Triassic Yanchang Formation Research Articles

Pore structure and its control on reservoir quality in tight sandstones: a case study of the Chang 6 member of the Upper Triassic Yanchang Formation in the Jingbian oilfield in the Ordos Basin, China

This study applied vacuum-impregnated casting thin sections, fluorescence slices, scanning electron microscopy (SEM), pressure-controlled mercury porosimetry (PCP), rate-controlled mercury porosimetry (RCP), X-ray diffraction of clay minerals, overburden pressure, and conventional physical property strategies to determine the microscopic characteristics of the Chang 6 member, a typical tight sandstone reservoir in the Jingbian oilfield in the Ordos Basin, China. We also analyzed the controlling effects of pore structure on reservoir quality and oiliness. The results showed that the pore types of the Chang 6 sandstone reservoir can be divided into four categories: residual intergranular pores, dissolution pores, intercrystalline pores between clay minerals, and microfractures. The pore size of the Chang 6 sandstone reservoir ranged from 20 to 50 μm. We employed PCP and RCP strategies to characterize the pore structure of the Chang 6 reservoir. The pore radius was less than 2 μm, and on average, the throat radius was less than 0.3 μm. The reservoir physical properties were affected by diagenesis, particularly compaction, and the average porosity failure rate was 56.3%. Cementation made the reservoir more compact, dissolution improved the physical properties of the reservoir locally, and fracturing effectively improved the reservoir seepage ability; however, its influence on porosity was limited. The pore structure controlled the quality of the reservoir. The physical properties of the reservoir were closely related to the oil-bearing properties. The lower limits of porosity and permeability of industrial oil flow in the reservoir were 7.5% and 0.15 mD, respectively. These results provide an additional resource for the exploration and development of tight oil in the Ordos Basin.

Paleo-depositional environment, origin and characteristics of organic matter of the Triassic Chang 7 Member of the Yanchang Formation throughout the mid-western part of the Ordos Basin, China

A comprehensive geochemical and petrographic study was performed on 23 clay-rich shale samples collected from 18 wells drilled in the mid-western part of the Ordos Basin to investigate the characteristics and thermal maturity of organic matter as well as the paleo-depositional environment of the Chang 7 Member within the Triassic Yanchang Formation. The samples are characterized by moderate to very high organic carbon (TOC) contents ranging from 0.24% to more than 24.83% (2.72% on average) and total sulfur (TS) contents of 0.06% to 6.51%. Rock-Eval S1 and S2 values range from 0.13 to 3.70 mg HC/g rock and 0.62 to 92.50 mg HC/g rock, respectively, implying good to very good hydrocarbon generation potential for most of the samples. Rock-Eval data and petrographical observations reveal that the kerogen is mainly composed of type II to type III, consisting of a mixture of aquatic and terrigenous organic matter. The aliphatic fractions show variable monomodal to bimodal distributions of normal alkanes. Different parameters such as TOC/TS and molecular biomarker data imply that the Chang 7 member is a typical clay-rich source rock deposited in a sulphate-poor lacustrine to fluvio-deltaic environment under oxic to sub-oxic conditions with substantial input of organic matter derived from higher land plants, mainly conifers. Low values of gammacerane further indicate a low salinity of the paleo-lake. Average Tmax values of 445 °C, vitrinite reflectance values between 0.68% and 0.88%, biomarker parameters, such as sterane ratios of C29 ααα 20S/(20S + 20R) and C29 ββ/(ββ + αα), and maturity indices based on aromatic compounds, indicate a thermal maturity within the oil window for the studied samples, showing some regional variability within the mid-western Ordos Basin.

