Bar Sandstones Research Articles

Geosite Pripiri, Coração de Jesus, Minas Gerais, Brazil: paleontological relevance, quantitative assessment, and initiatives for its promotion

The Quiricó Formation is the richest fossiliferous unit of Sanfranciscana Basin. Its paleoasis context (a paleolake record inserted in a desert setting) includes vertebrates, diverse microfossils, and paleobotanical content. This study presents new findings regarding the fossil content of the formation, focusing on discoveries made at the Pripiri Geosite, located in Coração de Jesus municipality, northern part of the state of Minas Gerais, Brazil. Among the discoveries are poorly informative fragments of putative archosaurs and novel microfossiliferous taxa. The microfossil assemblage includes dinocysts and Euglenophyceae, both associated with microbial mat-forming cyanobacteria. These, in turn, comprise the filamentous Pseudoanabaena sp. and the coccoidal cells of Aphanocapsa sp. or Chroococus sp. These microbial mats are recurrent throughout the analyzed stratigraphic succession, primarily observed in sandstone bars featuring distinct evaporitic features. This assemblage provides ecological insights concerning the biota resistance to extreme climatic conditions, and their occurrence in oxygen-rich waters. The preservation of these cells in evaporite-rich rocks is also of great interest for astrobiological studies and the search for past life on Mars. Therefore, due to the importance of Pripiri locality, a quantitative analysis of its geodiversity and geoheritage value were performed, revealing that Pripiri comprises a geosite of national relevance in terms of its scientific, touristic and educational values. In this sense, some initiatives have been carried out to geoconservation and to promote the local geodiversity, aiming to contribute to the paleontological knowledge of the Early Cretaceous, as well as to the social and economic development of Coração de Jesus.

Depositional facies controls on reservoir rock typing and heterogeneity: Devonian siliciclastic succession, south central Ghadames basin, Libya

The Devonian sedimentary succession hosts the most potential reservoir targets in Ghadames Basin, Northwest Libya. This succession consists of a complex pattern of siliciclastic facies deposited during successive episodes of sea level fall and rise, thereby leading to a large scale heterogeneity in sandstone facies connectivity and distribution. The present study integrates core sedimentology, wireline logs, and routine and special core analysis to establish a framework for predicting the distribution of the best Devonian reservoir rock types (RRTs). A lowstand amalgamated fluvial sandstone bars only exist in the Lower Devonian Tadrart Formation; the other Devonian rock units consist of transgressive and highstand tidally-influenced estuarine and deltaic facies. The pore system characteristics are controlled by initial depositional attributes where open, megapores with entry displacement pressures <10 psi occur in the quartzose, high energy fluvial sandstone bars, delta distributary channels and mouth bars. These facies are classified as RRTI and are mostly associated with lowstand and late highstand facies. In contrast, the other RRTs display tight pore system with elevated irreducible water content >20%. RRTI rocks account for greater than 90% of the fluid flow capacity in the Devonian reservoirs. A link between the facies depositional setting, RRT distribution, vertical amalgamation and lateral compartmentalization of flow conduits, barriers and baffles zones has been proposed, thereby promoting the predictability of the best reservoir targets.

Integrated seismic-stratigraphic, sedimentological and petrophysical approaches for characterizing the reservoir potential of early Cretaceous Alam El Bueib formation in the Meleiha concession of North Western Desert, Egypt

The Early Cretaceous late syn-rift sediments of Alam El Bueib Formation host potential, however poorly-explored reservoir targets in the north Western Desert of Egypt. The controls on depositional evolution and architecture of Alam El Bueib sedimentary facies, as well as the influence of depositional attributes on reservoir facies heterogeneity, are poorly constrained. Therefore, this study integrates seismic, sedimentological, and petrophysical data from Meleiha concession in the north Western Desert to build evolutionary models for Alam El Bueib succession and predict the architecture of the potential reservoir facies. Several channel systems were observed with a notable upward transition from complex-filled to flat-filled channels. The complex-filled channels correspond to the early highstand tidally-influenced estuarine, deltaic/estuarine sediments of Alam El Bueib-IV and Alam El Bueib-IIIG units. Matruh canyon and Alam El Bueib-IIIG fluvial sand bars were developed during a phase of forced regression, and the canyon was infilled by hemiplegic mudstones during the Late Valanginian sea level rise. The late highstand and lowstand facies of Alam El Bueib-IIIG unit present the main fluid flow conduits. The minimal degree of reservoir heterogeneity occurs in the lowstand amalgamated fluvial sandstone bars, whereas lateral and vertical reservoir heterogeneity increases in the deltaic/estuarine and estuarine successions of Alam El Bueib-IIIG and Alam El Bueib-IV, respectively. The lateral compartmentalization of the Alam El Bueib-IIIG fluvial facies against Alam El Bueib Matruh canyon shales produced excellent stratigraphic traps that should be tracked throughout the entire north Western Desert.

