Favorable Lithofacies Research Articles

Evaluation technology and practice of continental shale oil development in China

This paper analyzes the differences in geological and development characteristics between continental shale oil in China and marine shale oil in North America, reviews the evaluation methods and technological progress of the continental shale oil development in China, and points out the existing problems and development directions of the continental shale oil development. The research progress of development evaluation technologies such as favorable lithofacies identification, reservoir characterization, mobility evaluation, fracability evaluation, productivity evaluation and geological-mathematical modeling integration are introduced. The efficient exploration and development of continental shale oil in China are faced with many problems, such as weak basic theoretical research, imperfect exploration and development technology system, big gap in engineering technology between China and other countries, and high development cost. Three key research issues must be studied in the future: (1) forming differentiated development technologies of continental shale oil through geological and engineering integrated research; (2) strengthening the application of big data and artificial intelligence to improve the accuracy of development evaluation; (3) tackling enhanced shale oil recovery technology and research effective development method, so as to improve the development effect and benefit.

Multi-Scale Pore Structure Characterization of Silurian Marine Shale and Its Coupling Relationship With Material Composition: A Case Study in the Northern Guizhou Area

Investigation of pore structure is vital for shale reservoir evaluation and also “sweet spot” prediction. As the strong heterogeneity in pore types, morphology, and size distributions of organic matter-rich shales, it is essential to combine different approaches to comprehensively characterize them.Field emission-scanning electron microscopy (FE-SEM), low-pressure gas (CO2 and N2) adsorption, and high-pressure mercury intrusion (HPMI) were employed to systematically investigate the pore structure of the lower Longmaxi shale reservoirs in the northern Guizhou area. The results show that the shales can be divided into four lithofacies based on mineral composition, namely, siliceous shale (SS), clay shale (CS), carbonate shale (CAS), and mixed shale (MS), among which siliceous shale is the primary lithofacies of the Longmaxi shale. Numerous organic matter (OM)-hosted pores, clay interlayer pores, interparticle pores, and intraparticle pores were identified within shale reservoirs. The specific surface area ranges from 11.3 to 27.4 m2/g, with an average of 18.1 m2/g. It exhibits a strong positive correlation with TOC contents, suggesting that organic matter is the major contributor to the specific surface areas. A wide range of pore size distribution was measured by integration of gas adsorption and HPMI. It is shown that the pore size is primarily distributed within ∼100 nm, corresponding to micropores, mesopores, and part of macropores. The total pore volume, which is mostly derived from the contribution of micropores and mesopores, remains within a range of 0.11 to 0.025 ml/g, with an average of 0.018 ml/g. Furthermore, the volume of micropores and mesopores is mainly controlled by organic matter contents. The dissolution pore contributes most to the macropore space within shale reservoirs, based on the positive correlation with macropore volume and easily dissolved minerals, including carbonate and feldspar. Also, the total pores volume is mainly dominated by organic matter and carbonate contents. This is possibly attributed to the easily dissolved and rigid features of carbonate, which can protect the primary interparticle pores due to its high compression resistance and is conducive to forming abundant dissolution pores. OM-rich carbonate-bearing mixed shale may be the most favorable lithofacies for gas storage in the northern Guizhou area.

Determination of Favorable Lithofacies for Continental Shale Gas: A Case Study of the Shahezi Formation, Changling Fault Depression, Songliao Basin

In order to determine the optimum lithofacies for continental shale, the reservoir characteristics of different lithofacies types were studied based on a series of experiments. The lacustrine organic-rich shale of the Shahezi Formation is divided into siliceous (ORS), argillaceous (ORA), calcareous (ORC), and mixed (ORM) shales. The ORS, ORA, ORC, and ORM shales all carried out comprehensive reservoir comparative analysis. The results showed that the moderate content of clay minerals (45%) can significantly improve porosity, and high and low clay mineral contents are not conducive to the improvement of porosity. The ORM shale tends to have better pore connectivity than the ORS and ORA shales, and the ORC shale has the poorest pore-throat connectivity in micron-size. Internal pores in bitumen and clay shrinkage cracks are the dominant pore type and are well developed in ORS and ORM shales. The two types of pores are less developed in the ORA shale; however, dissolution pores are better developed than those of ORS and ORM shales. Inorganic pores are well-developed in the ORC shale, but organic pores are not. Organic and inorganic pores tend to be better connected in the ORM shale than those of the other three types, which could help improve the gas storage capacity. The ORM shale may have more irregular surfaces and lower liquid/gas surface tension. The higher capillary condensation on pore surfaces is more likely to occur in the ORA shale. The ORS shale offers fewer adsorption sites for CH4 and has lower adsorption capacity. The ORC shale has weak heterogeneity of the pore structure. Therefore, the ORM shale may be the most favorable lithofacies for shale gas enrichment and development, which has high porosity, good pore connectivity, moderate brittleness, and strong gas adsorption and storage capacity, followed by the ORS and ORA shales. The ORC shale is the worst.

