Porosity Preservation Research Articles

Recent exploration results in the Lower Triassic, Bedout Sub-basin: Australia's next petroleum province?

Commencing in 2014, Quadrant Energy and partners have undertaken an active exploration program in the Bedout Sub-basin with a 100% success rate, discovering four hydrocarbon accumulations with four wells. The primary exploration target in the basin, the Middle Triassic Lower Keraudren Formation, encompasses the reservoirs, source rocks and seals that have trapped hydrocarbons in a self-contained petroleum system. This petroleum system is older than the traditional plays on the North-West Shelf and before recent activity was very poorly understood and easily overlooked. Key reservoirs occur at burial depths of 3500–5500 m, deeper than many of the traditional plays on the North-West Shelf and exhibit variable reservoir quality. Oil and gas-condensate discovered in the first two wells, Phoenix South-1 and Roc-1, raised key questions on the preservation of effective porosity and productivity sufficient to support a commercial development. With the acquisition and detailed interpretation of 119 m of core over the Caley Member reservoir in Roc-2 and a successful drill stem test that was surface equipment constrained to 55 MMscf/d, the productive potential of this reservoir interval has been confirmed. The results of the exploration program to date, combined with acquisition of new 3D/2D seismic data, have enabled a deeper understanding of the potential of the Bedout Sub-basin. A detailed basin model has been developed and a large suite of prospects and leads are recognised across a family of hydrocarbon plays. Two key wells currently scheduled for 2018 (Phoenix South-3 and Dorado-1) will provide critical information about the scale of this opportunity.

The effect of oil emplacement on quartz cementation in a deeply buried sandstone reservoir

Quartz is an important, porosity-occluding cement in sandstone reservoirs that have been subjected to elevated temperature (>80°C to 100°C) for a substantial period of time. The effect of oil emplacement on quartz cementation in reservoir sandstones is controversial; some studies have concluded that early oil emplacement can inhibit quartz cementation, leading to the preservation of porosity, whereas other studies have concluded that quartz cementation appears largely unaffected by oil emplacement. Here we have studied shallow marine, Upper Jurassic sandstones from Ula field, Norwegian North Sea, with reservoir temperatures of approximately 150°C, to determine whether oil emplacement had a significant impact on diagenesis with particular attention to quartz cementation. Following sedimentological description of cores, samples above and below the oil–water contact have been collected, adjacent to core-analysis plug points. These samples then underwent a series of studies, including petrographic point counting with a transmitted light microscope, scanning electron microscopy (SEM), backscattered electron microscopy, SEM-cathodoluminescence microscopy, and fluid-inclusion studies. These data were integrated with routine core-analysis and petrophysical log data. Density and resistivity log data have been used to determine the precise oil saturation of each sample studied. The distributions of all potential controls on porosity and permeability, such as grain size, sorting, matrix clay content, and degree of bioturbation, the presence of grain coatings and dolomite cement, and the amount of quartz cement, have been assessed. The presence of primary oil inclusions within quartz cement shows that oil ingress into the Ula reservoir commenced prior to the onset of quartz cementation. Very fine–grained, matrix-rich, bioturbated, microquartz-cemented sandstones have uniformly low quartz-cement contents irrespective of oil saturation. Medium-grained, graded, matrix-poor, microquartz-poor sandstones have quartz cement ranging from 1% to greater than 17%, associated with core porosities of approximately 22% and 7%, respectively. Higher oil saturations equate to higher porosities and permeabilities in the medium-grained, graded, matrix-poor, microquartz-poor sandstones, which cannot be explained by any control other than the amount of quartz cement as a function of pore fluid type. Oil emplacement therefore appears to have inhibited quartz cementation at high oil saturations and can be viewed as an important control on reservoir quality. The significance of this study is that the presence of oil in a sandstone reservoir at the time that quartz cement was growing can have a considerable impact on reservoir quality. Models that seek to predict quartz cement and reservoir quality in sandstones need to account for the timing of oil emplacement compared with other diagenetic processes.

Diagenetic Preservation and Modification of Porosity in Aptian Lithic Reservoirs from the Sergipe–alagoas Basin, Ne Brazil

