Fine Sandstone Research Articles

Dynamic Crack Propagation and Fracture Behavior of Pre-cracked Specimens under Impact Loading by Split Hopkinson Pressure Bar

Deformation and fracture of brittle materials, especially crack propagation, have drawn wide attention in recent years. But dynamic crack propagation under impact loading was not well understood. In this paper, we experimentally tested Brazilian disk (BD) fine sandstone specimens containing pre-cracks under cyclic impact loading by theΦ74 mm diameter split Hopkinson pressure bar (SHPB) test device. The pre-cracked specimens were named central straight through crack flattened Brazilian disk (CSCFBD). By using the low air-pressure loading conditions (0.1 MPa, equal to the impact velocity of 3.76 m/s), a series of dynamic impact tests were detected successfully, and the effects of pre-cracks on dynamic properties were analyzed. Experimental results show that the multiple cracks mostly initiate at/or near the pre-crack tips and then propagate in different paths and directions varying by inclination angles, leading to the ultimate failure. Compared to static or quasi-static loading, dynamic crack propagation and fracture behavior are obviously different. Furthermore, we characterized the crack propagation paths, directions, and fracture patterns and discussed the influences of the pre-cracks during the breakage process. We concluded that the results obtained are significant in investigating the failure mechanism and mechanical properties of brittle materials under impact loading.

Selecting sweet spots for the exploitation of tight oil reservoirs from logs: case studies

The optimal selection of sweet spots for the exploitation of tight oil reservoirs is very important when formulating a development programme. Sweet spots for the exploitation of tight oil reservoirs are closely related to reservoir quality, hydrocarbon source rock quality and completion quality. In this study, criteria for the classification of sweet spots are established using a tight reservoir of fine sandstone in the first member of the Qingshankou Formation in Daqingzijing Oilfield, China as the study site. The results indicate that the vast majority of the research area includes type I and type II sweet spots. Type I sweet spots are located in the west and northwest of the research area and in the central region around wells H71 and H116; type II are located in the northeast and south of the research area, as well as in the region around the type I sweet spots. In the well region of type I sweet spots, the oil/gas is relatively enriched, and the fracability of the reservoir stratum is strong; these are key areas for tight oil development in the research area. Comparison of the evaluation results with well testing data indicates that the effect of well fracturing is obvious for type I sweet spots and the corresponding oil yield is high. The results of well fracturing are also adequate for type II sweet spots and the oil output can reach industry standards, although the corresponding yield is relatively low. Wells in non-sweet spots are unlikely to be fractured successfully, and even if they are, the oil output cannot reach industry standards.

Geochemical characteristics of the Permian marine mudstone and constraints on its provenance and paleoenvironment in the Fenghai area, Fujian Province, southeastern China

Permian marine strata have gradually become a research focus in the world. The marine strata of the Late Permian Dalong Formation (P3d) in the Fenghai area, Fujian Province, have become more and more important as their geochemical characteristics record important geological information and are a good indicator for recovering and reconstructing the paleo-sedimentary environments and tectonic attributes. The major elements, trace elements and rare earth elements were analyzed by XRF and ICP-MS, respectively. Based on the results of detailed field geological surveys, profile measurements as well as typical sample collection, the tectonic setting and provenance of Permian marine mudstone were comprehensively discussed. The results showed that the Dalong Formation (P3d) was deposited in an active continental margin tectonic environment as revealed by the relation between Fe2O3 + MgO and TiO2 and Al2O3/SiO2. The fingerprint characteristics of Mn, Fe, Co, Ni and REE and the ratio of U/Th, V/Cr, Sr/Ba, (La/Yb)N and V/(V + Ni) indicated that the sedimentary provenance was mainly derived from potassium feldspar, followed by muscovite. Sedimentary water bodies showed a gradually decreasing depositional rate trend, water depth gradually shallowing and paleo-salinity and productivity gradually increasing. Moreover, since transient delamination occurred during sedimentary processes, sedimentary water bodies showed obvious neritic characteristics. It was consistent with the results revealed by lithological and geochemical characteristics. Calcareous mudstone and siltstone transitioned into fine sandstone from bottom to top, indicating paleo-water bodies became shallow. The research results provided good reference and guidance for understanding Permian paleo-sedimentary environments and tectonic attributes of the Yong’an area, Fujian Province, southeastern China.