In situ S isotope analysis and source tracing of pyrite from lacustrine hydrothermal sedimentary rocks: the Chang 7-3 sub-member, Triassic Yanchang Formation, Ordos Basin

The Ordos Basin developed a large depression-type lake basin in the Late Triassic. Here, we report on pyrites from lacustrine hydrothermal sedimentary rocks that were found at the bottom of a black shale layer from the Chang 7-3 sub-member of the Triassic Yanchang Formation in Tongchuan and Yaoqu and Well Z9 sections, on the southern margin of the Ordos Basin. Observations of field outcrops and underground cores were used to identify vein and brecciated pyrites that are distributed within water-explosive dolomite breccias or sandwiched in a textured layer of hydrothermal sedimentary rock. Based on previous work, scanning electron microscopy observations, electronic probe microanalysis, nano-SIMS trace-element mapping and in situ sulfur isotope analyses were completed. The crystal form of pyrites is mainly cubic with surfaces mostly smooth and clean. Pyrites contain trace elements, such as As, Au, Co, Cu, Cr and Zn. The pyrite sulfur isotope values are enriched in 34S (δ34SV-CDT = 7.31–10.05‰; average 8.49‰) and show that the pyrite is related to hydrothermal deposition. Late Triassic hydrothermal activity at the bottom of the lake provided sulfate for pyrite formation, and hydrothermal sulfide may have also been an important sulfur source. KEY POINTS Pyrites from lacustrine hydrothermal sediments were formed at the bottom of a black shale layer from the Chang 7-3 sub-member of the Triassic Yanchang Formation on the southern margin of the Ordos Basin. The pyrite contains trace elements such as As, Au, Co and Ni. In situ pyrite sulfur isotope values are enriched in 34S and show that the pyrite is related to hydrothermal deposition with sulfur derived from sulfate and hydrothermal sulfide

Geochemical studies on high-efficiency hydrocarbon expulsion of Triassic lacustrine shale in the Ordos Basin, China: implications for tight and shale oil accumulation

The organic-rich sediments developed at the bottom of Chang7 Member of the Upper Triassic Yanchang formation, which can be divided into oil shale and black mudstones, are dominate source rocks for low permeability and tight oil reservoirs in Mesozoic petroleum system of Ordos Basin. The hydrocarbon expulsion characteristics of the organic-rich sediments have been investigated by geology and geochemistry data. By comparison with the geochemical parameters of crude oil samples and mudstone extracts, Chang7 shale has many geochemical evidences on high rates of hydrocarbon expulsion. Lower conversion rate of chloroform extracted bitumen “A”, especially for the samples with TOC higher than 10%; lower productivity indexes and lower hydrogen indexes reducing with TOC increasing; lower ratios of saturates/aromatics but higher contents of polar components for residual bitumen “A”; heavier stable carbon isotopes of residual hydrocarbons. It should be noted that the source rock evaluation and oil-source correlation would be misunderstood based on these abnormal geochemical data caused by high-efficiency hydrocarbon expulsion. Calculation for rock samples have shown that the expelled rates of the shale samples averaging 70% are significantly higher than those of the mudstone samples. Chang 7 shale with large amounts of hydrocarbons generated and expelled is critical for hydrocarbon accumulation in low-permeability and tight oil reservoirs. Also, Chang7 organic-rich rocks is an important exploration field of shale oil in the future.

Comparison of diagenetic characteristics and pore evolution in outcrops and cores of tight sandstone reservoirs in the Triassic Yanchang Formation, the Ordos Basin, China

The sedimentary and diagenetic control on tight sandstone reservoir quality is a key focus of hydrocarbon exploration. This paper compares reservoir characteristics, such as diagenetic processes, pore types, and porosity evolution, of cores and outcrops of the Yanchang Formation, the Ordos Basin, northwestern China, to identify diagenetic trends and processes responsible for improving reservoir quality and to develop a model for identifying reservoir sweet spots in tight sandstone formations. Samples of the Yanchang Formation sandstone are taken from wells and adjacent outcrops in the Ordos Basin. No significant differences were found in porosity-destructive diagenesis such as compaction and cementation between the core and outcrop samples. However, the effects of dissolution are far more pronounced in cores than in the outcrop, resulting in a higher core sample porosity. High-density sampling of the outcrop revealed that the reservoir sweet-spot model is controlled by sedimentation, whereas three distinct vertical diagenetic zones were identified in adjacent wells. A comprehensive analysis of cores and outcrop samples shows that both areas experienced similar sedimentation and early diagenetic patterns. However, the diagenetic processes experienced by core and outcrop sandstones diverged during the later part of burial and uplift, with dissolution being responsible for the contrasting reservoir porosities between core and outcrop.

