Southern Ordos Basin Research Articles

Diagenesis and hydrocarbon emplacement in the Upper Triassic Yanchang Formation tight sandstones in the southern Ordos Basin, China

ABSTRACTThe Upper Triassic Yanchang Formation in the southern Ordos Basin is a tight sandstone but productive hydrocarbon exploration target. A variety of analyses, including thin-sections, cathodoluminescence (CL), scanning electron microscope (SEM), X-ray diffraction (XRD), fluorescence, isotope and fluid inclusion, have been used to analyse the reservoir petrology and diagenesis to understand the evolution of reservoir porosity. The sandstones are mostly feldspathic litharenites and lithic arkoses with low porosities (7.6% on average) and extremely low permeabilities (0.49 mD on average). A complex diagenetic alteration history of the reservoir caused different kinds of reduction in reservoir porosity. The carbonate cements are sourced from adjacent organic-rich mudstone and precipitated with a higher content near the edges of sandstone units and a lower content at the centres of sandstone units, resulting in two different diagenetic evolution paths. The analysis of porosity evolution history and hydrocarbon emplacement produced the following conclusions: (1) during the eodiagenesis period, mechanical compaction reduced the primary porosity dramatically from 40 to 19%; and (2) during the mesodiagenesis period, the first-phase oil charged the reservoir (porosity ranging from 14 to 19%) as bitumen along detrital grains forming a preferential pathway for subsequent oil emplacements. In the second-phase of oil charge, the reservoir porosity ranged from 9 to 14% limiting water–rock reactions. During the third-phase of oil charge, porosity of the reservoir remained at 9%. In general, the densification period (with a porosity close to 10%) of the reservoir occurs at the same time as the major oil emplacement and may be an explanation for the low oil saturation of the Chang 8 tight sandstone.

Rare earth elements geochemistry characteristics and their geological implications of lacustrine oil shale from Chang 7 oil layer in southern Ordos Basin, China

Located in the middle‐western area of China, Ordos Basin is a large continental basin with vast petroleum resources. Oil shale of Chang 7 oil layer from Triassic Yanchang Formation represents the typical lacustrine oil shale in China. Oil shale samples were collected from Chang 7 oil layer to study the characteristics of elements geochemistry. The rare earth elements (REEs) concentration of oil shale samples varies from 138.51 to 206.36 μg/g with an average of 162.64 μg/g, close to the average REEs content of the North American shale Composite and slightly higher than the upper continental crust. The chondrite‐normalized and North American shale composite‐normalized REEs distribution patterns explain that oil shale samples, together with silty mudstone samples, may have been derived from a similar terrigenous source. The oil shale deposited mainly in early diagenetic stage B period and the paleoclimate condition was warm and humid with moderate chemical weathering. The whole sedimentary rate of oil shale was low. The source rocks of oil shale were mainly from the felsic rocks and deposited in the continental island arc tectonic setting. The total REE of oil shale samples show positive correlations with Al, Si, K, and ash yield concentration and negative correlations with oil yield, Fe, P, and total sulfur concentration, illustrating that REEs concentration in clay minerals is more than that in organic matter. In addition, the light REEs and heavy REEs are both present in clay minerals and controlled by land‐derived detritus.

Geometric Heterogeneity of Continental Shale in the Yanchang Formation, Southern Ordos Basin, China

Favorable prospects for the exploration of shale gas have been demonstrated in the Ordos Basin, China. Outcrop and core observations indicate that there are abundant laminas in the shale strata, which exert a great influence on hydro-fracture propagation, gas storage and fluid flow. In this study, the continental shale of the Chang 72 Member, collected from the south of Ordos Basin, was investigated to characterize the geometric heterogeneity. Laminas at multiple scales were observed and measured using conventional logging, borehole TV, core analysis, scanning electron microscopy, and the Particle and Crack Analysis System. These measurement tools correspond to the meter scale, decimeter scale, centimeter scale, millimeter scale and ten-micrometer scale, respectively, with measured thicknesses of 2.26 m, 2.09 dm, 1.70 cm, 1.48 mm and 11.70 μm, respectively. Fractal theory was used to analyze the power exponent distribution of the lamina thickness, with a resulting fractal dimension of 1.06. Finally, a geometric heterogeneity model was proposed for the Upper Triassic Yanchang Formation in the study area and verified by a modeled thickness of 26.30 m for the Chang 72 Member at the 10-m scale. The model facilitates cross-scale analysis and provides parameter guidance for heterogeneity characterization in the numerical simulation and model test of the shale gas reservoir.

