High-porosity Sandstone Research Articles

Experimental study on impedance dispersion characteristics and electrical resistivity of high temperature granite

Electromagnetic methods are widely used in geothermal exploration. The electrical resistivity and impedance dispersion characteristics of high temperature rocks are the basis for the inversion interpretation of electromagnetic responses from geothermal reservoirs. Alternating current impedance spectroscopy experiments on granite at different temperatures and water contents were carried out. The impedance spectrum from these tests show that the resistivity of both dry and wet granite is mainly caused by grain boundaries or microfractures, rather than the mineral grains. The resistivity of granite decreases dramatically with the appearance of water, and decreases with increasing temperature. When temperature increases from 25 °C to 150 °C, the resistivity of dry granite decreases from 5690–5812 Ω·m to 5519–5638 Ω·m, while that of wet granite decreases from 3123–3278 Ω·m to 2618–2736 Ω·m. The relationship between temperature and resistivity can be described by an empirical equation, in which the temperature coefficient of dry granite is about 2.8 × 10−4, and that of wet granite is about 1.7 × 10−3. It shows the resistivity of wet granite is more significantly affected by temperature than that of dry granite. A mixing model describing resistivity of wet granite was established based on the resistivity of dry granite and water. This mixing model can also take temperature into consideration. Even though the equivalent porosity of granite is quite small, the resistivity of granite can still be described by Archie's equation, which was initially developed for high porosity sandstone. The coefficients in Archie's equation for granite were calculated from experimental results, with a tortuosity factor a = 0.005 and a cementation exponent m ranging from 1.50 to 1.75.

An experimental investigation of the characteristics of cataclastic bands in high-porosity sandstones

Cataclastic bands in high-porosity sandstones significantly influence fluid flow, thus impacting the exploration and development of oil and gas. However, little experimental research has been conducted on the main factors controlling the formation, evolution, and physical properties of cataclastic bands. Moreover, it is difficult to use field surveys to discern variations and trends in the structural and physical properties of cataclastic bands formed during different deformation processes. In this study, we used a high-pressure and low-velocity ring-shear apparatus to analyze high-porosity, pure sandstone. Multiple sets of ring-shear experiments were carried out using the effective normal stress or shear displacement as a single variable. The experimental samples were analyzed based on physical property tests and thin sections. Our results indicate that the particles in the cataclastic bands generally have better roundness and are smaller (by at least two to three orders of magnitude) than the host rock. The porosity and permeability of the cataclastic bands are ∼70% lower and two to three orders of magnitude lower than those of the host rock, respectively. The characteristics of the cataclastic bands are controlled by two main factors, namely, the effective normal stress and shear displacement. The effective normal stress controls the intensity of the cataclasis, and the shear displacement controls the physical properties of the grains and indirectly controls the evolutionary stage, which corresponds to the intensity of cataclasis. As the effective normal stress or shear displacement increases, the cataclasis in the cataclastic bands intensifies, and the grain size decreases; then, the decrease in the porosity gradually declines, and the permeability decrease and thickness increase and then plateau. The results of this study reveal the evolutionary mechanisms of the structural and physical properties of cataclastic bands in high-porosity sandstones and lay a theoretical foundation for determining the effect of these bands on fluid flow in oil and gas reservoirs.

The effect of chlorite rims on reservoir quality in Chang 7 sandstone reservoirs in the Ordos Basin, China

Chlorite rims have been of interest to petroleum geologists over the past several decades, as a significant number of abnormal high-porosity sandstone reservoirs at great depths have been related to chlorite rims. To clarify the contribution of chlorite rims to porosity preservation in sandstones, the relationship between chlorite rims and porosity evolution in Chang 7 sandstones of the Upper Triassic Yanchang Formation in the Ordos Basin in north-central China was investigated using thin sections, scanning electron microscopy, energy dispersive X-ray spectroscopy, and X-ray diffraction analyses. Based on the detrital composition and diagenetic evolution pathway analyses, it was concluded that chlorite rims inhibited the formation of quartz cementation. However, in addition to quartz cementation, compaction and other cements (commonly carbonates and clays) also control the porosity evolution of sandstones. In sandstones in which porosity reduction is largely controlled by compaction or other cements, chlorite rims may have a limited effect on porosity preservation. Thus, chlorite rims can play an important role in porosity preservation only in sandstones in which quartz cementation is the main process of porosity destruction. The results of this study indicate that the development of high-porosity sandstones is often controlled by many factors, and the effects of chlorite rims on porosity preservation are not always significant.

