Mudstone Reservoirs Research Articles

Novel insights to unconventional carbonate mudstone reservoir with quantitative nanoporosity characterization and modeling of Tuwaiq Mountain Formation

Organic-rich carbonate mudstones have emerged as a significant unconventional play globally, particularly the Middle Jurassic carbonate mudstones of the Arabian plate. Despite their importance, these mudstones are highly heterogeneous, composed of organic-lean and organic-rich deposits, and often exhibit similar reservoir quality, making them challenging to characterize. While most studies have attributed the reservoir quality of these mudstones to the organic-related pores, little attention has been paid to the origin of the pore structure and network of the organic-lean counterparts. To address these issues, we performed high-resolution quantitative pore characterization and modeling by coupling micro-computed tomography and focused ion beam scanning electron microscopy to gain insight into the matrix-related pore structure, size, and networks on the organic-lean mudstones. Our study showed that in the organic-lean mudstones micropores are present but isolated, while the nanoscale pore structure appears to be well-connected, hence higher permeability. The pore-network model result has a median pore radius of 189–226 nm and a median throat radius of 91–112 nm, rarely exceeding 1000 nm and 500 nm, respectively. The frequency plots demonstrate positively skewed distributions indicating that large pores and throats are anomalous. Our study highlights that the organic-lean mudstone nanoscale pores are comparable to previously reported nanopores in the organic-rich zone and other mudstone plays around the globe. It emphasizes coexisting organic and matrix-related nanopores, suggesting the significance of matrix-bound nanopores' influence on carbonate mudstones unconventional play. The findings can enhance our understanding of the nano-scale pore structure and provide insights to evaluate and build development strategies of carbonate mudstones in the Arabian Intrashelf Basin and elsewhere.

Laminae Characteristics and Their Relationship with Mudstone Reservoir Quality in the Qingshankou Formation, Sanzhao Depression, Songliao Basin, Northeast China

Lamination is the predominant and widely developed sedimentary structure in mudstones. Similar to organic pores in shale gas reservoirs, the inorganic pores in the laminae of shale oil reservoirs are equivalently important high-quality reservoir spaces and flow channels. The laminae characteristics are strongly heterogeneous, being controlled by both deposition and diagenesis. However, the origin of this diversity is poorly understood. A detailed examination of cores, thin sections, and scanning electron microscopy analyses were conducted on the lacustrine mudstone of the Qingshankou Formation in the Songliao Basin to study the influence of deposition and diagenesis on laminae characteristics and their relationship to reservoir quality. Three types of laminae are mainly developed, namely thick siliceous laminae, thin siliceous laminae, and thin siliceous and argillaceous mixed laminae. Deposition controls the type and distribution of laminae. The thin siliceous and argillaceous mixed laminae are controlled by climate-driven seasonal flux variations. The thick siliceous laminae and thin siliceous laminae are controlled by bottom current or gravity-driven transport processes due to increased terrestrial input. The thin siliceous laminae have the optimum reservoir properties, followed by the thin siliceous and argillaceous mixed laminae, while the thick siliceous laminae have the worst properties. Diagenesis controls the pore evolution of the laminae. Different laminae have different paths of diagenesis. The thin siliceous laminae are mainly cemented by chlorite, preserving some primary porosity. The clay mineral content of the thin siliceous and argillaceous mixed laminae is high, and the primary pores are mainly destroyed by the strong deformation of the clay minerals during compaction. The thick siliceous laminae are intensely cemented by calcite, losing most of the porosity. The present study enhances the understanding of reservoir characteristics in laminae and provides a reference for shale oil exploration.

