Junggar Basin Research Articles

Organic geochemical characteristics of the Permian source rocks in Mahu Depression, Junggar Basin: depositional environment significance

Organic geochemical characteristics of the Permian source rocks in Mahu Depression, Junggar Basin: depositional environment significance

Discrete element method simulation of the fold-and-thrust belts along strike various compression in the southern margin of the Junggar Basin, China

The fold-and-thrust belts developed in the southern margin of the Junggar Basin in China can be divided into different segments and oblique rows in the strike and dip direction respectively. The internal structural patterns and linkage types of different sub-segments usually affect the oil and gas accumulation. In this work, we study on the propagation and linkages of the structural patterns involved in the fold-and-thrust belts due to the differential compression by the Discrete element method (DEM). Two simulations are configured to use four velocity modules, which can move with different velocity at the same time: (i) the homogenous module filled with brittle simulating sediment is used to investigate the structural response to a velocity difference and (ii) the module with ductile detachment at the front of each module is used to test how the weak layer affects deformation. The results are illustrated by topography of the simulations, such as cross sections perpendicular and parallel to the shortening direction, forward and lateral displacements seen from the top view, and sections of the velocity field. The results show that (i) resistance from adjacent modules promotes the development of back-thrusts in faster modules, and drag from faster moving module hinders nucleation in slower modules, and (ii) two types of transfer zones caused by shear stress appear in the piedmont region and in front of the fold belts. The detachment simulation produces results are similar to the western segment of the southern margin of the Junggar Basin, which can explain the several rows of anticlines that appear oblique in the plane. These results are useful to analyze the formation process of the oil and gas trap developed in the similar compressional area.

Paleoenvironment evolution of the Permian Lucaogou Formation in the southern Junggar Basin, NW China

The Early-Middle Permian is regarded as a critical period for the Junggar Basin in marine to continental transition. Lacustrine organic-rich shale successions were deposited in the Lucaogou Formation in the southern Junggar Basin (SJB) at that time. Although the paleoenvironment conditions of the Lucaogou Formation have been discussed from diverse perspectives, their spatial and temporal evolution remains controversial. This study systematically investigated 11 outcrop sections and cores in three sags from the SJB, namely the Chaiwopu Sag, the Fukang Sag, and the Jimsar Sag, respectively. Paleoclimatic indicators (i.e., CIA values and Sr/Cu ratios) show that the Lucaogou Formation was generally deposited under semiarid to arid conditions, among which the Jimsar Sag was more arid than the other two sags. Meanwhile, paleoredox indices (V/Cr, V/(V + Ni) ratios, and MoEF, UEF values) illustrate dysoxic-anoxic conditions prevailing during its deposition. Paleosalinity indicators (Sr/Ba and B/Ga ratios) suggest high salinity levels in the three sags, among which the Jimsar Sag exhibited the highest salinity. Our results suggest that, relative to the Chaiwopu and Fukang sags, the Jimsar Sag experienced a more arid, more restricted, and saltier condition during deposition of the Lucaogou Formation. There is a generally lake transgression trend, with relatively less arid climate and less salty water columns in the upper Lucaogou in all the three sags. This transgression may be related to the deglaciation of Gondwanaland, which cause an increase in global temperature and more rainfall. The SJB was composed of a cluster of lakes that were relatively independent and rarely connected with each other. However, lake level rising may have coalesced to form a giant paleo-lake when lake transgression took place.

Fracture Characteristics and Distribution in Slant Core from Conglomerate Hydraulic Fracturing Test Site (CHFTS) in Junggar Basin, Northwest China

Hydraulic fracture networks, especially fracture geometry, height growth, and proppant transport within the networks, present a critical influence on productivity evaluation and optimization of fracturing parameters. However, information about hydraulic fracture networks in post-fractured formations is seldom available. In this study, the characteristics (density and orientation) of hydraulic fractures were obtained from field observations of cores taken from conglomerate hydraulic fracturing test site (CHFTS). A large number of fractures were observed in the cores, and systematic fracture description was carried out. The fracture analysis data obtained includes fracture density, fracture depth, fracture orientation, morphology, fracture surface features, apertures, fill, fracture mechanical origin (type), etc. Our results show that 228 hydraulic fractures were intersected in a span of 293.71 m of slant core and composed of irregularly spaced single fractures and fracture swarms. One of the potential sources of the observed fracture swarms is near-wellbore tortuosity. Moreover, for regions far away from the wellbore, reservoir heterogeneity can promote complex hydraulic fracture trajectories. The hydraulic fractures were mainly cross-gravel and high-angle fractures and align with maximum horizontal stress (SHmax) ± 15°. The fracture density, orientations, and types obtained from the core fracture description provided valuable information regarding fracture growth behavior. For the near-wellbore area with a transverse distance of less than 25 m from the hydraulically-fractured wellbore, tensile fractures were dominant. While for the area far away from the wellbore, shear fractures were dominant. Our results provide improved understanding of the spatial hydraulic fracture dimensions, proppant distribution, and mechanism of hydraulic fracture formation. The dataset acquired can also be used to calibrate numerical models and characterize hydraulic fracture geometry and proppant distribution.

