Southwest Ordos Basin Research Articles

Response characteristics and novel understandings of dual induction logging of horizontal wells in fractured reservoirs

Previous studies commonly used dual laterolog or induction logging in vertical wells to study fractured reservoirs. However, there are few studies about the response characteristics of dual induction logging of horizontal wells in fractured reservoirs, which interferes with the evaluation of horizontal wells in fractured reservoirs. In this study, firstly, the finite element method is used to establish the fractured reservoir model and the dual induction instrument of horizontal well. Then, the response characteristics of dual induction logging in fractured reservoirs with different fracture conductivity, fracture dip angle, fracture width and fracture length are simulated and analyzed. The results show the response characteristics as follows: (1) The apparent conductivity of dual induction logging increases first and then decreases along with the increase of fracture conductivity, when the fracture conductivity is lower than formation conductivity, the apparent conductivity is similar to formation conductivity; (2) The fracture dip angle has little effect on the apparent conductivity of induction logging, when the fracture conductivity is high, the apparent conductivity decreases with the increase of the fracture dip angle; (3) The effect of fracture width and fracture length on apparent conductivity is similar. The larger the fracture width or length, the greater their influence on the apparent conductivity, which is different from the response characteristics of laterologs. In applications, these characteristics can be demonstrated by two wells in the southwest Ordos Basin, China. Finally, advantages and disadvantages of the simulation method, applications, logging responses discussed in detail, as well as the response characteristics and the shielding effect of fractures on electromagnetic waves of dual induction instrument are discussed in detail. The applications and discussions show that this study can provide response characteristics and novel understandings for the evaluations of horizontal wells in fractured reservoirs, especially in areas where dual induction logging is widely used.

Sedimentary characteristics, formation conditions, and distribution mechanisms of beach bar sand bodies of Chang 82 submember, southwest Ordos Basin

AbstractBeach bar sand bodies are a relatively new type of reservoir in oil exploration where they can be found in the proximity of the source rock. In this study, the genetic type of sand bodies in the Upper Triassic Yanchang Formation in the southwest Ordos Basin is systematically studied through the observation of outcrops, core description, microscopic inspection of thin sections, along with petrophysical properties, mineralogy, and geochemical analysis of trace elements. The results showed that a new sedimentary type of beach bar sand bodies are developed in the Chang 82 submember of the Yanchang Formation, that are generally characterized with a flat bottom and convex top, where the contact interface between the bottom of the single sand body and mudstone is flat, and the bottom lacks erosion and filling structure. Moreover, a variety of wave‐induced sedimentary structures are developed, showing a reverse sedimentary sequence from fine to coarse grains. The structural and compositional maturity of the beach bar sand bodies are higher than that of the river channel sand body. Moreover, reservoir pore type is mainly comprised of intergranular pores, and the porosity and permeability of the samples retrieved from the sand body are much higher than that of the river channel sand body. Finally, the main controlling factors of the formation and preservation of beach bar sands are found to be the lake basin bottom topography, the base level cycle, and the stable paleo‐lake shoreline. During the Chang 82 submember depositional period, a large area where the beach bar sand bodies were deposited parallel to the lake shoreline in the southwest of the basin was created. Such depositional environments had suitable conditions to host large‐scale oil‐bearing reservoirs, as major exploration and development targets in the Ordos Basin in the future.

A novel calculation model, characteristics and applications of Archie's cementation exponent in dual porosity reservoirs with intersecting dual fractures