Lithostratigraphic and Paleoenvironmental Characterization of the Senonian Series of the Errachidia-Boudenib-Erfoud Basin, South-East Morocco

Six lithostratigraphic sections distributed in different parts of the Errachidia-Boudenib-Erfoud basin (SE Morocco) were surveyed bed by bed and sampled in order to study stratigraphy, analyzed the facies, and characterized the depositional environment of the Senonian series. This study shows that this series is mainly constituted of three lithostratigraphic units: the first one is mixed, attesting to an alluvial plain environment disturbed by marine incursions at the base. The second is dominated by "kerkoub" sandstone bars intercalated by silt-clay intervals, reflecting a floodplain environment. The third is dominated by gypsum versicolored facies, marking an evaporitic playa environment. The correlation of the studied sections shows that the Senonian sedimentary prism opens progressively to the East. The series ends with Tertiary deposition: western marine carbonates and eastern continental conglomerates. The non-deposition of these faces in the center of the basin could be the result of the presence of a paleorelief at the Tinejdad-Goulmima transition.

The Late Paleozoic Ice Age in western equatorial Pangea: Context for complex interactions among aeolian, alluvial, and shoreface sedimentary environments during the Late Pennsylvanian – early Permian

The evolution of depositional environments in the Late Pennsylvanian-early Permian of the Paradox Basin in Utah, USA, is investigated through detailed sedimentological and high-resolution sequence stratigraphic analyses, in order to define a model of landscape evolution, to discuss the stratigraphic model, and to evaluate the significance of the cyclicity in the paleoclimatic context. Forty high-resolution cycles integrated in 15 minor and two major cycles are observed for the first time throughout the Late Pennsylvanian-early Permian units. A-three steps landscape evolution is recognized. First, the lower Cutler beds, mainly corresponded to a marine environment, with longshore bar, subtidal, tidal, mouth-bar, and with locally fluvial deposits. The upper part of the lower Cutler beds also contains an aeolian dune. Second, the Cedar Mesa Sandstone, corresponded to broad erg deposits which are present across the entire study area, whereas longshore bar, subtidal, mouth-bar, and some fluvial deposits are mainly preserved in the northern part of the studied area. Third, the Organ Rock Formation records decreasing aeolian dune field preservation. To the south, the aeolian environments are interbedded with shoreface deposits, whereas to the north, fluvial deposits with some mouth-bars are more developed. Semi-arid climatic conditions persisted, as indicated by the presence of calcretes. Everywhere in these three steps, root traces within sandstone bars indicate that a soil was once present above the sand dunes. This new model of landscape evolution documents complex interactions between aeolian, fluvial and marine environments within the entire Permian succession of the Paradox Basin. High-resolution stratigraphic analyses allow to discuss the significance of the cyclicity in a scenario that take into consideration sea-level variation in the Late Paleozoic Ice Age paleoclimatic context. This Late Pennsylvanian to early Permian succession reflects both relative sea-level fluctuations and the variability of sediment supply.

