Reservoir Properties Of Sandstones Research Articles

Characteristics of Rotliegend sediments in view of X-ray microtomohraphy and optical microscopy investigations

The article presents the results of microtomographic and petrographic investigations of Rotliegend sandstones collected from core material from wells located in the area of the Fore-Sudetic Monocline in its central and northern parts and also on the border of the Fore-Sudetic Monocline and the Mogilno - Łódź Synclinorium. Three areas were selected for the study: Czarna Wieś–Parzęczewo (19 samples), Środa Wielkopolska–Kromolice (21 samples) and Siekierki–Miłosław (21 samples). The aim of the petrologic and microtomographic studies was to investigate how the different reservoir properties of sandstones will be reflected in the results of the methods used. Strong differences between samples from the region of Środa Wielkopolska–Kromolice and the other studied areas have been demonstrated. In the case of this area several key factors were noticed: the highest average value of effective porosity; the highest average content of pores belonging to class VII (pore volume subsystem classification); three times higher average value of the CT porosity coefficient and the largest length of the average chord. Also in terms of petrography (composition of grains, cement type) it is a region where major differences, especially compared to the region of Czarna Wieś–Parzęczewo, can be seen. Based on the obtained results, it was possible to rank the examined regions in terms of their reservoir properties – from the worst (Czarna Wieś–Parzęczewo) to the best (Środa Wielkopolska–Kromolice). These conclusions are also confirmed by other petrophysical analyses (eg. mercury porosimetry, permeability analysis). Combination of the obtained microtomographic and petrographic results allowed to obtain a full characterization of the investigated samples – both in terms of the mineralogical composition of grains, as well as the development of the pore space. These data, especially in combination with the results of density and porosity analyses (helium pycnometry and mercury porosimetry), open up a number of possibilities to carry out different types of modeling (porosity, permeability) both on the scale of the sample itself, as well as the scale of a single well or even the whole basin, which is crucial for creating a hydrocarbon exploration strategy.

Effects of sedimentary processes and diagenesis on reservoir quality of submarine lobes of the Huangliu Formation in the Yinggehai Basin, China

Submarine lobe deposits of the upper Miocene Huangliu Formation in the Yinggehai Basin, characterised by high-temperature (132.4–141.2 °C), high overpressure (1.69–1.99), and inorganic CO2 accumulations, offer an excellent opportunity to investigate how reservoir quality is linked to sedimentary processes and diagenesis. This study examines the control on reservoir quality of submarine lobes using petrographic, geochemical, drilling test, and measured petrophysical data. Lobe axis deposits contain high-density turbidites (HDTs) that exhibit good reservoir quality, a higher degree of sandstone amalgamation, and larger grain sizes and pore-throat radii. The lobe off-axis deposits are composed of HDTs and low-density turbidites (LDTs) with moderate reservoir quality. In contrast, the lobe fringe deposits mainly comprising muddy debrites have the lowest reservoir quality. Sedimentary processes macroscopically controlled the initial grain size segregation, variation of textural properties, and reservoir quality throughout the submarine lobes. Flow transformation from turbidity currents in the lobe axis areas into muddy debris flows in the lobe fringe areas led to the high clay content of lobe fringe deposits, due to the incorporation of sediment eroded from the muddy seafloor. Mechanical compaction is the major diagenetic factor reducing reservoir properties of sandstones in the lobe axis and lobe off-axis positions. Calcite and ferrocalcite cements in the sandstones from the CO2-poor DF-B gas field were linked with thermal decarboxylation of organic matter in the adjacent mudstones. In contrast, the diagenetic processes in the CO2-rich DF-A gas field were more likely to be affected by inorganic CO2 accumulations. As a whole, the CO2-water-rock reactions in the CO2-rich DF-A area can not only lead to the precipitation of late dolomite and ankerite, and formation of authigenic clay minerals, but also cause intense dissolution of unstable compositions, resulting in an insignificant increase in porosity. In addition, the high-overpressure in both gas fields has effectively protected the primary porosity by reducing the effective stress exerted on the detrital grains. This study has implications for the assessment of reservoir potential in carbon sequestration and storage, and hydrocarbon exploration and production.

