Cemented Sandstone Research Articles

Coupled experimental/numerical workflow for assessing quantitative diagenesis and dynamic porosity/permeability evolution in calcite-cemented sandstone reservoir rocks

Some of the world best hydrocarbon reservoirs (carbonates and siliciclastics) are also believed to be valuable for subsurface storage of CO2 and other fluids. Yet, these reservoirs are heterogeneous in terms of their mineralogy and flow properties, at varying spatial-temporal scales. Therefore, predicting the porosity and permeability (flow properties) evolution of carbonates and sandstones remains a tedious task. Diagenesis refers to the alteration of sedimentary rocks through geologic time, mainly due to rock-fluid interactions. It affects primarily the flow properties (porosity and permeability) of already heterogeneous reservoir rocks. In this project a new approach is proposed to calculate/quantify the influence of diagenetic phases (e.g. dissolution, cement plugging) on flow properties of typical sandstone reservoir rocks (Early Jurassic Luxembourg Formation). A series of laboratory experiments are performed in which diagenetic phases (e.g. pore blocking calcite cement in sandstone) are selectively leached from pre-studied samples, with the quantification of the petrophysical characteristics with and without cement to especially infer permeability evolution. Poorly and heavily calcite-cemented sandstone samples, as well as some intermediate cemented samples were used. The results show a distinctive dissolution pattern for different cementation grades and varying Representative Elementary Volumes (REVs). These conclusions have important consequences for upscaling diagenesis effects on reservoirs, and the interpretation of geochemical modelling results of diagenetic processes. The same approach can be applied on other type of cements and host-rocks, and could be improved by integrating other petrophysical analyses (e.g. petroacoustic, NMR).

A natural cranial endocast with possible dicraeosaurid (Sauropoda, Diplodocoidea) affinities from the Lower Cretaceous of Patagonia

A partially preserved sauropod natural cranial endocast (MLL-003) was collected from the Lower Cretaceous Mulichinco Formation in Argentina. The cast consists of cemented sandstone that has preserved in detail the general endocranial morphology, but not the cranial nerves. The dural expansion, the dorsal portion of the medulla oblongata, the cerebrum and the optic lobes are preserved, and suggest the specimen belongs to a sauropod dinosaur. The presence of the pineal foramen, and possibly post-parietal foramen and floccular process, suggests this specimen is more likely related to dicraeosaurids than it is to Titanosauriformes. This makes specimen MLL-003 particularly relevant because the paleoneurology of Dicraeosauridae is poorly known.

Diagenesis of Chang6 Formation of Hu2 block, Huanjiang oilfield, Ordos Basin

ABSTRACTThree mian diagenetic processes, compaction, cementation and dissolution of Chang6 sandstone in Hu2 block of Huanjiang oilfield were identified and their impact on the tight reservoir quality were analyzed. Four pore types, intergranular, intragranular, moldic and micropores were identified. Porosity and permeability of low-energy facies with high level of muddy content were generally lower. A rapid loss of primary porosity was caused by compaction, proved by the abrupt change tendency at approximate depth of 2130m. Calcite was the dominant cement which plugged the intergranular pores, whereas other minerals like secondary overgrowth, kaolinite and illite also contributed to tightening the reservoir.

Isotopically zoned carbonate cements in Early Paleozoic sandstones of the Illinois Basin: δ18O and δ13C records of burial and fluid flow

