Permeability can effectively represent the ability of fluids to flow in porous media, and is a crucial physical parameter for the exploration of unconventional oil reservoirs. In low-permeable porous ...media, such as tight rocks, not all pores function as fluid flow channels; meanwhile, the pore surface tends to form unevenly adsorbed water lay. To date, the two phenomena above have not been considered in previous studies on permeability estimation models. In this study, a new fractal permeability model with variable pore sizes was proposed resulting in two advancements: (1) unevenly adsorbed water lay attached to pore surfaces was considered; and (2) instead of the total pore space, the movable fluid space obtained from nuclear magnetic resonance (NMR) experiments was used as the fluid flow channels in the porous media. Helium permeability data of 26 mixed rock samples from tight oil reservoirs were used to establish the accuracy of the proposed permeability model. The results of a sensitivity analysis show that high porosity and large pore diameters play a positive role in generating high permeability in porous media. The permeability decreased with an increase in the fractal dimension of the total pore space and the tortuosity fractal dimension of the total pore space and movable fluid space, whereas it first decreased and then increased as the fractal dimension of the movable fluid space increased. Furthermore, the proposed model indicates that the movable fluid space and unevenly adsorbed water lay greatly influence the fluid flow, specifically at lower permeabilities (K＜0.1 mD), while the influence of the unevenness of the adsorbed water lay on fluid flow is negligible at high permeabilities. Overall, the accuracy of the permeability estimation of the proposed model is reliable both in low-permeable and conventional porous media, and can be effectively applied to unconventional oil reservoirs.
•Movable fluid space and unevenly adsorbed water lay affect rock permeability.•High porosity and large pores play a positive role in generating high permeability.•Influence of hydrophilic minerals on the permeability cannot be ignored.•Proposed model helps to estimate the permeability of unconventional oil reservoirs.
Lamellation fractures, as important reservoir space and fluid flow channels, have significant effects on reservoir quality and hydraulic fracturing engineering in shale oil reservoirs. Core ...observation and this section reveal that there are three types of fractures identified, including tectonic fractures, lamellation fractures, and abnormal pressure fractures. Lamellation fractures are parallel along with the weak bedding (laminae) planes, and they can appear as open, partly sealed, and fully filled. Morphology of lamellation fractures surface is various, such as continuous, intermittent or pinched even or bifurcated, bent, or straight. Conventional petrophysical log suites are used to recognize lamellation fractures. The results indicate that lamellation fractures result of a positive deviation from deep and shallow resistivity logs, and they are associated with low gamma ray values. In addition three porosity logs are not sensitive to the lamellation fractures in shale oil reservoirs. According to the image logs, borehole-wall structural interpretation is used for detecting geological objects and determining their strike, dip, and dip direction. Geological objects types and their distribution are determined in a single well. The results show that the strata have a low dip angle with an average value of 1.5°. The maximum horizontal stress refers toward the EW direction, which is coinciding with the direction of the regional compressive stress field. The strike of lamellation fractures is consistent with the bedding planes and their dip angle is low in the majority value of 1.5°. Furthermore, the relationships between lamellation fractures and reservoir quality are clarified according to petrophysical experiment analysis including mercury injection capillary pressure (MICP), two-dimensional nuclear magnetic resonance (2D NMR), and physical property analysis. As a result, the development of lamellation fractures favors the connectivity of the pores and throats, and the closer to the lamellation fractures, the larger the diameter of the pores. Thus, lamellation fractures are favorable for high reservoir quality of shale oil reservoirs. In addition, the charge of oil and gas in the early stage during hydrocarbon generation and expulsion plays a crucial role in the preservation of lamellation fractures. Recognition and prediction of lamellation fractures are therefore vital in sweet spots prediction and mechanism of oil accumulation analysis in shale oil reservoirs.
•Lamellation fractures can be recognized by resistivity logs, gamma ray, and image logs. Three porosity logs are not sensitive to them.•Lamellation fractures improve the connectivity of the pores and throats, and the closer to the lamellation fractures, the larger the diameter of the pores.•The formation of lamellation fractures occurs during hydrocarbon generation and expulsion.Lamellation fractures provide the utmost effective reservoir space in shale oil reservoirs.