The development of cataclastic bands in high-porosity sandstones: Insights from ring shear experiments

Investigating the deformation mechanism of cataclastic bands in high-porosity sandstone is crucial for understanding the juxtaposition sealing ability of sandstones in a bed sequence. However, deformation bands that are developed in the field and rock cores do not reflect the evolutionary history of the host rock; therefore, the continuous deformation of high-porosity sandstone cataclastic bands cannot be observed. This paper analyzes the deformation mechanism that affects a cataclastic band when high-porosity sandstone faults form. Based on the latest independently developed high-pressure/low-speed ring shear experimental apparatus, the formation and evolutionary process of cataclastic bands in high-porosity sandstone were studied through the similarities among artificial cores. During the experiments, the effective normal stresses and fault displacement were used as single variables. After the experiments, plunger samples were drilled in the lateral direction, and thin sections were prepared to observe and analyze the thickness and particle characteristics of the deformation bands, thereby characterizing the deformation process of the damage zone. The experimental results reveal that high-porosity sandstone undergoes different intensities of cataclasis during the shearing process, and the evolutionary characteristics can be divided into four primary stages. The particles in the sandstone are successively subjected to rotation, rolling, cracking, cataclasis, and other changes after forced deformation. In terms of the macrostructure, a deformation band can be divided into two layers: an inner zone and an outer zone. From the host rock and the outer zone to the inner zone, the orientational arrangement of the particles along the shear direction becomes increasingly clear, and a greater displacement indicates a stronger orientational arrangement effect of the particles. In terms of the microstructures, the particle sizes in the deformation band are 2–3 orders of magnitude smaller than those in the host rock and exhibit greater roundness. With increasing shear displacement, the thickness of the cataclastic band first increases and then remains constant. With increasing effective normal stress, the thickness first decreases slightly, then increases and finally decreases slightly. Therefore, stress and displacement are important factors controlling cataclasis. The influence of displacement on cataclasis is clear at the initial stage, and stress determines the maximum intensity of cataclasis. Additionally, the increase in displacement will be favorable for roundness when the host rock is poorly sorted. These experimental results provide a theoretical basis for future studies on the effects of cataclastic bands and fluid flow.

The Effects of Porosity on The Anisotropy Parameters of The Slope Fan Facies Sand Reservoirs in The Deepwater Kutai Basin, East Kalimantan, Indonesia

For good interpretation and modelling, knowledge about the effects of anisotropy and its relationship with reservoir properties is needed. Related to this issue, this study evaluated the effects of porosity on the anisotropy parameters for sand reservoirs deposited in the upper slope fan facies. The study objects are obtained from core plug samples of sand reservoirs in the deep-water Kutai Basin. Core plug samples were collected from 13602.5 ft to 13704.5 MD depth. The ε, γ, δ, and η anisotropy parameters data were obtained by ultrasonic measurements, and porosity data were obtained by laboratory measurement with Coreval 700 apparatus and Boyle’s law. The analysis results show that the relationship of the anisotropy parameters with porosity appears when high-porosity sandstone and low-porosity sandstone are separated. The plots of anisotropy ε, γ, and δ, show trends for greywacke, with increasing anisotropy value the porosity increases. The effect of porosity on the high porosity (29%-37%) sandstone shows a steeper change than on the lower porosity (12%-13%) sandstone. The analysis also shows that the higher composition of lithic mineral grain reduces the effect of anisotropy on porosity.

Lithology and Porosity Distribution of High-Porosity Sandstone Reservoir in North Adriatic Using Machine Learning Synthetic Well Catalogue

Reservoir characterization on offshore fields often faces specific challenges due to limited or unevenly distributed well data. The object of this study is the North Adriatic poorly consolidated clastic reservoir characterized by high porosity. The seismic data indicate notable differences in reservoir quality spatially. The only two wells on the field drilled the best reservoir area. Seismic data, seismic reservoir characterization, and accurate integration with scarce well data were crucial. This paper demonstrates how the application of machine learning algorithms, specifically a Deep Forward Neural Network (DFNN), and the incorporation of pseudo-well data into the reservoir characterization process can improve reservoir properties prediction. The methodology involves creating different reservoir porosity and thickness scenarios using pseudo-well data, synthetic pre-stack seismic data generation, seismic inversion, and DFNN utilization to improve porosity prediction. This study also highlights the importance of lithology discrimination in the geological model to better constrain reservoir properties distribution in the entire reservoir volume. Facies probability analysis was utilized to define interdependence between litho–fluid classes established from the well data and acoustic impedance volume. Apart from the field well data, seismic inversion results, and DFNN porosity volume as main inputs, acknowledgments from the neighboring fields also had an important role.