Petrography and geochemical characteristics of Chang 7 sandstone in Yanhe profile, Ordos Basin: Implication for paleoenvironment and tectonic background

The Chang 7 oil layer from the upper Triassic Yanchang Formation is an important layer for hydrocarbon exploration. Most studies on the Chang 7 oil layer have focused on the source rocks, while research on the sandstone is still inadequate, especially on the petrography and geochemical characteristics. Using seven sandstone samples of the Chang 7 oil layer in the Yanhe profile, the grain-size analysis, major elements, trace elements, and rare earth elements were tested. The results find that the sandstone of fine-grained sediments of the Chang 7 oil layer is dominated by arkose with a minor number of lithic arkose. The range of grain size (Mz) is from 2.72 to 3.92 Φ, and the C value and M value of the sandstone samples suggest characteristics of turbidity deposition. The Al/Si ratios of all of the samples imply high clay mineral content. The results of trace and rare earth elements demonstrate the reducing condition, freshwater, and cold and dry weather. The provenance of the sandstone samples is mainly from island arc acidic volcanic rock, and the type of provenance is mixed with sedimentary rock, granite, and alkaline basalt. The tectonic background is continental island arc. This study provides a systematic geologic foundation for the formation of sandstone of Chang 7 oil layer in Ordos Basin.

Geochemical characteristics and sedimentary setting of chang 9 shale in the Upper Triassic Yanchang Formation of southeastern Ordos Basin (NW China)

In addition to the organic-rich shale in the Chang 7 Member (Ch7 shale) of the Ordos Basin, the shale hosted in the Chang 9 Member (Ch9 shale) is another source rock and potential unconventional target. However, there is a general lack of consensus on the features and potential of the Ch9 shale. In order to determine the potential of the Ch9 shale, this study analyzes its paleoenvironment and sedimentary setting based on the integrated analysis of geochemistry and sedimentary facies. A comparison of geochemical features between the Ch9 shale and the organic-rich Ch7 shale is made to evaluate the factors controlling organic matter (OM) enrichment of the Ch9 shale. The results show that the Ch9 shale has good hydrocarbon potential, and terrestrial OM is important component of the OM in the Ch9 shale. The Ch9 shale was deposited in climatic transition period from cold and dry to warm and humid, when the water body was suboxic-anoxic and freshwater. The relatively cold and dry climate and suboxic-anoxic water condition in the Ch9 period could not provide the same excellent conditions for OM enrichment as that in the Ch7 period. The Ch9 shale is composed of sediments mixed with felsic rocks that originated from continental island arcs and active continental margins. The active tectonic setting resulted in event deposition (e.g. turbidity flows and hydrothermal fluid activity) during the deposition of Ch9 shale. The input of terrestrial OM carried by turbidity flows also promoted OM enrichment, however, the very high depositional rates caused by turbidity flows diluted the OM, which probably offset the effect of hydrothermal on OM enrichment. Our results are helpful to revealing the source potential of the Ch9 shale, providing a theoretical basis for the shale oil and gas exploration in the Ordos Basin.

