Triassic Yanchang Formation Research Articles

Multi-scale pore structure, pore network and pore connectivity of tight shale oil reservoir from Triassic Yanchang Formation, Ordos Basin

The accurate evaluation of microscopic pore structure characteristics of shales is of great significance for shale oil development. In order to gain a better understanding of complicated pore structure of lacustrine shales, silty shale samples from the Upper Triassic Yanchang formation in Ordos Basin were investigated using FE-SEM, N2 adsorption, MIP, helium pycnometry and Nano-CT. FE-SEM observations show that five typical pores (interparticle pores, intraparticle pores, dissolution-related pores, organic matter pores and microfractures) are developed in shale samples. The full-scale pore size distributions, determined by combining N2 adsorption, MIP and Nano-CT, show that the pore sizes of the samples range from 2 nm to more than 20 μm, while the dominant pores are in the range of 20–100 nm. A certain difference exits in porosity obtained by Nano-CT, N2 adsorption, MIP, helium pycnometry and the combination methods. The N2 adsorption tends to underestimate the total pore volume and porosity. The MIP may overestimate or underestimate the porosity, due to the matrix compression or limited penetration of mercury, respectively. The combined porosity using multiple methods is reasonably consistent with helium pycnometry. Pore network models of these samples are extracted using the centerline algorithm. From the pore network model, the proportion of connected pores of the studied samples ranges from 15.51% to 37.45%. The coordination numbers obtained from pore network range from 1.34 to 1.84, indicating the relatively poor pore connectivity of the studied samples. Samples rich in organic matter have high proportion of connected pores in the region of interest (ROI) of 3D network. Pore throats show preferential orientation in XY plane (parallel to bedding) and then their two diagonal directions, while the Z axis of the pore structure (perpendicular to bedding) is less connected. These parameters improve the understanding on storage space and transport property of lacustrine shale oil.

Fine Logging Evaluation of Fractures in Continental Tight Oil Sandstone Reservoirs of Yanchang Formation in the Ordos Basin

Fractures are the main seepage channels in tight reservoirs, and they affect the distribution of high-quality reservoirs and the enrichment of hydrocarbons. Fine logging identification of fractures in the strongly heterogeneous continental tight oil reservoirs of the Upper Triassic Yanchang Formation in the Ordos Basin is a hot and difficult point in the field of petroleum geology. In this paper, taking the Yanchang Formation as an example, a fractal model was constructed to identify fractures in tight oil reservoirs using a large number of cores, conventional and micro-resistivity imaging logging data. The results show that high-angle, vertical and bedding fractures are mainly developed in the Yanchang Formation tight sandstones. There is a negative correlation between sand body thickness and fracture development degree. The fracture sensitivity parameters were used to construct a coupled fractal fracture index. The fractal model incorporates logging information from natural gamma, acoustic wave time difference, rock density, and shallow lateral resistivity. In addition, the constructed fracture fractal index realizes the functions of multi-conventional logging information fusion, which can effectively identify fracture development segments in sandstone. According to the statistics, the fracture identification rate is 83.3%. The study also found that with the increase of sandstone brittleness index, the fracture index has a “S” shape increasing trend. Therefore, the content of brittle mineral components in sandstone is an important factor affecting the development degree of natural fractures, and fractures are more likely to occur in high brittle, thin sand bodies. The highly brittle framework minerals have strong stress-supporting capacity, which can keep fractures open by resisting high overlying loads.

Linking Hydrothermal Activity With Organic Matter Accumulation in the Chang 7 Black Shale of Yanchang Formation, Ordos Basin, China

The Chang 7 black shale in the Upper Triassic Yanchang Formation is the principal source rock of Mesozoic oil-bearing system in the southwest Ordos Basin, containing high abundances of organic matter and hydrocarbon potential. Our study discusses the role of lake-bottom hydrothermal activities in the enrichment of organic matter during the deposition of the Chang 7 black shale. A large number of basement faults developed in the interior and margin of the Ordos Basin, which provided channels for the upwelling of deep hydrothermal fluids. Moreover, the strong tectonic activities during the Chang 7 sedimentary period provided dynamic conditions for the activation of the faults and the upwelling of hydrothermal fluids. The occurrence of hydrothermal activities in the Chang 7 sedimentary period is proved by the evidences of mineralogy petrology, stable isotopes, major, and trace elements in the black shale. Abundant nutrients that were transported from the lake-bottom hydrothermal fluids into lake water promoted the lacustrine surface primary productivity, and then increased the supply of sedimentary organic matter. At the same time, the degradation of a large number of organic matters increased consumption of oxygen in the water column, resulting in the formation of bottom-water anoxic environments. The accumulation of organic matter in sediments was controlled by the lake-bottom hydrothermal activities by the means of increasing the lacustrine surface paleoproductivity and promoting the formation of anoxic environments.