Fractured rock samples are difficult to use for ground conductive experiments, and there are few related simulation researches of Archie's cementation exponent in the reservoirs with intersecting dual fractures. In order to obtain the cementation index for saturation evaluation when no experiments and methods are available in such reservoirs, we focus on the researches of the cementation exponents in these reservoirs. Based on the Maxwell-Garnett mixing rule and the morphological characteristics of fractures in reservoirs, the method for obtaining the cementation exponents in reservoirs with parallel fractures at any inclination angle are first reviewed. secondly, the rock model of the dual porosity reservoirs with two intersecting fractures is constructed. The rock model is divided into four equal parts through equivalent processing, and the respective cementation exponents of the four parts can be calculated. Based on the series and parallel connection of Ohm's law, a novel calculation model of the overall cementation exponent of the rock model is established. Thirdly, using the novel model, the characteristics of cementation exponent of the rock model affected by different fracture dips, fracture porosities and included angles between the two fractures are calculated. In terms of detail and overall, the trend curves of these characteristics exhibit different nonlinear characteristics. The combined effect of the dip angles of the dual fractures, the included angle between them, the porosities of them and the porosity of matrix makes the cementation exponent characteristics more complex than that of single fracture or parallel fractures. Fourthly, the novel calculation model is used to evaluate the cementation exponent and fluid saturation of the two wells in the Chang 8 Formation in the southwest Ordos Basin, China, and the application results can demonstrate the reliability and availability of the novel model of cementation exponent in the reservoirs with intersecting dual fractures. At last, the application effect, influencing factors and applicable conditions of the novel calculation model of cementation exponent are discussed in detail. These applications and analyses show that it can provide an alternative solution for cementation exponent estimations in such fractured reservoirs, especially when experiments are not available.

Logging Identification and Distribution of Bauxite in the Southwest Ordos Basin

In recent years, with the discovery of oil and gas in the Carboniferous bauxite reservoir in the Ordos basin, the formation and distribution of bauxite and bauxite reservoirs have attracted the attention of oil and gas explorers. Based on the slightly equidistant core testing of minerals and the porosity on the formation and calibration on the logging curves, we established a logging identification method for bauxite in the study area and fitted the formula for calculating the diaspore content and porosity using logging data. By applying this formula and a large amount of logging data, thickness and porosity maps of the Taiyuan formation of bauxite in the southeastern part of the basin were produced. Then, according to the thickness of the earliest deposited Taiyuan formation on the unconformity surface, we analyzed the paleogeomorphology of the bauxite deposition. Finally, a sedimentary facies map of the Taiyuan formation was developed based on the content of sandstone, coal seams, and carbonate rocks contained there. The results show that the diaspore content of bauxite is positively correlated with the natural gamma logging (GR) values. According to the diaspore contents and the GR values, the aluminiferous rocks in the Taiyuan formation can be divided into three categories: ① Bauxite for GR values > 450 API and diaspore content >70%; ② Argillaceous bauxite for GR values = 300–450 API and diaspore content = 25%–70%; and ③ Bauxitic mudstone for GR values = 220–300 API and diaspore content = 0%–25%. Bauxite mainly occurred in the middle and deep lagoon environments in the lower part of the Taiyuan formation. The lagoon is distributed along the paleokarst groove in the NW strike, with a width of approximately 30–40 km and a length of approximately 150–200 km, among which the thickness of pure bauxite is 9 m, argillaceous bauxite 26 m and alumina mudstone 6 m. Bauxite with a high porosity mainly existed in pure bauxite. The lagoon bauxite in the lower part of the Taiyuan formation gradually changes upward into a tidal flat swamp and carbonate platform environment.

Identification of an oil-bearing layer by formation water resistivity: A case study of the Jurassic reservoir, Southwest Ordos Basin

In the southwestern Ordos Basin, the formation water salinity and resistivity of Jurassic sandstone reservoirs are characterized by rapid changes and great variations, which lead to difficulties in the identification of reservoir fluid types and the development and exploration of the hydrocarbons in this area. To explore appropriate methods for reservoir fluid identification provided with complex formation water salinity and resistivity, we have conducted three research steps. First, using formation water testing and well-logging data, the formation water salinity, spontaneous potential, and reservoir resistivity are used to calculate the formation water resistivity. Second, the calculation results were used to separate the reservoir fluid types. Third, the calculated formation water resistivities are classified and the relationship between the formation water resistivity and the oil-producing interval is established. The results indicate that, for oil-producing intervals, the formation water resistivity calculated using the reservoir resistivity is the highest, whereas that using the formation water salinity is the lowest. Based on this understanding, two identification methods of reservoir fluid types are developed, including the qualitative curve overlap method and the quantitative ternary graph method. The curve overlap method can be applied to visually distinguish reservoir fluid types through the differentiation of formation water resistivity. Compared with some earlier quantitative methods, the ternary graph method can achieve an efficiency of 87.7% in identifying the type of reservoir fluid. Finally, the reliability of both methods is verified by case studies.

