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.

Experimental investigation on pyrolysis products and pore structure characteristics of organic-rich shale heated by supercritical carbon dioxide

The efficient pyrolysis and conversion of organic matter in organic-rich shale,as well as the effective recovery of pyrolysis shale oil and gas,play a vital role in alleviating energy pressure.The state of carbon dioxide(CO_(2))in the pyrolysis environment of shale reservoirs is the supercritical state.Its unique supercritical fluid properties not only effectively heat organic matter,displace pyrolysis products and change shale pore structure,but also achieve carbon storage to a certain extent.Shale samples were made into powder and three sizes of cores,and nitrogen(N_(2))and supercritical carbon dioxide(ScCO_(2))pyrolysis experiments were performed at different final pyrolysis temperatures.The properties and mineral characteristics of the pyrolysis products were studied based on gas chromatography analysis,Xray diffraction tests,and mass spectrometry analysis.Besides,the pore structure characteristics at different regions of cores before and after pyrolysis were analyzed using N_(2) adsorption tests to clarify the impact of fracturing degree on the pyrolysis effect.The results indicate that the optimal pyrolysis temperature of Longkou shale is about 430℃.Compared with N_(2),the oil yield of ScCO_(2) pyrolysis is higher.The pyrolysis oil obtained by ScCO_(2) extraction has more intermediate fractions and higher relative molecular weight.The ScCO_(2) can effectively improve the pore diameter of shale and its effect is better than that of N_(2).The micropores are produced in shale after pyrolysis,and the macropores only are generated in ScCO_(2) pyrolysis environments with temperatures greater than 430℃.The pore structure has different development characteristics at different pyrolysis temperatures,which are mainly affected by the pressure holding of volatile matter and products blocking.Compared to the surface of the core,the pore development effect inside the core is better.With the decrease in core size,the pore diameter,specific surface area,and pore volume of cores all increase after pyrolysis.

Investigation of gas content of organic-rich shale:A case study from Lower Permian shale in southern North China Basin,central China

Measuring gas content is an essential step in estimating the commerciality of gas reserves. In this study,eight shale core samples from the Mouye-1 well were measured using a homemade patented gas desorption apparatus to determine their gas contents. Due to the air contamination that is introduced into the desorption canister, a mathematical method was devised to correct the gas quantity and quality.Compared to the chemical compositions of desorbed gas, the chemical compositions of residual gas are somewhat different. In residual gas, carbon dioxide and nitrogen record a slight increase, and propane is first observed. This phenomenon may be related to the exposure time during the transportation of shale samples from the drilling site to the laboratory, as well as the differences in the mass, size and adsorptivity of different gas molecules. In addition to a series of conventional methods, including the USBM direct method and the Amoco Curve Fit(ACF) method, which were used here for lost gas content estimation, a Modified Curve Fit(MCF) method, based on the 'bidisperse' diffusion model, was established to estimate lost gas content. By fitting the ACF and MCF models to gas desorption data, we determined that the MCF method could reasonably describe the gas desorption data over the entire time period, whereas the ACF method failed. The failure of the ACF method to describe the gas desorption process may be related to its restrictive assumption of a single pore size within shale samples. In comparison to the indirect method, this study demonstrates that none of the three methods studied in this investigation(USBM, ACF and MCF) could individually estimate the lost gas contents of all shale samples and that the proportion of free gas relative to total gas has a significant effect on the estimation accuracy of the selected method. When the ratio of free gas to total gas is lower than 45%, the USBM method is the best for estimating the lost gas content, whereas when the ratio ranges from 45% to 75% or is more than 75%, the ACF and MCF methods, are the best options respectively.

Lower Es3 in Zhanhua Sag, Jiyang Depression： a case study for lithofacies classification in lacustrine mud shale

Oil and gas exploration in lacustrine mud shale has focused on laminated calcareous lithofacies rich in type Ⅰ or type Ⅱ1 organic matter, taking into account the mineralogy and bedding structure, and type and abundance of organic matter. Using the lower third member of the Shahejie Formation, Zhanhua Sag, Jiyang Depression as the target lithology, we applied core description, thin section observations, electron microscopy imaging, nuclear magnetic resonance, and fullbore formation microimager （FMI） to study the mud shale lithofacies and features. First, the lithofacies were classified by considering the bedding structure, lithology, and organic matter and then a lithofacies classification scheme of lacustrine mud shale was proposed. Second, we used optimal filtering of logging data to distinguish the lithologies. Because the fractals of logging data are good indicators of the bedding structure, gamma-ray radiation was used to optimize the structural identification. Total organic carbon content （TOC） and pyrolyzed hydrocarbons （S2） were calculated from the logging data, and the hydrogen index （HI） was obtained to identify the organic matter type of the different strata （HI vs Tmax）. Finally, a method for shale lithofacies identification based on logging data is proposed for exploring mud shale reservoirs and sweet spots from continuous wellbore profiles.