Insights into the mechanical stratigraphy and vertical fracture patterns in tight oil sandstones: The Upper Triassic Yanchang Formation in the eastern Ordos Basin, China

Vertical fracture patterns are mapped in the outcrops to analyze the impact of mechanical stratigraphy on vertical fractures in the relatively undeformed Upper Triassic tight oil sandstones of the Yanchang Formation along Yanhe River in the east Ordos Basin, China. The relationship between natural fractures and different bounding interfaces are analyzed based on rock relative strength measurement by N-type Schmidt Hammer, fracture and facies description in the outcrops. According to the probability of the bounding interface terminating natural fractures being more than 20%, seven types of mechanical interfaces terminating fracture longitudinal propagation are identified, namely 1) the interface of laminae-set or laminae co-sets, 2) bedding surface interface, 3) accretionary interface, 4) mudstone interbed, 5) mudstone barrier, 6) calcareous barrier and 7) depositional scour interface. According to fracture sizes and the mechanical interfaces which constrain fracture propagation, the vertical fractures are divided into micro-scale, small-scale, meso-scale and macro-scale ones. Three possible vertical fracture patterns in various sedimentary microfacies of the shallow water delta deposits are discussed based on field outcrop observation, the restriction capacities of different mechanical interfaces and the temporally changing tectonic stress. Finally, the application of vertical fracture patterns for subsurface fracture prediction is carried out in the Ansai Oilfield in the eastern Ordos Basin, China. The predicted fractures in wells are consistent with the fractures obtained from cores and conventional logs. This study links sedimentary stratigraphy, mechanical stratigraphy and natural fractures in the tight oil sandstones within a relatively undeformed setting, meanwhile unravels the vertical fracture patterns in different sedimentary microfacies of the shallow water delta deposits in the study area. The results will thus guide subsurface fracture prediction between wells in the tight oil sandstones of the eastern Ordos Basin.

Late Triassic–Early Jurassic paleogeomorphic characteristics and hydrocarbon potential of the Ordos Basin, China, a case of study of the Jiyuan area

Abstract Paleogeomorphology is the surface morphology formed by the deposition of strata in the basin in historical period, which is greatly affected by the regional tectonic movement; however, the effect of basin paleogeomorphology on reservoir has been underexplored. This study documents the basin paleogeomorphic terrain features and their implications for the depositional processes and oil migration and accumulation in the Jiyuan area of the Ordos Basin. Stratigraphic classification is achieved through well logging data analysis. A seismic fine interpretation of the Triassic–Jurassic paleogeomorphology is conducted. Typical drilling core observations were utilized in a well section analysis of the strata at the top of the Yanchang formation sedimentary facies characteristics. Studies have shown that the Chang 1 subsection on top of the Triassic Yanchang formation in the Jiyuan area was eroded by an ancient Jurassic river, and the residual strata are primarily distributed in the Dingbian and Jiyuan plateaus. The Chang 1 reservoir in the Jiyuan area is dominated by a delta distributary channel sand body. The reservoir is concentrated in a high reservoir sand body structure position, and is controlled by a combination of structural and rock characteristics. This study suggests that the Indosinian movement in the late Triassic caused the entire basin to uplift and denudate, and the strata at the top of the Yanchang formation suffered from weathering and river erosion. The residual strata in the upper part of the Yanchang formation are controlled by the late Triassic–early Jurassic paleogeomorphology.