High resolution sequence stratigraphy of the Middle–Late Triassic Al Aziziyah formation, northwest Libya

This study offers the first integration of facies distribution, isotope and sequence stratigraphy studies of the Middle-Late Triassic (Ladinian-Carnian) Al Aziziyah Formation at its exposure at the Jifarah Basin of northwest Libya. Eight sections were measured along a depositional dip cross-section of the Al Aziziyah Formation. The Al Aziziyah Formation is a 2nd-order sequence, consisting of mid-ramp and outer ramp carbonate facies, with inner ramp facies restricted to the southernmost sections. The inner ramp peritidal facies indicate arid depositional system characterized by mudcracks, evaporite nodules and abundant cryptalgalaminites. Ramp crest shoals were predominantly peloidal and oolitic packstone-grainstone. However, mid-ramp facies show low to high-density bioturbation. Outer ramp is thin-medium hummocky cross-stratified beds, and interbedded shale. Changes from fine sandstone and red clay to carbonate mark the transition from the underlying Kurrush Formation to the Al Aziziyah Formation. In the most updip location, the transition from the Al Aziziyah Formation to the overlying Abu Shaybah Formation is an iron-rich, phosphatic surface. Seven sequences (S1-S7) within the Ghryan Dome section were identified based on facies stacking patterns, field observations and carbon stable isotope profiles. North of the Ghryan Dome section, there are three mainly subtidal sequences (S8S10) that may not correlate to the south. Higher-order (4th-5th) parasequences are present in the Al Aziziyah Formation, but correlating these parasequences between sections is difficult due to internal erosional discontinuities or autocyclicity.

ФИБРОБЕТОН ДЛЯ 3D-ПЕЧАТИ

The article deals with the possibility of using 3D-printing in the construction industry. The analysis of the work performed in this area in the world. It also presents the main advantages of this method of construction of structures and buildings in general. Experimental studies were carried out using the components of the mixture: CEM I 42.5H Sebryakovcement, fine ground quartzitic sandstone with a specific surface of 700 m2/ kg using gypsum. Sand was used as fine aggregate. To make the mixture plasticity, such additives as plasticizer PFM-NLK and Murapor Kombi 756 were used. Basalt fiber was used as a reinforcing agent. Were developed compositions of the concrete mixture dispersed reinforced with basalt fiber. The work was able to develop the composition of fiber-reinforced concrete mixture, which can be used for 3D- printing. Its strength characteristics were determined, allowing to conclude that this composition meets all the requirements for this technology.

Impact of rock fabric on flow unit characteristics in selected reservoir sandstones from West Baram Delta Offshore, Sarawak

The successful application of enhance oil recovery (EOR) technology requires an accurate understanding of the internal architecture of reservoirs and complex distribution of flow unit characteristics within different lithofacies. However, flow unit characteristics in the reservoir lithofacies of West Baram Delta Offshore, Sarawak, remain unexplored. Therefore, this paper investigates potential hydraulic flow units (HFUs) present within the five (5) identified sandstone lithofacies using thin sections, scanning electron microscopy (SEM), two-dimensional (2D) petrographic image analysis and helium porosimetery. The identified lithofacies include massive coarse-grained sandstone (MCGS), massive medium-grained sandstone (MMGS), massive fine-grained sandstone (MFGS), massive friable fine sandstone (MFFS) and massive very fine grained sandstone lithofacies (MVFGS). A plot of reservoir quality index (RQI) against normalized porosity (фz) showed five main hydraulic flow units (HFUs) within the identified lithofacies. The MFGS, MFFS and MVFGS lithofacies exhibited moderate to high matrix fractions and moderate to poor sorting, suggesting high tortuosity and hence characterized by low flow zone indicator (FZI). On the other hand, the MCGS and MMGS lithofacies exhibit very low to low matrix fractions, medium to coarse grain sizes and moderate to well sorting, signifying lower tortuosity, and hence characterized by relatively higher flow zone indicator (FZI). Thus, a hydraulic flow unit (HFU) can be formed by more than one lithofacies that exhibit similar fluid characteristic attributes, regardless of the difference in depositional environment and diagenetic processes undergone. This study concludes that HFU scattered plot can serve as a qualitative model for the characterization of flow unit characteristics in a well-comprising different lithofacies, particularly for field development plans in the West Baram Delta.

