Dissolution Of Carbonate Cement Research Articles

Reservoir characteristics and controlling factors of Permian glutenite in Northern Zhongghai uplift, Junggar Basin, China

The Permian Sandstone is well preserved in the Junggar Basin, China. It is consisting of fine to medium sandstone with interbedded shale. The present study is focused on to evaluate the reservoir characteristics and controlling factors of the Permian sandstone of Jinlong 51 well, Junggar basin, China. To fulfil the objectives of the current study, geological field, petrographic analysis, SEM, analysis, mercury injection capillary pressure, and progressed geochemical analysis has been used to evaluate the characteristics and diagenetic history of the Permian sandstone to evaluate its potential. Thin section analysis shows that rock comprises of medium to fine-grained lithic sandstone, and glutenite. Pores are primary isolated into intergranular pores, intergranular pores and intra granular pores. The SEM analysis reveals that the major diagenesis types are compaction, cementation, dissolution, and fractures. Rock is moderately compacted, with intermediate cementation as well as feldspar, rock fragment and carbonate cement dissolution. According to core NMR analysis evaluated as low porosity, low permeability of the reservoirs, some of them are low porosity and low permeability reservoirs. The mercury injection capillary pressure shows that pore throat is intermediate pores with moderate pore throat. Permian sandstone consisting of average porosity 17.8% and permeability is from 5.2 to 8 mD, which recommended that the studied formation consisting of moderate reservoir characteristics. Compaction, cementation and fractures fills with clastic material decreases, whereas primary intergranular pores, dissolution pores and fractures increases the reservoir quality. Finally, the impact of diagenesis on the physical properties of the reservoirs under study has been investigated, the compactions and cementation along with other constrains effect the porosity as well as permeability of the glutenite reservoirs.

Chlorite characterization and diagenetic evolution as primary controls on Zhuhai Formation sandstone reservoir quality in Zhu-III sag, Pearl River Mouth Basin, south China sea

Zhu-Ⅲ sag in the Pearl River Mouth Basin is rich in hydrocarbon resources. The Oligocene Zhuhai Formation is a key exploration horizon in Zhu-Ⅲ sag. However, the exploration results show that the reservoir is deeply buried, the diagenesis is complex and the reservoir heterogeneity is strong. Therefore, looking for high-quality reservoirs, which are the zones with relatively high porosity and permeability, is a key point of resource exploration. Based on the previous research results, this study analyzed the reservoir characteristics and diagenesis characteristics, established the pore evolution model of the study area, and defined the main controlling factors of reservoir development and used core observation, thin section, scanning electron microscope, X-ray diffraction and so on. The research shows that compaction is the main reason for the reservoir densification of Zhuhai Formation. Chlorite in Zhu-III sag consists of two occurrence states and characteristics. These two kinds of chlorite have a great influence on reservoir quality in three key zones. Pore-lining chlorite can effectively improve the reservoir quality. Another is chlorite cement which can destroy the pore structure. Meanwhile, fluid-controlled carbonate cement also has an obvious effect on the reservoirs in key zones. Fluid in early stage leads to the formation of carbonate cement and that in late stage leads to the dissolution of carbonate cement. In general, we clarify the diagenesis and pore evolution in key zones in Zhu-Ⅲ sag and its reservoir is mainly affected by compaction, dissolution, chlorite and hydrocarbon charging. This study of reservoir diagenetic evolution process and pore characteristics is crucial and helpful in the exploration and development of reservoir in Pearl River Mouth Basin and other basin.

Research on Diagenetic Evolution and Hydrocarbon Accumulation Periods of Chang 8 Reservoir in Zhenjing Area of Ordos Basin

The Mesozoic Chang 8 Section in the Zhenjing area is a typical low permeability-tight sand reservoir and is regarded as the most important set of paybeds in the study area. Guided by the principles of basic geological theory, the diagenetic evolution process and hydrocarbon accumulation periods of the Chang 8 reservoir in the study area were determined through various techniques. More specifically, core observation, scanning electron microscopy (SEM), X-ray diffraction (XRD), and vitrinite reflectance experiments were performed in combination with systematic studies on rock pyrolysis and the thermal evolutionary history of basins, the illite-dating method, and so on. The Chang 8 reservoir is dominated by feldspar lithic and lithic feldspar sandstones. Quartz, feldspar, and lithic fragments are the major clastic constituents. In clay minerals, the chlorite content is the highest, followed by illite/smectite formation and kaolinite, while the illite content is the lowest. The major diagenesis effect of the Chang 8 reservoir includes compaction, cementation, dissolution, metasomatism, and rupturing. The assumed diagenetic sequence is the following: mechanical composition → early sedimentation of chlorite clay mineral membrane → early cementation of sparry calcite → authigenic kaolinite precipitation → secondary production and amplification of quartz → dissolution of carbonate cement → dissolution of feldspar → late cementation of minerals such as ferrocalcite. Now, the study area is in Stage A in the middle diagenetic period. Through the inclusion of temperature measurements, in conjunction with illite dating and thermal evolutionary history analysis technology in basins, the Chang 8 reservoir of this study was determined as the phase-I continuous accumulation process and the reservoir formation epoch was 105~125 Ma, which was assigned to the Middle Early Cretaceous Epoch.