County in Southwest Ordos Basin Block 1 Period of Sand Body Distribution Characteristics of Mountains

Huanxian block is one of the key oil and gas exploration and development blocks in Longdong Gas field of Changqing Oilfield Company. The Mesozoic exploration is about to enter the saturation state, and it is urgent to explore new fields. Therefore, the seismic characteristics, reflection structure characteristics and reservoir sand body distribution characteristics of Shan-1 member of Permian Shanxi Formation of Upper Paleozoic in this block are studied comprehensively. By using the data of drilling core, well logging curve and 3D seismic interpretation, the regional small beds are finely divided and correlated, and the unified stratigraphic framework of the area is established. The structural characteristics of the bottom of Shan-1 member of Shanxi Formation are studied, and the distribution characteristics of reservoir sand bodies and effective sand bodies of Shan-1 member are analyzed and studied.

Geochemical Characteristics of Graptolite Shale in the Pingliang Formation of the Ordos Basin, China: Implications for Organic Matter, Thermal Evolution, and Hydrocarbon Reservoir

Graptolite-rich shale is the main layer of shale gas resources in the southern marine sedimentary basin. Recently, shale gas resources were discovered in the Ordovician marine graptolite-rich strata in the Ordos Basin. The graptolite shale in the study area is different from the marine graptolite shale in the Yangtze plate in southern China, and further exploration is needed. This paper presents core samples of the graptolite-rich shale of the Pingliang Formation in the southwest Ordos Basin as research objects. The graptolite genus and graptolite shale characteristics were studied using core observation, electron microscopy, scanning electron microscopy, and geochemical analysis. We determined the role of the sedimentary environment and thermal maturation of graptolite shale in hydrocarbon formation and explored the possibility of hydrocarbon generation. Many graptolite epidermises provide buried organic matter. The quiet sea and low-energy marine environment create favorable conditions for preserving organic matter. The tectonic process resulted in the evolution stage in the oil generation window. Different types of pores formed the spaces of hydrocarbons. Therefore, the shale of the Pingliang Formation has shale oil exploration potential, which complements the shale gas in the northwestern margin of the basin, and provides new venues for shale oil and gas exploration in northern China.

Coupling Relationship Between Densification Characteristics and Tight Oil Accumulation of the FGSR Reservoir: A Case Study of the Chang 7 Member of the Triassic Yanchang Formation in the Zhenyuan Area of the Southwest Ordos Basin, Central China

Fine-grained sedimentary rock (FGSR) reservoirs in a deep-water sedimentary environment from the Late Triassic Yanchang Formation in the Ordos Basin in central China have huge potential for tight oil production. According to the comprehensive research of fluid inclusion experiment, scanning electron microscopy, cathodoluminescence, thin section identification, and whole rock analysis, the petrographic characteristics, fluorescence spectrum parameters, and the homogenization temperature of oil inclusions and associated brine inclusions in the FGSR reservoir were analyzed to explore the coupling relationship between densification characteristics and tight oil accumulation of the FGSR reservoir. The results show the FGSR reservoir diagenesis has experienced compaction, cementation, and multiple stages of dissolution. A comprehensive analysis of burial history shows that the Chang 7 Member FGSR reservoir in the study area has three phases of oil charging, corresponding to the geologic time range of 132–124, 120–102, and 98–97 Ma. The organic acid produced by the first-stage oil charging accelerated the dissolution of feldspar and cuttings, and the dissolution pores increased significantly. Then, the organic acid in the second-stage oil charging accelerated the dissolution, and the abnormally high hydrocarbon expulsion pressure inhibited the damage of compaction on the pores of FGSR reservoir. Finally, the thermal evolution degree of source rocks reached the highest in the third-stage oil charging. The constructive effect of large-scale organic acids on pores and the protective effect of the chlorite membrane on pores inhibited the densification process of the FGSR reservoir. In short, the FGSR reservoir has experienced a cyclical reservoir densification process of “reducing porosity–increasing porosity–preserving porosity.” The coupling of pore evolution and oil charging of the FGSR reservoir is the key to the extensive accumulation and large-scale reservoir formation of tight oil in the Chang 7 Member of the Yanchang Formation in the Zhenyuan area, southwest Ordos Basin. The significance of the study may help to understand the process of the tight oil accumulation and predict the tight oil of the FGSR reservoir distribution.

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.