TOC estimation of shale oil reservoir by combining nuclear magnetic resonance logging and nuclear physics logging

Abstract The evaluation of source rock properties has become a vital step in logging interpretation. Total organic carbon (TOC) content is the key to estimating the quality and hydrocarbon generation potential of source rocks. In the shale oilfield of the Junggar Basin, the conventional method of calculating the TOC of hydrocarbon source rocks cannot satisfy logging evaluation requirements. This paper predominantly deals with a method for the quantitative estimation of TOC in source rocks via nuclear physics and nuclear magnetic resonance (NMR) logs. According to this method, the total hydrogen index of the source rock is the sum of the response of kerogen, clay minerals and fluid, expressed by corrected neutron porosity. The hydrogen index of fluid and clay minerals is indicated by the effective porosity of NMR and the estimated clay content, respectively. To eliminate the hydrogen index of fluid, the effective NMR porosity is subtracted from the corrected neutron porosity. On this basis, a new and overlapping method suitable for clay-rich rocks and oil reservoirs is proposed. This method was developed by overlaying the scaled clay content curve on the hydrogen index curve. In non-source rocks, the two curves regularly overlap. However, in organic-rich rocks the two curves will separate. The separation distance between the two curves was used to estimate TOC continuously. Possessing sound application and benefiting from the measured results of sweet spots, this method provides new insights for TOC quantitative prediction in shale oil reservoirs.

Pressure Transient Analysis for Hydraulically Fractured Wells with Changing Conductivity in Stratified Reservoirs: Case Study in Xinjiang Oilfield.

The oil development has been oriented toward deep-layer reservoirs and the commingling production and the separate-layer fracturing are important development methods. Currently, limited attention is given to the pressure transient analysis (PTA) of the fractured wells located in a stratified reservoir. Moreover, the proppant is very difficult to move inside the hydraulic fracture in the deep-layer reservoir, leading to the uneven fracture conductivity along the hydraulic fracture and increasing the complexity of PTA. To fill this gap, this work presented a fully analytical well test model for hydraulically fractured wells with changing fracture conductivity in stratified reservoirs, which is convenient to be used for interpreting the recorded pressure data from the oilfield due to its analytical nature. The establishment of this model is based on the trilinear flow model, Duhamel theorem, and pressure superposition principle. A systematic verification is conducted to ensure the validity of the proposed model. Furthermore, we offer a sensitivity analysis to investigate the effect of crucial parameters on pressure and pressure derivative, including the fracture extension, fracture conductivity, transmissibility factor, and storativity factor. Finally, a field case of a four-layer fractured well from Xinjiang Oilfield in Junggar Basin is interpreted to demonstrate the practicability of the presented model.

Differential Characteristics and the Main Controlling Factors of Shale Oil Sweet Spot Reservoirs in Lucaogou Formation, Jimsar Sag, Junggar Basin