Effects of precipitation and dissolution of carbonate cements on the quality of deeply buried high-temperature and overpressured clastic reservoirs: XD 10 block, Yinggehai Basin, South China Sea

Clarifying the precipitation and dissolution processes of carbonate cement is of great significance for reconstructing the history of reservoir diagenesis, quantitatively evaluating reservoir quality, and enhancing acidification-related oil recovery. In this study, comprehensive experiments were performed, including thin section observation, cathodoluminescence, scanning electron microscopy, electron probe, QEMSCAN, carbon and oxygen isotopes, and fluid inclusions, to investigate the carbonate cements from the lower Huangliu Formation in the XD10 block of the Yinggehai Basin in the South China Sea. The types, distributions, paragenetic framework, and formation mechanisms of carbonate cements were studied systematically to further reveal their impacts on clastic reservoir quality. The results were as follows: (1) The average absolute content of carbonate cement was 7.5%, which accounted for 84.3% of the total authigenic minerals and showed a negative correlation with reservoir properties. The early-stage calcite and dolomite being controlled by sedimentation with the δ18OPDB values ranging from −12.28‰ to −5.28‰, filled in the intergranular pores with poikilotopic crystals, which resulted in almost 90% of the primary porosity to be lost; the late-stage ferrocalcite and ankerite that were characterized by δ18OPDB values varying from −17.0‰ to −9.02‰ filled up approximately 30% of the secondary pores caused by minerals dissolution. (2) The overpressure of deeply buried clastic reservoirs resulted from CO2-rich fluid charging, water saturation, and their distributions, which directly affected the stability of the reservoir carbonate cements. For a gas layer containing bound water, spot-like selective dissolution developed because the film-type bound water contained only a small amount of CO2. For the water layer, the carbonate cements suffered from strong dissolution owing to the long-term sufficient CO2 supply. (3) In situ dissolution experiments of carbonate cement, periodically monitoring the relationship between injection pressure and dissolution time, indicated that the intrusion pressure represented a periodic leaping change. This is attributed to the periodic leap between the “throat blocking” resulted from the precipitation of dissolved carbonate grains and the “throat opening” caused by the further dissolution of carbonate grains. (4) Analysis of 3D petrophysical properties and 2D images showed that the early-stage carbonate cements dissolution enhanced the porosity of the reservoir from 3% to 28%, which increased the permeability by 2000–4000 times. The dissolution of late-stage carbonate cement improved the porosity from 8% to 16% and resulted in a permeability increase of 5–100 times. The mean radius of 2D pores were extended from 68.4 to 266.3 μm, the mean radius of 2D throat were increased from 14.9 to 35.1 μm. Systematic analysis of the influence of carbonate cements in deep tight reservoirs with high temperature and overpressure (HTOP) on the rock quality provided some insights into the analysis of quality control factors and the later development plan of carbonate cement-rich deep tight reservoirs.

Investigation on pore structure characteristics of ultra-tight sandstone reservoirs in the upper Triassic Xujiahe Formation of the northern Sichuan Basin, China