Laminar characteristics of lacustrine organic-rich shales and their significance for shale reservoir formation: A case study of the Paleogene shales in the Dongying Sag, Bohai Bay Basin, China

• Six types of the organic-rich shale laminae are reported in the Dongying Sag. • Discontinuous laminae were formed under high-energy hydrodynamic conditions. • Shale laminae containing favorable reservoir storage space are in the order of wavy laminar > lenticular laminar > weak laminar. • Thick and wavy laminae are the most favorable types for shale oil exploration. Shale oil exploration in China has demonstrated that laminated shales are favorable lithofacies for producing shale oil effectively. It is of great significance to study the type and reservoir properties of shales with different laminar structures. In this study, the laminae of Paleogene organic-rich shales in the Dongying Sag, Bohai Bay Basin were divided into two categories and six types based on thin section petrography, scanning electron microscopy (SEM), and geochemical characterization. The laminar types include thin parallel laminae, thick parallel laminae, wavy laminae, lenticular laminae, weak laminae, and sandy laminae. The first three types of laminae have good continuity and were formed under low-energy hydrodynamic conditions. In comparison, the latter three types are discontinuous and were formed under relatively high-energy hydrodynamic conditions with a strong terrigenous input. Significant differences were observed in the reservoir space types, microcracks, and oil-bearing properties among the different laminae. Analyses of porosity and oil saturation further revealed that laminated shales with high laminar continuity exhibit higher porosity, permeability, and oil saturation values than shales with poor laminar continuity. Shales with thick parallel laminar shale and wavy laminar shale possess favorable petrophysical parameters. Thus, from the perspectives of reservoir and oil-bearing properties, laminated shales with high laminar continuity are better than shales with poor laminar continuity for shale oil exploration in the Dongying Sag. The new findings provide a reference for the optimization and evaluation of favorable continental shale oil reservoirs in eastern China.

Identification of lithofacies and prediction of mineral composition in shales – A case study of the Shahejie Formation in the Bozhong Sag

The identification of shale lithofacies is the basic work of shale gas exploration and development. Accurate quantitative characterization of different mineral components in fine-grained mixed shale is of great significance for the identification and classification of lithofacies types, the enrichment conditions of shale oil and gas (hydrocarbon generation, reservoir, occurrence and preservation), and the evaluation of shale oil potential (reservoir, oil bearing, fracturability and oil mobility). When organic-rich argillaceous laminae are moderately mixed with brittle laminae, the laminated shale is not only the favorable interval of shale oil and gas enrichment for hydrocarbon generation (organic-rich argillaceous laminae) and storage (brittle laminae), but also the excellent “sweet spots” for continental shale oil and gas optimization. Different shale lithofacies have distinct rock texture, fabric and composition, leading to different brittleness and rock physical properties. For the identification of shale lithofacies containing various components, the overlaps of different wire-line logging responses and the vague boundaries between various logging data cause the large deviation in the logging prediction of lithofacies by traditional methods. In this study, shale samples from the lower part of the third member of Shahejie Formation in Bozhong Sag of the Bohai Bay Basin are selected to carry out the reservoir characterization and then the data mining of logging information by a Back Propagation (BP) neural network coupled with Atomic Search Optimization (ASO) algorithm. The BP algorithm based on the identified shale lithofacies (expected output) and logging data index (input) is used to train the neural network. The complex and unrecognized nonlinear relationship between shale lithofacies and logging data is mapped onto the high-dimensional identifiable nonlinear quantitative relationship to establish the prediction model of the relative content of clay minerals, silicate minerals and carbonate minerals. This study reveals the main lithologic characteristics of lacustrine shale lithofacies from Shahejie Formation in Bozhong Sag, and the main controlling factors for shale lithofacies prediction based on logging data. Our results show that the main mineral composition of shale lithofacies associations can be effectively predicted through the whole rock X-ray diffraction data, wire-line log data and neural network analysis, which provides the basis for lithofacies identification shale interval in well locations lacking core and test data. • Based on the optimized algorithm, a neural network prediction model of mineral composition is established to mine the hidden information in conventional logging data to predict lithofacies, and the accuracy is improved compared with the conventional neural network model. • The prediction of lithofacies mineral composition data for a wider range of non-coring wells by neural network provides more geological basis for the selection and prediction of favorable lithofacies zone - dessert area in subsequent exploration.