Linking the influence of diagenetic properties and clay texture on reservoir quality in sandstones from NW Borneo

Reservoir studies in the Sandakan sub-basin NW Borneo are challenging and inexplicable due to complex diagenetic effects such as compaction, cementation, dissolution and depositional means including grain size, sorting and packing with reference to clay texture. These diagenetic effects together impart significant amounts of uncertainty on reservoir quality assessment. In this regard, we documented the mineralogical, diagenetic and reservoir properties of sandstones from the Sandakan Formation, NW Borneo through thin sections, SEM, XRD, XRF, He-porosimeter and poroperm. In order to analyze the influential parameters, we used matrix, grain rearrangement and clay coating around detrital grains for the reservoir quality. Our results show that the sub-litharenites to quartz arenite that dominate in the sandstones are drawn from recycled orogenic sand. Furthermore, reservoir properties of the sandstones were ascertained to be controlled by diagenesis, micro quartz with clay coatings and pressure dissolution of feldspar with the alteration of lithic fragments. The result shows that the physical compaction, rearrangements and ductility of grains were the reasons that reduced porosity and permeability. The main reason for porosity reduction in the present study is pressure dissolution that resulted in the form of chemical compaction induced by micas with grain contacts. Nonetheless, through SEM and petrography, clay coatings around the grains and dissolution of feldspar were found to optimally contribute to the preservation of reservoir properties. Dissolution of feldspar and kaolinization resulted in the formation of secondary porosity that allows meteoric water to percolate through the open and detachment spaces of quartz or other grains. This percolation is also responsible in enhancing the pore throat radii in the reservoir media of the analyzed sandstones. These traits are obliging to diagenetic effects on porosity and permeability, which are linked with the physical compaction and clays coating to reduce and preserve the reservoir properties respectively of same type of reservoirs globally.

The differential diagenesis controls on the physical properties of lithofacies in sandstone reservoirs from the Jurassic Shaximiao Formation, western Sichuan depression, China

The Jurassic Shaximiao Formation on the eastern slope of western Sichuan depression in western Sichuan Basin of China develops a single or overlapping channel sandstones with strong internal heterogeneity. This study aims to reveal the origin of the differences of physical properties within the sandstone reservoirs under different assemblies of lithofacies. Based on analysis of sedimentary structure and grain size, the lithofacies are classified into nine types. Based on the classification of lithofacies formed under different energy of sedimentary environment and mechanical differentiation, the effect of early compaction on physical properties of sandstone reservoirs is confirmed. It is suggested that the physical properties of lithofacies can be influenced by differential diagenesis in different lithofacies assemblies. The rate of early compaction depends on the grain sorting and compositional maturity. The massive bedded lithofacies are prone to be of good grain sorting and high compositional maturity with relatively low compaction rate of 70.91%–72.75%. There are relatively high porosities and permeabilities of sandstones in the lithofacies assembly containing massive bedded lithofacies. Because of the export of diagenetic fluid along the bedding plane of parallel bedded sandstones, there would be relatively intensive dissolution of sandstones developed with parallel bedded lithofacies. Even if the development of parallel bedded sandstones could enhance the dissolution, yet they are subjected to relatively strong compaction, leading to the low proportion of intergranular pores. The permeability of sandstones within the lithofacies assembly containing parallel bedded lithofacies seems to be low. The porosity and permeability of sandstones within the lithofacies assembly containing horizontal bedded lithofacies are both poorer than other assemblies, caused by strong compaction, cementation and weak dissolution. The direction of bedding, argillaceous contents, and the capacity of resistance to compaction are the key factors controlling the physical property of high-quality sandstone reservoirs. The research conclusions can be used to interpret the genesis of different physical properties of sandstone reservoirs in other similar gas fields.

Will coupling low salinity water and steam flooding for heavy oil affect the rock properties of sandstone reservoirs? An experimental and simulation study