Research Article| November 14, 2017 Diagenetic Preservation and Modification of Porosity in Aptian Lithic Reservoirs from the Sergipe–alagoas Basin, Ne Brazil Argos B.S. Schrank; Argos B.S. Schrank Geosciences Institute, Federal University of Rio Grande do Sul, Avenida Bento Gonçalves, 9500 Porto Alegre, Brazil Search for other works by this author on: GSW Google Scholar Sabrina D. Altenhofen; Sabrina D. Altenhofen Geosciences Institute, Federal University of Rio Grande do Sul, Avenida Bento Gonçalves, 9500 Porto Alegre, Brazil Search for other works by this author on: GSW Google Scholar Luiz F. De Ros Luiz F. De Ros Geosciences Institute, Federal University of Rio Grande do Sul, Avenida Bento Gonçalves, 9500 Porto Alegre, Brazil Search for other works by this author on: GSW Google Scholar Author and Article Information Argos B.S. Schrank Geosciences Institute, Federal University of Rio Grande do Sul, Avenida Bento Gonçalves, 9500 Porto Alegre, Brazil Sabrina D. Altenhofen Geosciences Institute, Federal University of Rio Grande do Sul, Avenida Bento Gonçalves, 9500 Porto Alegre, Brazil Luiz F. De Ros Geosciences Institute, Federal University of Rio Grande do Sul, Avenida Bento Gonçalves, 9500 Porto Alegre, Brazil e-mail: argosssb@hotmail.com Publisher: SEPM Society for Sedimentary Geology First Online: 14 Nov 2017 Online Issn: 1938-3681 Print Issn: 1527-1404 Copyright © 2017, SEPM (Society for Sedimentary Geology) Journal of Sedimentary Research (2017) 87 (11): 1156–1175. https://doi.org/10.2110/jsr.2017.64 Article history First Online: 14 Nov 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Argos B.S. Schrank, Sabrina D. Altenhofen, Luiz F. De Ros; Diagenetic Preservation and Modification of Porosity in Aptian Lithic Reservoirs from the Sergipe–alagoas Basin, Ne Brazil. Journal of Sedimentary Research 2017;; 87 (11): 1156–1175. doi: https://doi.org/10.2110/jsr.2017.64 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyJournal of Sedimentary Research Search Advanced Search Abstract Litharenites composed of sedimentary and low-grade metamorphic rock fragments are rarely good hydrocarbon reservoirs. Most of their primary porosity is lost by ductile grain deformation during mechanical compaction. However, the lithic Carmópolis Member of the Muribeca Formation from the Sergipe–Alagoas Basin, NE Brazil, is one of the most important oil reservoirs of the country. The high reservoir quality of the Carmópolis Member, despite its unusual composition rich in low-grade metamorphic fragments, indicates the occurrence of effective porosity preservation processes. The understanding of the occurrence and distribution of these porosity preservation processes are of paramount importance to optimize the recovery from producing oilfields, for the further development of these reservoirs, and for the exploration for similar reservoirs. The Carmópolis Member consists of alluvial conglomerates and sandstones intercalated with fine-grained lacustrine and alluvial-plain sediments deposited during the Aptian, in an arid environment. Detailed petrographic analyses were performed on 135 thin sections taken from cores of six wells, with support from SEM and EDS analyses. The reservoirs correspond mainly to conglomerates and to medium- to coarse-grained conglomeratic sandstones, classified as litharenite phyllarenites, subordinately feldspathic litharenites, and lithic arkoses. Dolomite is the most abundant cement. Other volumetrically significant processes are K-feldspar and calcite precipitation, feldspar dissolution, and deformation of ductile grains due to compaction. Twenty reservoir petrofacies were defined, based on the main aspects of primary composition and texture and diagenesis that control the porosity. Although partial dolomite cementation was previously considered as the main process of porosity preservation in the Carmópolis reservoirs, the present study suggests that other mechanisms such as early oil saturation may have been important for preserving their quality. Variations in depositional texture and composition, and in the distribution of these processes, resulted in the various defined reservoir petrofacies. Oil and the fluids that precipitated dolomite migrated up along major faults from deeply buried fine sediments to the shallow reservoirs. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Contrasting diagenetic evolution patterns of platform margin limestones and dolostones in the Lower Triassic Feixianguan Formation, Sichuan Basin, China

Abstract Deeply-buried carbonate-reservoirs from the Lower Triassic Feixianguan Formation in the Sichuan Basin of China host extensive natural gas resources. These reservoirs are predominantly found in oolitic shoals, with the reservoir quality of dolomitized zones being higher than that of undolomitized limestone counterparts. Here we present a combination of petrographic, isotopic, fluid inclusion, and quantitative porosity data in order to understand and predict the diagenetic processes that have impacted the reservoir quality of dolostones and limestones. The porosity of limestones has been reduced to ∼7.5% due to calcite cementation, whereas the porosity in oolitic dolostones is not cemented with calcite and typically has ∼23.5% porosity. Dolomitization and concurrent early-diagenetic gypsum growth played crucial roles on the development and preservation of high porosity in the oolitic dolostone, first by stabilizing the rock fabric to inhibit loss of porosity during burial, and secondly through the generation of new porosity by dissolution of carbonate and anhydrite. A negative shift of δ 18 O and salinity values ( 13 C values (∼−20‰ VPDB) and δ 18 O (∼−10‰ VPDB) together with elevated calcite precipitation temperatures (>110 °C). It is likely that TSR was responsible for the formation of enlarged dissolution vugs that increased porosity by ∼2% in dolostones due to: i) anhydrite dissolution, ii) production of significant amounts of water resulting in formation water undersaturated with respect to calcite and dolomite, iii) generation of H 2 S, and CO 2 , and the consequent reaction of H 2 S with the siderite (FeCO 3 ) component in calcite and dolomite. This study demonstrates the importance of diagenesis in the formation of deeply-buried, high-quality reservoirs in ooid-dominated grainstones influenced by the presence of evaporites. Our results should be useful for guiding future exploration and reservoir developments in similar paleogeographic and diagenetic settings.