Characteristics of Oil and Gas Produced by In Situ Heating of Shale: A Case Study of the Chang 7 Member, Ordos Basin, China

Shale in the Chang 7 member of the Triassic Yanchang Formation of Ordos Basin is just suitable to shale oil development by in situ heating for its low maturity. In this study, continuous in situ heating simulations at three different heating rates were undertaken to investigate oil and gas characteristics of the Chang 7 shale. Results indicated that the shale has an oil production potential of up to 43–44 kg/t shale when heated to 450 °C. The heating rate had a minor effect on oil yield. The produced oil was light oil with a C8–C10 main peak. Saturated and aromatic hydrocarbons took up a total proportion of 87–90%. The saturated hydrocarbon contents in produced oil decreased with the increasing temperature accompanying aromatic increasing. Total gas yields for the three heating rates after heating to 450 °C were not significantly different, with total yields of 15.68–16.68 m3/t shale. However, yields of different gas components varied with the heating rate. The yield of hydrocarbon gas at a fixed temperature increased with the decreasing heating rate. Our experimental results also indicated that slow heating was favorable to reduce harmful gas formation by an in situ heating method during Chang 7 shale oil development.

Characteristics and Genetic Mechanism of Chang Eight Low Permeability and Tight Reservoir of Triassic Yanchang Formation in Central-East Ordos Basin

In this article, the characteristics of Chang 8 reservoir of Triassic Yanchang Formation in northern Shaanxi are studied by using polarizing microscope, field emission scanning electron microscope, image particle size, X-ray diffraction analysis of clay, and constant pressure mercury intrusion. The study shows that the target layer is in a relatively stable and uniform sinking burial period after deposition, and the lithology composition in the area is relatively complex, mainly composed of debris–feldspar sandstone and feldspar sandstone, with the characteristics of fine grain and high content of interstitial material. The porosity of the reservoir is generally between 4% and 12%, with an average of 8.05%. The permeability is generally between 0.03 × 10−3 and 0.5 × 10−3 μm2, with an average of 0.16 × 10−3 μm2. Strong compaction and well-developed cementation of calcareous, siliceous, and authigenic illite are important reasons for the formation of extra-low porosity and extra-low permeability reservoirs. But at the same time, because of the protective effect of chlorite film, some residual intergranular pores are preserved, which makes the some reservoirs with relatively good physical property, forming a local relatively high-porosity and high-permeability section of the “highway.”

Development Characteristics and Finite Element Simulation of Fractures in Tight Oil Sandstone Reservoirs of Yanchang Formation in Western Ordos Basin

Structural fractures have a significant control effect on the large-scale accumulation of hydrocarbons in the Yanchang Formation. Previous studies have affirmed the important role of fractures in hydrocarbon accumulations in strongly deformed zones. However, for low-amplitude structural areas, the degree of fracture development is relatively low, and their control on sweet spots of hydrocarbons has not yet formed a unified understanding. In this paper, taking the Upper Triassic Yanchang Formation in the western Ordos Basin as an example, the development characteristics, prediction method, and the distribution of fractures in tight sandstone reservoirs in low-amplitude structural areas have been systematically studied using a large number of cores, thin sections, paleomagnetism, FMI logging, acoustic emission, productivity data, and finite element method. The research results showed that the Yanchang Formation in the study area mainly develop high-angle and vertical fractures, which were formed by regional tectonic shearing. Fractures are mainly developed in the fine-grained and ultra-fine-grained sandstones of the distributary channel and estuary bar microfacies, while the fractures in the medium-grained sandstones of the distributary channel and the mudstones of the distributary bay are relatively underdeveloped. The core fractures and micro-fractures of the Yanchang Formation all have the regional distribution characteristics, and the fracture strikes are mainly between NE50° and NE 70°. Moreover, the finite element method was used to predict the fractures in the target layer, and the prediction results are consistent with the actual distribution results of the fractures. The coupling analysis of fractures and tight oil sandstone distribution showed that the existence of fractures provided conditions for the accumulation of hydrocarbons in the Yanchang Formation. The confluence and turning areas of the river channels were repeatedly scoured by river water, and the rocks were brittle and easy to form fractures. The thickness of the fractured sandstone in these areas is usually greater than 0.4 m. Moderately developed fracture zones are prone to form hydrocarbon accumulation “sweet spots,” and the fracture indexes of these areas are usually distributed between 0.8 and 1.2. However, when the fracture index exceeds 1.2, over-developed fractures are unfavorable for the accumulation of hydrocarbons.

