Formation Of Secondary Porosity Research Articles

Using iron speciation to track hydrocarbon migration in bleached rocks and its implications for reservoir reconstruction

Bleaching of red rock beds by hydrocarbons can provide valuable insights for hydrocarbon migration and reservoir transformation. This study investigated the bleaching effects of hydrocarbons on various rock types in the Dushanzi mud volcanic area. Bedrock and bleached samples of different lithologies, including mudstone, sandy mudstone and sandstone, were systematically collected and analyzed for their iron species, mineralogical and chemical compositions using Mössbauer spectroscopy, X-ray diffraction and X-ray fluorescence, respectively. The results indicate that the iron species of the studied samples were dominated by para-Fe3+ (from 44% to 76%), with variable contents of mag-Fe3+, para-Fe2+ and pyr-Fe2+. When compared to the related unbleached rocks, the bleached rocks exhibited a higher proportion of reduced Fe (para-Fe2+ and pyr-Fe2+). The pyr-Fe2+ in the sandstones and sandy mudstones was significantly higher than that of the mudstone interlayers. The observed variations in iron species were consistent with changes in mineralogical and chemical compositions, which can be correlated with the petrological properties of rocks. The increase of quartz and decrease of feldspar and carbonates in the bleached rocks could be attributed to the dissolution of feldspar and the subsequent secondary enlargement of quartz. Geofluids containing crude oils and natural gases preferentially dissolved K-feldspar over albite. The bleached rocks had a significant decrease in the proportion of kaolinite and a slight decrease in the proportion of chlorite, while the proportions of smectite and illite were increased. The dissolution of K-feldspar could provide sufficient K+ for the formation of illite, and the appearance of smectite in the bleached samples might be related to the dissolution of albite in an alkaline environment. The dissolution of carbonates and feldspar, along with the transformation of clay minerals, had a notable positive effect on the formation of secondary porosity, facilitating hydrocarbon migration. These findings provide valuable insights into the interactions between hydrocarbons and different lithologies, informing the impact of hydrocarbon migration on reservoir reconstruction in similar geological settings.

Determination of swelling operation parameters to improve the hierarchy of natural zeolite Lampung after synthesis

The swelling method constitutes a component of the hierarchical synthesis step of Natural Zeolite Lampung (NZL). The literature review results indicate that NZL swelling is a pretreatment integral to the synthesis process of NZL. The synthesis of Lampung natural zeolite (NZL) was conducted to form hierarchical zeolites. NZL, as a hierarchical zeolite, is defined as a system containing additional pores. The formation of secondary porosity distinguishes it, reduction of alkali and alkaline earth metals, removal of impurities, transformation of symmetric to asymmetric strains, increase in acidity, and most importantly, the occurrence of dealumination and desilication processes as the cause of the achievement of these indicators. Generally, the initial post-synthesis modification sequence, which typically involves acid and base treatment, is carried out separately. In this study, however, the dealumination and desilication modifications will be carried out simultaneously in a single process, namely by the swelling method. Consequently, it is essential to ascertain the optimal operating parameters at the requisite atmospheric pressure, thus enabling the simultaneous dealumination and desilication processes within the NZL swelling method. In this case, the swelling method is a pretreatment analogous to the initial zeolite activation conducted in previous studies. The swelling NZL method can potentially overcome the laborious and expensive synthesis procedures that currently limit the scale-up of this material into large-scale production.

Impact and implications of hydrothermal fluids migration in the Frolov hydrocarbon province in West Siberia