Origin of organic matter and paleo-sedimentary environment reconstruction of the Triassic oil shale in Tongchuan City, southern Ordos Basin (China)

Owing to the lack of conventional oil resources, oil shale from the Triassic Yanchang Formation in the southern Ordos Basin has attracted unprecedented attention. The origin of organic matter and the paleo-sedimentary environment of oil shale are discussed. Based on the results of total organic carbon (TOC), total sulfur (TS), rock pyrolysis, organic elements, vitrinite reflectance (Ro), biomarkers, major elements, and trace elements of oil shale from Yishicun Profile, Tongchuan City. The oil shale from the Chang 7 sub-unit has high TOC (0.29–24.68%) and low TS content (0.13–0.61%), exhibiting middle-grade criterion and extra-low sulfur quality. The Ro, together with discrimination diagrams of C29steraneββ/(ββ+αα)-αααC29sterane20S/(20S+20R) and OEP-CPI, suggests that the oil shale has reached maturity. C27−C29 regular steranes distribution, (C19+C20tricyclic terpane)/C23tricyclic terpane, and C23tricyclic terpane/(C23tricyclic terpane+C30hopane) all indicate that the parent materials of the organic matter in the oil shale are algae, phytoplankton, and terrestrial plants. The Pr/Ph, V/Cr, V/(V+Ni), U/Th, δU, and TOC-TS-TFe2O3 ternary diagrams indicate that oil shale mainly deposits in reducing environment with a relatively poor preservation conditions. TS content, gammacerane/αβ-C30hopane, gammacerane/0.5C31αβ(22R+22S), and Equivalent Boron accurately explain that marine transgression did not occur and the paleosalinity of the lake indicates predominantly fresh water. The paleoproductivity of the lake during oil shale sedimentation is very high in terms of U and Mo concentrations and Ba/Al ratio. After quantitative calculation, the lake is classified as an extremely eutrophic lake. The water depth of oil shale sedimentation varies from 26.00 to 108.6m, indicating a semi deep- deep lake.

Influence on lacustrine source rock by hydrothermal fluid: a case study of the Chang 7 oil shale, southern Ordos Basin

Hydrothermal fluid activity during sedimentation of the Triassic Yanchang Formation in the Ordos Basin and the impact of said activity on formation and preservation conditions of source rocks have received little attention. Oil yield, major element, trace element, rare earth element, and total sulfur (TS) data from the oil shale within the Yanchang are here presented and discussed in the context of hydrothermal influence. Oil shale samples returned relatively high total organic carbon (TOC), in the range of 4.69%–25.48%. A high correlation between TS and TOC suggests TS in the oil shale is dominated by organic sulfur and affected by organic matter. The low Al/Si ratio of oil shale samples implies quartz is a major mineralogical component. Si/(Si + Al + Fe) values suggest close proximity of the oil shale to a terrigenous source. δEu; Fe versus Mn versus (Cu + Co + Ni) × 10; and SiO2/(K2O + Na2O) versus MnO/TiO2, Fe/Ti, and (Fe + Mn)/Ti are evidence of hydrothermal fluid activity during oil shale sedimentation, and δU and U/Th of the oil shale indicate reducing conditions. The Sr/Ba of oil shale samples suggests fresh-water deposition. The high correlations of Fe/Ti and (Fe + Mn)/Ti with δU, U/Th, and TS demonstrate that hydrothermal fluid activity promotes reducing conditions. Sr/Ba ratios had low correlation with Fe/Ti and (Fe + Mn)/Ti, implying that hydrothermal fluid activity had little impact on paleosalinity. Fe/Ti, (Fe + Mn)/Ti, δU, U/Th, and Cu + Pb + Zn all exhibited high positive correlation coefficients with TOC in oil shale samples, suggesting that more intense hydrothermal fluid activity improves conditions in favor of formation and preservation of organic matter.

