Reservoir Basin Research Articles

Tectonic controls on geomorphological dynamics and sediment dispersal in source-to-sink systems in the Qingdong Sag, Bohai Bay Basin

Well-log and three-dimensional (3D) seismic data are used to investigate the tectonic controls during the depositional periods of the Kongdian and Shahejie formations from Ek–Es 4 L to Es 4 U–Es 3 L–M on strata distribution, depositional systems and sediment dispersal in the Qingdong Sag, Bohai Bay Basin, East China, from a source-to-sink perspective. This tectonic transition is characterised by the Tanlu Fault switching from sinistral to dextral strike-slip as the fault system changed from a dominant NW–WNW trend during the Ek–Es 4 L period to a dominant WNW–EW trend during the Es 4 U and Es 3 L–M periods. These changes subsequently controlled the framework and areal extent of sediment dispersal in the inner-basin source-to-sink systems and resulted in a change in trend of inner-basin secondary structural belts, from primarily being NW-trending during the Ek–Es 4 L period to E–W-trending during the Es 4 U and Es 3 L–M periods. The tectonic transition also resulted in the western and southern source-to-sink systems expanding with time and controlled the flow path and sediment redistribution within each source-to-sink system with migration of the subsidence centre of the Qingdong Sag and even the region. The significant changes in the basin structure controlled the source area, flow path, sediment transport distance and direction, and have a significant impact on the interpretation of depositional environments and reservoirs in rift basins with a multi-phase evolution history. KEY POINTS The Qingdong Sag, Bohai Bay Basin, experienced a tectonic transition during sediment deposition in the Eocene. The tectonic transition resulted in changes to the fault system and basin geomorphology. Fault movement and changes in basin geomorphology affected source-to-sink systems.

Nitrogen and Phosphorus Retention Risk Assessment in a Drinking Water Source Area under Anthropogenic Activities

Excessive nitrogen (N) and phosphorus (P) input resulting from anthropogenic activities seriously threatens the supply security of drinking water sources. Assessing nutrient input and export as well as retention risks is critical to ensuring the quality and safety of drinking water sources. Conventional balance methods for nutrient estimation rely on statistical data and a huge number of estimation coefficients, which introduces uncertainty into the model results. This study aimed to propose a convenient, reliable, and accurate nutrient prediction model to evaluate the potential nutrient retention risks of drinking water sources and reduce the uncertainty inherent in the traditional balance model. The spatial distribution of pollutants was characterized using time-series satellite images. By embedding human activity indicators, machine learning models, such as Random Forest (RF), Support Vector Machine (SVM), and Multiple Linear Regression (MLR), were constructed to estimate the input and export of nutrients. We demonstrated the proposed model’s potential using a case study in the Yanghe Reservoir Basin in the North China Plain. The results indicate that the area information concerning pollution source types was effectively established based on a multi-temporal fusion method and the RF classification algorithm, and the overall classification low-end accuracy was 92%. The SVM model was found to be the best in terms of predicting nutrient input and export. The determination coefficient (R2) and Root Mean Square Error (RMSE) of N input, P input, N export, and P export were 0.95, 0.94, 0.91, and 0.93, respectively, and 32.75, 5.18, 1.45, and 0.18, respectively. The low export ratios (2.8–3.0% and 1.1–2.2%) of N and P, the ratio of export to input, further confirmed that more than 97% and 98% of N and P, respectively, were retained in the watershed, which poses a pollution risk to the soil and the quality of drinking water sources. This nutrient prediction model is able to improve the accuracy of non-point source pollution risk assessment and provide useful information for water environment management in drinking water source regions.

Fluid Inclusion Evidence for Oil Charge and Cracking in the Cambrian Longwangmiao Dolomite Reservoirs of the Central Sichuan Basin, China

The Cambrian Longwangmiao gas reservoirs in the Moxi area of the Sichuan Basin have a complex associated gas accumulation history. Based on core and thin section examination, fluid inclusion analyses and 1-D burial-thermal modelling, the diagenetic evolution and hydrocarbon accumulation processes in the Longwangmiao Formation have been reconstructed. Various diagenetic events were identified, making up a complete pore fill sequence as follows: solid bitumen/dolomite→ nonfluorescent solid bitumen → dolomite → quartz/yellow fluorescent oily bitumen → residual hole. Analyses of oil inclusions and bitumen-bearing inclusions are key to the understanding of the hydrocarbon accumulation processes. The Th values of the aqueous inclusions that are contemporaneous with hydrocarbon inclusions range from 74.3 to 214.3°C. In conjunction with burial-thermal history modelling results, the results indicate that there were two stages of oil charge and three stages of natural gas accumulation in the Longwangmiao carbonate reservoirs. The two stages of oil charge occurred in the Late Silurian and Middle Triassic, respectively. Three gas accumulation events occurred in the Middle to Late Triassic, Middle Jurassic to Early Cretaceous, and Late Cretaceous, respectively.

