Tight sandstone reservoirs typically contain a wide pore throat sizes ranging from the nano-scale to micro-scale, and have complex pore geometry and pore throat structure. Microscopic pore throat ...structures are the most important factors affecting the macroscopic reservoir quality and fluid flow in tight sandstones. Evaluation and characterization quantitatively the microscopic pore structures, including pore geometry, pore size distribution, and pore connectivity, are of great importance for maintaining and enhancing petroleum recovery. This paper critically reviews the pore throat structures of tight sandstones, as assessed from peer reviewed papers in the literature as well as from the authors' personal experiences, in the particular contexts of comprehensive characterization and description of the entire pore throat structure using various complementary techniques. The depositional controls and diagenetic imprints on reservoir quality and pore structure are firstly discussed. The pore systems including pore throat type, pore geometry, pore size and connectivity, which are related to the depositional attributes and diagenetic modifications, are summarized. Then the theories and procedures of various testing techniques commonly used for pore structure characterization of tight sandstones are reviewed. Additionally, the pore throat structure characteristics in tight sandstones are obtained from various techniques such as MICP, NMR, N2GA and XCT. Pore throat distribution and capillary parameters of tight sandstones are examined, and the relationship between pore throat size distribution and permeability is overviewed. The pore size distribution and 3D pore connectivity are evaluated from NMR and XCT analysis. The NMR spectrum is also linked to the macroscopic performance, and the pore network is determined from N2GA. Then fractal theory is introduced to explain the irregularity and heterogeneity of pore throat structure characteristics, and the models for fractal dimension calculation through various techniques are summarized. Lastly the integration of various techniques is encouraged to fully characterize the entire pore size spectrum in tight sandstones by considering the complex pore structures and limitations of a single experiment in pore throat structure evaluation. This review will provide important insights into the microscopic pore structure characteristics of tight sandstones, and address the gap in comprehensive and quantitative evaluation of the heterogeneity in tight sandstones with complex microscopic pore structures.
Image logs provide important information on lithology, sedimentary textures, paleoflow directions, fractures and in situ stress analysis. Lacking of criteria used for establishing the image log ...facies limits the applications of image logs in sedimentary reservoir interpretation. This paper provides a thoroughgoing review focusing on the recent applications of borehole image logs for sedimentological and structural description and interpretation, and aims to establish image log facies which can provide guidelines in sedimentary reservoir interpretation. This paper firstly summarizes the principles and basic characteristics of various imaging logging tools, and then briefly introduces the pre-processing workflow of the image log data. Then the generated images are used for depth and orientation shifts of cores by calibrating individual sedimentary and structural features. Descriptive and concise image log facies are established based on combinations of image textures including dip type, dip pattern, and color scheme, and the characteristics as well as physical criteria for each individual image facies are summarized. The established image log facies are then interpreted in terms of structural and sedimentological features such as lithology, sedimentary structures, vugs, fractures and faults. The image log facies and its stacking patterns are then used to interpret the lithofacies associations by calibrating with cores and conventional logs. Natural fractures and induced fractures are recognized by image logs, and the principles of breakouts and drilling-induced fractures for in-situ stress analysis are reviewed. Then the applications of image logs in investigation of fracture attitudes and states, as well as in computation of fracture parameters are summarized. The procedures to evaluate fracture effectiveness through image logs are discussed. At last, the application of image logs for structural dip analysis is reviewed, and image logs are used for recognizing faults, fracture sets, and attitudes of stratum through the dip patterns. The basic procedures for paleocurrent reconstruction, which includes dip picking of cross beddings, structural dip determination and structural dip removal, are reviewed. Then the paleocurrent directions of the Lower Cretaceous Bashijiqike Formation in the Kuqa depression were reconstructed, which could help further understanding of the depositional systems. This review will help extend the utility of image logs in interpreting small to large -scale sedimentary and structural features, and bridges the gaps between well log analysis and sedimentary and structural interpretation.
•The principles and basic characteristics of imaging logging tools were summarized.•Descriptive and concise image log facies are established.•The procedures to evaluate fracture effectiveness through image logs are discussed.•Image logs are used for structural dip analysis and paleocurrent reconstruction.
