Logging Response Characteristics Research Articles

Lithology identification and reservoir characteristics of the mixed siliciclastic-carbonate rocks of the lower third member of the Shahejie formation in the south of the Laizhouwan Sag, Bohai Bay Basin, China

In the Shahejie Formation in the D oilfield area of the Laizhouwan Sag, Bohai Bay Basin, a mixed sedimentary structure of terrigenous clastic and carbonate rocks has developed. Scholars at home and abroad have performed much research on clastic and carbonate lithology identification. However, studies on lithology identification and reservoir characteristics of mixed siliciclastic-carbonate rocks in the study area have not been conducted thus far. Due to the complex logging response characteristics of mixed sedimentary rocks, it is difficult for conventional lithology identification methods to clearly reflect the lithological characteristics of mixed siliciclastic-carbonate rocks. In this study, the sensitivity of acoustic time difference, natural gamma ray, density, deep lateral resistivity and compensated neutron logging curves to each lithology was analysed using the conventional cross-plot method in the D oilfield. On this basis, a decision tree method is used to further screen and identify the lithology layer by layer, and the process and results of identifying a mixed lithology with logging curves are clearly reported. In addition, imaging logging and core data are used to verify the identified lithology, and a lithology identification template and technical method consisting of conventional logging cross plots, decision trees, imaging logging and core calibration are established to improve the accuracy of lithology identification of mixed rocks. Moreover, the influence of different lithologies on the reservoir characteristics in the study area is analysed.

Analysis on Log Response Characteristics of Oil Shale and In-situ Mining Geological Evaluation in Qingshankou Formation of South of Songliao Basin

In order to study the log response characteristics of oil shale in Qingshankou formation of the south of Songliao basin, the geology of in-situ mining of oil shale in this area is preliminarily evaluated, so as to provide scientific basis for in-situ mining of oil shale in this area. Based on the log response principle of oil shale, the Qingshankou formation oil shale in this area is identified qualitatively, and the in-situ geological parameters of oil shale are analyzed in combination with the characteristics of oil shale. The results show that the oil shale of Qingshankou formation in the south of Songliao basin is with the log response characteristics of high natural gamma, high resistivity, high acoustic time difference, and low density, and the organic carbon owns the best correlation with resistivity and density logs. Through analysis of the geological parameters affecting in-situ oil shale mining, the geological parameters affecting in-situ oil shale mining include oil content, thickness, burial depth, mineral type and its content, organic matter type and its maturity, water content, sulfur content, rock assemblage characteristics, ore body distribution characteristics, and dip distribution characteristics, etc.

Reunderstanding and significance of high-quality reservoirs of the inner Dengying Formation in the Anyue Gas Field

The overall development of the top of the gas reservoir of the Dengying Formation, Upper Sinian in the Anyue Gas Field of the Sichuan Basin, has been deployed, and the inner Dengying Formation is the next potential succession layer. However, the high-quality reservoirs of inner Dengying Formation are undeveloped and cannot be identified on the seismic profiles, which restricts its deepened understanding, exploration and development. In this paper, the logging and seismic response characteristics of high-quality reservoirs were analyzed using the mud logging and well logging data of the Anyue Gas Field, and a seismic response model for high-quality reservoir was established by means of seismic forward modeling. Then, the target processing was carried out for the problem of weak effective signals. Finally, the plane distribution of high-quality reservoirs of inner Dengying Formation and the favorable exploration areas were predicted. And the following research results were obtained. First, there are two types of high-quality reservoirs inside the Dengying Formation, i.e., concentrated vug type and independent cave type. Vertically, the independent cave reservoirs are mainly distributed within 100 m above the mudstone floor of the third Member of Dengying Formation. Horizontally, karst zones are developed along the bed. Laterally, they are scattered and beaded. And the scale is small. Second, seismic response of high-quality reservoirs inside the carbonate rocks is characterized by single-peak concealed “beaded” reflection, whose peak energy is relatively weak and can be easily covered by the noise of seismic data. Third, deep-seated weak signal recovery and processing technology can be used to deal with the weak seismic response of high-quality cave reservoirs, improve the amplitude preservation and S/N ratio of seismic data, and obtain the beaded reflections on seismic profiles so as to realize effective identification of high-quality cave reservoirs. In conclusion, the deep-seated weak signal recovery and processing technology is proved to be operable because the achieved results conform to the drilling data and the seismic forward modeling conclusions. In addition, high-quality cave reservoirs are developed at the bottom of the fourth Member of Dengying Formation, and they are locally distributed in a large scale. Therefore, it's one of the favorable succession layers for the following natural gas exploration and development of the Anyue Gas Field.