Microfacies characteristics and reservoir potential of Triassic Baikouquan Formation, northern Mahu Sag, Junggar Basin, NW China

By using data such as lithology, thin sections, sedimentary structure, particle size, logging facies signs, mercury intrusion, porosity and permeability from 34 wells, 11 types of sedimentary microfacies were divided in this study. Upon analysis, sedimentary microfacies exert significant control on reservoirs, and high quality reservoirs are primarily developed with the following sedimentary microfacies: gray sandy conglomerate in underwater distributary channels microfacies of fan delta fronts sub-facies, river-estuary dams and gray sandstones of distal bars on the front edge of the fan delta that have strong hydrodynamic conditions and good stability. Relatively good reservoirs are primarily distributed in the braided channel in fan-delta plains with brown sandy conglomerates, main channel of underwater distributary channels on the front edge of the fan delta with gray conglomerates and underwater distributary channel ends on the front edge of the fan delta with gray fine sandstones. With the stable tectonic background, Triassic sedimentary microfacies in the Mahu sag control the distribution and development of reservoirs. Therefore, sites of high quality reservoirs can be predicted by sedimentary microfacies, and this study provides guidance for oil and gas exploration.

Development of a streamflow-dominated alluvial-fan system in the southwestern margin of Gyeongsang Basin (Lower Cretaceous): implications for initial basin-fill history

This paper focuses on the initial basin-fill processes in an extensional nonmarine back-arc basin. Sedimentary facies analysis in the southwestern part of the Gyeongsang Basin (Jinju Subbasin), Hapcheon area reveals that the succession consists of four facies assemblages. The assemblage I mainly consists of disorganized conglomerate deposited by flash floods in a streamflow-dominated alluvial fan. The assemblage II is dominated by stratified conglomerates and massive, stratified, and cross-stratified sandstones, showing architecture of stacked channels and bars. The assemblage III largely comprises massive, stratified to laminated, and trough cross-stratified sandstones, which are organized into channel-fill and bar-accretion structures. It most likely formed in sandy sinuous rivers. The assemblage IV comprises gray mudstone interlayered with fine sandstone, representing water-logged floodplains. Conglomerate and sandstone bodies formed in the fluvial systems appear to be randomly distributed within floodplain fines, displaying a great lateral and vertical lithologic variation. This study helps understand the complexity of basin-fill history, i.e., switching of fluvial networks in time and space, in contrast to the simple notion of margin-parallel stacking of lithostratigraphic units.

Genesis of Xinzhai Sandstone-Type Copper Deposit in Northern Laos: Geological and Geochemical Evidences

Xinzhai sandstone-type copper deposit located in northern Laos lies in the Jiangcheng-Phongsaly-Phrae Mesozoic basin (JPMB), which is regarded as southern extension of the Lanping-Simao Mesozoic basin in China. The copper deposit belt is bounded by the Ailaoshan-Heishui River fault and the Dian Bien Phu-Luang Prabang fault at the east and Lancang River-Bannankan faults at the west. Two types of orebodies are identified in the Xinzhai area based on geological investigation. One is lamellar copper orebody hosted by the fine lithic feldspar sandstones and feldspar lithic sandstones; another is vein-type orebody. The sulfur isotopic compositions of the chalcopyrite and tetrahedrite are from -11.6‰ to -1.8‰, indicating that sulfur is derived from bacterial sulfate reduction (BSR). δD values of fluid inclusions in ore-bearing quartz samples are from -99‰ to -78‰. The calculated δ18OH2O values of ore-forming fluid vary from -2.3‰ to 0.4‰ using the quartz-water fractionation equations and the mineralization temperature. Oxygen and hydrogen isotopic compositions show that the ore-forming fluid was derived from basin fluid. Rock-mineral identifications show that both of the mineral grain maturity and the structural maturity are high in the Jurassic Huakaizuo Formation, reflecting a far-source accumulation and lake facies sedimentatary environment. Based on tectonic determination diagram of the Al2O3/SiO2-TFe+MgO, the sandstone samples collected from the Huakaizuo Formation were plotted in the passive continental margin. The collision of the Indian and Eurasian blocks during the Cenozoic formed large-scale strike-slips and thrust nappe structures in margin of the basin. With the tectonic movement, Cu-rich basin fluid from the basement of basin migrated upward along the contemporaneous fault and into the high porosity strata. At the same time, in organic matter-riched condition, bacterial sulfate reduction (BSR) has been triggered, forming a large number of S2- ions, and then precipitation of sulfide started. This mechanism describes the process of copper mineral deposition in the Xinzhai deposit.

