Gamma-ray Values Research Articles

The Ediacaran–Cambrian boundary in the Tarim Basin, NW China: Geological data anomalies and reservoir implication

Abstract In this study, unconformities (T90) formed during the succession from the Ediacaran to the Cambrian were identified based on five types of data, namely, field outcrops, C and O isotopes, major and trace elements, well logs and seismic data. The outcrop data show notable lithological changes above and below the T90 interface. The C and O isotope data show significant negative δ13C and δ18O excursions at the T90 interface. The negative δ13C excursion at the T90 interface occurred concurrently with the negative δ13C excursions that occurred globally during the same period. The average values of Ca, Mg, Nd, Pr, Ce and La were higher below the T90 interface than that above the T90 interface and that the average mass fractions of Zr, Sr, Zn, Ba, Mn, S, Si, Ni, Fe, Al, Rb, K, Cl and P were lower below that interface than that above it. The well log data show that at the T90 interface, the concentrations of U and Th and the mass fraction of K increased suddenly (from 97.66 to 104.308 ppm, from 210.005 to 232.086 ppm and from 11.609% to 12.99%, respectively) and the gamma ray value increased significantly, reflecting a brief land exposure of the interface. Onlap and truncation patterns were visible on the seismic sections above and below the T90 interface. Various types of unconformities have developed at the T90 interface in various areas of the Tarim Basin. Paraconformities are mainly distributed in the Kuluketage and Kalpin outcrop areas as well as the depressions in the northern Tarim Basin and a large area north of the Tarim Basin. Angular unconformities are distributed near the north wing of the central uplift belt. The paleogeomorphology of the T90 unconformity interfaces limited the distribution of sedimentary facies in the overlying strata, thereby affecting the distribution of the hydrocarbon source rocks of the Yurtus Formation, and controlled the development of karst reservoirs in the underlying strata.

Gamma-Ray-Tool Characterization Avoids Traditional Limitations

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 191717, “Is API Enough for Gamma Ray Logs, or Do We Need More?” by Feyzi•Inanc, SPE, and Andreas Vogt, SPE, Baker Hughes, a GE Company, prepared for the 2018 SPE Annual Technical Conference and Exhibition, Dallas, 24–26 September. The paper has not been peer reviewed. A major issue with gamma-ray (GR) logs is that the API definition is valid only if the tool is run in a 4.89-in. borehole filled with fresh water. Although all GR tools are intended to provide the same results for such a well, in the field there is no single one-size-fits-all concept. In this paper, the authors discuss the characterization process for GR tools and how they behave in boreholes different from the one used in the University of Houston (UH) GR characterization pit. Proposals for developing correction strategies so that GR logs become quantitative logs, rather than the qualitative logs of the past, also are outlined. Introduction One of the issues inherent in ensuring repeatability of GR logs starts with tool characterization. For decades, no common industry standard existed that defined the scale for measuring radioactivity. The chaos created by different tools providing radioactivity in different units in the same environments ended with the creation of the API unit in 1959. The definition of the unit was based on work done with the UH GR characterization pit. This specific pit has a 4.89-in. borehole and a casing to ensure stability within the surrounding synthetic formation. One downside of the API definition is that different combinations of radioactive isotopes, environmental conditions, and tool designs may yield comparable count rates. UH GR pit characterizations of GR tools lose their validity as the wellbore diverges from this previously described specific borehole size and environment. Thus, environmental corrections are needed to bring readings to a reference borehole used for tool characterization. Another issue with the UH GR-pit-based characterizations became evident with the emergence of logging-while-drilling (LWD) GR tools. The LWD tools were supposed to provide natural GR values comparable with those of wireline tools. Recommended practices for wireline tools did not have a provision for calibration of LWD GR tools that did not fit into the UH GR pit. Consequently, characterizations of LWD tools to provide results comparable with those of wireline tools became a topic of interest. Background Although the UH GR characterization pit still exists, it suffers from many issues. The facility is in a state of disrepair, with significant corrosion. In addition, its activity level is decreasing because of the presence of short-lived Ra-226 as one of the main ingredients in the high-activity zone. The radionuclide distribution is not even, causing gamma readings to change with the height, and rock dimensions do not correspond to those of an infinite rock environment.