Quantitative evaluation of the transporting capacity of unconformities: A Case Study from the Zhongguai Area, Junggar Basin, NW China

Abstract Unconformities are special geologic bodies with a three-layer structure including the rock above the unconformity, a weathered clay layer, and a leached rock. The Upper Wuerhe (Upper Permian) bottom unconformity in Northwest Junggar Basin is the main pathway system linking the Zhongguai (ZG) lift with hydrocarbon source kitchens. Based on the log response characteristics of the unconformity's vertical structure and core calibration, a principal component analysis (PCA) was applied to extract and identify the evaluation parameters of the unconformity's vertical structure. On the basis of Darcy's Law, the quantitative characterization formula of the unconformity's transport capacity with an actual geological significance was established after taking several factors into consideration. The migration probability of the unconformity was used to evaluate the possibility that the unconformity acted as a migration pathway. Consequently, a relationship between the migration probability and the evaluation parameters was developed to analyze the effectiveness of these parameters in characterizing the transporting capacity of the unconformity. This study shows that the quotient of the two principal components extracted from the conventional logging curves (Pc1/Pc2) have different reference values in the leached and parent rocks. The distribution of thicknesses of the weathered clay layer and the leached rock is well matched with the distribution of paleogeomorphology. The parameters representing only a single geological factor are not enough to determine whether the unconformity acted as a migration pathway. The flow coefficient (Cf) containing multiple factors can effectively and clearly identify the transporting capacity of the unconformity, considering the positive correlation between the parameter Cf and the migration probability of the unconformity. The oil and gas shows of the area with a parameter Cf greater than 10 × 10−4 cm2/s are abundant, which is the superior transporting area of the unconformity, and there is a good matching relationship between the distribution of the superior transporting area of the unconformity and that of the hydrocarbon distribution.

The reservoir characteristics and their significance for deliverability in metamorphic granite buried hill: a case study from the JZS oil field in the Liaodong Bay Basin, NE China

The previous studies show that the fractures are the main reservoir space of metamorphic rocks. But the oil and gas production capacity of the fracture segment is obviously different in JZS oil field. Demarcating the reservoir zones finely and finding the high quality seepage of reservoirs are the keys to explore and develop the metamorphic rock oil field efficiently. Based on the data of cores, thin section, physical property, conventional logging, imaging logging, and production logging, the reservoir types and their logging response characteristics of the JZS oil field in Liaodong Bay Basin were analyzed, and the differences in capacity of different zones of reservoirs are discussed. The results show that (1) the reservoir of the study area can be divided into weathering crust zone, natural fractures zone, and structural breccia zone, and different reservoirs have different logging response characteristics; (2) the weathering crust zone and the structural breccia zone are the main effective seepage reservoirs, and the seepage capacity of the fractured reservoir is greatly changed; and (3) the reservoir type, fracture density, and the relationship between fracture and that of maximum principal stress control the deliverability difference of the metamorphic rock reservoir.

Evaluation of weathered crust reservoirs in buried hills in western uplift belt of JB block in Indonesia