The microscopic differences in characteristics and formation mechanism of shale oil reservoirs in the upper and lower sweet spot sections of the Lucaogou Formation in the Jimsar Depression, which has been identified as a national shale oil demonstration area in China, are still unclear. In this study, the characteristics and the main controlling factors of reservoir differences in different sweet spots of Lucaogou Formation were specified based on core observation, thin-section observation, X-ray diffraction, Rock-Eval, microscopic fluorescence of hydrocarbon inclusions, and temperature measurement of saline inclusions. Results show that the Lucaogou Formation mainly develops dissolution and primary intergranular pores. The dissolution transformation leads to obvious differences between the upper and lower sweet spots. Specifically, the upper sweet spot section mainly develops primary intergranular pores and partially develops dissolution pores; the lower sweet spot section mainly develops dissolution pores, including intergranular, intragrain, and intergranular pores. Geochemical data such as inclusions indicate that hydrocarbon generation began in the Triassic, and a large number of hydrocarbons were charged in the Middle-Late Jurassic-Early Cretaceous. As the key fluid that triggers reservoir dissolution modification, it is mainly derived from organic acids generated by thermal evolution of source rocks within shale formations. The scale and quality of source rocks in the lower sweet spot are better than those in the upper sweet spot. The former has stronger hydrocarbon generation potential, which lays a foundation for the scale difference of organic acid output in the upper and lower sweet spots. At the same time, the source rocks in the lower sweet spot are more mature due to magmatic-hydrothermal upwelling. This condition accelerates the release of organic acids from source rocks, which results in the scale difference of dissolution effects in the upper and lower sweet spots.

Multiscale fracture prediction technique via deep learning, seismic gradient disorder, and aberrance: Applied to tight sandstone reservoirs in the Hutubi block, southern Junggar Basin

Fracture prediction is an important and active area of research for oil and gas exploration in fractured unconventional reservoirs. Traditional seismic fracture prediction techniques come in one of two flavors, prestack anisotropy-based or poststack edge-enhancement attributes such as ant tracking and maximum likelihood. Inaccurate predictions may result from an apparent low signal-to-noise ratio in the prestack domain approaches or from cumulative effects that are misrepresented in poststack data; there also are shortcomings from using a single pre- or poststack domain. We propose a comprehensive multiscale prediction framework to delineate major faults (using deep learning), associated minor faults (using seismic gradient disorder), and fractures (using seismic aberrance). The principles of deep learning, seismic gradient disorder, and aberrance are introduced and their application effects are verified through the study of tight sandstone reservoir fractures in the Hutubi area, the southern margin of the Junggar Basin.

Phylogeography of the desert scorpion illuminates a route out of Central Asia.

A comprehensive understanding of phylogeography requires the integration of knowledge across different organisms, ecosystems, and geographic regions. However, a critical knowledge gap exists in the arid biota of the vast Asian drylands. To narrow this gap, here we test an "out-of-Central Asia" hypothesis for the desert scorpion Mesobuthus mongolicus by combining Bayesian phylogeographic reconstruction and ecological niche modeling. Phylogenetic analyses of one mitochondrial and three nuclear loci and molecular dating revealed that M. mongolicus represents a coherent lineage that diverged from its most closely related lineage in Central Asia about 1.36 Ma and underwent radiation ever since. Bayesian phylogeographic reconstruction indicated that the ancestral population dispersed from Central Asia gradually eastward to the Gobi region via the Junggar Basin, suggesting that the Junggar Basin has severed as a corridor for Quaternary faunal exchange between Central Asia and East Asia. Two major dispersal events occurred probably during interglacial periods (around 0.8 and 0.4 Ma, respectively) when climatic conditions were analogous to present-day status, under which the scorpion achieved its maximum distributional range. M. mongolicus underwent demographic expansion during the Last Glacial Maximum, although the predicted distributional areas were smaller than those at present and during the Last Interglacial. Development of desert ecosystems in northwest China incurred by intensified aridification might have opened up empty habitats that sustained population expansion. Our results extend the spatiotemporal dimensions of trans-Eurasia faunal exchange and suggest that species' adaptation is an important determinant of their phylogeographic and demographic responses to climate changes.

The relation of the “four properties” and fluid identification of the carboniferous weathering crust volcanic reservoir in the Shixi Oilfield, Junggar Basin, China

This study addresses the poorly understood physical properties of the Shixi Oilfield reservoir, which consists of a weathered Carboniferous volcanic rocks with strong heterogeneity and in which logging identification and evaluation are difficult. Using the lithology, lithofacies, and reservoir space characteristics of volcanic materials, this comprehensive study uses core, well logging, mud logging, and production testing data to analyze the relationship among the lithology, physical properties, electrical properties, and oil-bearing properties (referred to as the “four properties”) of weathered Carboniferous volcanic crust in addition to fluid identification. 1) The lithology of Carboniferous volcanic crust is dominated by breccia lava, agglomerate, banded lava, and compact tuff, and the lithofacies are mainly effusive facies. Secondary pores and tectonic fissures are important reservoir spaces, and the corrosion-fracture pores are significant for reservoir properties. 2) The “four properties” of volcanic reservoirs in the study area have clear relationships. On this basis, data on the electrical properties of the material, such as interval transit time, density, and neutron, were used to establish a logging interpretation model of the properties and oil saturation of the volcanic rock. 3) Using the resistivity-porosity cross-plot method, normal probability distribution method, and Rt/Rxo-Rt cross-plot method, volcanic reservoir fluids were identified with coincidence rates of 80%, 63.63%, and 63.63%, respectively. The cross-plot method determines lower limits of the reservoir’s physical properties and oil saturation, yielding porosity>9%, permeability>0.2 mD, and oil saturation>45%.