Origin of different chlorite occurrences and their effects on tight clastic reservoir porosity

The effects of chlorite on tight sandstone reservoirs have not been fully understood yet. In this paper, two different chlorite occurrences are observed in the Upper Triassic Xujiahe tight sandstones of Guang'an Area, Sichuan Basin (grain coating) and in the Permian Lucaogou tight sandstones of Jimusar Sag, Junggar Basin (pore filling), respectively. The effects these two chlorite occurrences having on reservoir quality are evaluated. Results show the influence of chlorite on porosity of tight clastic reservoir depends on its occurrences. Grain-coating chlorite has positive effect on porosity preservation, while pore-filling chlorite plays a negative role.Analysis of grain size indicates that grain-coating and pore-filling chlorites always occur in high-energy and low-energy environments, respectively. The water current carries an abundance of medium-grained saltating contents in strong hydrodynamic tractive current environment that leads to a relative motion between the saltating population and suspended clay in water. As indicated by Bernoulli's theory, the saltating grains are coated by clay on the surface because of the local flow pressure resulting from relative velocity. However, in the case of a weak hydrodynamic tractive current depositional environment, sand is entrained with clay, i.e., both sand grains and clays are in suspension and they have no relative velocity; thus, the clay cannot completely coat the grain surface. As the precursor of chlorite, occurrence of smectite-dominant clays determines the occurrence of chlorite during diagenesis, and therefore, determines whether the chlorite has a positive effect on porosity of tight clastic reservoirs.

Controls on porosity preservation within a Jurassic oolitic reservoir complex, UAE

The Upper Jurassic Arab Formation, onshore southern UAE, comprises an upward-shallowing succession of slope to basinal wackestones (lower Arab D Member) oolitic grainstone (upper Arab D Member) and peritidal - subtidal carbonate-evaporites (Arab A-C Members). This study focuses on the upper Arab D Member, the volumetrically most important reservoir unit in the field, which is dominated by oolitic and skeletal oolitic grainstones with high porosity and moderate to low permeability. Through much of the upper Arab D Member, porosity has been inverted so that ooids are porous and interparticle macropores cemented by pre-compactional non-ferroan calcite. Cementation is most pervasive within cross-bedded skeletal oolitic grainstones, occluding primary interparticle macropores and biomoulds and penetrating the margins of altered ooids. In contrast, bioturbated pack-grainstones are often only partially cemented, and retain a well-connected primary interparticle macropore network.Ooid alteration and porosity inversion in oolitic grainstones is often explained by open-system recrystallization by meteoric fluids during platform emergence. In this study, however, deposition took place in an arid basin that was eventually desiccated and hence the supply of groundwater would have been limited. The high concentrations of Mg and Sr within both the bulk rock limestone and cements suggest recrystallization could have occurred by marine pore fluids whilst, petrographical and geochemical homogeneity, including a narrow range of δ18Ocalcite (−5%0 ± 1%0), can be explained by closed-system stabilization of aragonite. Subsequent calcite cemented fractures and drusy, sparry pore filling calcite contain petroleum and aqueous inclusions with homogenisation temperatures up to 150 °C The preservation of the highest macropore volumes on the palaeocrest of the field, and an off-flank reduction in porosity and permeability, suggests that oil emplacement terminated calcite cementation.The results of this study demonstrate that a major reorganization of porosity can occur during the stablisation of metastable grains without invoking open-system circulation of meteoric water. Overall, the data provides an insight into possible controls on so-called porosity inversion, a poorly understood phenomenon in many hydrocarbon-bearing oolitic grainstone reservoirs.

Deep burial diagenesis and reservoir quality evolution of high-temperature, high-pressure sandstones: Examples from Lower Cretaceous Bashijiqike Formation in Keshen area, Kuqa depression, Tarim basin of China