Effect of cascade damming on microplastics transport in rivers: A large-scale investigation in Wujiang River, Southwest China

Rivers are the important channels for transporting microplastics into the ocean from land. Prosperous dam construction changed the connectivity of rivers, thereby reducing the flux of microplastics to the ocean. However, this process currently lacks verification for the large-scale watersheds. In this study, we investigated the Wujiang River in China to evaluate the interception of cascade dams on microplastics. The results showed that: 1) The midstream exhibits a high abundance of microplastics (606.6–1046.2 items·kg−1) while the upstream and downstream reach exhibits relatively low pollution levels. The small-sized microplastics of 0–0.5 mm are easily migrated into downstream while the large-sized microplastics of 0.5–5 mm tend to deposit. 2) Ten kinds of plastic materials were found, in which polyethylene and polypropylene, originated from the developed tourism and fishery, account for 74.2% in all samples. 3) The earliest microplastics were found in the sediments of 1962. The abundance of microplastics in the sediments in seven reservoirs increased over time, impling the contribution of increasing human activities. 4) Positive correlations between the abundance of microplastics in sediments and local gross domestic product (GDP) (n = 33, R2 = 0.89, p < 0.05) and negative correlations between microplastics abundance and reservoir basin area (n = 33, R2 = 0.42, p < 0.05) revealed that GDP and watershed area are the key factors that control the distribution of microplastics. Our results help to understand the migration of microplastics between terrestrial and marine ecosystems.

Comparison of Different Artificial Neural Network Methods in Determining Reservoir Capacity

Making estimation of river flow with hydrological and methodological data of the past period and using in water resources project studies have been used for a long time in many studies in our country and in the world. In addition to this, along with frequent drought problems in recent years, it is also important to store and use the water in the reservoirs correctly. This study aims to research how two separate Artificial-Neural-Network-Functions, which are generated based on the study of neural networks in the human brain, can be used in areas with a certain reservoir capacity such as dams or ponds and to provide an example of the most appropriate ANN model for predicting the level changes that will occur depending on the next years. In this context, two separate functions have been evaluated. These are the Levenberg-Marquardt-(LM) training function and Gradient-Descent-with-Momentum(GDM) training functions. Here, instead of the best results obtained from the models, the best three model outputs were evaluated and the models were considered from a wide frame. Gökçe dam basin in Marmara region was selected as the Model Study Area. For the period of December 31, 2019 - January 1, 2000, the monthly average data prepared with daily data from DSI and different architectural ANN training models were used to forecast the monthly basin capacities for 2019. It can be stated that LM Training models created in terms of Basin reservoir level estimates give results closer to the measured value.

Coupling relationship between reservoir densification and hydrocarbon emplacement in the Jurassic Badaowan Formation sandstone reservoirs of the southern Junggar Basin, China

Understanding the interrelationship between diagenesis and hydrocarbon emplacement is crucially important for investigating tight sandstone reservoir‐forming processes. In this paper, efforts were made to elucidate this interrelationship in the Jurassic Badaowan Formation tight sandstone reservoirs of the southern margin of the Junggar Basin (SJB), China. Four lithofacies were clarified, including quartz cemented‐dissolution facies (QCDF), carbonate cemented facies (CCF), authigenic clay mineral facies (ACMF), and matrix‐caused tightly compacted facies (MCTF). Weak hydrocarbon migration forces and poor hydrocarbon preservation conditions were identified to be the main reasons for the limited hydrocarbon accumulation scale. The reservoir densification process, mostly before 100 Ma, was associated with four aspects, including the high matrix content, the intensive carbonate cementation, the limited dissolution and alteration, and the development of authigenic clay minerals. Hydrocarbons in the Badaowan Formation reservoir were demonstrated to be emplaced after reservoir densification. Overpressure resulting from hydrocarbon expulsion and tectonic compression should have acted as the main driver for hydrocarbon migration. Particularly, the tight sandstone in the Qigu anticline was charged with overpressurized hydrocarbons sourced from the middle‐lower Jurassic source rocks at the footwall of the Qigu north fault since the early Jurassic. The tectonic compression in the late Neogene eroded the core of the Qigu anticline, which destroyed the initially large palaeo‐oil reservoir into smaller ones. The first‐stage hydrocarbon emplacement greatly affected the diagenesis and physical properties of the reservoir. It played an important role in inhibiting cementation, promoting dissolution, and altering the petrophysical properties of the tight sandstone reservoir. The densification process controlled the wettability and capillary pressure of pores in the tight sandstone. Specifically, the high kaolinite cement raised the hydrophilia, while the ubiquitous densification increased the capillary pressure of pores.