A composite model based on semi-analytical method for multiwell horizontal pad with stimulated reservoir volume

The multiwell horizontal pad is an effective method to enhance oil recovery for unconventional reservoirs. Microseismic evidence indicates that the stimulated reservoir volume (SRV) may have originated in the overlap region created by multi-well fracturing. However, the majority of SRV modeling studies have concentrated on a single well, ignoring the interference from offsetting wells. In this study, a novel model for modeling the SRV and multiwell horizontal pad in unconventional reservoirs is developed using an effective semi-analytical method. The proposed semi-analytical model is capable of simulating the behaviors of a multiwell horizontal pad with an irregularly shaped SRV and an impermeable boundary. The transient pressure behaviors of the multiwell horizontal pad with SRV are significantly different than that of a single isolated well, as demonstrated by the results. The linear flow between wells, the pseudoradial flow (PRF) of the multiwell pad in SRV, the transitional flow, and the final pseudoradial flow (PRF) of the multiwell pad all correspond to an extremely unusual transient pressure response. Due to the high flow capability of SRV, pressure diffusion occurs at a faster rate, resulting in the appearance of these flow regions in advance. Sensitivity analysis confirms that as the scale of SRV increases, the duration of the flow influenced by the SRV increases. The SRV and compensating wells result in a negligible PRF, while the pressure behaviors exhibit a gradual rise. This phenomenon may serve as a demonstration of the extremely rare occurrence of radial flow or PRF in unconventional reservoirs. The horizontal line value for the PRF in SRV is proportional to the total production of the well pad. The horizontal line value for the final PRF is also related to the unstimulated region's mobility ratio, the well pad's production rate and 0.5. The case in Junggar Basin shows that the natural fracture permeability in the SRV is 2.5 mD, the matrix permeability is 0.044 mD, the rectangular SRV side lengths are 1600 m and 880 m, the mobility ratio and dispersion ratio are 15, and the wellbore storage coefficient is 0.5 m3/MPa.

Heterogeneity of alkaline lacustrine source rocks of the lower Permian Fengcheng Formation in the Mahu Sag, Junggar Basin, northwestern China

The alkaline lacustrine source rocks in the lower Permian Fengcheng Formation in the Mahu Sag, Junggar Basin, China, exhibit significant heterogeneity. However, most previous studies have focused on the vertical variability of the source rocks, and the lateral variability is poorly understood. This study investigated recently drilled samples of the Fengcheng Formation in the Manan area to assess the lateral variability of the source rocks. The geochemical characteristics of the Fengcheng Formation source rocks in the Manan area are variable, with low–high abundances of organic matter, which comprises types I–III kerogen that is mature to high-mature. These features differ from the typical Fengcheng Formation source rocks in the Wuxia–Fengcheng area, which have a high abundance of organic matter with substantial hydrocarbon and shale oil generation potential. Biomarker data show that the organic matter in the Fengcheng Formation source rocks in the Manan area is a mixture of higher plants, algae, and bacteria, which differs from the Wuxia–Fengcheng area where algae and bacteria are dominant. The Fengcheng Formation source rocks in the Manan area were deposited in an oxic to anoxic environment characterized by large variations in water salinity and stratification. Mudstones were deposited rather than carbonate rocks. This differs from the strongly reducing, hypersaline, and water-stratified depositional environment of the Fengcheng Formation source rocks in the Wuxia–Fengcheng area. These two areas are separated by the Dazhuluogou Fault, and there are no alkaline lacustrine deposits in the Manan area. The distribution of the alkaline lacustrine source rocks was controlled by the paleogeography and sedimentary environment, which was characterized by significant spatial heterogeneity.