Imaging Lowstand prograding stratigraphic pinch-out traps of a Lower Cretaceous clastic system using inverted seismic-based porosity simulation of SE-Asian Basin: Implications for petroleum exploration

Prograding sandstone-filled incised valleys of the Lowstand System tract (LST) form stratigraphic plays. The band-limited seismic data interpretation techniques constrain the prediction of porous and gas-bearing reservoir facies. Therefore, the development of a cost-economic petroleum-bearing reservoir system is a challenge for reservoir geophysicists. This research is intended to predict the porosity and thickness of incised valleys by executing the continuous wavelet transforms (CWT)-based spectrum decomposition (SD), seismic static inverted simulation (SWM), and inverted seismic-based porosity simulation (IVPS) on a Lower Cretaceous LST system of Pakistan. The seismic, root-mean-square (RMS), and absolute average seismic attributes have provided a limited capability to image the gas-bearing sandstones. The 24-Hz tuning frequency has imaged the progradational sandstone-filled incised valley and retrogradational fluvial point bars. The 37-Hz has imaged the aggradational barrier bars. The amalgamation of multiple attributes has accurately demarcated the reservoir sandstone and sealing shale. The SWS has resolved a 32-m-thick prograding incised valley, 30-m-thick aggrading barrier bars, and 23-m-thick retrograding point bars within the complete fluvial hierarchy. The well correlation has provided evidence for the distribution of point bars, barrier bars, and shallow-marine incised valley reservoir facies. The 24-Hz CWT waveform-based conventional IVPS has predicted 94 and 42 m-thick valleys and "point bars" between 25 and 13% pseudo-porosity wells. The 24-Hz waveforms-based treatment of conventional IVPS has predicted enhanced thicknesses of 75 and 29 m for valley and point bar deposits with porosities between 25 and 13%. The 24-Hz IVPS has resolved two valleys compared to the conventional IVPS during the rapid fall of sea level and negligible rise, which have accumulated the coarse-grained point bar deposits of a fluvial meandering stream. The integration of IVPS and GR (API) has provided strong implications for the exploration of a pure stratigraphic trap within a shallow marine sedimentary system of Pakistan. A NE to SW oriented regional change was observed in the lithology, thickness, and depositional environments of the petroleum system. The dominant lithology in the eastern zones was point bar sandstones. The dominant lithology in the central zones was barrier bar sandstones. The dominant lithology in the western zone was incised valley sandstone. The thickness of the reservoir was 18 m thick in the west, which was reduced to 8 m in the eastern flanks of this gas field. The depositional environment in the northeast was fluvial, which changed to shallow-marine in the southwestern zones of the Indus basin. These lateral fluctuations have proved that a pure stratigraphic fairway was present within the LST of the Indus Basin of the sedimentary system of Pakistan. The porous incised valley stratigraphic reservoirs are embedded inside the non-porous shales, which validates the presence of a pure stratigraphic petroleum play inside the Indus Basin, Pakistan.

Seismic attributes and static reservoir simulation applications for imaging the thin-bedded stratigraphic systems of the Lower-Cretaceous Lower Goru fluvial resources, Pakistan