SEM/SIMS imaging and analysis of δ18O and δ13C in sandstones from a transect through the Illinois Basin (USA) show systematic μm-scale isotopic zonation of up to 10‰ in both carbonate and quartz cements of the middle-Ordovician St. Peter and Cambrian Mt. Simon formations. Quartz δ18O values are broadly consistent with the model of Hyodo et al. (2014), wherein burial and heating in the Illinois Basin is recorded in systematically zoned quartz overgrowths. Observations of zoned dolomite/ankerite cements indicate that they preserve a more extended record of temperature and fluid compositions than quartz, including early diagenesis before or during shallow burial, and late carbonates formed after quartz overgrowths. Many carbonate cements show innermost dolomite with δ18O values (21–25‰ VSMOW) that are too low to have formed by deposition at low temperatures from ancient seawater (δ18O>−3‰) and most likely reflect mixing with meteoric water. A sharp increase in Fe content is commonly observed in zoned carbonate cements to be associated with a drop in δ18O and an abrupt shift in δ13C to higher or lower values. These changes are interpreted to record the passage of hot metal-rich brines through sandstone aquifers, that was associated with Mississippi-Valley Type (MVT) Pb-Zn deposits (ca. 270Ma) of the Upper Mississippi Valley. Local variability and individual trends in δ13C are likely controlled by the sources of carbon and the degree to which carbon is sourced from adjacent carbonate units or thermal maturation of organic matter. Quartz overgrowths in sandstones provide an excellent record of conditions during burial, heating, and pressure-solution, whereas carbonate cements in sandstones preserve a more-extended record including initial pre-burial conditions and punctuated fluid flow events.

Discrete Element Modeling of Micro-scratch Tests: Investigation of Mechanisms of CO2 Alteration in Reservoir Rocks

The injection of CO2 into geological formations leads to geochemical re-equilibrium between the pore fluid and rock minerals. Mineral–brine–CO2 reactions can induce alteration of mechanical properties and affect the structural integrity of the storage formation. The location of alterable mineral phases within the rock skeleton is important to assess the potential effects of mineral dissolution on bulk geomechanical properties. Hence, although often disregarded, the understanding of particle-scale mechanisms responsible for alterations is necessary to predict the extent of geomechanical alteration as a function of dissolved mineral amounts. This study investigates the CO2-related rock chemo-mechanical alteration through numerical modeling and matching of naturally altered rocks probed with micro-scratch tests. We use a model that couples the discrete element method (DEM) and the bonded particle model (BPM) to perform simulations of micro-scratch tests on synthetic rocks that mimic Entrada sandstone. Experimental results serve to calibrate numerical scratch tests with DEM–BPM parameters. Sensitivity analyses indicate that the cement size and bond shear strength are the most sensitive microscopic parameters that govern the CO2-induced alteration in Entrada sandstone. Reductions in cement size lead to decrease in scratch toughness and an increase in ductility in the rock samples. This work demonstrates how small variations of microscopic bond properties in cemented sandstone can lead to significant changes in macroscopic large-strain mechanical properties.

Quantifying Porosity through Automated Image Collection and Batch Image Processing: Case Study of Three Carbonates and an Aragonite Cemented Sandstone

Modern scanning electron microscopes often include software that allows for the possibility of obtaining large format high-resolution image montages over areas of several square centimeters. Such montages are typically automatically acquired and stitched, comprising many thousand individual tiled images. Images, collected over a regular grid pattern, are a rich source of information on factors such as variability in porosity and distribution of mineral phases, but can be hard to visually interpret. Additional quantitative data can be accessed through the application of image analysis. We use backscattered electron (BSE) images, collected from polished thin sections of two limestone samples from the Cretaceous of Brazil, a Carboniferous limestone from Scotland, and a carbonate cemented sandstone from Northern Ireland, with up to 25,000 tiles per image, collecting numerical quantitative data on the distribution of porosity. Images were automatically collected using the FEI software Maps, batch processed by image analysis (through ImageJ), with results plotted on 2D contour plots with MATLAB. These plots numerically and visually clearly express the collected porosity data in an easily accessible form, and have application for the display of other data such as pore size, shape, grain size/shape, orientation and mineral distribution, as well as being of relevance to sandstone, mudrock and other porous media.

Prediction of diagenesis and reservoir quality using wireline logs: evidence from the Upper Triassic (Raethian) fluvial reservoir tags - Toual field, Gassi Touil Area, SE Algeria

Reservoir quality is mainly controlled by environment deposit type and diagenesis processes. To investigate such subject we usually proceed to microscopic techniques. Absence of outcrops and missing of core samples let us use conventional wireline logs and core lab measurements as primary data. Direct lecture of well logs calibrated by core photos and wellsite description, well logs correlation and cross plots were our main methodology axis. Target of this study is the fluvial Shaly Sandstones of Upper Triassic (Rhaetian) reservoir (abreviated in French; TAGS) located in Toual field – SE of Algeria. The present investigation confirms the absence of clean sandstone levels in the studied reservoir. It detects also the presence of halite and clays (Illite) as cements in sandstone. The non-influence of illite on permeability was confirmed whereas porosity is less affected.