A Laboratory Approach Considering Salinity to Investigate Archie’s Parameters in Deep Shale: A Case Study in Longmaxi Formation, Southern Sichuan

Archie’s parameters are substantial to be investigated in the evaluation of water saturation. Many researchers adhered to the opinion that Archie’s equation can still be applied to water saturation calculation in shale if the accuracy of relative parameters could be improved. External conditions, such as temperature, confining pressure, water salinity, wettability, and displacement, may influence the determination of Archie’s parameters. The aim of the study is to investigate the effect of salinity on Archie’s parameters and their correlation with mineral composition and pore structure. The mineral contents and petrophysical properties were firstly acquired through X-ray diffraction (XRD) and basic measurements. Rock-electric experiments under different salinity were conducted on deep shale samples taken from Longmaxi (LMX) Formation in Luzhou (LZ). The results indicate that Archie’s parameters of “ a ,” “ b ,” “ m ,” and “ n ” under actual brine salinity are assigned to 1.47, 1, 1.26, and 1.6, respectively. Our cementation factor is lower than that in other studies including (shaly) sandstones, carbonates, and shales, probably due to the extremely low porosity in study area. Salinity has a positive effect on cementation factor ( m ) and saturation exponent ( n ), suggesting the traditional assignment to Archie’s parameters is inappropriate. It was concluded that the complicated pore structure and high porosity mainly associated with clay mineral may trigger the increased cementation factor. Further mathematical derivations confirmed that the rock resistivity is inextricable from pore system and establish a physical model. The paper provides a solid evidence base for further understanding and evaluation of water saturation in unconventional shale reservoirs, more significantly, innovatively unveils the research into Archie formula in deep shale reservoirs of southern Sichuan basin.

Experimental analysis of matrix moveable oil saturation in tight sandstone reservoirs of the south Ordos Basin, China

Tight oil reservoirs in the south Ordos Basin are characterized by fractured, heterogeneous oil-bearing strata (an oil saturation of less than 55% on average), normal pressure (0.8±) and extra-low permeability (less than 0.3 mD). In the Chang 8 tight sandstone reservoir in Honghe oilfield, micro- and nano-pores, especially those with a pore-throat radius of less than 1 μm, account for more than 90%. Fluid flow in the matrix is non-linear and crude oil flow rates are very low under normal pressure gradients. An improved understanding of oil mobility in a tight matrix is key to further development of normal-pressure tight-oil resources in the continental basin. In this study, constant-velocity and high-pressure mercury injection experiments were conducted using samples of typical tight sandstone cores obtained from the south of Ordos Basin. A new method for reconstructing the full-scale pore-throat distribution characteristics of tight sandstone reservoirs was established successfully, based on which multistage centrifugal tests, tests of low-pressure differential displacement of oil by water, and nuclear magnetic resonance tests were conducted in order to obtain the distribution characteristics of moveable fluid in different pores. The moveable oil saturation (MOS) and degree of oil recovery (i.e. ratio of accumulative oil production to producing geologic reserves) of the core samples under different differential pressures for displacement were determined. As for the tight oil reservoirs in the south Ordos Basin, the moveable fluids are mainly stored in sub-micron (0.10–0.5 μm) pores. For Type Ⅰ reservoirs (k > 0.1 mD), the volume percentage of moveable fluid in pores with a radius larger than 0.5 μm is relatively high (greater than 40%). The degree of oil recovery of water flooding serves as the basis for forecasting recoverable reserves for tight oil reservoirs. Recoverable reserves under water flooding, mainly occur in pores with a radius greater than 0.5 μm. The contribution of Type Ⅰ reserves to oil production is observed to be greater than 60%, and the degree of oil recovery reaches up to 17.1%. These results help improve our understanding on the evaluation and classification of Chang 8 tight sandstone reservoirs in Honghe oilfield and serve as theoretical basis for pilot tests to explore effective injection media and development methods to improve the matrix-driven pressure differences and displacement efficiency for oil.