Application of Geophysical Logs to Estimate the Source Rock Quantity of Ratawi Formation, Southern Iraq: A Comparison Study

Quantitative detection of the parameters of source rock that targeted Ratawi Formation in the south of Iraq is achieved via the Geophysical log data, precisely the resistivity, porosity, and gamma-ray log of the well X in the northern part of Mesopotamian basin, which is so called Euphrates subzone. Total organic carbon and source rock productivity is detected by mathematically interpreting of the log parameters and that lead to assigning and identification of the content of organic matter and source rock intervals. The responses of log curvatures are relevant to the rise of total organic carbon, which can be identified via logs indicators. Thereby, the organic carbon can be expected with a surge in values of sonic log, gamma-ray values, neutron or total porosity log, and resistivity as well as a decrease in the density log. The adoption of sonic/resistivity overlay technique for the determination of total organic carbon content has indicated the likely lithology of the intervals of interest where the logs initially could indicate the lithology. As such, high organic carbon content is significant to environments that have relatively low energy.

Discovery and prediction of high natural gamma sandstones in Chang 73 Submember of Triassic Yanchang Formation in Ordos Basin, China

The large scale development of high natural gamma sandstones has been discovered in the Chang 73 Submember of the Triassic Yanchang Formation in the Ordos Basin, China. High natural gamma sandstones consist of terrigenous detrital rocks with particle sizes ranging from silt to sand. They represent turbidite deposits characterized by high gamma ray values that are more than 180 American Petroleum Institute (API) units on a natural gamma ray log profile. For a long time, very high natural gamma sandstones had been identified as high-quality source rocks, such as oil shales, from conventional well log profiles, such as natural gamma ray well logs. Therefore, predicting the distribution of high natural gamma sandstones was studied. The sedimentary, lithological, and well log characteristics, as well as the genesis of the high radioactivity of high natural gamma sandstones were analyzed in the Chang 73 Submember. Thorium (Th), uranium (U) and other radioactive elements were found, carried by deep hydrothermal activity, and probably resulted in the formation of a relatively high radioactive zone in the cross-section, where high natural gamma sandstones usually develop in large quantities. This caused many turbidite sand bodies, which should have a continuous distribution in the cross-section, to appear to have a discontinuous distribution, when using conventional well log profiles, such as natural gamma ray well logs. From the above mentioned apparent discontinuous distribution of turbidite sand bodies in the cross-section, a continuous distribution can be predicted. It is obvious that the prediction of areas of continuous turbidite sand bodies in the cross-section usually corresponds with areas where high natural gamma sandstones are developed in large quantities. Exploration and development practice demonstrated that the developed method is fast and effective in predicting high natural gamma sandstones in the Chang 73 Submember.

Integrating Geomechanical Data Optimizes Completions Design

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 191768, “Integrating Rock Properties and Fracture-Treatment Data To Optimize Completions Design,” by Eric Bruesewitz, SPE, Hawkwood Energy; Jessica Iriarte, SPE, Well Data Labs; Joel Mazza, SPE, Carrie Glaser, and Eric Marshall, Fracture ID; and Scott Brooks, Hawkwood Energy, prepared for the 2018 SPE Liquids-Rich Basins Conference—North America, Midland, Texas, USA, 5–6 September. The paper has not been peer reviewed. In the process of analyzing treatment responses that occur during hydraulic fracturing, several variances in treating pressure exist that are not readily explained by examining the surface pressures and pipe friction in isolation. These variances are also apparent when looking at bottomhole injectivity. This paper demonstrates how engineers can take advantage of their most-detailed completions and geomechanical data by identifying trends arising from past detailed treatment analyses. Introduction The Eagle Ford Shale was deposited in the Late Cretaceous Period in a marginal to open marine setting. The Lower Cretaceous part can be divided into two second-order transgressive/regressive cycles that have been labeled lower and upper Eagle Ford. The deposition of these units varies across the formation as a result of topography at the time of deposition. Therefore, operators in the Eagle Ford and other shale plays must account for changing stratigraphy and facies to locate horizontal wells properly for optimal perforation intervals. This study is based on integrated geomechanical data, metered high-frequency treatment data, and post-job reports. The data set used for this study corresponds to six wells completed in the Lower Bench of the Eagle Ford formation in Brazos and Burleson counties in Texas. Integration of completions, drilling, and geomechanical data has been analyzed before in the literature. Those authors observed an increase in pressure in several stages with higher Young’s modulus and lower gamma ray values, in which they were forced to pump an extra wellbore volume of clean fluid (sweep) to place the designed proppant volume. These sweeps accounted for more than 20,000 bbl of additional fluid pumped. The wells analyzed in this study presented data showing multiple examples of difficult-to-treat stages, in which unplanned sweeps had to be pumped and screenouts occurred frequently. Because the treating-pressure anomalies were correlated with mechanical rock properties in other unconventional plays, it was suspected that similar investigation may yield savings, efficiency, and productivity improvements in the Eagle Ford. Methodology The six wells of this study similarly consist of an approximately 7,600-ft lateral completed with 5.5-in. casing and approximately 150-ft stage lengths for a total of 283 stages analyzed. The fracture-treatment design consisted of proppant ramps of 100-mesh and 40/70 white sand placed primarily with slickwater. However, the completion design of four wells included cross-linked gel to place the highest proppant concentrations ranging between 3.0 and 4.0 lbm/gal. All six wells were recorded and processed for geomechanical properties such as Young’s modulus and Poisson’s ratio; a subsequent petromechanical model; and high-frequency fracture-treatment data (treating pressures, rates, and fluid and proppant volumes) and perforation data. The geo mechanical data gathered along the wellbore were divided and assigned to each stage, matching the measured depths to compare variables across data sets. Then, these data sets were standardized to a common format, screened for quality control, normalized, and analyzed with a data-management application.