Both the prolific source rocks of the Bazhenov Formation and the reservoir-prone units of the Abalak Formation cover most of the West Siberian region. Hydrothermal fluids circulation has been suggested to affect hydrocarbon potential and reservoir properties at some localities of this vast hydrocarbon province. However, integrated studies are currently lacking. Here we present an extensive multidisciplinary study combining regional studies with core logging properties and sample analyses (i.e. geochemistry, petrography, organic geochemistry) to reveal processes related to the hydrothermal activity that affected both formations. We also focus on the factors and implications for secondary unconventional reservoir formation within the deposits from the Kamennaya summit of the Frolov oil and gas region. Three boreholes corresponding to type sections located in different parts of the study area (elevated, transitional and deep) were investigated. The elevated and transitional parts of the basin are adjacent to the weakened contact zones of a granite massif. These tectonic discontinuities acted as conductors for hydrothermal fluids circulation on the area. Lithological and stable isotope studies revealed the presence of intensive hydrothermal secondary alterations in various types in rocks, including massive metasomatic carbonatization, sulfatization, sulfide mineralization and leaching processes. This hydrothermal fluid activity resulted in reservoir formation within siliceous radiolarite layers, while in the deep part of the basin potential reservoirs were completely healed. Organic matter maturity varies in different parts of the basin. We identify several mechanisms related to the various hydrothermal processes that influenced rocks composition, structure and organic matter maturity: (1) A weakened zone located on the periphery of the granite massif in the basin basement is characterized by extensive fault damage zones facilitating the migration of high temperature fluids along deposits during post-sedimentation stages; (2) The thickness, mineral composition of the rocks overlying the basement control fault attenuation and fluid workflow; (3) The fluids composition, their acidity/alkalinity, prompted various changes in rock structure, especially in secondary porosity formation. The identification of various hydrothermal processes and their influence on pore space allowed to determine the approach for the spatial distribution prediction of unconventional reservoirs in the studied formations, and subsequently improve the efficiency and effectivity of the exploration process not only at the Kamennaya summit, but also on the other similar West Siberian basin areas.

Volcanoclastic and epiclastic diagenesis of sandstones associated with volcano-sedimentary deposits from the upper Jurassic, Lower cretaceous, Paraná Basin, southern Brazil

The opening of South Atlantic Ocean produced an immense volume of lava that covered an active aeolian system during Lower Cretaceous. The contact of Botucatu Formation sandstones with the Serra Geral volcanic flows generated a variety of volcano-sedimentary features and deposits. During volcanism, the temperature contrast between lava and wet or water-saturated sediments formed volcanoclastic features and related sedimentary deposits. In moments of magmatic quiescence, epiclastic deposits were generated due to the return of sedimentation. Volcanoclastic deposits include volcanic breccias with sandy matrix, and epiclastic deposits include conglomerates and conglomeratic sandstones with intraclasts. Quantitative petrography of the sandy matrix samples of these deposits was analyzed, in order to discuss the paragenesis of volcano-sedimentary interactions. Regular burial diagenetic processes were discussed for epiclastic sandstones and contact diagenesis processes for volcanoclastic sandstones. The original composition of volcanoclastic and epiclastic sandstones is heterogeneous. However, the main difference between them is the presence of textural fluidization indicators found in the volcanoclastic sandstones, along the lava-sediment interface. These textural characteristics indicate that the unconsolidated sediments were probably wet or saturated in water during the interaction with the volcanic flow. Epiclastic sandstones have infiltrated clays and neoformed smectites as their main diagenetic constituent due to alteration of available volcanic lithoclasts. For volcanoclastic sandstones, the formation of opal, chalcedony and megaquartz cements through burial is unlikely, due to the intergranular volume occupied by these cements, indicating near-surface early diagenetic conditions. Therefore, the mechanisms of dissolution and repreciptation of the siliceous fluids, incorporated in the system, formed mainly the chalcedony cementation. Furthermore, zeolite cementation, which occurs along the lava-sediment contact, is another important indicator of contact diagenesis for volcanoclastic sandstones. During telodiagenesis, the formation of secondary porosity by dissolution of volcanic lithoclasts and detrital K-feldspar grains was of great importance in volcanoclastic and epiclastic sandstones.

Meteoric freshwater leaching and its significance to reservoir quality in a buried hill of lower-middle Jurassic fluvial sandstones: A case study from the Huanghua Depression, Bohai Bay Basin, China