Abstract Recently Al-Saedi and Flori et al. (2018d) studied the potential of low salinity alternating steam flooding (LSASF) in laboratory sandstone cores, and the results were promising for increasing heavy oil recovery [1]. In this study, we investigate the effect of LSASF on rock properties for the Bartlesville Sandstone Reservoir. These core samples contain high viscous oil (600 cp) with an average permeability of 80 mD. Combining low salinity (LS) water flooding and steam flooding is a novel idea that takes advantage of the relative strengths of both methods. To investigate the proposed new method and the role of LSASF, core flooding experiments, contact angle measurements, spontaneous imbibition tests, zeta potential tests, and reactive transport model were performed. The laboratory experiments showed that optimum recovery is achieved by diluting the FW 40 times and using the same water in a shorter steam cycle. Permeability measurements before and after core flooding revealed that the higher the salinity, the more permeability decreased. Steam can enhance the performance of LS water by reducing precipitation, increasing permeability, and dissolving minerals, and in turn, increasing oil recovery. Flooding the core with both LS water and steam resulted in more water wetness in the sandstone cores. Comparing the cores that were flooded with only LS water with the cores flooded with both LS water and steam, lower water wetness was obtained for the cores that were flooded only with LS water. The results were deliberated concerning wettability change processes by LS Water and steam. The zeta potential tests confirmed that as the salinity decreases, the zeta potential decreases too. Also, measuring contact angle before and after core flooding resulted in more water wetness. Spontaneous imbibition tests were in line with all other tests. The reactive transport modeling confirmed the crucial role of diluting FW in decreasing the number of the most effective kaolinite edges Si O and Al O , which explains why lowering the salinity gives additional oil recovery.

Illite distribution and morphology explaining basinal variations in reservoir properties of Upper Jurassic sandstones, Danish North Sea

Abstract Growth of fibrous illite may destroy an otherwise promising sandstone reservoir, even sandstones characterized by microquartz coatings. Therefore detailed petrographical and diagenetic investigations were performed in order to understand the factors that control illite precipitation and other factors affecting the reservoir properties in the Upper Jurassic Heno Formation. The Heno Formation is encountered in large parts of the Danish Central Graben, where its lower part – the Gert Member – was mainly deposited in back-barrier environments and its upper part – the Ravn Member – was deposited in a shoreface environment. In total, 215 core samples from 15 wells were investigated petrographically by optical and scanning electron microscopy. This was supplemented by plug porosity and permeability measurements and X-ray diffraction of bulk and clay fraction. The investigations show, among other things, that microquartz coatings may help to preserve porosity and permeability, whereas the permeability is reduced when microquartz occurs combined with fibrous illite. Illite precipitation is largely governed by the amounts of K-feldspar present during burial. Consequently, the initial abundance of K-feldspar (in the sediment source area) and its alteration during and immediately after deposition, in particular during subaerial exposure (at sequence boundaries), has a major influence on the final reservoir properties of the sandstones. The risk of illite precipitation can largely be predicted from the available sediment source material, depositional environment and burial depth.

Impacts of petrophysical properties of sandstone reservoirs on their irreducible water saturation: Implication and prediction

Irreducible water saturation (Swir) is an important parameter in reservoir evaluation that can affect hydrocarbon saturation, permeability prediction models and estimation of original oil in - place. The Swir is usually obtained from capillary pressure data. In this study, routine and special core analyses for sandstone reservoirs in four wells were used to investigate the relationship between irreducible water saturation and the petrophysical properties of the reservoir rocks. The results indicated that the magnitude of Swir is controlled by a complex relationship of reservoir rock wettability, reservoir quality, clay content, clay mode of distribution, and capillary pressure. Reservoir rock wettability can obscure the effect of other factors in the case of oil–wet rocks. On the other hand, irreducible water saturation from published data were used together with the studied four wells data, providing 110 sandstone samples covering different geologic ages and facies, to examine the applicability of the traditional empirical models that were commonly used in Swir prediction as well as to introduce a new simple equation by which Swir can be predicted. The study indicated that the Timur equation gave the best applicable empirical model to predict Swir. The study indicated also that the flow zone indicator can slightly improve the prediction strength of the Swir. Using other parameters characterizing capillary pressure curves such as displacement pressure and Swanson parameter didn't improve the relationship prediction strength.