Characteristics and controlling factors of LORS from the Chang 7–3 section of the Triassic Yanchang Formation in the Ordos Basin

Lacustrine organic-rich shale (LORS) developed in the Chang 7–3 section of the Triassic Yanchang Formation in the Ordos Basin. LORS has been proven to be a source of crude oil and tight oil, as well as a source and reservoir for shale oil in the basin. A total of 35 LORS samples were collected from Well Z233 and analysed by mineralogy and element geochemistry to reveal the characteristics and controlling factors of LORS. The following results were obtained: ① In the oil shale, the organic matter is composed of type I and type II kerogen, with a predominance of type II. The organic matter is mature and reaches high levels of abundance. ②Through further analysis. The values of w(U)/w(Th), w(V)/w(Cr), w(Ni)/w(Co), the size and morphology of pyrite, indicates that the lake water of the LORS deposition period in the study area is anoxic or suboxic state. the TOC of the samples was found to have significant positive correlations with the w(Mo) and w(Babio) contents. Paleoproductivity can be inferred to be the key factor controlling organic matter enrichment in the LORS. ③Volcanic and hydrothermal activities promote the development of LORS, provide nutrients needed by organisms, promote the growth of organisms, and improve paleoproductivity. Hydrothermal activity leads to the formation of an anoxic environment at the bottom of the lake, and tuff will cover the sediments, which promotes LORS preservation. This study reveals in detail paleoproductivity is the main factor in the formation of LORS during the evolution of the basin. Volcanic and hydrothermal activities promoted the increase of paleoproductivity, and finally the oil shale layers with the richest organic matter have been formed. And thereby deepens our theoretical understanding of organic matter enrichment.

Experimental evaluation of live oil oxidation together with its physical properties during air injection in a tight oil reservoir

In this paper, an integrated and pragmatic framework has been developed to experimentally determine the live oil oxidation with its physical properties and evaluate performance of air injection in a tight reservoir. In addition to continuously measuring interfacial tension, slim tube tests were performed to monitor and quantify miscibility between air and live oil collected from the Upper Triassic Yanchang Formation of the Chang 7 tight oil reservoir in the Ordos Basin, China. The displacement experiments showed that oil recovery factor of air injection is obviously higher than that of nitrogen injection in core plugs collected from the tight oil reservoir, while static and dynamic oxidation experiments were then conducted to identify the inherent oxidation mechanisms. The comparative core displacement experiments of live oil by air and nitrogen injection under reservoir temperature and pressure were carried out to evaluate the enhanced oil recovery (EOR) performance of air injection. It is found that heat released by the oxygen addition reaction under the formation conditions of 60.0 °C and 16.00 MPa leads to the bond scission reaction. The gas generated by the bond scission reaction forms a flue gas displacement front together with the remaining nitrogen contained in the air. Although miscibility cannot be achieved, air injection followed by the oxygen addition reaction and the bond scission reaction can reduce oil viscosity and thus increase its flowability. As for the commonly experimental studies with dead oils excluding porous media, the oxidation degree and the corresponding physical properties are normally overestimated since light components are limited in the dead oils and porous media constrain the corresponding oxidation.

The genesis model of carbonate cementation in the tight oil reservoir: A case of Chang 6 oil layers of the Upper Triassic Yanchang Formation in the western Jiyuan area, Ordos Basin, China

Abstract Carbonate cementation is one of the significant tightness factors in Chang 6 reservoir of the western Jiyuan (WJY) area. Based on the observation of core and thin sections, connecting-well profile analysis as well as carbon and oxygen isotope analysis, it is found that ferrocalcite is the main carbonate cements in the Chang 6 reservoir of the WJY area. The single sand body controls the development of carbonate cements macroscopically. Both carbonate cements and calcite veins hold similar diagenetic conditions: the dissolution of plagioclase is the main calcium source and the de-acidification of organic acids is the main carbon source. The diagenetic stage is identified as the mesogenetic A stage. The sedimentary environment is of low salinity. Accordingly, the development model of carbonate cementation in Chang 6 reservoir is summarized into three types: “eggshell pattern,” “cutting pattern,” and “favorable reservoir pattern.” The development degree of carbonate cementation affects the physical properties of reservoir.