Study on the Permeability of Weakly Cemented Sandstones

Fractured rocks are a type of complex media that widely exist in various projects including energy, hydraulic, and underground space engineering, whose permeability properties are a hotspot in current rock mechanics domain. Aiming at investigating the seepage characteristics of the fracture surfaces in different rock strata, uniaxial compressive test and permeability test were performed on single-fracture homogenous and heterogeneous rocks. Specifically, rock’s physical and mechanical parameters were measured in uniaxial tests while the initial width of the single fracture was determined through CT scanning. In combination with test results and the calculation model of the displacement of single-fracture heterogeneous rock under triaxial stress condition, the calculation formula of the permeability coefficient of single-fracture heterogeneous rock was derived. Results show that hydraulic pressure in the fracture can affect the permeability coefficient of the fractured rock. Hydraulic fracturing effect occurred with the increase of hydraulic pressure in the fracture, which then generates slight normal deformations of the rock masses on both two sides of the fracture surface, decreases the contact area in the fracture, and leads to the increases of both fracture width and permeability coefficient. For single-fracture rock, the lithological properties of the rock masses on both two sides of the fracture surface impose significant effects on the permeability coefficient. Under same hydraulic pressure and confining pressure, the permeability coefficient of single-fracture coarse sandstone is greatest, followed by that of single-fracture heterogeneous rock, and finally by single-fracture fine sandstone. Theoretical calculation results agree well with the test results, suggesting that the derived theoretical formula can adequately describe the variation tendencies of permeability coefficient with confining pressure and hydraulic pressure in the fracture.

X-ray Microtomography to Quantify Morphological Sandstones Properties

Abstract The sandstones of the Furnas Formation in the State of Parana, south of Brazil, are located in regions with great importance due to the natural heritage found in the rocky forms and associated flora and fauna. This paper presents a detailed analysis of the internal structure of diverse sandstones of the Furnas Formation. X-ray microtomography allowed 3D characterization of the number, pore size and shape distributions of the sandstones. Images with voxel size of 603 µm3 were utilized to evaluate the sandstone’s macroporosity. The main results obtained indicate that higher macroporosities were observed in coarser and some types of silicified sandstones. In general, fine sandstones were characterized by the largest number of pores. As a general conclusion the size and shape distribution of pores seem to have a variable relation with genetic factors as grain size and cement (siliceous and/or ferruginous).

The quality of geothermal reservoir of the Lower Jurassic aquifer in the Mogilno-Łódź Trough (Polish Lowlands)

The assessment of the geothermal potential should always take an accurate identification of the geological conditions into account. Petrographic and petrophysical investigations of the aquifers allow to estimate their productivity and infectivity. The area of the Mogilno-Łódź Trough is next to the Podhale basin, one of the most prospective regions in terms of geothermal potential in Poland. The Lower Jurassic aquifers are characterized by the largest dispositional resources among geothermal aquifers in the Polish Lowlands. The Lower Jurassic geothermal reservoir consist of fine and mixed grain-size sands and sandstones layers from 10-650 m thick, depending on the depth. The water within the reservoir exhibits mineralization ranging from 2 to over 250 g/L and its temperature ranges from 30 to 100°C. Boreholes profiles selected for study consist Lower Jurassic formations of the Polish Lowlands, which are perspective for the heat production in Poland. Sandstones are represent mainly by quartz arenites and subordinate by quartz wackes. Parameters measured include among others porosity, permeability and surface area. The results of the petrographic and mineralogical studies of core samples confirmed the presence of rock with favorable parameters for geothermal water accumulation.