• Constructed workflow is a vital case for discovery of gas-bearing fluvial stratigraphic traps. • The SRSN resolves the thin-bedded point bar sandstone deposits in the basin. • AI and thicknesses for point bars sandstone are -0.214 to -0.219 g/c.c.*m/s and 17 to 19 m. • SRSN image sedimentary facies during the sea-level rise and fall events. Fluvial depositional systems accommodate a variety of primary stratigraphic-based trapping pools (STP) such as the channelized sands (CS) and point bars (PB). When they are encased within the non-porous and compacted shales, they develop hydrocarbon-bearing reserves. However, these STP are developed during the rapidly rising sea level and very slow fall, which creates hurdles in the development of accommodation space to accumulate the thin-bedded (hydrocarbon-bearing) and coarse-grained PB. PB are the primary stratigraphic traps within the fluvial hierarchy. Therefore, they require sophisticated tools such as reflection-based impedance reservoir simulations to simulate the thickness, accommodation space, proximal and distal locations, and sealing configurations of PB from a 3D perspective. The conventional seismic amplitudes are not sufficient enough to quantitatively portray these seismic sedimentological attributes due to a lack of preserved amplitudes and tuning frequency content aimed at fluvial-dominated clastic sequences. Therefore, the ultimate goal of conducting this research work is to detect thin-beds of PB and CF aimed at accumulating and generating hydrocarbon-posture STP. These thin-bedded reservoir facies are very delicate in tuning the frequency of the sub-surface systems. They require amplitude-based seismic data interpretation tools such as amplitude-based seismic attributes and inverted reservoir simulations. These tools can image the exact thickness of the reservoir lithology and types of fluids compared to the bandlimited geometrical attributes. Generally, the inverted reservoir simulations reconstruct the reflection-based impedances for the stratigraphic reservoirs, which helps in the quantification of the sub-surface thin-bedded stratigraphic systems. Therefore, the amplitude-based seismic attributes and thin-bedded wedge static reservoir simulation (SRSN) tools in Pakistan's Indus Basin. The amalgamated root-mean-square (RMS)-based seismic amplitude and frequency highlighted a full of twist continuous meander channel with a sinuosity index (S.I) > 2.7. Strong low-frequency shadows are observed at 23 Hz magnitudes that authorize the restoration of petroleum-posture fluvial-dominated PB STP. The amplitude attenuation model using the continuous wavelet transform (CWT) showed attenuation of spectral energy below the PB. The SRSN determined a 16 m thickness of STP using a constant velocity of 3449 m/s, which pinched out in the east of the Indus Basin. The predicted physical attributes from SRSN and their cross-plots against the thickness constraint have discriminated against the dominant sedimentary facies of the fluvial system during the significant rising and negligible falling sea levels. The predicted physical parameters have enabled vigorous controls aimed at developing accommodating intervals that have stored a significant amount of hydrocarbon-bearing PB. The quantitative characterization has enhanced acoustic impedance [AI] [g/c.c.*m/s] and thickness of PB with a strong correlation coefficient of R 2 >0.93. The predicted AI [g/c.c.*m/s] and thicknesses [m] aimed at PB were -0.214 to -0.219 g/c.c.*m/s and 16 to 19 m, respectively. Therefore, the SRSN can be used as a dynamic instrument for the accurate prediction of AI [g/c.c.*m/s] and thickness for the characterization of fluvial systems. This workflow may serve as a vital example for exploring the oil and gas-bearing stratigraphic system within the Indus Basin and similar basin settings.

Sedimentary Architecture Analysis of Deltaic Sand Bodies Using Sequence Stratigraphy and Seismic Sedimentology: A Case Study of Jurassic Deposits in Zhetybay Oilfield, Mangeshrak Basin, Kazakhstan

Three-dimensional (3D) seismic data and well log data were used to investigate the sandstone architecture of the Middle Jurassic deltaic reservoirs of the Zhetybay Oilfield, Mangeshrak Basin, Kazakhstan. The base-level cycles of different scales were identified and divided using well log and 3D seismic data. Five types of sedimentary boundaries were identified in the mouth bar sandstones. The boundaries divide single mouth bars. Vertically, the spatial distribution of sand bodies can be divided into superposed, spliced, and isolation modes. Laterally, contact modes can be divided into superposition, lateral, and isolation modes. We found that the base-level cycle controls the evolution of the delta front sand body architecture. In the early decline or late rise of the base-level cycle, the superimposed or spliced modes dominate the sand body. By contrast, the lateral or isolation modes dominate the sand body in the late decline or early rise of the base-level cycle. This paper proposes an architecture model of the delta front sand bodies controlled by the base-level cycle. The spatial distribution and morphological variation of deltaic sand bodies could be linked to the base-level cycles.

Investigating the Frequency Spectrum Characteristic of Stress Wave under Multistage Loading Stress

The stability of rock mass is destroyed by natural or human activities and leads to stress redistribution, causing the rock mass in a certain stressful environment. This study conducted a small disturbance impact tests on sandstone bar under loading stress by modified split Hopkinson pressure bar (SHPB). The results show that the reflection and transmission characteristics of stress wave are affected due to the loading stress changes in the sandstone porosity. The loading stress has a specific effect on the frequency spectrum distribution of the stress wave. The frequency spectrum curve has gone through the three stages, a gradual increase, then rapid attenuation, and finally a smooth development with the frequency increasing, and its dominant frequencies are mainly concentrated in 0∼2 kHz. The loading stress has a significant influence on the variation tendency of the dominant frequency. The dominant frequency experiences a slow increase and then tends to be stable, and the total energy of the frequency band shows a fast attenuation and then a gentle development, and its stress boundary point is σ/σc = 30%. The total energy attenuates as a first‐order exponential function and its attenuation rate shows an exponential‐linear function with the increasing loading stress, the farther away from the shock end, the faster the total energy attenuation is. The sandstone can filter the high‐frequency wave and the low‐frequency wave can penetrate rock media better. The closer the distance to the impact source, the greater the total energy of the frequency band. The frequency band energy is mainly concentrated in 0∼36.62 kHz, the higher the frequency of the frequency band is, the smaller the energy ratio is. Therefore, those conclusions can provide a reference for the evolution analysis of the stress wave spectrum in an excavated rock mass.