After nearly 20 years of exploration, the western uplift belt of the JB block is now at a high exploration level, and the buried hill has become another new exploration area besides lithologic traps. On the basis of the results of well logging of 17 wells, core of 6 wells and logging interpretation of 17 wells, the differences in the vertical texture features of the basement reservoir are fully considered. According to the differences of logging and seismic response characteristics of 500 km2 between weathered zone and the fractured zone, the colored inversion technique and spectral imaging technique combined with seismic attribute technology are used to predict the plane distribution of porous reservoir in weathered crust. The pre-stack elastic wave impedance inversion technique is preferred for oil-gas-bearing detection evaluation. The results show that the lithology of the B paleo-uplift is granite, and that of the P paleo-uplift is phyllite. The vertical zoning of the buried hill reservoir is obvious, which can be divided into weathered crust zone, fractured zone and tight zone from top to bottom. From the top of the weatheredzone to the fractured zone, the porosity becomes smaller gradually, and distributes in the shape of the discontinuous strips laterally. The planar distribution of the pore-type reservoirs in the weathered zone is obviously controlled by the paleo-structure, and is in the shape of strips and flakiness with obvious zoning characteristics. The porous reservoir is relatively developed onthe crest of the paleo-structure, and the weathered crust reservoir quality of B uplift is superior to that of the P uplifts. Hydrocarbon mainly distributes in the high parts of the two paleo-structures in the shape of strip and flakiness. The results provide a scientific basis for the follow-up drilling decision-making.

Quantitative identification of relatively high-level radioactive sandstone in the Upper Triassic Yanchang Formation of the Ordos Basin, China

Most relatively high-level radioactive sandstone (HRSS) reservoir has considerable oil (or gas) resource potential. HRSS is often wrongly identified due to its similar logging response characteristics as mudstone, which leads to the omission of effective reservoirs. In this paper, a quantitative identification method for HRSS is proposed after the analyzing of the response characteristics and relationship between spontaneous potential log and natural gamma-ray log in conventional sandstone and mudstone strata. Take the Upper Triassic Yanchang Formation in Ordos Basin as an example: the responses of spontaneous potential log and the responses of natural gamma-ray log are synchronized and positively correlated in conventional sandstone and mudstone strata, but they are not synchronized in HRSS. Quantitative identification of HRSS was realized based on this synchronization feature, and a “virtual compensation” of natural gamma-ray log was performed. At the same time, logging evaluation method about HRSS has been discussed. The final results shows that this identification method work effectively, and can reduce the misjudgment and omission of effective reservoirs.

Coal seam porosity and fracture heterogeneity of marcolithotypes in the Fanzhuang Block, southern Qinshui Basin, China

Marcolithotypes play a significant role in coal seam porosity and fracture heterogeneity. The optical microscopy, CO2 and N2 adsorption, and X-ray CT were used for reconstructing the pore-fracture system of different marcolithotypes. The results show that the microfractures are well developed in the bright coal with the characteristics of high fracture density, good connectivity, and strongest fractal dimension, corresponding to the highest permeability and the best seepage capacity. The adsorption capacity measured with N2, CO2, and CH4 indicates that the specific surface area of micropores (<2 nm) increases linearly from the bright to the dull coal, and the bright coal is favorable for gas adsorption by providing more available surface sites. The difference in physical properties of various macrolithotypes causes different logging response characteristics, including the compensated density (DEN), the gamma ray (GR), and the acoustic time (AC). Based on that, the index (N = AC/(GR × DEN)) was put forward to forecast the distribution of coal macrolithotypes quantitatively (N ≤ 1.3, Partings; 1.3 < N ≤ 3, Dull coal; 3 < N ≤ 5, Semi-dull coal; 5 < N ≤ 8, Semi-bright coal; N > 8, Bright coal), and its accuracy was verified by coal cores from different areas, i.e. Fanzhuang, Zhengzhuang, and Hancheng blocks.

New method for predicting coal seam gas content

ABSTRACTCoal seam gas (CSG) content is an important factor affecting the safety related to gas explosions, and it is also an indicator of natural gas resource assessment. The conventional method of establishing a network model for predicting gas content is mainly to predict the gas content of an individual location point, and achieve gas content prediction in the target area through an interpolation algorithm, which cannot accurately control the interpolation beyond the prediction point. Therefore, we selected a certain study area as an example, conducted a comprehensive analysis of CSG enrichment logging and seismic response characteristics, to determine the relative sensitive parameters, and used a support vector machine (SVM) network for sensitive parameters training based on genetic constraints. The algorithm (GA) optimizes the penalty parameters and kernel functions of the prediction model, completes the prediction of the gas content at the drill site, and uses neural network inversion to train and predict the target gas curve. The distribution of the gas content in the three-dimensional volume of the coal seam was obtained. A variety of gas content influencing factors and sensitive parameters of gas enrichment were integrated to establish a set of prediction methods for the volume gas content, which provided a theoretical basis for accurately predicting coal seam gas content. Comparison with subsequent measured data verified that this prediction method has a higher accuracy in this study area.