Influence of differential diagenesis on pore evolution of the sandy conglomerate reservoir in different structural units: A case study of the Upper Permian Wutonggou Formation in eastern Junggar Basin, NW China

Abstract In order to study the differential diagenesis of sandy conglomerate reservoirs in different tectonic units in eastern Junggar Basin, and establish the differential temporal sequence of burial – diagenesis – hydrocarbon charging – pore evolution, the Upper Permian Wutonggou Formation sandy conglomerate reservoir in the Dongdaohaizi Sag and Baijiahai Uplift, eastern Junggar Basin are studied, based on observation of thin sections under microscope, measurement of scanning electron microscopy (SEM), X-ray diffraction (XRD), physical properties, cathodoluminescence (CL), micro beam fluorescence, and analysis of fluid inclusions, combined with previous research results. The result shows that the reservoir displays a differential diagenetic process as they are situated at different tectonic units of the Dongdaohaizi Sag and the Baijiahai Uplift. The Dongdaohaizi Sag is dominated by continuous subsidence, the reservoir buried in relatively deep depths experienced three stages of hydrocarbon charging, which is more strongly affected by compaction, fracturing, dissolution, and late-stage cementation, the effect of middle-stage cementation is relatively weak. The pore evolution experienced four stages, including decreased porosity by shallow burial compaction, decreased porosity by moderate burial compaction and middle-stage cementation, decreased and increased porosity by moderate to deep burial middle-stage cementation and dissolution, and increased and decreased porosity by deep burial dissolution and late-stage cementation. The diagenetic stage had reached A2 sub-stage of mesodiagenesis, and the present porosity is relatively low; however, the developed structural fractures in the reservoir has played a good role in improving the reservoir seepage capacity. By contrast, the Baijiahai Uplift is characterized by multi-stage uplift, the reservoir buried in relatively shallow depths experienced two stages of hydrocarbon charging, which is more strongly affected by middle-stage cementation, with relatively weak compaction and dissolution, fracturing, and late-stage cementation was limited. The pore evolution experienced three stages, including decreased porosity by shallow burial compaction, decreased porosity by moderate burial compaction and middle-stage cementation, and decreased and increased porosity by moderate-deep burial middle-stage cementation and dissolution. The diagenetic stage has reached A1 sub-stage of mesodiagenesis, and the present porosity is relatively high. Two types of favorable reservoirs are developed in the study area. The first one is matrix pore favorable reservoir, which is mainly located in the area of the Baijiahai Uplift of the reservoir with relatively high porosity and permeability. The second one is structural fracture developed favorable reservoir, which is mainly located in the fault development area of the Dongdaohaizi Sag.

Stratigraphic Characteristics and Sediment-Filling Process of the Early Permian Fengcheng Formation in the Northwestern Margin of the Junggar Basin, Northwest China

The Permian Fengcheng Formation in the Mahu sag of the Junggar Basin is a crude oil reservoir and source rock. However, its stratigraphic characteristics, the boundary marks of the three members (lower, middle, and upper), and the sediment-filling processes are unclear. This study presents the sedimentary systems and sediment-filling processes in an intracontinental rift basin of this area using sedimentary-faces analysis, sequence stratigraphy, well logs, and two-dimensional seismic interpretations. The results show that the Fengcheng Formation consists of three third-order sequence stratigraphy (SQf1, SQf2, and SQf3). The lowest member of the Fengcheng Formation is composed of gray to dark gray thin middle layers of tuff, lacustrine dolomitic mudstone, and argillaceous dolomite near the depocenter. Tuff, siltstone, a small amount of fan–delta glutenite, volcanic breccia, and basalt are present near the fault zone. The logging curve is characterized by a high gamma-ray (GR) value, high amplitude, wavelength growth, and medium frequency. Near the depocenter, the middle member of the Fengcheng Formation comprises gray and gray–black thin layers of lacustrine muddy dolomite and dolomitic mudstone and thin sandstone and mudstone interbedded between tuff and gravel near the orogenic belt. The logging curve displays high-to-low GR values, high amplitude, short wavelength, and high frequency. The upper member of the Fengcheng Formation is not characterized by lesser tuffaceous and dolomitic components but by an increased fan–delta sandy content. Next, the bottom of the upper member is composed of lacustrine mudstone and siltstone interbedded between dolomitic mudstones; whereas, the top of the upper member is dominated by fan–delta coarse sandstone and sandy conglomerate. The logging curve shows low GR values, small amplitude, and continuous stability. The sediment-filling process of the Fengcheng Formation is controlled by fault activity. Overall, the lower and middle members of the Fengcheng Formation expand during the sediment-filling process of the lake basin of the Mahu sag; whereas, the upper member contracts. The strong uplift of the Horst near the Baiquan and Mahu orogenic belts led to an increase in provenance supply, resulting in a contraction of the southwestern margin of the lake basin during the sedimentation period from lower member to upper member of Fengcheng Formation.