Microstructural observations of clay-hosted pores in black shales: implications for porosity preservation and petrophysical variability

AbstractTypical unconventional gas/liquid plays of China were studied using field-emission/focused ion beam scanning electron microscopy (SEM) for clay mineralogy and microstructural development. The SEM microstructural investigations of clay-rich shale and mudstone reservoirs provided significant information about clay mineral type, size, distribution and aggregates, which allows for interpretations regarding porosity preservation and petrophysical variability. The major clay-hosted porosity types are interparticle clay pores, intraparticle clay pores and aggregate pores. Interparticle clay pores occur in open spaces of the clay aggregates and include four subtypes: (1) elongated pores, (2) packed pores, (3) jagged pores and (4) card-house pores. Intraparticle clay pores are present within clay particles and have mostly secondary origin. These pores are diagenesis dependent and are restricted to secondary illite particles during the transformation of clay minerals from smectite to illite. Intraparticle clay pores constitute only isolated porosity and could not contribute to hydrocarbon molecule storage and migration. Aggregate pores were predominantly encountered in association with organic–clay and pyrite–clay aggregates. In places, organic–pyrite–clay aggregates can also display polymerization, but they do not contribute significantly to overall porosity and permeability. Combining SEM morphological analyses with the software ImageJ is critical in clay microstructure and porosity analyses via semi-quantitative characterization of the 3D pore surface, 2D pore profile, pore quantity, pore size, areal porosity, etc. These visual and semi-quantitative results highlight the significance of jagged pores and pyrite–clay aggregate pores in shale gas/liquid reservoirs because they may be important facilitators of gas storage and transmission.

Study on Elastoplastic Damage Coupling of Reservoir Mudstone Considering Permeability Change

Mudstone, a common complex medium in oil and gas reservoirs and with widely distributed micro-pore and micro-fissures, is liable to produce significant damage evolution and plastic deformation under high buried depth stress environments. Based on the analysis of the physical characteristics, the elastoplastic damage coupling mechanical characteristics of mudstone in a high buried depth reservoir for oil and gas engineering are discussed. Firstly, conventional triaxial compression tests under different confining pressures were performed to calculate the damage variable and obtain the damage evolution. The damage evolution included the elastic damage stage, the plastic-dominated elastoplastic damage coupling stage and the damage-dominated elastoplastic damage coupling stage. Secondly, a coupled elastoplastic damage mechanical model for mudstone was proposed, which was based on the degradation of the damage stiffness and plastic flow caused by the plastic and damage internal variables and considered the elastic damage coupling and elastoplastic damage coupling during the loading process. Thirdly, the elastoplastic damage coupling mechanical characteristics of mudstone were simulated. The simulation results are in good agreement with the experimental results, which reflects well the mechanical characteristics of mudstone, including the transition from volume compression to expansion, plastic hardening, damage softening and residual strength, etc. Finally, based on the relevant research results, a permeability evolution model of mudstone based on the damage was proposed, and the secondary development was carried out based on ABAQUS. UMAT and USFLD subroutines were compiled, and seepage–stress coupling simulation verification was carried out. The relevant results provide a reliable basis for engineering theory research and stability analysis of deep mudstone reservoirs.

Particle size distribution analysis of mudstone based on digital image processing

Mudstone is becoming increasingly important for unconventional oil and gas development. The morphological characteristics of clastic are not only contributions to the 3D framework of mudstone, but they can clearly affect the physical properties of a mudstone reservoir, including surface area, pore-size distribution, and porosity. However, it is very difficult and time-consuming to measure the particle size distribution (PSD) of mudstone because of its small particle size and strong cementation. However, because of the importance of oil and gas extraction, it is urgent to develop a fast, accurate, and objective analysis method to measure the PSD of mudstone. Based on a comparison of various PSD measurement methods commonly used in the energy industry and geology, the best PSD measurement method for mudstone should be digital image processing. Two imaging methods, trainable Weka segmentation (TWS) and black mudstone particle size measurement, were used to analyze sections of the early Silurian Longmaxi mudstone of China’s Sichuan Basin. The PSD data obtained by manual measurement, TWS, and the contribution method are compared. Image analysis finds that the particle sizes of all samples fall in the range of coarse silt to clay, and the average sizes fall in the range of coarse silt to fine silt. The skewness, kurtosis, standard deviation, and other distribution characteristics parameters find minor errors, and the relative error is less than 15%. The Pearson correlation coefficient of the 10th quantile of TWS, black mudstone particle size measure, and manual measurement were calculated, which found R2 values typically ranging between 0.64 and 0.87. Kolmogorov-Smirnov test results find that the data obtained by the three measurement methods are from the same distribution at the level of 0.05. Analytic results find that the method presented is effective, cost-efficient, and could avoid artificial errors.