Organic-rich source rock characterization and evaluation of the Cretaceous Qingshankou Formation: results from geophysical logs of the second scientific drilling borehole in the Songliao Basin, NE China

The second continental scientific drilling (SKII east) borehole in the Songliao Basin has been planned to be the deepest borehole to drill through the Cretaceous continental strata under the framework of the International Continental Scientific Drilling Program (ICDP) up to date. This borehole was designed not only to explore the potential relationships between dinosaur extinction and climate environment during the Cretaceous but also to achieve new breakthroughs in oil and gas exploration. The high hydrocarbon (oil and gas) potential of a source rock is highly dependent on its organic content. We used geophysical log data of Borehole SKII east to evaluate the organic content of organic-rich source rock. In the period of the first member in the Qingshankou Formation (K2qn1) from the study area, high accommodation space and anoxic environment could promote organic matter deposition. Firstly, based on geological information and abundant geophysical log data, the basic geophysical characteristics (including petrophysical, lithological, mineralogical, and sedimentary properties) of the Qingshankou Formation in Borehole SKII east were studied. Secondly, geophysical log response characteristics (including resistivity, porosity, radioactivity, mineral and element) of organic-rich source rocks were analyzed. Thirdly, we tried to obtain suitable methods to predict total organic carbon (TOC) content of the target formation with geophysical log data. The laboratorymeasured TOC values of core samples from Borehole SKI south were used to make calibrations with calculated TOC values from geophysical logs. Results from improved ΔlogR technique and Dual_Vsh method are consistent, and indicate that these two methods are effective in this formation. The calculated TOC values from these two methods are relatively desirable, and show that the organic-rich source rocks with high TOC content occurred in the K2qn1 (1646.00~1669.00 m). The highest TOC content can reach 9.15%. The bed thicknesses of organic-rich source rocks are totally up to 7.88 m. These organic-rich source rocks can be considered as excellent. This study demonstrated that improve ΔlogR technique can be applied to evaluate source rocks in the formations without maturity data, and the new method Dual_Vsh is also valid in the Qingshankou Formation. The organic-rich source rock evaluation results could promote further exploration and development of oil and gas resources in the upper Cretaceous formations, Songliao Basin.

Tectonic stress estimation from ultrasonic borehole image logs in a coal bed methane well, northeastern Qinshui Basin, China

Small-scale structures and tectonic stress are known to affect the production of coalbed methane. Ultrasonic borehole image logging (UBI) can be used to identify fractures and breakout zones around the wellbore and to analyze tectonic stress fields. In this study, a variety of image processing and automated fracture detection methods were used to process and interpret the ultrasonic logging data in a coal bed methane well. Combined with other logging data in the well, this study analyzed the ultrasonic logging response characteristics of the formation lithology and structure. Statistical analysis has been completed to identify the fracturing pattern. The results showed that the dip angle and orientation of the natural fracture were related to regional tectonics. The paleo-stress field, reflected by the natural fractures, was consistent with the geological conditions, while the current stress field, reflected by the breakout zones, was found to be different from the paleo-stress field. This study proposed a new method by using fractures with associated conjugate joints, which are obtained from UBI logs, to estimate the paleo-stress orientation.

Petrophysical characteristics and logging evaluation of asphaltene carbonate reservoirs: A case study of the Cambrian Longwangmiao Formation in Anyue gas field, Sichuan Basin, SW China

Abstract The petrophysical and logging response characteristics of asphaltene carbonate reservoirs were examined based on the measurement of porosity and permeability, density, compressional and shear wave slowness, resistivity and Nuclear Magnetic Resonance transverse relaxation time (T2) of cores before and after the bitumen dissolving. The results show that (1) the asphalt can damage the pore structure of the reservoir and cause reduction of effective reservoir space and permeability; (2) with the increase of asphalt content, the compression and shear wave slowness generally decrease while the density and resistivity increase; (3) with the increase of asphalt content, the compressional wave slowness and density change less, while the shear wave slowness and resistivity change larger; and (4) the T2 values of asphalt are generally less than 3 ms, and the higher the maturity of the asphalt, the lower the T2 value. Based on these experiments, a method based on conventional and special logging methods was presented to evaluate asphalt content, effective porosity and water saturation in asphaltene carbonate reservoirs. The method has already been applied to 80 wells in the Longwangmiao Formation of the Anyue gas field in the Sichuan Basin to pick out zones rich in asphalt on the plane, which has effectively guided the selection of well location in the gas reservoir development.

