Clay Mineral Cements Research Articles

Diagenetic controls over reservoir quality of tight sandstone in the lower Jurassic reservoir in the Lenghu area, the north margin of Qaidam basin

The Lower Jurassic Reservoir (LJR) in the Lenghu area on the northern margin of the Qaidam Basin (NMoQB) has become the most promising target for hydrocarbon exploration. The reservoir has experienced complex diagenesis; however, the porosity evolution and the influence of different diagenesis events on reservoir densification remains unclear. In this study, various analytical methods was first used to clarify the diagenetic stage and sequence, establish a porosity evolution model, quantitatively analyze the time and influence of different diagenesis events on reservoir densification, and illustrate the densification mechanism of tight sandstone reservoir in the Lenghu area. The results showed that the dominant rock types in the LJR were feldspathic litharenite, followed by litharenite, lithic arkose, and a small amount of subarkose and sublitharenite. The reservoir is a typical tight sandstone reservoir (TSR), with average porosity and permeability of 5.5% and 0.08 mD, respectively. The pore types were primarily secondary pores, followed by residual intergranular pores and occasional fractures. Diagenesis of LJR has entered the meso-diagenetic stage A, with minor progression into meso-diagenetic stage B. Quantitative calculation showed that the initial porosity of LJR is 32.2%. In eo-diagenetic stage A, compaction is the main factor for porosity reduction. In eo-diagenetic stage B, the porosity loss rates caused by compaction and cementation were 81.5% and 11.8%, respectively. In this stage, the LJR in the Lenghu area has almost been densified, mainly affected by compaction, calcite cementation and clay minerals cementation. Additionally, cementation is an important factor causing reservoir heterogeneity and is dominated by calcite and clay mineral cementation. After entering the meso-diagenetic stage A, dissolution is the main diagenesis event, which can increase porosity by 6.0%. This stage is the critical period for forming high-quality reservoir of LJR in the Lenghu area, mainly in the 4,400 m depth. In meso-diagenetic stage B, the influence of dissolution gradually decreased. Simultaneously, in this stage, the cementation of iron-bearing calcite further strengthened the densification of the LJR in the Lenghu area.

Diagenetic Evolution and Its Impact on Reservoir Quality of Tight Sandstones: A Case Study of the Triassic Chang-7 Member, Ordos Basin, Northwest China

The Upper Triassic Chang-7 Member of the Ordos Basin contains typical tight sandstone reservoirs. Reservoir quality is affected by diagenesis, which is a critical factor in tight oil exploration. In this study, the Chang-7 Member tight sandstones were studied by a variety of experimental methods, including thin sections, scanning electron microscopy (SEM), and X-ray diffraction (XRD), to determine the reservoir characteristics and diagenesis and discuss their influences on the reservoir quality. The Chang-7 Member sandstones are mainly lithic arkose and feldspathic litharenite with an average porosity and permeability of 7.17% and 0.13 mD, respectively. The pore type is mainly primary intergranular pores, and the secondary pores are feldspar dissolved pores, which are relatively developed, with pore radii of 2–20 µm. Diagenesis of Chang-7 Member tight reservoirs mainly includes compaction, quartz cementation, carbonate cementation, clay mineral cementation, and dissolution. The diagenetic stage develops into Mesodiagenesis A. The average porosity loss from compaction and cementation of sandstone in the Chang-7 Member is 15.93% and 18.67%, respectively. With the increase in burial depth, the porosity and permeability of the reservoir gradually decrease. In mesodiagenesis, the authigenic illite and carbonate cementation compacts the reservoir. The acid fluid carried by the two stages of oil and gas filling during diagenesis dissolved feldspar and carbonate cement, which plays a certain role in transforming the tight reservoir.