Pore Structure Characteristics, Genesis, and Its Controlling Effect on Gas Migration of Quaternary Mudstone Reservoir in Qaidam Basin

The Sanhu Depression in Qaidam Basin is the largest Quaternary biogenic gas exploration area with the shallowest burial depth in the world. The shallow high abundance of gas reservoirs and high-quality pure methane have become the main production areas. The development characteristics of loose mudstone reservoirs are restricted by extreme heterogeneity. Therefore, the Quaternary Qigequan Formation mudstone in Qaidam Basin is selected as the research object. Based on the study of the sedimentary background, petrological characteristics, and pore structure characteristics, by comparing the characteristics of different mudstone reservoirs, the controlling factors of the development of different mudstone reservoirs and the control of gas migration are clarified. Research shows the following: (1) The mudstone reservoirs of the Qigequan Formation mainly develop intergranular pores, clay mineral pores, and intragranular pores. The pore size distribution varies from nanometers to micrometers, and mesopores mainly contribute to the specific surface area. (2) Rigid minerals and clay minerals are the main controlling factors for the pore structure of mudstone reservoirs. The increase in the content of rigid minerals is conducive to the development of macropores or larger micropores, while the increase in the content of clay minerals is conducive to the development of mesopores and provides an important specific surface area for gas adsorption. (3) The gas migration form of pure mudstone is mainly dominated by Fick diffusion and slippage flow, which has the characteristics of self-sealing accumulation. The gas migration form of silty mudstone is the coexistence of Fick diffusion, slippage flow, and Darcy flow, which has the features of self-sealing and Darcy flow accumulation. The gas migration form of sandy mudstone is mainly Darcy flow, only with Darcy flow accumulation characteristics. The flow form of gas creates different accumulation modes of mudstone biogas. The Quaternary mudstone reservoir shows different particularity under different material components, and the exploration targets should be treated differently according to specific mudstone types.

NMR T1-T2 Logging in Unconventional Reservoirs: Pore-Size Distribution, Permeability, and Reservoir Quality

The rock-pore textural properties in unconventional fine-grained reservoirs are much more complicated than in conventional reservoirs. In fine-grained rocks, this complexity derives from very small component grain assemblages (clay and silt fractions) and very small pores (nano- to microsizes) overprinted by diagenesis. It is further compounded by the pores being developed in intergranular, grain dissolution, and organic-associated-intragranular voids. Additionally, because locally sourced hydrocarbons in unconventional reservoirs, in contrast to migrated hydrocarbons that are present in conventional reservoirs, exist within similar pore sizes as the original saturating water phase, they create geometrically complex fluid distributions. These imply that, in the context of nuclear magnetic resonance (NMR) logging and petrophysical interpretation of tight oil unconventional reservoirs, pore size can no longer be considered a proxy for fluid type and vice versa. This means using T2 or T1 cutoffs may give inaccurate predictions. Current industry applications of NMR T1-T2 logging have demonstrated reliable interpretation of fluid types and water saturation. However, because petrophysical controls of advanced relaxation effects (such as surface and bulk fluid relaxation properties, pore-size distribution, and wettability) are not properly understood, these applications have been limited to estimating fluid saturations and have not been applied to estimating pore sizes. We extend the application of NMR T1-T2 measurements in tight oil unconventional reservoirs to model pore-size distributions by using apparent surface relaxivities and bulk relaxation times that have been jointly estimated from poro-fluid relaxations. To test these predictions, modeled pore-size distributions are compared to rock-pore textural properties from petrographic images of core samples. Such comparison allows us to derive insights into controls of mudstone reservoir quality (RQ) in conjunction with the impact of rock fabric endmembers (e.g., matrix-supported clays, grain-supported framework, diagenetic cements, and solid organic matter) on pore-size distributions. With these assumptions in mind, it follows, based on the Kozeny-Carman formulation, that unconventional rock permeability is reliably predicted from the NMR-based pore-size distributions. Furthermore, we deduce the impact of mineral assemblages in the Herron permeability model to infer influence on rock textural properties and ultimately predict permeability in limited data environments. This paper introduces a novel approach to estimate pore-size distributions per fluid type (water and hydrocarbons) from NMR T1-T2 measurements in unconventional reservoirs. Additionally, petrophysical controls of RQ, storage, and transport properties are derived from NMR-based pore-size distributions and mineral assemblages. This enables more reliable RQ and permeability assessment than typical T2- or T1-cutoff methods.