Characteristics and Evolution of Strike-Slip Faults in a Stable Cratonic Block: Southwest Ordos Basin, China

Characteristics and Evolution of Strike-Slip Faults in a Stable Cratonic Block: Southwest Ordos Basin, China

Sedimentary sequence evolution and organic matter accumulation characteristics of the Chang 8–Chang 7 members in the Upper Triassic Yanchang Formation, southwest Ordos Basin, central China

Lacustrine organic–rich shales are widely developed in the Chang 7 member of the Yanchang Formation in the Ordos Basin. These shales have been proven to be the source rocks for crude oil and shale oil. The study of sedimentary evolution and the characteristics of organic matter (OM) accumulation is an important factor for the exploration and development of oil and gas resources. Based on the lithological assemblages, lithofacies, mineral contents, log curves and geochemical data, the sequence stratigraphic framework of the Chang 8–Chang 7 members of the Yanchang Formation in the southern margin of the Ordos Basin were established. A complete third–order sequence was identified, and it included four systems tracts: a lowstand systems tract (LST), a transgressive systems tract (TST), a highstand systems tract (HST) and a regressive systems tract (RST). By using organic petrology, organic geochemistry and elemental geochemistry, the abundance and source of OM, paleoclimate, paleowater conditions, paleoproductivity, and deposition rates of the different systems tracts of the Chang 8–Chang 7 are studied in detail. Various factors controlled the accumulation of OM in the different systems tracts. The abundance of OM in the LST is low, the kerogen type is mainly type III (including vitrinite); the abundance of OM increased gradually in the TST, the kerogen types are III and II2 (including vitrinite and sporophyte); the OM is mainly concentrated in the HST, and the kerogen type is mainly type II1 (including alginite), biomarker compounds show that OM has a mixed source of land plant and planktonic material; the abundance of OM in the RST is relatively high, the kerogen type is mainly type II2 and III (including vitrinite and alginite). The main controlling factor for OM accumulation was paleoclimate, which affected the primary productivity of lake and the chemical properties of the water column (redox and paleosalinity), leading to differences in OM accumulation under different climatic conditions. Hydrothermal activities contributed to anoxia and a salinity increase in lake bottom waters, with volcanic ash providing large amounts of essential nutrients for algal blooms. Based on the V/Cr, Sr/Ba, P/Ti and (La/Yb)N ratios with TOC values, the abundance of OM is positively correlated with paleoproductivity and water column reducibility, and negatively correlated with paleosalinity. The abundance of OM first increases and then decreases with increasing sedimentation rate. In general, the accumulation of OM was promoted under warm and humid climates, anoxic fresh to brackish water and adequate paleoproductivity.

Pore–throat structure and fractal characteristics of tight sandstones in Yanchang Formation, Ordos Basin

Pore–throat structure is a key factor that influences the storage and fluid flow capacity of tight sandstone reservoirs. By performing petro-physical characteristic tests, scanning electron microscopy, and casting thin sections, the main pore types of tight sandstone reservoir samples collected from the Upper Triassic Yanchang Formation in the Baibao area of the Southwest Ordos Basin in China were identified to be dissolved pores, intergranular pores, intercrystalline pores, and micro-cracks. Meanwhile, the size of the pore–throat was widely distributed, whereas its shape varied. Given the advantages and disadvantages of various technical methods, high-pressure mercury injection (HPMI) and nuclear magnetic resonance (NMR) technology were used to study the distribution characteristics of a full-size pore–throat. All 10 samples showed bimodal and unimodal distribution characteristics, and their distribution characteristics were consistent with the results of the petro-physical characteristics, high-pressure mercury injection, and movable fluid saturation tests. The throat distribution characteristics of the reservoir were studied by applying constant-rate mercury injection (CRMI), and the throat radius ranged from 0.43 μm to 9.76 μm with an average of 3.34 μm.The fractal features of the pore–throat and throat were quantitatively characterized by CRMI. The fractal curves of the pore–throat showed self-similarities, and the fractal dimension D ranged between 2.6297 and 2.9529 with an average of 2.7865. The fractal curves of the throats were broken down into two segments, and the fractal dimensions corresponded to two ranges, namely, the small (Dt1) and large throats (Dt2) fractal dimensions. The average of fractal dimension Dt1 was 2.9170, and the average of fractal dimension Dt2 was higher than that of Dt1 with an average of 2.9828, thereby indicating the complex structure and strong heterogeneity of the former. The fractal dimensions D, Dt1, and Dt2 were negatively correlated with permeability, movable fluid saturation, average throat radius, clay mineral content, and laumontite content, positively correlated with quartz content, and showed poor correlation with feldspar. Compared with Dt2, the fractal dimension Dt1 had a higher correlation with pore–throat parameters and mineral content. Given the higher self-similarity of small throats, their distribution is considered highly homogeneous.