Structure and fracture-cavity identification of epimetamorphic volcanic-sedimentary rock basement reservoir: a case study from central Hailar Basin, China

Taking the epimetamorphic volcanic-sedimentary complex basement in Beier Depression, Hailar Basin, China, as an example, the lithology, diagenesis, reservoir characteristics, and vertical zoning structure division are studied by using core, thin sections, cathodoluminescence, SEM, LSCM, well logging, FMI, diplog, and 3D seismic data. The reservoirs including fractures and cavities are identified and predicted separately under the control of vertical zoning structure, by well logging response, seismic reflection features, and multiseismic attributes. The results show that the assemblage of fine siltstone, tuffaceous sandstone, and tuff is the best lithology assemblage for reservoir development. The basement reservoir structure is divided into four zones, and zone II developing both fractures and pores has the best reservoir quality, followed by zone III having fractures dominated. Fracture and pore-cavity reservoirs have decreasing resistivity, density, and other features in conventional well log, and pore-cavity reservoirs have relatively high amplitude and bead or lenticular-shaped seismic reflection. The seismic attributes of curvature, coherence, and amplitude extracted under the control of specific zonation are effective reservoir prediction methods. Reservoir distribution including fracture and pore-cavity is controlled by the palaeogeomorphology of basement buried hill and fault activity and property. For the buried hills with high palaeogeomorphology and long-term structural activity controlled by main faults, developing high-quality reservoirs of fractures as well as pores, are the optimal targets for basement drilling, in the condition of effective hydrocarbon migration path.

A new method for clastic reservoir prediction based on numerical simulation of diagenesis: A case study of Ed1 sandstones in Bozhong depression, Bohai Bay Basin, China

The use of seismic exploration technique to provide reliable reservoir information is a conventional method. However, due to its quality and resolution reasons, it cannot satisfy the detailed research and characterization of reservoirs, especially the clastic reservoir with thin sand body. Diagenesis is a fundamental process in the development and formation of all petroleum reservoirs and is a major contributor to their ultimate physical properties. Based on numerical simulation of diagenesis, a new prediction method called geology prediction techniques is presented to simulate the evolution of the diagenetic stages, diagenetic facies and porosity of clastic reservoirs and ultimately for favorable reservoir prediction. It emphasizes the idea of dynamic quantitative research dominated by process recovery, the most important of which is the establishment of mathematical models, including mineral dissolution models, mineral cementation models and sediment compaction models using the experimental data in study area and the results of previous studies. The essence of this method is illustrated, and its effectiveness is proved using Ed1 clastic sandstones in the Bozhong depression, Bohai Bay Basin, China. At present, the reservoir is in the early diagenetic stage B (IB) and the middle diagenetic stage A1 (IIA1). The major diagenetic processes that influence the porosity of the sandstones in study area are mechanical compaction, carbonate cementation, quartz cementation, clay cementation, feldspar dissolution and carbonate dissolution. There are three types of sandstones including fine sandstone, siltstone, and argillaceous siltstone, and the variation range of primary porosity of these sandstones is from 26% to 38%. Compaction and carbonate cementation are the main reasons for porosity reduction, with porosity loss percentage by compaction (P-Com) and porosity loss percentage by cementation of carbonate (P-C-Car) being 53.1% ~ 7.8% (av. 41.9%) and 53.1% ~ 7.8% (av. 18%), respectively, while carbonate dissolution and feldspar dissolution can greatly improve reservoir physical property, with porosity increase percentage by dissolution of carbonate (P-D-Car) and porosity increase percentage by dissolution of feldspar (P-D-Fel) being 0 ~ 9.9% (av. 8.9%) and 0 ~ 27.8% (av. 9.4%), respectively. The predicted porosities match the measured porosities well. Cited as : Qian, W., Yin, T., Hou, G. A new method for clastic reservoir prediction based on numerical simulation of diagenesis: A case study of the Ed1 clastic sandstones in the Bozhong depression, Bohai Bay Basin, China. Advances in Geo-Energy Research, 2019, 3(1): 82-93, doi: 10.26804/ager.2019.01.07

The excavation of the Modaoshan Site of the Paleolithic Age in Yunan County, Guangdong