The deep high-temperature, high-pressure Lower Cretaceous Bashijiqike sandstone (buried to depths as great as 6.5-7.1km) is an important natural gas reservoir in Keshen gas field, Kuqa depression of the Tarim Basin. Reservoir quality is a critical risk factor in the development of these ultradeep reservoirs. Integrated approaches incorporating routine core analyses, mineralogical, petrographic, and geochemical analyses have been used to investigate the diagenetic history of these rocks and their impact on reservoir quality with the aim to unravel the mechanisms for maintaining anomalously high porosities in sandstones that are buried to such a great depth. These sandstones are dominantly fine to medium-grained, moderately to good sorted lithic arkoses and feldspathic litharenite. Most primary pores have been lost by mechanical compaction or carbonate cementation, and the reduction of porosity by mechanical compaction was more significant than by cementation. Dissolution of framework grains contributed to the enhancement of reservoir quality. Eogenetic diagenetic alterations mainly include mechanical compaction, precipitation of calcite cements and grain-coating clays; mesogenetic diagenesis is characterized by dissolution of framework grain by organic acids and subsequent precipitation of clay minerals and quartz; infiltration of meteoric water related to teleodiagenesis would result in dissolution of the framework grains. The meteoric leaching events during teleodiagenesis are of great importance for the Bashijiqike sandstones. Grain-coating clay minerals (mixed-layer illite/smectite clays) help to preserve porosity at depth by retarding quartz cementation and pressure solution. The unique burial regime as early-stage shallow burial with late-stage rapid deep burial contributes to porosity preservation in eodiagenesis. Fluid overpressure caused by intense structural compression in the middle Himalayan movement retarded compaction and helped preserve porosity in the late rapid deep burial stage. Anomalously high porosities are mainly found in medium-grained, good sorted sandstones with grain-coating clays, but with low clay and carbonate cement content, of which the porosity is preserved primarily and enhanced secondarily. The lowest porosities are associated with sandstones that are tightly compacted or cemented with carbonates or rich in detrital matrix. Porosity-depth trends may vary significantly with lithofacies due to its differences in textural and compositional attributes. Five lithofacies are defined in terms of detrital composition and texture, type and degree of diagenesis. The reservoir quality prediction models of various lithofacies are constructed, and the results of this study provide insights into mechanisms for maintaining anomalously high porosity and permeability in high-temperature, high-pressure sandstone reservoirs, and may help explain hydrocarbon distribution.

Diagenesis of tight oil sand reservoirs: Upper Triassic tight sandstones of Yanchang Formation in Ordos Basin, China

Upper Triassic sandstones in the Ordos Basin, northern‐central China, comprise tight oil reservoirs. Using a combination of thin sections, SEM, BSE, EDS, XRD, and fluid inclusion analyses, 24 core samples from 13 wells were collected to study the petrology, paragenesis, and diagenetic processes and implications for reservoir quality. Quartz cement usually occurs as overgrowths or euhedral quartz. Extensive dissolution and albitization of K‐feldspar can be observed. Five types of carbonate cements, ferrocalcite, ankerite, dolomite, calcite, and siderite, occur during different diagenetic stages. Two main types of illite and 5 main habits of chlorite are observed in this study. Kaolinite mainly occurs as booklets and vermicular aggregates. Diagenetic illite, chlorite, biotite, mixed‐layer illite/smectite (I/S), and other minor minerals are also observed. The diagenetic processes include compaction, alteration of volcanic materials and mica, clay mineral transformation, cementation (silica, aluminosilicate, and carbonate), and dissolution of feldspars and rock fragments. Compaction was a significant porosity‐reducing agent, and the presence of carbonate cement exerts a dominant impact on the reduction of porosity. Quartz cement and authigenic clays are less important; however, it is worth mentioning that pore‐lining clays are conducive to porosity preservation. In this study, most of the porosity variation is caused by a combination of compaction, carbonate cements, quartz cement, and authigenic clays. This study gives insights into diagenetic alterations within tight sandstones and has implications for reservoir quality prediction in similar settings.

The influence of pore geometry and orientation on the strength and stiffness of porous rock

The geometry of voids in porous rock falls between two end-members: very low aspect ratio (the ratio of the minor to the major axis) microcracks and perfectly spherical pores with an aspect ratio of unity. Although the effect of these end-member geometries on the mechanical behaviour of porous rock has received considerable attention, our understanding of the influence of voids with an intermediate aspect ratio is much less robust. Here we perform two-dimensional numerical simulations (Rock Failure Process Analysis, RFPA2D) to better understand the influence of pore aspect ratio (from 0.2 to 1.0) and the angle between the pore major axis and the applied stress (from 0 to 90°) on the mechanical behaviour of porous rock under uniaxial compression. Our numerical simulations show that, for a fixed aspect ratio (0.5) the uniaxial compressive strength and Young's modulus of porous rock can be reduced by a factor of ∼2.4 and ∼1.3, respectively, as the angle between the major axis of the elliptical pores and the applied stress is rotated from 0 to 90°. The influence of pore aspect ratio on strength and Young's modulus depends on the pore angle. At low angles (∼0–10°) an increase in aspect ratio reduces the strength and Young's modulus. At higher angles (∼40–90°), however, strength and Young's modulus increase as aspect ratio is increased. At intermediate angles (∼20–30°), strength and Young's modulus first increase and then decrease as pore aspect ratio approaches unity. These simulations also highlight that the influence of pore angle on compressive strength and Young's modulus decreases as the pore aspect ratio approaches unity. We find that the analytical solution for the stress concentration around a single elliptical pore, and its contribution to elasticity, are in excellent qualitative agreement with our numerical simulations. The results of our numerical modelling are also in agreement with recent experimental data for porous basalt, but fail to capture the strength anisotropy observed in experiments on sandstone. We conclude that the alignment of grains or platy minerals such as clays exerts a greater influence on strength anisotropy in porous sandstones than pore geometry. Finally, we show that the strength anisotropy that arises as a result of preferentially aligned elliptical pores is of a similar magnitude to that generated by bedding in porous sandstones and foliation in low-porosity metamorphic rocks. The modelling presented herein shows that porous rocks containing elliptical pores can display a strength and stiffness anisotropy, with implications for the preservation and destruction of porosity and permeability, as well as the distribution of stress and strain within the Earth's crust.