Seismic stratigraphy, sedimentary facies analysis and reservoir characteristics of the Middle Jurassic syn-rift sediments in Salam Oil Field, north Western Desert, Egypt

The Middle Jurassic syn-rift sediments (Khatatba Formation) of the Western Desert, Egypt hosting potential reservoir intervals are prospective targets for hydrocarbon exploration and development campaigns. However, their depositional evolution and factors controlling distribution of the best reservoir faces are not very well-constrained. This work integrates seismic, well logs and conventional core dataset in order to define the syn-rift sedimentary patterns and their impact on distribution and quality of the reservoir facies. Khatatba Formation comprises two discrete units; the lower unit constitutes lower progradational and upper retrogradational sedimentary packages that are interpreted in terms of discrete rift phases. Nine sedimentary facies associations (FA) were recognized and their stacking patterns are governed by the rift evolution and the interplay between variation in fault offsets and sediment supply. Distal sand bar sandstones, marine shales and organic-rich shales were accumulated during periods of high subsidence. Successive alternation between shallow and deep facies at the base of the lower unit reveals repeated short periods of progradation and retrogradation with an overall progradational trend reflecting an elevated rate of sediment supply.Episodic fault-controlled retrogradation and progradation continued during the deposition of the Khatatba upper unit where the progradational deltaic facies changes upward into retrogradational marine sediments and aggradational-progradational aeolian and shoreface sandstones. Five hydraulic flow units (HFUs) were identified and their distribution is mostly controlled by the rift evolution. The lowest reservoir quality HFU4 and HFU5 are often associated with the retrograding distal facies accumulated during highest rates of subsidence. These zones do not contribute to more than 10% of the reservoir flow capacity. On the other hand, the progradational deltaic facies accounts for approximately 85% of the reservoir flow capacity in both lower and upper Khatatba units.

Origin and distribution of grain-coating and pore-filling chlorite in deltaic sandstones for reservoir quality assessment

Grain-coating chlorite preserves porosity and permeability through the inhibition of quartz cement whereas pore-filling chlorite blocks pore-throats and diminishes reservoir quality. The aim of this study is to determine the origin and principal mechanisms which govern the distribution of grain-coating and pore-filling chlorite in Jurassic deltaic sandstones (Tilje Formation, Smørbukk field, Mid Norwegian Shelf). The study focussed on very high-density sampling for petrographic analysis, from three sections of sandstone core from the same well with contrasting reservoir quality, rather than the low-density sampling approach typically employed. The aim was to gain new understanding of specific controls on porosity and permeability based on core description, core analysis measurements and a suite of petrographic techniques. Results of this study show grain-coating chlorite originated from the thermally-driven recrystallisation of detrital clay coats and/or clay mineral precursors. Pore-filling chlorite has principally derived from the ductile deformation of chlorite-rich Fe-ooids, that were possibly reworked from a proximal evaporitic setting. The distribution of chlorite precursor material, detrital clay coats, and subsequently the distribution of grain-coating and pore-filling chlorite, were controlled by the relative dominance of tidal and fluvial processes active during sediment deposition. Optimum grain-coating chlorite is found in tidal-fluvial sandstones with moderate fluvial influence. Pore-filling chlorite is pervasive in tidal-fluvial channel sandstones deposited during periods of high fluvial discharge, or proximal to the central turbidity maximum zone; marked by an abundance of fluid mud. Tidal channel sandstones with no fluvial influence are pervasively quartz cemented due to an absence of grain-coating chlorite. Grain-coating chlorite and good reservoir quality occurs in heterolithic distributary mouth bar sandstones, however mixing of mud- and sand-prone facies due to intense bioturbation has reduced permeability. Results from this study can be used to predict reservoir quality in the Smørbukk field and in analogous shallow-marine sandstones worldwide.