The nature, origin, and predictors of porosity in the Middle to Late Devonian Horn River Group of the Central Mackenzie Valley, Northwest Territories, Canada

The characterization of porosity is an essential step in the evaluation of resource-bearing porous media. Here, we focus on the Devonian Hare Indian and Canol Formations, two potential unconventional mudstone reservoirs, in a core from the Horn River Group of the Central Mackenzie Valley, Northwest Territories, Canada. By combining bulk porosity, low-pressure N2 adsorption, and scanning electron microscopy (SEM) results with composition and lithofacies datasets, we assess the porosity in these successions to understand pore types, size, distribution, degree of connectivity, and controls and predictors of porosity. Mineral matrix pores (interparticle and intraparticle), organic matter pores, and lithofacies-dependant natural fractures are present. All pore types display limited connectivity in two dimensions. Mineralogy is the most significant control on porosity with trends in porosity present among lithofacies. No relationship is observed between porosity and total organic carbon (TOC), suggesting that mineral matrix pores, rather than organic matter pores, are dominant in this unit. We compare these results to other mudstone reservoirs in North America and show that the Bluefish Member (Hare Indian Formation) and the Canol Formation are characterized by comparable bulk porosity, lower N2 mesopore volume, and higher quartz content relative to the other units considered. In contrast, compared to the other unconventional reservoir examples, the Bell Creek Member of the Hare Indian Formation exhibits lower quartz and higher clay content, average bulk porosity, and lower N2 pore volume. The results collectively suggest that high quartz and low clay content are the best predictors of porosity in the Horn River Group. Natural fractures may serve as flow pathways to induced fractures; however, these units lack the network of interconnected organic matter pores that can be present in other successions.

Porosity and Pore Networks in Tight Dolostone—Mudstone Reservoirs: Insights from the Devonian Three Forks Formation, Williston Basin, USA

Porosity and Pore Networks in Tight Dolostone—Mudstone Reservoirs: Insights from the Devonian Three Forks Formation, Williston Basin, USA

Depositional and diagenetic controls on reservoir quality of microporous basinal lime mudstones (Aptian), United Arab Emirates

This petrographic and petrophysical study constrains parameters controlling porosity-permeability heterogeneity in unconventional microporous lime mudstones reservoir of the Lower Bab Member (Aptian) deposited in intrashelf Bab basin, Abu Dhabi, United Arab Emirates. Depositional texture and diagenetic processes control the petrophysical properties of the mudstones. The high concentration of coccoliths with relatively large intragranular pores causes increase in porosity and permeability. Diagenetic control on reservoir quality includes the degree of development and thickness of syntaxial micro-overgrowths around the micrite particles. Mudstones with well-developed and coalesced overgrowths, which are most common in the vicinity of stylolites, have the lowest porosity (5–8%) and permeability (0.01–0.1 mD) and narrowest pore throats (around 0.05 μm). Conversely, mudstones with relatively poorly developed syntaxial calcite micro-overgrowths have greater porosity (20–24%), permeability (0.3–2 mD) and pore-throat size (0.5 μm). Scattered moldic pores formed by the dissolution of aragonitic bivalves caused increase in porosity but not permeability of the wackestones. Combining petrographic and petrophysical properties revealed that depositional parameters and diagenetic processes resulted in three distinct unconventional microporous lime mudstone reservoir types.

History of exploration and research of Miocene biosiliceous mudstone reservoirs, preliminary study of the reservoir properties in the thermally-matured biosiliceous mudstones of the Onnagawa Formation

History of exploration and research of Miocene biosiliceous mudstone reservoirs, preliminary study of the reservoir properties in the thermally-matured biosiliceous mudstones of the Onnagawa Formation

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.