Origin and Distribution of Grain Dolostone Reservoirs in the Cambrian Longwangmiao Formation, Sichuan Basin, China

Dolostones in the Cambrian Longwangmiao Formation have become one of the most significant gas exploration domains in China. Over a trillion cubic meters of gas reservoirs have been discovered in the Gaoshiti-Moxi area; however, the origins and distribution of the dolostone reservoirs are not well understood. This work discussed the geology and geochemistry of the dolostone reservoirs in the Longwangmiao Formation to determine their origin and distribution. Two understandings are acquired: firstly, a carbonate ramp provided excellent conditions for grain beach deposition, while the presence of a hypersaline lake was favorable for the contemporaneous dolomitization of grain beach deposits. Petrographic and geochemical evidence further confirm that the Longwangmiao dolostone was formed during the contemporaneous stage. Secondly, the reservoir characteristics indicate that the grain beach sediments provide material basis for the development of the Longwangmiao dolostone reservoirs. Reservoir dissolution simulation experiments show that the porosity of the reservoirs was formed by dissolution during contemporaneous and burial stages. The dissolution pores formed during the contemporaneous stage were controlled by sequence interfaces. The large scale dissolution vugs formed during the burial stage subsequently spread along the pre-existing porosity and fracture zones. This study therefore identified that the development of grain dolostone reservoirs in a shallow water ramp under arid climatic conditions generally met the following conditions: (1) reefal beach deposits lay a foundation for reservoir development; (2) superficial conditions are an important determining factor for reservoir porosity; and (3) burial conditions provide environment for porosity preservation and modification.

Depositional and diagenetic controls on anomalously high porosity within a deeply buried CO2 storage reservoir—The Cambrian Mt. Simon Sandstone, Illinois Basin, USA

Current diagenetic models predict that sandstone porosity and reservoir quality decrease with burial depth and formation age. However, the opposite trend is true for the Cambrian Mt. Simon Sandstone. The lowermost section of the Mt. Simon is highly porous—it has an average log-derived effective porosity of 17.0%. To understand the possible reason(s) for this anomaly, the amount of porosity reduction due to compaction porosity loss (COPL) and cementation porosity loss (CEPL) was analyzed. The COPL and CEPL were coupled with sedimentary characteristics in order to ascertain which factors controlled porosity and reservoir quality in the Mt. Simon. The underlying Argenta sedimentary unit disconformably underlies the Mt. Simon and is included for diagenetic and depositional comparison.Similar to expected trends from other deep basin reservoirs, the COPL generally increases with depth in the Mt. Simon. The CEPL is significantly lower in the lower half of the Mt. Simon compared to the upper half of the Mt. Simon, indicating that higher porosity is attributed to having a lower volume of intergranular cement. Early episodes of diagenesis within the arkosic lower half of the Mt. Simon, such as clay infiltration and feldspar alteration, produced diagenetic grain-coating clays, which inhibited the development of quartz cements. Thus, these grain coatings assisted in porosity preservation within the Lower Mt. Simon by reducing CEPL. The upper half of the Mt. Simon is more heavily cemented as a result of changes in the depositional setting and thus changes in the lithology. Later, largely post-compaction feldspar dissolution further increased porosity in the Lower Mt. Simon Sandstone.

Diagenesis and reservoir quality of overpressured deep-water sandstone following inorganic carbon dioxide accumulation: Upper Miocene Huangliu Formation, Yinggehai Basin, South China Sea