Porosity and permeability correction of laumontite-rich reservoirs in the first member of the Shaximiao Formation in the Central Sichuan Basin

The development of a large amount of laumontite cement in clastic reservoirs generate pseudo fractures at normal temperatures and pressures conditions on the ground surface, causing porosity and permeability values to be significantly higher than the actual values of buried conditions. Taking the first member of the Shaximiao Formation in the Middle Jurassic of the Central Sichuan Basin as an example, this study analyzes the material composition, reservoir porosity and permeability, diagenesis, and other characteristics of reservoirs rich in laumontite using the petrology, geochemistry, porosity and permeability characterization, and overburden pressure porosity and permeability tests and further discusses a method for correcting porosity and permeability at normal temperatures and pressures conditions on the ground surface. The reservoir rich in laumontite in the first member of the Shaximiao Formation in the Central Sichuan Basin primarily comprises lithic arkose sandstone, with porosity ranging from 2.12% to 17.48%, with an average of 10.28%, and permeability ranging from 0.004 to 120.351 mD, with an average of 2.691 mD. There are various types of diagenesis, including laumontite cementation, calcite cementation, siliceous cementation, clay mineral cementation, laumontite dissolution, feldspar dissolution, etc. Laumontite is mainly exhibited as pore filling cementation, and the extensive cementation and dissolution of laumontite are the key factors affecting porosity and permeability of reservoirs. By comparing the porosity and permeability difference between overburden pressure condition and normal temperatures and pressures condition, the influence of granular marginal pseudo fractures on the porosity and permeability of the reservoir is corrected. The corrected porosity and permeability values at normal temperatures and pressures conditions on the ground surface are linearly correlated with those before the correction. In different laumontite content intervals, the permeability of the reservoir rich in laumontite accords with the linear correction correlation. The higher the laumontite content, the smaller the correction coefficient is. Therefore, this correction guarantees accurately predict the porosity and permeability of the reservoir rich in laumontite at normal temperatures and pressures conditions on the ground surface.

Illitization in the Mt. Simon Sandstone, Illinois Basin, USA: Implications for carbon dioxide storage

Clay mineral cementation is one of the most important controls on sandstone reservoir properties. In this paper, the diagenetic history of the Mt. Simon reservoir complex is studied to reveal the origin, timing, and controls of illitization. Samples of sandstone and shale from Mt. Simon reservoir complex were acquired from the Illinois Basin–Decatur Project (IBDP), a CO2 storage demonstration project in the central Illinois Basin. Petrographic, SEM, TEM, XRD analyses and K/Ar age dating were completed to identify the major detrital and diagenetic components of the samples and reveal illite to be the major clay component in all samples. Illitic clay coatings in the lower Mt. Simon reservoir are identified as a major control on reservoir properties by inhibiting major precipitation of authigenic quartz during illitization, resulting in highly permeable sandstone, essential for CO2 storage. The coating box-work morphology and mineralogy are indicative of a detrital smectite origin with subsequent illite growth associated with feldspar dissolution and kaolinite alteration. The mineralogy of bulk material and separate grain size fractions (2–0.6 μm; 0.6–0.2 μm; < 0.2 μm) with illite polytypes 2M1, 1M, and 1Md were quantified and age dated via 40K-40Ar methods. The shales or reservoir seals contain the highest proportions of detrital illite with illite in the lower Mt. Simon Sandstone reservoir identified as solely diagenetic. Two major events of illitization are identified throughout the Mt. Simon with more porous reservoir rock exhibiting the older event from approximately 360 to 315 Ma and tighter sandstone units with reservoir properties too low to be considered reservoir exhibiting illite dates from 250 to 220 Ma. This partitioning of illitization is attributed to varied mineralogy controlled by depositional changes and evolution of the greater basin.