Source rock potential, main depocenters, and CO2 occurrence in the pre-salt section of Santos Basin, southeast Brazil

The absence of expressive hydrocarbon columns and high CO2 content observed in the pre-salt of Santos Basin reservoirs over the last decade reveal the risk of hydrocarbon charge and contamination to be a relevant concern in exploration activities in the basin. Therefore, this study was focused on elucidation of the source potential, basin depocenters, and CO2 occurrence in the pre-salt of the Santos Basin, aimed at reducing risk of upcoming exploration activities. Based on geochemical analysis, our study revealed that Late Barremian-Early Aptian fine-grained rock intervals, rich in organic matter, from the Itapema Formation and secondarily, from the Piçarras Formation, are likely the main source rocks of the Santos Basin. Seismic interpretation revealed five wide pre-salt depocenters. The Main Depocenter, followed by the Central and North depocenters, could be the major hydrocarbon sources for pre-salt reservoirs. Based on the CO2 distribution data, low amounts of the contaminant (under 5%) were detected in the post-salt and proximal pre-salt reservoirs, with intermediate values (from 10 to 50%) in the main pre-salt accumulations. High CO2 amounts (over 50%) were encountered mainly in a northeast to southwest trend, linking Peroba-Júpiter-Libra areas, and could be related to a deep-seated fault system associated with mantle contamination.Accordingly, the northern and central parts of the Central Santos sub-basin have a low risk of hydrocarbon charge and contamination, whereas its eastern portion has a high risk because of the absence of an expressive hydrocarbon column and elevated CO2 content. The Southwest Santos sub-basin presented a high risk for pre-salt discoveries because of salt windows which limit the top seal for pre-salt-sourced hydrocarbons. In the Southeast Santos sub-basin, the risk of hydrocarbon charge and contamination is significant because of the absence of mapped depocenters and its location on a crustal domain favorable to mantle CO2 access.

Lithium-rich geothermal brines in Europe: An up-date about geochemical characteristics and implications for potential Li resources

Lithium (Li) is a strategic metal - especially for batteries in electric vehicles - for which worldwide demand is constantly increasing. Presently, several investigations are examining if a part of lithium could be extracted from deep European geothermal fluids. Among the data from geothermal and hydrocarbon wells found in the literature review carried out by BRGM and EIFER, only six areas stand out with deep fluids containing high Li concentrations from 125 to 480 mg/l in Italy, Germany, France and the United-Kingdom. Except the UK fluid, which has a relatively low salinity (TDS = 19 g/l) and reservoir temperature (around 52 °C), these deep Li-rich fluids are Na-Cl brines (TDS ≥ 56 g/l) with high concentrations of Na (> 18 g/l) and Cl (> 25 g/l) and high temperatures (≥ 120 °C and up to 380 °C in Italy). If high TDS and temperature values seem to be key factors for triggering high Li concentrations in such fluids, these two factors alone cannot be sufficient. Indeed, our study confirms that Li concentrations not only depend on temperature and fluid salinity, but also on the type of reservoir rock and its mineralogical constituents, as demonstrated by the good results obtained using two different Na-Li thermometric relationships existing in the literature. One fits well with the brines from low to high temperature (120–250 °C) reservoirs in deep tectonic sedimentary basins over a crystalline basement (France, Germany) and the other with the fluids from ultra-high temperature (≥ 300 °C) reservoirs in volcano-sedimentary environment (Italy). If the high chloride concentrations values in these brines mainly depend on the fluid origin (evaporated seawater or freshwater, halite dissolution, primary neutralization fluids or parent-geothermal fluids in high-temperature and -pressure volcanic environments, water mixing, etc.), the other aqueous major species are mostly controlled by hydrothermal water-rock interaction processes. At these temperatures (≥ 120 °C), the fluid-rock interaction processes are generally dominated by plagioclase and K-feldspar dissolution, followed by albitization of these minerals, dissolution of white micas and biotite, precipitation of illite, and chloritization. According to the two Na-Li thermometric relationships, existing mineralogical and isotopic data, this study suggests that the main sources of Li are white micas and biotite dissolution. Among the European areas, it shows that the Upper Rhine Graben (URG) along the French/German border is probably the most promising zone. For the URG geothermal brines, the main source and control of lithium, at about 225 °C, at reservoir depth, is probably the up to 450-m-thick micaceous continental sandstone of Triassic Buntsandstein. A minor contribution from the granite basement can also not be excluded. Though the Li concentration values of these brines (≥ 150 mg/l) seem to be favourable for geothermal Li exploitation, it is essential to make an as accurate as possible estimate of the Li resource of these brines, design the Li extraction process, and examine the economic conditions of its exploitation.