The petrological characteristics and signif icance of organic-rich shale in the Chang 7 member of the Yanchang Formation,south margin of the Ordos basin,central China

The organic-rich shale of the Chang 7 member is the most important source rock in the Ordos basin.The sedimentary environment and the controlling factors of organic matter enrichment,however,are still in contention.In this investigation,the Yishicun outcrop,located on the south margin of the Ordos basin,has been considered for the study.X-ray diffraction,polarizing microscopy,field emission scanning electron microscopy and cathodoluminescence(CL)were used to investigate the petrological features of the organic-rich shale.The content of volcanic ash and the diameter of pyrite framboid pseudocrystals were measured to illustrate the relationship between oxygen level,ash content and the enrichment of organic matter.It has been found that the diameter of pyrite framboid pseudocrystals has a strong correlation with the total organic carbon,demonstrating that the redox status degree of the water column has a positive impact on the enrichment of organic matter.Additionally,with an increase in the ash content,the content of organic matter increased at first and then decreased,and reached a maximum when the ash content was about 6%,illustrating that the ash input has a double effect on the enrichment of organic matter.

A rock physics model for the characterization of organic-rich shale from elastic properties

Kerogen content and kerogen porosity play a significant role in elastic properties of organic-rich shales. We construct a rock physics model for organic-rich shales to quantify the effect of kerogen content and kerogen porosity using the Kuster and Toks6z theory and the selfconsistent approximation method. Rock physics modeling results show that with the increase of kerogen content and kerogen-related porosity, the velocity and density of shales decrease, and the effect of kerogen porosity becomes more obvious only for higher kerogen content. We also find that the Poisson＇s ratio of the shale is not sensitive to kerogen porosity for the case of gas saturation. Finally, for the seismic reflection responses of an organic-rich shale layer, forward modeling results indicate the fifth type AVO re- sponses which correspond to a negative intercept and a positive gradient. The absolute values of intercept and gradient increase with kerogen content and kerogen porosity, and present predictable variations associated with velocities and density.

The application of machine learning under supervision in identification of shale lamina combination types——A case study of Chang 7_(3)sub-member organic-rich shales in the Triassic Yanchang Formation,Ordos Basin,NW China

Organic rich laminated shale is one type of favorable reservoirs for exploration and development of continental shale oil in China.However,with limited geological data,it is difficult to predict the spatial distribution of laminated shale with great vertical heterogeneity.To solve this problem,taking Chang 73 sub-member in Yanchang Formation of Ordos Basin as an example,an idea of predicting lamina combinations by combining'conventional log data-mineral composition prediction-lamina combination type identification'has been worked out based on machine learning under supervision on the premise of adequate knowledge of characteristics of lamina mineral components.First,the main mineral components of the work area were figured out by analyzing core data,and the log data sensitive to changes of the mineral components was extracted;then machine learning was used to construct the mapping relationship between the two;based on the variations in mineral composition,the lamina combination types in typical wells of the research area were identified to verify the method.The results show the approach of'conventional log data-mineral composition prediction-lamina combination type identification'works well in identifying the types of shale lamina combinations.The approach was applied to Chang 73 sub-member in Yanchang Formation of Ordos Basin to find out planar distribution characteristics of the laminae.

