Sedimentary Rhythm Research Articles

Research on CO2-WAG in Thick Reservoirs: Geological Influencing Factors and Random Forest Importance Evaluation.

In the development process of thick reservoirs, the impact of various geological factors on the effectiveness of the CO2 water alternating gas (CO2-WAG) flooding technology remains unclear. This paper establishes multiple CO2-WAG flooding models for thick reservoirs to study the effects of sedimentary rhythm, dip angle, matrix permeability, high-permeability streaks (HPS), and barrier layers on the effectiveness of CO2-WAG flooding and then uses the random forest algorithm to rank the importance of these geological factors. The results show that different geological factors have varying degrees of impact on the distribution of water and gas migration and recovery rates during the CO2-WAG flooding process. The ranking of the importance of various factors obtained by reservoir numerical simulations and the random forest algorithm is HPS, sedimentary rhythm, dip angle, matrix permeability, and barrier layers. These research findings will provide effective guidance and a reference for the optimal selection of CO2-WAG flooding schemes for similar thick reservoirs under different geological conditions.

Numerical Investigation on Alkaline-Surfactant-Polymer Alternating CO2 Flooding

For over four decades, carbon dioxide (CO2) has been instrumental in enhancing oil extraction through advanced recovery techniques. One such method, water alternating gas (WAG) injection, while effective, grapples with limitations like gas channeling and gravity segregation. To tackle the aforementioned issues, this paper proposes an upgrade coupling method named alkaline-surfactant-polymer alternating gas (ASPAG). ASP flooding and CO2 are injected alternately into the reservoir to enhance the recovery of the WAG process. The uniqueness of this method lies in the fact that polymers could help profile modification, CO2 would miscible mix with oil, and alkaline surfactant would reduce oil–water interfacial tension (IFT). To analyze the feasibility of ASPAG, a couples model considering both gas flooding and ASP flooding processes is established by using the CMG-STARS (Version 2021) to study the performance of ASPAG and compare the recovery among ASPAG, WAG, and ASP flooding. Our research delved into the ASPAG’s adaptability across reservoirs varying in average permeability, interlayer heterogeneity, formation rhythmicity, and fluid properties. Key findings include that ASPAG surpasses the conventional WAG in sweep and displacement efficiency, elevating oil recovery by 12–17%, and in comparison to ASP, ASPAG bolsters displacement efficiency, leading to a 9–11% increase in oil recovery. The primary flooding mechanism of ASPAG stems from the ASP slug’s ability to diminish the interfacial tension, enhancing the oil and water mobility ratio, which is particularly efficient in medium-high permeability layers. Through sensitivity analysis, ASPAG is best suited for mid-high-permeability reservoirs characterized by low crude oil viscosity and a composite reverse sedimentary rhythm. This study offers invaluable insights into the underlying mechanisms and critical parameters that influence the alkaline-surfactant-polymer alternating gas method’s success for enhanced oil recovery. Furthermore, it unveils an innovative strategy to boost oil recovery in medium-to-high-permeability reservoirs.

Jurassic sedimentary evolution model and its implication for the sandstone-type uranium mineralization in the Kamusite area in eastern Junggar Basin, NW China