Elemental characteristics and paleoenvironment reconstruction: a case study of the Triassic lacustrine Zhangjiatan oil shale, southern Ordos Basin, China

Using trace elements to reconstruct paleoenvironment is a current hot topic in geochemistry. Through analytical tests of oil yield, ash yield, calorific value, total sulfur, major elements, trace elements, and X-ray diffraction, the quality, mineral content, occurrence mode of elements, and paleoenvironment of the Zhangjiatan oil shale of the Triassic Yanchang Formation in the southern Ordos Basin were studied. The analyses revealed relatively high oil yield (average 6.63%) and medium quality. The mineral content in the oil shale was mainly clay minerals, quartz, feldspar, and pyrite; an illite–smectite mixed layer comprised the major proportion of clay minerals. Compared with marine oil shale in China, the Zhangjiatan oil shale had higher contents of quartz, feldspar, and clay minerals, and lower calcite content. Silica was mainly in quartz and Fe was associated with organic matter, which is different from marine oil shale. The form of calcium varied. Cluster analyses indicated that Fe, Cu, U, V, Zn, As, Cs, Cd, Mo, Ga, Pb, Co, Ni, Cr, Sc, P, and Mn are associated with organic matter while Ca, Na, Sr, Ba, Si, Zr, K, Al, B, Mg, and Ti are mostly terrigenous. Sr/Cu, Ba/Al, V/(V + Ni), U/Th, AU, and δU of oil shale samples suggest the paleoclimate was warm and humid, paleoproductivity of the lake was relatively high during deposition of the shale—which mainly occurred in fresh water—and the paleo-redox condition was dominated by reducing conditions. Fe/Ti ratios of the oil shale samples suggest clear hydrothermal influence in the eastern portion of the study area and less conspicuous hydrothermal influence in the western portion.

The net primary productivity of Mid-Jurassic peatland and its control factors: Evidenced by the Ordos Basin

Using the large-scale thick 4# coal seam from the Mid-Jurassic in the southern Ordos Basin as an example, this paper studied the net primary productivity (NPP) level of the Mid-Jurassic peatland, and discussed its control factors. Geophysical logging signals were used for a spectrum analysis to obtain the Milankovitch cycle parameters in coal seam. These were then used to calculate the accumulation rate of the residual carbon in 4# coal seam. The carbon loss can be calculated according to the density and residual carbon content of 4# coal seam. Then, the total carbon accumulation rate of the peatland was further derived, and the NPP of peatland was determined. The results show that the NPP of Mid-Jurassic peatland is higher than that of Holocene at the same latitude. Comprehensive analysis indicates that the temperature, carbon dioxide and oxygen levels in atmosphere are the main control factors of the NPP of Mid-Jurassic peatland.

Calibration of the mudrock compaction curve by eliminating the effect of organic matter in organic-rich shales: Application to the southern Ordos Basin, China