Controls on anomalously high porosity/permeability of Middle Jurassic deeply buried tight sandstones in the Taibei Sag, Turpan-Hami Basin, northwestern China: Implications for reservoir quality prediction

The lower member of the Middle Jurassic Xishanyao Formation (J2x1+2) sandstones are significant exploratory targets for hydrocarbon resources in the Taibei Sag of the Turpan-Hami Basin, northwestern China. Formation of anomalously high porosity/permeability in deeply buried J2x1+2 sandstones and reservoir quality prediction were investigated using a variety of petrographic analyses. These results show that the J2x1+2 sandstones are mostly feldspathic litharenites and litharenites, which are characterized by moderate-to good sorting, silty to medium granularity, and point- to long grain contacts. The J2x1+2 sandstone has low porosity (avg 6.0%) and permeability (avg 1.12 mD), but shows anomalously high porosity/permeability at depth interval of 3850–4050 m. There is a noticeable variation in pore types and sizes from intergranular-intragranular dissolution pores with a size mode of 2.0 μm in anomalously high porosity (AHP) sandstones, to major intragranular dissolution pores with a size mode of 0.5 μm in normally high porosity (NHP) sandstones, to most micropores with a size mode of 0.25 μm occurred in the normally low porosity (NLP) sandstones. The compaction is the main cause of porosity destruction, resulting in an average porosity loss of 89.6%. However, the authigenic minerals have relatively little influence on reservoir quality. The combination of nappe tectonics and well-sorted particles alleviated the compaction and thus preserved more intergranular pores in the AHP sandstones. Dissolution further increases the porosity and eventually forms the AHP sandstones. The strong carbonate-cemented facies (SCC), tightly compacted facies (TC), moderately compacted with moderate dissolution facies (MCMD), and weakly compacted with strong dissolution facies (WCSD) are determined in the J2x1+2 sandstones. The sandstones with SCC and TC are recognized as the NLP reservoirs. The MCMD and WCSD correspond to the NHP and AHP reservoirs, respectively. The reservoir quality predicted using the constructed diagenetic facies charts is in good agreement with the photomicrograph observations and physical property tests. Additionally, the AHP reservoirs are still developed in the deeply buried sandstones with depths larger than 4000 m.

Validation of the Crack Mode-Changing Stress in Circular Tunnels Under Generalized Triaxial Tensile Stress States

In the past century, mining depths have increased profoundly from a few hundred meters to several kilometres following improvements in drilling techniques and instrumentation methods. In highly-stressed tunnels, the predominant failure mechanism of underground rock is controlled by stress-induced fractures parallel to the excavation boundaries in terms of spalling failures or, in some severe cases, a violent slab ejection that forms a V-shaped notch around the excavation walls. Such failures can seriously affect production and shut down the operation temporarily or even permanently. To better characterize, predict, and prevent these catastrophic failures, many laboratory tests and scaled model tunnels have been conducted under uniaxial and biaxial loading conditions, with limited studies also available under true triaxial stress states. This paper extends the previous studies on the impact of true triaxial loading conditions on the spalling failure in high porosity sandstone and investigates further the validation of a new Crack Mode-Changing Stress (CMCS) parameter, defined as the minimum principal stress required to change rock fracturing from splitting to a sliding failure mode. The results revealed that the minor principal stress can significantly affect the propagation of the stress-induced fractures of spalling failure along the tunnel sidewall.