Natural fractures and their contribution to tight gas conglomerate reservoirs: A case study in the northwestern Sichuan Basin, China

Well-developed natural fractures in the tight gas conglomerate reservoirs of the northwestern Sichuan Basin, China, are proved to greatly contribute to the porosity and permeability of such reservoirs. Natural fractures can be storage space for hydrocarbons in the reservoir and also serve as fluid flow conduits. Outcrops, cores, and image logs were assessed in order to understand the natural gas production from the tight conglomerate reservoirs. Based on the type of interaction between fractures and clast grains, here we present three types of fractures in the conglomerate tight gas reservoirs: intergranular fractures, intragranular fractures, and grain-edge fractures. These fractures may be tectonic, diagenetic, or combined tectonic-diagenetic. Factors affecting natural fracture development in conglomerates include composition of grain and interstitial material, grain size, contact behavior of grains, and structural position. Well-developed open fractures make up over 70% of the porosity within the tight conglomerate reservoir, while the local permeability can be 3-5 orders higher than that of the matrix reservoir. Natural fractures can also significantly impact the natural gas productivity from individual wells. Production data suggests that higher natural fracture intensity correlate with higher natural gas productivity from wells. Compared with the intensity of intragranular fractures and grain-edge fractures, the high intensity of intergranular fractures shows a notably stronger correlation with high gas productivity. The impact of fractures on natural gas productivity varies greatly with fracture orientation. East-West fractures are predominant flow channels under the existing East-West stress field, contributing significantly to gas production. Northeast-Southwest-oriented fractures are of secondary importance.

Why is it easy to form high-quality reservoirs in a mixed siliciclastic-carbonate system? Evidence from diagenetic characteristics

Mixed siliciclastic-carbonate reservoirs are considered as new important high-quality exploration targets because giant hydrocarbon accumulations have continually been discovered in these reservoirs in continental rift basins. This study used a combination of reservoir geological and geochemical evidence to investigate the characteristics and formation mechanisms of mixed siliciclastic-carbonate reservoirs in the first and second members of the Shahejie Formation in the Bozhong Sag, Bohai Bay Basin, China. The results show that bioclastic-dominated mixed rocks represent high-quality reservoirs because they did not only preserve a amount number of primary pores but also preserved a fair proportion of secondary pores that contain organic matter fabrics or associated secondary pores. The main diagenetic processes recognized in the mixed sediment reservoirs are micritization, cementation, dissolution and compaction. Micritization in the contemporaneous stage, dolomitization in the penecontemporaneous stage and leaching of meteoric freshwater in the early diagenetic process are the main controlling factors leading to the formation of high-quality reservoirs. The micritic envelope effectively resists compaction, preserving a high amount of primary porosity, and the early leaching of meteoric freshwater can produce a large amount of secondary dissolution pores. This study does not only reveal the genetic mechanism of high-quality reservoirs of mixed rocks in continental basins but also provide significant guides that can be utilized to search and produce from new types of high-quality reservoirs.

Effects of Afforestation Projects on Tradeoffs between Ecosystem Services: A Case Study of the Guanting Reservoir Basin, China

Evaluating the responses of ecosystem services (ESs) to local land-use changes is critical for understanding the effects of ecological projects related to land planning. Change patterns in the interrelationships between ESs delivered by land-use changes, which are helpful for formulating future strategies, have not been well studied. In this study, we quantified four ESs, namely water yield (WY), water and soil conservation, nonpoint pollution control, and carbon sequestration services, based on the soil and water assessment tool model (SWAT) in the Zhangjiakou section of the Guanting Reservoir watershed, a region with a high concentration of afforestation projects. The impacts of land-use changes on changes in ESs and interrelationships of ESs were investigated by redundancy analysis. The results showed that, along with afforestation, regional water conservation and soil organic carbon content increased by 3.22% and 1.08%, respectively, whereas sediment output, WY, phosphorus output, and nitrogen output decreased by 1.82%, 3.07%, 8.08%, and 12.51%, respectively. Significant tradeoffs of regional ESs were observed between WY and other ESs, while synergies existed between other ESs. Increased areas of evergreen and deciduous forests helped in conserving water, fixing carbon, and regulating runoff. Evergreen forests tended to conserve more water than deciduous forests. With the increase in grassland area, most of the ESs can be improved while introducing fewer tradeoffs compared with those of most of other land-use types. This study provided a better understanding of the effects of afforestation on ESs tradeoffs and benefits to develop better ecological conservation strategies in afforestation regions.