A new lithologic classification method for tight sandstone reservoirs based on rock components and logging response characteristics

The original lithology classification method for tight sandstone reservoirs has a low prediction accuracy and it does not accurately reflect reservoir characteristics. We propose a new method that uses thin sections, logging curves, and core physical data to classify the lithology of the Ordos Basin, China. First, the relationship between the rock components and physical properties in the study area was analyzed and we found that quartz and rock debris played an active role in the properties of the reservoir, while feldspar minerals in the reservoir had a negative effect on it. Second, we synthesized the logging response characteristics of various rock components and divided the lithology into (1) high feldspar content, low quartz content, low rock debris content, and (2) low feldspar content, high quartz content, high rock debris content, using ((quartz + rock debris)/feldspar) as an index for lithologic classification. The lithology identification model was established using a support vector machine approach with a regression accuracy of 84.62%. Applying the model to the well in the study area to distinguish the lithology, results were in good agreement with thin section data, the physical properties of the reservoir, and the production capacity. Lithology can effectively reflect reservoir characteristics and play an important guiding role in the identification of reservoirs and the evaluation of productivity.

Sedimentary control on the formation of a multi-superimposed gas system in the development of key layers in the sequence framework

Based on core observations, well logs and test results of siderite-bearing mudstone from the Benxi Formation to the Member 2 of the Shanxi Formations in the Linxing block, northeastern Ordos Basin, a logging identification model for siderite-bearing mudstone (key layer) was established. The porosity characteristics and sealing property were quantitatively evaluated by logging data. Sedimentary control on the formation of multi-superimposed gas-bearing system in the development of key layers in the sequence framework was also discussed. The results showed that the siderite-bearing mudstone has obvious logging response characteristics, e.g., high photoelectric absorption cross-section index (PE), high density (DEN), high amplitude natural gamma ray (GR), low acoustic (AC), low resistivity (M2RX) and low neutron porosity (CNCF). The quantitatively evaluated results of the porosity characteristics and sealing property for the key layer showed that the key layer has the characteristics of low porosity (with an average of 1.20 percent), low permeability (with an average of 2.29 × 10−8μm2), and high breakthrough pressure (with an average of 12.32 MPa) in the study area. This layer acts as an impermeable gas barrier in a multi-superimposed gas system. The results also indicated that the material composition of the multi-superimposed gas-bearing system can be established by the sequence stratigraphic framework. The sedimentary evolution results in a cyclic rhythm of material composition vertically. The spatial distribution of the corresponding transgressive event layer near the maximum flooding surface (MFS) in the sequence framework restricts the spatial distribution of the key layer with high breakthrough pressure and low porosity, which constitutes the gas-bearing system boundary. The siderite-bearing mudstone formed near the MFS in the second-order sequence and constitutes a stable comparison of the first-order gas-bearing system boundary, which has a wide range of regional distribution and stable thickness. The siderite-bearing mudstone formed near the MFS in the third-order sequence is often incompletely preserved due to the late (underwater) diversion channel erosion and cutting. This layer forms the coal-bearing reservoirs, which we termed as a second-order gas-bearing system in adjacent third-order sequences to form a uniform gas-bearing system.

Predicting the Porosity of Natural Fractures in Tight Reservoirs

Predicting the fracture porosity of tight fracture reservoirs represents a difficult problem for log interpretation, although studying conventional log responses of tight fracture reservoirs provides helpful insights. Comparisons and analyses of the log response characteristics of the natural fracture zone of a tight reservoir are performed, and the results indicate that the acoustic (AC) and density (DEN) data are highly sensitive to fractures. First, the log response of the AC and DEN data are sensitive to tight fracture reservoirs, and differences in the AC and DEN log responses are obvious. Second, based on these differences, a model for calculating the fracture porosity is constructed, and an improved Willie difference method for the AC and DEN responses is proposed to calculate the fracture porosity. Finally, we apply the methodology to a case study of a tight reservoir in the Sichuan Basin, China, and then calculate the AC–DEN correlation coefficient and fracture porosity. The calculated fracture porosity is consistent with the fracture porosity from the full borehole micro-resistivity imaging results and core porosity. The results indicate that the method is applicable for predicting the porosity of fractures.