Evolution and controls of high reservoir quality in Oligocene sandstones from moderate to deep burial, Nanpu Sag, Bohai Bay Basin, China

The Oligocene sandstones characterized by highly variable reservoir quality are the main hydrocarbon exploration and development targets in the Nanpu Sag, Bohai Bay Basin, China. In this study, the evolution and controls of reservoir quality in Oligocene sandstones from moderate to deep burial were investigated using petrographic and petrological data. The results show that the Oligocene sandstones are mostly fine-to coarse-grained, subangular to subrounded roundness, moderately sorted feldspathic litharenites and lithic arkoses with an average Q/F/R distribution of 48.4: 29.1: 22.5. Three anomalously high porosity/permeability zones (AHPZs, hereinafter named AHPZ-I, AHPZ-II, and AHPZ-III, respectively) are developed at depth ranges of 2300–3100 m, 3200–3550 m, and 3700–4200 m, respectively. Notably, the three zones are featured by distinctly different pore types: mainly primary pores in AHPZ-I; pore combination (average primary pore volume > average secondary pore volume) in AHPZ-II; primary pores with a few microfractures in AHPZ-III. The pore-throat sizes controlling permeability in anomalously high porosity sandstones (AHPS) decrease steadily from AHPZ-I to AHPZ-III-1 and then increase significantly at AHPZ-III-2. The AHPS reservoirs are characterized by low displacement pressure, large pore throat and mercury intrusion saturation, and high free fluid index (FFI) and reservoir quality index (RQI).Compaction greatly degraded the reservoir quality of Oligocene sandstones; carbonate cementation further restricts the formation of high-quality reservoirs. The sandstones with low content (<10 %) of carbonate cements tend to have larger primary porosity and secondary porosity, hence a larger RQI. The formation of AHPZ-I and AHPZ-II is attributed to higher content of rigid particle components, lower carbonate cements, and strong dissolution. Moreover, good sorting, responsible for a higher original porosity, also plays a positive role in the cause of AHPZ-I. The coarse-grained particles and early petroleum emplacement are the keys to the preservation of primary porosity in AHPZ-III. The petroleum emplacement effectively inhibits the feldspar dissolution and kaolinite precipitation in the deeply buried sandstones with depths larger than 4000 m. In addition, the generation of crushed microfractures in coarse grains leads to relatively low porosity and high permeability.

The effect of grain size and porosity on the nature of compaction localisation in high-porosity sandstone

Compaction bands are a type of deformation band with negligible shear in porous rocks such as sandstones and are important because they can impede subsurface fluid flow. However, the microstructural properties which favour their growth are not fully understood. We experimentally investigate the effect of porosity and grain size on compaction localisation using synthetic sandstones that permit control of the starting porosity (27%, 32%, 37%) and mean grain size (ranging from 314 to 987 μm). Each sample is subjected to the same triaxial test conditions and shortened by 5% axial strain at an effective confining pressure of 85% of their grain crushing pressure (P*). Discrete compaction bands oriented normal to the axial loading direction are found only in the sample with the lowest starting porosity (27%) and smallest grain size (314 μm), while diffuse bands are observed for the same porosity at a larger grain size of 411 μm. No compaction bands are observed for any grain size in either the 32% or 37% starting porosity samples. Porosity analysis indicates grain size reduction does not necessarily correspond to porosity reduction indicating that compaction by grain rearrangement is as effective as localisation through comminution for these high-porosity synthetic sandstones.

Porosity prediction using semi-supervised learning with biased well log data for improving estimation accuracy and reducing prediction uncertainty

SUMMARY Porosity characterization is of profound significance for seismic inversion and hydrocarbon prediction. Although semi-supervised learning (SSL) based methods have been used to boost prediction accuracy and lateral continuity of supervised learning (SL) inverted subsurface properties, their variations are relatively limited since the relationships between the data and the parameter model are straightforward in most reported cases. To further figure out their essential differences, we proposed the SSL-based network (SSLBN) for reservoir porosity prediction using seismic and well log data with disparate complexity and quality, and compared it with the SL-based network (SLBN). The SSLBN comprises a data-driven inverse model named decoder and a data-driven forward model named encoder based on the bidirectional-gated recurrent units. The architecture of the SLBN is the same as the encoder. Trained by several seismic-to-well pairs and numerous unlabelled seismic logs, the SSLBN learns the physical process from input single-trace observed seismic log to the intermediate porosity log, and the inverted porosity to the output generated seismic log. We first prepare the porosity model with biased or unbiased labels, the convolution model (CM) and reverse time migration (RTM) based synthetic seismic data, and then implement SL- and SSL-based statistical tests. The synthetic data examples demonstrate that the SSLBN has significant preponderance over the SLBN in the scenes of the RTM imaged seismic data and biased porosity labels. Compared with the SLBN, the physical regularization of the data misfit in the SSLBN improves estimation accuracy and reduces prediction uncertainty of porosity. Finally, statistical tests on a braided river deposited field data example illustrate that the SSLBN can generate more geologically trustworthy porosity models and indicate the oil layers of high porosity sandstone reservoirs.