The mudrock log-derived compaction curve is a significant tool for investigating the primary migration of hydrocarbon, predicting fluid overpressure, estimating formation erosion thicknesses and restoring the buried history and paleo-structure of a basin. However, the presence of kerogen in organic-rich shales can create typically high logging values of the acoustic transit time. Thus, the abnormally high values of the acoustic transit time for organic-rich rocks may not truly reflect the porosity variations of subsurface rocks, leading to great uncertainties in the understanding of the mudstone compaction and a certain amount of error in the abnormal fluid pressure estimation when using the mudrock log-derived compaction curve. Therefore, it is necessary to recalibrate the mudstone compaction curve by eliminating the increment of the acoustic transit time caused by the kerogen content of organic-rich mudstones. Taking the southwest Ordos Basin as an example, this paper presents a new equivalent volume model based on the composition of organic-rich shale in which the kerogen content is also considered. Based on the quantitative relationship between the rock composition and the acoustic transit time, a quantitative formula for calculating the acoustic transit time increment caused by the kerogen is derived. This formula shows that the increment depends not only on the organic content but also on the occurrence state, pore size, pore fluid composition and other factors. X-ray diffraction (XRD) data were used to determine the main mineral composition of the mudstone and to calculate the acoustic transit time of the rock skeleton. Then, the mudstone compaction curve in the Zhenjing area was calibrated by combining the measured porosity and total organic carbon (TOC) of the mudstone based on the correction formula. The compaction characteristics varied significantly between before and after the calibration. The slope of the normal compaction trend (NCT) line decreased by 30–55%, and the acoustic transit time deviation from the NCT in the undercompaction interval decreased significantly. The overpressure at the maximum burial depth estimated by the equivalent depth method is in better agreement with the results obtained by numerical simulation after the calibration, and the porosity determined from the well log after the calibration is also closer to the true measured value. The method proposed in this paper is of great significance for improving the reliability and accuracy of compaction research on organic-rich mudstones, especially for the accurate estimation of abnormal pressure in the source rock layer. Additionally, it can be used as an effective reference for mudstone compaction studies in similar geological settings areas or basins.

Seismic expression of delta to deep-lake transition and its control on lithology, total organic content, brittleness, and shale-gas sweet spots in Triassic Yanchang Formation, southern Ordos Basin, China

The Triassic Yanchang Formation is currently a hot spot for lacustrine shale-gas exploration in the Ordos Basin, China. A seismic-sedimentological study using a newly acquired 3D survey was conducted to advance the interpretation and prediction of the shale-gas reservoirs. The seismic volume was converted into a log-lithology volume at the thin-bed level. The inverted acoustic impedance volume was stratal sliced to extract seismic geomorphologic information. Deltaic facies and depositional history were interpreted using lithologic and planform morphology of seismic attributes on stratal slices. Detailed core analysis data including total organic content (TOC), mineral composition, and mechanical properties of sandstones and shales were correlated with seismic-derived lithology and facies. Microseismic data were evaluated for reservoir performance in production wells. Finally, shale-gas sweet spots, or optimal drilling targets, were predicted by mapping relatively high-brittleness lithofacies in 3D using seismic attributes. In the three submembers of the Chang 7 member of the Yanchang Formation, the high-TOC Ch7-1 shale unit is not ideal for shale-gas development because of its ductile nature and tunnel effect when fracturing. Ch7-2 prodelta sediments are shale-gas sweet spots because they are thick, relatively brittle, and in close proximity to high-TOC Ch7-1.

Study on the distribution of extractable organic matter in pores of lacustrine shale: An example of Zhangjiatan Shale from the Upper Triassic Yanchang Formation, Ordos Basin, China

Shale oil and gas have been discovered in the lacustrine Zhangjiatan Shale in the southern Ordos Basin, China. To study the distribution of extractable organic matter (EOM) in the Zhangjiatan Shale ([Formula: see text] ranges from 1.25% to 1.28%), geochemical characterization of core samples of different lithologies, scanning electron microscope observations, low-pressure [Formula: see text] and [Formula: see text] adsorption, and helium pycnometry were conducted. The content and saturation of the EOM in the pores were quantitatively characterized. The results show that the distribution of the EOM in the shale interval is heterogeneous. In general, the shale layers have a higher EOM content and saturation than siltstone layers. The total organic content and the original storage capacity control the EOM content in the shale layers. For the siltstone layers, the EOM content is mainly determined by the original storage capacity. On average, 75% of the EOM occurs in the mesopores, followed by 14% in the macropores, and 11% in the micropores. The EOM saturation in the pores decreases with the increase in pore diameter. The distribution of EOM in the shale pores is closely related to the pore type. Micropores and mesopores developed in the kerogens and pyrobitumens and the clay-mineral pores coated with organic matter are most favorable for EOM retention and charging.