The Junggar Basin in the northern Xinjiang Province, northwestern China, is a large hydrocarbon basin with tremendous reserves of oil, gas and coals. Tens of sandstone-type uranium showings, occurrences and/or anomalies have also been discovered in the Junggar Basin since the 1960s, yet hitherto only one large uranium deposit, the Kamusite deposit, has been identified in the Middle Jurassic Toutunhe Formation sandstones in the eastern part of the basin. Sand bodies with high-permeability are the prerequisite for the formation of the in situ leachable sandstone-hosted uranium deposit, nevertheless the spatial distribution and genetic types of the sand bodies, as well as the deposition model in Jurassic time in the Kamusite area both remain unclear, which largely hindered the understanding of the sedimentary evolution and uranium exploration directions in the region.In this contribution, based on the outcrop observation, core logging and well logging data interpretation, and comprehensive analysis, the sedimentary characteristics, sedimentary facies types, sand body morphology, plane distribution characteristics, as well as the sedimentary evolution history in Jurassic time in the Kamusite area have been illustrated in details. From early to late, the Jurassic sediments in Kamusite can be divided into the Badaowan, Sangonghe, Xishanyao, Toutunhe and Qigu formations, of which the sand bodies favorable for the sandstone-type uranium mineralization have been systematically analyzed, and the depositional model in the Kamusite area have been established. The Early Jurassic Badaowan Formation is a set of coal-bearing coarse-grained clastic sediments of the braided river–braided river delta facies, followed by the lake transgression and fine-grained lacustrine deposition in the Sangonghe Formation. The Middle Jurassic Xishanyao Formation is a set of braided river delta facies coal-bearing clastic sediments, while the uranium orebody-bearing Toutunhe Formation underwent the paleoclimate transition from warm and humid to hot and arid, and developed a set of braided river delta to meandering river facies sequences with an overall positive sedimentary rhythm. The Qigu Formation is dominated by a series of meandering river to lacustrine facies fine-grained sediments and largely denuded by later tectonic activities. Three sets of sand bodies favorable for the sandstone-type uranium mineralization are recognized, including those in the lower Toutunhe Formation, the lower Xishanyao Formation, and the upper Badaowan Formation, from bottom to top. A four-stage sedimentary evolution model from the Badaowan Formation to the Toutunhe Formation has been established thereafter, and general exploration suggestions were proposed in the last of the contribution. More drilling is recommended to the south of the Kalasayi Fault, aiming the lower Toutunhe, lower Xishanyao and upper Badaowan formations. To the north of the Kalasayi Fault, more uranium mineralization could also be expected, and the exploration is suggested to focus on the Jurassic Toutunhe, Xishanyao formations and Cretaceous Tugulu Group.

Identification of Reservoir Water-Flooding Degrees via Core Sizes Based on a Drip Experiment of the Zhenwu Area in Gaoyou Sag, China

In order to identify the degree of water flooding in a reservoir and to discover any remaining oil-enriched areas, in this paper, a systematic study on the water flooding of cores in obturated coring wells is carried out. With observations and testing data of the cores, based on the notion of sedimentary facies, the water-flooding degrees of 4–7 sand groups in member one of the Paleogene Sanduo Formation (E2s14–7) of the Zhenwu area in the Gaoyou Sag are determined. Overall, the results show that the study area is formed under the background of lake regression, with various sedimentary systems, mainly including delta facies, braided fluvial facies, and meandering fluvial facies. The degree of water flooding is determined using a point-by-point drip experiment of the core. Combined with the testing results of the core, the water-flooding degrees of the different sedimentary facies are quantitatively determined. Identification standards for the water-flooding degree of delta facies, braided river facies, and meandering river facies are established. The water-flooding degree of the delta sand body is generally weak, with an oil saturation rate of 24.1–40.2%, essentially indicating no water flooding or weak water flooding. The water-flooding degree of the braided fluvial sand body significantly changes, and the variation range of the oil and water saturation measurement results is also large. The water-flooding degree of the meandering fluvial sand body is weaker than that of the braided fluvial sand body, which is mostly not flooded or weakly flooded. The water-flooding degree is obviously controlled by the sedimentary rhythm and the sedimentary type. The top of the positive rhythm, the bottom of the sludge bed in the braided fluvial point bar, the deltaic front subaqueous distributary channel, and the point bar in the meandering fluvial have relatively low water-flooding degrees. They are the subjects of subsequent development adjustment and the remaining oil potential tapping.

Sedimentary records of palaeofloods during the late Holocene along the lower Yellow River, China: A comprehensive study based on multi-proxy