Automated mineralogy (SEM-EDS) approach to sandstone reservoir quality and diagenesis

Porosity and permeability define the reservoir quality of a sandstone. Porosity and permeability are controlled by primary sedimentary characteristics and subsequent diagenetic modification. Grain size, sorting, detrital mineralogy, and matrix content are defined at the point of deposition. Bioturbation, infiltration, continued alteration of reactive minerals and soft sediment deformation can occur soon after deposition and, together with the primary characteristics, these condition, or define, how a sediment will behave during subsequent burial. Diagenetic modifications include compaction, initially mechanical and then chemical, mineral growth and, in some cases, grain dissolution and development of secondary pores. Porosity and permeability typically decrease as diagenesis progresses. Scanning Electron Microscopy-Energy Dispersive Spectroscopy (SEM-EDS) approaches can be applied to study many aspects of sandstone diagenesis including detrital mineralogy, grain size, sorting, grain shape, grain angularity and matrix content. SEM-EDS is also useful for defining quantities and location in the pore network of cements that are mineralogically distinct from detrital grains (e.g., calcite, dolomite, siderite, or anhydrite). SEM-EDS can be useful for studying clay mineral cements, especially when they occur in patches bigger than 5–10 μm. In sandstones, SEM-EDS is not so useful when the cements are mineralogically identical to detrital grains (e.g., quartz cement in quartz sandstones) where additional signals such as cathodoluminescence (CL) may be required. Macro- and meso-pores can be quantified using SEM-EDS but, on its own, it cannot automatically measure micro-porosity as it is below the minimum 1 µm spatial resolution of the approach. Mechanical compaction, a key process that causes porosity-loss in sandstones, is not easily quantified using SEM-EDS. Nonetheless, together with additional routine techniques, SEM-EDS can be used to solve most problems associated with sandstone diagenesis and reservoir quality.

The Study on Diagenetic Characteristics of Coal Measures Sandstone Reservoir in Xishanyao Formation, Southern Margin of the Junggar Basin

The reservoir physical properties, pore types, diagenetic characteristics and reservoir quality controlling effect of the Xishanyao formation coal measure sandstone in the southern margin of the Junggar basin were discussed in this study based on thin section observation, high pressure mercury injection, low-temperature nitrogen adsorption and scanning electron microscope observation. The result shows that the porosity and permeability of the sandstone are generally low with a medium-high texture maturity and low compositional maturity. The sandstone storage space is mainly composed of residual intergranular pores, secondary dissolution pores, inter-crystalline pores and micro-fractures. The diagenetic stage of coal measure sandstone is in the mesodiagenesis A1-A2 stage, and their diagenetic interaction types mainly include compaction, cementation and dissolution. The reservoir quality of the coal measure sandstone deteriorates by compaction due to high matrix content and plastic debris content. Because of the large amounts of organic acids generated during the thermal evolution of the coal measure source rock, the coal measure sandstone suffers from strong dissolution. The secondary dissolution pores formed by the massive dissolution of feldspar, lithic fragments and early carbonate cementation in the sandstone significantly improved the reservoir quality. In the coal measure sandstone, clay mineral cementation is the most developed cementation form, followed by quartz cementation and carbonate cementation. Although kaolinite cementation and dolomite cementation can generate a small number of inter-crystalline pores, cementation deteriorates the reservoir quality. The Xishanyao formation coal measure sandstone formed in a lacustrine-delta environment, and its composition and texture make it susceptible to the influence of compaction and dissolution during diagenesis.

Integrated Diagenesis Study of Tight Gas Sandstone: The Permian Lower Shihezi Formation, Northern Ordos Basin, China

Abstract Diagenesis varies greatly from basin to basin and has been considered as the key control of the reservoir quality of tight sandstones. In this study, we analyzed the petrographic characteristics, diagenesis, and pore types and characteristics of the Permian Lower Shihezi Formation in the Ordos Basin. Results show that most of sandstones are litharenites with minor sublitharenites and feldspathic litharenites. The tight sandstones have ultralow permeabilities (averaging 0.67 mD) and porosities (averaging 8.35%), with pore-throat sizes ranging from 0.035 to 13.29 μm with 94.12% less than 5 μm. Overall, the tight sandstone reservoirs have undergone complex diagenetic alteration. Compaction and clay mineral cements are the two crucial diagenetic factors that controlled the tightness of the reservoirs. The compaction destroyed most of primary porosity, and the effective pores are mainly dissolved micropores, intragranular dissolved micropores, and micropores in clay minerals and mineral grains. The chlorite coating might have helped preserve a certain amount of porosity, but pore-lining chlorites significantly obstructed pore throats and reduced permeability. Results of the study provide insights and direct implications for the future success of exploration and production of the tight gas sandstone in the Ordos Basin and other similar tight sandstone reservoirs.