Tectonic sequences of Triassic strata in the southern Ordos Basin, China

Tectonic sequences of Triassic strata in the southern Ordos Basin, China

Fracture prediction in the tight-oil reservoirs of the Triassic Yanchang Formation in the Ordos Basin, northern China

It is important to predict the fracture distribution in the tight reservoirs of the Ordos Basin because fracturing is very crucial for the reconstruction of the low-permeability reservoirs. Three-dimensional finite element models are used to predict the fracture orientation and distribution of the Triassic Yanchang Formation in the Longdong area, southern Ordos Basin. The numerical modeling is based on the distribution of sand bodies in the Chang 71 and 72 members, and the different forces that have been exerted along each boundary of the basin in the Late Mesozoic and the Cenozoic. The calculated results demonstrate that the fracture orientations in the Late Mesozoic and the Cenozoic are NW–EW and NNE–ENE, respectively. In this paper, the two-factor method is applied to analyze the distribution of fracture density. The distribution maps of predicted fracture density in the Chang 71 and 72 members are obtained, indicating that the tectonic movement in the Late Mesozoic has a greater influence on the fracture development than that in the Cenozoic. The average fracture densities in the Chang 71 and 72 members are similar, but there are differences in their distributions. Compared with other geological elements, the lithology and the layer thickness are the primary factors that control the stress distribution in the study area, which further determine the fracture distribution in the stable Ordos Basin. The predicted fracture density and the two-factor method can be utilized to guide future exploration in the tight-sand reservoirs.

Comparative Studies of Three Nonfractured Unconventional Sandstone Reservoirs with Superlow Permeability: Examples of the Upper Triassic Yanchang Formation in the Ordos Basin, China

Unconventional sandstone reservoirs with ultralow permeability (1 mD < Kair < 10 mD, where Kair represents the permeability in air) and superlow permeability (0.1 mD < Kair < 1 mD) in China are widely distributed in major basins and have significant economic potential. (Here, D is a permeability unit: 1D = 1000mD = 1 μm2.) A comparative study has been performed here on three Upper Triassic sandstone reservoirs with superlow permeability in the southern Ordos Basin to understand the characteristics of each reservoir, the differences in reservoir porosity and permeability, and their causes. The study shows that pore type is the main factor that controls the physical property of nonfractured, superlow permeability sandstone reservoirs in the area of study. The main pore type observed in all three reservoirs is dissolution-enhanced intergranular, which comprises a maximum of 70% of the total porosity. Remnant intergranular and intragranular dissolution pores are also observed in these reservoirs. The pore thr...