Sedimentary and geochronological studies on tuff levels from Triassic strata in the Nanzhao Basin, North Qinling Orogenic Belt, and their geological significance

The Ordos Basin is the second largest oil‐ and gas‐bearing sedimentary basin in China. This basin formed during the Triassic. At present, the location of the basin margins is still highly debated in the literature, especially its eastern and southern margins. The Triassic Yanchang Formation extensively outcrops in the Ordos Basin, and its distribution can provide a significant constraint on the basin extension. In this study, we focus on the measurement and correlation of the Middle–Late Triassic sedimentary strata from Nanzhao Basin, which is located to the south‐east of the present‐day Ordos Basin. The results of this study indicate that the sedimentary sequence of Triassic strata from Taizishan Formation and Taishanmiao Formation in the Nanzhao Basin is comparable with that of the Yanchang Formation in the Ordos Basin. Tuff intercalations in clastic rocks from Taizishan Formation in the Nanzhao Basin were studied with zircon chronology and Hf isotope analysis. The results show that the tuff sample has concord U–Pb age of 234.6 ± 1.5 Ma, which is considered as the boundary age between Middle and Late Triassic with a similar age in respect to that of the Yanchang Formation. The TDM2 values for the studied sample range from 1.0 to 1.4 Ga with the peak at 1.2 Ga, which is also similar to TDM2 values for tuffs encountered in the Yanchang Formation. On the other hand, the corresponding εHf(t) values of tuff in Nanzhao Basin range from −1.1 to +4.2 with an average of 1.2 and peak at 2, which are different with respect to those of the Yanchang Formation. According to the geochronological data, the tuffs in Nanzhao Basin might be mainly derived from a depleted‐mantle source from Late Meso‐Proterozoic period, and contaminated by a continental Early–Middle Meso‐Proterozoic crust. Therefore, we suggest that in Middle–Late Triassic, the southeastern margin of the Ordos Basin may have extended to Nanzhao area and the different εHf(t) values between tuffs in Nanzhao Basin and present Ordos Basin may be the result of scissor‐type collision of North China Block and South China Block during the Triassic.

Geological characteristics and exploration of shale oil in Chang 7 Member of Triassic Yanchang Formation, Ordos Basin, NW China

Abstract A set of shale-dominated source rocks series were deposited during the heyday of lake basin development in the Member 7 of Triassic Yanchang Formation, Ordos Basin, and the thickness is about 110 m. Aimed at whether this layer can form large-scale oil enrichment of industrial value, comprehensive geological research and exploration practice have been carried out for years and obtained the following important geologic findings. Firstly, widely distributed black shale and dark mudstone with an average organic matter abundance of 13.81% and 3.74%, respectively, lay solid material foundation for the formation of shale oil. Secondly, sandy rocks sandwiched in thick organic-rich shale formations constitute an oil-rich “sweet spot”, the average thickness of thin sandstone is 3.5 m. Thirdly, fine-grained sandstone and siltstone reservoirs have mainly small pores of 2–8 μm and throats of 20–150 nm in radius, but with a large number of micro-pores and nano-throats, through fracturing, the reservoirs can provide good conductivity for the fluid in it. Fourthly, continued high-intensity hydrocarbon generation led to a pressure difference between the source rock and thin-layer reservoir of up to 8–16 MPa during geological history, driven by the high pressure, the oil charged into the reservoirs in large area, with oil saturation reaching more than 70%. Under the guidance of the above theory, in 2019, the Qingcheng Oilfield with geologic oil reserves of billion ton order was proved in the class I multi-stage superimposed sandstone shale reservoir of Chang 7 Member by the Changqing Oilfield Branch through implementation of overall exploration and horizontal well volume fracturing. Two risk exploration horizontal wells were deployed for the class II thick layer mud shale interbedded with thin layers of silt- and fine-sandstones reservoir in the Chang 73 submember, and they were tested high yield oil flows of more than 100 tons per day, marking major breakthroughs in petroleum exploration in class I shale reservoirs. The new discoveries have expanded the domain of unconventional petroleum exploration.