Research on the Non-Point Source Pollution Characteristics of Important Drinking Water Sources

In recent years, freshwater resource contamination by non-point source pollution has become particularly prominent in China. To control non-point source (NPS) pollution, it is important to estimate NPS pollution exports, identify sources of pollution, and analyze the pollution characteristics. As such, in this study, we established the modified export coefficient model based on rainfall and terrain to investigate the pollution sources and characteristics of non-point source total nitrogen (TN) and total phosphorus (TP) throughout the Huangqian Reservoir watershed—which serves as an important potable water source for the main tributary of the lower Yellow River. The results showed that: (1) In 2018, the non-point source total nitrogen (TN) and total phosphorus (TP) loads in the Huangqian Reservoir basin were 707.09 t and 114.42 t, respectively. The contribution ratios to TN export were, from high to low, rural life (33.58%), farmland (32.68%), other land use types (20.08%), and livestock and poultry breeding (13.67%). The contribution ratios to TP export were, from high to low, rural life (61.19%), livestock and poultry breeding (21.65%), farmland (12.79%), and other land use types (4.38%). The non-point source pollution primarily originated from the rural life of the water source protection zone. (2) Non-point source TN and TP pollution loads and load intensities showed significantly different spatial distribution patterns throughout the water source protection area. Specifically, their load intensities and loads were the largest in the second-class protected zone, which is the key source area of non-point source pollution. (3) When considering whether to invest in agricultural land fertilizer control or rural domestic sewage, waste, and livestock manure pollution control, the latter is demonstrably more effective. Thus, in addition to putting low-grade control on agricultural fertilizer loss, to rapidly and effectively improve potable water quality, non-point source pollution should, to a larger extent, also be controlled through measures such as establishing household biogas digesters, introducing village sewage treatment plants, and improving the recovery rate of rural domestic garbage. The research results discussed herein provide a theoretical basis for formulating a reasonable and effective protection plan for the Huangqian Reservoir water source and can potentially be used to do the same for other similar freshwater resources.

Paleoenvironmental insights from the deposition and diagenesis of Aptian pre-salt magnesium silicates from the Lula Field, Santos Basin, Brazil

ABSTRACT Since the discovery of giant Aptian pre-salt reservoirs in Brazilian margin basins, the study of lacustrine carbonates has drawn great attention from the scientific community. Comparatively, minor attention was given to the characterization and genesis of the Mg-silicates (e.g., stevensite, kerolite) which are commonly associated with these carbonates. A systematic petrological study was performed in the Aptian Barra Velha Formation (BVF) within distinct structural compartments of the giant Lula Field in the Santos Basin, in order to recognize the patterns of primary formation and diagenetic alteration of these Mg-silicates. Mg-silicates occur as peloids, ooids, intraclasts, and fine-grained laminated deposits, either mixed in variable proportions with other particles, such as carbonate bioclasts and volcanic rock fragments, or constituting specific intrabasinal deposits. In the BVF interval, clay peloids and laminated deposits are associated with spherulitic and fascicular calcite aggregates, as substrate and hosts for these precipitates. Ooids are interpreted as formed at the sediment–water interface by the nucleation of concentric envelopes on the surface of particles (heterogeneous nucleation), through repeated rolling under gentle wave and current action. Laminated deposits, interpreted as precipitated directly from the water column (homogeneous nucleation) in highly supersaturated and low-hydrodynamic-energy environments, constitute extensive deposits in the BVF. Peloids were probably formed in intermediate energy conditions. Some ooidal arenites show porosity from the dehydration and contraction, and/or the dissolution of ooids. In some rocks, these pores are filled with fibrous calcite, while the remaining Mg-silicates are replaced by dolomite, calcite, or silica. A similar diagenetic pattern occurs in the laminated deposits, where magnesite and dolomite fill shrinkage pores formed along their characteristic wavy laminae. Owing to their elevated solubility, most of the Mg-silicates were dissolved, or intensely replaced by calcite, dolomite, or silica. The detailed petrologic analysis indicates that the original volumes of Mg-silicates were substantially larger, and that their deposition was widespread in the basin, including on structurally high areas. The types and intensity of diagenetic alteration of the Mg-silicate deposits are distinct for each structural compartment, being more intense towards the highs and closer to the overlying evaporites, which imposed a strong influence on reservoir quality.