Braided deltas and diagenetic control on tight sandstone reservoirs: A case study on the Permian Lower Shihezi Formation in the southern Ordos Basin (central China)

The Lower Shihezi Formation, situated in the south of the Chinese Ordos Basin, is a tight sandstone reservoir that is generally influenced by its sedimentary facies and diagenesis, corresponding to distinct petrophysical reservoir properties for each lithofacies type. The existing lithofacies types within the Lower Shihezi Formation were determined through observations in field profiles and drill cores that were combined with the analysis of logging data. Diagenetic processes and their effects were analyzed by using different methods, such as conventional thin section, cast thin section, and scanning electron microscope (SEM) analyses. The results of this contribution indicate that the Lower Shihezi Formation in the southern Ordos Basin is dominated by braided delta facies and locally developed lacustrine facies. During diagenesis of this studied formation, compaction and cementation led to a reduction in porosity with compaction probably forming the main factor. Existing cementation is mainly manifested in the form of clay mineral cements, siliceous and carbonate cements, and induced a somewhat lower pore reduction effect on the reservoir compared to compaction. Minor dissolution led to the break-down of rock fragments, feldspars, and other soluble minerals, and thereby may have caused a subsequent formation of secondary pores, thus effectively improving the reservoir property. Consequently, due to the compositional differences and corresponding unique diagenetic effects, the distributary channels and underwater distributary channels display the highest reservoir properties. These are followed by the mouth bars and sand sheets. The lowest reservoir properties are shown by the natural levee microfacies.

Diagenesis and reservoir space types of alkaline lake-type shale in Fengcheng Formation of Mahu Sag, Junggar Basin, China

Using observations and descriptions of drilling cores, image logging, microscopic section, argon ion polishing field emission scanning electron microscopy, X-ray diffraction, and whole-rock trace element analysis, the study of shale reservoir diagenesis and space types in the Fengcheng Formation of the Mahu Sag was conducted. Considering the trace element contents and their ratios (Sr/Ba, V/Ni, Th/U, V/(V + Ni), U/Mo, and Sr/Cu), the Fengcheng Formation is formed in a dry and hot continental lacustrine basin with a paleoenvironment of saltwater and anoxic/lean oxygen conditions. The shale reservoirs of the Fengcheng Formation with the characteristic of multisource mixed sedimentation include terrigenous clastic, volcaniclastics, and carbonate rocks. Currently, the Fengcheng reservoir of the Mahu Sag is in the middle of diagenetic-stage B. The principal factors for reservoir densification are compaction, dissolution, carbonate mineral cementation, and clay mineral cementation. The Fengcheng Formation develops multiple reservoir storage space types, such as rock fractures, stylolites, and micro–nanoscale pore-throat systems. The macroreservoir space types include tectonic, induced, bedding, and dissolution–expansion fracture types. The microreservoir space types include microfractures, stylolites, and micro–nanoscale pore throats. The research showed that the Fengcheng Formation has tectonic fracture-pore systems (tectonic fracture-type reservoirs) and stylolite-matrix pore-tectonic microfracture systems (shale oil reservoirs), forming the shale oil preponderant charging channel network. Reservoir space type and its spatial distribution are the principal factors for shale oil accumulation of the Fengcheng Formation in the Mahu Sag.