Application of Rare-Earth Elements and Comparison to Molecular Markers in Oil–Source Correlation of Tight Oil: A Case Study of Chang 7 of the Upper Triassic Yanchang Formation in Longdong Area, Ordos Basin, China

The biomarker features of 10 Chang 7 crude oil samples were investigated by gas chromatography–mass spectrometry (GC–MS), and the rare-earth element (REE) compositions of 16 Chang 7 and Chang 8 crude oil samples were determined by inductively coupled plasma–mass spectrometry (ICP–MS) for the first time in Longdong area. Oil–source correlation analysis was improved by biomarkers and REEs. The distribution and relative ratios of a series of biomarker parameters in saturated hydrocarbons and aromatic hydrocarbons of crude oil samples indicate that Chang 7 tight oil has already reached the mature stage. The organic matter mainly comes from lower aquatic organisms of algae, with some contribution of micro-organisms and bacteria, while the forming environment of tight oil is mainly the transitional environment of sub-oxidizing to sub-reducing. The V/(V + Ni) and Ni/Co ratios of crude oil samples suggest that the specific redox conditions of Chang 71 and Chang 72 samples were slightly oxic, while Chang 73 and Chang 8 samples were formed under an anoxic environment. The results of both biomarker-based and REE-based oil–source correlation analysis indicate that Chang 71 and Chang 72 tight oils come from Chang 7 mudstone, while most of the Chang 73 tight oils are from Chang 7 oil shale, with part of mixed from Chang 7 mudstone. This recognition may indicate that Chang 7 mudstone and oil shale are two relatively independent hydrocarbon self-generation and near-storage systems. The analysis results demonstrate that the REE composition in crude oil is an efficient and accurate tool for oil–source correlation in the petroleum system.

Diagenetic Facies Study of Chang 8 Reservoir in Triassic Yanchang Formation in Jiangjiachuan Area of Ordos Basin

Diagenetic Facies Study of Chang 8 Reservoir in Triassic Yanchang Formation in Jiangjiachuan Area of Ordos Basin

Multifractal characteristics of shale and tight sandstone pore structures with nitrogen adsorption and nuclear magnetic resonance

Based on the experiments of nitrogen gas adsorption (N2GA) and nuclear magnetic resonance (NMR), the multifractal characteristics of pore structures in shale and tight sandstone from the Chang 7 member of Triassic Yanchang Formation in Ordos Basin, NW China, are investigated. The multifractal spectra obtained from N2GA and NMR are analyzed with pore throat structure parameters. The results show that the pore size distributions obtained from N2GA and NMR are different, and the obtained multifractal characteristics vary from each other. The specific surface and total pore volume obtained by N2GA experiment have correlations with multifractal characteristics. For the core samples with the similar specific surface, the value of the deviation of multifractal spectra Rd increases with the increase in the proportion of large pores. When the proportion of macropores is small, the Rd value will increase with the increase in specific surface. The multifractal characteristics of pore structures are influenced by specific surface area, average pore size and adsorption volume measured from N2GA experiment. The multifractal characteristic parameters of tight sandstone measured from NMR spectra are larger than those of shale, which may be caused by the differences in pore size distribution and porosity of shale and tight sandstone.

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.

Qualitative and quantitative characterization of multiple factors that influence movable fluid saturation in lacustrine deep-water gravity-flow tight sandstones from the Yanchang Formation, southern Ordos Basin, China