Characteristics of hydrocarbon migration and accumulation in the Lower Jurassic reservoirs in the Tugerming area of the eastern Kuqa Depression, Tarim Basin

The hydrocarbon migration and accumulation process in the Kuqa Depression are complex. In recent years, the Jurassic strata have recorded hydrocarbon discoveries in the Tugerming area of the eastern Kuqa Depression. However, it is a new exploration area, and the characteristics of hydrocarbon accumulation are unclear, restricting oil and gas exploration and development. In our study, the deeply-buried and shallow-buried Jurassic strata in the Tugerming area are the research objects. Quantitative fluorescence technique and the observation of fluid inclusions are implemented on the selected samples of Jurassic reservoirs to determine the stages and times of hydrocarbon charging. Gas chromatography-mass spectrometry analysis for saturated hydrocarbons is used to identify the origin of organic matter in oils. Combined analytical data with the reconstructed histories of burial, thermal evolution and tectonic evolution, allow simulating the hydrocarbon migration and accumulation process in the Tugerming area. The results show that two categories of hydrocarbon accumulation mechanisms (Burnham and Sweeney, 1989). The first type is acted as the type of “self-generation and self-storage and late accumulation”. Three stages of hydrocarbon charging have occurred in the deeply-buried Lower Jurassic reservoirs, represented by two wells (Tudong 2 and Tuge 4). The charging time is the early-middle stage of Miocene Kangcun Formation (18–9 Ma), forming the yellow or yellow-green fluorescent oil inclusions and paleo-oil reservoir. During the sedimentary period of the late Kangcun Formation to the early Kuqa Formation (9–5 Ma), the reservoir experienced massive condensate oil migration, corresponding to blue-white fluorescent oil inclusions. Large-scale natural gas was expelled into the reservoirs since the deposition stage of Kuqa Formation (5–0 Ma), and then accumulated in the trap controlled by the late Himalayan movement, forming the current gas reservoir. Moreover, the hydrocarbons are generated from Jurassic coal measure source rocks in the Well Tudong 2 based on the characteristics of biomarkers (Burruss et al., 1991). The second type is characterized by “lower source rock and upper reservoir and accumulation in the late or recent period”, developed in the shallow-buried Lower Jurassic reservoirs containing two wells (Tuxi 1 and Mingnan 1). In this case, there are two stages of crude oil charging: mature oil charging in the early-medium Miocene (18–12 Ma) and condensate oil migration in the late Miocene (8–5 Ma). The strata suffered denudation due to the stratigraphic uplifting in the orogenic movement, resulting in the leakage of the early oil reservoir. In both categories, it suggests that the Lower Jurassic reservoirs in the Tugerming area have the favorable potential for oil and gas exploration. The understanding of hydrocarbon migration and accumulation in the Lower Jurassic reservoirs in the Kuqa Depression of Tarim Basin can contribute to the knowledge of the potential to form large oil and gas reservoirs Tugerming Area, and reservoirs of basins with a similar tectonic settings, such as the Jungar Basin.

Fracability evaluation of the upper Cretaceous Naparima Hill Formation, Trinidad

The upper Cretaceous Naparima Hill Formation, a primary source rock for conventional oil and gas reservoirs in Southern and Columbus basins, Trinidad is now considered an unconventional reservoir. Effective fracability evaluation is critical to the entire process of unconventional reservoir exploitation. At present, there exists no information on the fracability of the Naparima Hill Formation. A recent study have shown that the Formation consist of four lithofacies (siliceous-calcareous mudstone, calcareous mudstones, carbonate-rich mudstone and siliceous mudstone) that are highly brittle, implying that they are easily fractured. This is debatable because brittle rocks can have a higher fracture toughness, making them more difficult to fracture. In this study, an existing fracability evaluation model that incorporates brittleness, fracture toughness, and minimum horizontal insitu stress was used to evaluate the fracability of the four lithofacies within the Naparima Hill Formation. Through a series of laboratory testing (measurements of P- and S-wave velocities, uniaxial compression tests and Brazilian tests) of the dry outcrop samples, elastic properties (Young's modulus and Poisson's ratio), uniaxial compressive strength and tensile strength were determined. X-ray diffraction analysis (XRD) was also performed on the outcrop samples to determine their mineral compositions. Brittleness indices based on the rock elastic properties, rock strength and mineral compositions were used to evaluate the rock brittleness, and the fracture toughness was estimated from the tensile strength. The study results indicate that all lithofacies are highly brittle, which is consistent with the previous study. The fracability evaluation results showed that the siliceous calcareous mudstones and siliceous mudstones are more fracable than the calcareous and carbonate-rich mudstones. The key factors that control the fracability of the Formation were found to be the amount of quartz, and the rock strength that is influenced by calcite cementation. • This paper presents fracability of the Naparima Hill Formation mudstones • Brittleness alone is not sufficient to evaluate the fracability of mudstones • High quartz content increases the fracability of the mudstones. • High fracture toughness and tensile strength lowers the fracability of the mudstones.