Sedimentological, Lithostratigraphic, Clayey and Exoscopic Study of the Senonian Series of Arg N’Sidi Ali Ou Bourk, High Central Atlas of Morocco

The sedimentological study of the red formations (Senonian series) represents a challenge and an exciting subject that needs to be explored. This paper will present the sedimentological, stratigraphic, and paleogeographic aspects of the Upper Cretaceous of the ARG N'SIDIALI OR BOURK, Central High Atlas Morocco. In this region, a marker section was realized bed by bed and sampled in a very straightforward way in order to study stratigraphy, to describe, to interpret and to characterize the sedimentary environment of the senonian succession in the ARG N'SIDIALI OU BOURK region, more particularly the analysis of the clays by X-ray diffractometer (XRD) has allowed understanding the depositional environments of the Upper Cretaceous formations. Therefore, the Senonian of ARG N'SIDIALI OU BOURK corresponds to continental lagoon deposits. It is characterised by sedimentation in the continental environment (fluvial or flooded), with some rare marine ascents marked by the installation of whitish sandstone bars.

Spalling of a one-dimensional viscoelastic bar induced by stress wave propagation

As a stress wave propagates in rock, the amplitude of the stress wave will attenuate, and the waveform will dissipate, which can be equivalently characterized by the stress wave propagation in a viscoelastic material. The stress wave propagation may induce rock damage when the stress is greater than the rock strength. In this study, viscoelastic wave propagation in rock and the damage and failure process of rock induced by viscoelastic wave propagation, which are usually examined separately in existing theoretical and numerical models, were investigated under a unified framework. Based on viscoelastic damage theory and the theory of stress wave propagation in a one-dimensional viscoelastic bar, theoretical equations for spalling fracture of a viscoelastic bar were obtained. Then, the numerical model implemented by the one-dimensional wave propagation program was validated by reproducing the dynamic spalling of green sandstone bar; after that step, the program was employed to study the behavior of viscoelastic bar spalling. Based on the numerical simulation, the condition, position and time of the dynamic spalling induced by quarter sine wave propagation in the viscoelastic bar were examined, and the influence of the amplitude and frequency of the quarter sine wave and the viscosity coefficient and length of the viscoelastic bar on the spalling were quantified.

Facies associations, detrital clay grain coats and mineralogical characterization of the Gironde estuary tidal bars: A modern analogue for deeply buried estuarine sandstone reservoirs

Abstract Estuarine tidal bar sandstones are complex reservoir geobodies commonly exploited by the oil and gas industry. In order to better predict the reservoir potential of these geobodies, this study provides a modern-day reservoir analogue, describing tidal bars in the inner and outer Gironde estuary from the microscopic to the macroscopic scale. The originality of this work lies in the multi-scale study of modern estuarine tidal bars based on numerous piston cores extracted in a high-energy environment. This work demonstrates that these tidal bars are composite sedimentary bodies made up of individual reservoir sand units separated by thick muddy layers. Their vertical facies associations and internal architectures are controlled by local hydrodynamic variations and seasonal river influxes. Detrital clay grain coats are notably characterized using a portable and handheld mineral spectrometer from the base to the top of the tidal bars. X-ray diffraction and electron microscopes reveal that these coats are mainly composed of di-octahedral smectite, illite, chlorite and kaolinite associated with other components such as diatoms or pyrite. The best reservoir geobodies are those with the minimum clay permeability barriers at the macro and mesoscale. An optimum coated grain content and clay fraction volume is also needed for generating authigenic clay coatings and inhibiting quartz overgrowth. These conditions are met within the tidal sand bars of the outer estuary funnel that are expected to be the best reservoir geobodies in deeply buried sandstones.