Factors Controlling Hydrocarbon Accumulation in Jurassic Reservoirs in the Southwest Ordos Basin, NW China

AbstractThe sedimentary, paleogeomorphological and reservoir characteristics of the Jurassic Yan'an Formation in the southwestern Ordos Basin, northwestern China, were studied by means of casting thin sections, scanning electron microscopy, inclusion analysis and identification of low‐amplitude structures. A model for reservoir formation is established, and the controlling effects of sedimentary facies, paleotopography, low‐amplitude structures and formation water on oil reservoirs are revealed. There are significant differences in the sedimentary characteristics, structural morphology and paleowater characteristics between the reservoirs above the Yan 10 Member and those in the Yan 9 to Yan 7 Members. The Yan 10 Member contains fluvial sediments, whereas the Yan 9 to Yan 7 members contain delta‐plain anastomosing‐river deposits. The distribution of high‐permeability reservoir is controlled by pre‐Jurassic paleogeomorphology and sedimentary facies. Some of these facies exhibit high porosity and high permeability in a low‐permeability background. The main hydrocarbon accumulation period was the late Early Cretaceous, filling was continuous, and the charging strength altered from weak to strong and then from strong to weak. The Yan 10 reservoir is mainly controlled by the paleogeomorphology: hydrocarbons migrated upward at a high speed through the unconformity surface, and accumulated in the favorable traps formed by paleogeomorphic structural units, such as gentle slopes or channel island. Furthermore, groundwater alternation in these areas was relatively stagnant, providing good reservoir preservation conditions. The reservoirs in the Yan 9 and higher members are controlled by the sedimentary facies, low‐amplitude structure and paleowater characteristics. Hydrocarbons migrated through the three‐dimensional delivery system, influenced by favorable sedimentary facies and high‐salinity groundwater, then accumulated in the favorable low‐amplitude structural traps that formed during the hydrocarbon production period.

A Novel Method to Obtain Permeability in a Dual-Pore System Using Geophysical Logs: A Case Study of an Upper Triassic Formation, Southwest Ordos Basin, China

Fractured tight sandstone reservoirs are dual-pore systems including matrix pores and natural fractures, which both contribute to the system permeability. However, most previous studies either calculated the matrix permeability or obtained the fracture permeability to represent the system permeability in the logging evaluation of fractured tight sandstones because existing logging methods cannot distinguish the two types. In this study, a novel method is proposed to estimate the system permeability in fractured tight sandstones using geophysical logs. First, the fracture characteristics in the Upper Triassic Chang 8 member of the Yanchang Formation, southwest Ordos Basin, China, were analyzed. Based on the hydraulic flow unit approach, the formation classification criteria and the corresponding permeability–porosity models were established; then, the pure matrix permeability in the dual-pore system was calculated using geophysical logs. Based on the fracture characteristics, the relative pure fracture permeability was obtained using the Sibbit and Faivre method. By applying the Parsons’ model in boreholes, the system permeability was then calculated by coupling the relationship between the two permeabilities. Finally, two field applications in the study area demonstrate the feasibility of the proposed method, and the logging responses, application effects and applicable conditions of this method are discussed in detail. These applications indicate that the proposed method is suitable for tight reservoirs with fracture widths less than 200 μm, and considered to be dual-pore systems.