Turbidite deposits, overpressure, hydrothermal fluid activity, and CO2 accumulations are the basic geologic characteristics of the Upper Miocene Huangliu Formation sandstone reservoir in the Dongfang gas field, Yinggehai Basin. This study aims to unravel the timing and origin of dissolution in sandstone and evaluate the effects of inorganic CO2 accumulation on reservoir quality. A comparative study between area DFX-1 (CO2-rich) and area DFX-2 (CO2-poor) has illustrated the differences in diagenetic alteration and the effects of CO2 accumulation on the sandstone reservoir. Diagenetic alteration of sandstones in area DFX-1 was characterised by intense dissolution and medium cementation. Dissolution of silicates following inorganic CO2 charging in the DFX-1 area created a higher percentage of dissolution porosity (41.0%–53.9% of total visible porosity), compared with minor dissolution porosity in the DFX-2 area (16.7%–29.6% of total visible porosity). Small amounts of kaolinite, authigenic quartz, ankerite, and siderite were precipitated by CO2-water-rock reactions in area DFX-1; these minerals are almost absent in area DFX-2. The carbonate cement of the Huangliu Formation sandstone in area DFX-1 is characterised by light δ13C (−3.3‰ to −1.6‰, PDB), which is consistent with formation from inorganic carbon. The timing of dissolution by inorganic CO2 is suggested to be at about 0.4 Ma based on the previous simulation experiments. In the two areas studied, overpressure was caused by pressure transmission along deep open faults and was not related to mudstone undercompaction. Overpressure did a little to contribute to porosity preservation. CO2 accumulating at low partial pressures related to hydrothermal fluid flow cannot make a net increase in porosity in the Huangliu Formation sandstone of the Dongfang gas field.

Diagenesis, plagioclase dissolution and preservation of porosity in Eocene and Oligocene sandstones at the Greeley oil field, southern San Joaquin basin, California, USA

Abstract The Vedder (Oligocene) and Kreyenhagen (Eocene) sandstones at the Greeley oil field consist of arkosic to subarkosic arenites and wackes deposited in shallow marine environments. Burial depths of the Vedder sandstones exceed 3150 m and the reservoir temperature is 124°C. The Kreyenhagen sandstones are buried to greater than 3920 m and the reservoir temperature is estimated to be c. 135°C. These sandstones are currently at or very near their deepest burial depths. The textural relationships of the diagenetic minerals suggest syndepositional formation of glauconite, phosphate and pyrite, followed by early precipitation of pore-lining clay coatings and carbonate cements along with framework-grain fracturing and possibly dissolution. With increasing burial, dissolution of the framework grains continued, accompanied by the albitization of feldspars, the formation of K-feldspar and quartz overgrowths, the precipitation of kaolinite and other clays and possibly the precipitation of late carbonate cements. Finally, hydrocarbon migration and the formation of pyrite occurred during late diagenesis. Porosity preservation and reservoir quality are primarily the result of plagioclase dissolution occurring as the strata approached their current burial depths. Mass balance calculations indicate the significant export of aluminium out of the sands. Thus secondary porosity produced by plagioclase dissolution has replaced the primary porosity destroyed by compaction, and now accounts for the majority of the porosity in these rocks.

Controls on reservoir quality of Lower Cretaceous tight sandstones in the Laiyang Sag, Jiaolai Basin, Eastern China: Integrated sedimentologic, diagenetic and microfracturing data

The Jiaolai Basin (Fig. 1) is an under-explored rift basin that has produced minor oil from Lower Cretaceous lacustrine deltaic sandstones. The reservoir quality is highly heterogeneous and is an important exploratory unknown in the basin. This study investigates how reservoir porosity and permeability vary with diagenetic minerals and burial history, particularly the effects of fracturing on the diagenesis and reservoir deliverability. The Laiyang sandstones are tight reservoirs with low porosity and permeability (Φ<10% and K < 1 mD). Spatial variations in detrital supply and burial history significantly affected the diagenetic alterations during burial. In the western Laiyang Sag, the rocks are primarily feldspathic litharenites that underwent progressive burial, and thus, the primary porosity was partially to completely eliminated as a result of significant mechanical compaction of ductile grains. In contrast, in the eastern Laiyang Sag, the rocks are lithic arkoses that were uplifted to the surface and extensively eroded, which resulted in less porosity reduction by compaction. The tectonic uplift could promote leaching by meteoric water and the dissolution of remaining feldspars and calcite cement. Relatively high-quality reservoirs are preferentially developed in distributary channel and mouth-bar sandstones with chlorite rims on detrital quartz grains, which are also the locations of aqueous fluid flow that produced secondary porosity. The fold-related fractures are primarily developed in the silt–sandstones of Longwangzhuang and Shuinan members in the eastern Laiyang Sag. Quartz is the most prevalent fracture filling mineral in the Laiyang sandstones, and most of the small-aperture fractures are completely sealed, whereas the large-aperture fractures in a given set may be only partially sealed. The greatest fracture density is in the silt–sandstones containing more brittle minerals such as calcite and quartz cement. The wide apertures are crucial to preservation of the fracture porosity, and the great variation in the distribution of fracture-filling cements presents an opportunity for targeting fractures that contribute to fluid flow.