Factors controlling organic-rich shale development in the Liushagang Formation,Weixinan Sag,Beibu Gulf Basin:Implications of structural activity and the depositional environment

The mechanisms of lacustrine organic-rich shale formation have attracted attention due to its association with global shale oil and shale gas exploration.Samples of general-quality and excellent-quality source rocks,and oil shale from the Beibu Gulf Basin were analyzed to investigate their organic geochemistry,palynofacies,and trace elements.Hydrocarbon potential was higher in the oil shale(29.79 mg/g)than in the general-quality source rock(3.82 mg/g),and its kerogen type wasⅠ-Ⅱ2.Hydrogen-rich liptinite(cutinite and sporinite)components derived from terrigenous higher plants provided most of the hydrocarbon potential of excellent-quality source rock and oil shale.Under the influence of depressioncontrolling fault activity,a deeper subsidence center promotes the deposition of excellent-quality source rock and oil shale in brackish-hypoxic bottom water.A shallower subsidence center,due to subsag-controlling fault activity,promotes the formation of excellent-quality source rock under freshbrackish and weak oxidation-weak reducing conditions.The local uplift and shallow-slope led to the formation of general-quality source rock,under freshwater weak-oxidation conditions.A model was established for organic matter(OM)accumulation in organic-rich shales,accounting for fault activity,terrigenous hydrogen-rich OM,and the preservation conditions,to predict the development of excellent-quality source rock from areas with low levels of exploration.

Laminae combination and shale oil enrichment patterns of Chang 73 sub-member organic-rich shales in the Triassic Yanchang Formation, Ordos Basin, NW China

The Chang 73 sub-member of Triassic Yanchang Formation in the Ordos Basin was taken as an example and the lamina types and combinations,reservoir space features and shale oil enrichment patterns in organic-rich shale strata were investigated using core observation,thin section analysis,XRF element measurement,XRD analysis,SEM,high solution laser Raman spectroscopy analysis,and micro-FTIR spectroscopy analysis,etc.According to the mineral composition and thickness of the laminae,the Chang 73 organic-rich shales have four major types of laminae,tuff-rich lamina,organic-rich lamina,silt-grade feldspar-quartz lamina and clay lamina.They have two kinds of shale oil-bearing layers,"organic-rich lamina+silt-grade feldspar-quartz lamina"and"organic-rich lamina+tuff-rich lamina"layers.In the"organic-rich+silt-grade feldspar-quartz"laminae combination shale strata,oil was characterized by relative high maturation,and always filled in K-feldspar dissolution pores in the silt-grade feldspar-quartz laminae,forming oil generation,migration and accumulation process between laminae inside the organic shales.In the"organic-rich+tuff-rich lamina"binary laminae combination shale strata,however,the reservoir properties were poor in organic-rich shales,the oil maturation was relatively lower,and mainly accumulated in the intergranular pores of interbedded thin-layered sandstones.The oil generation,migration and accumulation mainly occurred between organic-rich shales and interbedded thin-layered sandstones.

Geochemical characteristics and sedimentary environment of the Middle Devonian organic-rich shales in the Northwest of Guizhong Depression, Southwest China

In order to figure out the redox conditions and paleo-sedimentary environment of the Middle Devonian shales in the northwest of Guizhong Depression,the trace element analysis was conducted on the Middle Devonian cores(320.35–938.50 m)of the typical shale gas investigation well(GY-1)at a 1.50 m sampling interval through X-ray fluorescence spectroscopy(XRF)and inductively coupled plasma mass spectrometry(ICP-MS).According to the test result,the average values of V/(V+Ni),V/Cr and Ni/Co in Nabiao formation(Fm.)are larger than 0.67,4.65 and 7.71 respectively,and Nabiao Fm.is rich in biological assemblages such as tabasheer,ammonite,etc.These evidences indicate the rising sea level rose relatively in the sedimentation period of Nabiao Fm.and a deepwater shelf environment,which was favorable for the preservation of organic matters.The V/(V+Ni),V/Cr and Ni/Co in Luofu Fm.and Tangting Fm.are 0.38–0.65,0.73–4.10 and 3.70–6.72 respectively,indicating that the sea level dropped relatively in their sedimentation period,during which the water bodies became shallow,and the sedimentary environment was a weak oxidizing shallow water shelf environment.In addition,the variation of TOC has a high correlation with the enrichment degree of Ba element,indicating the favorable conditions for the enrichment and preservation of organic matters under an oxygen-deficient environment.Moreover,according to the identification of trace element indexes,the northwest of Guizhong Depression experienced the sedimentary cycle of relative rise to relative fall of sea level from bottom to top in the Middle Devonian sedimentation period.The relative sea level rose to the highest in the sedimentation period of Nabiao Fm.,in which the organic-rich shales with stable thickness and high organic content were deposited.Hence,the Nabiao Fm.could be regarded as the favorable exploration target interval in this area.