Determination of coal layers using geophysical well-logging methods for correlation of the Gelik-Zonguldak and Kazpınar-Amasra (Bartın) coalfields, Turkey

The most commonly used well-logging methods for determining coal layers are radioactive (gamma ray, neutron and gamma–gamma) and electrical (resistivity, spontaneous potential and inductive polarization). Other methods that can be used are sonic, thermal, density, caliper and dipmeter logs. In the determination of the coal beds, the evaluation of the well-logs and the interactions between the boreholes are very important. The boundaries of the geological strata are determined by the logs, which are also spatially correlated. The decrease in gamma-ray values, low neutron density, high resistivity, characterize coal layers. Due to the presence of large coal reserves in this area, 81 boreholes have been drilled at the determined points in the long term. Radioactive and electrical well-logs have been applied to these boreholes in order to determine lithology and coal beds. In this study, three boreholes from Gelik-Zonguldak region and four boreholes from Kazpinar-Amasra (Bartin) region were selected and evaluated. Geophysical data were used for the determination of coal beds and correlation. Regional assessments were made with the lithological, structural and tectonic data.

Research and Application of Element Logging Intelligent Identification Model Based on Data Mining

Underground strata are reflected in various information sources in petroleum exploration, including good logging and drilling data. Real-time measurement parameters obtained from mud logging can provide data support for the early discovery of oil and gas resources and the prevention of safety accidents. It plays a forward-looking role in the drilling process. In this paper, we aim at the defection of fuzzy and random characteristics of the big data of drilling element parameters in the current drilling process. A new method named grey wolf optimization-support vector machine (GWO-SVM) is proposed by analyzing the relationship between logging data and formation to solve the serious problem of formation misjudgment. Using element content and Gamma-ray value, data mining is performed by a large number of real-time data obtained from the drilling site. The obtained information is used for comprehensive estimation and prediction of strata. First, the data is normalized, and then, the best $\zeta $ and $\sigma $ values are found through the optimization of gray wolf algorithm, next the SVM training is carried out, finally, the formation prediction model is established, and the error analysis of the results was conducted. In the paper, the algorithm model is subsequently applied to three actual wells. The GWO-SVM model based on drilling data is used to predict the formation, and the error analysis showed that the error range of the GWO-SVM algorithm is within 10%. Compared with the GWO-SVM, the model accuracy of SVM, Particle Swarm Optimization-Support Vector Machine (PSO-SVM) algorithm is lower 53% and 23%, respectively. The GWO-SVM has higher robustness, reliability, and achieves faster convergence speed, stronger generalization effect, and improves the identification accuracy of elements for the formation. The average accuracy of the GWO-SVM in stratum dynamic identification is 93.5%. This model is implemented to support the logging system to improve application strength.

