Facies Types Research Articles

Research on Sequence Stratigraphy of Nantun Formation in Bayanhushu Depression of Hailar Basin and Favorable Target Prediction

Exploration in Bayanhushu depression has been published from time to time, showing great exploration potential, but the degree is low at present. We devided Nantun formation into two third sequences through comprehensive analysis and studied on the types and distribution of sedimentary facies. The conclusion shows that Nantun formation is mainly comprised of fan delta, braid delta and lacustrine facies. The fan delta sedimentary facies mainly distribute in the western steep slope zone, while the braided river delta develops in the eastern gentle slope zone, and the lacustrine facies mainly develops in the middle trough zone and the edge of the fan delta. The sedimentary pattern has the characteristics of “east-west zoning”. From the steep slope to the gentle slope, the types of sedimentary facies gradually change from fan delta to shore-shallow lake, semi-deep lake to deep lake and then to braided river delta. The study of source-reservoir-cap assemblages and accumulation conditions shows that fan delta front and braided delta front have good physical properties and are close to high grade source. So it is easy to form inversion structural reservoirs in the steep slope break zone, while fault-lithologic reservoirs in gentle slope zone, a lithologic reservoirs near trough areas.

Diagenesis and reservoir quality of sandstones in the Eocene Shahejie Formation, Raoyang Sag (Jizhong Depression), Bohai Bay Basin, China

Deeply buried (3.5–4.5 km) fan deltaic and braided deltaic sandstones of the Eocene Shahejie Formation are important hydrocarbon reservoirs in the Raoyang Sag of the Bohai Bay Basin (North China). An integrated approach incorporating petrophysics, thin‐section petrography, scanning electron microscope (SEM), cathodoluminescence (CL), and well log analysis was applied to investigate diagenesis, diagenetic minerals, and their impacts on reservoir quality. The sandstones of the Sha‐3 (the third Member of the Shahejie Formation) are dominated by lithic arkoses and arkoses with a low‐moderate compositional maturity and a moderate textural maturity. The pore systems are dominated by secondary pores resulting from dissolution of framework grains, micropores among authigenic clays, and minor amounts of primary intergranular pores. Chemically unstable framework grains such as feldspar and lithic fragments experienced alteration and resulted in formation of authigenic kaolinite. Dissolution and alteration of feldspars and rock fragments and pressure solution of detrital quartz grains were the main sources for quartz cements. Mechanical compaction and cementation by calcite, dolomite, quartz, kaolinite, and mixed‐layer illite/smectite destroyed primary pores, whereas dissolution of framework grains generated secondary pores. Five types of diagenetic facies are identified based on framework mineralogy, texture, diagenetic minerals, and pore systems. These comprise the following: (a) quartz‐cemented sandstone, (b) tightly compacted sandstone, (c) carbonate‐cemented sandstone, (d) clay‐mineral‐cemented sandstone, and (e) clean dissolved sandstone. Diagenetic processes and evolution sequences for the five diagenetic facies were reconstructed based on textural relationships from thin section, SEM, and CL studies. The reservoir quality evolution of various diagenetic facies is predicted by considering variations in grain size, sorting, shape, and matrix content of sandstones. This work, which investigates the diagenetic sequence of sandstones, will provide insights into reservoir quality prediction for low‐permeability sandstones with similar tectono‐depositional settings.

A pattern-matching method for flow model calibration under training image constraint

Modern geostatistical modeling techniques are developed to simulate complex geologic connectivity patterns (e.g., curvilinear fluvial systems) at the grid-level using a training image (TI) that encodes multiple-point statistical (MPS) information. A challenging aspect of using MPS methods is conditioning the resulting models on nonlinear flow data. We develop a pattern-matching method for calibration of MPS-based facies models subject to the TI constraint. Since the exact statistical information in the TI can only be expressed empirically, flow data conditioning and pattern matching are carried out in two iterative steps, using an alternating-direction algorithm. Flow data integration is formulated through a regularized least-squares by taking advantage of learned k-SVD sparse parametrization and l1-norm sparsity-promoting regularization methods. The TI constraint is enforced through a MPS-based pattern-matching algorithm that uses the identified model calibration solution to generate a corresponding facies model that is consistent with the TI. The pattern-matching algorithm uses a local search template to scan the TI to find facies patterns with smallest distances from the corresponding local patterns in the parameterized approximate solution. The identified patterns for each location in the model are stored and used to estimate local conditional probabilities for assigning the facies types to each grid cell. The resulting solution is passed to the flow data conditioning step as a regularization term to perform the next iteration. The process is repeated until the MPS facies model provides an acceptable match to the data. Numerical experiments are presented to evaluate the performance of the pattern-matching method for calibration of complex facies models.

