clay intergranular pores, intercrystalline pores and intracrystalline pores in the Lower Silurian Longmaxi Formation shale of southeast Chongqing. Organic pores mainly include organopore and fossil pore. The total porosity, organic porosity and inorganic porosity of organic-rich shale samples can be quantitatively evaluated using this method. The total porosity, organic porosity and inorganic porosity values of the Longmaxi Formation shale samples from the well Pyl in southeast Chongqing lie in 2.75%-6.14%, 0.08%-2.52% and 1.41%-4.92% with average values of 4.34%, 0.95% and 3.39%, respectively. The contributions of the inorganic pores to the total porosity are significantly greater than those of the organic pores.

Source Rock and Cap Rock Controls on the Upper Ordovician Wufeng Formation–Lower Silurian Longmaxi Formation Shale Gas Accumulation in the Sichuan Basin and its Peripheral Areas

Objective The Upper Ordovician Wufeng Formation-Lower Silurian Longmaxi Formation is one of the priority interval for shale gas exploration in the Sichuan Basin and its peripheral areas, and commercial shale gas has been discovered from this interval in Jiaoshiba, Changning and Weiyuan shale gas fields in Sichuan Province. However, there is no significant discovery in other parts of the basin due to the different quality of black shale and the differences of tectonic evolution. Based on the progress of shale gas geological theory and exploration discoveries, as well as the theory of ＂source rock and cap rock controls on hydrocarbon accumulation＂, of the Upper Ordovician the main controlling factors Wufeng Formation-Lower Silurian Longmaxi Formation shale gas enrichment in the Sichuan Basin and its peripheral areas were analyzed, and the source rock and cap rock controls on the shale gas were also discussed. The results can provide new insights for the next shale gas exploration in this area.

A Strain Rate Dependent Constitutive Model for the Lower Silurian Longmaxi Formation Shale in the Fuling Gas Field of the Sichuan Basin,China

Shale,as a kind of brittle rock,often exhibits different nonlinear stress-strain behavior,failure and timedependent behavior under different strain rates.To capture these features,this work conducted triaxial compression tests under axial strain rates ranging from 5×10-6 s-1 to 1×10-3 s-1.The results show that both elastic modulus and peak strength have a positive correlation relationship with strain rates.These strain rate-dependent mechanical behaviors of shale are originated from damage growth,which is described by a damage parameter.When axial strain is the same,the damage parameter is positively correlated with strain rate.When strain rate is the same,with an increase of axial strain,the damage parameter decreases firstly from an initial value(about 0.1 to 0.2),soon reaches its minimum(about 0.1),and then increases to an asymptotic value of 0.8.Based on the experimental results,taking yield stress as the cut-off point and considering damage variable evolution,a new measure of micro-mechanical strength is proposed.Based on the Lemaitre’s equivalent strain assumption and the new measure of micro-mechanical strength,a statistical strain-rate dependent damage constitutive model for shale that couples physically meaningful model parameters was established.Numerical back-calculations of these triaxial compression tests results demonstrate the ability of the model to reproduce the primary features of the strain rate dependent mechanical behavior of shale.

The new multistage water adsorption model of Longmaxi Formation shale considering the spatial configuration relationship between organic matter and clay minerals

The water adsorption by shale significantly affects shale gas content and its seepage capacity.However,the mechanism of water adsorption by shale is still unclear due to its strong heterogeneity and complicated pore structure.The relationship between the adsorbed water content at different relative humidities(RHs)and shale compositions,as well as shale pore structure and the spatial configuration relationship between organic matter(OM)and clay minerals,was investigated to clarify the controlling factors and mechanisms of water adsorption by Longmaxi Formation shale from the Southern Sichuan Basin in China.Consequently,the water adsorption process could be generally divided into three different stages from 0%RH to 99%RH.Furthermore,the Johnston’s clay mine ral interlayer pore structure model(JCM),the Freundlich model(FM)and the Dubinin-Astakhov model(DAM)were tested to fit the three water adsorption stages from low RH to high RH,respectively.The fitting results of the JCM and FM at lower RHs were far from good,while the fitting results of DAM at higher RHs were acceptable.Accordingly,two revised models(LRHM and MRHM)considering the spatial configuration relationship between OM and clay minerals were proposed for the two stages with lower RHs,and performed better fitting results indicating the pronounced effect of the spatial configuration relationship between OM and clay minerals on the water adsorption process of Longmaxi Formation shale.The outcomes of this study will contribute to clarifying the water distribution characteristics in the pore network of shale samples with variable water contents.

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.