Within polycyclic sedimentary basins, meteoric freshwater leaching plays an important role in the formation of secondary porosity. Whereas, when superimposed by other late diagenesis processes, it is often difficult to identify its diagenetic indicators. The Jurassic clastic reservoirs in the “Wmy buried hill” of the Huanghua Depression testify from such meteoric freshwater diagenesis. The “buried hill” is named after a village called “Wmy” and nowadays buried deeply into a Paleogene lacustrine sag, which displays like a highland or hill. These reservoir sandstones were studied based on a variety of approaches including 3-D seismic interpretation, core and thin section observations, X-ray fluorescence analysis, Electron Probe Microanalysis and stable isotope analysis, combined with reconstruction of the burial and thermal histories. At last, the evolutionary model of reservoir quality was addressed. Difference of sedimentary fabrics in sand bodies leads to the heterogeneity of pores distribution, making the permeability in coarse-grained and fine-grained sandstones largely different. The meteoric water leaching caused the loss of Fe and Ti, leading to a zonation of core colors with white and red zones. In addition, the dissolution by meteoric water of biotite and volcanic fragments as well as feldspar grains produced large amounts of secondary porosity without formation of diagenetic by-products, which thus points to an open diagenetic system. The calcite cement of meteoric freshwater origin displays relatively low Fe and high Mn content exhibiting a low Fe/Mn ratio. The origin of Mn is probably dissolution of volcanic fragments. The evolutionary process of reservoir quality can be divided into four stages. The second stage is the crucial period for formation of secondary porosity whilst faults derived from the compressional stress during tectonic uplif associate coal-bearing source rocks with Jurassic reservoirs. Moreover, the last stage is the critical period for petroleum and/or gas migration and accumulation because the underlying coal-measures entered the hydrocarbon generation window and overlying Paleogene sealing rocks were intensely compacted. Scale of potential reservoirs in the whole well profile are evaluated by statistical methods, the total thickness of which can reach about 25 m. Reservoirs close to the unconformity surface between the Mesozoic and Cenozoic display more effective porosity than the deeper ones, which should be the target for the subsequent hydrocarbon exploration.

Classification and Diagenetic Characteristics of the Cretaceous Sandstones in the Southern Bredasdorp Basin, Offshore South Africa

AbstractA systematic petrographic and geochemical studies of 92 representative sandstone samples from exploration wells E‐AH1, E‐AJ1, E‐BA1, E‐BB1 and E‐D3 in the southern part of the Bredasdorp Basin was undertaken to classify the sandstones as well as unravel the main diagenetic processes and their time relations. Petrographic study shows that the sandstones are largely subarkosic arenite and arkosic litharenite, which have underwent series of diagenetic processes as a result burial, rifting and subsequent uplift. The main diagenetic processes that have affected the reservoir properties of the sandstones are cementation by authigenic clay, carbonate and silica, growth of authigenic glauconite, dissolution of minerals and load compaction. The major diagenetic processes reducing the porosity are calcite cementation in the subarkosic arenite, and compaction and quartz cementation in arkosic litharenite. On the other hand, the formation of secondary porosity due to the partial to complete dissolution of early calcite cement, feldspars and minor grain fracturing has improved the reservoir property of the sandstone to some extent. The clay minerals in the sandstones commonly acts as pore choking cement, which reduces porosity. In general, there is no particular diagenetic process that exclusively controls the type or form of porosity evolution in the sandstones.

Reservoir geology of the Berea Sandstone (uppermost Devonian), eastern Kentucky

Berea reservoirs in eastern Kentucky are developed in tight siltstone and sandstone. Precision horizontal drilling provides access to thin ( 0.1 md, porosity is generally >10%. Cement in Berea samples includes quartz overgrowths, ferroan dolomite, siderite, pyrite, and kaolinite. In addition, these rocks were subject to varying degrees of intergranular compaction, quartz overgrowths, and secondary porosity formation (grain dissolution). In thin section, three pore types were encountered: (1) primary intergranular pores, (2) secondary moldic pores, and (3) microporosity. Intergranular porosity is most common. Mercury capillary injection tests show that moderate-permeability (0.02–0.2 md) Berea has pore size distributions of 1 to less than 0.1 μm, but mostly from 0.5 to 0.6 μm, which are commonly found in gas reservoirs. Higher-permeability samples (1.2–2.0 md) have pore-throat diameter distributions of 2 to >0.1 μm. The highest pore volumes (near 3.25%) have pore diameters of 1–2 μm, which is more characteristic of oil-producing reservoirs.