Abnormal open-hole natural gamma ray (GR) log in Baikouquan Formation of Xiazijie Fan-delta, Mahu Depression, Junggar Basin, China

Abstract Based on large amounts of cores, open-hole conventional logs and mineral components analysis, abnormal natural gamma ray (GR) log showing high values in conglomerates and low values in fine-grained sediments, are described and explained in Baikouquan Formation of Xiazijie Fan-delta, Mahu Depression, Junggar Basin. After observing cores, normalizing the GR log and correcting depth errors between both, the GR log values of individual grain-sized lithology are extracted and counted. When grain-size decreases, the average GR values of different sized grains increase generally. The GR values of conglomerates are mostly between 50 and 80 API, while the values of fine-grains are mainly between 70 and 100 API. However, abnormal GR log features exist in the cores and wells of Baikouquan Formation prevalently. A great deal of high radioactive intermediate-acid volcanic minerals, such as volcanic tuff, felsite, andesite, granite, rhyolite, et al., distribute widely in the conglomerates, which results in abnormal high GR values in conglomeratic intervals. Low radioactive quartz components exit widely in high percentage in mudstones, which is the primary mineral explanation for the abnormal low GR values in reddish-brown siltstones and sandstones intervals.

Fault control on depositional systems and sequence stratigraphic architecture in a multiphase, rifted, lacustrine basin: A case study from the paleogene of the central Bohai Bay Basin, northeast China

Abstract The stratigraphic evolution of rift basins is complicated by out-of-phase displacement on boundary and intrabasinal faults and this analysis of Paleogene strata in the central Bohai Bay Basin of eastern China exemplifies this conundrum. The spatial and temporal distribution of depositional systems and sequences in the central Bohai Bay Basin are evaluated by using seismic reflection data in conjunction with drill cores and well logs. Three second-order composite sequence boundaries are identified on seismic profiles on the basis of truncations below and onlap relationships above and by abrupt changes abrupt changes in gamma-ray values on well logs. Composite sequences are comprised of third-order sequences identified as distinctive seismic packages. Variations in thickness and distribution of third-order sequences are consistent with out-of-phase displacement of boundary and intrabasinal faults which resulted in migration of depocenters in space and time. A range of depositional systems are recognized including fan delta, braid delta, river delta, sublacustrine fan, and shallow and deep lacustrine. During rift-climax stages, lowstand fan deltas developed on the hanging wall adjacent boundary faults to the north, braid deltas on the footwall ramps to the south and sublacustrine fans in the central basin. Following transgression, highstand braid deltas prograded from the north and south whereas river deltas during the late stages of basin fill. Both tectonic activity and climate were important controls on accommodation and the balance between sediment supply and accommodation determined the sedimentary architectures. Faulting, uplift and erosion as well as fault-block rotation controlled the formation of sequence boundaries and sequence geometries whereas variations in sediment flux can be attributed to climate changes. High-quality reservoirs are associated with deltaic channel-fill and sublacustrine fan sandstones. Intrabasinal faults provided pathways for oil migration whereas prodeltaic, lacustrine mudstones are source rocks as well as seals. This study demonstrates that the spatial and temporal distribution of deposystems in rift basins, in general, requires an understanding of the history of boundary and intrabasinal faults within these compartmentalized basins.

Field gamma-ray patterns and stratigraphic reinterpretation of offshore well-log data from Lower Jurassic organic-rich units of the Lusitanian Basin (Portugal)