Petrophysical properties of deep Longmaxi Formation shales in the southern Sichuan Basin, SW China

Deep shale layer in the Lower Silurian Longmaxi Formation,southern Sichuan Basin is the major replacement target of shale gas exploration in China.However,the prediction of"sweet-spots"in deep shale gas reservoirs lacks physical basis due to the short of systematic experimental research on the physical properties of the deep shale.Based on petrological,acoustic and hardness measurements,variation law and control factors of dynamic and static elastic properties of the deep shale samples are investigated.The study results show that the deep shale samples are similar to the middle-shallow shale in terms of mineral composition and pore type.Geochemical characteristics of organic-rich shale samples(TOC>2%)indicate that these shale samples have a framework of microcrystalline quartz grains;the intergranular pores in these shale samples are between rigid quartz grains and have mechanical property of hard pore.The lean-organic shale samples(TOC<2%),with quartz primarily coming from terrigenous debris,feature plastic clay mineral particles as the support frame in rock texture.Intergranular pores in these samples are between clay particles,and show features of soft pores in mechanical property.The difference in microtexture of the deep shale samples results in an asymmetrical inverted V-type change in velocity with quartz content,and the organic-rich shale samples have a smaller variation rate in velocity-porosity and velocity-organic matter content.Also due to the difference in microtexture,the organic-rich shale and organic-lean shale can be clearly discriminated in the cross plots of P-wave impedance versus Poisson’s ratio as well as elasticity modulus versus Poisson’s ratio.The shale samples with quartz mainly coming from biogenic silica show higher hardness and brittleness,while the shale samples with quartz from terrigenous debris have hardness and brittleness less affected by quartz content.The study results can provide a basis for well-logging interpretation and"sweet spot"prediction of Longmaxi Formation shale gas reservoirs.

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.

The application of Gaussian distribution deconvolution method to separate the overlapping signals in the 2D NMR map

The 2D NMR(T_(1)-T_(2))mapping technique,which can be used to separate different proton populations from various sources(hydroxyls,solid organic matter,free water,and free HC)has gained attention in petroleum industry.To separate proton contributions,a fixed straight line is commonly employed to separate different regions representing proton sources on the map.However,some of these regions(Region 1 and 2)might overlap which makes extracting the NMR signal amplitude from these regions inaccurate.In order to solve this issue,in this study,we applied the Gaussian distribution deconvolution method to separate the T_(1)and T_(2)relaxation distributions and then derived the signal amplitude of each region instead of following the common fixed line approach.Next,we employed this method to analyze several shale samples from the literature and compared the results following both methods to verify our methodology.Finally,samples from the Bakken Shale were studied to separate signals from Region 1 and Region 2 and corelated the results with geochemical properties that were obtained from programmed(Rock Eval)pyrolysis.Results demonstrated an improvement in their relation when our approach is employed compared to the fixed line technique to differentiate signal from overlapping regions.This means the Gaussian distribution deconvolution method can be used with confidence to provide us with more accurate petrophysical and geochemical understanding of complex formations.

Formation mechanism of a rainfall triggered complex landslide in southwest China

This case study is about a landslide that occurred after 4 days of heavy rainfall,in the morning of June 29,2012,in Cengong County,Guizhou Province of China,geographical coordinated 108°20′-109°03′E,27°09′-27°32′N,with an estimated volume of 3.3×106 m3.To fully investigate the landslide process and formation mechanism,detailed geotechnical and geophysical investigations were performed including borehole drilling,sampling,and laboratory tests coupled with monitoring of displacement.Also,a combined seepage-slope stability modeling was performed to study the behavior of the landslide.After the heavy rainfall event,the sliding process started in this area.The landslide development can be divided into different parts.The man-made fill area,spatially distributed in the south side of the landslide area with low elevations,slid first along the interface between the slope debris and the strongly weathered bedrock roughly in the EW direction.Consequently,due to severe lateral shear disturbance,the slope in the main sliding zone slid next towards the SW direction,along the sliding surface developed within the strongly weathered calcareous shale formation located at a depth of 25-35 m.This means it was a rainfall triggered deep-seated landslide.Finally,retrogressive failure of a number of upstream blocks occurred,which moved in more than one direction.The initial failure of the man-made fill area was the‘engine’of the whole instability framework.This artificial material with low permeability,piled up in the accumulation area of surface and sub-surface and destroyed the drainage capacity of the groundwater.The numerical modeling results agreed with the analysis results obtained from the laboratory and field investigations.A conceptual model is given to illustrate the formation mechanism and development process of the landslide.