Cementation and structural diagenesis of fluvio-aeolian Rotliegend sandstones, northern England

Quartz cementation in sandstones is closely linked to grain coating phases and diagenetic alteration. Grain coatings consisting of illite–smectite stained with iron oxides and hydroxides are able to preserve large amounts of porosity by preventing the formation of syntaxial quartz overgrowth cement. The Penrith Sandstone Formation was chosen as an analogue for Rotliegend reservoirs to test the impact of grain coatings on quartz cementation. This adds to an existing model of cementation. Differences of grain coating coverage can be linked to grain size. Extensive grain coatings are present in finer grained laminae in some samples. Coarser grained laminae contain less extensive grain coatings. The analysis of grain coatings based on standard petrographic analyses is combined with high-resolution QEMSCAN® analyses. Structural features include deformation bands of different ages. Diagenetic alterations around faults, recorded by grain coatings, allow the delineation of relative temporal relations, revealing at least two generations of deformation band formation associated with normal faulting. In the Vale of Eden succession one normal faulting event postdates burial diagenetic quartz cementation as is evident by fault focused fluid flow and associated bleaching of iron and absence of quartz overgrowth.

Experimental interaction of hydrothermal Na-Cl solution with fracture surfaces of geothermal reservoir sandstone of the Upper Rhine Graben

The Upper Rhine Graben (URG) is a Tertiary rift structure in central Europe offering favorable conditions for geothermal energy utilization. Relatively high heat flow combines with sufficiently high permeability of the hydrothermal reservoirs. One of the target stratigraphic levels is the lower Triassic sandstone formation (Buntsandstein), where hot water resources at temperatures up to 250 °C can be utilized. Extensional neotectonics and hydraulic stimulation form new fracture surfaces in the reservoir rocks (enhanced geothermal system, EGS). The exposed fresh rock fracture surface reacts with the highly saline reservoir brines (Na-Cl up to 200 g l−1) with consequences for the permeability of the reservoir.In order to better understand the dynamic evolution of fault systems caused by fluid-rock interaction, we conducted batch-type experiments in a stirred autoclave system and reacted arkosic sandstone with synthetic 2 molal Na-Cl solution at temperatures of 200 °C and 260 °C. After 45–55 days reaction time altered rock samples were compared with the starting material and the geochemical-mineralogical processes were deduced with the help of XRD, SEM methods and EMP measurements. Fluid compositions were examined by ICP-MS, ICP-OES and IC.The arkosic sandstones show a surprisingly high reactivity during the experiments. Quartz grain surfaces show deep dissolution features and all reaction fluids were saturated with respects to quartz. Illite and kaolinite from the primary sandstone cementation completely dissolved from the sample surface. Perfectly euhedral crystals of metastable analcime formed during the experiment as separate crystals on quartz, as groups or clusters and as surface covering mats. The overall net transfer process dissolves quartz + illite + kaolinite ± K-feldspar and precipitates analcime + chlorite ± albite. The process is accompanied by a total volume increase of the solids of 20–30 vol%. K-feldspar dissolution is hampered by albitization rims shielding K-feldspar and efficiently preventing an equilibration of the Na–K exchange with the fluid. The experiments show changes on the rock surface, leading to an increase of the aperture of a single fracture during the early phases of reaction and later to a decrease as the fluid-rock reaction progresses. Alteration of the fracture surface also generates loose fragments and altered minerals. This fine material may efficiently reduce the fracture aperture at narrow points along the fracture.