МОДЕЛИРОВАНИЕ ФОРМИРОВАНИЯ КОЛЛЕКТОРОВ В КРЕМНИСТЫХ ИЗВЕСТНЯКАХ ПОД ВОЗДЕЙСТВИЕМ ГИДРОТЕРМАЛЬНЫХ ФЛЮИДОВ (СЕРИИ С Н2О2, НСL И СОЛЯМИ)

The results of modeling the formation of secondary reservoirs (hydrothermal silicites) in siliceous limestones with a wide areal distribution in the pre-Jurassic complex of Western Siberia, exposed to hydrothermal fluids in the H2O-H2O2, H2O-HCl and H2O-salt systems are considered. Experiments with the hydrogen peroxide were carried out at temperatures of 295 and 390°C and H2O2 concentrations from 29 to 43%. Experiments with hydrochloric acid were carried out at a temperature of 410°C, and its concentration varied from 0.05 to 1.0N. Experiments with salts were carried out at a temperature of 400°C, CO2 concentration of 0.48 g / l and a salt content of 36.8 g / l NaCl and 3.1 g / l CaCl2. It was found that in the first system (Н2О-Н2О2), metals that compose the autoclave take an active part in reactions of hydrothermal synthesis, that, to a certain extent, complicates the study of resulting mineral associations. Hematite, anhydrite, native sulfur and sulphates, as well as chromium and nickel oxides are the newly formed (hydrothermal) mineral phases in this system. Dolomite, pyrite and calcite are unstable initial minerals. Dolomite, calcite, and pyrite are also unstable minerals in the water-hydrochloric acid system, and the formation of new (hydrothermal) associations depends on the acidity of the fluid: the stable association is the kaolinite-siderite one under ultra-acidic conditions, and chlorite is formed in more alkaline conditions. Acid feldspar, zeolite, wollastonite and sodium-calcium carbonates are formed in the water-salt system. Pyrite, dolomite, calcite, illite and partially biogenic quartz are decomposed. In all experiments, the formation of secondary porosity was observed due to the dissolution of unstable minerals. It is determined that forming secondary pores are very small in size, which leads to a high content of residual water in these secondary reservoirs. The main regularities of the secondary reservoirs formation – hydrothermal silicites - are revealed by Paleozoic siliceous limestones.

About a Role of Microorganisms in Destruction of Rock Structure of An Oil Reservoir

Microorganisms as the biotic factor are capable to participate in processes leaching of a rock of an oil layer directly, and also by secretion of various agents leaching -gases, acids, spirits, slime, etc. The basic mechanism leaching of a rock is formation of chelates by microbial agents with minerals of a rock. The order of carrying out of elements from minerals of rock of a class alumosilicates basically is defined by structure and property of the mineral. The elements making cristallic a lattice of a mineral will least intensively be mobilized; isomorphically replaced or exchange ions which are taking place on peripheral sites of mineral particles will most intensively be mobilized . Knowledge of physical and chemical features of a collector - the nature and a cementing mineral of a rock, his maintenance in a rock, type of cement (bazalic or pore type etc.) and other parameters, in the certain degree allow to predict probability of shaking of structure of a rock of a collector and speed of his destruction during technological influence. At selective leaching minerals of collector properties of rock can be modified both aside deterioration, and aside improvements of filtrational properties of pore spaces of a reservoir. Deterioration of collector properties can be shown, for example, in display sand recovery to wells-bottom (most likely in the first in sandy collectors with the small maintenance of a cementing material of montmorillonite type). In collectors with the high maintenance of a cementing material (bazalic or pore type of cement) leaching can promote substantial improvement of collector properties in connection with increase of permeability due to formation of secondary porosity.

Modeling the hydrothermal impact on the Jurassic deposits of the West Siberian sedimentary basin (series with H2O and H2O2)