The tight sandstones are characterized by low porosity, low permeability, complex pore structure and strong heterogeneity due to the extensive diagenetic modifications they experienced. Understanding ...of the impact of diagenetic alterations on reservoir quality is crucial to the hydrocarbon exploration and production in tight sandstones. Diagenetic facies, which is the comprehensive description of the diagenesis and diagenetic minerals, determines the formation and distribution of sweet spot. By correlating the diagenetic facies to well log responses, the subsurface distribution of porosity and permeability can be predicted. However, the prediction of diagenetic facies and reservoir quality via well logs in tight sandstones remains a challenging task. This paper critically reviews the impact of diagenesis and diagenetic minerals on reservoir quality in tight sandstones, and establishes a model for prediction of diagenetic facies via well logs, as assessed from peer reviewed papers in the literature as well as from the authors' personal experiences. This review begins with reviewing the impacts of compaction, cementation, dissolution and various types of diagenetic minerals on reservoir quality evolution. The definition and classification schemes of diagenetic facies are then discussed, and the reservoir quality as well as diagenetic evolution sequence of various diagenetic facies is summarized. The same diagenetic facies commonly display similar compositional and textural attributes, matrix and cement, as well as porosity systems. The well log responses (GR, AC, DEN, CNL, and RT) of various diagenetic facies are summarized by the calibration of log values with cores and related thin sections. By translating the diagenetic facies to conventional well logs, a predictable model, which can be used for subsurface reservoir quality prediction, is established. Then the theory of ECS logs is reviewed, and the application of ECS logs in diagenetic facies evaluation is discussed. At last, the quantitative characterization for various type and degree of diagenesis is reviewed, and the subsurface diagenetic facies is predicted by quantitative calculation of the compactional porosity loss, cementational porosity loss and dissolution porosity content via well logs. Correlating the diagenetic facies to well logs provides a powerful tool to predict the distribution of high quality reservoirs in tight sandstones. This review will provide insights into the reservoir quality evaluation and sweet spot prediction via well logs in tight sandstones.
The Fengcheng Formation in the Mahu Sag of the Junggar Basin is composed of a set of fine-grained sedimentary rocks located under a layer of alkaline lacustrine rocks. There are alternating dolomite ...lamina, siltstone and organic matter laminae that show strong heterogeneity. The lamina structure determines reservoir quality and oil-bearing potential in shales. Therefore, the identification of lamina structure is a crucial basis for identifying reserves of shale oil. Identified as massive, layered and laminated, three different types of lamina structures are divided according to core observations in terms of the density of the laminae and the overlapping relationships. High-resolution processing is also performed on dynamic image log data to form slab images for recognizing the various types of lamina structures. Compared to core slab photos, it is believed that slab images offer an excellent thin-layer resolution of up to a millimeter in scale and can effectively identify the lamina structure of the shale reservoir. There are no evident beddings on the core of the massive pattern, and the slab image shows a bright or dark block pattern. There are centimeter-level felsic bands of fine silt sand on the thin-layered sedimentary fabric core, and the slice image shows a band-like pattern. A large number of millimeter-level dolomitic laminae are enriched on the core of the laminated type, and the slab image shows millimeter-level light and dark lamina. Slab images were used for the division and identification of lamina structures in a single well. Then, the relationships between the three types of lamina structures and the two-dimensional nuclear magnetic resonance logs and oil test data were recorded. The results show that massive and laminated pores are dominated by intergranular pores with low porosity and movable oil content. Therefore, only low oil flow can be obtained during an oil test. However, the layered type due to centimeter-level silty bands, T1 and T2, is bimodal with high porosity, movable oil content, and high levels of hydrocarbon production. Therefore, the optimal areas of shale reservoirs are closely related to the layered type. The more significant the proportion of layered types in shale reservoirs, the higher the quality of the reservoir. The present study is intended to identify lamina structure-controlled reservoir quality and oil accumulation and to provide insights into the exploration and development of shale oil.
•Three types of multi-scale lamina structure are divided.•The continuous multi-scale lamina structure type is recognized by slab images.•Relationship between lamina structure and reservoir quality are unraveled.
The deeply buried (5.5–7.1 km) Lower Cretaceous Bashijiqike sandstones are important fractured reservoirs in the Kuqa depression. In these high temperature-high pressure drilling environments, ...oil-based muds are used to limit drilling risks and improve efficiency, making facture detection using resistivity-based image logs difficult. The maximum horizontal stress (SHmax) trends close to North-South(0–10°) based on orientations of the drilling-induced fractures and borehole breakouts. Core descriptions, thin sections and image logs used to describe the fractures in the Bashijiqike sandstone reservoirs show that opening-mode natural fractures are common, and the fractures range from fully open to sealed by calcite and clay minerals. The open fractures are dominantly steeply dipping or vertical and primarily strike north-south. Locally horizontal fractures or fractures with low angle dip are detected.
We compare fracture visibility on image logs acquired in the same KS 2-2-12 key well in oil-based mud and water-based mud. FMI-HD logs successfully detect large (0.1 mm aperture) vertical fractures in oil-based mud better than OBMI image logs, and the two imaging tools have a similar ability in detecting the high dip angle fractures. Since the UBI tool is not affected by oil-based muds, most of the fractures can be picked out on UBI images. Minor negative deviations of bulk density could be observed in fractured zones, and the presence of fractures obviously causes an increase in sonic transit time. Full waveforms from DSI tools combined with electrical image logs detect fractured zones. Both the velocity and amplitude of the sonic waves are sensitive to fractures, and the fractured zones could be identified according to the attenuation of Stoneley waves. The combination of cores, image logs, and full-wave sonic logs offers comprehensive detection of fractures in oil-based mud environments.