Pore and permeability properties of reservoir sandstone under a uniaxial compression CT test

For underground gas storage projects that use depleted gas reservoirs, cyclic gas injection will change the stress fields and cause rock deformation. In-depth knowledge of the stress-induced deformation of reservoir rocks is important for secure gas storage. In this study, we performed a uniaxial compression computed tomography (CT) test with a reservoir sandstone sample to investigate the influence of rock deformation on the pore and permeability properties. Based on scanned 3D images, we analyzed the axial strain evolution by using the digital volume correlation (DVC) technique and the variations in pore properties with the microstructures and permeability properties that were extracted by the pore network modeling method. The DVC analysis shows that strain localization develops gradually with increasing axial stress. The initial porosity distribution with sample height is inhomogeneous. The local porosity variations are not completely consistent with the global porosity variations under increasing axial stress. The level of inconsistency will decrease with increasing axial stress in both the volumetric compression and dilation stages. Increases in axial stress mainly induce the variations in pore numbers and pore volumes. The variations in pore connectivity are not significant. For the permeability evolution, X-ray CT scanning combined with pore network modeling can simulate the permeability decreases during volumetric compression. For high porosity sandstones (>15%), the permeability still decreases in the volumetric dilation stage due to grain size reductions and tortuosity increases. However, this technique has limitations in analyzing the permeability reductions in the volumetric dilation stage. The microcracks that are induced inside the grain particles are too small to be recognized. The variations in the relative gas and water permeabilities at different axial stresses are not significant.

THE IMPACT OF PLASTERING ON THE HYGROTHERMAL BEHAVIOUR OF HISTORICAL SANDSTONE LOCATED IN THE COASTAL REGION OF LEBANON

In an unusual trend in the conservation practice of built heritage in Lebanon, the external plaster layer, which protects the sandstone from weathering factors, is being removed for aesthetic reasons. These buildings are in a coastal region, hot humid climate in the summer and moderate cold weather in the winter. This paper discusses the importance of external and internal plastering of the historical sandstone bearing wall, by the mean of a computational tool that underlines the role of the plastering in the hygrothermal behaviour of the historical stone, in the aim to validate the practice of the ancestors in covering the envelope of their buildings to avoid the degradation of these natural stones. The methodology adopted in this paper is a comparative quantitative study, using a hygrothermal simulation modelling tool, named Wufi Pro, which is a standard program for evaluating moisture conditions in building envelopes developed by the Fraunhofer Institute for Building Physics (IBP) Stuttgart, Germany. The results are in the form of values for the heat and moisture fluxes through the high porosity sandstone assembly that composes the building envelope. These values represent the hygrothermal behaviour of the stone, and they are compared between two cases, one without plastering while the second has the inner and outer layers of traditional plaster. The analysis of the results is guiding to a conclusion that favours one of the two cases, taking into consideration the interstitial condensation threat avoided by the means of plastering.

A coupled elastic constitutive model for high porosity sandstone

An elastic constitutive model is developed to predict the mechanical unloading response for high porosity sandstone. The resulting model is sufficiently complex to represent known mechanical response, yet reasonably straightforward to calibrate. A mechanical dataset from a suite of true triaxial tests on Castlegate sandstone is used to develop, calibrate, and validate the model, which incorporates the experimentally observed evolutions of the elastic bulk and shear moduli with stress and plastic strain. Furthermore, to properly represent the hydrostatic unloading response, a coupled term is required, in which the plastic volume strain (accumulated during both hydrostatic and deviatoric loading) modifies the nonlinear stress dependence of the bulk modulus. Omission of this coupling term causes significant over prediction of the plastic volume strain and bulk modulus after complete unloading. However, the coupled term is not required to model deviatoric response and shear modulus evolution. While a suite of 28 tests was used to develop the model, only one carefully controlled test is required to calibrate the model for a chosen high porosity sandstone.