Dissolution versus cementation and its role in determining tight sandstone quality: A case study from the Upper Paleozoic in northeastern Ordos Basin, China

Physical and chemical alterations, particularly the balance between cementation and dissolution, is significant in recognizing and predicting sandstone properties. Evidence of quantitative porosity evolution and direct chemical variation was acquired based on thin section observation, mercury porosimeter, scanning electron microscopy, vitrinite reflectance and Rock-Eval tests. A whole succession of tight gas sandstone from the Upper Paleozoic in the northeastern Ordos Basin, China, was investigated. This included the Carboniferous Taiyuan (C2t), Permian Shanxi (P1s) and Xiashihezi (P2x) Formations. The sandstones are mainly lithic, lithic quartz, and lithic feldspar sandstone, characterized by low porosity and low permeability. The observed pores are mainly of intergranular dissolution, intragranular dissolution, and inter-crystalline pores. The sandstones exhibit a meso-diagenesis stage A to B transition with deeper strata. The P2x Sandstone had widely distributed K-feldspar dissolution and quartz overgrowths. In its underlying P1s, kaolinite-illite transformation as well as extensive siliceous cementation were observed. The deeper C2t had a largely distributed illite, while kaolinite disappeared. The porosity of P2x Sandstone varied by −16.50, −22.38, and +2.13% during the compaction, cementation, and dissolution processes, respectively. A criterion for quantitatively classifying diagenesis intensity was proposed. The P2x Sandstone was located in the medium compaction to medium cementation and medium dissolution range, while the P1s and C2t were in the medium compaction to weak cementation and strong dissolution range. Two dissolution-cementation systems were recognized; the weak cementation-strong dissolution and medium cementation-strong dissolution systems. The dissolved feldspar generally coexisted with clay mineral cementation. Although P1s and C2t sandstones belong to the strong dissolution range, it is highly insufficient to offset the damage caused by the cementation, the secondary porosity being low.

A Simplified Physical Model Construction Method and Gas-Water Micro Scale Flow Simulation in Tight Sandstone Gas Reservoirs

Accuracy defects exist when modeling fluid transport by the classical capillary bundle model for tight porous media. In this study, a three-dimensional simplified physical model construction method was developed for tight sandstone gas reservoirs based on the geological origin, sedimentary compaction and clay mineral-cementation. The idea was to reduce the porosity of the tangent spheres physical model considering the synergistic effect of the above two factors and achieve a simplified model with the same flow ability as the actual tight core. Regarding the wall surface of the simplified physical model as the boundary and using the Lattice Boltzmann (LB) method, the relative permeability curves of gas and water in the simplified model were fitted with experimental results and a synergistic coefficient could be obtained, which we propose for characterizing the synergistic effect of sedimentary compaction and clay mineral-cementation. The simplified physical model and the results simulated by the LB method are verified with the experimental results under indoor experimental conditions, and the two are consistent. Finally, we have carried out a simulation of gas flooding water under conditions of high temperature and high pressure which are consistent with the actual tight sandstone gas reservoir. The simulation results show that both gas and water have relatively stronger seepage ability compared with the results of laboratory experiments. Moreover, the interfacial tension between gas and water is lower, and the swept volume is larger during placement. In addition, the binding ability of the rock surface to the water film adhered to it becomes reduced. The method proposed in this study could indicate high frequency change of pores and throats and used to reflect the seepage resistance caused by frequent collisions with the wall in microscopic numerical simulations of tight sandstone gas reservoirs.

Diagenetic controls on the reservoir quality of the tight gas Collyhurst Sandstone Formation, Lower Permian, East Irish Sea Basin, United Kingdom