Calculation of safe drilling mud density window for shale formation by considering chemo-poro-mechanical coupling effect

It is difficult to define safe drilling mud density window for shale sections.To solve this problem,the general Biot effective stress principle developed by Heidug and Wong was modified.The Weibull statistical model was used to characterize the hydration strainrelated strength damage.Considering drilling fluid sealing barrier on shale,a calculation method of safe drilling mud density has been established for shale formation under drilling fluid sealing-inhibition-reverse osmosis effect,combined with a flow-diffusion coupling model.The influence of drilling fluid sealing and inhibiting parameters on safe drilling mud density window was analyzed.The study shows that enhancing drilling fluid sealing performance can reduce the pore pressure transmission and solute diffusion;the inhibiting performance of drilling fluid,especially inhibition to strength damage,is crucial for the wellbore collapse pressure of shale section with significant hydration property.The improvement of drilling fluid sealing and inhibition performance can lower collapse pressure and enhance fracturing pressure,and thus making the safe drilling fluid density window wider and the collapse period of wellbore longer.If there is osmosis flow in shale,induced osmosis flow can make the gap between collapse pressure and fracturing pressure wider,and the stronger the sealing ability of drilling fluid,the wider the gap will be.The safe drilling mud density window calculation method can analyze the relationships between collapse pressure,fracturing pressure and drilling fluid anti collapse performance,and can be used to optimize drilling fluid performance.

Types of biogenic quartz and its coupling storage mechanism in organic-rich shales: A case study of the Upper Ordovician Wufeng Formation to Lower Silurian Longmaxi Formation in the Sichuan Basin, SW China

Biogenic quartz in the Upper Ordovician Wufeng Formation to Lower Silurian Longmaxi Formation(WufengLongmaxi)shale layers in the Sichuan Basin and its periphery is qualitatively analyzed and quantitatively characterized by organic petrologic,mineralogic,and geochemical methods to find out the coupling effect between organic matter and quartz.(1)There are two types of biogenic quartz in the shale layers:Type I quartz is submicron quartz appearing in clusters around the organic matter.Type II quartz is in nano-scale grain size and floats in spherical shape on organic matter,with grains in point-to-point or surface-to-surface contact;this type of quartz is mainly biologic origin and slightly affected by hydrothermal activity in local parts.(2)The reservoirs in the Wufeng-Longmaxi formations is consistent in distribution with biogenic silica content in them,and mainly concentrated at the bottom of the Wufeng-Longmaxi formations,and is thinner in the Changning and Weiyuan regions,while thicker in the Fuling region.(3)The biogenic quartz in the Wufeng-Longmaxi shale worked through the entire evolution process of hydrocarbon generation.The presence of biogenic quartz can enhance the development of organic matter pores and microcracks,and can effectively preserve the organic matter pores and residual intergranular pores,forming"biological silicon intergranular pores,organic pores and micro-fractures".This would benefit later hydraulic fracturing and result in high production/stable production of well.The coupling effect between biogenic quartz development and organic matter evolution and hydrocarbon generation is a critical factor for high-quality shale reservoir development.

Shale Gas Formation and Occurrence in China： An Overview of the Current Status and Future Potential

Shale gas is one of the most promising unconventional resources both in China and abroad. It is known as a form of self-contained source-reservoir system with large and continuous dimensions. Through years of considerable exploration efforts, China has identified three large shale gas fields in the ruling, Changning and Weiyuan areas of the Sichuan Basin, and has announced more than 540 billion m3 of proven shale gas reserves in marine shale systems. The geological theories for shale gas development have progressed rapidly in China as well. For example, the new depositional patterns have been introduced for deciphering the paleogeography and sedimentary systems of the Wufeng shale and Longmaxi shale in the Sichuan Basin. The shale gas storage mechanism has been widely accepted as differing from conventional natural gas in that it is adsorbed on organic matter or a mineral surface or occurs as free gas trapped in pores and fractures of the shale. Significant advances in the techniques of microstructural characterization have provided new insights on how gas molecules are stored in micro- and nano-scale porous shales. Furthermore, newly-developed concepts and practices in the petroleum industry, such as hydraulic fracturing, microseismic monitoring and multiwell horizontal drilling, have made the production of this unevenly distributed but promising unconventional natural gas a reality. China has 10-36 trillion m3 of promising shale gas among the world＇s whole predicted technically recoverable reserves of 206.6 trillion m3. China is on the way to achieving its goal of an annual yield of 30-50 billion m3 by launching more trials within shale gas projects.