Insights on spatial variography and outcrop-subsurface relationship of sandstone reservoir properties: A case study from the Late Triassic Minjur Formation, central Saudi Arabia

Understanding the variography of outcrop properties may give a guide for predicting the subsurface reservoir quality, more significantly for the more stable and resistant reservoir rocks such as sandstones compared to the carbonates. This study was planned to investigate the lithofacies and reservoir properties of different sandstone lithofacies of the Late Triassic Minjur Formation outcrop, and attempted to develop a relationship between these aspects in outcrop and subsurface settings. Based on the field and petrographic analyses, sandstone lithofacies of Minjur Formation were categorized into five types upon their textures. These lithofacies were intensively investigated for porosity and permeability. Variography analyses were applied on composite outcrop lithofacies, and also on porosity and permeability data. A good relationship was observed between porosity and permeability, which coincides with the univariate statistics of each and confirmed by variography analysis. Combination of outcrop and subsurface reservoir properties showed an acceptable correlation between outcrop reservoir properties and data from central Saudi Arabia and Rub’ Al-Khali Basin in southern and southeastern Saudi Arabia. This correlation was further highlighted by the global sense of subsurface sandstone reservoir properties. In general, the current study reflects the possibility of sandstone outcrop data to act as a representative analog for its equivalent subsurface reservoir. This possibility, which is mainly controlled by the similarity in depositional setting, can be also attained for reservoir properties and quality. Obtaining more qualitative and quantitative data is always advantageous to get better and more robust correlation.

Diagenesis and reservoir properties of Middle Jurassic sandstones, Traill Ø, East Greenland: The influence of magmatism and faulting

The c. 500 m thick Middle Jurassic sandstones of the fluvial Bristol Elv and marine Pelion Formations of the East Greenland Basin are evaluated here in order to improve the understanding of the processes that influenced the diagenetic evolution. The study may help to predict the reservoir properties of sandstones affected by magmatism and faulting, both in general and specifically in undrilled areas on- and offshore East Greenland and, in the Vøring Basin on the Mid-Norwegian shelf. The study shows a variety of authigenic mineral phases dominated by quartz cement, carbonate cement, illite and iron-oxide. One of the clear differences between the two formations is the presence of early carbonate-cemented horizons in the marine sandstones; these horizons are inferred to reflect a primary concentration of biogenic clasts and fossil shells. Intense quartz cementation occurs primarily in the fluvial sandstones but the marine sandstones are also highly quartz-cemented. Two episodes of burial and uplift are recorded in the diagenetic sequence and widespread grain-crushing in coarse-grained intervals is believed to result from overpressure and subsequent compaction due to sudden pressure release along major faults. Maximum burial depths may only have been around 2000–2500 m. Cathodoluminescence analyses show that grain crushing was followed by intense quartz cementation. The quartz cement is to a great deal believed to have formed due to increased surface area from crushing of detrital quartz grains, creating fresh nucleation sites for the quartz. Cathodoluminescence investigations also show that only minor pressure dissolution has taken place between detrital quartz grains and that the ubiquitous quartz cementation displays several growth zones, and was thus in part the result of the introduction of silica-rich extra-formational fluids related to the flow of hot fluids along reactivated faults and increased heat flow and temperature due to magmatism. This interpretation is supported by fluid inclusion homogenization temperatures between 117 and 158 °C in quartz cements. In one of the two study areas, the development of macroscopic stylolites has significantly enhanced quartz cementation, probably in connection with thermal convection flow. As a result of the magmatic and fault-related quartz cementation and illitization, the reservoir quality of the sandstone formations deteriorated and changed drastically.

Flow Properties of Sandstone and Carbonate Rocks by X-Ray Computed Tomography

We report the use of X-ray micro-CT and numerical simulations for studying the reservoir properties of sandstone and carbonate rocks in the Melekess depression of the Volga-Ural anteclise, Russia. To simulate the flow of a viscous fluid in the pore scale, we use a mathematical model which consists of the continuity equation, stationary Navier-Stokes equations and Darcy law. As a result of computational micro-scale experiments on digital three-dimensional images of sandstone, we get satisfactory compatible permeability coefficients, which are calculated using Navier-Stokes and Kozeny-Carman equations. Carbonates with fracture porosity reveals significant variations in permeability coefficients, calculated by the two methods. The Kozeny-Carman equation can describe the flow properties of carbonates with a predominantly matrix porosity. It is shown that a pronounced heterogeneity of the pore space leads to the representative elementary volume for permeability coefficient in carbonates several times larger than in sandstones.