In this paper we propose a new super-comprehensive proxy that integrates the sedimentary rhythm indicators of grain size, black carbon, anthropogenic elements, and pollen for the division of alluvial sedimentary rhythms suitable for use on sediments that have or have not been disturbed by past human activity. The new proxy was applied in the analysis of sedimentary records from two boreholes (identified as JM and SZ) in the Kaifeng area of the lower Yellow River, an area that has received less research attention in terms of flood events and sedimentary records compared to its middle and upper reaches. From this analysis, flood events that occurred in the lower Yellow River since the late Holocene were reconstructed. The results suggest there are 15 and 14 sedimentary rhythms in the cores of JM and SZ, respectively. Based on historical documents, archaeological excavation data from the vicinity of the core sites, and AMS14C dating, we established the chronological framework of sedimentary rhythms in both JM and SZ cores. Using this information, we then reconstructed six Yellow River flood events impacting Kaifeng that occurred after the Warring States period in the years 225 BCE, 1387 CE, 1399 CE, 1461 CE, 1642 CE and 1841 CE. Among the six flood events, except for the flood of 225 BCE which was directly caused by human factors, the rest were caused by natural factors, mainly from prevailing climatic conditions but superimposed by human activities. The research results, therefore, can provide a scientific basis for flood prediction, prevention and risk assessment in the Yellow River Basin under the background of global change, and also provide support for clarifying the relationship between regional climate background and human activities in abnormal flood events.

Geostatistical seismic inversion constrained by reservoir sedimentary structural features

Geostatistical inversion is a technique widely used in seismic inversion. It builds the posterior distribution by incorporating prior information. Abundant prior information for geostatistical inversion is provided by the spatial variability of well log represented by variograms. However, a variogram can only describe the spatial variability of lithologic parameters at a single point. Describing the structural relationship of rock deposition in the vertical direction (the sedimentary rhythm) requires a continuous curve, rather than one single point. We have developed a geostatistical seismic inversion method constrained by sedimentary structural features to enhance the consistency between inversion results with the sedimentary features of a reservoir. This method first uses sparse representation to learn the sedimentary structural features of a reservoir. Then, the structural features are taken as prior information of geostatistical inversion to improve the accuracy of inversion results. The novelty of our method is that the element describing the spatial variability of model parameters is extended from a single point to a sedimentary structural block. We derive an analytical expression of posterior distribution of model parameters with constraints of sedimentary structural features and realize a data-driven geostatistical seismic inversion. Experiments on synthetic and field data indicate promising results consistent with the sedimentary features of reservoir.

Quantitative Prediction of Low-Permeability Sandstone Grain Size Based on Conventional Logging Data by Deep Neural Network-Based BP Algorithm

The median grain size of rock is the main basis for the identification of sedimentary facies, and the variation of the median grain size of rock can be used to obtain the stratum sedimentary rhythm and thus to classify the flow unit. Therefore, the median grain size of rock is an extremely important parameter for reservoir evaluation. However, there is no petrophysical method that can directly evaluate the median grain size of rock in the logging data. The predecessors used natural gamma logging data to calculate the median rock grain size (Md) based on linear and statistical analysis for medium-high porosity and permeability sandstone reservoirs work. However, for low-permeability sandstone reservoirs, the error in the fitted median grain size of rock using linear multiple regression methods is too large for the calculated results to be applied. Therefore, the calculation of the median grain size of low-permeability sandstone reservoirs is a difficult problem to be solved. In this paper, the sensitivity logging parameters of median rock grain size are optimized for low permeability sandstone reservoirs using principal component analysis obtained the grain size direction correlation curves (DEN, CNL,GR, and RD) in the study area, and the corresponding loss and activation functions are selected based on the learning characteristics of the nonlinear mapping of the logging data and the BP neural network to ensure that overfitting occurs. The best model was obtained by using decision tree, support vector machine, shallow and deep neural networks to model the median rock grain size and predict neighboring wells, and a comparative analysis showed that for the problem of predicting the median rock grain size in low-permeability sandstone reservoirs, the deep neural network improved significantly over the shallow one and was much stronger than other machine learning methods. The best model obtained a coefficient of determination ( R 2 ) of 0.9831. Machine learning of median grain size from conventional logging data was systematically carried out through conventional logging sensitivity curve optimization, algorithm modeling, network parameter optimization, median grain size prediction, and validation, and the relative error in its quantitative prediction met application requirements. This method takes into account the nonlinear mapping relationship between the logging data and the fitting of small sample data and provides a systematic way of thinking for the logging curve to predict the grain size of low-permeability sandstone.