Favorable lithofacies types and genesis of marine-continental transitional black shale: A case study of Permian Shanxi Formation in the eastern margin of Ordos Basin, NW China

Based on core description, thin section identification, X-ray diffraction analysis, scanning electron microscopy, low-temperature gas adsorption and high-pressure mercury intrusion porosimetry, the shale lithofacies of Shan23 sub-member of Permian Shanxi Formation in the east margin of Ordos Basin was systematically analyzed in this study. The Shan23 sub-member has six lithofacies, namely, low TOC clay shale (C-L), low TOC siliceous shale (S-L), medium TOC siliceous shale (S-M), medium TOC hybrid shale (M-M), high TOC siliceous shale (S-H), and high TOC clay shale (C-H). Among them, S-H is the best lithofacies, S-M and M-M are the second best. The C-L and C-H lithofacies, mainly found in the upper part of Shan23 sub-member, generally developed in tide-dominated delta facies; the S-L, S-M, S-H and M-M shales occurring in the lower part of Shan23 sub-member developed in tide-dominated estuarine bay facies. The S-H, S-M and M-M shales have good pore structure and largely organic matter pores and mineral interparticle pores, including interlayer pore in clay minerals, pyrite intercrystalline pore, and mineral dissolution pore. C-L and S-L shales have mainly mineral interparticle pores and clay mineral interlayer pores, and a small amount of organic matter pores, showing poorer pore structure. The C-H shale has organic micro-pores and a small number of interlayer fissures of clay minerals, showing good micro-pore structure, and poor meso-pore and macro-pore structure. The formation of favorable lithofacies is jointly controlled by depositional environment and diagenesis. Shallow bay-lagoon depositional environment is conducive to the formation of type II2 kerogen which can produce a large number of organic cellular pores. Besides, the rich biogenic silica is conducive to the preservation of primary pores and enhances the fracability of the shale reservoir.

Geochemical, Mineralogical, and Lithofacies Characterizations of Lacustrine Shale: A Case Study From the Cangdong Sag, Bohai Bay Basin, China

The study of lithofacies identification and its distribution characterizations of shales is essential for the geological evaluation of shale oil exploration and development. In this study, core description, optical microscope, total organic carbon (TOC) content, whole-rock X-ray diffraction (XRD), Rock-Eval pyrolysis, X-ray fluorescence (XRF), scanning electron microscope (SEM), and well logging data were used in the qualitative analyses of the geochemical and mineralogical characteristics of shales. Since the work of detailed lithofacies identification is still lacking in the previous studies, based on TOC, sedimentary structure, and ternary diagram of siliceous minerals, carbonate minerals, and clay minerals, eight predominant lithofacies were identified in the shale of the second member of the Kongdian Formation (Ek2): ① low to moderate organic matter content (referred to as “low to moderate TOC”) massive calcareous mudstone (C-1); ② low to moderate TOC laminated calcareous mudstone (C-2); ③ low to moderate TOC layered mixed calcareous mudstone (C-3); ④ moderate to high organic matter content (referred to as “moderate to high TOC”) laminated calcareous siliceous mudstone (S-1); ⑤ moderate to high TOC massive mixed siliceous mudstone (S-2); ⑥ moderate to high TOC laminated clayey siliceous mudstone (S-3); ⑦ low to moderate TOC massive mixed mudstone (M-1); ⑧ moderate to high TOC layered calcareous/siliceous mixed mudstone (M-2). The favorability of lithofacies was investigated based on TOC, pyrolysis parameters, pore structures, and specific mineral contents. S-1 was currently considered as the most favorable lithofacies with excellent hydrocarbon potential, high amount of free hydrocarbon, and abundant organic pores; S-2, S-3, and M-2 were considered as favorable lithofacies, while C-1 and M-1 were ranked as the least-promising lithofacies in the research area. The lateral distribution of the shale is quite different, and Ek23 contains the most favorable lithofacies for shale oil exploration.