Deformation band development as a function of intrinsic host-rock properties in Triassic Sherwood Sandstone

Abstract Deformation bands significantly alter the local petrophysical properties of sandstone reservoirs, although it is not known how the intrinsically variable characteristics of sandstones (e.g. grain size, sorting and mineralogy) influence the nature and distribution of deformation bands. To address this, cataclastic deformation bands within fine- and coarse-grained Triassic Sherwood Sandstone at Thurstaston, UK were analysed, for the first time, using a suite of petrographical techniques, outcrop studies, helium porosimetry and image analysis. Deformation bands are more abundant in the coarse-grained sandstone than in the underlying fine-grained sandstone. North- and south-dipping conjugate sets of cataclastic bands in the coarse-grained sandstone broadly increase in density (defined by number/m 2 ) when approaching faults. Microstructural analysis revealed that primary grain size controls deformation band density. Deformation bands in both coarse and fine sandstones led to significantly reduced porosity, and so can represent barriers or baffles to lateral fluid flow. Microstructural data show preferential cataclasis of K-feldspar grains within the host rock and deformation band. The study is of direct relevance to the prediction of reservoir quality in several petroleum-bearing Lower Triassic reservoirs in the near offshore, as deformation band development occurred prior to Carboniferous source-rock maturation and petroleum migration.

Strain localization in sandstone and its implications for CO2 storage

Geological storage of CO2 is a key technical solution to the climate-energy challenge, but it has a number of technological constraints (Baines and Worden 2004; Halland et al., 2011), broadly under the themes of assuring adequate storage capacity and long-term storage integrity. A suitable CO2-storage reservoir should consist of rock formations with sufficient porosity, permeability and connectivity in order to provide an adequate storage volume. The role of faults and their associated deformation structures (such as deformation bands and fractures) in controlling both storage capacity and long-term storage integrity is thus a key factor in achieving globally significant CO2 storage (Figure 1). Although some sedimentary basins on the Norwegian continental shelf already harbour operational CO2- injection and storage projects such as Sleipner (Zweigel et al., 2004) and Snohvit (Hansen et al., 2013), our understanding of reservoir fluid communication due to compartmentalization is far from complete and will be important for further use of the offshore basins for CO2 storage. In addition to the inherited structural features, elevated injection pressures may cause hydraulic fractures or stimulate fault reactivation which both point to the need to characterize the geomechanical response of the rock system to CO2 injection (Rutqvist, 2012; Iding and Ringrose, 2010). In the present work, we investigate the effects of faults and their related structures on the geomechanical and petrophysical properties of sandstone reservoirs. Important components of fault systems include fractures and deformation bands in the damage zone and fault core (Caine et al., 1996; Shipton and Cowie, 2003; Fossen et al., 2007). Fault systems may enhance or suppress fluid communication, which in turn may affect the storage capacity and conductivity of the candidate reservoirs (Figure 1). As a case study, a reservoir model of the Tubaen Formation at the Snohvit CO2 injection site in the Barents Sea (Grude et al., 2013; Hansen et al., 2013) was investigated using 4D seismic data and fault attribute analysis. The characteristics of deformation structures (e.g. sub-seismic faults, deformation bands and fractures) were investigated by field studies of outcrop analogues and by triaxial laboratory experiments to provide a basis for numerical modelling. Fault architecture within reactivated fault systems was studied by the use of analogue modelling. Key questions addressed in the work include: a) Where and when might strain localize in the reservoir? b) How does rock strain influence fluid communication? c) How might structural architecture affect CO2 storage effectiveness?

Depositional and Diagenetic Controls on Sandstone Reservoirs with Low Porosity and Low Permeability in the Eastern Sulige Gas Field, China