Pore structure is the key factor to control the quality and fluid flow of ultra-tight reservoir. The relationship between rock types, pore types, and pore sizes of ultra-tight sandstone reservoirs was identified by observing microscopic characteristics under a polarizing microscope and scanning electron microscope (SEM) and quantitively assessed using high-pressure mercury intrusion (HPMI) and fractal theory. By a comparative analysis of lithic sandstone and feldspar lithic sandstone reservoirs of Upper Triassic Xujiahe Formation of the Northern Sichuan Basin, we found that the pore type of the ultra-tight sandstone reservoir was linked to the rock type. Argillaceous lithic sandstone minly generates fracture pores, while fine-grained lithic sandstone and feldspar lithic sandstone formed dissolved pores by dissolution of carbonate cement, rock debris, and feldspar particles. Meanwhile, clay minerals such as kaolinite and chlorite in the reservoir possess pores between clay platelets and intraplatelet pores within clay aggregates, which are important pore types for ultra–tight sandstone reservoirs. A method to divided intervals of pore size distribution was proposed using high-pressure mercury intrusion (HPMI), and three intervals of pore diameter distribution were observed: 10–40 nm, 40 nm–1 μm, and 1–100 μm, which is largely controlled by pore type. The reservoir with only fracture pores is a one–interval reservoir with pore diameter in the range of 1–100 μm, while the reservoirs with dissolved pores and clay mineral-related pores have the above three intervals. Meanwhile, the fractal of pores in different scale range is different, and the larger the pore size, the stronger is the reservoir heterogeneity. A treelike pore structure model can be utilized to define pore development features in ultra–tight sandstone reservoirs. Micron–scale pores with diameters of 1–100 μm is mostly made up of fractures, cement and particle dissolution pores. However, nano–scale pores in the range of 40 nm–1 μm and 10–40 nm are predominantly pores formed between clay platelets and intraplatelet pores within clay aggregates, respectively.

Origin of carbonate cements in deep sandstone reservoirs and its significance for hydrocarbon indication: A case of Shahejie Formation in Dongying Sag

Abstract Carbonate cements are primary cement types formed in deep sandstone reservoirs of Dongying Sag. We have proposed three stages of carbonate cements with different origin and material sources: carbonate cements in early stage are rim-shaped high-Mg calcite, which is the product of quasi-contemporaneous period; and calcite filled with primary pores without obvious compaction and diagenetic transformation is mudstone compaction during the drainage process. Carbonate cements in middle stage are calcite and dolomite filled with feldspar secondary dissolved pores. The rich Ca2+, Mg2+, and CO 3 2 − {\text{CO}}_{3}^{2-} in overpressure fluid enter the reservoir and mix with Ca2+ in the original formation water. Carbonate cements in late stage are ferrocalcite and ankerite that filled the dissolution pores of early- and middle-stage carbonate cements. They were products of CO 3 2 − {\text{CO}}_{3}^{2-} formed by organic acid splitting decomposition in late diagenesis and CO 3 2 − {\text{CO}}_{3}^{2-} formed by dissolution of carbonate cements in early and middle stages, combined with Mg2+, Ca2+, Fe2+ plasma in pore fluid. Dissolution–reprecipitation of the lacustrine carbonate rocks is responsible for obvious positive drift in the δ 13CPDB‰ values of carbonate cements. Carbonate cements in middle stage and late stage, respectively, represent the early hydrocarbon charging of Dongying Formation and the end of Guantao Formation to the present.

Investigating the mechanism of pH effect on saturated permeability of remolded loess

The pollution of remolded loess caused by acids and bases greatly affects its engineering performance, and thereby seriously threaten the service life of loess buildings or structures. To explore the changes in saturated permeability of the remolded loess polluted by hydrochloric acid or sodium hydroxide solutions, a series of the saturated seepage tests were designed in presence of different pH values. In addition, the hydrochemical components in the leachate along with the zeta potentials in water-soil solutions under different pH were measured. The changes in mineralogical, chemical, and microstructural properties of the loess samples before and after the seepage were also investigated. The results demonstrate gradual increase in the permeability of the loess samples with the decrease of pH of the solution. When acid enters the loess, the dissolution of carbonate cement and chemical cement (Al 2 O 3 and Fe 2 O 3 ) lays the foundation for the enlargement of pores. The decrease in thickness of the diffuse double layer results in aggregation of clay particles and enlarges the space of gravity water moving in the free layer. Hence, the significant increase in the permeability takes place over time during the seepage at pH = 3. The change in permeability at pH = 7 shows similar behavior to the acidic condition, and this is mainly responsible for the leaching of carbonate. However, the alkaline (pH = 11) condition inhibits the dissolution of carbonate minerals, and calcite is precipitated due to the incongruous dissolution of dolomite and dedolomitization. Al(OH) 3 , Fe(OH) 3 , and H 2 SiO 3 colloids are formed and attached to the particles. These processes plug the pores during the seepage. Moreover, the development of the diffuse double layer results in wide distribution of the clay particles with highly cementation, which greatly reduces the effective connectivity of the pores. Therefore, its permeability decreases with the seepage time. These findings will offer reference for the practice of geological engineering in the loess areas. • The change laws of saturated permeability of remolded loess are investigated at different pH. • The geochemical process, chemical behavior, and electrokinetic property are explored during the seepage. • The similarities and differences of physicochemical interactions between soil and water are revealed at different pH. • The mechanisms of pH effect on saturated permeability of remolded loess are disclosed.