Petrophysical evaluation of reservoir in a selected well (Z) in an onshore oil field (X) in the Niger Delta Basin, Nigeria using wireline logs

: The petrophysical properties of a well selected from among the onshore oil fields in the Niger Delta Basin, Nigeria was evaluated using gamma ray log, resistivity log and neutron-density overlay log to analyze for the hydrocarbon potential of the well. Some petrophysical properties of the reservoir rocks and fluid characters such as porosity, shale volume, effective porosity, permeability, and formation resistivity factor, resistivity of water, hydrocarbon saturation, water saturation, and net pay thickness were evaluated. The results show that the volume of shale in each reservoir zone directly affects the effective porosity and the zones. The volume of shale is inversely proportional to the effective porosity, as an increase in the volume of shale will bring about a decrease in effective porosity. On the other hand, permeability is dependent on the effective porosity. Facies type identification of the reservoir sands were also carried out using the wireline log - Gamma Ray Log. Results of the work revealed the presence of hydrocarbon in 10 reservoirs across the well, and the hydrocarbon type was observed to be only oil. Oil/Water contacts occurred at 2288m, 2518mm, and 2989m, and the Net Pay Thickness was calculated to be 238m, that is the total thickness of the hydrocarbon reservoirs.Keywords: Petrophysics, Wireline logs, Facies, Reservoir rocks, Pay thickness.

Geospatial mapping and drinking water quality status of fluoride endemic hilly District Doda (J&K), India

In the present study, an attempt has been made to investigate the drinking water quality status of various surface and ground water sources and the supplied water from two water filtration plants supplying drinking water to Doda town and its adjoining areas by using Arithmetic Water Quality Index (WQI) and geospatial mapping techniques. Doda is fluoride endemic district of Jammu and Kashmir State where groundwater fluoride concentration upto 7.0 mg/L is recorded. Water samples collected from about twenty two drinking water sources from two blocks of Doda district (Bhagwah and Doda blocks) were analyzed at pre-determined locations marked using handheld GPS(Montana 650). These locations formed the attribute database for the study based on which fluoride distribution maps have been derived and integrated with Arithmetic WQI through Inverse distance weighted (IDW) interpolation technique. Prepared thematic maps have confirmed the vulnerability of the drinking water sources and water supply systems in the study area, thus, posing a serious public health concern. Based on the WQI, water quality status of Beoli filtration plant (WQI=43.26) falls under the category of good water whereas that of Moochan filtration plant (WQI=157.41) has been classified as unfit for drinking. The geochemical evolution of the water studied using Piper’s diagram has shown mixed type of hydrochemical facies. Coefficient of correlation(r) between different parameters has indicated significant correlation between several parameters. The study urges the concerned government authorities to make provisions for providing safe drinking water to public which is free of turbidity and low in fluoride concentration.

Sandstone Reservoir Distribution Mapping of Bekasap Formation Using Seismic Attributes and Identification of Stratigraphic Trap in ‘X’ Field, Central Sumatera Basin

The ‘X’ field is an oil-producing field in the Central Sumatra Basin. Central Sumatra Basin is the largest oil-producing sedimentary basin in Indonesia. The field is an area in the form of onlaping Bekasap Formation to the Basement High. The onlapping feature has potential for stratigraphic traps. This study aims to identify the potential stratigraphic traps for hydrocarbon accumulation, the distribution of sandstone reservoirs, facies types and the depositional environments. The data used in the study are 3D seismic data, well log, and core data. The research methods used including the analysis of electrofacies based on well log, seismic attributes, and conceptual geology model. The research shows that there is a stratigraphic trap in the form of intra-formation and regional traps. Intra-formation traps were formed vertically by the intersection of sandstone and shale lithologies and lateral facies changes, whereas the regional traps are formed by the shale layer of Telisa Formation and Bekasap Formation with onlapping on the Basement High. Based on AI inversion and RMS amplitude show the distribution of Top A sandstone reservoir is Northeast - Southwest on onlaping zone to the Basement High. The reservoir was deposited in depositional environment as wave dominated delta which is four facies associations: distributary channel, upper shoreface, lower shoreface, and offshore facies.