The results of hydrothermal modeling of the formation of epigenetic mineral associations and secondary reservoirs in various types of sedimentary rocks of the Jurassic age in Western Siberia in the H2O-H2O2 system are considered. The experiments were carried out at a temperature of 390°C, a pressure of 70 MPa and H2O2 concentrations varying from 0 to 37.5%. The duration of the experiments was 10 days. In all experiments, a pyrite mixture was added as a buffer. It was found that in experiments in which there was no H2O2, that is, the hydrothermal fluid consisted only of water, there was an intense formation of bitumen and volatile components from the organic matter present in the initial samples. The process of generation of bitumen and volatile components was accompanied by fluid fracturing of rocks and the formation of secondary porosity. Under these conditions, carbonates (calcite, siderite), plagioclases, and kaolinite turned out to be unstable. On the contrary, the formation of hydrothermal ferruginous chlorite was noted. Dissolution of unstable mineral phases by hydrothermal fluid led to the formation of additional secondary porosity. As the concentration of H2O2 and, accordingly, oxygen in the composition of the hydrothermal fluid increased, more and more complete oxidation of the organic matter present in the composition of the sediments was noted until its complete disappearance. With an increase in the oxygen concentration in the composition of the hydrothermal fluid, the appearance of first magnetite and then hematite, native sulfur, and anhydrite was observed. Under hydrothermal conditions with a maximum oxygen concentration, the formation of kaolinite and various sulfates in addition to anhydrite, represented by millosevichite, alum, as well as phases with the participation of elements that make up autoclaves, nickel and chromium sulfates, was noted. Intensive silicification of the sample surface is noted, as well as the constant presence of amorphous silica, which, along with various sulfates, is a quenching phase. These mineral phases are separated from the supersaturated hydrothermal fluid due to its rapid cooling and do not have time to form well-faceted crystals, but are present in the form of loose microporous aggregates. Along with kaolinite, the formation of hydrothermal film illite is noted. The results obtained indicate that the presence of bitumen in the Jurassic sediments is an important exploratory feature, indicating that in this place they were subjected to hydrothermal action, which means, firstly, secondary reservoirs with high filtration and capacity properties were formed, and second, there was an intensive generation of hydrocarbons and, as a consequence, the formation of hydrocarbon deposits.

DISTRIBUTION OF CARBONATE CEMENTS IN THE SANDSTONES OF THE SOUTHEAST OF THE DNIEPER-DONETSK DEPRESSION IN THE ASPECT OF FORMING SECONDARY POROSITY IN THEM AT HIGH DEPTHS

The main problem when searching for oil and gas at great depths is the lack of an economic reservoir. Since great depths correspond to high degrees of catagenetic transformations, the primary porosity of most rocks is reduced and all hopes are associated either with the relict primary porosity, the value of which progressively decreases with depth, or with secondary porosity. There are different views on the formation of secondary porosity in sandstones - the main reservoirs of the Dnieper-Donetsk Depression (DDD). Some of these views are briefly discussed in this paper. In all cases, carbonate cement plays an important role as its dissolution under the influence of various factors forms secondary porosity. The question arises as to how often sandstones with carbonate cement are found among the differences with a different cementing material, at what depths, in the sediments of what ages, are there many sandstones in which the formation of secondary porosity is possible by one or another mechanism. The aim of this work is to quantitatively assess the prevalence of carbonate sandstones among the sandy rocks of the southeastern part of the DDD, clarify the distribution of carbonate sandstones in depth and over the main stratigraphic horizons, and preliminary estimate the content of various carbonates in the cement of sandstones according to field and standard laboratory studies of core material. In the course of the work, data from more than 10 thousand samples were used, taken at various fields of the northern side and the northern near-zone zone of the southeastern part of the Far East region, approximately from the Belskoye field in the north-west to Kruzhilovskoye in the southeast inclusive. The sampling depth intervals for the samples used vary from 1 to 6 km, the age of the sediments they characterize is from Triassic to Devonian. The results obtained show that the content of carbonate in sandstone cement at great depths is high enough for its movement to cause the formation of secondary porosity on an industrially significant scale, which, in turn, provides good prospects for prospecting hydrocarbon deposits at great depths.

Sedimentological analysis and reservoir quality of the early Cenomanian transgressive sandstones, northern Western desert, Egypt