Role of cementation and compaction in controlling the reservoir quality of the Middle to Late Jurassic Sandstones, Jara dome, Kachchh Basin, western India

Thin‐section petrography, scanning electron microscopy (SEM) and X‐ray diffraction (XRD) analyses are applied to investigate the various diagenetic controls on the reservoir quality of the Ridge and Athleta Sandstone members (Chari Formation) of the Jara dome, Kachchh Basin, western India. The results show that the sandstones of Jara dome are quartz‐arenite to sub‐arkose, medium‐ to coarse‐grained, sub‐angular to sub‐rounded, poorly to moderately well‐sorted and having low‐ to medium‐sphericity. Diagenetic features such as mechanical compaction, pressure solution, cementation, and dissolution of unstable framework grains are analysed to synthesize the reservoir characteristics. The principal cementing materials are carbonates, iron oxides, silica overgrowth, and authigenic clays, mainly chlorite and kaolinite. Compaction and cementation are the controlling factors for the reduction in porosity and permeability of the sandstones. Dissolution of detrital feldspars is the main cause of porosity enhancement in these sandstones. The relationship between the intergranular volume (IGV) and cement volume indicates that cementation had played a more important role in the destruction of primary porosity than that of compaction. On the one hand, the ubiquitous development of carbonate cement advocates the loss in primary porosity at the early stage of diagenesis, while on the other hand it also prevented the further compaction of sandstone, thus preserving unfilled voids as primary porosity. Precipitation of carbonate cement is followed by development of pore‐lining and pore‐filling clayey cements. The pore‐filling kaolinite resulted in a considerable loss of porosity while the pore‐lining chlorite has helped in retaining the porosity by preventing syntaxial silica overgrowths. The reservoir quality of the studied sandstone is reduced by the authigenic clay cements, early carbonate precipitation and silica overgrowths, nonetheless, it is improved by the dissolution and alteration of unstable grains, in addition to partial dissolution of carbonate cements. The potential of the studied sandstone to act as a reservoir is strongly related to sandstone diagenesis.

Application of isotopic and geochemical signals in unconventional oil and gas reservoir produced waters toward characterizing in situ geochemical fluid-shale reactions

Optimizing hydrocarbon production and waste management from unconventional oil and gas extraction requires an understanding of the fluid-rock chemical interactions. These reactions can affect flow pathways within fractured shale and produced water chemistry. Knowledge of these chemical reactions also provides valuable information for planning wastewater treatment strategies. This study focused on characterizing reservoir reactions through analysis of produced water chemistry from the Marcellus Shale Energy and Environmental Laboratory field site in Morgantown, WV, USA. Analysis of fracturing fluids, time-series produced waters (PW) over 16 months of operation of two hydraulically fractured gas wells, and shale rocks from the same well for metal concentrations and multiple isotope signatures (δ2H and δ18O of water, δ7Li, δ11B, 87Sr/86Sr) showed that the chemical and isotopic composition of early (<10 days) PW samples record water-rock interactions during the fracturing period. Acidic dissolution of carbonate minerals was evidenced by the increase in TOC, B/Na, Sr/Na, Ca/Na, and the decrease in 87Sr/86Sr in PW returning in the first few days toward the 87Sr/86Sr signature of carbonate cement. The enrichment of 6Li in these early (e.g., day 1) PW samples is most likely a result of desorption of Li from clays and organic matter due to the injection of fracturing fluid. Redox-active trace elements appear to be controlled by oxidation-reduction reactions and potentially reactions involving wellbore steel. Overall, PW chemistry is primarily controlled by mixing between early PW with local in-situ formation water however certain geochemical reactions (e.g., carbonate cement dissolution and desorption of 6Li from clays and organic matter) can be inferred from PW composition monitored immediately over the first ten days of water return.

Genesis of authigenic kaolinite and its indicative significance to secondary pores in petroliferous basins: A case study in the Dongying Sag, Bohai Bay Basin, Eastern China