Abstract Lower Jurassic (Sinemurian–Pliensbachian) successions cropping out in onshore areas of the Lusitanian Basin provide detailed information of three well characterized organic-rich intervals: i) the Unit F from the Coimbra Formation, ii) the Polvoeira Member of the Agua de Madeiros Formation, and the iii) Marly Limestones with Organic-rich Facies (MLOF) Member of the Vale das Fontes Formation. Despite decades of research, these source rocks have not yet been properly identified and characterized in terms of extension and thickness in the basin's offshore. This paper compares detailed gamma-ray (GR) profiles from onshore outcrops (Coimbra, Montemor-o-Velho, Figueira da Foz, Monte Real, Sao Pedro de Moel and Peniche regions) with seven offshore wells. Our objective is to recognise Lower Jurassic organic-rich intervals in offshore areas of the basin, highlighting their lateral extension and improving the overall stratigraphic definition of the Lower Jurassic in the Lusitanian Basin. GR field measurements indicate that the highest GR values and U contents are consistently observed in the organic-rich intervals, reaching up to 134 API and 9.0 ppm of U. In contrast, the lowest API and U contents were determined in dominantly calcareous units, ranging between 5-34 API and 0.1-1.4 ppm, respectively. Wireline gamma-ray logs show a good correspondence between field patterns and offshore well logs, recognizing the organic-rich intervals in offshore areas. The highest GR values are interpreted to be linked with the 2nd- and 3rd-order maximum flooding intervals previously recognised in the Sinemurian–Pliensbachian successions in the western Iberian margin. Detailed GR maps linked to maximum flooding intervals highlight the variation of the vertical and lateral distributions of the organic-rich units in offshore areas of the Lusitanian Basin.

High resolution sequence stratigraphic reconstruction of mud-dominated systems below storm wave base; A case study from the Middle to Upper Devonian Horn River Group, British Columbia, Canada

The application of sequence stratigraphy to deep-water mud-dominated systems is more challenging than in shallow water siliciclastic settings, largely because mudstones display subtle contrasts in grain size and lithology that make identification of facies and recognition of major stratigraphic surfaces difficult. However, the integration of sedimentology, ichnology and geochemistry and an extensive suite of cores from the Middle to Upper Devonian Horn River Group shales allow us to better understand how deep-water mud-dominated systems respond to fluctuations in the relative sea level, manifested in sequence stratigraphic relationships. The Horn River Group consists of, in ascending order, the Evie Member and Otter Park Member of the Horn River Formation and the Muskwa Formation. Four systems tracts, including highstand, transgressive, lowstand and falling stage systems tracts (HST, TST, LST, FSST, respectively) are identified and mapped. Highstand and transgressive systems tracts are both represented by dominantly massive mudstone lithofacies, sparse bioturbation, high Mo/Al ratios and high TOC values (avg. 3.1 and 3.4 wt%, respectively), as well as low Al2O3 concentrations. They can be distinguished by upward increasing gamma ray values for transgressive systems tracts and upward decreasing values for highstand systems tracts. The falling stage and low stand systems tracts display more heterolithic and laminated lithofacies, sparse to intense bioturbation, lower Mo/Al ratio and lower TOC values (avg. 1.2 and 1.7 wt%, respectively) and high Al2O3, concentrations and relatively low gamma ray readings. Sequence boundaries are represented by a transition from unbioturbated units to moderately or intensely bioturbated units, suggesting increased bottom water oxygen levels, which we infer to represent shallower waters. They are gradational and clearly conformable contacts in distal parts of the basin. They are sharp in more proximal settings, but no erosional surfaces were identified that would indicate unconformable contacts. Maximum flooding surfaces are characterized by the highest TOC contents and an increased gamma ray values. Our results suggest that the Otter Park Member belongs to a long-period sea level lowstand (e.g., 2nd order cycle), whereas the Evie Member and Muskwa Formation belong to long-period sea level highstands.

A Comparison between Model Base Hardconstrain, Bandlimited, and Sparse-Spike Seismic Inversion: New Insights for CBM Reservoir Modelling on Muara Enim Formation, South Sumatra

Coal Bed Methane (CBM) as a newly developed resource in Indonesia is one of the alternatives to relieve Indonesia’s dependencies on conventional energies. Coal resource of Muara Enim Formation is known as one of the prolific reservoirs in South Sumatra Basin. Seismic inversion and well analysis are done to determine the coal seam characteristics of Muara Enim Formation. This research uses three inversion methods, which are: model base hard- constrain, bandlimited, and sparse-spike inversion. Each type of seismic inversion has its own advantages to display the coal seam and its characteristic. Interpretation result from the analysis data shows that the Muara Enim coal seam has 20 (API) gamma ray value, 1 (gr/cc) – 1.4 (gr/cc) from density log, and low AI cutoff value range between 5000-6400 (m/s)*(g/cc). The distribution of coal seam is laterally thinning northwest to southeast. Coal seam is seen biasedly on model base hard constraint inversion and discontinued on band-limited inversion which isn’t similar to the geological model. The appropriate AI inversion is sparse spike inversion which has 0.884757 value from cross plot inversion as the best correlation value among the chosen inversion methods. Sparse Spike inversion its self-has high amplitude as a proper tool to identify coal seam continuity which commonly appears as a thin layer. Cross-sectional sparse spike inversion shows that there are possible new boreholes in CDP 3662-3722, CDP 3586-3622, and CDP 4004-4148 which is seen in seismic data as a thick coal seam.