Experimental study on stress wave attenuation and energy dissipation of sandstone under full deformation condition

Excavation behaviours not only destroy the balance of original stress in rock mass but lead to the redistribution of initial stress, resulting in rock mass in a new stress environment. The static stress in rock mass has the characteristics of dynamic change. For the purpose of investigating the effect of increasing static stress on attenuation mechanism and energy dissipation of stress wave in sandstone under the overall deformation condition, small disturbance experiments on sandstone bar are carried out with a modified split-Hopkinson pressure bar test system. The stress waveform, p wave velocity, temporal and spatial attenuation of amplitude, and energy dissipation are studied under various static stresses. Results show that stress waveforms at different locations are approximately the same, those at the same locations vary greatly, and the tensile waves that appeared at the tails of stress waves are larger with increasing static stress. With increasing static stress, p wave velocity experiences a dramatic increase, then develops gently, and finally a sharp decrease; the stress demarcation points are σs/σc = 30% and σs/σc = 55%, respectively. P wave velocity can be predicted by a quadratic equation when σs/σc > 63.69%. The relation between longitudinal wave velocity and static stress can be described by a quadratic function. Stress wave amplitudes decrease exponentially with increasing distance and time. Values of temporal spatial response intensity (RI) and spatial RI are approximately equal and present the same development tendency of nonlinear and linear stages due to the same dimensions. Values of temporal attenuation characteristic (AC) and spatial AC are differed by 2 or 3 orders of magnitude although they present the same development tendencies. Ratios of RI and AC can indicate the sensitivity of temporal and spatial attenuation to different static stresses. Full-wave energy declines in an exponential function with increasing static stress and decays as a logarithmic function with increasing distance. Widespread attenuation characteristics like these should provide a theoretical basis for rock mass stability analysis.

Hydraulic - Electrical Flow Units and Water Saturation Calculation: Dam Formation, South Western Qatar, Arabian Gulf

Hydraulic-electric flow units represent two different concepts with different measuring units. They are frequently used to confirm each other’s for distinguishing the different reservoir flow zones but sometimes give undesirable results. 60 limestone samples were collected from the upper part of the Dam Formation exposed in the Al-Nakhsh area, Southwestern Qatar Peninsula. These samples represent the Upper Dam sediments which are considered as an important aquifer for underground water in its subsurface occurrences in Qatar and nearby areas. Porosity, gas permeability and electrical resistivity were laboratory measured for the collected samples. Saturation exponent (n), cementation exponent(m) and pore throat radius (r35) were calculated for each sample. The vertical profile of gas permeability and pore throat radius (R35) is accomplished. Also, gas-permeability versus porosity is performed. Numerous useful and applicable relationships have been found. The obtained reservoir flow units which are distinguished according to hydraulic flow indicators (RQI and Øz) are only three units. However, the hydraulic-electric cross-plots (i.e. n-R35 and n-FZI) indicate three and four reservoir flow units respectively. In conclusion, the obtained number of reservoir flow units in the study area ranges from 3-4. It means that the hybrid hydraulic-electric combination sometimes shows flow units in concordance with the single type parameter, but occasionally give dissimilar results. An innovative nomograph is constructed according to an empirical equation related to the Dam Formation [1] to estimate the water saturation (SW) in oolitic carbonate facies. The present nomograph is verified by carbonate facies, deltaic distributary channel and barrier bar sandstone. The obtained values of water saturation from the proposed model are acceptable in case of both resistivity ratio ( Rt/Rw) and porosity (Ø) are available.

Attenuation Characteristics of Stress Wave Peak in Sandstone Subjected to Different Axial Stresses

To investigate the effect of axial stress on the attenuation characteristics of stress wave peaks, stress wave propagation experiments with small disturbance of a sandstone bar were carried out by a modified split Hopkinson pressure bar test system. Then, effects of axial stress on the waveform, attenuation rate, temporal-spatial attenuation characteristics, and attenuation sensitivity factor of the peak were studied. The results showed that the presence or absence of axial stress has a significant effect on the waveform. With axial stress loading, both temporal and spatial attenuation rates undergo similar development stages, “nonlinear stage + linear stage,” in which the demarcation stress (σ/σc) is 30%. Under the same axial stress, the peak decreases exponentially with the propagation time and distance with different attenuation intensities. With increasing axial stress, the temporal and spatial response intensities also experience “nonlinear stage + linear stage.” However, the temporal and spatial attenuation coefficients undergo three stages, first a dramatic decrease, then gentle development, and finally a sharp increase, in which demarcation stresses (σ/σc) are 30% and 55%. The defined attenuation sensitivity factor can well describe the attenuation sensitivity of peaks to different axial stresses. The conclusions can provide a theoretical reference for rock mass stability analysis in blasting excavation.