Geophysical well log data are widely used in the field of structural geology, sedimentary geology and petroleum geology. Gaps and misunderstandings are still existing in the scientific interpretation ...of geophysical well logs. Logging environments and log curves need correction and standardization before interpretation, additionally, there are some special geological phenomena that will mislead the well log interpretation. This review critically highlights the typical misunderstandings existing in the well log data interpretation, and proposes countermeasures as well as scientific interpretation of well logs when encounter these misunderstandings. The factors that affect the well log data acquisition are summarized in terms of types of drilling muds, borehole stability and logging instrument rotation. The vertical resolution of various log series spans a wide range from 5 mm to about 10 m. In the field of structural geology, well logs can be used for determination of stratum attitude, fault recognition, fracture and in situ stress characterization as well as unconformity identification. Lithology and depositional facies can be interpreted using well logs. Well logs aim at finding hydrocarbons, and are used for source rock characterization and logging reservoir evaluation in the petroleum geology field. Then the typical misunderstandings and countermeasures in solving geological issues using geophysical well logs are reviewed from published papers as well as from the authors’ personal experiences. This review will provide insights into the scientific interpretation of geophysical well log data, and help solving geological issues for the petrophysicist and geologist.
Core, thin sections, cathodoluminescence (CL), scanning electron microscopy (SEM), conventional well logs and image logs are used to unravel depositional microfacies, diagenetic facies and fractures ...in the deeply buried (6000–8100 m) Cretaceous Bashijiqike (K1bs) and Baxigai (K1bx) Formations in Kuqa depression of the Tarim Basin. Results show that depositional microfacies in core and well log data include distributary channel, river mouth bar and distributary bay deposits of fan-braided delta fronts as well as lacustrine deposits. We establish well log predictable models of depositional microfacies using conventional and image logs. The Cretaceous sandstones experienced low to medium mechanical compaction and low intensity dissolution. Dominant diagenetic minerals are carbonate cements, clay minerals and minor amounts of quartz cements. The pore spaces are primarily intergranular pores and rare dissolution pores. We identify three diagenetic facies according to compaction state and diagenetic minerals, including carbonate cemented facies, tightly compacted facies, and facies having limited compaction and lacking cement (slightly cemented and compacted facies). We use well log characteristics to interpret diagenetic facies in areas without core data. Fractures are also elements of reservoir quality, including vertical opening-mode fractures, high to medium dip angle fractures, low angle fractures that probably include small faults and horizontal fractures. Fractures include open, partially open, and closed (sealed) fractures. We used image logs to identify fracture attributes, and calculate fracture density, porosity, aperture, and length. Depositional microfacies and diagenetic facies determine the primary intergranular pores and diagenesis subsequently modified secondary pore spaces. Natural fractures acted as both reservoir porosity (probably less than 1%) and as hydrocarbon flow channels accounting for elevated permeability. We evaluate and predict reservoir quality by superposing depositional microfacies, diagenetic facies and fracture occurrence.
•Reservoir quality is evaluated from depositional facies, diagenetic facies and fracture.•Hydrocarbon productivity is determined by fracture, and depositional-diagenetic controlled matrix pores.•Petrophysical facies is used for sweet spot prediction in deep buried tight sandstone.
Low-contrast oil reservoirs have a complicated origin story that frequently results from the interaction of several different factors. The low-contrast oil reservoirs in the Daqing Oilfield’s SaPu ...intercalation in the Longxi Region are the main subjects of this study. This work investigates the petrophysical origins of these low-resistivity oil reservoirs through a series of carefully planned petrophysical experiments. The results showed that the main determinants of the low-contrast oil reservoirs in the SaPu intercalation were the conductivity of clay minerals and a high irreducible water saturation. A low clay conductivity had a significant effect, resulting in a significant reduction in the formation resistivity, even in circumstances with a low porosity and a low formation-water salinity. Moreover, the results from the NMR tests revealed that the irreducible water saturation in the core samples primarily ranged from 0.6 to 0.8, suggesting a significant prevalence. This work provides strong petrophysical indices for evaluating low-resistivity oil reservoirs in the SaPu intercalation and useful information for the petrophysical evaluation of similar reservoirs.
Understanding diagenetic heterogeneity in tight sandstone reservoirs is vital for hydrocarbon exploration. As a typical tight sandstone reservoir, the seventh unit of the Upper Triassic Yanchang ...Formation in the Ordos Basin (Chang 7 unit), central China, is an important oil-producing interval. Results of helium porosity and permeability and petrographic assessment from thin sections, X-ray diffraction, scanning electron microscopy and cathodoluminescence analysis demonstrate that the sandstones have encountered various diagenetic processes encompassing mechanical and chemical compaction, cementation by carbonate, quartz, clay minerals, and dissolution of feldspar and lithic fragments. The sandstones comprise silt-to medium-grained lithic arkoses to feldspathic litharenites and litharenites, which have low porosity (0.5%–13.6%, with an average of 6.8%) and low permeability (0.009 × 10−3 μm2 to 1.818 × 10−3 μm2, with an average of 0.106 × 10−3 μm2).