Key Development Fields and Construction of Technical System for Logging of Continental Shale Oil

The research on continental shale oil has just started in China. The key development fields of logging, the link of geology-engineering integration, are not yet clear. There is no systematic establishment of a corresponding collection and evaluation technical system, which restricts the development and full exploitation of shale oil logging to a certain extent. After a systematic analysis of current status of shale oil logging and the requirements of the geology-engineering integration of shale oil, the contents, difficulties, and shortcomings of logging evaluation were analyzed in detail from the four aspects, including a quantitative analysis of mineral components and identification of favorable lithofacies while drilling, reservoir and oil-bearing property evaluation, mobility evaluation, and fracrability evaluation. It is proposed that the development of diffuse reflection infrared Fourier transform spectroscopy (DRIFTS), online nuclear magnetic resonance (NMR) logging for the oil-bearing property of drilling fluids, T1-T2 two-dimensional NMR logging of rock samples, and logging rock mechanics should be emphasized. On this basis, according to the principles of pertinence, effectiveness and economy, continental shale oil can be divided into two types: medium-low maturity and medium-high maturity, and the technical system for logging based on the evaluation of reservoir property, oil-bearing property, mobility, fracrability, and drillability was developed, in a way that can guide the development and production practice in shale oil logging.

The Role of Soluble Organic Matter in Shale Oil “Sweet Spots” Prediction: An Investigation of Shale With Different Lithofacies in the Dongying Sag

Lithofacies are the fundamental geological units for shale oil and gas exploration and development, and soluble organic matter (SOM) is most similar to crude oil in composition. Both aspects attract our attention in the interpretation of SOM in different lithofacies, which can provide direct evidence to predict shale oil “sweet spots”. Here, twenty-five shale samples were collected from the Eocene Shahejie Formation in the Dongying Sag and were subjected to X-ray diffraction, Rock-Eval pyrolysis, and SOM characterization. Comparison of the SOM contents in shales with different lithofacies revealed remarkable differences: 1) The contents of SOM, saturates and total hydrocarbons (THC) showed the order of detrital massive mudstone < homogenous massive mudstone < wide laminated shale < discontinuous laminated shale < fine laminated shale < gypsum-bearing mudstone, and the SOM content was controlled by lithofacies through differences in both OM and minerals. 2) The SOM in detrital and homogenous massive mudstones was mainly composed of saturates and resins. Saturates were the main component in wide and fine laminated shales. The SOM in discontinuous laminated shale was mainly composed of saturates and aromatics. The SOM in gypsum-bearing mudstone was mainly composed of saturates, and the percentage of asphaltenes was quite high. Based on the evaluation parameters of high-quality lithofacies in terms of abundance (i.e., SOM, THC or saturate contents) and quality (i.e., quality index and asphaltene percentage), the fine, wide and discontinuous laminated shales were regarded as relatively favorable lithofacies. Based on the lithofacies combination, the fine, wide and discontinuous laminated shales in Es3x and Es4ss (upper section of Es4s) in the Dongying Sag were interpreted as “sweet spots” for shale oil exploration and development. Thus, it is of great significance to study the characteristics of SOM in shale with different lithofacies for shale oil exploration and development.

Characteristics and main controlling factors of Ordovician deep subsalt reservoir in central and eastern Ordos Basin, China

In recent years, the subsalt dolomite reservoirs in the Ordovician strata of the Ordos Basin have indicated good exploration results and are expected to have a high hydrocarbon potential. This study targets the Ma 3 and Ma 4 reservoirs below the salt rock of the Ordovician in the central and eastern Ordos Basin. It investigates the reservoir characteristics and main controlling factors of high-potential reservoirs by combining core observations, cast slabs, scanning electron microscopy, physical property analysis, and mercury injection data. The results show that the Ma 3 reservoir is dominated by gypsum-bearing dolomite deposited in evaporation environments, whereas the Ma 4 reservoir is dominated by calcite dolomite under a transgressive environment. The types of Ma 4 reservoir space mainly include dissolved pores (dissolved holes), intra-crystalline pores, intra-crystalline dissolved pores, and microcracks. The types of Ma 3 reservoir space mainly include microcracks, gypsum mould pore, dissolved pores (dissolved holes), and intergranular pores. The pore structures of reservoirs are complex. The capillary pressure curves can be divided into four types: dissolved pore type, intra-crystalline (dissolved) pore type, microcrack type, and micro-porous type. Based on available information, the average porosity values of the Ma 3 and Ma 4 reservoirs are 2.1% and 2.3%, respectively, and the average permeability values are 0.19 × 10−3μm2 and 0.22 × 10−3μm2, respectively. The Central paleo-uplift, the Wushenqi secondary paleo-uplift, and the Shenmu low uplift controlled the sedimentary facies and favorable lithofacies distributions. Sedimentary microfacies control rock fabric, and the rock fabric affects the physical properties of the reservoirs. The limestone and dolomite reservoirs with gypsum and sand cutting texture have relatively good physical properties. Diagenesis is also a key factor in the development of sub-salt reservoirs. Intra-crystalline pores are mainly formed under various dolomization mechanisms. The surficial and buried diagenetic environments facilitate the formation of a large number of secondary pores, such as dissolved pores (dissolved holes), intra-crystalline dissolved pores, and gypsum mould pores. In addition, the Yanshan tectonic activity played a vital role in the development of reservoir microcracks. The microcracks can significantly improve reservoir physical properties. It is favored that the tectonic setting, rock fabric, diagenetic, and microcrack alterations collectively determine the sub-salt reservoirs' storage capacity and are the main controlling factors for forming high-quality reservoirs.