Evaluation of pore-scale wettability in the tight sandstone reservoirs of the Upper Triassic Yanchang Formation, Ordos Basin, China

Wettability is a key factor influencing multiphase fluid distributions and flow behavior in the reservoirs. The microscopic characteristics of the mixed wettability which is a special wettability type are unclear in sandstone reservoirs. An integrated study of petrographic, petrophysical and pore-scales wettability by Environmental Scanning Electron Microscope (ESEM) using the water condensation method were carried out on typical sandstone samples in the Chang-6 interval of Yanchang Formation, southeastern Ordos Basin, and the characteristics of mixed wettability were investigated. It shows that the oil-free sandstones are water-wet in general, while the oil-bearing sandstones are apparently mixed-wet. In the mixed-wet sandstones, the walls of different pores or even the same one pore display different wetting states. The mixed wetting nature of the tight sandstones was probably caused by the early phase oil charge. After oil charged into the reservoir, chlorite surfaces in contacting with pores adsorbed the polar compounds in the crude oil, and their wettability was altered locally from water-wet to oil-wet states. However, pore walls surrounded by quartz and feldspars mainly remained water-wet after oil charge.

Analyzing major controlling factors of shale oil 'sweet spots' in the Chang-7 member of the Triassic Yanchang Formation, Ordos Basin

The Chang-7 Member of the Upper-Triassic Yanchang Formation has favorable geological conditions (e.g., extensive distribution of source rock, broad distribution of fine-grained sand bodies, stable structure, etc.) for developing a large shale oil pool. Initial assessments have estimated that the Chang-7 has shale oil resources of 3.0 × 109 t approximately. The discovered Xin'anbian giant shale oil field has an estimated reserve of one billion tons. The shale oil in the Chang-7 Member has been developed by volumetric fracturing in long horizontal wells since 2011. By the end of 2021, more than 500 horizontal wells had been drilled, with average initial production of 9.6 t/d per well. Some large-scale effective development pilot zones (e.g., X233, Z183, A83, etc.) have been constructed. The source rock and reservoir in the Chang-7 Member are heterogeneous. Under an extensive distribution background of shale oil layers, there are some local 'sweet spots'. In order to reveal controlling factors for the productivity of shale oil, research involving the comparison of development effects, formation of geologic conditions, and technologies of the three typical pilot zones, has been conducted. The results showed that organic abundance in the source rock is the primary controlling factor for “sweet spot” distribution of the shale oil. The average TOC value in the black shale source rock in the Chang-7 Member is about 10% higher than that in the dark gray mudstone. The average hydrocarbon generation intensity is about 2.0 × 103 t/km2, the average hydrocarbon expulsion rate is 34.6%, and the production rate of gaseous hydrocarbons is 14.65–39.46 m3/t. In the vicinity of black shale with a high TOC value, the oil filling intensity in the shale oil reservoirs is high, with oil saturation of up to about 70%, and a gas-to-oil ratio >90 m3/t. Secondly, the 'sweet spots' with such petrophysical properties control the enrichment of reservoirs. Owing to conditions of low-porosityand low-permeability, the shale oil reservoirs in the Chang-7 Member of the Ordos Basin still developed 'sweet spots',with porosity larger than 9% and permeability larger than 0.08 × 10−3 μm2 . These reservoir sweet spots have a pyrolysis yield of hydrocarbons >9 mg/g and the development wells have high initial and total production, a slow decline rate, and low water cut. Moreover, the dissolved gas in shale oil in the Chang-7 Member of the Ordos Basin is an oil-type gas that was generated during the primary pyrolysis stage of sapropel kerogen after entering a mature period. The gas-to-oil ratio in crude oil is controlled by several factors, such as organic abundance and maturity in source rock, thickness, petrophysical properties, and fractures in reservoirs. The overlap of black shale with high TOC, source rock thickness >10 m, thermal maturity Ro > 0.8%, and type I and II high-quality reservoir beds are the pre-requisites for the development of sweet spots with high and stable production of shale oil. The produced gas-to-oil ratio in the development wells is stable (generally 19.68 m3/t). Some wells have produced oil continuously and stably at over 14 t/d for 5 years. This paper delineates the distribution patterns of shale oil sweet spots in the Chang-7 Member of the Ordos Basin and confirms that profitable and efficient development of shale oil can be achieved in this basin.