Dual-shale-content method for total organic carbon content evaluation from wireline logs in organic shale

AbstractOrganic shale is one of the most important unconventional resources all around the world. Total organic carbon (TOC) content is an important evaluation parameter of reservoir hydrocarbon source quality. The regular evaluation methods have higher requirements of well logs and core experiment data for statistical regression. Through analyzing the resistivity and gamma ray logging response characteristics of shale content and organic matters, combined with digital rock physics experiment simulation, we put forward and improve the dual-shale-content method for TOC content logging evaluation. The accuracy of this method is verified by actual data processing. The result shows the dual-shale-content method is simple to use and the error is small. And by comparing with the calculation results by using the

NUMERICAL SIMULATION AND CHARACTERISTICS ANALYSIS OF DUAL LATEROLOG IN CAVERNOUS RESERVOIRS ON THE BASIS OF 3D‐FEM

AbstractCavernous carbonate formations are characterized by strong heterogeneity, which results in the complex electric logging responses and the difficulty in the logging identification and evaluation. Numerical simulation of electric logging tools is helpful to figure out the logging response characteristics in these formations and could provide theoretical support for cave recognition and evaluation. In this paper, based on three dimensional finite element methods, a local refinement technique is adopted for the accurate discretization and simulation of complex sphere caves. Then, the response characteristics and sensitivity of dual laterolog to caves around and penetrated by wellbore are analyzed, finally the differences of sensitive scope between deep and shallow laterolog are studied. The results show that the dual laterolog response decreases significantly when cave exists. Affected by cave boundary and the investigation depth of dual laterolog tool, the logging response for caves penetrated by wellbore is very complicated, however it exhibits parabolic‐like characteristic for caves around wellbore. Compared with caves around wellbore, the dual laterolog is much more sensitive to caves penetrated by wellbore. For caves around wellbore, the maximum sensitive distance for deep and shallow laterolog between the cave boundary and well wall is 0.5 m and 0.3 m, respectively.

Well logging evaluation of water-flooded layers and distribution rule of remaining oil in marine sandstone reservoirs of the M oilfield in the Pearl River Mouth basin

In the marine sandstone reservoirs of the M oilfield the water cut is up to 98%, while the recovery factor is only 35%. Additionally, the distribution of the remaining oil is very scattered. In order to effectively assess the potential of the remaining oil, the logging evaluation of the water-flooded layers and the distribution rule of the remaining oil are studied. Based on the log response characteristics, the water-flooded layers can be qualitatively identified. On the basis of the mercury injection experimental data of the evaluation wells, the calculation model of the initial oil saturation is built. Based on conventional logging data, the evaluation model of oil saturation is established. The difference between the initial oil saturation and the residual oil saturation can be used to quantitatively evaluate the water-flooded layers. The evaluation result of the water-flooded layers is combined with the ratio of the water-flooded wells in the marine sandstone reservoirs. As a result, the degree of water flooding in the marine sandstone reservoirs can be assessed. On the basis of structural characteristics and sedimentary environments, the horizontal and vertical water-flooding rules of the different types of reservoirs are elaborated upon, and the distribution rule of the remaining oil is disclosed. The remaining oil is mainly distributed in the high parts of the structure. The remaining oil exists in the top of the reservoirs with good physical properties while the thickness of the remaining oil ranges from 2–5 m. However, the thickness of the remaining oil of the reservoirs with poor physical properties ranges from 5–8 m. The high production of some of the drilled horizontal wells shows that the above distribution rule of the remaining oil is accurate. In the marine sandstone reservoirs of the M oilfield, the research on the well logging evaluation of the water-flooded layers and the distribution rule of the remaining oil has great practical significance to the prediction of the distribution of the remaining oil and the optimization of well locations.