The role of post-orogenic normal faulting in hydrocarbon migration in fold-and-thrust belts: Insights from the central Apennines, Italy

The reconstruction of the hydrocarbon migration and entrapment history is fundamental for reducing risks and identifying traps and migration pathways during oil exploration and exploitation in fold-and-thrust belts, which host 14% of the world's discovered oil reserves. Within fold-and thrust belts, faults and folds have fundamental roles in controlling hydrocarbon migration, entrapment, and/or leakage. Past studies have mainly focused on the role of syn-orogenic thrusts, tear faults, and folds in controlling hydrocarbon migration and entrapment. On the contrary, little attention has been dedicated to the role of normal faults in hydrocarbon remobilization during the post-orogenic extension. We combine field and subsurface data with geological evidence from the Ripi oil field, central Apennines (Italy), and propose a conceptual model for hydrocarbon remobilization along post-orogenic normal faults and potential entrapment within shallow reservoirs. Results show that post-orogenic normal faults promoted up-dip hydrocarbon remobilization from oil traps generated during fold-and-thrust belt development. Where pre-orogenic deposits are still covered by syn-orogenic deposits, hydrocarbons moved up-dip along post-orogenic normal faults and impregnated high-porosity sandstone lenses, generating discontinuous and heavy oil reservoirs. On the contrary, where syn-orogenic deposits were eroded in response to orogenic uplift, hydrocarbons leaked at the Earth's surface. Results from this study can be applied to reduce risks during hydrocarbon exploration and appraisal in similar tectonic settings, such as Hellenides and Carpathian fold-and-thrust belts, as well as the southern Apennines of Italy, where the Tempa Rossa and Monte Alpi oil fields, two of the largest onshore oil fields in Western Europe, were discovered in the last decades.

Catalog of Coefficients for Estimating Bulk and Shear Moduli as a Function of Lithology

The purpose of this paper is to present correlation coefficients for a variety of rock types that can be used in a suitable petroelastic model (PEM). The correlation coefficients for different rock types facilitate the application of a petroelastic model in reservoir flow models. By combining the correlation coefficients and the PEM, it is possible to obtain low-cost estimates of reservoir geophysical attributes. The rock types include dolomite, limestone, high porosity sandstone, poorly consolidated sandstone, tight gas sandstone, and well consolidated Gulf Coast sandstone.

Estimating Reservoir Properties from 3D Seismic Attributes Using Simultaneous Prestack Inversion: A Case Study of Lufeng Oil Field, South China Sea

Summary Lufeng oil field was discovered in the 1980s in the Pearl River Mouth Basin (PRMB) of the South China Sea and has entered the stage of secondary oil recovery. One major problem that has restricted the subsequent exploration of the oil field is the unclear regional reservoir and caprock distribution, because practitioners have been using post-stack attributes and acoustic impedance inversion to analyze the distribution of sandstone (reservoir) and mudstone (caprock). In current geophysical research on reservoirs and caprocks, prestack inversion has been widely used because of its advantage over post-stack inversion. However, the accuracy of using single P/S-wave velocity ratio (VP/VS) or density (Vden) inverted by prestack to predict lithology is still insufficient. In this study, we created a new attribute VRDEN, the sum of VP/VS and the weighted Vden, to capture the lithology variation of the reservoir. We integrated 3D seismic data and well log data and applied simultaneous prestack inversion and multiattribute regression analysis to determine reservoir properties, such as sand thickness, effective porosity, and distribution of sandstone and mudstone of the Lufeng oil field. Then, we calculated the new attribute VRDEN from VP/VS and Vden obtained from simultaneous prestack inversion to determine the lithology variation. The multiattribute regression analysis, combining prestack attributes and post-stack attributes, indicates the effective porosity and sand volume in the Enping Formation, which contains the main oil-bearing reservoirs in the Lufeng oil field. Results show that when the sandstone thickness is greater than 12.5 m, the prediction error of VRDEN is the lowest compared with VP/VS and Vden. In En-2 member of the Lower Enping Formation, medium- to high-porosity (14 to 17%) sandstone (19.5 m thickness) is widely distributed in the west and middle of the study area, with an area of nearly 300 km2. The high-porosity sand zones stretch from the east of the Lower Enping Formation to the west of the Upper Enping Formation, which is the result of westward progradation of the braided delta. Our workflow used a novel seismic attribute VRDEN in the simultaneous prestack inversion and multiattribute regression process to provide a more predictive spatial distribution of reservoir-nonreservoir features. The improved reservoir understanding will allow more efficient exploitation of the Lufeng oil field, and the improved workflow will facilitate exploration of other oil fields in the world.