Evolution of the chemical and isotopic composition of groundwater under a semi-arid climate; the case of the Cenomano-Turonian aquifer within the Essaouira basin (Morocco)

This study evaluated how the chemical and isotopic composition of the groundwater in the Cenomano-Turonian aquifer of the Essaouira basin from Morocco has changed in response to low annual rainfall associated with climate change during the period 1995–2018. The study of the chemical composition of groundwater shows a Cl–Ca–Mg type facies which was maintained during the observation period (1995–2018). The electrical conductivity (EC) and the Cl− content of groundwater increased during the study period in response to low rainfall with a mean values range from 1214 (1995) to 2482 µs/cm (2018) for EC and from 11.36 (1995) to 14 meq/L (2018) for Cl−. Stable isotope measurements indicate that the Cenomano-Turonian aquifer is recharged by rainfall of Atlantic origin. Levels of 18O during the period 2007 and 2016 were generally stable with slight fluctuations (≈ 1‰) that may be due to analytical errors, suggesting that climate change has no effect on the isotopic signature of groundwater in the study area. Tritium levels in groundwater decreased between 2007 and 2016, possibly due to low recharge rates associated with low annual rainfall during this period.

Integrated geophysical and hydrochemical investigations for seawater intrusion: a case study in southwestern Saudi Arabia

The objective of the present study was to assess the deterioration in groundwater quality due to saline water intrusion in the Ad-Darb region of southwestern Saudi Arabia. An integrated approach using geochemical and geophysical methods was applied to assess the extent of saline water intrusion. Geochemical methods involved the determination of main groundwater facies present in the region using Piper and extended Durov plots. The geophysical methods used in the study included seismic refraction (SR), seismic refraction tomography (SRT), vertical electrical soundings (VES), and electrical resistivity tomography (ERT). Two SR and SRT surveys, three ERT surveys, and nine VES surveys were carried out. Piper plot shows that the water mainly belongs to the SO4-Cl type of anionic facies. The Durov plot indicates base ion exchange, linear mixing, and saline water intrusion as the main factors influencing the groundwater chemistry of the area. The TDS increases towards the coast, with values reaching as high as 9000 mg/l. These results confirm the interpretation of the vertical electrical sounding (VES) and electrical resistivity tomography (ERT) sections which indicates the presence of saline water intrusion, with the thickness of the intruded zone increasing towards the sea. The geoelectric resistivity results indicate four geoelectric resistivity layers. The average thickness of upper layer is 2.5 m and the resistivity ranges from 22.5 to 280 Ωm. The P wave velocity of this layer varies from 488 to 787 m/s. The second geoelectric resistivity zone consists of relatively disconnected layers with 10 to 33 m thickness and 52 to 120 Ωm resistivity. The third zone is a low resistivity zone, increasing in thickness westwards towards the sea and with resistivities of 0.3 to 19.5 Ωm. This is the main zone (consisting of unconsolidated Quaternary sediments) affected by saline water intrusion. This zone is intercalated with muddy marine intercalations, as indicated by its low resistivity (as low as 0.3 Ωm) The second and third geoelectric resistivity layer is represented by a single seismic zone with P wave seismic velocities ranging from 1231 to 1524 m/s, indicating the presence of water. The resistivity of the lowest layer ranges from 56 to 822 Ωm and corresponds to the basement. This bedrock layer has seismic velocities varying between 2315 and 3164 m/s.

Local and Target Exploration of Conglomerate-Hosted Gold Deposits Using Machine Learning Algorithms: A Case Study of the Witwatersrand Gold Ores, South Africa