The Use of Integrated Fluid Inclusion Studies for Constraining Petroleum Charge History at Parsons Pond, Western Newfoundland, Canada

This study, based on fluid inclusion petrography, microthermometry and ultraviolet microspectroscopy of inclusion oil, investigates the petroleum charge history at Parsons Pond, western Newfoundland. To address this matter, drill core and cuttings samples of allochthonous and autochthonous strata in the Parson’s Pond area were collected from three exploration wells. Fluid inclusions were examined from fragments of calcite and quartz veins, diagenetic cements in sandstone, and in large hydrothermal dolomite and calcite crystals. Primary aqueous inclusions in authigenic sandstone cements indicate that cementation occurred at relatively shallow depths and low temperatures (<50 °C). Hydrocarbon-bearing fluid inclusions (petroleum, wet gas and gas) are generally restricted to calcite and quartz veins, indicating that petroleum and gas migration at Parson’s Pond is fracture-controlled. No hydrocarbons were observed in the diagenetic cements of the essentially tight sandstones. Fluid inclusion microthermometry and ultraviolet microspectroscopy indicate the presence of multiple generations of hydrocarbon fluid, ranging in composition from ~33 API gravity petroleum to pure CH4. Petrographic evidence suggests that hydrocarbons were generated multiple times during progressive burial and heating. In addition, the distribution of hydrocarbon bearing inclusions with depth suggests that deeper levels are gas-prone, with petroleum confined to relatively shallow depths. Although only gas flow was encountered during the drilling of exploration wells at Parson’s Pond, the presence of petroleum-bearing fluid inclusions in calcite and quartz veins indicates that the historical production from shallow wells in the Parsons Pond area likely tapped small reservoirs of fractured petroliferous strata.

Climate fluctuations recorded in phreatic and vadose calcretes of the Lower Carboniferous Clyde Sandstone Formation of Machrihanish, Kintyre Peninsula, SW Scotland

The Galdrings cliffs of Machrihanish, in the Kintyre Peninsula of Scotland, expose a Lower Carboniferous clastic succession that hosts a wide variety of calcretes, including thick and massive host-replacing phreatic calcrete hardpans (HRPCHs), which are geologically rare, and which are the products of a thorough replacement of host minerals by calcite in the mixing zone between fresh and evaporitic groundwaters. Isotopic values of 53 samples from various calcretes distributed in a c. 45 m thick succession provided well-defined trends of covariance between carbon and oxygen isotopes, thus delineating trends of aridification and humidification from the interplay of precipitation and evaporation rates. Although values from HRPCHs are more constrained, all other forms of calcrete in the succession (pedogenic nodules, pillar calcrete, laminar calcrete and invasive phreatic calcrete cement in sandstone) follow similar stable isotopic trends. An aridification trend preceded the formation of each HRPCH occurrence, corroborating studies on modern equivalents that suggest that they develop in the more arid range of calcrete formation. The better constrained isotopic values of HRPCHs compared with those of more common forms of calcrete are interpreted to be indicative of more specific environmental requirements, which may in part explain their rarity in the geological record.

Diagenesis and reservoir quality of sandstones with ancient “deep” incursion of meteoric freshwater——An example in the Nanpu Sag, Bohai Bay Basin, East China