Experimental Investigation on Hydraulic Fracture Morphology of Inter-Salt Shale Formation

The inter-salt shale in the Qianjiang formation of Jianghan Basin in China is characterized by multiple salt-shale bedding planes, various rock strength, and high heterogeneity of rock mechanics. In this paper fracturing experiments under different conditions were carried out to study the effects of the injection velocity, type of fracturing fluid and interface strength on the propagation law of hydraulic fracture in the salt sedimentary rhythm there. In the meantime, Acoustic emission system and radial strain sensor were applied to monitor experimental process. The result indicates that 1) compared with the shale, there are four fracture propagation modes mainly being observed: passivating type (Mode I), “I”-type (Mode II), penetration type (Mode III) and mixed type ((Mode IV)), among which the mixed type is the relatively complex crack propagation mode. 2) With the increase of injection rate and viscosity of fracturing fluid, the hydraulic fracture will penetrate cementation surface more easily. 3) The increase of flow rate and viscosity will increase the breakdown pressure. The breakdown pressure of high strength cementation surface is 16.70% higher than that of low strength.

Pillar stability of salt caverns used for gas storage considering sedimentary rhythm of the interlayers

In view of the low permeability and good damage self-healing properties of rock salt, salt caverns are thought to be ideal media to hold natural gas. However, widely distributed salt formations in China were formed bedded by lacustrine sedimentation with significant sedimentary rhythm characteristics. The various interlayers in bedded salt formations may cause instability of the cavern pillar due to their uncoordinated deformation, thus threatening the safety of the whole gas storage caverns. Combining theoretical analysis and numerical simulation, the influence of different characteristics of interlayers on the stability of salt cavern pillars were detailed analyzed. Some main conclusions were summarized as follows: 1) In a sedimentary rhythm, the rock salt and the soft interlayers can be considered as one category, and the hard interlayers can be treated as another category. Compared with the hard interlayers, rock salt and the soft interlayers contributes more to the shrinkage of salt cavern; 2) With the decrease of pillar width or the increase of operating time, the principal stress curves (both σ1 and σ3) in the interlayers along the line between the centers of the two caverns generally experience the process of "Bimodal type - Saddle type - Unimodal type"; 3) The entire plastic zone expansion process in hard interlayers of the cavern pillar shows the periodic law of "interfaces - middle interlayer - interfaces - middle interlayer"; 4) As to site selection of salt caverns for gas storage, bedded salt formations with too many interlayers should be avoided. If the interlayers cannot be avoided, the salt cavern should be constructed with interlayers located far away from the cavern center.

Influence of interbedded rock association and fracture characteristics on gas accumulation in the lower Silurian Shiniulan formation, Northern Guizhou Province

Abstract The northern Guizhou area, located near the southwestern margin of the Yangtze Block, is a promising area for shale gas exploration and development. The Lower Silurian Shiniulan Formation as a new discovery stratum of natural gas marks an exciting breakthrough in natural gas exploration in northern Guizhou area. Based on several field investigations and samples analyses, the lithology and fracture characteristics were systematically analyzed in the lower Shiniulan Formation, and the reservoir specificity and its influence on natural gas accumulation were determined. The characteristics of the relatively fractures and lithology assemblages were identified as key factors controlling the natural gas accumulation. The lower Shiniulan Formation is deposited as calcareous shale and marlstone with frequent centimeter-scale interlayers. This is reflective of a shallow sea shelf strata with decreasing sedimentary rhythm and gradual weakening of sedimentary changes and developed calcareous shale and marlstone with frequent centimeter scale interlayer changes. The gas reservoir is dominated by calcareous mudstone, controlled by the interbedded rock association (calcareous mudstone and limestone), characterized by the raw-storage and the accumulation-reservoir interbedded system. The reservoir is located in the central part of the syncline and is characterized by strong sealing of the stratum, large proportion of free gas, and high abnormal pressure. The Lower Shiniulan Formation is formed between the shale layer with horizontal fractures and dense limestone with underdeveloped fractures. Among them, the shale section generally develops diagenetic shrinkage fractures, which provide good storage space for natural gas and act as the main body of natural gas. The pore sizes in limestone (2.8 nm) are significantly smaller than those in mudstone (7.5 nm), which results in a good capping and preservation of shale gas. This paper reports on results that are of significance for supplementing the theory of unconventional natural gas accumulation and guiding shale gas exploration in similar areas.