Determining the gold grade and facies type in areas with little geological information and sparse exploration samples is fraught with uncertainties and often results in high operational costs. Point-wise gold grade data are commonly used to guide exploration and resource estimation with the application of spatial interpolation techniques such as kriging. Within this environment of data scarcity, the application of kriging leads to significant grade estimation errors, as high nugget thresholds reduce the effectiveness of kriging, a good example being the gold deposits in the Witwatersrand Basin of South Africa. To reduce the impact of subjective grade interpolation and geological interpretation, as well as to exploit currently unused geological descriptions, we present a novel machine learning-based algorithm called GS-Pred. It combines both sedimentological and gold assay data for point-wise gold grade prediction and automated facies identification in a conglomerate-hosted gold deposit. For this application, GS-Pred requires an input database of sedimentological descriptions, spatial information and gold grades and makes predictions of gold grades at any point within the spatial coverage of the input database, provided that it has appropriate sedimentological descriptions. In essence, GS-Pred examines the spatial and non-spatial variability of metal grades and provides information of the estimated resource below the nugget threshold. This proposed algorithm has been validated on subsets of data on gold grade and sedimentological characteristics of conglomerates in the Witwatersrand Basin. Validation results suggest that GS-Pred is more accurate than current machine learning techniques and ordinary kriging. The clustering result shows that there are four or at most five facies which can be distinguished from the clustering results within the dataset, which maximises the contrast in the inter-cluster prediction behavior. These clusters have a good spatial correspondence with the known geology, and the method, combined with gold grade predictions, was able to identify probable mineralization patterns, thus assisting in target exploration. This novel machine learning algorithm is entirely data driven. We have shown its successful application in a complex geological setting as the Witwatersrand Basin.

Using GIS methods for determining the spatial distribution of groundwater hydrochemical facies: a case study for a shallow aquifer in Fuyang, China

The determination of groundwater hydrochemical facies (GHF) is useful for analyzing the chemical composition of groundwater, tracking sources of groundwater recharge and assessing how its chemical composition changes along groundwater flowpaths due to water–rock interactions. This paper proposes a geographical information systems (GIS)-based method for identifying and mapping the spatial distribution of GHF categories in an aquifer. Several procedures including spatial interpolation, raster reclassification, raster-to-vector conversion, spatial union overlay, and generalization methods are coupled in GIS tools to determine the type and distribution of GHF categories. This method was implemented to distinguish shallow GHF categories for a shallow aquifer near Fuyang City in Anhui Province, China. The results obtained from the GIS assessment of water quality data from 30 wells show there were seven types of hydrochemical facies identified in the study area. The major GHFs categories are HCO3–Na + K + Ca + Mg, HCO3–Ca + Mg, HCO3–Na + K + Ca, and HCO3–Na + K, which cover 94% of the total area. The GIS method correctly identified the GHF categories determined in 80% of the wells in the area. The major GHFs categories progressively change in the direction of groundwater flow from HCO3–Ca + Mg to HCO3–Na + K+Ca + Mg, then to HCO3–Na + K + Ca, and then to HCO3–Na + K. The changes in the proportions of major ions in groundwater, coupled with salinity increases in the direction of groundwater flow are due to the combined effects of water–rock interactions in the aquifer and human activities. The method can relatively efficiently identify and map GHF zones for aquifers.

Hydrogeochemical characteristics and quality assessment of groundwater in Balikligol Basin, Sanliurfa, Turkey

The Balikligol Basin (Sanliurfa) is located in southeastern Turkey. The basin groundwater is used for drinking, domestic use and irrigation. In this study, the seasonal groundwater from wells and springs of the basin was analyzed hydrogeochemically and evaluated for irrigation suitability for 1 year. The study consists of geochemically controlling the dissolved matter concentration of the groundwater in the basin as well as the decomposition of silicate into Ca and Mg values. In terms of hydrochemical facies, six types of facies have been identified in the Piper classification and the predominant water is Ca–HCO3 water. According to some indexes and classifications such as EC, TDS, Na%, TH, SAR, RSC, MH, PI (except N4), US salinity level and Wilcox, the groundwater in the Balikligol Basin was characterized as being suitable for irrigation. The groundwater has not exceeded the maximum acceptable values of national and international classifications in terms of drinking and irrigation. As a result, it can be stated that groundwater resources in the basin are affected by water–rock interaction rather than anthropogenic factors.