Influence of differential diagenesis on primary depositional signals in limestone-marl alternations: An example from Middle Permian marine successions, South China

The diagenetic alteration of rhythmic successions remains one of the main challenges in paleoclimatology and cyclostratigraphy. To address this very important problem, thin sections and geochemical data of the limestone-marl alternations from Middle Permian carbonate successions in South China are examined to identify differential diagenetic effects in the petrography, organic and inorganic compositions, strontium concentrations and stable isotopic compositions of limestone beds and marl interlayers. In the Lengshuixi outcrop and the JY66-1 drill core, the most conspicuous features are that the limestone beds exhibit dense cementation, high strontium concentrations, negative oxygen isotopic values and positive carbon isotopic values. In contrast, the marl interlayers contain talc and exhibit strongly deformed and oriented bioclasts, low strontium concentrations, positive oxygen isotopic values and negative carbon isotopic values.Although differential diagenesis has occurred, some paleoclimate proxies (e.g., computed gamma-ray values) still preserve their primary signals and thus are suitable for cyclostratigraphic research. Eccentricity rhythm and millennial-scale rhythm are obviously recorded in Middle Permian limestone-marl alternations in South China. The variations in the oxygen and carbon isotopic values of limestone-marl alternations from different basins represent responses to different processes of differential diagenesis. This knowledge assumes that differential diagenesis occurs in diagenetically closed systems under the deep sea-floor. However, this assumption may not be valid in diagenetically open systems, where exotic fluids may add unexpected disturbances. Hence, it is crucial to understand diagenetic systems by determining the variations in strontium concentrations, which are as important as variations in carbon and oxygen isotopic values.

Quantification of Organic richness through wireline logs: a case study of Roseneath shale formation, Cooper basin, Australia

The late Carboniferous-Middle Triassic, intracratonic Cooper basin in northeastern South Australia and southwestern Queensland is Australia's foremost onshore hydrocarbon producing region. The basin compromises Permian carbonaceous shale like lacustrine Roseneath and Murteree shale formation which is acting as source and reservoir rock.The source rock can be distinguished from non-source intervals by lower density, higher transit time, higher gamma ray values, higher porosity and resistivity with increasing organic content. In current dissertation we have attempted to compare the different empirical approaches based on density relation and Δ LogR method through three overlays of sonic/resistivity, neutron/resistivity and density/resistivity to quantify Total organic content (TOC) of Permian lacustrine Roseneath shale formation using open hole wireline log data (DEN, GR, CNL, LLD) of Encounter 1 well. The TOC calculated from fourteen density relations at depth interval between 3174.5–3369 meters is averaged 0.56% while TOC from sonic/resistivity, neutron/resistivity and density/resistivity yielded an average value of 3.84%, 3.68%, 4.40%. The TOC from average of three overlay method is yielded to 3.98%.According to geochemical report in PIRSA the Roseneath shale formation has TOC from 1 – 5 wt %.There is unpromising correlations observed for calculated TOC from fourteen density relations and measured TOC on samples. The TOC from average value of three overlays using Δ LogR method showed good correlation with measured TOC on samples.

An application of variogram modelling for electrical resistivity soundings to characterize depositional system and hydrogeology of Bannu Basin, Pakistan