Study on Sedimentary Characteristics of Chang 9 Member of Triassic in Ordos Basin

The Triassic Yanchang Formation in the Ordos Basin is one of the main exploration targets. Based on the detailed exploration of the outcrop at the southern margin of the Ordos Basin, combined with previous results, a systematic analysis of lithology, rock assemblage, sedimentary facies characteristics and temporal evolution is carried out. The results show that the Changchang 9 oil-bearing group on the southern margin of the Ordos Basin can be divided into 9 lithofacies types, and three types of sedimentary facies, meandering river, meandering river delta, and braided river delta are developed. The meandering river facies develops riverbed retention deposits, beaches, and There are four types of microfacies in natural dikes and rupture fans. There are five types of microfacies in meandering river deltas: underwater distributary channels, underwater natural dikes, divert bays, estuary dams, and remote sand dams. Braided river deltas develop underwater distributary channels, There are two kinds of microfacies between distributary channels; the Chang 9 oil-bearing group has a coarse-fine-coarse compound rhythm in the vertical direction; meandering river sand bodies and delta front sand bodies are the main places for oil and gas migration and accumulation.

Testing origin of reservoir quality difference of tight sandstones in the Yanchang Formation, Ordos Basin, China

The Chang 7 and Chang 8 members of the Upper Triassic Yanchang formation are high-yielding tight sandstone oil reservoirs deposited in different sedimentary environments in the Ordos Basin. Lacustrine sediments are dominant in Chang 7 members, while delta front sediments are dominant in Chang 8 members. The causes of reservoir quality differences in the different sedimentary environments are not clear, which renders tight-oil exploration and development difficult. In this study, X-ray diffraction analysis (XRD), optical microscopy, scanning electron microscope (SEM), mercury injection tests and oil-water relative permeability tests were used to study the origin of reservoir quality differences between the Chang 7 and 8 reservoirs. The results show the following: 1) the Chang 8 and Chang 7 reservoirs are dominated by primary intergranular pores and dissolution pores, respectively. 2) Grain-coating chlorite is present in the Chang 8 reservoir, which is known to enhance the ability of reservoirs to resist compaction and prevent the growth of quartz. However, the reservoir pore space could be preserved only when the abundance of grain-coating chlorite is below the threshold of 4 vol%. 3) Carbonate cementation occurs pore-throat spaces, and the porosity of the reservoir tends to improve with carbonate content increase in the range of 0–10 vol%. This phenomenon is caused by carbonate minerals formed in different times. 4) The strong heterogeneity of pore sizes is the reason for rapid rise of capillary force of pore mercury saturation curve in Chang 8 reservoir. The throat volume controls the mercury saturation of the pore throats. The Chang 8 reservoir exhibit larger throat volume and higher permeability than Chang 7 reservoir. 5) The Chang 7 reservoir has small pore radius and poor connectivity. Therefore, it is difficult for water to displace the oil phase. As a result, the water phase permeability increases rapidly, the residual oil saturation is high, and the water productivity of the reservoir increases rapidly. In conclusion, the reservoir quality of Chang 8 Member is better than that of Chang 7 Member, and this can be attributed to the following factors: i) Chang 8 reservoirs have larger grain sizes and more abundant grain-coating chlorite, ii) the low content of illite and carbonate cement reduces reservoir cementation capacity and iii) the pore connectivity is better, because the throat radius and effective throat volume are larger. The results provide guidance for the efficient development of tight oil in these reservoirs.

Pore system and gas adsorption potential of lacustrine Yanchang Mudstone, Ordos Basin, China

Understanding the pore system and the geological factors controlling the adsorption of gas in mudstones and shales is a key element in predicting the gas accumulation potential of an unconventional reservoir. A series of integrated methods, including basic geochemistry, X-ray diffraction, scanning electron microscopy, low-pressure gas (CO2 and Ar) physisorption, and high-pressure methane adsorption were performed on a set of mudstones from the Triassic Yanchang Formation in Ordos Basin, to characterize the pore system and gas adsorption capacity and their geological controlling factors. The results show that the studied Yanchang Mudstones are mainly siliceous rocks with high total organic carbon concentration (average of 7.63%) and low to moderate thermal maturity (vitrinite reflectance of 0.55–0.92 %). The microscopic visible pores are mainly intergranular pores between organic matter and rigid mineral grains, while organic matter pores are less well-developed. The pore volume is mainly contained in micropores (diameter < 2 nm) and mesopores (diameter = 2–50 nm), while much of the specific surface area occurs in the micropores, mainly in the organic matter pores. The occurrence of rigid minerals can inhibit the compaction of the intergranular pores. The abundant organic matter can provide potential adsorption sites for gas molecules, while clays and rigid minerals also have a limited positive effect on adsorption capacity. The adsorbed methane is mainly stored in micropores, which accounts for about 55%–90% of the total adsorbed methane content.