Exceptional reservoir quality in HPHT reservoir settings: Examples from the Skagerrak Formation of the Heron Cluster, North Sea, UK

As the exploration of hydrocarbons moves into more complex and deeper basinal settings the need to understand the effect of high temperatures and high pressures on reservoir quality and rock properties becomes even more important. The fluvial channel sandstone reservoirs of the Skagerrak Formation in the Central North Sea exhibit anomalously high porosities and permeabilities considering their depth of burial (>3500 m below sea floor). Despite the complex depositional setting, diagenetic history, high overpressure and temperatures encountered in the Skagerrak Formation, hydrocarbons are currently being produced. The Skagerrak Formation reservoirs used in this study have encountered overpressures of >40 MPa and temperatures up to ∼185 °C at present day maximum burial. To identify the role played by the high pressure and high temperature encountered in the reservoir sandstones a multidisciplinary approach involving petrographic, fluid inclusion, and burial history modelling studies has been adopted. Our interpretation of the results is that the generation of shallow overpressure in these fields limited mechanical compaction and also played an important role in minimizing pressure solution in the chemical compaction regime as evidenced by reduced quartz cementation. Fluid inclusions found in the quartz overgrowths indicate late-stage development with temperatures of formation in the range 130 to 170 °C coincident with late-stage deeper burial. Hydrocarbon emplacement occurred after quartz cementation and has had little to no effect on porosity preservation. The formation of well-developed authigenic chlorite (>70% surface coating) and, to a lesser extent illite clay coats with burial had a positive effect on porosity preservation even though permeability was marginally reduced in the illite-rich sandstones. A schematic porosity and quartz cement evolution model has been developed which allows for pre-drill prediction of reservoir quality in the Heron Cluster and provide valuable insights for other complex high-pressure high-temperature reservoirs.

Effects of early petroleum charge and overpressure on reservoir porosity preservation in the giant Kela-2 gas field, Kuqa depression, Tarim Basin, northwest China

Kela-2 is a giant gas field with a proven reserve of 597 tcf in the Kuqa depression, northern Tarim Basin. Widespread overpressures have been encountered in the Eocene and Cretaceous sandstone reservoirs of the field, with pressure coefficients up to 2.1 from drill-stem tests and well-log data analysis. Disequilibrium compaction associated with horizontal tectonic compression may be the dominant overpressure mechanism in the sandstone reservoirs, because the overpressured sandstone with a maximum burial depth over 6000 m (19,685 ft) displays anomalously high porosity and low density. The causes for sandstone reservoirs with anomalously high porosity in the Kela-2 gas field were studied based on an integrated investigation of sandstone reservoir characteristics, paleo oil–water contact, petroleum charge history, and overpressure evolution. Collective evidence indicates that early oil charge had retarded the porosity reduction of the reservoir sandstone and resulted in disequilibrium compaction from overburden rocks, and overpressure from disequilibrium compaction and horizontal tectonic compression at the beginning of the rapid subsidence and deposition in the Kela-2 gas field again contributed to the preservation of the reservoir porosity: (1) overpressured mudstones in the Kela-2 gas field are characteristic of normal compaction, and overpressure was generated by horizontal tectonic compression instead of disequilibrium compaction; (2) the reservoir sandstones with high porosity and permeability are associated with high paleo oil saturation, as indicated by quantitative grain fluorescence (QGF) responses and anomalous QGF on extract intensity; (3) sandstone units below the paleo oil–water contact have very low porosity and permeability; and (4) three episodes of oil and one episode of gas charge are identified in the sandstone reservoirs of the Kela-2 gas field, and the later two episodes of oil charge occurred circa 5.5–4.5 Ma, which corresponds to the beginning of the rapid tectonic subsidence and deposition in the Kuqa depression. The initially charged oil in the sandstone reservoirs was subsequently displaced by gas at circa 3–2 Ma through fault activation at the edge of the anticline trap. The overpressure evolution for the K1bs reservoir sandstone in the Kela-2 gas field indicates that the apparent overpressure development in the sandstone reservoir began at 5 Ma following the major oil charge and has been maintained to the present. Overpressure development from 5 Ma in the sandstone reservoirs of the Kela-2 gas field is believed to be the dominant cause of the porosity preservation.

Acoustic Behavior of Subfloor Lightweight Mortars Containing Micronized Poly (Ethylene Vinyl Acetate) (EVA).

This paper aims to contribute to acoustical comfort in buildings by presenting a study about the polymer waste micronized poly (ethylene vinyl acetate) (EVA) to be used in mortars for impact sound insulation in subfloor systems. The evaluation method included physical, mechanical and morphological properties of the mortar developed with three distinct thicknesses designs (3, 5, and 7 cm) with replacement percentage of the natural aggregate by 10%, 25%, and 50% EVA. Microscopy analysis showed the surface deposition of cement on EVA, with preservation of polymer porosity. The compressive creep test estimated long-term deformation, where the 10% EVA sample with a 7 cm thick mortar showed the lowest percentage deformation of its height. The impact noise test was performed with 50% EVA samples, reaching an impact sound insulation of 23 dB when the uncovered slab was compared with the 7 cm thick subfloor mortar. Polymer waste addition decreased the mortar compressive strength, and EVA displayed characteristics of an influential material to intensify other features of the composite.