Characterization of favorable lithofacies in tight sandstone reservoirs and its significance for gas exploration and exploitation: A case study of the 2nd Member of Triassic Xujiahe Formation in the Xinchang area, Sichuan Basin

Abstract By using core, logging curves, and experiment data, favorable lithofacies types in the 2nd Member of Triassic Xujiahe Formation in the Xinchang area, Sichuan Basin were classified, standard of the favorable lithofacies was established, planar distribution regularities of the favorable lithofacies were identified, and forming mechanisms of the favorable lithofacies and their control effect on production were examined. (1) The 2nd Member of Xujiahe Formation has twelve types of lithofacies, among which multiple layer medium-coarse grain sandstone lithofacies, parallel bedding medium-coarse grain sandstone lithofacies, massive bedding medium-coarse grain sandstone lithofacies, inclined bedding medium-coarse grain sandstone lithofacies, and charcoal-bearing medium-coarse grain sandstone lithofacies with better physical properties and higher gas content are favorable lithofacies; they feature low gamma, low neutron porosity, low resistivity, and high acoustic travel time on logging curves. (2) The sedimentary process controls spatial distribution of sand bodies which are the material basis of the favorable lithofacies; post diagenetic fluids would differentially reconstruct the favorable lithofacies; tectonic activities and abnormal formation pressure made strata slide along the weakness plane, giving rise to fractures in different types of rocks, which can enhance the reservoir permeability significantly. (3) The development degree of favorable lithofacies is a major factor affecting stable production of gas well.

Accumulation contribution differences between lacustrine organic-rich shales and mudstones and their significance in shale oil evaluation

Abstract The differences in organic matter abundance and rock composition between shale and mudstone determine the discrepancy of their contributions to the formation of conventional and shale oil/gas reservoirs. The evaluation criteria of source rocks are different in the future exploration in self-sourced petroleum systems. Shales are deposited in deep/semi-deep lacustrine, with low sedimentation rate and chemical depositions of various degrees, while mudstones are mostly formed in shallow lacustrine/lakeside, with high deposition rate and density flow characteristics. Three factors contribute to the enrichment of organic matter in shales, including the “fertility effect” caused by volcanic ash deposition and hydrothermal injection, excessive and over-speed growth of organisms promoted by radioactive materials, and deep-water anaerobic environment and low sedimentation rate to protect the accumulation of organic matter from dilution. Lamellations in shales are easy to be stripped into storage space, and acid water produced during hydrocarbon generation can dissolve some particles to generate new pores. The massive mudstones with high clay content are of poor matrix porosity. Shales with high total organic carbon, developed laminations, relatively good reservoir property, and high brittle mineral content, are the most favorable lithofacies for shale oil exploration and development. It is necessary to conduct investigation on the differences between shale and mudstone reservoirs, to identify resources distribution in shale and mudstone formations, determine the type and standard of “sweet-spot” evaluation parameters, optimize “sweet-spot areas/ sections”, and adopt effective development technologies, which is of great significance to objectively evaluate the total amount and economy of shale oil resources, as well as the scale of effective exploitation.