Sedimentary and geochemical characteristics of the Triassic Chang 7 Member shale in the Southeastern Ordos Basin,Central China

The Ordos Basin is the largest petroliferous basin in China, where the Chang 7 Member shale serves as the major source rock in the basin, with an area of more than 100,000 km^2 So far, sedimentary and geochemical characterizations have rarely been conducted on the shale in shallow(< 1000 m) areas in the southeastern part of the basin, but such characterizations can help identify the genesis of organic-rich shale and promote the prediction and recovery of shale oil. In this paper,several outcrop sections of the Chang 7 Member in the Tongchuan area were observed and sampled, and sedimentary and geochemical characterizations were conducted for the well-outcropped YSC section. The study results show that the Chang7 Member shale is widely distributed laterally with variable thickness. The organic-rich shale is 7-25 m thick in total and exhibits obvious horizontal variation in mineral composition. In the eastern sections, the shale contains organic matter of TypeⅡ_2-Ⅲ and is low in thermal maturity, with high clay mineral content, low K-feldspar content, and no pyrite. In the western sections, the shale contains Type Ⅱ_1 organic matter and is low in thermal maturity, with high clay mineral, K-feldspar, and pyrite contents. The YSC section reveals three obvious intervals in vertical mineral composition and organic abundance.The Chang 7 Member organic-rich shale(TOC > 10%) contains mainly sapropelite and liptinite, with Type Ⅱ kerogen. It is generally characterized by a hydrocarbon potential of more than 70 mg/g, low maturity, and shallow-semideep lacustrine facies. In the western sections, the shale, still in a low maturity stage, has a higher hydrocarbon potential and is optional for shale oil recovery. However, the Chang 7 Member shale in the study area is highly heterogeneous and its shale oil recovery is practical only in the organic-rich intervals.

Molecular Structure of Kerogen in the Longmaxi Shale: Insights from Solid State NMR, FT-IR, XRD and HRTEM

Kerogen plays an important role in shale gas adsorption,desorption and diffusion.Therefore,it is necessary to characterize the molecular structure of kerogen.In this study,four kerogen samples were isolated from the organic-rich shale of the Longmaxi Formation.Raman spectroscopy was used to determine the maturity of these kerogen samples.Highresolution transmission electron microscopy(HRTEM),13 C nuclear magnetic resonance(13 C NMR),X-ray diffraction(XRD)and Fourier transform infrared(FT-IR)spectroscopy were conducted to characterize the molecular structure of the shale samples.The results demonstrate that VReqv of these kerogen samples vary from 2.3%to 2.8%,suggesting that all the kerogen samples are in the dry gas window.The macromolecular carbon skeleton of the Longmaxi Formation kerogen is mainly aromatic(fa’=0.56).In addition,the aromatic structural units are mainly composed of naphthalene(23%),anthracene(23%)and phenanthrene(29%).However,the aliphatic structure of the kerogen macromolecules is relatively low(fal*+falH=0.08),which is presumed to be distributed in the form of methyl and short aliphatic chains at the edge of the aromatic units.The oxygen-containing functional groups in the macromolecules are mainly present in the form of carbonyl groups(fac=0.23)and hydroxyl groups or ether groups(falO=0.13).The crystallite structural parameters of kerogen,including the stacking height(Lc=22.84?),average lateral size(La=29.29?)and interlayer spacing(d002=3.43?),are close to the aromatic structural parameters of anthracite or overmature kerogen.High-resolution transmission electron microscopy reveals that the aromatic structure is well oriented,and more than 65%of the diffractive aromatic layers are concentrated in the main direction.Due to the continuous deep burial,the longer aliphatic chains and oxygen-containing functional groups in the kerogen are substantially depleted.However,the ductility and stacking degree of the aromatic structure increases during thermal evolution.This study provides quantitative information on the molecular structure of kerogen samples based on multiple research methods,which may contribute to an improved understanding of the organic pores in black shale.