Porosity of sandstone generally declines as the formation burial depth rises, during which transformation of unstable minerals (e.g. feldspar) in sandstone significantly contributes to abnormally high porosity zones. It is still controversial that whether transformation of feldspar being massive in sandstone can lead to formation of considerable secondary pores. As the main diagenetic product of feldspar transformation, namely authigenic kaolinite, in terms of the physical and chemical features, acts as indicators of the transformation process of feldspar and the consequent formation of secondary pores. In this study, a series of analyses are implemented on sandstone samples collected from the Dongying Sag, Bohai Bay Basin, Eastern China, including microscopic observation, SEM, EDX, XRD, isotope, and fluid inclusion analysis, with specific attention to genesis and formation process of authigenic kaolinite as well as their indicative significance to secondary pores. It is demonstrated that there are mainly three types of authigenic kaolinite (denoted as K1, K2, and K3), the occurrence and geochemical characteristics of which are greatly different. Based on contact relationship between authigenic kaolinite and other diagenetic products, the three authigenic kaolinite are distinguished into two stages, namely the first-stage K1and the second-stage K2 and K3, which are of obviously different genesis. Specifically, K1 is formed under the effect of acidic fluids that are rich in carboxylic acids. In contrast, K2 and K3 are formed under the effect of acidic fluids that are rich in CO2. During formation of K1, plagioclase dissolution pores are formed and well preserved, and carbonate cements get well preserved, showing no sign of dissolution. During formation of K2 and K3, K-feldspar is kaolinitized as K2, and a small amount of Si and Al elements migrates with short-distance and precipitates as K3 in the vicinity of K2. During this process, K-feldspar is transformed into kaolinite, accompanied by dissolution of carbonate cements and massive formation of secondary pores.

Permeability of rock discontinuities and faults in the Triassic Sherwood Sandstone Group (UK): insights for management of fluvio-aeolian aquifers worldwide

Fluvio-aeolian sedimentary successions host groundwater aquifers at shallow depths (<~0.15 km), which overlie geothermal and shale-gas reservoirs, and nuclear waste repositories at intermediate depths (~0.15–2.0 km). Additionally, such deposits represent petroleum reservoirs at greater depths (~2.0–4.0 km). The need to improve conceptual understanding of the hydraulic behaviour of fluvial-aeolian sandstone successions over a large depth interval (~0–4 km) is important for socio-economic reasons. Thus, the hydraulic properties of the Triassic Sherwood Sandstone aquifer in the UK have been reviewed and compared to similar fluvio-aeolian successions. The ratio between well-scale and core-plug-scale permeability (Kwell-test/Kcore-plug) acts as a proxy for the relative importance of fracture versus intergranular flow. This ratio (which typically varies from ~2 to 100) indicates significant contribution of fractures to flow at relatively shallow depths (<~0.15 km). Here, permeability development is controlled by dissolution of calcite-dolomite in correspondence of fractures. The observed ratio (Kwell-test/Kcore-plug) decreases with depth, approaching unity, indicating that intergranular flow dominates at ~1 km depth. At depths ≥ ~1 km, dissolution of carbonate cement by rock alteration due to groundwater flow is absent and fractures are closed. Aeolian and fluvial deposits behave differently in proximity to normal faults in the Sherwood Sandstone aquifer. Deformation bands in aeolian dune deposits strongly compartmentalize this aquifer. The hydro-structural properties of fluvio-aeolian deposits are also controlled by mineralogy in fault zones. A relative abundance of quartz vs. feldspar and clays in aeolian sandstones favours development of low-permeability deformation bands.

RESERVOIR QUALITY EVOLUTION OF UPPER CARBONIFEROUS (WESTPHALIAN) TIGHT GAS SANDSTONES, LOWER SAXONY BASIN, NW GERMANY

This paper investigates reservoir quality development in tight Upper Carboniferous fluvial sandstones (Westphalian C/D) in the Lower Saxony Basin, NW Germany. The study integrates data from three outcrops (Piesberg, Woitzel and Hüggel) in the south of the basin with that from two wells (Wells A and B) located at gas fields approximately 50 km to the north. Petrographic and petrophysical data are related to the diagenetic evolution of the sandstones and the burial and structural history of the Lower Saxony Basin. The outcrop and subsurface data sets are compared in order to investigate the factors controlling reservoir quality evolution.Upper Carboniferous fluvial sandstones from the Woitzel and Hüggel outcrops and from Wells A and B have similar matrix permeabilities (0.01 to 10 mD), but matrix porosities vary between Well A (average 6%), Well B (average 10%), Woitzel (average 15%), and Hüggel (average 19%). Permeability reduction during burial is related to the formation of clay mineral cement, which was mainly controlled by variations in both the palaeo‐climate and in the sandstones’ depositional composition. Matrix porosity was controlled by local differences in burial history related to basin inversion tectonics. The greater amount of inversion‐related uplift at Well B (about 2.8 km) resulted in lower thermal exposure of the Westphalian sandstones at this location, which thus show higher matrix porosities than the sandstones at Well A which were uplifted by only about 1.2 km. Further increases in porosity in the outcrop sandstones may be related to the dissolution of carbonate cement during late‐stage uplift in near‐surface conditions.Upper Carboniferous fluvial sandstones from the Piesberg quarry show the poorest reservoir characteristics compared to the samples from the subsurface and the other outcrops, with matrix porosities averaging 6% and permeabilities &lt;0.01 mD. Reservoir quality reduction was controlled by thermal anomalies associated with a large fault at the Piesberg quarry. By contrast, a few outliers in the sample data sets from Well B and the Piesberg quarry, which have permeabilities of more than 100 mD, show that faulting or natural fracturing may enhance reservoir quality within a particular area. Faults/fractures may act as potential migration pathways for leaching fluids, or may provide fracture‐permeability systems with production potential.Depositional setting, burial‐related diagenetic processes and structural characteristics in the Lower Saxony Basin need to be carefully evaluated in order to provide an improved understanding of the reservoir quality of the Upper Carboniferous sandstones.