This study describes a method of combining geostatistical analysis with geophysical inversion of 1D electrical resistivity data acquired in the Bannu Basin, northwestern Himalayas of Pakistan. The data has been integrated and broadened from single dimension resistivity data into a 2D model that can be fully visualized and interpreted in a spatial sense. By interpreting and calibrating the electrical resistivity curves with the lithologies and geophysical logs of boreholes, it has been possible to identify unique sedimentary accumulations that occur throughout the basin. Through the use of variogram modelling the spatial extent of these accumulations can be identified and associated with measured hydraulic properties. It has become possible to map the extents and facies of the alluvial systems of the Bannu Basin formed during periods of heightened tectonic activity. The coarser sediments are associated with higher levels of resistivity as measured in the electrical surveys, whereas the finer sediments exhibit characteristically lower resistivities. Thus, the zones of high and low resistivity values become indicative of sediments associated with alluvial fans and lacustrine environments, respectively. The sediments of alluvial fans show relatively low gamma ray levels and higher transmissivities, whereas those of the piedmont deposits as well as lacustrine and sandy plains tend to show medium to high gamma ray values and lower transmissivities. Gross transmissivities were estimated by studying regression relationships between transmissivities measured in boreholes and the apparent formation factors of sediments. These relationships indicate an exponential increase in transmissivity with increasing apparent formation factor. These trends suggest a contrast in transmissivities between alluvial fan and other environments where lacustrine and sandy plains are characterized by low transmissivities. These estimated transmissivities do not represent absolute values but are merely indicative of any high or low trends in transmissivity of sediments, throughout the basin.

Genesis of tuff interval and its uranium enrichment in Upper Triassic of Ordos Basin, NW China

Recently measured high gamma ray values in the Yanchang Formation of the Upper Triassic in the Ordos Basin have added an interesting and controversial twist to the study of the formation’s uranium enrichment and genesis. High uranium and thorium contents in the tuffaceous layer cause high gamma ray values in the Yanchang Formation. Petrographic studies, major elements, rare earth elements (REEs), and trace elements have been systematically analyzed to determine the composition, geochemical environment, and diagenetic processes of the layer. The observed color of the tuffaceous layer in the study area varies from yellow to yellowish brown. The tuff consists of matrix supported with sub-rounded to sub-angular lithic fragments. These lithic fragments probably derived from pre-existing rocks and incorporated into the tuffaceous layer during volcanic eruption. Quartz, plagioclase, and biotite were observed in well to poorly sorted form, in addition to framboidal pyrite and organic laminae. Measured ratios of SiO2/Al2O3 ranged from 3.277 to 6.105 with an average of 3.738. The ratio of TiO2/Al2O3 varied from 0.037 to 0.201 with an average of 0.061, indicating that the sediments of the tuffaceous layer originated from an intermediate magma. REE distribution patterns show sharp negative Eu anomalies, indicating a reducing environment, which is suitable for uranium deposition. A reducing environment was confirmed by black shale in the base of the Yanchang Formation. Such black shale has high organic matter content that can take kerogene from mudstone and provide a reducing environment for uranium enrichment in the tuffaceous layer. Moreover, negative Eu anomalies and the REE patterns indicate a subduction-related volcanic arc environment as the magma source of the tuffaceous layers. High values of Rb, Ba, and Sr might be the result of fluid phase activities; low values of Hf and Eu indicate the involvement of crustal material during diagenesis of the tuff. Discrimination diagrams (Th/Yb vs Ta/Yb, Th/Hf vs Ta/Hf) suggest an active continental margin as the tectonic setting of source volcanoes. Plots of Nb versus Y, Rb versus Y + Nb, TiO2 versus Zr, and Th/Yb versus Nb/Yb of the tuffaceous content point to calc-alkaline continental arc-related magmatism. We concluded that uranium enrichment in the tuffaceous layer was supported by oxidation–reduction.

Integrated core-log facies analysis and depositional model of the gas hydrate-bearing sediments in the northeastern continental slope, South China Sea

Host sediments may exert a significant influence on the formation of gas hydrate reservoirs. However, this issue has been largely neglected in the literature. In this study, we investigated the types, characteristics and the depositional model of the fine-grained gas hydrate-bearing sediments in the northeastern margin of the South China Sea by integrating core visual observations and logging-while-drilling downhole logs. The gas hydrate-bearing sediments consist dominantly of muddy sediments formed in the inter-canyon ridges of the upper continental slope, including hemipelagites, debrites (mud with breccia) and fine-grained turbidites. Cold-seep carbonates and associated slumping talus, muddy breccia debrites, as well as coarse-grained turbidites, may locally occur. Four classes and six sub-classes of log facies were defined by cluster analysis. Core-log correlation indicates that gas hydrates are majorly distributed in fine-grained sediments with high resistivity and low acoustic transit time (AC) log responses, which are easily differentiated from the fine-grained background sediments of high gamma-ray (GR), high AC, and low resistivity log values, and the seep carbonates characterized by low GR, high resistivity, high density, low AC and low porosity log values. The primary host sediments consist of fine-grained hemipelagic sediments formed by deposition from the nepheloid layers of river material and from the microfossils in seawater column. Most of the hemipelagic sediments, however, might have been extensively modified by slumping and associated gravity flow processes and were re-deposited in the forms of debrites and turbidites. Locally developed seep carbonates associated with gas hydrate dissociation and leakage provided additional sources for the gravity flow sediments.