Abstract In 2014, Guangdong Provincial Institute of Cultural Relics and Archaeology and other institutions excavated Locality 1 of the Modaoshan (Whetting Hill) Site in Yunan County, Guangdong. The excavation recovered almost 400 pieces of lithic products mostly made of quartz, sandstone and quartzite, plus a few made of fine sandstone and metasandstone, the categories of which included blanks, cores, flakes, tools, chunks, debris and utilized gravels, containing the entire procedure from blank fetching, stone tool making and refuse abandoning, and the finished tools took small proportion. The date of Locality 1 was the early stage of the Middle Pleistocene, belonging to the Lower Paleolithic Age. The Modaoshan Site is the first open site of the Paleolithic Age scientifically excavated in Guangdong Province and the earliest cultural remains of the early human beings in Guangdong Province confirmed to date.

A comprehensive analysis of the sedimentology, petrography, diagenesis and reservoir quality of sandstones from the Oligocene Xiaganchaigou (E3) Formation in the Lengdong area, Qaidam Basin, China

The sandstone from the Oligocene Xiaganchaigou Formation (E3) in the Lengdong area, Qaidam Basin, China, is extensively distributed and acts as an effective regional reservoir for hydrocarbon accumulation. Based on thin sections, scanning electron microscopy (SEM), X-ray diffraction (XRD) image analysis and cathodoluminescence (CL), a comprehensive analysis of the sedimentology, petrography, diagenesis and reservoir quality of the Oligocene sandstone reservoirs of the Xiaganchaigou (E3) Formation in the Lengdong area, Qaidam Basin, are studied. The sandstones in the Lengdong area are classified as argillaceous sandstone, silty sandstone, fine sandstone, medium sandstone, pebbly sandstone, conglomeratic sandstone and conglomerate based on their grain sizes, degrees of sorting, and matrix contents. Five sedimentary facies were identified in the Xiaganchaigou Formation (E3) of the Lengdong area, Qaidam Basin: distributary channel, interdistributary bay, mouth bar, central bar and distal sand sheet facies. Their major diagenetic processes include compaction, carbonate cementation and feldspar and calcite cement dissolution. The porosity and permeability values of the fine sandstone, conglomerate, and conglomeratic sandstone are the highest, followed by those of the medium sandstone and silty sandstone, and the argillaceous sandstone and pebbly sandstone have the lowest porosity and permeability values. The diagenetic sequence in the study area ranges from the eodiagenesis B stage to the mesodiagenetic A stage. Compaction has significantly reduced the primary porosity. Carbonate cements, mainly calcite cements, occlude pores by precipitating in intergranular and intragranular spaces. Dissolution largely contributes to increasing the secondary porosity. The well-sorted and relatively coarser-grained sandstones that formed in higher-energy sedimentary facies (distributary channel, mouth bar and central bar) have a better reservoir quality than the poorly sorted, clay-rich sandstones that formed in low-energy environments (interdistributary bay and distal sand sheet). Based on the comprehensive analysis of petrographic data, diagenesis and sedimentary facies, the reservoir in the Lengdong area is classified into three types: type A, type B and type C. Type A, located in the center of the study area, is the most favorable reservoir, while type C sandstone is the last to consider when looking for a good reservoir. This study can guide hydrocarbon exploration in the Lengdong area, as well as in northwestern China and similar areas throughout the world.