An example of diagenesis and reservoir quality of buried sandstones with ancient incursion of meteoric freshwater is presented in this study. The interpretation is based on information including porosity and permeability, petrography, stable isotopic composition of authigenic minerals, homogenization temperatures (Th) of aqueous fluid inclusions (AFIs), and pore water chemistry. These sandstones, closely beneath or far from the regional unconformity formed during the late Paleogene period, are located in the thick Shahejie Formation in the Gaoliu area of Nanpu Sag, Bohai Bay Basin, East China. Early-diagenetic calcite cements were leached to form intergranular secondary pores without precipitation of late-diagenetic calcite cements in most sandstones. Feldspars were leached to form abundant intragranular secondary pores, but with small amounts of concomitant secondary minerals including authigenic quartz and kaolinite. The mass imbalance between the amount of leached minerals and associated secondary minerals suggests that mineral leaching reactions occurred most likely in an open geochemical system, and diagenetic petrography textures suggest that advective flow dominated the transfer of solutes from leached feldspars and calcites. Low salinity and ion concentrations of present pore waters, and extensive water rock interactions suggest significant incursion of meteoric freshwater flux in the sandstones. Distances of the sandstones to the regional unconformity can reach up to 1800 m, while with significant uplift in the Gaoliu area, the burial depth of such sandstones (below sea level) can be less than 800–1000 m during the uplift and initial reburial stage. Significant uplift during the Oligocene period provided substantial hydraulic drive and widely developed faults served as favorable conduits for downward penetration of meteoric freshwater from the earth's surface (unconformity) to these sandstone beds. Extensive feldspar leaching has been occurring since the uplift period. Coupled high Th (95∼115 °C) of AFI and low δ18O(SMOW) values (+17∼+20‰) within the quartz overgrowths show that quartz cementation occurred in the presence of diagenetic modified meteoric freshwater with δ18O(SMOW) values of −7∼−2‰, indicating that authigenic quartz only have been formed during the late reburial stage when meteoric fresh water penetration slowed down. Secondary pores in thin sections and tested porosity suggest that meteoric freshwater leaching of feldspars and calcite minerals generated approximately 7–10% enhanced secondary porosity in these sandstones. Meteoric freshwater leaching reactions cannot be ignored in similar sandstones that located deep beneath the unconformity, with great uplift moving these sandstones above or close to sea level and with faults connecting the earth's surface with the sandstone beds.

Petrological features of selected components of the Cergowa sandstones (Outer Carpathians) recorded by scanning electron microscopy – preliminary study

The scanning electron microscope analysis of the Cergowa sandstones brings new data on their petrological features and chemical composition. Previous work in standard petrographic examination, e.g. polarising (PL) or cathodoluminescence (CL) microscopy, displayed limited information on grain surface topography and only assumptions to their geochemistry. Both identification and characterisation of minerals are fundamental in the progress of mining and minerals processing systems. Detrital grains of the Cergowa sandstones are bound by calcite and dolomitic cement and commonly corroded by diagenetic fluids, however, in varying degrees, which is illustrated here by feldspar, quartz and dolomite minerals. Dissolution processes of marginal parts of these mineral grains resulted in corrosion, which increased the contact surface between the grains and the cement. The difference in resistance to these processes was observed not only among distinct groups of minerals, but also within the group of feldspars: between K–feldspars and minerals of plagioclase. That combination resulted in exceptionally strong cementation of the Cergowa sandstones, which is expressed by their high hardness and resistance to abrasion, freezing, and thawing. Inherent parameters of sandstones are characterised by their petrographical properties.

Origin and Distribution of Carbonate Cement in Tight Sandstones: The Upper Triassic Yanchang Formation Chang 8 Oil Layer in West Ordos Basin, China

Two generations of carbonate cement as Type I (microcrystalline calcite and dolomite) and Type II (mainly Fe-calcite and Fe-dolomite) are recognized in Chang 8 sandstones, Ordos basin. Carbonate cement in Chang 8 sandstones is closely related to the dissolved carbon from thermal maturation of organic matters. Carbonate cement in the loosely packed framework grains precipitated shortly after deposition, and late-stage ferroan calcite and ferroan dolomite formed with progressive burial. The early diagenetic carbonate cement is partially to completely replaced by late-stage ferroan calcite and ferroan dolomite. Carbonate cement is much more commonly observed in sand bodies adjacent to Chang 7 source rocks. With increasing distance from the Chang 7 oil layers, the carbonate cement content gradually decreases. However, some tight carbonate cemented zones also occur at the sandstone-mudstone interfaces. Dissolution of Ca-feldspars by organic acids-rich fluids, together with clay mineral transformations such as illitization of smectite, would provide Ca2+ and Mg2+ ions for carbonate cementation. Organic acids and CO2 rich fluids would charge into the reservoirs with the hydrocarbons, and when the CO2 and acids were buffered by the framework grain dissolution, carbonate cement would precipitate with a decrease in CO2 concentration.