Clay mineral transformations of mesozoic mudstones in the central Junggar Basin, northwestern China: Implications for compaction properties and pore pressure responses

The clay mineral transformation of deeply buried mudstones is closely related to the mudstone microstructure, petrophysical properties, and pore fluid pressure (pore pressure). The influence of the burial history on the microstructure and compaction behavior of mudstones has been the focus of recent research. In this study, the mineral composition, mudstone microfabric and petrophysical properties of Mesozoic overpressured mudstones in the central Junggar Basin were analyzed to explore the influence of clay mineral diagenesis on compaction processes and overpressure responses in diagenetically altered mudstones. The clay minerals of these Mesozoic mudstones are mainly composed of mixed smectite/illite layers (I/S) and illite with small amounts of kaolinite and chlorite, but they do not contain discrete smectite. The Mesozoic mudstone underwent clay mineral diagenesis, the smectite in I/S transformed to illite at depths of 4000–4800 m (temperatures of 90–105 °C), and the I/S changed from the R0 type to the R3 type. The recrystallization and growth of clay minerals continued with increasing stress with depth, leading to the substantial realignment of clay particles observed at the microscale. The combination of the preferential alignment of clay particles and precipitation of authigenic cements caused significant decreases in the porosity of the mudstone. The results of this study support the viewpoint that the effective stress continues to work in diagenetically altered mudstones, although porosity loss is enhanced by clay fabric destabilization caused by clay mineral transformation. The top depth of smectite-to-illite transformations is consistent with the top of the overpressure, indicating that overpressure occurs simultaneously with clay diagenesis in diagenetically altered mudstones. The decrease in porosity and increase in density of mudstone caused by clay diagenesis changes the overpressure logging response to a certain extent and affects the accuracy of the pressure prediction results. The sonic transit time-density cross plot can be used to effectively distinguish the compaction stage and the cause of overpressure in diagenetically altered mudstones. Clay mineral diagenesis cannot be ignored in pore pressure analysis and pressure prediction in deeply buried mudstones. This research is of great importance for deepening the theory of mudstone compaction and has practical significance for pore pressure prediction in deeply buried layers.

Shale Lithofacies and Its Effect on Reservoir Formation in Lower Permian Alkaline Lacustrine Fengcheng Formation, Junggar Basin, NW China

Lower Permian Fengcheng Formation is considered to be a high-quality alkaline lacustrine shale oil resource in the Junggar Basin, NW China. Based on core and thin section observation, X-ray diffraction, scanning electron microscope, low-pressure N2 adsorption, and high-pressure mercury intrusion porosimetry, different shale lithofacies, and pore structures were examined. According to the mineral composition, shales in well My 1 are divided into five types: dolomitic mudstone, calcareous mudstone, siliceous mudstone, tuffaceous mudstone, and argillaceous mudstone, each of which shows its pore structure distribution. Intragranular pores, inter-crystalline pores associated with clays and pyrites, dissolution pores, and microfractures were commonly observed. There are three segments of pore structures including <50 nm, 50 nm-4 μm, and >4 μm. Clay minerals mainly contribute to mesopores, especially in argillaceous mudstones. The dissolution of carbonate minerals and feldspars is significant for macropores predominantly in dolomitic mudstones and tuffaceous mudstones, respectively. Micron-scale microfractures associated with laminae dominate in dolomitic mudstones. Therefore, the dolomitic mudstones, especially with lamination, and tuffaceous mudstones are proposed to be favored shale lithofacies with great exploration potential in the Mahu Sag.