Depicting depositional models of transgressive sandstone reservoirs is challenging due to their internal complexity and limited case studies. This study presents an example for transgressive sandstone reservoirs from the Bahariya Formation at Hayat-Yasser oil field, Shushan Basin (northern Western Desert, Egypt). The upper part of the Bahariya Formation accumulated in an estuarine setting as a consequence of a successive sea level rise. Detailed reservoir characterization, employing sequence stratigraphic and sedimentological approaches, revealed that the studied reservoir consists of a mixed siliciclastic-carbonate transgressive estuarine-shelf succession. Four conventional core samples of this succession at Khalda ridge (Shushan Basin), permit the analysis of their sedimentology and internal architecture (mesoscopic heterogeneities) along correlation lines parallel or slightly oblique to depositional dip. The succession consists of six backstepping parasequence sets that record the overall shift from tidal-dominated to wave-dominated estuaries/bays. The transgressive component of the parasequences sets includes bay mudstones, flood-tidal deltas, and shoreface/barrier deposits; while the regressive component includes estuarine point bars, bay head deltas, and tidal deltas. The reservoir properties show significant variation according to the facies assemblages. The tidally-dominated estuarine channels show low net-to-gross, as it is characterized by lenticular sandstone bodies with several lateral and vertical baffles and barriers. On the other hand, the shoreface/barrier and tidal delta sandstones show high net-to-gross and consist of amalgamated sandstone bodies with little mud barriers or baffles. Petrographically, the upper Bahariya reservoir includes intergranular and secondary porosities. The microscopic heterogeneities are mainly due to the fine grain nature, poor sorting, presence of matrix, and continuous mud barriers or baffles. Destruction of porosity, affecting the reservoir quality, occurred by silica overgrowths, ferroan calcite, grain coating chlorite, and kaolinite cementation, as well as compaction. Feldspar dissolution is the main diagenetic process responsible for the formation of secondary porosity, enhancing the reservoir quality.

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.

On the question of creation of secondary porousity in large-depth sediments of the south-east of the Dneprovsk-Donetsk depression

Introduction. Currently, most researchers associate the main prospects of oil and gas potential with great depths. The most important component of the problem of oil and gas potential at great depths is the problem of reservoir availability. This problem still has no clear solution. From the point of view of the classical theory of stadial catagenesis, with depth, the primary porosity naturally and irreversibly decreases, and we can only talk about how quickly this happens. In this case, large depths appear to be a zone of fading oil and gas potential without any special prospects. The theory of superimposed catagenesis comes to the rescue, which shows that there are processes that naturally lead to the formation of secondary porosity, a secondary reservoir, and then large depths represent a special deep zone of gas accumulation. According to the views of many researchers, including the authors of this work, quartz sandstones with regenerative quartz cement – quartzite sandstones - are the most promising in terms of reservoir properties at great depths. The aim of the work is to establish the regularities of compaction of quartzite sandstones with depth, to identify anomalies in the compaction process and to try to interpret them using mineral indicators of hypogenic-allogeneic porosity formation – one of the varieties of superimposed catagenesis of great depths. Previous study. The theory of metasomatic reservoir formation as a result of hypogenic-allogeneic porosity formation is one of the most developed. It is based on the penetration of a certain favorable composition of deep aggressive fluids into the reservoir during the period of tectonic-magmatic activation with leaching of some components and deposition of others. It is assumed that this process is accompanied by the crystallization of certain indicator minerals. Result of work. The graphs constructed in the course of this work reflect both a natural change in the porosity of this type of Sandstone with depth, and show an anomaly associated with some superimposed process that allows for different explanations. It is natural to use indicator minerals to solve the problem, but the analysis shows that specific indicator minerals of hypogenic-allogeneic decompression have not yet been found, and all the minerals involved can be interpreted as minerals of one of the stages of stadial catagenesis. This does not disprove the possibility of the formation of a secondary metasomatic reservoir by the above mechanism, but it is not possible to prove its implementation by the indicator minerals noted in the studied samples. Relevance. The question of the formation of secondary porosity is of great practical importance for assessing the prospects of gas content: either large depths are represented by the area of naturally decreasing gas accumulation, or the area of naturally occurring secondary reservoirs with good oil and gas prospects, which can be distinguished as a separate deep zone with its own specific power sources, reservoirs and cap rocks.

Characteristics of organic acids in lacustrine organic-rich shale, Ordos Basin, China

Organic acids are critical to studying the formation of secondary porosity in tight lacustrine reservoirs, Ordos Basin. The expelled fluids produced through hydrocarbon evolution in a semi-closed system under different pressure and temperature settings (16.9 MPa, 250 °C; 22.1 MPa, 300 °C; 32.5 MPa, 350 °C; 37.7 MPa, 370 °C; 42.9 MPa, 400 °C; and 52.2 MPa, 450 °C) were collected. In this study, 12 different organic acids were identified using ion chromatography, and both the concentration and species of organic acids indicate a two-step process.