New early Eocene vertebrate assemblage from western India reveals a mixed fauna of European and Gondwana affinities

The Ypresian Cambay Shale Formation at Vastan and Mangrol lignite mines in Gujarat, western India, has yielded a rich vertebrate fauna with numerous taxa of European affinities. Here we report a new, approximately contemporary vertebrate assemblage from two fossiliferous layers in the nearby mine of Tadkeshwar. These layers have yielded a similar mammal fauna with the co-occurrence of the perissodactyl-like cambaytheriid Cambaytherium thewissi, the adapoid primates Marcgodinotius indicus and cf. Asiadapis cambayensis, and the hyaenodontid lndohyaenodon raoi. The presence of these species in both Vastan and Tadkeshwar mines and at different levels suggests that the deposits between the two major lignite seams represent a single land mammal age. Apart from the aforementioned species there is a new, smaller species of Cambaytherium, and a new genus and species of esthonychid tillodont. This fauna also contains the first large early Eocene vertebrates from India, including an unidentified Coryphodon-like pantodont, a dyrosaurid crocodyliform and a new giant madtsoiid snake. Among the Tadkeshwar vertebrates several taxa are of Gondwana affinities, such as Pelomedusoides turtles, dyr- osaurids, and large madtsoiids, attesting that the early Eocene was a crucial period in India during which Laurasian taxa of European affinities co-existed with relict taxa from Gondwana before the India-Asia collision. Our results suggest that terrestrial faunas could have dispersed to or from Europe during episodes of contact between the Indian subcontinent and different island blocks along the northern margin of the Neotethys, such as the Kohistan Ladakh island-arc system. Gondwana taxa might represent remnants of ghost lineages shared with Madagascar, which reached the Indian subcontinent during the late Cretaceous; alternatively they might have come from North Africa and passed along the southern margin of the Neotethys to reach the Indian subcontinent. These dispersals would have been possible as a result of favourable paleogeographic conditions such as the particular Neotethys conformation during the beginning of the early Eocene.

Methane Adsorption Capacity Reduction Process of Water-Bearing Shale Samples and Its Influencing Factors: One Example of Silurian Longmaxi Formation Shale from the Southern Sichuan Basin in China

Due to the existence of water content in shale reservoir,it is quite meaningful to clarify the effect of water content on the methane adsorption capacity(MAC)of shale.However,the role of spatial configuration relationship between organic matter(OM)and clay minerals in the MAC reduction process is still unclear.The Silurian Longmaxi Formation shale samples from the Southern Sichuan Basin in China were prepared at five relative humidity(RH)conditions(0%,16%,41%,76%,99%)and the methane adsorption experiments were conducted on these water-bearing shale samples to clarify the MAC reduction process considering the spatial configuration relationship between clay minerals and OM and establish the empirical model to fit the stages.Total organic carbon(TOC)content and mineral compositions were analyzed and the pore structures of these shale samples were characterized by field-emission scanning electron microscopy(FE-SEM),N2 adsorption and high-pressure mercury intrusion porosimetry(HPMIP).The results showed that the MAC reduction of clay minerals in OM occurred at different RH conditions from that of clay minerals outside OM.Furthermore,the amount of MAC reduction of shale samples prepared at the same RH condition was negatively related with clay content,which indicated the protection role of clay minerals for the MAC of water-bearing shale samples.The MAC reduction process was generally divided into three stages for siliceous and clayey shale samples.And the MAC of OM started to decline during stage(1)for calcareous shale sample mainly because water could enter OM pores more smoothly through hydrophobic pathway provided by carbonate minerals than through hydrophilic clay mineral pores.Overall,this study will contribute to improving the evaluation method of shale gas reserve.

Logging prediction and evaluation of fracture toughness for the shales in the Longmaxi Formation, Southern Sichuan Basin

The mode-I fracture toughness is of great significance for evaluating the fracturing ability of shale reservoirs.In this study,the mode-I fracture toughness of the shale in the Lower Silurian Longmaxi Formation,Southern Sichuan Basin was determined with the Cracked Chevron Notched Brazilian Disc(CCNBD).Based on the experimental data,the relationships among the mode-I fracture toughness,the density,the acoustic time and the clay mineral content were analyzed.The shale samples for fracture toughness test from cores were commonly limited,the investigation of fracture toughness using well logs was necessary.Therefore,a prediction model was proposed by correlating the fracture toughness with well logs responses.The results indicate that the fracture toughness of shale samples is from 0.4744 MPa$m1/2 to 1.0607 MPa$m1/2 with an average of 0.7817 MPa$m1/2,indicating that the anisotropy of fracture toughness of the Longmaxi Formation shale.The clay mineral content and the density have a positive effect on the fracture toughness,whereas the acoustic time plays a negative role on the fracture toughness.The clay mineral content has an important effect on the relationships among fracture toughness,acoustic time and density.The prediction model can provide continuous data of mode-I fracture toughness along the wellbore for field hydraulic fracturing operation,and it has certain guiding significance in the exploration and development of oil and gas reservoirs.