This study suggests that diagenetic facies identified from petrographic observations can be up-scaled by correlation with wire-line log responses, which can facilitate prediction of reservoir quality at a field-scale. Four diagenetic facies are determined based on petrographic features including intensity of compaction, cement types and amounts, and degree of dissolution. Unstable and labile components of sandstones can be identified by low bulk density and low gamma ray log values, and those sandstones show the highest reservoir quality. Tightly compacted sandstones/siltstones, which tend to have high gamma ray readings and relatively high bulk density values, show the poorest reservoir quality. A model based on principal component analysis (PCA) is built and show better prediction of diagenetic facies than biplots of well logs. The model is validated by blind testing log-predicted diagenetic facies against petrographic features from core samples of the Upper Triassic Yanchang Formation in the Ordos Basin, which indicates it is a helpful predictive model.
•Low reservoir properties in Chang 7 reservoir unit result from various diagenesis.•Sandstones are divided into four relatively homogeneous diagenetic facies.•PCA technique is validated to be a helpful tool to predict diagenetic facies.
Marine dolostones of Carboniferous Huanglong Formation constitute major gas reservoir rocks in eastern Sichuan Basin. However, the investigation with respect to sources of dolomitizing and diagenetic ...fluids is relatively underexplored. The current study attempts to investigate the REE characteristics of dolomites using seawater normalization standard, and therefore discusses the origins of dolomitizing and diagenetic fluids, on the basis of continuous 47.33-m-long core samples from the second member of Huanglong Formation (C2h2) in eastern Sichuan Basin. Low Th, Sc, and Hf concentrations (0.791 × 10−6, 4.751 × 10−6, and 0.214 × 10−6, respectively), random correlation between total REE concentration (ΣREE) and Fe or Mn abundance, and seawater-like Y/Ho ratios (mean value of 45.612) indicate that the carbonate samples are valid for REE analysis. Based on petrographic characteristics, four dolomite types are identified, including micritic-sized dolomite (type Dol-1), fine-to medium-sized dolomite (type Dol-2), medium-to coarse-sized dolomite (type Dol-3), and coarse-to giant-sized saddle dolomite (type Dol-4). Dol-1 dolomites, characterized by positive Ce anomaly (mean value of 6.398), light REE (LREE) enrichment, and heavy REE (HREE) depletion with mean LREE/HREE ratio of 12.657, show micritic calcite-like REE patterns, indicating seawater origin of their dolomitizing fluids. Dol-1 dolomites were formed in sabkha environment whereas the dolomitizing fluids originated from evaporative brine water due to their micritic crystal sizes and tight lithology. Dol-2 dolomites, particularly subtype Dol-2a barely developing vuggy porosity, also show micritic calcite-like REE patterns, suggesting their dolomitizing fluids were seawater or seawater-derived fluids. This inference is confirmed by low Fe and Mn concentrations, which range from 651 μg/g to 1018 μg/g (mean value of 863 μg/g) and 65 μg/g to 167 μg/g (mean value of 105 μg/g), respectively, whereas homogenization temperatures (Th, mean value of 103 °C) indicate that Dol-2 dolomites were formed under burial environment. Dol-3 dolomites, in form of cements of Dol-2 dolomites, show similar REE patterns to their host minerals (i.e., Dol-2 dolomites), indicating their parent source was possibly derived from Dol-2 dolomites. Dol-3 dolomites have high Fe and Mn concentrations with mean values of 3346 μg/g (ranging from 2897 μg/g to 3856 μg/g) and 236 μg/g (ranging from 178 μg/g to 287 μg/g), respectively, indicating the involvement of meteoric water. Meanwhile, it confirms that the dissolution in Dol-2 dolomites was caused by meteoric water leaching. Positive Eu anomalies (mean value of 1.406) in Dol-4 dolomites, coupled with high homogenization temperatures (mean value of 314 °C), suggest that Dol-4 dolomites precipitated from hydrothermal fluids. High Fe and Mn concentrations (mean values of 2521 μg/g and 193 μg/g, respectively) in Dol-4 dolomites likely results from interactions of hydrothermal fluids with deep burial clastic rocks.