Secondary alterations of deep buried Lower Carboniferous terrigenous rocks in some areas of the Dnieper-Donets basin

Problematics. Due to the ambiguous understanding of the nature and void space distribution within reservoir rocks, prediction of prospective deep buried and low permeable reservoirs becomes more uncertain. There are many factors influencing on the pore space, such as primary rock composition, hydrogeological regime, tectonic processes, geological time, epigenetic processes etc. Moreover, in explanation of different factors the influence of secondary reservoirs formation, systematization problems become more essential. Either, it is important to identify ‘working’ conditions of influencing factors to make the prediction of hydrocarbon deposits at great depth more certain. Finding universal criteria of prospective secondary reservoir rocks identification will allow us to enhance the discovery rate of deep buried hydrocarbon deposits. Analysis of previous studies. Many researches were devoted to epigenetic processes in the second part of XXth century. There were described major secondary alterations which may occur at great depth. Recent publications showed the expansion of previous knowledge about epigenetic processes and their influence on the secondary deep buried reservoir formation. Some scientists state that major influencing factors on reservoir quality are tectonic and primary mineral composition, others bring epigenetic imposed processes to the top of influencing factors. Purpose. The purpose of the research is to study secondary alterations and find possible zonation based on the alteration intensity of Carboniferous (Viséan and Serpukhovian) rocks within Central part of the Dnieper-Donets basin. Methods. During this research we studied 43 thin sections of Carboniferous sandstones from cores of Central part of the Dnieper-Donets basin, using polarizing microscope. Alteration stage analyses were used for determination of mineral sequence transformations. Results. Carboniferous (Viséan and Serpukhovian) sandstones discovered by deep wells within Solokhivsko-Dykanskyi and Berezivsko-Kotelevskyi archs in Central part of the Dnieper-Donets depression have been studied. Microscopic studies have shown widespread development of imposed epigenetic alterations in terrigenous rocks. The involvement of laboratory evaluation of porosity and permeability, as well as the values of formation temperatures, allowed us to determine the main trends of secondary processes effect on reservoir properties of rocks. It is shown that the development of secondary microfracturing, kaolinitization and sideritization increases the value of porosity and permeability. Dissolution, regeneration and redeposition of quartz are accompanied by calcitization and lead to the significant decrease of the reservoir properties of terrigenous rocks and formation of secondary epigenetic seals. Sideritization facilitates the development of secondary reservoirs at great depth, meanwhile calcitization leads to decrease of the pore space. Bitumoids and pyrite presence may be the evidence of hydrocarbon migration process. Combination of the research results with thermobaric data retrieved directly from borehole measurements as well as theoretically calculated on Bilske gas condensate field shows that formation of epigenetic seal in the temperature interval of 110-120 °C is caused by quartz dissolution and intensive calcitization of terrigenous rocks. Consequently, secondary reservoir rocks below the epigenetic seal have local spreading caused by microfracturing, sideritization and calcitization. Specific secondary sandy reservoirs of Berezivske gas condensate field are located within 110 and 120 °C isotherm. They occurred in acid environment leading to dissolution of calcite cement and intense sandstone kaolinitization. Such favorable conditions for secondary porosity formation may occur as a result of possible tectonic movements inversion which triggered the development of regressive epigenesis.

GEOLOGICAL CONDITIONS OF SECONDARY POROSITY FORMATION AND ITS DISTRIBUTION IN ROCKS

The paper presents the problem of collectors secondary porosity, which is the main component of the total effective porosity. Different aspects of this problem are considered: the origin and distribution of secondary porosity in the rock, the dependency between the structure of the pore space and its filtration properties, the formation of hydrocarbons deposits at great depths, the role of post-sedimentation processes in the formation of secondary porosity and fractures, as well as the issue of secondary porosity prediction and its role in prospecting secondary oil and gas reservoirs.