•The maximum horizontal stress SHmax trends close to North-South.•Most of the fractures trend parallel to SHmax, and have a good connectivity.•FMI-HD and OBMI can detect fractures with high dip angle in oil based mud.•Low dip angle fractures are difficult to be detected by FMI-HD and OBMI.•Fracture in oil based mud is detected by a combination of image and sonic log.
Frequent droughts induced by climate warming have caused increasing impacts on the vegetation of the Loess Plateau (LP). However, the effects of drought on vegetation are highly dependent on when the ...drought occurs and how long it lasts during the growing season. Unfortunately, most of the existing drought indices ignore the differences in the drought effects on different vegetation growth stages. In this study, we first established a phenology-based vegetation condition index, namely weighted vegetation condition index (WVCI), which accounts for the differences in vegetation sensitivity to drought by assigning specific weights to different phenological stages of vegetation. Then, we used the WVCI to reveal the temporal and spatial variations in vegetative drought from 2001 to 2019 over the LP from the aspects of drought frequency, trend and relative deviation. The results showed that (1) the LP experienced frequent droughts during the study period, but mainly mild and moderate droughts. The drought frequencies decreased from southeast to northwest, and extreme droughts rarely occurred in mountainous areas and plains. (2) The droughts in most areas of the LP tended to ease, and only a few areas in the Hetao Plain, Ningxia Plain and Fenwei Plain showed an increasing trend of drought. (3) After 2012, the departure percentage of WVCI in most areas of the LP was positive, indicating above-average vegetation conditions. (4) Compared with the well-established vegetation condition index, the WVCI proved to have the ability to monitor and assess vegetative drought on an annual scale in the LP. As a result, our research could help develop and implement drought-resistance and disaster-prevention measures on the LP.
The complex tectonic activities and multi-stage diagenetic modifications lead to the formation of a heterogeneous pore network spectrum ranging from macropores (centimeter-size) to nanometer-size ...pores (<1 μm) in the deep Cambrian to Lower Ordovician dolostones in Tarim Basin, China. Thin section and Computed Tomography (CT) scanning were performed to obtain the petrography of dolomites, 2-D images of wide variety of pores and 3-D representation of the pore networks. Nuclear Magnetic Resonance (NMR) transverse relaxation time (T2) distributions at saturated and centrifugal conditions were performed to obtain the continuous pore size distribution, and derive NMR parameters including Bulk Volume of Immovable fluid (BVI), Free Fluid Index (FFI), T2gm (T2 weighted mean on a logarithmic scale), and T2cutoff (transition T2 value separating BVI from FFI).
Three types of matrix dolomites (dolomicrite, microbial dolomite and crystalline dolomites) and two types of dolomite cements (saddle dolomite or crystalline dolomites) are recognized in the Cambrian to Lower Ordovician dolostones. The pore systems include open vugs (enlarged dissolution pores), fabric dissolution pores, intercrystal (dissolution) pores, micro-fractures, as well as intracrystal pores, forming dual or multiple pore networks. The NMR T2 spectra are mainly characterized by bi-modal distributions and mostly right-skewed. The very low T2 components (<50 ms) are associated with the intracrystalline pores with small pore sizes (<10 μm). The left peaks with T2 components ranging from 50 ms to 500 ms relate to the intercrystal and intercrystal dissolution pores with pore sizes ranging from several microns to about 100 μm. The right peaks (T2 > 1000 ms) represent the vuggy pores with pore sizes spanning from hundreds of μm to thousands of microns. The transition zones (100 ms < T2 components <1000 ms) referred to the fabric dissolution pores with pore sizes spanning from tens of microns to about 200–300 μm. The micro-fractures have tail T2 distributions. The various pore types are identified in the whole-diameter core CT scanning and FMI (Fullbore Formation MicroImager) image logs. Fluorescent thin sections confirm the favorable oil-bearing potential in intercrystal and intercrystal dissolution pores as well as fabric dissolution pores and micro-fractures. The continuous distribution of pore networks in dolostones can be fully characterized by using multiple-approaches. This study highlights the challenge to describe the various pore types covering several orders of magnitude in size in the heterogeneous dolostones, and will support the hydrocarbon recovery enhancing in dolostones worldwide.
•The pore types, pore throat structure characteristics are revealed by thin section, NMR and CT analysis.•We describe the various pore types spanning several orders of magnitudes in pore size in the dolostones.•The various pore systems are calibrated with whole-diameter core CT and image logs.
Nowadays environmental issues have been of great concern to the world, among which the problem of global warming caused by greenhouse gas emissions is particularly prominent. All countries in the ...Kyoto Protocol and the Paris Agreement have committed to control greenhouse gas emissions, and China, as the largest carbon emitter, has assumed a heavier burden. China has been striving to develop low-carbon technologies such as hydrogen, nuclear, wind, and solar energy, but the most attention should be paid to CCUS, which many scholars have high expectations that CCUS can help China reduce emissions to some extent. Therefore, this paper presents a prediction that CCUS can reduce 3.8% of carbon emissions for China in 2040 when CCUS emission reductions increase at a rate of 30%. The power and chemical industries could reduce carbon emissions by 2.3% and 17.3%, respectively.