Analysis of lithofacies and evaluation of effective reservoirs of member 2 of Xujiahe Formation in the Xinchang area in Western Sichuan

In the Xinchang area in western Sichuan Basin, a total of 121.12 billion cubic meters of proved reserves of natural gas are discovered in Member 2 of Xujiahe Formation, but only 7.739 billion cubic meters of proved reserves of natural gas are produced. The main reason is that sandstones in Member 2 of Xujiahe Formation in the Xinchang area are generally tight, with strong heterogeneity, only intervals with relatively good physical properties have a certain production capacity; therefore, it is very necessary and important to evaluate effective reservoirs in Member 2 of Xujiahe Formation. Through analysis of cores, with consideration of color, sedimentary texture and grain size, lithofacies of Member 2 of Xujiahe Formation in the Xinchang area can be divided into 12 types. According to data of physical properties and thin section analysis, it can be concluded that the favorable lithofacies in the upper submember of Member 2 of Xujiahe Formation are the massive medium-coarse sandstone, the medium-coarse sandstone with cross bedding, and the medium-coarse sandstone with carbonaceous fragments. On the basis of electrical characteristics, well-logging interpretation standards for favorable lithofacies are established, in addition, favorable lithofacies and effective reservoir in the Xinchang area are determined, and distribution of favorable lithofacies (effective reservoir) is identified, which lay a solid foundation for next deployment of appraisal well and development of effective reserves.

Pore-scale oil distribution in shales of the Qingshankou formation in the Changling Sag, Songliao Basin, NE China

A total of 19 representative lacustrine shale (core) samples from the Upper Cretaceous Qingshankou Formation in the Changling Sag, Songliao Basin, northeast China have been characterized to determine the types of shale oil reservoir lithofacies and investigate pore structures and pore-scale oil distribution using an integrated petrographic, geochemical and petrophysical approach. Based on the sedimentary structures and mineral compositions, four lithofacies types have been recognized: foliaceous argillaceous mudstone (FAM), laminated diamictic mudstone (LDM), laminated felsic mudstone (LFM) and interbedded felsic mudstone (IFM). Sequential extraction experiments via a Soxhlet extractor were performed using different solvents to recover oils of varying polarities: weak-polar oil (WPO) was extracted with n-hexane under room temperature, while strong-polar oil (SPO) was extracted with a mix of dichloromethane and methanol (9:1 v/v) extraction. Low-pressure N2 sorption data acquired from the original and solvent extracted samples indicates that the pore volumes occupied by WPO (VWPO) decrease progressively in LFM (averaging 68.32 × 10−4 cm3/g), LDM (averaging 50.41 × 10−4 cm3/g), FAM (averaging 48.86 × 10−4 cm3/g) and IFM (averaging 39.39 × 10−4 cm3/g); while pore volumes occupied by SPO (VSPO) increase progressively in IFM (averaging 8.00 × 10−4 cm3/g), LFM (averaging 12.62 × 10−4 cm3/g), LDM (averaging 38.85 × 10−4 cm3/g) and FAM (averaging 65.08 × 10−4 cm3/g). Major factors controlling pore-scale oil distribution in the shale reservoirs include pore structures, TOC and clay mineral contents. Pore structures can affect the distribution of WPO and SPO to a great extent. WPO is mainly distributed in the mesopores, while SPO is present in all pores in the four lithofacies. SPO is mainly distributed in mesopores in FAM and LDM, and in macropores in LFM and IFM. TOC and clay mineral contents have strong impacts on the distribution of SPO. Carbonate minerals in LDM and IFM are dominated by ostracoda shells and authigenic calcite cement, respectively. The carbonate mineral content shows a positive correlation with WPO in LDM, in which the carbonate minerals are dominated by ostracoda shells with well-developed pores due to organic acid dissolution. TOC shows a positive correlation with WPO for TOC less than 1.2 wt%, while shows a negative correlation for TOC greater than 1.2 wt%. In the study area, LDM and LFM are identified as favorable lithofacies or “sweet spots” for shale oil exploration because they have relatively high TOC (0.7–2 wt%), high percentages of in situ WPO and a high mineral brittleness index.

Characterization and evaluation of shale lithofacies within the lowermost Longmaxi-Wufeng Formation in the Southeast Sichuan Basin

Based on drilling cores, thin sections, FIB-SEM and geochemistry data, shale lithofacies in the lowermost part of the Longmaxi-Wufeng Formation were studied in the Southeast Sichuan Basin. This formation is the main shale gas producing layer in Jiaoshiba and its surrounding areas. In this article, with the combination of the three-end-member method (using mineral components) and TOC, six lithofacies, namely, organic-rich siliceous shale, organic-rich mixed shale, organic-rich argillaceous shale, organic-lean siliceous shale, organic-lean mixed shale, and organic-lean argillaceous shale, were identified. Relevant parameters that could represent reservoir quality of the different lithofacies, including porosity, permeability, pore spaces, TOC and gas content, were analyzed and compared. In the end, some of those parameters (e.g., porosity, TOC and gas content), together with special mineral contents (e.g., QMF and clay), were used to establish shale lithofacies evaluation criteria. The results show that organic-rich siliceous shale could be considered as an excellent lithofacies for shale gas and is mainly concentrated in the first section; organic-rich mixed shale, organic-lean siliceous shale and organic-lean mixed shale are assumed to be moderately promising for shale gas, while organic-rich argillaceous shale and organic-lean argillaceous shale are ranked as the least-promising lithofacies in the research area. Consequently, the first section contains the most favorable lithofacies for shale gas exploration.