A saturation evaluation method in tight gas sandstones based on diagenetic facies

Abstract Water saturation is one of the key parameters in reservoir evaluation and calculation. Tight sandstone reservoirs with complex pore structures caused by strong diagenesis often have complex rock conductivity and response laws, resulting in the low accuracy of the water saturation values calculated using existing models. Taking a Permian block in the eastern margin of the Ordos Basin as an example, the core samples of five wells were analysed using X-ray diffraction, scanning electron microscopy, core mercury intrusion experiments, analyses of their physical properties, and rock and electric experiments. First, research on the diagenetic facies classification method was carried out. Second, the conduction mechanisms of different diagenetic facies were studied, and a saturation model was established. Finally, Deep AutoEncoder was used to automatically abstract and express the response characteristics of different diagenetic facies. Random forest, which is sensitive to small sample sizes, was used to establish the logging criteria of different diagenetic facies, and the longitudinal parameters of the saturation parameters of the exploration wells were obtained. Studies have shown that the division schemes of diagenetic facies based on diagenesis, pore types, pore structures and damage to or construction of the reservoir are reasonable and effective. The rock electrical parameters (m, n) in the study area are controlled by the diagenetic facies: the cementation index m of various diagenetic facies is mainly controlled by the pore structure, and the cementation index m and porosity of the three types of diagenetic facies fall on different trend lines. The saturation exponent n is mainly affected by the pore structure and clay; as the pore structure deteriorates, the clay content decreases, and the n value increases. The saturation model based on diagenetic facies was applied in the saturation log evaluation of the study block, and the predicted results were compared with those of five other saturation models suitable for different reservoir types. The saturation evaluation error was reduced from 40.28% to 12.4%. The reliability of the saturation model proposed in this paper was verified, and a new classification scheme is provided for the evaluation of the saturation of tight sandstone, which can assist in the evaluation of the saturation of tight sandstone reservoirs.

Stratigraphic architecture and hierarchy of fluvial overbank splay deposits

Splay deposits represent an important sand-prone component of the otherwise fine-grained stratigraphic record of fluvial overbank systems. This work presents a hierarchical approach to the classification and palaeoenvironmental interpretation of ancient preserved splay deposits supported by the analysis of the stratigraphic architecture of 11 exhumed examples from the Jurassic Morrison Formation and the Cretaceous Mesaverde Group (western USA) and analysis of the morphology of splays from nine modern fluvial systems. A hierarchical arrangement of splay deposits is proposed, categorized into lithofacies, beds, elements and complexes. Recognition criteria for each tier of the hierarchy include identification of bounding surfaces, thinning and fining trends of splay elements and complexes, and palaeocurrent variability. Progradational and compensational stacking trends control the stratal architecture of splay deposits, and these are influenced by the following factors: (1) the rate of local accommodation generation, which influences the erosive power of floodwaters and whether splay elements are laterally offset owing to compensational stacking; (2) the nature of the topographic confinement of the floodplain; (3) the preservation potential linked to migration direction of channel. Splay bodies can contribute volume to fluvial reservoirs and may form significant connectors that link otherwise isolated primary channel bodies, thereby enhancing reservoir connectivity. Supplementary material: Thickness and facies types with global positioning system (GPS) coordinates, length and width v. thickness, and modern data and coordinates are available at https://doi.org/10.6084/m9.figshare.c.4461026

Palaeoecology and microfacies correlation analysis of the Oligocene–Miocene Asmari formation, in the Gachsaran oil field, Dezful Embayment, Zagros Basin, Southwest Iran

The Asmari formation is a thick carbonate succession of Oligocene–Miocene age in the Zagros Basin, southwest Iran. This research is focused on the microfacies analysis and paleoecology interpretation (based on the distribution of benthic foraminifera) of carbonate sediments of the Asmari formation. Three sections of this formation studied in the Gachsaran oil field (wells 25, 31 and 37), Dezful Embayment, Zagros Basin (southwest Iran). These sections are composed of limestone, dolomite, and limy dolomite. The Asmari formation has a conformable contact with the overlying (Gachsaran formation) and the underlying (Pabdeh formation) units. Based on petrologic studies, identification of larger foraminifera and facies analysis of the Asmari formation allowed the recognition of 14 facies types. There are grouped into four depositional environments which correspond to the inner, middle, outer ramp, and intrashelf basin. According to this study, deposition of the Asmari formation with association of hyaline, lamellar, perforate large, and flat foraminifera first started in the intrashelf basin environment during the Early Oligocene. In the latest Oligocene–earliest Miocene time, the Asmari platform margin prograded basinward, sediments of shallow water were deposited.