Characteristics of late Miocene Lithofacies - Paleogeography in the Southeast Region of Hanoi Depression

The characteristics of late Miocene lithofacies in the southeast region of Hanoi depression have been revealed on the basis of sedimentary evolution in relation to sea level change and tectonic movement. During late Miocene, global sea level change has created one depositional sequence and three sedimentary systems tract. The lowstand systems tract (LST) is characterized by 5 rhythms of alluvial lithofacies. These rhythms are represented by rough, humoc seismic wave fields. This environment is not favorable condition for coal formation. The transgressive systems tract (TST) is characterized by 6 transitional lithofacies rhythms. Each lithofacies rhythm consists of 4 facies: the tidal flats sand facies of the bay, the mud facies of the river mouth lagoon, the coastal marshy mud facies creating coal and the bay greenish-gray clay facies. The pacing process involves changes in the local sea level caused by tectonic lift. Each tectonic subsidence phase takes place at a very slow velocity, so it is compensated for fine-grained sediment creating marshy mud facies to develop mangroves on a large scale. It is a prerequisite to create thick coal seams distributed near the end of each rhythm. A part of highstand systems tract (HST) are eroded due to the improved folding process, creating an angular unconformity boundary with Pliocene-Quaternary sediments. However, after reconstracting of deformated section, it was clear that this systems tract had only one rhythm including 2 facies: prodelta mud facies and alluvial fan sandstone facies.
 Keywords: Lithofacies-paleogeography, sedimentary systems tract, sedimentary rhythm

Cold water-flooding in a heterogeneous high-pour-point oil reservoir using computerized tomography scanning: Characteristics of flow channel and trapped oil distribution

There are relatively few studies that consider the influence of reservoir heterogeneity on reservoir wax deposition. This study conducts a computerized tomography scan to study the flow channels, heavy component retention, displacement efficiency, and trapped oil distribution in a high-pour point oil reservoir with a positive sedimentary rhythm under various water injection temperatures. This study greatly improves the understanding of the wax deposition influences on the flow channel. Three displacement models are proposed: water-flooding breakthrough model (49.5 °C), approximate-equilibrium piston-like displacement model (54.5 °C/59.5 °C), and non-equilibrium piston-like displacement model (64.5 °C). When the injection temperature is lower than the wax appearance temperature (59.5 °C), an elliptical wax deposition is formed at the inlet, the most serious deposition is in the middle-permeability layer. A wax deposition barrier is formed away from the inlet, and constrains the downward movement trend of the water injected above. Therefore, the moderate wax precipitation will improve the oil recovery from the top low-permeability layer. However, the heterogeneity in the spaces and strength of the barrier result in the oil recovery from the 100 mD layer at 59.5 °C begin higher than that at 54.5 °C; moreover, at 59.5 °C, the recovery at the inlet is better than that at the outlet. The vortex phenomenon increases the oil recovery from wax deposition area in the bottom layer at the inlet. The heavy component retention is significant, and the oil recovery decreases with decreasing injection temperature. After flooding, the formation mechanisms of the trapped oil include wax deposition, weak water washing, and reservoir sedimentary rhythms. The research results provide initial insights for the evaluation of cold damage.

Sedimentary rhythm of Mn-carbonate laminae induced by East Asian summer monsoon variability and human activity in Lake Ohnuma, southwest Hokkaido, northern Japan