Fluid inclusion and geochemical evidence for the origin of sparry calcite cements in Upper Permian Changxing reefal limestones, eastern Sichuan Basin (SW China)

Gas reservoirs in the Upper Permian Changxing Formation in eastern Sichuan Basin are found mostly in dolostones, rather than the contemporaneous reefal limestones that had most of their porosity occluded by pervasive calcite cementation. The sparry calcite, which was the latest but most abundant type of cement, was responsible for major porosity loss. Hence, this study focuses attention on the origin of sparry calcite by employing petrography (e.g. microscope, cathodoluminescence), geochemistry (carbon, oxygen and strontium isotopes, trace elements, and clumped isotope thermometry) and fluid inclusion analysis. The sparry calcite cements are interpreted to be of burial origin as shown by their enclosement of radiaxial calcite and host rock fragments, the presence of two-phase (liquid and vapor) fluid inclusions with higher homogenization temperatures, and relatively lower oxygen isotopic values. Sparry calcites show none to very dull cathodoluminescence, preserve similar carbon and strontium isotopic signatures to other chemical components of host rocks, and have low concentrations of Fe and Mn. These facts indicate that the source for precipitation of sparry calcites was most probably derived from chemical compaction of host rocks and/or adjacent limestone strata. The contrast in porosity and sparry calcite abundance between the reefal limestones and dolostones imply that pervasive dolomitization of carbonates in the Changxing Formation probably occurred prior to chemical compaction, rather than deep-burial dolomitization proposed in previous studies. Early dolomitization enhanced preservation of porosity because the dolostones were more resistant to chemical compaction than the limestones. Reefal limestones in the Changxing Formation gradually lost their porosity during progressive burial because of chemical compaction and the resultant sparry calcite cementation. This study emphasizes the importance of chemical compaction in carbonate cement precipitation and the resultant porosity occlusion in deep burial carbonate successions.

Enhanced porosity preservation by pore fluid overpressure and chlorite grain coatings in the Triassic Skagerrak, Central Graben, North Sea, UK

Abstract Current understanding of porosity preservation in deeply buried sandstone reservoirs tends to be focused on how diagenetic grain coatings of clay minerals and microquartz can inhibit macroquartz cementation. However, the importance of overpressure developed during initial (shallow) burial in maintaining high primary porosity during subsequent burial has generally not been appreciated. Where pore fluid pressures are high, and the vertical effective stress is low, the shallow arrest of compaction can allow preservation of high porosity and permeability at depths normally considered uneconomic. The deeply buried fluvial sandstone reservoirs of the Triassic Skagerrak Formation in the Central Graben, North Sea, show anomalously high porosities at depths greater than 3500 metres below sea floor (mbsf ). Pore pressures can exceed 80 MPa in the upper part of the Skagerrak Formation at depths of 4000–5000 mbsf, where temperatures are above 140°C. The Skagerrak reservoirs commonly have high primary porosities of up to 35%, little macroquartz cement and variable amounts of diagenetic chlorite grain coats. This research sheds light on the complex controls on reservoir quality in the fluvial sandstones of the Skagerrak Formation by identifying the role of shallow overpressure in arresting mechanical compaction and the importance of chlorite detrital grain coatings in inhibiting macroquartz cement overgrowth as temperature increases during progressive burial.

FORMATION OF AN OPEN FACE IN DIFFERENT GEOLOGICAL AND TECHNICAL CONDITIONS OF BUILDING MULTI-WELLS

The term formation of an open refers to the creation of well completion technologies, which are combined to a gradual deepening of the wellbore in the project horizon with simultaneous waterproofing aquifers, hardening of unstable rock intervals and preservation of porosity and hydrocarbon saturation properties of interlayers. The trend complication geological and field conditions completions in the late and final stages of oil and gas development toughens requirements for quality tailing deposits, tightness lining and hydrodynamic design excellence bottom wells. To solve arising in this complex technical problems in the abnormal geological and field conditions of technogenic origin in recent years is technology that combines the processes of destruction of rocks and deepen the bottom of wells with permeable walls mudding trunk in different ways To this end, developed advanced in permeable formations colmatation screens of different functionality, which can be divided into two main classes: Technology of deep colmatational areas with long-term and irreversible insulated reservoir fluids; Technology of shallow (penetration into the formation) shielding, temporarily protect reservoirs from the infiltration of process fluids and their effluents. The paper considered three technologies: The technology of selective treatment fluid jetting Fat productive technology is intended for long-term isolation of permeable formations unsealed as in the conventional technology (for repressionnom or pressure balanced), and jetting technology; Technology gentle mudding in the process of deepening the face and a device for its implementation; Primary drilling technology using struyno- wave kolmatatora and material quartz.