Hydrochemical characterization and vulnerability assessment of unconfined groundwater systems in Ore area, Southwestern Nigeria

The inhabitants of Ore and its environs, Southwestern Nigeria, rely exclusively on groundwater for their domestic, industrial and irrigation purposes such that there is the need to identify and subject the recharge and discharge areas for pollution potential integrity assessment. A total of 144 dug wells were selected for the study. Topography, water table levels and hydrochemical trend methods were employed to identify twelve recharge and eighteen discharge areas. Wells in each of the areas were selected and subjected to hydrogeochemical tests to determine their hydrochemical facies types. Results showed that concentrations of cations and anions are in the order of Ca2+> Na+> Mg2+> K+ and HCO3− > Cl− > SO42− > NO3−, respectively. The dominant hydrochemical facies is Ca–HCO3 type. These results were integrated with DRASTIC parameters to determine their pollution potentials. The results showed that both the recharge and discharge areas have low pollution potential based on their DRASTIC index values. Water quality indices result of Ore groundwater fall within good to excellent/very good categories with 31.43% and 68.57%, respectively. This study indicated that groundwater sourced from both recharge and discharge areas is safe for domestic and agricultural purposes and that the groundwater is generally less vulnerable to pollution.

Sequence Division and Sedimentary characteristics of Shahezi formation in Northwest Xujiaweizi fault depression

In the light of the unclear current conditions of the sequence stratigraphic framework division and sedimentary for Shahezi Formation in Northwest Xujiaweizi fault depression. Based on single well, well logging and seismic data,the stratigraphic sequences of the formatio n are confirmed with the help of Vail sequence stratigraphy theory. Based on the single well facies and source supply, the sedimentary facies types and the distribution characteristics of the Northwest of Xujiaweizi are analyzed. The studies show that Northwest Shahezi Formation can be divided into one second-class sequences and furthermore it can be classified into four third-class sequence; Fan delta sedimentary facies is developed in the west of study area, braided river delta sedimentary facies is developed in the east of study area, lacustrine facies is developed in the central sag; In the Northwest of the Shahezi formation, the area of the lake basin in the early stage of the sedimentary basin is small, and the latter is gradually enlarged, lake facies is the main one and the fan is mainly developed in the East and the West in this area.

Response of Middle Jurassic shallow-marine environments to syn-depositional block tilting: Isles of Skye and Raasay, NW Scotland

The Hebridean Province of NW Scotland provides insight into the interaction between tectonics and shallow-marine and tidal strait depositional environments in the Sea of the Hebrides and Inner Hebrides basins. The study tests the influence of syn-depositional block tilting on gross thickness, sand to mud ratio and the distribution of shallow-marine facies in the resulting succession. New Middle Jurassic palaeogeographical maps and stratigraphic correlations are presented that integrate both outcrop and well data and illustrate the evolution of the deltaic sedimentary system in a broad, semi-regional context. Results show that distance from the sediment entry point and the syn-rift tectonic geomorphology were the critical controls on gross thickness, sand to mud ratios and facies types. The impact of relative sea-level change is hard to detect in locations proximal to the Scottish hinterland, where sediment supply was large relative to accommodation (Ss > Ac), but becomes more influential in distal locations where eustasy and tectonic subsidence convolved to increase the influence of accommodation over sediment supply (Ac > Ss). Supplementary material: An outcrop to well log correlation exercise is available at https://doi.org/10.6084/m9.figshare.c.4397858

Towards a robust parameterization for conditioning facies models using deep variational autoencoders and ensemble smoother

Ensemble-based methods have been applied with remarkable success for data assimilation in geosciences. However, they sometimes fail to preserve the geological realism of the model, which is particularly evident in reservoirs with complex facies distributions. This occurs mainly because of the underlying Gaussian assumptions on model parameters that are inherent in these methods. This fact has encouraged an intense research activity to develop Gaussian parameterizations in a latent space that maps into geologically-realistic facies realizations. Despite the large number of publications, the development of robust parameterizations for facies remains an open problem. Deep learning techniques have been delivering impressive results in a number of different areas and the first applications in data assimilation in geoscience have started to appear in the literature. The present paper reports the current results of our investigations on the use of deep neural networks towards the construction of a continuous parameterization of facies, which can be used for data assimilation with ensemble methods. Specifically, we use a convolutional variational autoencoder and the ensemble smoother with multiple data assimilation. We tested the parameterization in three synthetic history-matching problems with channelized facies. We focus on this type of facies because they are among the most challenging to preserve after the assimilation of data. The parameterization showed promising results, outperforming previous methods and generating well-defined channelized facies. However, more research is still required before deploying these methods for operational use. In particular, it is necessary to investigate procedures to improve the reconstruction accuracy in three-dimensional cases and reduce the computational requirements to train the networks.