Past and ongoing precipitations of Mn-carbonate (rhodochrosite) have occurred in the freshwater Lake Ohnuma, which is situated about 8 km south of the Hokkaido-Komagatake volcano, southwest Hokkaido (northern Japan). Lake sediments from a surface and a longer ∼4 m core contain the sedimentary rhythms of rhodochrosite laminae and volcanic deposits which were identified by nondestructive micro-XRF fluorescence analyses. The three tephra ash layers from the Hokkaido-Komagatake volcano, characterized by peaks of Al, Si, K, Ca, and Ti, correspond to regionally recognized tephras (1640, 1929 and 1996 CE) based on ashfall isopachs and 137Cs, 210Pb and 14C ages determined from the two cores. The sedimentation rate showed a drastic change around 1929 CE; ∼0.3 cm/yr for 1640–1929 CE and ∼2.7 cm/yr for 1929–2012 CE. The Mn content record of Lake Ohnuma sediments, composed of rhodochrosite laminae, showed a periodic variation with a decadal–centennial scale over the interval 1640–2012 CE. Sediment petrography and hydrochemical measurements suggest that the metastable phase of rhodochrosite was deposited during summer after being precipitated, followed by the crystal growth of rhodochrosite within the anoxic sediments. On the basis of these results, high Mn content is interpreted as an indicator of strengthened summer stratification associated with reduced rainfall runoff. The Ti/Si ratios are representative of the relative abundance of detrital inputs from the watershed. The combined signals of elevated Mn content and Ti/Si ratio for 1935–2012 CE indicate eutrophication and hypoxia in the lake and enhanced land erosion in the watershed, associated with the post-1970s intensive land use such as livestock farming. For 1640–1935 CE, the Lake Ohnuma record shows an antiphase relationship between the Mn content and Ti/Si ratio, which implies that the detrital inputs reflected the strength of the East Asian summer monsoon (EASM), and the reduced rainfall runoff resulted in weaker mixing, followed by depleted O2 in the water column. The antiphase variation in Mn content and Ti/Si ratio suggest weak EASM during the Little Ice Age (∼1850 CE), followed by strong EASM between 1860 and 1929 CE.

Influence of formation heterogeneity on foam flooding performance using 2D and 3D models: an experimental study

The formation heterogeneity is considered as one of the major factors limiting the application of foam flooding. In this paper, influences of formation properties, such as permeability, permeability distribution, interlayer, sedimentary rhythm and 3D heterogeneity, on the mobility control capability and oil displacement efficiency of foam flooding, were systematically investigated using 2D homogeneous and 2D/3D heterogeneous models under 120 °C and salinity of 20 × 104 mg/L. The flow resistance of foam was promoted as the permeability increased, which thus resulted in a considerable oil recovery behavior. In the scenario of the vertical heterogeneous formations, it was observed that the permeability of the high-permeable layer was crucial to foam mobility control, and the positive rhythm appeared favorable to improve the foam flooding performance. The additional oil recovery increased to about 40%. The interlayer was favorable for the increases in mobility reduction factor and oil recovery of foam flooding when the low permeability ratio was involved. For the 3D heterogeneous formations, foam could efficiently adjust the areal and vertical heterogeneity through mobility control and gravity segregation, and thus enhancing the oil recovery to 11%–14%. The results derived from this work may provide some insight for the field test designs of foam flooding.

Climate change and tectonic implications during the Pliocene climate transition interval from lacustrine records in western Wei River Basin, central China

The Earth's climate had undergone a global transition around 4–3 Ma, from warm to cool. The western Wei River Basin, its late Cenozoic tectonic evolution was mainly controlled by the expansion of the Tibetan Plateau, is affected by both Monsoon climate and Westerlies circulation. A set of lacustrine sediments between 4.2 and 3.6 Ma with clearly sedimentary rhythm are good materials for studying the palaeoclimate/palaeoenvironment changes and exploring the contained tectonic implications.We present a high‐resolution grain‐sized, magnetic susceptible, and carbonate content records from the palaeolake sediments at the global climate transition interval (4.2–3.6 Ma) in the western Wei River Basin. The abrupt changes of climate indicators at about 4.14–4.16 Ma, which is a regional response to global cooling, indicate that both westerly and monsoonal circulations were enhanced and should be attributed to global cooling rather than the uplift of TibetanPlateau. From 4.14 to 3.6 Ma, the climate represented stepwise strengthening or weakening with frequent fluctuations on the millennium scale and extreme dry events at 3.77–3.80 Ma. Combined with regional geological environment and palaeoclimate evolution data, the palaeolake that rapidly receded at 3.6 Ma provides evidence for the understanding the major climatic and environmental changes produced by the uplift of the Tibetan Plateau in western Wei River Basin and surrounding areas.