Age of the Fontainebleau sandstones: a tectonic point of view

The age of the cementation of the Fontainebleau sandstones, located in the upper part of the Rupelian Fontainebleau Sand Formation and largely outcropping in the south of the center of the Paris Basin, remains a matter of debate: did the silicification occurred at early times during Miocene, following sedimentation, or did it occurred during Quaternary cold climate episodes? In this work, we determined an orthogonal fracture network (main directions N115° ±5° and N025° ±5°) over an area of ∼6000 km2. The fractures are oblique to the adjacent valley orientation and to the quarry working face orientation, discarding a gravitational origin. This tectonic fracturing is superimposed on regional scale antiforms and synforms that may be at least partly controlled by inherited basement faults reactivation during Alpine episodes. The whole Fontainebleau Sand Formation seems to be folded, including the Fontainebleau sandstones. We establish a relative chronology of the various phenomena and propose that silicification at the origin of the Fontainebleau quartzite occurred during early or middle Miocene. Alpine stresses then induced Fontainebleau sand and quartzite folding and fracturing during late Miocene and Pliocene. Finally, the fracture network facilitated fluid circulations and secondary carbonate sandstones or quartzite precipitation probably during Quaternary cold climate episodes.

The Distribution and Origin of Carbonate Cements in Deep-Buried Sandstones in the Central Junggar Basin, Northwest China

Extremely high porosities and permeabilities are commonly discovered in the sandstones of the Xishanyao Formation in the central Junggar Basin with the burial depth greater than 5500 m, from which hydrocarbons are currently being produced. High content of carbonate cements (up to 20%) is also observed in a similar depth range. Our study aimed to improve our understanding on the origin of carbonate cements in the Xishanyao Formation, in order to provide insights into the existence of high porosity sandstones at greater depths. Integrated analyses including petrographic analysis, isotopic analysis, fluid-inclusion, and core analysis were applied to investigate the distribution and origin of carbonate cements and the influence of high fluid pressure on reservoir quality. Textural evidences demonstrate that there are two generations of carbonate cements, precipitated at the temperature of 90°C and 120°C, respectively. The carbonate cements with low δCPDB13 ranging from −19.07 to -8.95‰ dominantly occurred near the overpressure surface and especially accumulated at approximately 100 m below the surface. Our interpretation is that high content of carbonate cements is significantly influenced by early carbonate cements dissolution and migration under overpressure. Dissolution of plagioclase resulted in the development of internal pores and porosities of as much as 10% at 6500 m depth presumably.

Trace Fossils in Clastic Beachrocks at the Yallahs Salt Ponds, Jamaica: Implications for Beachrock Cementation

ABSTRACT Edwards, T.C.P. and Mitchell, S.F., 2018. Trace fossils in clastic beachrocks at the Yallahs Salt Ponds, Jamaica: Implications for beachrock cementation. The extent and importance of trace fossils preserved in clastic beachrock from the southern section of the Yallahs Salt Ponds on the SE coast of Jamaica are described. The beachrock contains a seaward-dipping facies of cemented sandstones and conglomerates and a landward facies consisting of cemented sandstones packed with the trace fossil Psilonichnus, which is produced by the ghost crab Ocypode quadrata (Fabricius). The seaward-dipping bedded facies represents more than 95% of the beachrock at the Yallahs Salt Ponds and has an exposed height ranging from 6 to 50 cm and lateral continuity of 1 to 200 m. The bioturbated facies, which represents less than 5% of the beachrock at the Yallahs Salt Ponds, was only found at three localities. It has an exposed height of 40 to 60 cm, with outcrops traceable for 6 to 30 m along the beach. These bioturbat...