Porosity Distribution Characteristics and Geological Analysis of Brine Storage Medium in the Qaidam Basin, Western China

Underground brine is an unusual water resource that contains abundant mineral resources. It is distributed widely in the Qaidam Basin, western China, a hyperarid inland basin located in the northern Tibetan Plateau. Pores in the brine storage medium act as storage space and transmission channels of underground brine. Therefore, the porosity of brine storage medium determines its ability to store brine. In this study, Mahai Salt Lake was used as the research area as a modern saline lake located in the north area of the Qaidam Basin. A total of 100 porosity samples were collected from eight sampling points in two profiles of the research area at sampling depths of 1.30–314.78 m. The porosity distribution characteristics and influencing factors in brine storage medium were analysed according to the measured porosity data. Based on analysis of the pore structure characteristics, the brine storage medium contains intercrystalline pores, unlike conventional freshwater storage mediums. Moreover, the primary salt rock is susceptible to dissolution by lighter brine, facilitating the formation of secondary porosity. Due to the formation of secondary pores, a porosity greater than 20% remains even at buried depths greater than 100 m. Based on the geological statistical analysis, due to the geographic location, salt formation time, and depositional environment, the porosity values of Mahai Salt Lake do not exhibit a wider distribution, but also show more extreme values than a nearby salt lake. Based on the porosity characteristics by depth, due to the presence of secondary pores, flooding, stratigraphic static pressure, and other factors, porosity shows fluctuations with increasing depth.

Erratum to: Carbonate diagenetic products and processes from various diagenetic environments in Permian paleokarst reservoirs: a case study of the limestone strata of Maokou formation in Sichuan Basin, South China

Understanding the processes and products of carbonate diagenesis is crucial to the exploration and optimum development of palaeokarst hydrocarbon reservoirs in Permian carbonate rocks. Several well-documented outcrops and well drills in the Permian marine carbonate strata around the Sichuan Basin, South China, encompassing well-developed paleokarst landforms in the Middle-Upper Maokou Formation have been investigated. The sections and cores recording paleokarst reservoirs exhibit very similar carbonate diagenetic products and processes from various diagenetic environments by much most cementation and formation of secondary porosity in carbonates. The major diagenetic environments experience the zones of seawater, surface or near-surface atmospheric water, and subsurface mixed water between marine and meteoric. Characteristic diagenetic textures produced in each environment are described and different generations of calcite cements related to the process are responsible for their formation by sedimentary petrology and stable isotope geochemistry. Paleokarst under the constrained condition of surface and near-surface atmospheric water is the most favorable for oil–gas accumulation and presents some karst-related pores-caves-fissures with less or no fillings. Dissolution and corrosion of carbonate by atmospheric freshwater can alter the accumulation spaces and develop better fractures-caves system. These characteristic could exhibit some substantial data for better predicting porosity trends in Permian carbonate rocks and also provide important geological information for further exploring oil–gas resource in marine carbonate strata.

Depositional and diagenetic controls on deeply-buried Eocene sublacustrine fan reservoirs in the Dongying Depression, Bohai Bay Basin, China

This paper investigates the reservoir potential of deeply-buried Eocene sublacustrine fan sandstones in the Bohai Bay Basin, China by evaluating the link between depositional lithofacies that controlled primary sediment compositions, and diagenetic processes that involved dissolution, precipitation and transformation of minerals. This petrographic, mineralogical, and geochemical study recognizes a complex diagenetic history which reflects both the depositional and burial history of the sandstones. Eogenetic alterations of the sandstones include: 1) mechanical compaction; and 2) partial to extensive non-ferroan carbonate and gypsum cementation. Typical mesogenetic alterations include: (1) dissolution of feldspar, non-ferroan carbonate cements, gypsum and anhydrite; (2) precipitation of quartz, kaolinite and ferroan carbonate cements; (3) transformation of smectite and kaolinite to illite and conversion of gypsum to anhydrite. This study demonstrates that: 1) depositional lithofacies critically influenced diagenesis, which resulted in good reservoir quality of the better-sorted, middle-fan, but poor reservoir quality in the inner- and outer-fan lithofacies; 2) formation of secondary porosity was spatially associated with other mineral reactions that caused precipitation of cements within sandstone reservoirs and did not greatly enhance reservoir quality; and 3) oil emplacement during early mesodiagenesis (temperatures > 70 °C) protected reservoirs from cementation and compaction.