Predictive model for water absorption in sublayers using a machine learning method

The sedimentary rhythm of Chinese oilfields is complicated and the heterogeneity is extremely strong. Allocating water absorption of each sublayer by dividing coefficient or numeric simulation cannot accurately reflect the actual water injection of the reservoir. Calculation based on water absorption profile monitored on site is the most commonly used method in oil field. However, access to these type of data is limited due to its cost and time related to acquisition. In this study, a machine learning approach was adopted to predict water absorption in sublayer based on geologic and production parameters of injectors and producers. On the one hand, it can save test costs. On the other hand, it can continuously predict water absorption of sublayers, and make up for water injection wells with insufficient injection profiles. A handful of training observations are obtained from on-site monitoring. Interwell connectivity is first conducted to identify connected producers for injectors. Introducing interwell connectivity helps to constitute predictor variables and yield significant improvements in feature selection. Connectivities in the well group are represented by similarity between injection sequence and production sequence, which is computed by Dynamic Time Warping. Average importance of predictors are then measured based on Mean Decrease Impurity, Mean Decrease Accuracy, and Ridge regression. Some relative important features are selected to consist the final predictors. The Extreme Gradient Boosting model is developed and then trained for making predictions given any set of observations. The proposed approach is validated by using actual field case from SL oilfield, China. Results show a significant correlation between predictions and actual value from on-site monitoring.

Tight reservoir oiliness numerical simulation based on a Markov chain Monte Carlo (MCMC) method: A case study of the upper Triassic Yanchang-6 formation (T3ch6 Fm.) outcrop of Ordos Basin

Numerical simulation of the tight reservoir oiliness has always been an important issue in tight oil resources exploration. It is hard to take some special situations into simulations such as the micro-nano thermodynamic system, the multi-fluid phase behavior, and the superimposed tectonic stress field of tight oil reservoirs. In our research, we use MCMC methods to simulate the tight reservoir oiliness constrained by some visualized, spatial-related and interacted geological parameters based on the statistic Bayesian theory, instead of taking the complicated accumulation mechanism into consideration. In this research, we built a posterior probability density distribution (Post-PDD) matrix using the spatial relation between oiliness, fracture, and lithofacies parameters of a typical Yanchang tight oils outcrop. In the simulation, one Monte Carlo Step (MCS) was settled N = 80 × 36 × 5000, where 80 and 36 represent the horizontal and longitude grid number respectively, and 5000 represents the sampling times for each grid. In total, simulations of 50000 MCSs were made for each realization. For the simulation results, the variation in the fluctuating trends of the fitted and actual values matched well with the analytic variogram for the MCMC realizations. Additionally, we found that the simulation uncertainty was consistent with the reservoir heterogeneity. Even though the lamina, intercalation or the barrier was not defined in the grid model, the oiliness realization of the different types was remarkably consistent with the reservoir heterogeneity, which may be caused by the sedimentary rhythm similitude between different scale sedimentary cycles, particularly in the lacustrine tight reservoir.

The characteristics of tight oil sand reservoir-source assemblage in lacustrine basins: A case study of the Cretaceous Qingshankou Formation, Songliao Basin, northeast China

The complicated source-reservoir-assemblage characteristics of lacustrine tight oil sand in China are the main controlling factors of tight reservoir oiliness (i.e., oil richness). Several studies have focused on qualitative description of source-reservoir-assemblage characteristics without quantitative assessment. In this study, reservoir-source-assemblage (RSA) has been evaluated quantitatively by fitting the RSA log in the evaluation of Qijia Depression in the Songliao Basin. Total organic carbon (TOC) and sand volume (Vs) logs are used to fit the RSA log in three steps: (1) TOC and Vs log fitting and normalization, (2) RSA log fitting, and (3) extraction of root-mean-square (rms) amplitude and frequency (Frq(0)) information from the RSA log. The rms represents the reservoir capability and hydrocarbon potential, and Frq(0) represents the interbedding frequency that changes with the lake level. Positive values (0–1] of the RSA log correspond to a high lake level, whereas negative values [[Formula: see text], 0) correspond to a low lake level. Based on RSA log values, we defined the parameter RSAsuf, a product of rms and Frq(0), to quantitatively evaluate the tight oil sweet spot. RSAsurf serves as tight oil sweet spot indicator and correlates positively to oil richness. As a result, four types of effective reservoirs (RI, RII, RIII, and RIV), two types of effective sources (SI and SII), and three types of RSAs (R-S-R, S-R-S, and S-S-R) are identified based on cores and RSA logs. High RSAsuf values on the isoline map indicate the sweet spot zones around the G933 and J392 well areas, which correlates very well with the oilfield test data. The approach is appropriate for lacustrine basins with complicated RSA, in which RSA logs serve as indicator for the sedimentary rhythm, reservoir capability, and hydrocarbon potential.