Geochemical characteristics and origin of Chang-9 crude oil of Triassic Yanchang Formation in Zhidan area, Ordos Basin, China

Geochemical characteristics and origin of Chang-9 crude oil of Triassic Yanchang Formation in Zhidan area, Ordos Basin, China

New Online Shunt Acidification for Water Injection Increasing Technology and Its Application in Huanjiang Oilfield

The poor physical property and strong heterogeneity of Triassic Yanchang formation in Huanjiang oilfield of Ordos Basin are the main reasons for uneven water absorption, partial injection wells underinjection at high pressure, and decline of production. Previously, large numbers of conventional acidifications were used for plugging removal in the reservoir, but the effect was not so good and effective period was short. Aiming at the geological characteristics of Huanjiang oilfield, an online shunt acidification and augmented injection technology which does not stop water injection, pull original production strings out, and continuously inject acid and diverting agent has been proposed. A chelating acid COA-1S with low corrosion rate (0.3675 g/(m2·h)), good retardation capacity (hydrolysis constant = 1.2 × 10−6), and effective chelating ability (precipitation inhibition rate &gt;95%) has been developed, as well as a diverting agent COA-1P with good dispersion in acid solution, diversion effect, and particle size (10–100 μm), which behaves well in COA-1S acid. It has been proved that the online acid system has a good diversion acidizing ability and plugging removal performance in a deep area in the laboratory core physical simulation test. The field test results show that the online shunt acidizing and augmented injection technology could reduce the injection pressure significantly (4.2 MPa) and increase water injection by 10 m3/d for the measured well (H5) and improve the water injection profile prominently. The online shunting acidification and augmented injection technology have the following advantages: simple procedures, fewer equipment needed, high efficiency of depressurization, and increasing water injection, which could effectively improve the profile of water wells, and there is a bright future of the technology.

The application of machine learning under supervision in identification of shale lamina combination types — A case study of Chang 73 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.

Triassic hydrothermal chimneys from the Ordos Basin of Northern China

Because few well-preserved hydrothermal chimneys have been found in terrestrial sedimentary rocks, research on paleo-thermal vents in geological history is relatively sparse. In this study, we present our original discovery of “hydrothermal chimneys” from the Chang 7 source rocks of the Triassic Yanchang Formation in the Ordos Basin, China, and provide the best evidence for deciphering hydrothermal activity preserved in the geological record (i.e., sedimentary rocks). Three possible chimney samples (i.e., samples 1551.6, 1551.6–2 and 1574.4) were collected for this study; they were interbedded with mudstones and oil shales, indicative of a deep-lake sedimentary environment. All three samples consist mainly of anhydrite, pyrite, and dolomite with the formation of mineral zoning across the walls of these structures, suggesting a sulfate-dominated stage and a carbonate-sulfide replacement stage. Moreover, their in situ geochemistry is characterized by high Eu, U, Th, Sr, Mn and U/Th ratios, which are typical indicators of hydrothermal vents. In addition, their S isotopes range from 7.89% to 10.88%, near the values of magma sulfur, implying a possible magmatic trigger for these hydrothermal vents. All this evidence shows that the Triassic sedimentary rocks of the Ordos Basin probably contain hydrothermal chimneys. Comparing ancient hydrothermal chimneys to modern hydrothermal chimneys, we should note the important implications of ancient chimneys; their formation mechanism may have been related to oil production, and they are possible indicators for future oil investigations. Further, they have great significance for studying the hydrothermal properties of primary dolomite.