Impact of thermal maturity on the concomitant evolution of the ultrafine structure and porosity of marine mudstones organic matter; contributions of electronic imaging and new spectroscopic investigations

This study aims to better understand the evolution of the organic matter (OM) ultrafine structure (designating the nanoscopic structure of the OM/macerals) and porosity with increasing thermal maturity of mudstones source-rocks. To this end, the particulate fractions of kerogen from organic-rich Kimmeridge clay mudstones were isolated by acidic treatment before and after laboratory thermal maturation. The evolution of the composition, the chemical structure and the porosity of kerogen from the immature stage (Ro = 0.42%) to the dry gas zone (Ro = 2.12%) were documented using a combination of elemental analysis, vitrinite reflectance measurements, Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM), Raman spectroscopy and Small Angle X-Ray Scattering (SAXS). The evolution of the porosity of OM was then compared to the evolution of the pore volume of the total rock measured by nitrogen adsorption. The results show that the progressive densification, reorganization and aromatization of the amorphous kerogen particles into a more ordered, but heterogeneous carbon-rich residue is responsible for significant variations in the kerogen porosity with increasing maturity. Contrary to the previous observations on these Kimmeridge clay mudstones, the variations that occur during the peak of oil generation (Ro of ca. 0.90%) mark the onset of the OM-hosted pores development. There appears to be a natural close relationship between the evolution of OM porosity and total pore volume of organic-rich mudstones during gas generation. Indeed, a similar alternation of pore collapse and pore development is observed in response to gas generation. The development and the evolution of pores in these organic-rich mudstones seems thus mainly driven by the evolution of the chemical structure and the composition of OM during thermal maturation. In the dry gas zone, the porosity and the specific surface area of the kerogen are significant (19% and 57.1 m2.g−2 respectively). This highlights the importance of the OM content, type and composition in the porosity and gas storage capacities of mudstone reservoirs, increasingly discussed in the available literature in recent years.

Laboratory Investigation on Optimum Selection of a Sand Control Method for an Interbedded Sandstone and Mudstone Reservoir

It is critical to select an optimized sand control method for an interbedded sandstone and mudstone reservoir (ISMR) due to its serious sand production hazards. However, currently, most general sand control methods cannot meet the requirements of sand control in interbedded sandstone and mudstone reservoirs (e.g., Bohai Bay oil and gas fields from China). Ensuring efficiency of sand control and increasing the oil and gas production rate in this interbedded sandstone and mudstone become more and more important. In this paper, a “multilayer rotatable sand control experimental device” for the interbedded sandstone and mudstone reservoir was developed. A series of sand control experimental studies were conducted by using the proposed device. The net-to-gross ratio (NTG) and well inclinations are two major factors considered in the experimental analysis. In addition, a sensitivity analysis regarding formation particle size distribution (PSD), clay content, and mineral compositions is performed in the experiment under a moderate sand control mode. With systematic experimental test results in this work, combined with numerous existing sand control models, a set of optimum sand control design and the associated optimization template for ISMR were developed, which have been successfully applied in Bohai Bay. Field application results show that NTG and well inclination are two critical parameters in the design of sand control in ISMR. The optimal indexes of a sand control mode are determined as NTG of 0.4 and well inclination of 45°. The introduction of these two key factors in sand control design broadens the application range of moderate sand production.