•Sweater standard as a normalization method for marine carbonate REE patterns is proposed.•Sample validity is examined for REE analysis.•REE characteristics normalized to seawater are discussed.•Origins of dolomitizing and diagenetic fluids are discussed.
Low oil saturation reservoirs are increasingly encountered in superimposed sedimentary basins, however, the uncertainty about the origin of the low oil saturation becomes a major factor impeding ...further hydrocarbon exploration. Routine core analysis, (fluorescence) thin section, scanning electron microscope (SEM) and nuclear magnetic resonance (NMR) measurements were used to understand the characteristics and formation mechanisms of the low oil saturation reservoirs in the Paleogene Dongying Formation in the Nanpu sag, Bohai Bay Basin, East China. Irreducible water saturation was determined from sealed coring oil saturation testing and NMR logs. Wide ranges of porosity and permeability were observed, and the samples with moderate reservoir quality have the best oil bearing property. Low oil saturation oil layers are characterized by low oil saturation (<50%) but high content of irreducible water (>40%), with only oil being produced whereas no water or only minor amounts of water being produced. The low oil saturation oil-water reservoirs refer to the low oil saturation layers in which both oil and water will be produced during oil testing.
Irreducible water saturation increases with total clay content, with the authigenic clays (kaolinite, illite and mixed layer illite/smectite) blocking pore spaces, and resulting in high irreducible water saturations. Fluorescence thin sections combined with NMR measurements show that the NMR T2 (transversal relaxation time) distribution is uni-modal, with large intergranular pores being rare. Almost the entire pore-size range is oil bearing for the low oil saturation oil reservoirs. The large intergranular pores are fluorescence-free, while the intragranular dissolution pores and micropores emit fluorescence in the low oil saturation oil-water reservoirs. The NMR T2 distribution of low oil saturation oil-water reservoirs is bi-modal and right-skewed, and the main oil-bearing NMR pore size distribution ranges from 0.1 to 100 ms. High irreducible water content was encountered in the low oil saturation oil reservoirs and low oil saturation oil-water reservoirs, and only part of the large intergranular pores are oil saturated, and in some cases only the intragranular pores are saturated with oils forming low oil saturation oil reservoirs. The results help improve the knowledge of characteristics and formation of the low oil saturation oil layers, and could provide insights in enhancing oil recovery.
•Definition and characteristics of the low oil saturation oil reservoirs are clarified.•Formation mechanisms of the low oil saturation oil reservoirs are revealed.•Oil occurrences of low oil saturation oil reservoirs are investigated.
Knowledge of the state of the in-situ stress field provides important insights into the petroleum engineering and geology. The in-situ stress orientation and magnitudes in the high and steep ...structure of Lower Cretaceous Bashijiqike Formation in Kuqa Depression were analyzed based on well log calculations, and the impacts of in-situ stress field on reservoir quality (macroscopic reservoir properties and microscopic pore structure as well as fracture aperture) were investigated. The maximum horizontal stress (SHmax) orientation was determined as nearly north-south according to the image log derived drilling induced fractures and borehole breakouts. The vertical stress (Sv), SHmax, and minimum horizontal stress (Shmin) magnitudes were calculated via well logs. The result reveals a dominant strike-slip stress regime (SHmax > Sv > Shmin) in the Bashijiqike Formation in the Kuqa Depression. The Sv at the depth ranging from 5000 to 7100 m is 150–180 MPa, and horizontal stresses (SHmax and Shmin) vary significantly from 130 to 210 MPa. The horizontal stress difference (Δσ) is in the range from 40 to 70 MPa, and fluid overpressures are commonly encountered. The effective stress (σe) has a positive relationship with the Δσ when the Δσ is less than 50 MPa. Intergranular pores can be preserved in layers with low Δσ values. The microscopic pore structure will be complex, and macroscopic reservoir quality will be poor with the increasing Δσ values. In addition, fractures are mainly corresponding with the layers where the Δσ value is low, and the fracture aperture and porosity decreased significantly with the effective stress σe. By determining the magnitude and orientation of in-situ stresses around the wellbore, insights can be provided into the wellbore instability, optimum drilling trajectories and reservoir quality evaluation.
•The three components of the in situ stresses are calculated.•The impact of the in situ stress on reservoir quality is determined.•The impact of the in situ stress on fracture parameter is evaluated.
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