Lithofacial models of productive deposits of the Moshakov formation within the southwesten part of the Siberian platform

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How important is carbonate dissolution in buried sandstones: evidences from petrography, porosity, experiments, and geochemical calculations

Burial dissolution of feldspar and carbonate minerals has been proposed to generate large volumes of secondary pores in subsurface reservoirs. Secondary porosity due to feldspar dissolution is ubiquitous in buried sandstones; however, extensive burial dissolution of carbonate minerals in subsurface sandstones is still debatable. In this paper, we first present four types of typical selective dissolution assemblages of feldspars and carbonate minerals developed in different sandstones. Under the constraints of porosity data, water–rock experiments, geochemical calculations of aggressive fluids, diagenetic mass transfer, and a review of publications on mineral dissolution in sandstone reservoirs, we argue that the hypothesis for the creation of significant volumes of secondary porosity by mesodiagenetic carbonate dissolution in subsurface sandstones is in conflict with the limited volume of aggressive fluids in rocks. In addition, no transfer mechanism supports removal of the dissolution products due to the small water volume in the subsurface reservoirs and the low mass concentration gradients in the pore water. Convincing petrographic evidence supports the view that the extensive dissolution of carbonate cements in sandstone rocks is usually associated with a high flux of deep hot fluids provided via fault systems or with meteoric freshwater during the eodiagenesis and telodiagenesis stages. The presumption of extensive mesogenetic dissolution of carbonate cements producing a significant net increase in secondary porosity should be used with careful consideration of the geological background in prediction of sandstone quality.

Characteristics and genetic mechanism of carbonate cements in sandstones near the overpressure top surface: A case study of the Niuzhuang Depression in Bohai Bay Basin

Carbonate cements are important authigenic minerals in the sandstone reservoir near the overpressure top surface in the Niuzhuang Depression. However, their genetic mechanism has not been studied in detail. This study uses observations of casting thin sections and cathodoluminescence and micro-area electron probe compositional data to determine that the diagenetic sequence of these carbonate cements is penecontemporaneous dolomite → calcite filling primary pores → calcite/iron-bearing calcite filling secondary dissolution pores → ankerite. The genetic mechanisms of these carbonate cements are analyzed based on the fluid inclusion temperature at different phases of carbonate cementation, carbon and oxygen isotope composition, simulation of the thermal evolution of the hydrocarbon source rock, and elemental chemical composition. Results show that the carbon isotopic compositions of the carbonate cements are affected not only by organic acids released during the maturation of organic matter but also by the dissolution of lacustrine carbonate rock in the fourth member of the Shahejie Formation. Dissolution–reprecipitation of the lacustrine carbonate rocks is responsible for an obvious positive drift in the δ13CPDB values of carbonate cements near the overpressure top surface. There are four phases of carbonate cementation near the top surface of the overpressured zone, with phase IV carbonate cements mainly occurring near the overpressure top surface. Phase I carbonate cements are mainly dolomite, and their formation is related to the decomposition of organic matter by methanogenic bacteria. Phase II carbonate cements are related to Ca2+ and CO32− supersaturation during pore fluid concentration. Phase III carbonate cements result from overpressured fluid that is rich in Ca2+ and Mg2+ and feldspar dissolution caused by overpressure fluid intrusion. Phase IV carbonate cements are derived from CO32− formed by organic acid pyrolysis in late diagenesis, CO32− formed by early carbonate cement dissolution, and Mg2+ and Fe2+ plasma formed by the dissolution of metamorphic rock debris into pore fluid. The formation of phase III and IV carbonate cements is most affected by overpressured hydrocarbon fluids.