Overpressure controlling factors for tectonic fractures in near-source tight reservoirs in the southwest Ordos Basin, China

A typical tight sandstone reservoir with a huge reserve of oil has been discovered in the eighth member of the Upper Triassic Yanchang Formation in Jinghe Oilfield, southwest Ordos Basin, China. In this tight reservoir, the tectonic fractures have been proved to be a critical controlling factor for the distribution of “sweet spots”. How to accurately describe the distribution of the tectonic fractures, however, is still challenging due to the poor understanding of the driving mechanisms for fracture formation. In this paper, the effects of the adjacent source rocks on the development of tectonic fractures were studied based on field data and theoretical analysis. Our results indicated that the overpressure transferred from the adjacent source rocks can significantly influence the development of the tectonic fractures in the Jinghe Oilfield. This conclusion was drawn according to two main facts. Firstly, the density and distribution of the tectonic fractures present a high correlation with the development of the overpressure of the adjacent source rocks. In the vertical direction, the density of fractures of the target reservoir gradually decreases along with the increase of the distance away from its above source rocks. Secondly, the period of main hydrocarbon charging and the formation of massive tectonic fractures overlaps very well. Thus, as a typical case, it implies that the effect of the transferred overpressure from source rocks should not be neglected for accurately predicting the spatial distribution of tectonic fractures in tight reservoirs in the southwest Ordos Basin. To do that, pore pressure stress coupling is an inevitable aspect and both overpressure evolution and stress regime should be considered as important controlling factors.

Discovery and geological knowledge of the large deep coal-formed Qingyang Gas Field, Ordos Basin, NW China

Abstract After 50 years of oil and gas exploration in Longdong area of southwest Ordos Basin, NW China, a deep coal-formed gas field was discovered for the first time in Qingyang area. Through observation of field outcrops and core, analysis of common, cast and cathode thin sections, Ro and other geochemistry indexes, carbon isotope, electron microscope and other supporting tests and analyses, the hydrocarbon generation, reserves formation and reservoir formation characteristics of gas reservoirs at different buried depths in Yishaan slope were examined and compared. The deep gas reservoir has an average buried depth of more than 4 200 m, and the main gas-bearing formation is the Member 1 of Lower Permian Shanxi Formation, which is characterized by low porosity, low permeability, low pressure and low abundance. It is believed that hydrocarbon generation in thin seam coal source rocks with high thermal evolution can form large gas fields, high-quality sandstone reservoirs with dissolved pores, intergranular pores and intercrystalline pores can still develop in late diagenetic stage under deep burial depth and high thermal evolution, and fractures improve the permeability of reservoirs. High drying coefficient of natural gas and negative carbon isotope series are typical geochemical characteristics of deep coal-formed gas. The integrated exploration and development method has been innovated, and the economic and effective development mode of gas fields of “dissecting sand body by framework vertical wells, centralized development by horizontal wells” has been formed. The discovery and successful exploration of the large gas field has provided geological basis and technical support for the construction of natural gas fields of 100 billion cubic meter scale in the southwest of the basin, and has important guidance for exploration of coal-derived gas with deep buried depth and high thermal evolution widely distributed in China.

Multichannel systems in an ancient river-dominated delta: case study of the lower Yanchang Formation, southwest Ordos Basin, China

Distributary channels in large deltas can form a channel pattern similar to braided fluvial system or anastomosed fluvial system that have multichannel systems. Although both systems are of generally comparable platforms, their geometry, sedimentology, and facies associations may exhibit unique characteristics. Many ancient multichannel systems have been interpreted as braided patterns, but some are certainly anastomosed patterns. A reevaluation of ancient multichannel architectures and sedimentology patterns is needed to improve discrimination of braided and anastomosed patterns of multichannel systems. This study examines the characteristics of two modern anastomosed pattern channel systems. Those modern systems are compared to ancient examples in the lower Yanchang Formation, southwest Ordos Basin. This comparison indicates that the multichannel systems of the delta, southwest Ordos Basin, exhibit greater similarity to modern anastomosed channel systems of shallow-water deltas. Systems of low-sinuosity distributary channels and interdistributary bays or swamp islands are developed mainly between the channels, and there are no mouth bar deposits. Both modern and ancient multichannel systems suggest that low gradient slope is a significant controlling factor in the formation of anastomosed pattern channels in river-dominated deltas. The identification of anastomosed patterns plays a significant role in reservoir characterization and hydrocarbon exploration and production in delta systems.