Integration of Formation Evaluation Data Overcomes Offshore Coring Challenges

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 195866, “Overcoming Coring Challenges in a New Unconventional Play Offshore by Integration of Formation Evaluation Data,” by Thomas Bradley, SPE, Baker Hughes, and Per Henrik Fjeld and Jonathan Scott, Aker BP, et al., prepared for the 2019 SPE Annual Technical Conference and Exhibition, Calgary, 30 September-2 October. The paper has not been peer reviewed. A well was drilled into a prospective unconventional mudstone play offshore Norway. Two of five coring runs were successful while the rest yielded little to no core recovery. Subsequent investigation of the core substantiated that the coring issues largely had natural causes. This understanding is being applied to two imminent coring operations and has driven selection of drilling, coring, and wireline technology and procedures and is informing casing design. Introduction The Valhall field is in Block 2/8 of the Norwegian North Sea. The primary reservoir is Cretaceous chalk, located at approximately 2400 m true vertical depth. Production of this reservoir began in 1982. Above this reservoir are other formations, including a shallower potential unconventional mudstone reservoir. A production well was drilled targeting the Cretaceous chalk reservoir. As part of the drilling program, logging-while-drilling (LWD) and wireline logs - including LWD density, neutron, gamma ray and acoustic, wireline spectral gamma ray, cross-dipole acoustic, nuclear-magnetic-resonance, and formation-testing and -sampling - were run across the potential mudstone reservoir to appraise the standard petrophysical properties. A conventional core was also attempted; however, recovery was poor, with repeated jams. Coring Challenges During coring operations, significant problems were encountered when coring the mudstone intervals. These challenges are illustrated in Fig. 1, which shows a section of the core from xx14 m, demonstrating the typical condition of the recovered core when still in the core barrel. At the time of operation, poor recovery was attributed to the coring practices used and no further coring was attempted. Formation Evaluation The mudstone interval was not a primary target for the well. Therefore, limited formation evaluation of these intervals was performed, mainly to estimate the petrophysical properties. However, at the time of initial logging, more-advanced formation evaluation data (e.g., images, full waveform acoustic, and nuclear-magnetic-resonance data) were not evaluated fully. A few years after initial operations, the log data were revisited - specifically, detailed interpretation was conducted for all recorded log measurements. On examination of the results over much of the interval, the integration of the conventional and advanced volumetric measurements gave considerable extra value in assessing the petrophysical properties of the mudstone. However, detailed discussion of these results is outside the scope of this paper. In addition to the extra information gleaned from the advanced volumetric interpretation, over certain intervals, some unusual log responses and interpretation results were seen that were deemed worthy of further investigation.

Source/reservoir characteristics and shale gas “sweet spot” interval in Shahezi mudstone of Well SKII in Songliao Basin, NE China

Samples from the Shahezi mudstone of Well SongkeII (SKII) in the Songliao Basin were analyzed for mineral composition, source rock geochemistry, and reservoir pore structure. It is revealed that the Shahezi mudstone is a typical clay rock, with an average TOC of 2.12% (dominantly Type II–III organic matters) and a vitrinite reflectance Ro of 2.28–2.34%. The pore structure of the Shahezi mudstone reservoir mainly comprises intergranular pores and interlayer fractures in clay minerals, and also organic pores (including fractures at the particle-organic edges and also pores in some unstructured organic matters). The Shahezi mudstone has a specific pore volume of 0.01–0.04 cm3/g. Correlation analysis of specific pore volume and mineral composition reveals that the pore development of the Shahezi mudstone is mainly controlled by inorganic minerals. The nitrogen adsorption and high-pressure mercury intrusion tests were combined to obtain the full-scale pore size distribution and specific pore volume of the Shahezi mudstone, and the shale gas “sweet spot” interval is inferred to be 3,500–3,700 m. The study results can be referential for understanding the pore characteristics of highly-mature continental mudstone and locating the “sweet spot” of shale gas globally.

Reservoir characteristics and logging evaluation of gas\u2212bearing mudstone in the south of North China Plain

Mudstone is very similar to shale except it lacks sheet bedding. Shale gas is widely concerned and successfully exploited commercially in the world, while gas-bearing mudstone is rarely paid attention. To evaluate the reservoir characteristics and exploitation potential of gas-bearing mudstone, a total of 127 mudstone samples from the Shanxi formation were tested by X-ray diffraction (XRD), scanning electron microscope (SEM), gas content, etc., and the qualitative identification and quantitative evaluation of gas-bearing mudstone reservoirs were performed on four wells using the logging curve overlay method and reservoir parameter calculation equations. The results showed that: (1) the average total gas content of core measurement is 1.81 m3/t, and the total content of brittle minerals is 44.2%, which confirms that mudstones can also have good gas content and fracturing performance; (2) logging evaluation the average thickness of gas-bearing mudstone is 55.7 m, the average total gas content is 1.6 m3/t, and the average brittleness index is 38.1%, which indicates that the mudstone of Shanxi formation in the study area is generally gas-bearing and widely distributed. All the results reveal that gas-bearing mudstone with block bedding has the same exploitation potential as shale with sheet bedding,which deserves more attention.