Connotation and strategic role of in-situ conversion processing of shale oil underground in the onshore China

In-situ conversion processing (ICP) of shale oil underground at the depth ranging from 300 m to 3 000 m is a physical and chemical process caused by using horizontal drilling and electric heating technology, which converts heavy oil, bitumen and various organic matter into light oil and gas in a large scale, which can be called"underground refinery". ICP has several advantages as in CO2capture, recoverable resource potential and the quality of hydrocarbon output. Based on the geothermal evolution mechanism of organic materials established by Tissot et al., this study reveals that in the nonmarine organic-rich shale sequence, the amount of liquid hydrocarbon maintaining in the shale is as high as 25%in the liquid hydrocarbon window stage (R o less than 1.0%), and the unconverted organic materials (low mature-immature organic materials) in the shale interval can reach 40%to 100%. The conditions of organic-rich shale suitable for underground in-situ conversion of shale oil should be satisfied in the following aspects, TOC higher than 6%, R o ranging between 0.5%and 1%, concentrated thickness of organic-rich shale greater than 15 meters, burial depth less than 3 000 m, covering area bigger than 50 km2, good sealing condition in both up-and down-contacting sequences and water content smaller than 5%, etc. The shale oil resource in China’s onshore region is huge. It is estimated with this paper that the technical recoverable resource reaches 70-90 billion tons of oil and 60-65 trillion cubic meters of gas. The ICP of shale oil underground is believed to be a fairway to find big oil in the source kitchen in the near future. And it is also believed to be a milestone to keep China long-term stability of oil and gas sufficient supply by putting ICP of shale oil underground into real practice in the future.

Fine-grained gravity flow sedimentation and its influence on development of shale oil sweet sections in lacustrine basins in China

The geological conditions and processes of fine-grained gravity flow sedimentation in continental lacustrine basins in China are analyzed to construct the model of fine-grained gravity flow sedimentation in lacustrine basin,reveal the development laws of fine-grained deposits and source-reservoir,and identify the sweet sections of shale oil.The results show that fine-grained gravity flow is one of the important sedimentary processes in deep lake environment,and it can transport fine-grained clasts and organic matter in shallow water to deep lake,forming sweet sections and high-quality source rocks of shale oil.Fine-grained gravity flow deposits in deep waters of lacustrine basins in China are mainly fine-grained high-density flow,fine-grained turbidity flow(including surge-like turbidity flow and fine-grained hyperpycnal flow),fine-grained viscous flow(including fine-grained debris flow and mud flow),and fine-grained transitional flow deposits.The distribution of fine-grained gravity flow deposits in the warm and humid unbalanced lacustrine basins are controlled by lake-level fluctuation,flooding events,and lakebed paleogeomorphology.During the lake-level rise,fine-grained hyperpycnal flow caused by flooding formed fine-grained channel–levee–lobe system in the flat area of the deep lake.During the lake-level fall,the sublacustrine fan system represented by unconfined channel was developed in the flexural slope breaks and sedimentary slopes of depressed lacustrine basins,and in the steep slopes of faulted lacustrine basins;the sublacustrine fan system with confined or unconfined channel was developed on the gentle slopes and in axial direction of faulted lacustrine basins,with fine-grained gravity flow deposits possibly existing in the lower fan.Within the fourth-order sequences,transgression might lead to organic-rich shale and fine-grained hyperpycnal flow deposits,while regression might cause fine-grained high-density flow,surge-like turbidity flow,fine-grained debris flow,mud flow,and fine-grained transitional flow deposits.Since the Permian,in the shale strata of lacustrine basins in China,multiple transgression-regression cycles of fourth-order sequences have formed multiple source-reservoir assemblages.Diverse fine-grained gravity flow sedimentation processes have created sweet sections of thin siltstone consisting of fine-grained high-density flow,fine-grained hyperpycnal flow and surge-like turbidity flow deposits,sweet sections with interbeds of mudstone and siltstone formed by fine-grained transitional flows,and sweet sections of shale containing silty and muddy clasts and with horizontal bedding formed by fine-grained debris flow and mud flow.The model of fine-grained gravity flow sedimentation in lacustrine basin is significant for the scientific evaluation of sweet shale oil reservoir and organic-rich source rock.