The manufacture, mechanical properties, and microstructural analysis of synthetic quartz-cemented sandstones

A key issue in the field of porous rock geomechanics is finding suitable suites of samples in which petrophysical characteristics systematically vary, in order to determine the controlling factors on the mechanical behavior. To resolve this problem a new methodology has been developed, whereby high-porosity synthetic sandstone samples are produced from initially incohesive sand, by using the chemical reaction between sodium silicate and hydrochloric acid to generate amorphous quartz cement and thus provide cohesion. This enables the production of sandstone samples for laboratory testing which have reproducible petrophysical properties that can be systematically controlled, including porosity, grain size, cement content, grain shape and grain size distribution. Here, poorly-sorted, high-porosity (36–38%) sandstones were produced and subjected to a range of geomechanical tests, including uniaxial and triaxial experiments, to see if their microstructure and mechanical response was similar to equivalent natural samples. The 36–38% porosity synthetic sandstones have realistic and reproducible uniaxial compressive and tensile strengths of 5.18 (± 0.76) MPa and 0.167 (± 0.117) MPa respectively, as well as hydrostatic yield strengths (P*) of 67–69 MPa. They also exhibit yield curves with a comparable geometry to natural sandstones of similar porosity and grain size and display elastic moduli within the expected range for natural sandstones. The methodology outlined in this contribution allows for the production of sandstones for laboratory testing with reproducible behavior, that can be utilized in future studies to systematically investigate how different petrophysical properties control the mechanical response of sandstone to elastic and inelastic deformation.

Deformation Bands and Associated FIP Characteristics From High-Porous Triassic Reservoirs in the Tarim Basin, NW China: A Multiscale Analysis

Deformation bands are widely formed and distributed in Triassic high-porous rocks as a result of multistage tectonic movement. In this research, core observation, the rock thin section (fluorescence and casting thin section), FIB-SEM, X-ray diffraction, Raman laser, and thermometry of fluid inclusions were employed to describe the macro- and micro characteristics of deformation bands and their associated relationship with microfractures. Results indicate that the main types of deformation bands formed in the Lunnan Triassic high-porosity sandstone during the Yanshanian and Himalayan periods under different temperature and pressure conditions are compaction shear bands, and their quantity increases evidently with the distance of thrust faults. The density of deformation bands near the fault is about 15/m; porosity and permeability decrease sharply compared with those of the host rock. Microscopically, two obtained fluid-inclusion planes (FIPs) can be distinguished as 51 samples collected from 12 wells by the cutting relationship and mechanical characteristics. The homogenization temperature of associated aqueous inclusion is generally characterized by two peaks, mainly 70–80°C and 110–120°C, which were formed in the Late Yanshanian and Late Himalayan periods. The formation period of deformation bands induced by the intragranular microfractures improved the reservoir seepage capacity. In the later stage, as the interlayer and barrier with low porosity and low permeability affects the distribution of oil and gas, which is an important factor in this study of the local fluid dynamic field and high-quality reservoir evolution distribution.

Empirical Formula for Dynamic Biot Coefficient of Sandstone Samples from South-West of Poland

In this research, two empirical correlations have been introduced to calculate the dynamic Biot coefficients of low-porosity and high-porosity sandstone samples from two open pit mines located in South-West Poland. The experiments were conducted using an acoustic velocity measurement apparatus. Under the undrained condition, firstly, the confining pressure was increased in increments of 200 psi, and the values of pore pressure and dynamic elastic modulus were recorded. This experiment was continued until the Skempton coefficient remained in the range of 0.98–1. Secondly, an experiment on the same sample was conducted under drained conditions, and the corresponding dynamic elastic moduli were calculated. Then, using the calculated dynamic elastic moduli, the dynamic Biot coefficient was determined for each sample under different confining pressure. Finally, two empirical correlations were formulated for each sandstone category. The results demonstrate that, as the confining pressure increases, the Biot coefficient decreases from 0.79 to 0.50 and from 0.84 to 0.45 for low-porosity and high-porosity samples, respectively. Furthermore, as the porosity increases, the sandstone behavior increasingly approaches that of soil. The empirical correlations can be used for sandstone formations with the same porosity in projects where there is not a measurement method for the Biot coefficient.