Tidal modulation of river-flood deposits: How low can you go?

Abstract Quantification of the interaction between river discharge and tides is vital to characterize fluvio-deltaic systems, to identify diagnostic elements of tidal signatures in the rock record, and to reconstruct paleogeographies. In modern systems, even microtides can significantly influence delta morphodynamics; yet, many fundamental processes, particularly in prodeltaic settings, remain elusive. Here, by combining a unique process-product data set acquired during a flood event of the Po River (Italy) with numerical modeling, we show that tidal signatures are recorded in the open-water prodelta zone of a microtidal system. Based on the analyses of box-cores collected before and after a flood off the main distributary channel, we interpreted storm beds, tide-modulated flood strata of alternating normal and inverse graded beds, and rapid bioturbation. Modeling of the river discharge indicates that, at the peak of the flood, the steepening of the water-surface profile forced by 0.15 m lowering of sea level during low tides generated an 8% increase in river flow velocity. The alternation of profile steepness and associated cyclicity in flow strength during consecutive tidal cycles controlled the sediment load of the plume and, consequently, led to the deposition of tidal-modulated strata. Formation of microtidal signals appears to be enhanced in fluvio-deltaic successions characterized by multiple distributaries and in basins where river floods are out of phase with storm-wave activity. Bioturbation of sediment, which can start during the waning stage of the flow, and erosion by storm waves hamper the preservation of tidal signals, unless rapid burial occurs. The recognition of tidal-modulated strata in river-dominated settings may facilitate the characterization of mudstone reservoirs and reconstruction of paleogeographic conditions during deposition.

Miocene Biogas Generation System in the Carpathian Foredeep (SE Poland): A Basin Modeling Study to Assess the Potential of Unconventional Mudstone Reservoirs

This paper presents the results of a research project aimed at evaluating the unconventional natural gas potential of the autochthonous Miocene sediments in the Polish part of the Carpathian Foredeep. The primary objective of the study was to re-evaluate the biogenic gas generation system within Miocene sediments, paying special attention to unconventional gas resources accumulated in tight mudstone formations. The four-dimensional (4D) petroleum system modeling method (PetroMod software) was used to reconstruct the basin geometry and three-dimensional (3D) evolution through a geological timescale, in particular the progress of gas generation, migration, and accumulation processes, as well as their consequences for gas exploration and development. Special attention was paid to the dynamics of gas generation processes and the advancement of sediment compaction and their time dependence, as well as to the progress and outcomes of gas migration and accumulation processes. The results indicate significant potential for unconventional gas accumulations in mudstone reservoirs. However, part of the biogenic gas resources occurs in a dispersed form. Analysis of the dynamics of biogenic gas generation and accumulation conducted on a basin scale and within particular sedimentary complexes and depth intervals allowed an indication of the premises regarding the most favorable zones for mudstone–claystone reservoir exploration.

English

Modern Rate Transient Analysis (RTA) is the hottest technology in recent years in reservoir engineering. Its main working principle is combining production data with bottom-hole pressure data based on the assuming reservoir model and well model. It will evaluate the reserves (Oil Drainage Radius and STOIIP), reservoir properties (permeability), wellbore condition (skin) and forecast production performance for single well based on matching Blasingame and Log-Log type-curve. The RTA was found to be extremely powerful and popular particularly with high resolution to obtain equivalent results with PTA rather than limited shutting well. With benefit of continued monitoring of production and FBHP or FWHP, it is possible to evaluate the wells’ interference and boundary in distance besides conventional reservoir properties. In this paper, taking a carbonate reservoir in the Middle East as an example, this technology was applied to diagnose and analyze the abnormal production of pilot horizontal wells. Based on the analysis results, combining with the understanding of geological study, material balance analysis, numerical simulation and adjacent vertical well’s performance, it is clear that the distribution of mudstone interlayer and reservoir poor connectivity in member-A are the main interior factors of the abnormal production of pilot horizontal wells. According to the research, this paper put forwards optimization of development strategy for subsequent producing subzones, well location and well type of new well in interest area. Key words: Rate transient analysis, type-curve, reservoir properties, carbonate reservoir, Middle East