Paleogene-Neogene Cap Rocks and its Relationship with Hydrocarbon Accumulation in the Zhanhua Sag

To analyse the Zhanhua Paleogene–Neogene cap rocks and its relationship with hydrocarbon accumulation, the seal lithology, the relationship between compaction of argillite rock and its sealing capacity, and its destruction by faults and fractures were studied. The results indicate that there are four types of cap rocks: argillite rock and silty mudstone, microcrystalline carbonate, dense cemented sandstone and dense cemented carbonate. Among these cap rocks, argillite rock is the main type in the Zhanhua Sag. According to the evolutionary characteristics of the argillite rock and its destruction by fractures and faults, the argillite cap can be classified into three categories: porosity cap, fracture transformation cap and the fault transformation cap. Among their sealing capacities, the porosity cap is the best, followed by the fracture transformation cap, and the fault transformation cap is the worst. Through the analysis of the relationship between existing oil & gas reservoirs and the distribution characteristics of the Paleogene–Neogene cap rocks in the Zhanhua Sag, it was found that the cap combination which was below or above the reservoir together controlled the hydrocarbon accumulation and preservation. It means that the destruction of the cap below or down-dip the reservoir is a necessary condition for hydrocarbon accumulation, and only when the sealing capacity of the cap rock above or up-dip the reservoir is better than that of below or down-dip the reservoir, hydrocarbon could be efficiently stored in reservoirs, thus could be effectively enriched.

Origin and significance of secondary porosity: A case study of upper Triassic tight sandstones of Yanchang Formation in Ordos basin, China

The Chang7 and Chang6 sandstones in the southwestern Ordos basin are the typical tight oil sandstones in China. Pore textures and their genetic classes of secondary porosity were studied in detail using a combination of optical petrography, SEM and BSE. In order to understand generation process of secondary porosity better, the diagenetic history was reconstructed by a variety of methods, including thin section observation, SEM, BSE and fluid inclusion. Three main diagenetic events is considered to be most likely related to the generation of secondary porosity, these are: 1) meteoric water flushing; 2) formation of authigenic clay minerals; 3) leaching of feldspars, rock fragments and carbonate cements in deep burial sandstones. Particular attention was paid to the generation of secondary porosity and mass transformation in these diagenetic events. This study suggests only meteoric water flushing, which occurs at shallow depths, may result in a net increase of porosity because dissolved minerals can be removed out of sandstones in such an open system. However, below the zone of freshwater flow, dissolved minerals would precipitate as new diagenetic minerals at nearby pore spaces in sandstones, suggesting secondary porosity formed by dissolution only represents a local redistribution of the solids rather than a net increase of porosity.

Origin and timing of Dauphiné twins in quartz cement in fractured sandstones from diagenetic environments: Insight from fluid inclusions

Electron backscattered diffraction techniques (EBSD) show that Dauphiné twins in quartz are widespread in many tectonometamorphic environments. Our study documents that under diagenetic temperatures (<200°C) and burial depths <5km Dauphiné twins are common in isolated fracture quartz deposits spanning between fracture walls (i.e., quartz bridges) in low-porosity quartz-cemented sandstones. Using examples from East Texas and Colorado cores, we show that twins are associated with crack-seal microstructure and fluid inclusions. Fracture wall-parallel and wall-normal inclusion trails contain coexisting aqueous and hydrocarbon gas inclusions, so homogenization temperatures of aqueous inclusions record true trapping temperatures. Inclusions in alignments normal to fracture walls are large and irregularly shaped compared to those aligned parallel to walls, but both show similar liquid-to-vapor ratios. Stacking transmitted light images with scanning electron microscope cathodoluminescence (SEM-CL) and EBSD images demonstrates that Dauphiné twin boundaries are localized along wall-normal inclusion trails. Trapping temperatures for wall-normal inclusion trails are usually higher than those aligned parallel to the fracture wall. Wall-normal fluid inclusion assemblage temperatures typically match the highest temperatures of wall-parallel assemblages trapped during sequential widening, but not necessarily the most recent. In context of burial histories for these samples, this temperature pattern implies that wall-normal assemblages form at discrete times during or after crack-seal fracture widening. Localization in isolated, potentially high-stress quartz deposits in fractures is compatible with a mechanical origin for these Dauphiné twins. Punctuated temperature values and discrepant sizes and shapes of inclusions in wall-normal trails implies that twinning is a by-product of the formation of the wall-normal inclusion assemblages. The association of Dauphiné twins and fluid inclusion assemblages from which temperature and possibly timing can be inferred provides a way to research timing as well as magnitude of paleostress in some diagenetic settings.