In order to determine the genesis and the factors that control the low-porosity and low-permeability sandstone reservoirs in the eastern Sulige Gas Field in the Ordos Basin, systematic studies on the sedimentary facies and diagenesis were conducted by means of analysis of cores, thin sections, fluid inclusions, X-ray diffraction, cathode luminescence and scanning electron microscope. It was found that the sand bodies of the major gas reservoirs in the Shan1 section (P1S1) and the He8 section (P2H8) were formed during the Permian as sedimentary facies such as braided-channel bars, braided-river channels and point bars of a meandering river. Four types of diagenetic facies developed subsequently: in order from the best to the poorest properties these are type A (weak compaction, early calcite cement-chlorite film facies), type B (moderate compaction, quartz overgrowth-feldspar corrosion-kaolinite filling facies), type C (strong compaction, late calcite cement-quartz corrosion facies) and type D (matrix filling and strong compaction facies). This diagenesis is undoubtedly the main reason for the poor reservoir properties of sandstone reservoirs, but the sedimentary facies are the underlying factors that greatly affect the diagenesis and thus the reservoir performance. Favorable diagenetic facies developed mainly in relatively small lithofacies such as braided-river channels, channel bars and point bars. The vertical distribution of the physical properties and the diagenetic facies of the reservoirs are related to the stratigraphic succession. Most of the sandstones between mudstones and thin beds of sandstone are unfavorable diagenetic facies. Analyses indicate that siliceous cementation can hardly be stopped by hydrocarbon filling. Authigenic chlorite could hardly protect the primary porosity. It not only occupies pore space, but also blocks pathways through sandstone reservoirs, so that it has significant influence on the permeability. Authigenic chlorite cannot be used as a marker for a specific sedimentary facies because it can be formed in different sedimentary facies, but it indicates high hydrodynamic conditions and presence of favorable reservoirs.

Factors controlling Oiliness of Jurassic Lianggaoshan tight sands in central Sichuan Basin, SW China

Abstract By comprehensively analyzing oil-bearing and non-oil-bearing tight sandstone reservoir properties, pore structure characteristics and contact between source rock and reservoir, reservoir fluorescent photographs of typical wells of Jurassic Lianggaoshan Formation in central Sichuan Basin, the research tries to find out factors controlling the oiliness of the Jurassic Lianggaoshan tight sandstone reservoir. The oiliness of the Jurassic Lianggaoshan tight sandstone reservoir in central Sichuan Basin is controlled by the following factors: (1) The migration pathway between the source rock and reservoir: when there exist sandstone and mudstone transition zones with high mud content and poor physical property between the source rock and reservoir, without fracture connection, it is difficult for oil and gas to migrate into the overlying reservoir; (2) The reservoir physical property: if the reservoir contacts directly with source rock and has good physical properties, oil and gas can migrate and accumulate directly in the reservoir. (3) Development degree of fractures: when the reservoir physical properties are poor, rich fractures can improve porosity and permeability of the reservoir, oil and gas can still migrate and accumulate in the reservoir; if the reservoir physical property is poor and no fractures developed, oil and gas are difficult to migrate and accumulate in the reservoir. Based on the sedimentary and reservoir characteristics of Jurassic Lianggaoshan Formation in central Sichuan Basin and the distribution of source rocks, Yingshan-Guang'an and Hechuan-Baimiao-Luodu two favorable exploration zones are selected.

Geostatistical relationships between mechanical and petrophysical properties of deformed sandstone

Petrophysical and mechanical properties of sandstone reservoirs are likely to change as a result of faulting. In this paper, we investigate the distribution of deformation features (structures) such as fractures and deformation bands in the Navajo and the Entrada sandstones in the fault core and damage zones of two faults in two localities in southeast (Cache Valley) and central (San Rafael Swell) Utah. These two localities had different degree of calcite cementation and hence are of interest to study the mechanical and petrophysical properties of these localities, in order to find out the impact of cementation on these properties and their possible relations. We have performed in-situ measurements by Tiny-Perm II and Schmidt hammer to examine the distribution of permeability and strength/elasticity of rock within the damage zone of these faults. We have studied the statistical relation between (i) Tiny-Perm II measurements and Schmidt hammer values, (ii) permeability and uniaxial compressive strength, and (iii) permeability and Young's modulus of deformed rocks. The statistical results demonstrate that there are correlations between the studied parameters, but the dependencies vary with the degree of calcite cementation in mineralogically similar sandstones (quartz sandstone). Statistical results demonstrate to first approximation that an exponential law is more suitable for description of the relations (i), (ii) and (iii) of non-cemented Navajo sandstone whereas for cemented Navajo sandstone these relations are better approximated by power law.