Effects of pore-throat structures on the fluid mobility in chang 7 tight sandstone reservoirs of longdong area, Ordos Basin

Pore structure and fluid mobility are important factors affecting reservoir quality evaluation and effective hydrocarbon resource assessment in tight sandstone reservoirs. In this research, 14 tight sandstone samples from the Chang 7 member of the Upper Triassic Yanchang Formation in the Longdong area of the Ordos Basin were analyzed through high-pressure mercury injection (HPMI), low-temperature nitrogen adsorption (LTNA) and nuclear magnetic resonance (NMR) experiments to clarify the pore-throat structure characteristics of tight sandstone reservoirs and their influence on the occurrence characteristics and mobility of reservoir fluids. The results suggest that the different distributions and combinations of pore and throat types and sizes resulted in three pore-throat structures and four pore-throat spaces in the study area, corresponding to different occurrence characteristics of movable fluid. Among them, the type I pore-throat structure is characterized by large pore throats, high pore-throat connectivity and weak heterogeneity, with the highest corresponding average movable fluid saturation (MFS) and movable fluid porosity (MFP). The pore-throat size distribution (PSD) of type II pore-throat structure has bimodal characteristics with a low pore-throat sorting degree but a good pores and throats configuration relationship with the higher average MFS and MFP. The type III pore-throat structure has developed intercrystalline pores with a smaller average pore-throat radius, the highest fractal dimension and the lowest fluid mobility. Micropores and mesopores, as the main pore-throat spaces in the three types of pore-throat structures, are the main contributors to the petrophysical properties and the main occurrence place for movable fluids. Nanopores with strong heterogeneity and macropores with high permeability both occupy small pore-throat spaces and have little influence on reservoir fluid mobility. It is thus cleat that the pore-throat structure of tight sandstone reservoirs is the main factor affecting reservoir fluid mobility. More specifically, a pore-throat structure with a large pore-throat radius, good pore-throat connectivity and weak heterogeneity is favorable to the occurrence and seepage of movable fluids. Moreover, reservoir fluid mobility is comprehensively influenced by reservoir petrophysical properties and various clay minerals. Lower clay mineral contents and higher reservoir quality are conducive to the high flowability of tight sandstone reservoir fluids.

Investigation into the Pore Structure and Multifractal Characteristics of Shale Reservoirs through N2 Adsorption: An Application in the Triassic Yanchang Formation, Ordos Basin, China

To understand the pore structure and heterogeneity of pore size distribution (PSD) is essential for revealing fluid mechanics and evaluating the utilization of unconventional resources. In this study, there are multiple shale examples collected from the Chang 7 section in the Ordos Basin for the investigation was conducted on the basis of various experiments on total organic carbon (TOC), X-ray diffraction (XRD), and nitrogen gas adsorption, through scanning electron microscopy (SEM) and multifractal method. The multifractal characteristic parameters, including the width of singularity spectra ( Δ α ), Hurst exponent ( H ), D 1 / D 0 , and nitrogen gas adsorption, were used to find out about the characteristics of pore development and to quantify the complexity and heterogeneity of pore structure. Depending on the exact mineral composition, the Yanchang Formation of Chang 7 shales is classified into either silty mudstone (SM) or muddy siltstone lithofacies (MS). According to the investigative results, the Chang 7 lacustrine shale features a complex pore system with the pores ranging from 1.5 to 10 nm in diameter. Besides, mesopores contribute significantly to the total pore volume (TPV) and total surface area (TSA). As for TPV and TSA of the SM lithofacies in the samples under investigation, they are nearly 1.09–1.78 and 0.80–1.72 times greater as compared to the MS lithofacies samples. The dominant types of reservoir spaces include organic matter (OM) pore and interparticle pore which are related to inorganic minerals. The value of Δ α is higher for MS lithofacies than for SM lithofacies, indicating a greater heterogeneity of PSD in the MS lithofacies. The pore structure of MS lithofacies is determined mainly by TOC and siliceous mineral content, whereas the influencing factors for SM lithofacies are TOC and clay mineral content. There is a significant relationship between multifractal parameters and pore structure parameters for both SM and MS lithofacies. The TOC of SM and MS lithofacies exhibits a close correlation with Δ α , suggesting that the pores in organic matter are dominated by those nanopores with a complex and heterogeneous pore structure. The rock composition of the lithofacies can affect Δ α to a varying extent, which means that the minerals have an evident impact on the heterogeneity of MS and SM lithofacies.

Occurrence State of Soluble Organic Matter in Shale Oil Reservoirs from the Upper Triassic Yanchang Formation in the Ordos Basin, China: Insights from Multipolarity Sequential Extraction

Occurrence State of Soluble Organic Matter in Shale Oil Reservoirs from the Upper Triassic Yanchang Formation in the Ordos Basin, China: Insights from Multipolarity Sequential Extraction