Facies Architecture of the Fluvial-Aeolian Buchan Formation (Upper Devonian) and Its Implications on Field Exploration: A Case Study from Ardmore Field, Central North Sea, UK

The Upper Devonian Buchan Formation in the Central North Sea is a typical terrestrial deposit and predominantly comprises fine to medium-grained sandstones with occasional conglomerates and mudstones. The Buchan Formation has been previously described as being made up mostly of braided fluvial sandstones; however, this study confirms the presence and significance of aeolian sandstones within this fluvial-dominated sequence. Facies architecture is investigated through analogue outcrop study, well log curves and numerical facies modelling, and the results show contrasting differences between fluvial and aeolian facies. The fluvial facies is composed of multiple superimposed and sand-dominated fining-upward cycles in the vertical direction, and laterally an individual cycle has a large width/thickness ratio but is smaller than the field scale. However, the high channel deposition proportion (CDP, average value = 72%) in fluvial-dominated intervals means that it is likely all the sand bodies are interconnected. Aeolian facies comprise superimposed dune and interdune depositions and can be laterally correlated over considerable distances (over 1 km). Although the aeolian sandstones are volumetrically minor (approx. 30%) within the whole Buchan Formation, they have very high porosity and permeability (14.1% - 28%, 27 - 5290 mD) and therefore are excellent potential reservoirs. The fluvial sandstones are significantly cemented by quartz overgrowth and dolomite and by comparison with the aeolian sandstones are poor reservoirs. Aeolian sandstones can be differentiated from fluvial sandstones using several features: pin-stripe lamentation, good sorting, high visible porosity, friable nature and lack of muddy or conglomeratic contents; these characteristics allow aeolian sandstones can be tentatively recognized by low gamma ray values, high sonic transit time and low density in uncored wells. The thin, laterally correlatable and permeable aeolian sandstones within the Buchan Formation are effective reservoirs and could form important exploration targets when the Devonian is targeted elsewhere in the North Sea.

Paleokarst system based on FMI facies analysis: a case study in the Yingshan Formation at the Tazhong uplift in the Tarim Basin, Northwest China

FMI (Formation Micro-Scanner image) is a widely used technique to analyze reservoir properties with the advantages of being high resolution and providing a visible and quantifiable image. In this study, eleven FMI facies are classified based on integrating borehole images, cores, thin sections, and well logs. The facies include such reservoirs as caves and tectonic fractures, sedimentary interbeds, such as high-resistivity facies, and two facies of fractures and holes. In addition to the description of karst systems in the borehole images, anomalies in the GR (gamma ray) well log curve may complement the results from the FMI images. The GR curve shows that the composite facies of fractures and holes could be an effective reservoir instead of caves with high GR values because the holes filled with oil and gas are connected by fractures. Two suites of karst systems formed in the SQ4, while small scale caves and holes formed in the SQ3 because of their lithological and structural locations. The results of this study demonstrate that unconformities and tectonic geomorphology control the differences between the paleokarst systems.

Prediction of lateral variations in reservoir properties throughout an interpreted seismic horizon using an artificial neural network

Successful use of an artificial neural network is shown to predict lateral variations of seismic velocity, density, thickness, and gamma rays associated with sand dune reservoirs identified on a previously interpreted seismic horizon. The work is presented in two main sections. Section one is a feasibility analysis based on synthetic data. A known geologic model is used, performed by pseudowells, in which lateral variations in seismic velocity, density, and gamma ray values are related to the dunes. The synthetic seismic model and the attributes derived are used as training input in the neural network. Section two is a real case example where the methodology is applied to a real seismic data set. Results indicate that using a set of data and attributes restricted to a time interval corresponding to a previously interpreted seismic horizon is more efficient than using a whole data cube, involving a very large volume of data.