Diagenesis and quartz cement distribution of low-permeability Upper Triassic–Middle Jurassic reservoir sandstones, Longyearbyen CO2 lab well site in Svalbard, Norway

Reservoir properties of Upper Triassic–Middle Jurassic sandstones, Spitsbergen, are studied as part of a CO2 storage pilot project in Longyearbyen. The reservoir formations show large contrasts in sandstone compositions, with unexpected low permeability despite moderate porosity values. Petrographic analyses were performed to investigate the influence and distribution of diagenesis. It is concluded that, because of various compaction, cementation, and dissolution processes, the sandstone porosity is mainly isolated molds and micropores and associated with fibrous illite and chamosite, explaining the low permeability. Diagenesis and the distribution of quartz cement is influenced by lithofacies and detrital compositions. Mineralogically immature sandstones (De Geerdalen Formation) show a homogeneous distribution of quartz cement overgrowths on quartz grains, distributed interstitial to labile grains and other cements (e.g., late calcite). The main silica source was from the dissolution of adjacent feldspar and labile grains as part of the chemical compaction. In contrast, quartz-dominated sandstones (Knorringfjellet Formation) show a heterogeneous patchy distribution of quartz cement influenced by the sedimentary bioturbation pattern, with silica sourced also from dissolution at clay-rich microstylolites. Phosphatic beds at the base and top of the formation are strongly influenced by marine eogenesis and reworking processes and associated with concentration of iron-rich authigenic minerals. The highest porosity appears in sand-supported conglomerate where moldic clay-mineral ooids contributed to reduce quartz cementation. The stratigraphic change from mineralogical immature (Triassic) to mature (uppermost Triassic–Jurassic) sandstone compositions is detected in wide areas of the Barents Shelf and has considerable implications for the distribution of sandstone reservoir properties.

Identification of waterflooded zones and the impact of waterflooding on reservoir properties of the Funing Formation in the Subei Basin, China

This paper describes the identification of waterflooded zones and the impact of waterflooding on reservoir properties of sandstones of the Funing Formation at the Gao 6 Fault-block of the Gaoji Oilfield, in the Subei Basin, east China. This work presents a new approach based on a back-propagation neural network using well log data to train the network, and then generating a cross-plot plate to identify waterflooded zones. A neural network was designed and trained, and the results show that the new method is better than traditional methods. For a comparative study, two representative wells at the Gao 6 Fault-block were chosen for analysis: one from a waterflooded zone, and the other from a zone without waterflooding. Results from this analysis were used to develop a better understanding of the impact of waterflooding on reservoir properties. A range of changes are shown to have taken place in the waterflooded zone, including changes in microscopic pore structure, fluids, and minerals.

Comparative analysis of facies and reservoir characteristics of Miri Formation (Miri) and Nyalau Formation (Bintulu), Sarawak

This study on the sedimentological and reservoir petrophysical properties (porosity, permeability, density, sonic velocity) of sandstones were conducted on the sedimentary rocks belonging to the Miri Formation (Middle Miocene) from Miri and Nyalau Formation (Middle Miocene) from Bintulu in Sarawak. The objectives are i) to investigate and determine the facies characteristics and reservoir properties of the different types of sandstones, and ii) to establish the relationships between the facies characteristics and petrophysical properties. Two lithofacies have been sampled from the outcrops of Nyalau Formation and four lithofacies from the outcrops of Miri Formation based on lithology, sedimentary structures, fossil traces and bed geometry. These are grouped into four major lithofacies: (i) hummocky cross-stratified sandstones (HCS); (ii) trough cross-bedding sandstones (TCB); (iii) bioturbated sandstones; and (iv) swaley cross-stratified sandstones (SCS). Hummocky and swaley cross-stratified sandstones, trough cross-bedded sandstones and some bioturbated sandstones of Miri formation recorded the highest poro-perm values, with relatively lower density values (as compared Bintulu samples). The TCB sandstones are well sorted sandstones, with minimal mud content. This has contributed to the high poro-perm values. The bioturbated swaley cross-stratified sandstone (MF4) shows high permeabilities in some samples and low permeabilities in others. This reflects the heterogeneity in facies characteristics and poro-perm distribution of bioturbated samples. Bioturbated HCS, the HCS and TCB sandstones of Bintulu show moderate poro-perm values. The Nyalau Formation rocks are older, and thus are expected to have undergone greater compaction than the younger Miri Formation. This is also reflected in the higher density values recorded for all the Bintulu samples. INtrodu Ct Io N In general, facies characteristics determine the overall reservoir properties of sandstones, while diagenetic changes impose a secondary control on their poro-perm characteristics. The purpose of studying the reservoir sedimentological characteristics and petrophysical properties of Tertiary sandstones from Sarawak is to investigate and determine the facies characteristics, and reservoir properties of the different types of sandstones, and to establish the relationships between the facies characteristics and petrophysical properties. This study on the sedimentological and reservoir petrophysical properties (porosity, permeability, density, sonic velocity) of sandstones were conducted on the sedimentary rocks belonging to the Miri Formation (Middle Miocene) from Miri and Nyalau Formation (Middle Miocene) from Bintulu in Sarawak.