Laboratory study of nonlinear propagation of transient signals in a sandstone bar

Experiments are reported on finite-amplitude, impact propagation in a thin bar of Texas moss sandstone. Wideband, unipolar pulses were generated by excitation with a pendulum hammer at one end of the bar, in contrast to prior work with either tone-burst excitation or resonance methods. The bar was rectangular, 8 cm on each side and 175 cm in length, with a density of 2.0 g/cc. Impacts had a center frequency around 3 kHz, amplitude of 10 to 130 microstrain, and propagated repetitively between reflections at each end. Measurements were made with a laser Doppler vibrometer at multiple locations along the bar, while ultrasonic tone-burst probes were used at a central location, transverse to the axis. The sound speed and effective modulus were reduced in proportion to the impact amplitude as the finite-amplitude impact pulse propagated through the transverse and other measurement sites. The attenuation of the impact signal was also found to follow a power law in the frequency range of 1–6 kHz, in proportion to impact intensity. Although amplitude-dependent attenuation and softening of the elastic modulus are both found in micro-inhomogeneous materials, the present results were enhanced, up to an order of magnitude, compared to those in previously reported experiments.

Diagenetic differences in tight sandstone reservoirs in two delta fronts: an example from the Chang 4 and 5 members of the Yanchang Formation in the Longdong area, Ordos Basin, China

With the intensification of oil and gas exploration, tight sandstone reservoirs have received an increasing amount of attention, particularly with regard to the genesis of tight reservoir rock. The Upper Triassic Yanchang Formation in the Longdong area of the Ordos Basin has developed a typical tight, oil-bearing, clastic reservoir (lithic arkose and feldspathic litharenite, grain size is mainly 0.1~0.3 mm in diameter). During the depositional period of the Chang 4 and 5 members, the two provenance systems of the southwest and northeast developed in the study area. In the southwest, sandstones in the lower part of distributary channels are coarser with fewer quartz overgrowth and ankerite and better reservoir quality (porosity about 12%, permeability about 1 mD). In the northeast, chlorite coating is thicker (> 4 vol%) in the underwater channel sandstones (porosity is about 14%, permeability is about 2 mD) than in the mouth bar sandstones. Sandstones in the upper part of distributary channels are finer with lower permeability (about 0.1 mD). Authigenic ankerite mainly appears around detrital dolomite as an overgrowth. The SiO2 in the quartz overgrowth most likely came from the transformation of smectite to illite and the dissolution of feldspar. In the northeast, only 2 vol% of chlorite rims significantly inhibited quartz overgrowth, but they probably blocked and delayed the dissolution of feldspars by acids. We present results here that show the diagenetic differences in sand bodies in delta fronts are influenced by sediment size, maturity, and the composition of framework grain; the materials that compose authigenic minerals mainly come from the alteration of sandstones. As a whole, the formation of tight reservoir rocks in the study area is closely related to sedimentary facies, composition of framework grain, cement type and content, and development of dissolution.

Hydrocarbon Potential in Sandstone Reservoir Isolated inside Low Permeability Shale Rock (Case Study: Beruk Field, Central Sumatra Basin)

Upper Red Bed, Menggala Formation, Bangko Formation, Bekasap Formation and Duri Formationare considered as the major reservoirs in Central Sumatra Basin (CSB). However, Telisa Formation which is well-known as seal within CSB also has potential as reservoir rock. Field study discovered that lenses and layers which has low to high permeability sandstone enclosed inside low permeability shale of Telisa Formation. This matter is very distinctive and giving a new perspective and information related to the invention of hydrocarbon potential in reservoir sandstone that isolated inside low permeability shale. This study has been conducted by integrating seismic data, well logs, and petrophysical data throughly. Facies and static model are constructed to estimate hydrocarbon potential resource. Facies model shows that Telisa Formation was deposited in deltaic system while the potential reservoir was deposited in distributary mouth bar sandstone but would be discontinued bedding among shale mud-flat. Besides, well log data shows crossover between RHOB and NPHI, indicated that distributary mouth bar sandstone is potentially saturated by hydrocarbon. Target area has permeability ranging from 0.01-1000 mD, whereas porosity varies from 1-30% and water saturation varies from 30-70%. The hydrocarbon resource calculation approximates 36.723 MSTB.