Wave processes in relation to the tectono-sedimentary of the Tarim basin: A new method for basin study

This study gives insights into the tectonic and sedimentary evolution of the Tarim basin, the largest petroleum-bearing basin in Western China, by analyzing sedimentary wave process using multi-scale decomposition of the wavelet transform method, and the mathematical defects of sliding-window method in the process of sedimentary wave analysis was discussed. The filtering analysis, in the time domain, was carried out on cores collected from 14 wells in the Tarim basin. The results show that the evolution of the Tarim basin was controlled by various wave processes. The periods of 212 ± Ma, 102 ± Ma and 33 ± Ma indicated a strong sedimentary rhythm, which displayed a spatial and temporal evolution of wave characteristics. The 33 ± Ma period represented a third-order sequence of the stratigraphic division, which identified with the unconformities of the Tarim basin; 102 ± Ma period corresponded to an important period of sedimentation and the plate collision around the Tarim basin, which marked a second-order period of the basin evolution. The centers of subsidence showed periodic shifts since the Cambrian, and the time for periodic migration (completing a circle) of the depocenters or the centers of subsidence is traced as 212 ± Ma, which corresponded to the first-order period of the basin evolution. The coupling relationships between the wave processes and the evolution and distribution of source, reservoir and cap rocks in the basin were given by three kinds of patterns, which were important in predicting the space-time distribution of source, reservoir and cap rocks. This study gives a new insight on predicting the formation and distribution of petroleum, which could be used for selecting petroleum exploration targets effectively.

Sedimentary laminae in muddy inner continental shelf sediments of the East China Sea: Formation and implications for geochronology

The sedimentary rhythm is well developed in the environment which is controlled by strong seasonal climate changing condition. This rhythm is important for sediment core dating purpose. In this study, we examine muddy sediments on the inner continental shelf of the East China Sea that are influenced under seasonal change of the East Asian monsoon. We use Ensemble Empirical Mode Decomposition (EEMD) method to process sediment grain size and X-radiograph negatives grayscale value data in three sediment cores from this area and use the results to analyze the patterns of sedimentary variability and the causes of cyclic variability of the sedimentary process. The variability in the grain size and grayscale values of the core sediments has three components: the high-frequency variability, low-frequency variability and a secular trend. Among these components, the high-frequency component reflects variability on annual time scale can be well recognized, we suggest that the annual variability is derived from seasonal variation in the sediment flux from the Yangtze River to the sea, as well as the East Asian monsoon's impact on the sedimentary dynamics of the East China Sea's inner continental shelf. However, the other variability is hard to identify which cycles there are. This variability may reflect decadal winter monsoon oscillation and human activity of dam construction in the Yangtze River. Each annual variability presents a varve in the sediment sequence, which can be used to determine the sediment deposition rate and accurately date the core sediment. We found that the average deposition rates of the three cores from north to south on the inner shelf were 3.1 cm/yr, 1.0 cm/yr, 0.9 cm/yr, indicating that the Yangtze-derived sediment decreased with the increasing distance from the Yangtze estuary to the shelf. The deposition rate of each core varies over time, and the maximum deposition rate can be 3–5 times greater than the minimum rate. The sediment ages obtained from varve chronology and radiometric dating are fairly consistent on a centennial scale, differing only by 2–5 years. Thus, varve counting is a useful supplementary approach to radiometric age dating for constructing chronologies for core sediment in shelf seas influenced by monsoons.