A FACIOLOGIA DO GRUPO PASSA DOIS NO RIO GRANDE DO SUL

The study of geological sections in the outcropping area of the Passa Dois Group in Rio Grande do Sul, complemented with the study of cores from drillings for coal, showed the relations in the State, which are related bellow. 1) The Passa Dois Group is constituted by the Irati and Estrada Nova Formations; the Rio do Rasto Formations is lacking. 2) The thickness of the group is about 100 m. 3) The contact with the subjacent formation is characterized by the occurrence of limestone concretions immersed in a grey silty shale, very similar to the Palermo Formation. 4) The Irati Formation has two facies. One characterized by the occurrence of two beds of pyrobituminous shale associated with fossiliferous limestone lenses. These two beds are intercalated with a grey silty shale with concoidal fracture, and with yellowish limestone concretions immersed in it. This facies, when present, occupies the lower part of the formation and was called Tiaraju facies. The second facies, partially synchronous with the first one, is constituted by grey silty shales with limestone concretions and West of Pantano Grande, is cut by oblique veins of silica. Toward the east it is predominantly siltic, showing cross-lamination. This facies ends the Irati deposition in the State and was called Valente facies. 5) The Estrada Nova Formation also has two lithologic facies, one being pellitic; the other, sandy, which, when present, occupies the top. The pellitic facies was called Caveiras facies and is characterized by the development of pink and red colours alternating with grey. The sandy facies, also reddish, is characterized by lenticular bodies of very fine sandstone, by occurrences of limestone lenses with irregular shapes, and also by large limestone concretions. This facies was called Armada facies. 6) The upper limit of the group was set on the contact of the above mentioned lithologies with a pink medium sanstone, with planar and channal cross stratifications, and with intraformational conglomerates.

Acoustic emission characteristics of creep fracture evolution in double-fracture fine sandstone under uniaxial compression

Acoustic emission (AE) experiments were carried out to study short-time creep behavior under uniaxial compression for cuboid-shaped fine sandstone specimens with two pre-existing cracks (the sample scale was 70 mm × 70 mm × 140 mm and the lengths of cracks were 2a = 20 mm and 2a = 30 mm) using the RLJW-2000 servo-controlled triaxial rheological test system of rock and PCI-II AE system. Based on the experimental results of creep curves, the mechanical properties of specimens with different pre-existing crack lengths were analyzed. The experimental results showed that compared to the intact specimen as the standard, the specimens with pre-existing crack lengths of 20 and 30 mm exhibited a decrease in the creep damage strength from 160.4 to 100 and 55 MPa, respectively. The pre-existing crack length significantly affected the strength of the specimen. Based on the experimental results of the spatial evolution of AE events, the three-dimensional evolution law of micro-cracks in the specimens was studied. The experimental results showed that the AE events could well reflect the crack evolution process of specimens with cracks. The occurrence of AE event points was mainly due to the initiation and propagation of micro-cracks within the sample; and with the increase of stress levels, the distribution of event points exhibited different characteristics. In the first two stages of creep loading, AE events were produced to a lesser extent and were evenly distributed inside the sample. With the increase of the creep stress level, the AE activity increased, in particular at the four ends of pre-existing cracks, and evolved from the ends of the fissures, thereby enabling the prediction of the penetration position of the micro-cracks within the fractured rock mass. Moreover, AE location results also directly reflected the interior stress field propagation process. AE location results directly reflected the spatial position, direction, and spatial curved face of crack propagation in the rock sample with pre-existing cracks, which are extremely significant to study the mechanism of rock failure.

English

The study area comprises Injana Formation (Late Miocene), exposed on the hills nearby of Tharthar Lake and about 120 km north of Baghdad city. This study depends on sedimentologic and facies analysis to recognize paleoenvironment and recognize the kinds of vertebrate bone fossils during Late Miocene. Sedimentologic and facies analysis showed many sedimentary facies: facies (Se) of scoured erosional surface, facies of (Sp) cross- bedded sandstones, facies (Fs) of fine sandstone facies, facies of (Fc) claystone, and facies of (C) calcareous clay. Facies analysis referred to the sub environments which are: point bar, over bank and floodplain in addition to fining upward cycles of deposition, which refers to meandering fluvial depositional environment.

Thermal and mechanical coupling effects on permeability of weakly cemented sandstone

The weakly cemented sandstone is the host rock in the Cretaceous and Jurassic strata with low intensity, high water-bearing capacity and sensitivity to disturbance. In order to investigate the influence of temperature and confining pressure on the permeability of weakly cemented sandstone, scanning electron microscope were used to characterize microstructure of the particles, a series of triaxial creep tests were performed on coarse sandstone, medium sandstone and fine sandstone separately, and the effect of particle size on the permeability of sandstone was also analyzed. The results indicate that the increase of confining pressure led to the plastic deformation between the rock particles, that permeability was irreversible as the confining pressure changed, and that the permeability reduction rate of coarse sandstone was lower than that of fine sandstone. The sensitivity of weakly cemented sandstone to temperature and confining pressure increased as the sizes of the particles decreased, and the effect on permeability was more obvious.