Authigenic silica in continental lacustrine shale and its hydrocarbon significance

Taking lake basin shales of the Triassic Yanchang Formation in the Ordos Basin,NW China and the Cretaceous Qingshankou Formation in the Songliao Basin,NE China as research objects,the characteristics and origins of different types of silica in the shales have been studied by means of core observation,thin section identification,cathodoluminescence,X-ray diffraction analysis,scanning electron microscope(SEM),electron probe and rock pyrolysis.The results shows that the origins of silica include felsic mineral dissolution,tuffite devitrification,clay mineral transformation and siliceous mineral metasomatism.The silica formed by feldspar dissolution commonly appears as spots and veins,with low degree of crystallization,and is largely aqueous opal mineral,with an average Si O2 content of 67.2%.Silica formed by devitrification of tuffite mainly occurs in two forms,amorphous silica and authigenic quartz with better crystal shape.The authigenic silica formed during the transformation of clay minerals is embedded in the clay minerals in the form of micron-scale plates and small flakes,or mixed with clay minerals in a dispersed state.The authigenic quartz formed by siliceous mineral metasomatism is in better angular crystal shape,and has an average Si O2 content of 87%.The authigenic siliceous mineral content is positively correlated with the content of terrigenous felsic minerals.The pressure solution of felsic minerals is the main source of authigenic siliceous minerals,followed by the transformation of clay minerals,and the organic matter has some boost on the formation of authigenic silica.The authigenic siliceous materials of different origins have different geological characteristics and occurrence states from terrigenous quartz,which would affect the storage performance,seepage capacity and fracturing effect of continental shale.Although the organic-rich shale has high silica content,different from terrigenous quartz,authigenic silica in this kind of shale mostly floats and disperse in clay minerals,which would have negative effect on the formation of complex fractures in fracturing,fracture support ability after fracturing,and formation of effective seepage channels.Calculating the brittleness index of shale intervals only based on the composition of brittle minerals cannot accurately characterize mechanical characteristics of continental shale oil reservoirs,and would affect comprehensive evaluation and selection of continental shale oil“sweet spots”.

负压振动筛原理及在油基钻井液钻屑减量中的应用

经过几十年的发展,振动筛性能有很大提高,但其对钻屑的脱液能力已达极限。40年来,应用于钻井固控,效果不明显。十多年前,经过尝试与高频低幅振动相结合的真空过滤技术应用于钻井固控时,钻屑的脱液能力显著提高,排放钻屑体积大幅减小。近年来,多种负压振动筛在国内钻井中开展了应用试验。经过对常规振动筛复合负压过滤的负压振动筛的原理、主要技术参数、现场应用与国内外类似技术的对比分析及现场应用表明,所使用的负压振动筛的处理量、钻屑含液率、钻屑排放减少量、筛网寿命、能效、设备成本、使用经济效益等都均有明显优势。负压振动筛的应用对节能减排和实现“碳中和”具有重要作用,其逐渐替代常规振动筛,将是钻井固控技术发展的必然趋势。

织纳地区龙潭组煤系页岩气储层特征与勘探潜力评价

为查明织纳地区龙潭组页岩气勘探前景,通过分析Z2井龙潭组泥页岩样品的矿物组成、有机地化特征、微观孔隙特征、钻孔泥页岩厚度、分布情况及其含气性特征,研究龙潭组页岩气储层特征并对页岩气勘探潜力进行评价。结果表明:泥页岩有机质类型为Ⅱ_(2)-Ⅲ型,且以Ⅱ_(2)为主,总有机碳含量为1.34%~8.25%,平均值为3.68%,有机质成熟度为0.92%~3.24%,平均值为2.45%,属于高-过高成熟演化阶段,具有较好的生气潜力。扫描电镜结果显示泥页岩样品主要发育四类微观孔隙,包括粒内孔、粒间孔、有机质孔和微裂隙,内部孔隙发育,为页岩气提供良好的储集空间。Z2井泥页岩脆性矿物含量为24.40%~63.08%,平均值为41.22%,泥页岩总厚度为170.74 m,占比42%,发育5个有效连续厚度大于15 m的泥页岩层,与邻井可形成2个泥页岩层段,适宜进行压裂改造。织纳地区龙潭组泥页岩具备良好的生气条件、储集条件和可压裂性,具有较大勘探潜力。