Method for calculating the fracture porosity of tight-fracture reservoirs

Identifying fractures and evaluating the parameters of tight reservoirs are important problems. Developing methods to accurately interpret logging data for tight sandstone and shale reservoirs is of great significance, especially when only conventional logging data can be obtained. Identifying natural fractures with limited available data is challenging. Comparing and analyzing the log response characteristics of the natural fracture zone of a tight reservoir indicate that acoustic (AC) and density (DEN) data are highly sensitive to fractures in tight reservoirs. First, we have obtained the characteristics and differences of the log response of natural fractures. Second, we established a model that is based on these sensitive log-response characteristics (AC and DEN) to identify fractures (correlation coefficient method with window lengths), and we analyzed the cut-off of the correlation coefficient. Then, we established a model (the revised Wyllie difference method) to characterize the fracture porosity based on the difference in the sensitive log responses of the fracture (the difference in the AC and DEN curves). Finally, we applied this methodology to a case study of a tight reservoir in the Sichuan Basin, China, and the AC-DEN correlation coefficient and fracture porosity are calculated. The AC-DEN correlation coefficient adequately identifies fractures, and the calculated fracture porosity is consistent with the fracture porosity from full borehole microresistivity imaging. Thus, this method is applicable to evaluating fractures in tight-fracture reservoirs.

Logging identification and characteristic analysis of the lacustrine organic-rich shale lithofacies: A case study from the Es3L shale in the Jiyang Depression, Bohai Bay Basin, Eastern China

The Es3L (lower sub-member of the third member of the Eocene Shahejie Formation) shale of Jiyang depression was a set of relatively thick and widely deposited lacustrine sediments with high organic richness and medium thermal maturation (0.7%≤Ro≤1.1%), which rapidly emerges an important and target shale-oil play in Eastern China. An important component for shale-oil reservoir evaluation is lithofacies that affect hydraulic fracture stimulation and constrain significant organic-matter and oil concentration. Based on core and thin section observation, X-ray diffraction and comparative analysis of well logging data, we can define the types of shale lithofacies in the Es3L and determine log response characteristics of each lithofacies to show effective methods for identifying or predicting the lithofacies from conventional well logs. Organic richness, porosity and permeability measurements and the analysis of scanning electron microscope (SEM) images were used to study the impact of lithofacies on total organic carbon (TOC), reservoirs properties and oil content. The Es3L shale is rich in carbonate minerals (most >50%). According to the mineral composition, sedimentary structure and genesis, the lithofacies can be divided into 5 types and 2 sub-categories: marl (including massive and bedded), calcareous shale (including massive and bedded), laminated bindstone, massive silty mudstone and argillaceous shale. By the analysis of log response characteristics of lithofacies from 17 wells, 8 sensitive logging curves (e.g., density (DEN), acoustic times (AC) and resistance (Rt)) were optimized to construct logging recognition models and conduct cross plots to qualitatively identify lithofacies. On this basis, using the Fisher Discrimination Analysis and Naïve Bayes Classification Function, 5 types of shale lithofacies were quantitatively identified and predicted. Combined with the identification of shale beddings from Formation MicroScanner Imaging (FMI), the 7 sub-categories of shale lithofacies can be further recognized. The statistics of lithofacies indicate that bedded marl and calcareous shale are the dominant lithofacies in the Es3L. TOC content has the wide variation between different lithofacies (0.06–12%). The argillaceous shale has the high TOC content, followed by calcareous shale and bindstone. TOC is positively correlated to clay and pyrite content, and negatively correlated to quartz. The measurements of reservoir properties indicate that bindstone and calcareous shale have high porosity (>7.0%) and good permeability. A general positive relationship between porosity and TOC, quartz or carbonate minerals indicates that organic matter (OM), recrystallized intercrystal and interparticle pores in BSE images can improve the porosity. Additionally, the oil accumulation index (OAI) of bindstone, calcareous shale and argillaceous shale is high. Considered reservoir properties and fracability, the laminated bindstone and bedded calcareous shale in the medium-low section of Es3L are the most advantageous lithofacies for shale-oil exploration and exploitation in this area.