Characteristics of faults and their significance in controlling the oil accumulations in Honghe field, Ordos Basin

The Chang 8 (C8) reservoir is a transitional oil pool type from a tight oil sandstone to a lithologic trap in the Upper Triassic Yanchang Formation of the Honghe oil field, southwest Ordos Basin, China. Vertical faults play important roles in the formation and distribution of the sweetness. To predict favorable high-yield well areas and guide horizontal well deployment, 3D seismic data are used to analyze these faults’ distribution, formation phase, stress property, and control on the oil accumulation. The results indicate that the X shape of the east–northeast and north–northeast conjugate shear faults were formed during the Indosinian epoch, and the northwest faults were formed during the Yanshan epoch. Meanwhile, the earlier east–northeast faults partially experienced reactivity and rework later but the north–northeast faults ceased since the Indosinian. The strike-slip faults in the area are classified into three categories based on their intensity, density, formation stage, and oil control effectives: types I, II, and III. The type I faults extended in the northwest and east–northeast directions and were cut from the Ordovician up to the top Cretaceous. They are characterized by the maximum extensions aerially and vertically thereby faulting the thickest strata. Type II faults mainly extended in the east–northeast direction and were cut from the Triassic up to the top Cretaceous in small intension. Type III faults mainly extended in the north–northeast direction and were cut from the Ordovician up to the Upper Permian. The faults connect the C8 sandstone reservoir and the Chang 7 source shale by penetrating the interlayered silty mudrock in between them. The low permeable reservoir also requires additional permeability imparted by the type I faults. High-yield horizontal wells are distributed along intensive type I faults. The water-bearing zones or oily water zones are mostly related to no fault or to the type II and III faults.

A method to determine nuclear magnetic resonance (NMR) T2cutoff based on normal distribution simulation in tight sandstone reservoirs

The T2cutoff is an important input parameter in nuclear magnetic resonance (NMR) applications. The accuracy of the T2cutoff affects the prediction reliability of parameters associated with the identification and evaluation of formations. Current methods are based on regional statistics; they have limited applications and the predicted values are not always reliable. In this study, a total of 36 core samples, drilled from Triassic tight sandstone reservoirs of the southwest Ordos Basin in China, were used for laboratory NMR measurements under fully brine saturated and irreducible water conditions. Based on the morphological character analysis of experimental NMR T2 spectra, we demonstrate that NMR T2 spectra can be fully simulated using the normal distribution function. In addition, a new method was proposed, which can predict various T2cutoffs based on the morphological differences of NMR T2 spectra, and the irreducible water saturation (Swirr) was calculated, which represents the ratio of the sum volume of clay bound water and capillary bound water to total pore volume. The reliability of this method was verified by comparing the predicted T2cutoff and Swirr values with those of core NMR experimental results. Finally, we extended this method into field applications in several tight sandstone reservoirs in China. The results show that total T2 distributions of the formation were fully simulated by the normal distribution function, and various T2cutoff and Swirr values were precisely predicted. Meanwhile, tight sands permeability curves, predicted based on the Timur-Coates model, were also accurately estimated. The results of our study may be applied to improve tight sandstone reservoir identification and evaluation using NMR logs.

Analysis of the charging process of the lacustrine tight oil reservoir in the Triassic Chang 6 Member in the southwest Ordos Basin, China

With the success of Bakken tight oil (tight sandstone oil and shale oil) and Eagle Ford tight oil in North America, tight oil has become a research focus in petroleum geology. In China, tight oil reservoirs are predominantly distributed in lacustrine basins. The Triassic Chang 6 Member is the main production layer of tight oil in the Ordos Basin, in which the episodes, timing, and drive of tight oil charging have been analyzed through the petrography, fluorescence microspectrometry, microthermometry, and trapping pressure simulations of fluid inclusions in the reservoir beds. Several conclusions have been reached in this paper. First, aqueous inclusions with five peaks of homogenization temperatures and oil inclusions with three peaks of homogenization temperatures occurred in the Chang 6 reservoir beds. The oil inclusions are mostly distributed in fractures that cut across and occur within the quartz grains, in the quartz overgrowth and calcite cements, and the fractures that occur within the feldspar grains, with blue–green, green, and yellow–green fluorescence colours. Second, the peak wavelength, Q650/500, and QF535 of the fluorescence microspectrometry indicate three charging episodes of tight oil with different oil maturities. The charging timings (141–136, 126–118, and 112–103 Ma) have been ascertained by projecting the homogenization temperatures of aqueous inclusions onto the geological time axis. Third, excess-pressure differences up to 10 MPa between the Chang 7 source rocks and the Chang 6 reservoir beds were the main driving mechanism supporting the process of nonbuoyancy migration.