Fluid mobility is one of the most important factors in evaluating the potential for recovering tight oil. However, quantitative effects of multiple factors that influence movable fluid saturation in different pore-throat combinations and their relationships with lithofacies in tight sandstones, especially for lacustrine deep-water gravity-flow deposits, remain controversial due to the strong heterogeneity and complex pore structure of tight reservoirs. Core samples obtained from the Upper Triassic Yanchang Formation in southern Ordos Basin were evaluated by using a variety of techniques, including nuclear magnetic resonance (NMR), scanning electron microscopy (SEM), X-ray diffraction (XRD), pressure-controlled porosimetry (PCP), rate-controlled porosimetry (RCP), impregnated thin sections, and helium porosity and nitrogen permeability measurements. Based on NMR T2 distributions under water-saturated and centrifugal conditions, four types of pore-throat combinations are identified by using the relaxation time thresholds Ts1 and Ts2 from NMR: large intergranular pore-dominated pore-throat combinations (LIP), small intergranular pore-dominated pore-throat combinations (SIP), intragranular pore-dominated pore-throat combinations (IAP), and micropore-dominated pore-throat combinations (MP). Among them, SIP is the dominant pore space favorable for fluid flow. Movable fluid saturation in SIP pore space varies among different lithofacies. The best fluid mobility commonly occurs in fine-grained, cross-bedded sandstones (Sc), whereas the worst fluid mobility usually appears in siltstones to very fine-grained sandstones (Ss). The quantitative effects of a total of 41 factors that influence movable fluid saturation of different pore-throat combinations were investigated through the analysis of the Pearson correlation matrix. The results demonstrate that porosity and permeability mainly affect the movable fluid saturation in the pore space of SIP and MP. In addition, maximum mercury intrusion saturation from PCP (Smax) and tortuosity (λ) are the critical pore structure parameters affecting the movable fluid saturation in the pore space of SIP and MP, whereas the average pore throat radius ratio (η) is the critical factor affecting the movable fluid saturation in LIP pore space. Overall, movable fluid saturation in different pore-throat combinations is characterized mainly by different microscopic pore structure parameters. A general pore network model for different lithofacies with different fluid mobility is established to facilitate assessment of the heterogeneity of tight sandstones and to further guide hydrocarbon exploration and development in similar lacustrine deep-water depositional settings.

Insights of the pore system of lacustrine shales from immature to late mature with the aid of petrology, mineralogy and porosimetry: A case study of the Triassic Yanchang Formation of the Ordos Basin, North China

The study of pore network evolution is an important aspect of tight shale reservoir characterization. Pore spaces in shales consist of three major types: interparticle pores and intraparticle pores associated with mineral matrix and organic-matter pores. In this study, the evolution of pore networks for each pore type in lacustrine shales during thermal maturation was studied based on a suite of 19 Triassic Yanchang Formation shale samples from the Ordos Basin across a maturation gradient from immature to late mature (vitrinite reflectance (Ro) from 0.36 to 1.30%). The results show that different pore types follow systematic evolutions during thermal maturation and that the evolution of pore type and pore size is critically controlled by multiple diagenetic processes such as mechanical compaction, cementation, mineral dissolution, clay mineral transformation, and organic matter thermal maturation. Specifically, at immature and early mature stages (Ro < 0.55%), pores associated with mineral matrix are abundant and have moderate pore sizes. Interparticle pores exhibit preferential orientation along clay platelets. At the mature stage (0.55% < Ro < 1.15%), pores associated with mineral matrix have the smallest size, and interparticle pores become less common and show reduced preferential orientation because of mechanical compaction and cementation. Organic-matter pores start to develop at a maturity level of Ro 0.77% and higher. The total porosity and BET specific surface area show minimum values at Ro 1.12%, mainly because pre-existing matrix-related pores are destroyed by mechanical compaction and filled by migrated bitumen and liquid hydrocarbons, and then they increase when Ro exceeds 1.12%, which likely results from the development of secondary organic-matter nanopores due to the generation and expulsion of gaseous hydrocarbons. An increase in pore size of matrix-related pores at the late mature stage (1.15% < Ro < 1.30%) could be a result of dissolution by organic acid generated during organic matter thermal maturation.