Proactive stress interference mechanism and its application in the Mahu oil area, Junggar basin

There exists a large horizontal principal stress difference (11–38 MPa) in the tight conglomerates in the Mahu oil area, China. It is difficult to form a complex fracture network via hydraulic fracturing under these conditions. To improve reservoir stimulation, the fracture formation mechanism of the complex conglomerate fracture networks was explored. Based on the geomechanics theory of fracture formation, the mechanism of the “stress wall” formed by fracturing in horizontal wells was analyzed in this paper. The inhibitory effect of the stress wall on the formation of tensile and shear fractures was studied. The reason for the decrease in the stress difference coefficient caused by fracturing fluid was analyzed through numerical simulation, which suggested that the complexity of a fracturing network is mainly controlled by the interference of externally applied stress and the reduction in the coefficient of internal stress difference. In this paper, innovative technologies were developed by proactively introducing stress interference in the application of the Ma131 small-well-spacing pilot area. The core technologies include optimization of the 3-D staggered small-well-spacing pattern, and synergetic optimization of multiple elements and zipper fracturing. The positive effects of proactive stress interference on improving fracturing volume, reserve utilization rate and recovery were discussed. Based on the concept of proactive stress interference, the “serial fracturing mode” of horizontal wells was proposed to reduce drilling and fracturing interference and improve the development effect.

A Transition Towards an Unusually Wet Condition Will Not Alleviate Water Scarcity Risk in Xinjiang, China

AbstractThe arid Xinjiang region of China exemplifies severe water scarcity regions with a burgeoning agriculture. Whether or not the ongoing wetting, unprecedented in instrumental hydroclimatic records, will become routine has implications for regional oasis economy and China's Go West plan. Here, we show that Xinjiang experienced a one‐third increase in precipitation since the 1980s, and that this increase is primarily due to sea surface warming in the Bay of Bengal‐west Pacific and tropical North Atlantic. Using the historical precipitation‐sea surface temperature relationship to constrain future precipitation changes under a moderate warming scenario, annual precipitation over Xinjiang is projected to increase by 56% relative to the present‐day, and the levels of wetness of exceptional years, such as 2010 and 2016, will potentially become a new norm in terms of regional mean within the next few decades. However, such a large precipitation‐induced increase in freshwater resources will still be insufficient to rescue the Tarim and Junggar basins from their severe water scarcity dilemma within the next few decades, unless the most‐efficient water‐saving irrigation strategies are implemented.

Sequence architecture, slope-break development and geological significance during the P–T transition in the Mahu Sag, Junggar Basin, China

During the P–T transition period (from the Late Permian to the Early Triassic), the Mahu Sag of the Junggar Basin transitioned from a foreland basin to a large depression lake basin, with the development of two sets of unconformities (T1/P and P3w/P2w) and two sets of lowstand deposits, the upper Wuerhe Formation (P3w) and the Baikouquan Formation (T1b) that now host a series of large or giant conglomerate oil fields. For the two third-order sequence boundaries T1/P and P3w/P2w, two types of slope-break zones associated with flexure or erosion have been identified: the flexural slope-break zone controlled by a persistent paleo-uplift that also controls the layer-by-layer “unidirectional onlapping” of strata above the sequence boundary towards the higher part of the slope; and the erosional slope-break zone controlled by the erosional remnants of paleo-mounds, typically characterized by “two-way onlapping” towards the residual mounds or filling along the lower part of the valleys. Both types of slope-break zones control stratigraphy and sedimentary changes, and they encourage the formation of lithostratigraphic traps of up-dip onlapping, lowstand filling, and truncated types. The findings have a significant impact on the exploration of large-scale lithostratigraphic plays controlled by paleotopography.

Thin-interbedded reservoirs prediction based on seismic sedimentology

Interference of thin-interbedded layers in seismic reflections has great negative impact on thin-interbedded reservoirs prediction. To deal with this, two novel methods are proposed that can predict the thin-interbedded reservoirs distribution through strata slices by suppressing the interference of adjacent layer with the help of seismic sedimentology. The plane distribution of single sand bodies in thin-interbedded reservoirs can be clarified. (1) The minimum interference frequency slicing method, uses the amplitude-frequency attribute estimated by wavelet transform to find a constant seismic frequency with the minimum influence on the stratal slice of target layer, and then an optimal slice corresponding the constant frequency mentioned above can be obtained. (2) The superimposed slicing method can calculate multiple interference coefficients of reservoir and adjacent layers of target geological body, and obtain superimposed slice by weighted stacking the multiple stratal slices of neighboring layers and target layer. The two proposed methods were used to predict the distribution of the target oil layers of 6 m thick in three sets of thin-interbedded reservoirs of Triassic Kelamayi Formation in the Fengnan area of Junggar Basin, Northwestern China. A comparison with drilling data and conventional stratal slices shows that the two methods can predict the distribution of single sand bodies in thin-interbedded reservoirs more accurately.