Laboratory Investigation of Effective Stresses' Influence on Petrophysical Properties of Sandstone Reservoirs During Depletion

Abstract An increase in effective stresses takes place in reservoirs as a consequence of fluid production, a well-known phenomenon in both shallow and deep reservoirs. It may seem reasonable to assume that permeability and porosity decrease as pore pressure declines, since both effective radial and axial stresses become intensified during reservoir depletion. However, laboratory results show that this is not always the case. Porosity certainly decreases as a result of the compaction process, which allows the breakage of grain-to-grain cement bonds. Grain particles will become more compacted as both lateral and axial effective stresses increase. On the other hand, permeability shows no definite trend. In this paper, a series of delicate experimental procedures were conducted to reveal some of the most intriguing phenomena in pore collapse and their impact on permeability. Sandstone samples were tested using a triaxial set-up. Based on the experimental results of this study, in weak reservoir formations, pore collapse does not occur suddenly. Rather, rocks gradually compact as grain-to-grain cement bonds break down. It was found that permeability indeed changes as effective stresses increase. However, the pathway to permeability was found to be much more complex than previously stipulated. It was discovered that enhancement or damage to permeability is not a function of pore collapse alone. Other factors, such as stress path, initial porosity, particle size, particle shape and particle distribution play a major role in determining what type of alteration in permeability occurs and to what magnitude that alteration would be. Introduction Traditionally, petrophysical properties data are very crucial for reserves assessment and fluid flow characterization of petroleum reservoirs. A great deal of money and effort are usually allocated in order to accurately estimate these properties. Permeability and porosity are among those properties and are by far the most important. Porosity is the key for reserves estimates while permeability is the main parameter to predict flow rates, design drawdown and, therefore, wellbore completion. Until recently, the common belief was that, once determined, these properties remain constant throughout the production life of the reservoir. Studies(1–3), including this paper, showed that this is not a realistic assumption. During the pressure depletion process, as production from the reservoir continues, effective stresses within the reservoir increase. It is understood that the effect of reservoir stresses on porosity and permeability of the reservoir is more severe when porosity and permeability are high, although some experimental studies like Hubbert and Willis(4), Voight and St. Pierre(5) and Rosepiler(6) showed this effect is still significant, even at low porosity and permeability. It is also understood that stress paths have a large influence on horizontal and vertical permeability, and also on porosity. The elastic uniaxial strain model is mostly used in reservoir engineering to describe production-induced changes in horizontal stress due to pore pressure decline (pressure depletion). It predicts the total horizontal stress by using overburden stress, reservoir pressure decrease and material mechanical parameters. The principal assumption in this model is that there is no lateral deformation (zero horizontal strain condition) during the depletion process.

Diagenesis influencing the porosity of Upper Jurassic reservoir sandstones, Danish North Sea

Upper Jurassic quartz-rich sandstones in the North Sea Basin are important reservoir rocks for oil and gas, and one of the latest discoveries of oil in the Danish sector was made in the area of the Hejre wells that penetrated such sediments (Fig. 1). The reservoir properties of sandstones are strongly influenced by diagenetic alteration, i.e. the mineralogical changes that take place during burial of the sediments. The diagenetic features depend on the source area, depositional setting, facies architecture and burial history of the sediment. The major diagenetic features influencing porosity in Upper Jurassic reservoir sandstones are feldspar dissolution and precipitation, precipitation of illite, calcite and quartz, and quartz stylolite formation. With regard to the Upper Jurassic sandstones in the Danish sector of the North Sea, the important question is: how can porosity be preserved in sediments buried at depths of more than 5 km? The Hejre-2 well penetrated the Upper Jurassic sediments (Fig. 2) before reaching pre-Upper Jurassic volcaniclastic conglomerates. The diagenetic features were studied in thin sections of core samples with traditional petrographic techniques using transmitted light microscopy supplemented by scanning electron microscopy (SEM) of rock chips and thin sections.