Utilizing macroscopic areal permeability heterogeneity to enhance the effect of CO2 flooding in tight sandstone reservoirs in the Ordos Basin

Carbon dioxide-enhanced oil recovery (CO2-EOR) technology has been proven to be effective approach for enhancing oil recovery and has been used in conventional reservoirs since 1970s. Tight sandstone reservoir has poor petrophysical properties, and the complex diagenetic processes that make the reservoir highly heterogeneous resulted in posing significant challenges to CO2-EOR technology. This paper investigated the effects of macroscopic permeability heterogeneity on the efficiency of CO2-EOR by laboratory experiments and field data analysis. The permeability heterogeneity of the reservoir matrix in the study area was evaluated. The results reveal that the matrix permeability range from 0.001 mD to 15 mD, with a mean approximant 1 mD, Beside common thin bedding laminations, the areal permeability varies significantly in a short distance, which is typical for a tight sandstone reservoir. The Triassic Yanchang Formation (200–1400 m thick) is a major petroliferous stratigraphic section in the Ordos Basin. It consists of complete lacustrine sedimentary cycles with clastic sediments. From bottom to top, it can be divided as Chang 10 to Chang 1 members. The low porosity, low permeability, low oil saturation, anomalously low reservoir pressure, and high reservoir heterogeneity makes application of enhanced oil recovery techniques in this formation challenging. The submerged distributary channel core sample with various permeability from the Chang 4+5 sandstone reservoir in Shanbei area of the Ordos Basin were utilized in the laboratory CO2-EOR experiments to study the impact of heterogeneity on CO2 flooding. The results show that the mode of high–low permeability (i.e., the injection well located in the higher permeability area and the production well located in the low permeability area) can ensure the gas injection capacity, maintain a high pressure level throughout the production period, and increase the oil displacement efficiency, which breaks the conventional understanding. Conventional understanding holds that high permeable pore path or fractures developed on the injection area are prone to CO2 channeling, and high permeable pore path or fractures developed on the production area are conducive to oil flow and output increase. Therefore, oil well fracturing is often carried out in the field, and fracturing of injection well is not recommended. However, our experimental research and field application found that the traditional guiding ideology is not conducive to improve oil recovery, but the high–low permeability model which is put forward in this paper can not only enhance CO2 injectivity, but also is beneficial to high formation pressure level and full interaction between gas and crude oil. It also can be confirmed from oilfield that the application of high-low permeability model with high-heterogeneity reservoir could effectively improve the effect of CO2 flooding in tight sandstone reservoirs.

Impact of detrital composition and diagenesis on the heterogeneity and quality of low-permeability to tight sandstone reservoirs: An example of the Upper Triassic Yanchang Formation in Southeastern Ordos Basin

Reservoir quality is key to hydrocarbon exploration and often poses challenges for the production of low-permeability to tight sandstone reservoirs. In this paper the Yanchang Formation in the southern Ordos basin was characterized by different reservoir types and heterogeneity through comprehensive analysis of petrological features, petrophysical properties, diagenetic sequences and hydrocarbon charging. Eventually we identified the high-quality reservoirs, more importantly, the controlling factors on their petrophysical properties. This study reveals that low-permeability to tight sandstone reservoirs in the study area exhibit strong heterogeneity. Four types of sandstones were identified and classified as calcareous, dark lamiae-rich, oil-bearing and water-bearing ones. Prior to oil charging, dark lamiae-rich sandstones and calcareous sandstones experienced strong compaction and cementation, respectively. Therefore, they are typical of poor petrophysical properties and usually exist as interlayers. On the contrary, although the oil-bearing and water-bearing sandstones underwent complex diagenetic processes, they maintained good petrophysical properties and became high-quality reservoirs. Now that the sediments have different provenances, the high-quality reservoirs in the southern provenance have more ductile grains, typical of compactional porosity loss. As a result, their reservoir properties are relatively inferior. In comparison, as the high-quality reservoirs in the northeast suffered less from compactional porosity loss, they were developed with superior porosity and permeability. But on the downside, the presence of large amounts of authigenic chlorite cements caused some damage to the reservoir quality.