Rayleigh wave phase velocities of South China block and its adjacent areas

Using records of continuous seismic waveforms from 609 broadband seismic stations in the South China Block and its adjacent areas in 2010–2012, empirical Green’s functions of surface waves were obtained from cross-correlation functions of ambient noise data between these stations. High quality phase velocity dispersion curves of Rayleigh waves were obtained using time-frequency analysis. These interstation dispersion curves were then inverted to build Rayleigh wave phase velocity maps at periods of 6–50 s. The results of phase velocity maps indicate that phase velocities at 6–10 s periods are correlated with the geological features in the upper crust. Major basins and small-scale grabens and basins display slow velocity anomalies; while most of the orogenic belts and the fold belts display high velocity anomalies. With the gravity gradient zone along Taihang Mountain to Wuling Mountain as the boundary for the phase velocity maps at period of 20–30 s, the western area mainly displays low velocity anomalies, while the eastern side shows high velocity anomalies. Phase velocities in the eastern South China Block south to the Qinling-Dabie orogenic belt is higher than that in the eastern North China Block to the north, which is possibly due to the differences of tectonic mechanisms between the North China Craton and the South China Block. The phase velocities at periods of 40–50 s are possibly related to the lateral variations of the velocity structure in the lower crust and upper mantle: The low-velocity anomalies in the eastern part of the Tibetan Plateau are caused by the thick crust; while the Sichuan Basin and the southern part of the Ordos Basin display distinct high-velocity anomalies, reflecting the stable features of the lithosphere in these blocks. The lateral variation pattern of phase velocities in the southern part of the South China Block is not consistent with the surface trace of the block boundary in the eastern Yunnan Province and its vicinities. The phase velocities in the Sichuan Basin are overall slow at short periods and gradually increase with period from the central part to the edge of the basin, indicating the features of shallower basement in the center and overall stable lithospheric mantle of the basin. The middle and upper crust of the southern Ordos Basin in the North China Block is heterogeneous, while in lower crust and the uppermost mantle the phase velocities mainly exhibit high anomalies. High-velocity anomalies are widespread at the middle of the Qinling-Dabie orogenic belt, as well as the areas in southeastern Guangxi with Caledonian granite explosion, but its detailed mechanism is still unclear.

Shear fracture direction and mechanical characteristics of the middle Yanchang Formation, southern Ordos Basin, China

Field observations, fractures identified from micro-resistivity logging images from six wells, and seismic anisotropy analysis all show that six sets of vertical shear fractures are developed in the middle Yanchang Formation in the southern Ordos Basin, comprising three orthogonal fracture systems. Of these, System I fracture sets predominate, trending approximately 75° and 150°, the result of NWW-SEE directed maximum compressive stress. System II fracture sets trend approximately 60° and 120°, the result of E-W directed maximum compressive stress, while System III fracture sets trend approximately 30° and 90°, the result of NEE-SWW directed maximum compressive tress. Fractures with a strike of 75° are most common in the middle Yanchang Formation because of rock heterogeneity, while three reservoir sets are present, comprising fluvial medium-to-fine grained blocky sandstones and thin interbeds of sandstone and shale from lacustrine-delta facies.

Effect of sedimentary heterogeneities on hydrocarbon accumulations in the Permian Shanxi Formation, Ordos Basin, China: Insight from an integrated stratigraphic forward and petroleum system modelling

Sedimentary heterogeneities are ubiquitous in nature and occur over a range of scales from core, reservoir to basin scales. They may thus exert significant influences on hydrocarbon generation, migration and accumulation. The sedimentary heterogeneities of the Permian Shanxi Formation in the Ordos Basin, China were modelled using Sedsim, a stratigraphic forward modelling program. The simulation results were then used to construct a 3D petroleum system model using PetroMod. The effects of sedimentary heterogeneities on hydrocarbon accumulations were evaluated by comparing the integrated Sedsim-PetroMod model with the classic 3D basin model. The Sedsim simulation shows that considerable sedimentary heterogeneities are present within the Shanxi Formation, as a result of the interplay of the initial topography, tectonic subsidence, base level change and sediment inputs. A variety of lithologies were developed both laterally and vertically within the Shanxi Formation at kilometre and metre scales, respectively, with mudstones mainly developed in the depositional centre, while sandstones developed in the southern and northern margin areas. A typical source-ward retrogradation is well developed within the Lower Shanxi Formation.A base-case classic 3D basin model was constructed to quantify the Permian petroleum system in the Ordos Basin. The geological and thermal models were calibrated using Vr and borehole temperature data. The source rocks of the Upper Paleozoic became mature (Ro > 0.5%) and high mature (Ro > 1.2%) in the late Triassic and late Jurassic, respectively, in the central and southern areas. During the Early Cretaceous, a tectonically induced geothermal event occurred in the southern Ordos Basin. This caused the source rocks to reach over maturity (Ro > 2.0%) quite rapidly in the early Late Cretaceous in the central and southern areas. All the source rock transformation ratios (TR) at present are greater than 70% in the P1 coal and P1 mudstone layers with TR values approaching 100% in the central and southern areas. The transformation ratios of the P1 limestone are close to 100% over the entire interval.In the base-case model, a large amount of hydrocarbons appear to have been expelled and migrated into the Shanxi Formation, but only a minor amount was accumulated to form reservoirs. In the model, the Shanxi Formation sandstone layer was set to be homogeneous vertically and there was no regional seal rocks present at the top of the Shanxi Formation. Therefore hydrocarbons could not be trapped effectively with only minor accumulations in some local structural highs where hydrocarbons are trapped both at the top and in the up-dip direction by the adjacent mudstone facies. In contrast, the integrated Sedsim-PetroMod model takes into account of the internal lithological and sedimentary facies heterogeneities within the Shanxi Formation, forming complex contiguous sandstone-mudstone stacking patterns. Hydrocarbons were found to have accumulated in multiple intervals of lithological traps within the Shanxi Formation. The results indicate that lithological distinctions, controlled by sedimentary heterogeneities in three dimensions can provide effective sealing in both the top and up-dip directions for hydrocarbon accumulations, with gas being mainly accumulated near the depocentre where lithological traps usually formed due to frequent oscillations of the lake level.