Formation mechanisms and distribution of weathered volcanic reservoirs: A case study of the carboniferous volcanic rocks in Northwest Junggar Basin, China

AbstractIn view of the complex reservoir heterogeneities within weathered volcanic rocks, taking the Carboniferous volcanic weathered profile reservoir of the northwest Junggar Basin as an example, this study aims to characterize weathered volcanic rocks by the core, thin slice, log interpretation, and experimental test. The Carboniferous volcanic rocks are mainly composed of volcanic lava, volcanic breccia, and tuff, etc, which formed in a volcanic arc environment. As a result of the weathering and denudation, the weathered profile can be divided into four layers: weathered clay layer, intensive weathered zone, weak weathered zone, and unweathered compacted zone. The preserved void space mainly includes primary porosity (both vesicular and fractures), secondary pores, and fractures enhanced by weathering and structural processes. Among them, the volcanic facies control the first‐order properties of the reservoir. The degree of weathering‐leaching controls the thickness of the weathered part of the reservoir. The faults and fractures control the seepage/permeability capacity of weathered volcanic reservoir. On the plane, the weathered volcanic reservoirs mainly distributed along the fault zone, tectonic zone, and favorable lithofacies zone. On the longitudinal direction, they are always within the weathered profile toward the top of a certain depth range (20‐240 m).

Effect of shale diagenesis on pores and storage capacity in the Paleogene Shahejie Formation, Dongying Depression, Bohai Bay Basin, east China

Abstract Compared to other marine-shale gas reservoirs in South China, the shale that developed in faulted basins in East China possesses peculiar characteristics regarding the mineral types and their evolution. The diagenesis of inorganic minerals, generation of hydrocarbons, and evolution of organic matter control the final features and quality of these shale reservoirs. Therefore, we must analyze this diagenesis based on the essential characteristics of the representative reservoir and discuss the influence of the diagenesis' evolution stage on the shale reservoir. This topic is also of practical value for contemporary shale-oil exploration and development. In this paper, we observe diagenesis, study its essential characteristics (such as the structure of the reservoir and pores), determine the key factors that control the reservoir's storage capacity, and analyze the role of diagenesis (organic and inorganic) in the evolution of pores and the reservoir's storage capacity. The mineral types are diverse, and the content of calcite and other carbonate minerals is generally very high. Organic matter is abundant, thermal evolution is low, and the hydrocarbon generation capacity is strong. Because of their high variability, some pores are hard to differentiate from microfractures. Dissolved pores, which are generated by intercrystalline pores and the dissolution of calcite, dolomite, and other carbonate mineral grains, are the major component of a macroporous matrix. The large proportion of hydrocarbons that are generated by solid organic matter leads to the accumulation of pressure. The shale generates a high number of microfractures, which form a reservoir space network system that increases the porosity of the reservoir. Dolomite has a unique supporting role for the matrix pores, leading to the development of a matrix pore-seam system. The reservoir's mineral composition, pore diameter, porosity, and organic-matter content determine the reservoir properties and oil saturation. Organic-rich laminated shale has the best physical properties, dominated by macro-pores and easily connected and merged pore seams. The massive shale has the poorest reservoir property. The development and evolution of pores are mainly controlled by the transformation of clay minerals, the content of carbonate minerals, the sedimentary structure and the intensity of diagenesis. Dissolution depends on the intensity of hydrocarbon generation, the expulsion of acid by organic matter, the state of the carbonate minerals and organic matter, and the smooth depression of the fluid environment. Organic diagenesis is closely related to the diagenesis and evolution of inorganic matter and is typically reflected in the recrystallization of calcite, abundance of organic matter, and hydrocarbon generation by organic matter. A local alkaline diagenetic fluid environment and reduction in the degree of hydrocarbon generation transform incremental porosity to pore reduction. Favorable lithofacies and advantageous layer sections should be selected by considering the stage of diagenesis evolution.