The Effects of Diagenesis on the Reservoir Characters in Ridge sandstone of Jurassic Jumara dome, Kachchh, Western India

ABSTRACT Ridge sandstone of Jurassic Jumara dome of Kachchh was studied in an attempt to quantify the effects of diagenetic process such as compaction, cementation and dissolution on reservoir properties. The average framework composition of Ridge sandstone is Q80F17L3, medium-to coarse grained and subarkose to arkose. Syndepositional silty to clayey matrix (3% average) is also observed that occurs as pore filling. The diagenetic processes include compaction, cementation and precipitation of authigenic cements, dissolution of unstable grains and grain replacement and development of secondary porosity. The major cause of intense reduction in primary porosity of Ridge sandstone is early cementation which include silica, carbonate, iron, kaolinite, illite, smectite, mixed layer illite-smectite and chlorite, which prevents mechanical compaction. The plots of COPL versus CEPL and IGV versus total cement suggest the loss of primary porosity in Ridge sandstone is due to cementation. Cements mainly iron and carbonate occurs in intergranular pores of detrital grains and destroys porosity. The clay mineral occurs as pore filling and pore lining and deteriorates the porosity and permeability of the Ridge sandstone. The reservoir quality of the studied sandstone is reduced by clay minerals (kaolinite, illite, smectite, mixed layer illite-smectite, chlorite), carbonate, iron and silica cementation but on the other hand, it is increased by alteration and dissolution of the unstable grain, in addition to partial dissolution of carbonate cements. The potential of the studied sandstone to serve as a reservoir is strongly related to sandstone diagenesis.

Changes in the composition and properties of Ashalchinskoye bitumen-saturated sandstones when exposed to water vapor

Ashalchinskoye bitumen deposit is an experimental platform for testing technology of high-viscosity oil extraction from reservoir rocks. Last time for enhanced of oil recovery in reservoir used pressurization a water vapor with a temperature of ∼ 180 ° C (SAGD technology). However, what happens in sandstone reservoir is little known. We did a study of the effects of water vapor on the structural components of bitumen saturated sandstone. In paper were studied the rock samples at base condition and after one week exposure by water vapour. The thermal analysis showed that steaming helps to removes light and middle oil fractions with a boiling point up to 360 ° C from oil saturated sandstones. Content of heavy oil fractions virtually unchanged. Studying the composition of water extractions of samples showed that the process of aquathermolysis of oil is accompanied by a lowering of the pH of the pore solution from 7.4 to 6.5 and rise content in several times of mobile cations Ca2+, Mg2+ and HCO3-, SO42- anions. Follows from this that the thermal steam effect by bitumen saturated sandstones leads to partial oxidation of hydrocarbons with to form a carbon dioxide. The source of sulfate ions were oxidized pyrite aggregates. Due to the increasing acidity of condensed water, which fills the pore space of samples, pore fluid becomes aggressive to calcite and dolomite cement of bitumen saturated sandstones. As a result of the dissolution of carbonate cement the pore fluid enriched by calcium and magnesium cations. Clearly, that the process is accompanied by reduction of contact strength between fragments of minerals and rocks. Resulting part of compounds is separated from the outer side of samples and falls to bottom of water vapor container. Decreasing the amount of calcite and dolomite anions in samples in a steam-treated influence is confirmed by X-Ray analysis. X-Ray analysis data of study adscititious component of rocks showed that when influenced of water vapor to bitumen saturated sandstones there are the processes of transformation of clay minerals. Mixed-illite-montmorillonite phase is primarily exposed to changes. In this case we fix initial stage of the destruction of polycrystalline particles mixed-mineral. Reducing the size of clay minerals particles along the normal to the layers (L001) is the result from lower energy costs of delamination and disintegration the crystallites along this direction, in comparison with others. Thus, using of SAGD technology at exploitation of Ashalchinskoye bitumen saturated reservoir will be followed by acidification of the pore fluid, activation processes of dissolution calcite cement and transformation of mixed-layers clays minerals.