Integration of soft data into multiple-point statistical simulation: re-assessing the probability conditioning method for facies model calibration

Conditioning multiple-point statistical (MPS) facies simulation on dynamic flow data is complicated by the complex relationship between discrete facies distribution and the flow response data. The process can be facilitated by using the probability conditioned method (PCM), in which an ensemble-based data assimilation approach is used to infer, from observed flow data, a facies probability map that can guide conditional simulation from a training image (TI). We discuss important aspects of flow model calibration based on facies probability maps and propose several improvements to the original PCM, including: (1) using a statistically consistent approach for inferring facies probability maps from updated ensemble of continuous hydraulic properties; (2) assigning spatially variable weights to simulation grids based on relative confidence in the information content of observed data; and (3) considering spatial patterns instead of marginal facies probabilities in constructing probability maps. We illustrate that, in simulating categorical facies, the pattern-imitating behavior of MPS can have a significant impact on soft data conditioning. In particular, in the Single Normal Equation SIMulation (SNESIM) algorithm, the initial points that are visited on the random path tend to determine the main global facies connectivity. Once these initial points are populated with simulated values, the conditional probabilities (inferred from the TI) for the subsequent cells on the random path tend to be very close to 0 and 1. Hence, the facies types in the remaining cells are known almost deterministically, making the soft data inconsequential at the later stages of simulation. This behavior suggests that, without compensating for this effect, the facies probability map is not effectively integrated into the SNESIM simulation, an issue that is reflected in the generated realizations. Several numerical experiments are presented and discussed to illustrate this behavior and its impact on soft-data conditioning.

Depositional facies and stratal cyclicity of carbonate successions in the Yingshan and Yijianfang Group (Lower-Middle Ordovician) in Yuejin-Tuoputai Region, Tarim Basin, NW China

The Lower-Middle Ordovician Yingshan and Yijianfang Group in the Tarim Basin are overwhelmingly composed of cyclic carbonates. Based on microscopic observation and facies analysis from two borehole sections in Yuejin-Tuoputai area, two main types of facies are recognized: semi-restricted subtidal, open-marine subtidal, and these are further subdivided into six and nine lithofacies in the Yuejin and Tuoputai area, respectively. In general, these facies are vertically arranged into shallowing-upward, meter-scale cycles. These cycles are commonly composed of a thinner basal horizon reflecting abrupt deepening, and a thicker upper succession showing gradual shallowing upwards. Based on the vertical facies arrangements and changes across boundary surfaces, two types of cycle: semi-restricted subtidal and open subtidal, are further identified. The semi-restricted subtidal cycles, predominating over the middle-upper Yingshan Group and the lower Yijianfang Group, commence with algal bindstone in the bottom and are capped by pelletoid grainstone and bindstone with peloidal grains. In contrast, the open subtidal cycles, dominating the upper Yijianfang Group, are dominated by clastizoic grainstone or clastizoic Bindstone. Based on vertical lithofacies variations, cycle stacking patterns, and accommodation variations revealed by Fischer plots, nine larger scale third-order depositional sequences (Sq3-Sq11) are recognized. These sequences generally consist of a lower transgressive and an upper regressive systems tract. The transgressive tracts are dominated by thicker than-average cycles, indicating an overall accommodation increase, whereas the regressive tracts are characterized by thinner-than-average cycles, indicating an overall accommodation decrease. The sequence boundaries are characterized by transitional zones of stacked thinner-than-average cycles, rather than by a single surface. These sequences can further be grouped into lower order sequence sets: the lower and upper sequence sets. The lower sequence set, including Sq3–Sq7, is at the rising–descending cycle in Fisher Curve plots. And this indicates the accommodation space keeps steady. The upper sequence set, including Sq8–Sq11, some features can be found that these Fisher Curves of sequences flunctuate strongly in the middle-lower Yijianafang Group while keep steady in the upper part. This indicates a phenomenon, where the accommodation space varies from violent to gentle.