Experimental study on hydration damage mechanism of shale from the Longmaxi Formation in southern Sichuan Basin, China

As a serious problem in drilling operation,wellbore instability restricts efficient development of shale gas.The interaction between the drilling fluid and shale with hydration swelling property would have impact on the generation and propagation mechanism of cracks in shale formation,leading to wellbore instability.In order to investigate the influence of the hydration swelling on the crack propagation,mineral components and physicochemical properties of shale from the Lower Silurian Longmaxi Formation(LF)were investigated by using the XRD analysis,cation exchange capabilities(CEC)analysis,and SEM observation,and we researched the hydration mechanism of LF shale.Results show that quartz and clay mineral are dominated in mineral composition,and illite content averaged 67%in clay mineral.Meanwhile,CEC of the LF shale are 94.4 mmol/kg.The process of water intruding inside shale along microcracks was able to be observed through high power microscope,meanwhile,the hydration swelling stress would concentrate at the crack tip.The microcracks would propagate,bifurcate and connect with each other,with increase of water immersing time,and it would ultimately develop into macro-fracture.Moreover,the macrocracks extend and coalesce along the bedding,resulting in the rock failure into blocks.Hydration swelling is one of the major causes that lead to wellbore instability of the LF shale,and therefore improving sealing capacity and inhibition of drilling fluid system is an effective measure to stabilize a borehole.

Evaluation of hydrocarbon generation of the Xiamaling Formation shale in Zhangjiakou and its significance to the petroleum geology in North China

The semi-closed pyrolysis simulation system under constant pressure was conducted to explore the characteristics and mechanisms of hydrocarbon generation from Xiamaling Formation shale in Xiahuayuan,North China.The experiment results indicate the oil generated by the Xiamaling Formation shale in oil window should be classified as "aromatic-intermediate" type,whereas the decreasing of dry coefficient can be ascribed to the cracking of residual bitumen in source rock in the stage of high to post maturity.The amount of hydrocarbon gas generated from residual bitumen can be up to 1-2 m3 per ton rock in high to post mature stage by calculating hydrogen contents in the kerogen,the expelled hydrocarbon,and the residual hydrocarbon.This reveals the importance of residual bitumen as a gas source during high to post mature stage of the kerogen evolution,and also as the broad exploration prospect of these gases.This research highlights the attention should be paid to oil/gas reservoirs sourced from residual bitumen of organic-rich source rock in high mature stage,even the primary oil/gas reservoirs considered as the main exploration targets in middle-upper Proterozoic sediments of North China.

Integrated feasibility analysis of shale inhibition property by utilization of pore pressure transmission equipment

Regarding the widespread utilization of formate-based drilling fluids in numerous operational circumstances,petroleum industries are in urgent need of decreasing wellbore inefficiencies by improving the procedures quality.Thereby,providing appropriate parameters such as safety,boosting the drilling speed and enrolling the proper procedure of optimizing performances of high activity water-sensitivity formation should be taken into consideration.The objective of this comprehensive study is to apply experimental evaluation tests on the shale inhibitor fluids by using prepared plug samples from the studied field due to the administration of pore pressure transmission(henceforth,PPT)equipment.Even though,PPT equipment is not able to simulate all the mechanisms which are applied to different drilling fluids on the shale layers’stability,it will control such parameters like the penetration of fluid filtration through the formation that subsequently reduce the wellbore problems.Consequently,the ubiquitous use formate fluid in drilling operations of exploratory oil wells in the studied field is successfully implemented to be addressed the current well inefficiencies adequately.

The Marine Redox Change and Nitrogen Cycle in the Early Cryogenian Interglacial Time: Evidence from Nitrogen Isotopes and Mo Contents of the Basal Datangpo Formation, Northeastern Guizhou, South China

Cryogenian Datangpo Formation was deposited during the interglacial time between the Sturtian and Marinoan ice ages. We studied nitrogen isotope compositions and contents of Mo of the black shales from the basal Datangpo Formation in northeastern Guizhou, South China, for an attempt to reconstruct the marine redox change and nitrogen cycle during the interglacial time. Based on lithostratigraphy as well as geochemical profiles, the basal black shales can be divided into four intervals： Interval 1 has the lowest δ^（15）N value（＋5.0‰）; in interval 2, δ^（15）N values vary between ＋6.4‰ and ＋7.4‰（the first peak）; interval 3 records stable values of δ^（15）N around ＋6‰; and interval 4 is characterized by its higher δ^（15）N values, between ＋6.7‰ and ＋7.8‰（the second peak）. The values of enrichment factor of Mo decrease from 56.8 to 2.6 with the ascending stratigraphic trend. It indicated that immediately after the Sturtian glaciations, the marine seawater above the transitional zone between the shelf to slope of the southern margin of the Yangtze Platform was stratified, with shallow seawater being oxic but deep water being sulfidic. Subsequently, high denitrification rates prevailed in expanded suboxic areas in spite of a short emergence of an oxic condition in the surface seawater, and the deep seawaters were still anoxic or even euxinic.