Provenance and sediment dispersal of the Triassic Yanchang Formation, southwest Ordos Basin, China, and its implications

The Ordos Basin in north central China records a transition from marine to non-marine deposition during the late Paleozoic to early Mesozoic. As a result, the northern and southern regions of the Ordos Basin show different tectonic histories and very distinctive sedimentation styles. Two deformation belts, the Qinling orogenic belt to the south and the Liupanshan thrust and fold belt to the west, controlled the structural evolution of the southern Ordos Basin during the early Mesozoic. Paleocurrent analysis, net-sand ratio maps, sandstone modal analysis, and U–Pb detrital zircon geochronology were used to document sediment sources and dispersal patterns of the Triassic Yanchang Formation in the southwest Ordos Basin. Paleocurrent measurements suggest that the sediments were mainly derived from the Liupanshan and the Qinling orogenic belts. Net-sand ratio maps show that several fan delta systems controlled sediment delivery in the south Ordos Basin. Both sandstone modal analysis and U–Pb detrital zircon geochronology suggest that the Yanchang Formation is locally sourced from both of the basin marginal deformation belts; the lower and middle sections are recycled Paleozoic sedimentary rocks mainly derived from the north Qinling orogenic belt, whereas for the upper section, the Qilian–Qaidam terranes and possibly the west Qinling orogenic belt began to shed sediments into the southwest Ordos Basin. Results have important implications for basin marginal tectonics and its controls on sedimentation of intracratonic basins in China and similar settings.

Re-discussion on the detrital zircon provenance of the lower Yanchang Formation in the southern Ordos Basin

AbstractThe Ordos Basin is the second largest sedimentary basin in China. The Yanchang Formation is the key oilbearing layer in the Ordos Basin. The stratigraphic time interval and the stratigraphic division of the Yanchang Formation has been highly debated with estimates ranging from Middle Triassic to Late Triassic. According to the latest studies on the stratigraphical division of Yanchang Formation, it was considered to be deposited as early as the Middle Triassic. Based on this new understanding, we reexamined the previous studies of the detrital zircons from the lower Yanchang Formation. The detrital zircons from the lower Yanchang Formation were divided into three groups based on their U-Pb ages: Paleozoic, Paleoproterozoic, and Neoarchean. The lack of Neoproterozoic U-Pb ages indicates no input from either the Qinling Orogen or the Qilian Orogen. The two older age groups (Paleoproterozoic, and Neoarchean) are likely derived from the North China Craton basement. The Paleozoic zircons were derived from the Inner Mongolia Paleo-uplift. The lower Yanchang Formation was mainly derived from the Inner Mongolia Paleo-uplift instead of being recycled from the previous sedimentary material from the central-eastern North China Craton as was previously hypothesized.