Reservoir characteristics and hydrocarbon accumulation of Carboniferous volcanic weathered crust of Zhongguai high area in the western Junggar Basin, China

Based on the drilling, cores, logs, seismic, laboratory analysis and so on, reservoir characteristics and hydrocarbon accumulation of Carboniferous volcanic weathered crust in Zhongguai high area are studied. Volcanic rocks were formed in an island arc environment. The lithology is mainly andesite and tuff; Reservoir spaces are mainly secondary pore, fracture and their combination forms, fractures have a better effect on reservoir seepage; There are four layer structures of volcanic weathered crust, weathered clay layer, strongly weathered zone, weakly weathered zone and unweathered zone and strongly weathered zone is the best, which is the main reservoir development zone; The development of reservoir is mainly affected by weathering-leaching, lithology and lithofacies, and fault (fracture); Effective reservoirs could reach to 480 m thickness (high quality reservoirs are within 240 m). Carboniferous volcanic reservoirs are distributed along three zones, which are near the fault zone, high structural part, favorable lithofacies development zone, and one plane, which is near the unconformity.

Evolution and migration of shale facies and their control on shale gas: A case study from the Wufeng-Longmaxi Formations in the Sichuan Basin and its surroundings

The Fuling gas field, the largest commercial discovery of shale gas in China, has been producing from the organic-rich shale in the Upper Ordovician Wufeng Formation and Lower Silurian Longmaxi Formation in the Sichuan Basin, China. A refined shale gas E&P and sweet spots identification require studies to be carried out not only at the sedimentary facies scale but also at the petrographic scale. This paper focuses on the characteristics of organic-rich shales. Based on data from outcrops, drilling cores, X-ray diffraction, thin sections, and scanning electric microscope, a systematic analysis of wells and well-correlation sections indicates that shales in the study area underwent two progressive and regressive cycles. In addition, differences in lithofacies and their organic or inorganic components are significant either in the vertical or in the horizontal direction, which generally controls the material basis of shale gas. The early-stage siliceous shale distributes in the east, whereas the late-stage carbonate-bearing shale extends to the west. In the southeastern Sichuan Basin, the siliceous composition decreases, whereas the clay minerals increase upward. In the central Sichuan uplift, there is a higher carbonate content. And with our new lithofacies maps, distribution of the sweet spots was identified. The northeastern Sichuan Basin has the siliceous shale in the Wufeng Formation and the silicon-bearing shale in the Longmaxi Formation, making it a play area with the greatest gas potential. The southern Sichuan Basin occurs as a favorable lithofacies area with the stacked siliceous shale in the Wufeng Formation and the calcareous shale in the Longmaxi Formation. The relatively favorable lithofacies area covers the regions extending from the periphery of southern Sichuan Basin toward the central Sichuan uplift and the northern Guizhou uplift, featuring stacked carbonate-bearing shale and clay mineral-bearing shale in the Longmaxi Formation. The shallow-marine areas close to the uplifts are dominated by mudstone and silty mudstone, thereby possessing poor material basis.

Petrological characteristics and shale oil enrichment of lacustrine fine-grained sedimentary system: A case study of organic-rich shale in first member of Cretaceous Qingshankou Formation in Gulong Sag, Songliao Basin, NE China

Abstract Taking organic-rich shale in the first member of Cretaceous Qingshankou Formation in the Gulong Sag, northern Songliao Basin as an example, this study examined the lithofacies classification, petrological characteristics, pore size distribution and their implications on the enrichment of shale oil of lacustrine detrital fine-grained shale. The spatial variation of lithofacies, controlled by Milankovitch cycle and influenced by sediment provenance, has an obvious sequence. The fine-grained sedimentary rocks of studied section could be classified into seven lithofacies according to a three-step classification criterion that consists of total organic carbon (TOC), sedimentary structure and mineral composition. Among them, the laminated siliceous mudstone lithofacies with moderate TOC has high hydrocarbon generation potential and abundant reservoir space, making it the most favorable lithofacies for the enrichment of matrix shale oil. Under the background of abnormally high pressure, the laminated siliceous mudstone lithofacies with moderate TOC deposited between the top of SSC2 and the bottom of SSC3 is stable in lateral distribution in the delta-lacustrine transition zone, with continuous thickness greater than 30 m. The massive siliceous mudstone lithofacies with high and moderate TOC developed in the middle of the these two cycles can act as the roof and floor seal for shale oil, therefore, the study area has good conditions for forming matrix type shale oil reservoirs.