Fluid geochemical response recorded in the alteration of marine carbonate reservoirs: the Silurian Shiniulan Formation, southeast Sichuan Basin, China

Natural gas resources in marine carbonate rocks comprise the most significant remaining oil and gas resources in China. Preservation conditions assessment and destruction mechanisms of high-quality carbonate reservoirs play an important role in petroleum resource evaluation in carbonate reservoirs. In the Silurian Shiniulan carbonate gas reservoirs of southeast Sichuan Basin, mineral sequences in fracture-cavity veins, calcite-precipitating fluid sources, and paleo-pressure evolution histories were investigated by Raman quantitative analysis, element characteristics and isotope compositions. The process of reservoir decompression indicated by inclusion analysis and the enrichment of trace elements are all due to the response of reservoir reconstruction. Petrographic observations show two stages of calcite vein occur commonly in fracture-cavity. Stage-1 calcite veins mineralized in a reduction environment as results of hydrothermal activity. Gas accumulation was accompanied by the crystallization and precipitation of the stage-1 calcite veins. Stage-2 calcite vein precipitated during Himalayan tectonics and derived from intensive invaded penetrating fluid, which might be affected by atmospheric freshwater. The density, pressure, and temperature for CH4-bearing fluid inclusions in the stage-1 calcite veins filling was determined by quantitative Raman spectroscopy. Thermodynamic simulations were conducted to constrain pore pressure evolution histories with the establishment of trapping condition of the geo-fluids. Our results suggest that gas generation and charging comprise the main mechanisms for paleo-overpressure development. Identification of fluid sources and understanding pore pressure evolution are keys to study the oil and gas migration and accumulation.

Aportes naturais e antrópicos de nutrientes em bacias hidrográficas de reservatórios do semiárido brasileiro

Abstract: Aim Estimate the input of loads of N and P emitted by natural (atmospheric deposition and soil denudation) and anthropogenic (agriculture, livestock and sewage) factors for the hydrographic basins of two reservoirs in the Brazilian semiarid region (Mendubim and Umari). Methods In the present work, we use georeferenced data provided by official agencies, data presented in academic papers, field samples and laboratory analysis of emission factors in the estimates of nitrogen and phosphorus inputs in reservoir basins of Brazilian semiarid region. Results Soil denudation was identified as the main natural source of N and atmospheric deposition as the main source of P in both basins. Among the anthropogenic sources, the main source of N and P, for the two basins, was livestock. The total loads (natural and anthropogenic) of N (579.01 tonne. year-1) and P (136.35 tonne. year-1) received by the Umari basin was, respectively, 43.90% and 22.10% higher than those received by Mendubim, with a predominance of anthropogenic sources in both nitrogen and phosphorus emission. Conclusions The results showed the importance of monitoring human activities that can enhance nutrient inputs, such as nitrogen and phosphorus, in basins of the Brazilian semiarid region. The quantification of the emission factors analyzed here can be a tool in the development of strategies to mitigate the problems that high concentrations of N and P can bring to the quality and use of water in semiarid reservoirs.

Comprehensive 3D reservoir modelling and basin analysis: An insight into petroleum geology to re‐evaluate the hydrocarbon possibilities in the Siwa Basin, North‐Western Desert, Egypt

Three‐dimensional (3D) reservoir modelling and basin analysis were performed for the Siwa Basin in the North‐Western Desert of Egypt. An integrative study from the available data were done, including geochemical data to construct a 1D basin model, petrophysical data to realize the characteristics of the reservoir, and geological data to understand the subsurface geologic setting, has been undertaken. Basin modelling was done for the Siwa A1‐X well to determine the burial and thermal history, sedimentation rate, level of maturity, and finally the event chart for the petroleum system. Siwa Basin comprises one petroleum system. The source rock is represented by the Palaeozoic Dhiffah, Desouky, and Zeitoun formations, with fair to good organic richness, producing oil and gas, which entered the oil window in 143.91, 162.87, and 167.31 Ma, and entered the wet gas in 138.75, 141.82, and 143.63 Ma, with sedimentation rate varying from low to high 543.96, 22.79, and 1,404.46 m/Ma, respectively. The Safa Member (Khatatba Formation) is the main reservoir, with hydrocarbon saturation reaching up to 80%, and with a low subsidence rate of 76.43 m/Ma. The Original Oil In Place for the new prospects is 197.177 Stock Tank Barrel (STB), while for the oil reserve is 50.618 Stock Tank Barrel (STB).