Palynofacies and geochemical analysis of the Triassic Yanchang Formation, Ordos Basin: Implications for hydrocarbon generation potential and the paleoenvironment of continental source rocks

The petroliferous basins that have formed in China since the Mesozoic are characterized by non-marine organic matter input, a characteristic that is significantly different from that of marine organic sources. The Triassic Yanchang Formation in the Ordos Basin is a suite of typical intra-continental lacustrine sediments that comprise the most important source rocks for the Triassic oil reservoirs. The Yanchang Formation is divided into ten subsections from top to bottom (Chan 1 to Chan 10). Three borehole successions in the southern Ordos Basin (located in the Huachi, Zhidan and Yichuan areas) cross the Chan 4+5 to Chan 10 subsections and have been studied using the palynofacies method combined with organic geochemistry data. Palynofacies analysis indicates that the sediments are rich in amorphous organic matter (AOM) and phytoclasts. The AOM content in all of the samples positively correlated with the hydrogen index (HI). In contrast, the transparent ligno-cellulosic fragments (TLF)+opaque particles (OP) content are negatively correlated with the HI values. Additionally, the gelified particles (GP) content has no linear correlation with the geochemistry data. Based on the quantitative composition of the particulate organic matter, three palynofacies types are identified, reflecting depositional settings in a distal dysoxic–anoxic deep basin, a shelf-to-basin transition zone and a proximal suboxic shelf. The palynofacies, total organic carbon (TOC), and Rock-Eval data together indicate that type I and II kerogen are abundant in the D48 and W22 wells in the Zhidan and Yichuan areas, respectively. These kerogen types are uncommon in the L94 well in the Huachi area, which suggests low hydrocarbon generation potential. In detail, the Chan 7 and Chan 9 subsections of the three wells contain abundant type I and II kerogen, indicating that these layers are likely the two primary source rocks in the southern Ordos Basin.

The role of the residual bitumen in the gas storage capacity of mature lacustrine shale: A case study of the Triassic Yanchang shale, Ordos Basin, China

The role of the residual bitumen in the shale gas storage capacity is investigated for potential terrestrial shale gas reservoirs in the Yanchang area, southern Ordos Basin, China. The Upper Triassic organic-rich Yanchang shales comprise of thermally mature Chang 7 Shale (average TOC 5.12 wt.%, Type I–II1 kerogen, Tmax 443–458 °C, Ro 0.83%–1.02%) and relatively mature Chang 9 Shale (average TOC 4.40 wt.%, Type I–II1 kerogen, Tmax 443–476 °C, Ro 0.88%–1.10%). The mineralogy of Yanchang shales is dominated by clay minerals (average 48.85%) and quarts (average 31.7%). A series of low pressure nitrogen adsorption/desorption and high pressure methane sorption experiments were conducted on pretreated fresh drilling cores (including three groups of core samples: one original group, one group extracted by dichloromethane and the other group extracted by trichloromethane) selected from Yanchang shales to demonstrate the role of the residual bitumen in the shale gas potential. Low pressure nitrogen sorption method was used to elucidate the effect of the residual bitumen on the pore structure of Yanchang Shales. The results show that the specific surface area and pore volume and pore surface area of shale samples after the extraction greatly increase and their growth was closely related with pores of >30 nm and <10 nm respectively. A negative correlation between the surface area and TOC was observed widespread in Yanchang shales, indicating that the residual bitumen that blocks the pores and pore-throats dramatically influences the methane sorption capacity in the mature shales. Based on nitrogen adsorption/desorption isotherms, ink-bottle-shaped micropores and mesopores likely acting as narrow necks of those pores well developed and were most likely influenced by the residual bitumen, which is favorable for adsorption accumulation but disadvantageous for the seepage of shale gas. The methane sorption isotherms measured on moisture-equilibrated shale samples suggest that the dissolution in the residual bitumen could be an important gas storage mechanism in Yanchang low mature lacustrine shales.