The role played by carbonate cementation in controlling reservoir quality of the Triassic Skagerrak Formation, Norway

Anomalously high porosities up to 30% at burial depth of >3000 m along with varying amounts and types of carbonate cements occur in the fluvial channel sandstone facies of the Triassic Skagerrak Formation, Central Graben, Norway. However, porosities of the Skagerrak Formation are lower in the Norwegian sector than in the UK sector. In this study, petrographic analysis, core examination, scanning electron microscopy, elemental mapping, carbon and oxygen isotope, fluid inclusion and microgeometry analysis are performed to determine the diagenesis and direct influence on reservoir quality, with particular focus on the role played by carbonate cementation. The sandstones are mainly fine-grained lithic-arkosic to sub-arkosic arenites and display a wide range of intergranular volumes (2.3%–43.7% with an average of 23.6%). Porosity loss is mainly due to compaction (av. 26.6%) with minor contribution from cementation (av. 12.1%). The carbonate cements are patchy in distribution (from trace to 20.7%) and appear as various types e.g. calcretes (i.e. calcareous concreted gravels), poikilitic sparite and sparry ferroan dolomite, and euhedral or/and aggregated ankerite/ferroan dolomite crystals. This study highlights the association of carbonate precipitation with the remobilisation of carbonate from intra-Skagerrak calcretes during early burial stage i.e. <500 m. During deeper burial, compaction is inhibited by carbonate cements, resulting high intergranular volume of up to 32% and 29% for fine- and medium-grained sandstones, respectively. Carbonate cement dissolution probably results from both meteoric water flow with CO2 during shallow burial, and organic CO2 and carboxylic acid during deep burial. The maximum intergranular volume enhanced by dissolution of early carbonate cements is calculated to 8% and 5% for fine- and medium-grained sandstones, respectively. Compaction continues to exert influence after dissolution of carbonate cements, which results in a loss of ∼6% intergranular volume for fine- and medium-grained sandstones. Reservoir quality of the Norwegian sector is poorer than that of the UK sector due to a lower coverage of clay mineral coats e.g. chlorite, later and deeper onset of pore fluid overpressure, lower solubility of carbonate compared to halite, and a higher matrix content.

Investigating effects of water salinity on geotechnical properties of fine-grained soil and quartz in a sandstone case study: Ajichay project in northwest Iran

The effect of water salinity on the geotechnical properties of a CL soil and mechanical properties of a quartz sandstone has been studied using samples from the Ajichay project, located in the northwest of Iran. The purpose of this investigation is to investigate the feasibility of using saline water in processing the clay core of earthen dams in this area. One-dimensional consolidation, swelling, and uniaxial compressive strength tests were performed on the soil with distilled, half-saline, and saline water. To evaluate the effect of water salinity on the sandstones placed in the abutments of the dams, the slake durability index and uniaxial compressive strength were investigated. Results indicated that the compressibility index decreased, hydraulic conductivity decreased, and uniaxial compressive strength of the soil increased with increasing water salinity. The soil swelling percent with all three waters was less than 1 % after 24 h. However, swelling percent increased by 23 % with saline water and decreased by 32 % with half-saline water. Some damage in the rock texture such as disaggregation, weathering, and corrosion of the feldspars along with the dissolution of carbonate cement was observed in thin sections after 6 months of immersion in saline water. The strength of the sandstones exposed to saline water for 5 months decreased by between 5 and 13 %.

Are changes in time‐lapse seismic data due to fluid substitution or rock dissolution? A CO2 sequestration feasibility study at the Pohokura Field, New Zealand

ABSTRACTSeismic methods are commonly used to monitor the subsurface when carbon dioxide (CO2) is injected into a reservoir. Besides fluid saturation and pressure changes, CO2–water mixtures may cause rock alteration. In this petrophysical study, we compare the elastic property changes due to fluid replacement and those due to mineral dissolution for carbonate‐cemented sandstones at the Pohokura Field, New Zealand. We quantify the effects of fluid substitution from fully brine to fully supercritical CO2 saturation and carbonate cement dissolution on the seismic signatures of the reservoir rocks by combining laboratory results, petrographic analyses, and geophysical well log data. We conclude that elastic property changes due to mineral dissolution are significantly greater than those due to fluid substitution alone. The northern part of the Pohokura Field has coarser‐grained sandstones, which experience the largest changes in wave speeds. Our hypothesis is that these changes result from carbonate cement dissolution in the presence of CO2–water–sandstone reactions. If time‐lapse seismic data were to be acquired in this field, the northern area could show P‐wave velocity reductions of up to 20% and a 131.7% increase in seismic amplitude from a brine‐saturated rock to an altered, fully CO2‐saturated rock. In comparison, the southern part of the field, where sandstones are mostly fine‐grained, we expect a P‐wave velocity decrease of 6% if such dissolution process took place. Finally, we show that the elastic properties of the reservoir rocks can be described with the constant‐cement model. The model is used to predict that the dissolution process reduces the volume of grain contact cement, on average, from 2.5% to 1.75% of the total rock mineral volume. Our analysis suggests that changes to the rock frame, which includes carbonate minerals, cannot be ignored for a CO2 injection scenario.