The Lower Permian Collyhurst Sandstone Formation is an important gas reservoir both offshore and onshore in the East Irish Sea Basin of north-western UK, and is considered to be a tight gas reservoir. Diagenetic processes have significantly influenced the reservoir quality of the sandstone. This study employs petrographic, mineralogical, and stable isotopic analyses on two different onshore core successions to investigate the composition and texture, the diagenetic history, and the impact of lithology and diagenesis on reservoir quality. The Collyhurst Sandstone Formation consists of fine- to very coarse-grained, poorly to well-sorted, sub-arkosic to lithic arenite sands. The reservoir properties of the sandstone are relatively poor, with average porosity and permeability of 6.6% and 0.387 mD respectively. On the basis of petrographic analyses, five sandstone lithofacies were identified: fine-grained, conglomeratic, carbonate-cemented, quartz-cemented and matrix-free. Contrasting characteristics between the reservoir units suggest the development of divergent diagenetic pathways. Features observed during early diagenesis include compaction, growth of pore-filling early carbonate cements (calcite I, dolomite and siderite I) and vermicular kaolinite. Late diagenetic features include: further carbonate cementations (ferroan dolomite/ankerite, calcite II and siderite II-III), clay mineral cementation (illite and chlorite) with both grain-coating and pore-filling textures. Also common in late diagenesis are syntaxial quartz overgrowths. Albite, barite and anhydrite are minor late diagenetic cement phases. Compaction processes rather than cementation accounts for the major destruction of the initial sandstone porosity. But cementation still accounts for up to 32% of porosity loss. Carbonate cements mostly fill intergranular pores, thus block pore throat and prevent migration of fluid. Clay minerals bridge and occlude pores, resulting in further destruction of the reservoir quality. The study provides insights into how lithofacies (primary composition, texture and sorting) and diagenesis can influence reservoir qualities of tight gas sandstones.

Diagenesis and evolution of the lower Eocene red-bed sandstone reservoirs in the Dongying Depression, China

Abstract The diagenetic evolution of the lower Eocene red-bed high-quality reservoirs in the Dongying Depression was systematically investigated through an integrated petrographic, petrophysical, fluid evolution and thermal history analysis. The reservoirs experienced several phases of diagenesis, including compaction, carbonate cementation, gypsum and ankerite cementation, feldspar and carbonate dissolution, quartz overgrowth cementation, and clay-mineral cementation and transformation. An early alkaline diagenetic environment caused significant cementation of calcite and gypsum in the reservoir sandstone units near the sandstone-mudstone contacts. Acidic formation fluids caused widespread dissolution of the feldspar and carbonate cements, leading to synchronous occurrences of primary intergranular pores and acidic dissolution pores. From the middle parts of the reservoir sandstone units to the sandstone-mudstone contacts, dissolution wanes gradually with ferro-carbonate progressively diminishing. Migration of exogenetic thermal fluid along faults may have contributed to the gradual decrease in dissolution and the gradual increase in cementation from the lower part to the upper part of fault blocks. The alternating alkaline and acidic diagenetic environments and the distribution of diagenetic products caused the development of the high-quality red-bed reservoirs in the middle part of the thick-layered sandstone units in the lower part of fault blocks. Because of the spatial and temporal relationships between the diagenetic stages and their products, lithological traps of a diagenetic origin were developed in the red-bed reservoirs.

Diagenetic facies quantitative evaluation of low-permeability sandstone: A case study on Chang 82 reservoirs in the Zhenbei area, Ordos basin

Quantitative characterization of diagenetic facies has great significance for reservoir evaluation and prediction. In order to find out the method to evaluate diagenetic facies, the author took Chang 82 reservoir low-permeability sandstone in Zhenbei area of Ordos basin as research object and divided the reservoir into six types of diagenetic facies by analysis of casting thin section, scanning electron microscope, cathode luminescence, and physical property. According to 14 quantitative evaluating parameters which were related with petrology characteristic, diagenesis strength, pore structure, etc. quantitative evaluation of diagenetic facies of low-permeability sandstone reservoir was done by data envelopment analysis. The result showed that in the Chang 82 reservoir low-permeability sandstone in Zhenbei area of Ordos basin, quantitative representative indexes of diagenetic facies ranged from 0 to 1.00. Various diagenetic facies and their indexes had interval corresponding relation. The diagenetic facies of weak corrosion with chlorite mat and the diagenetic facies of corrosion of unstable components had the best reservoir quality. Their diagenetic facies indexes ranged from 0.66 to 1.00. The reservoir quality of the diagenetic facies filled with kaolinite was not as good as the former. The indexes ranged from 0.50 to 0.66. The diagenetic facies of quartz secondary enlargement and the diagenetic facies of clay mineral cementation replacement had poor reservoir quality. Their diagenetic facies indexes ranged from 0.30 to 0.40. The diagenetic facies of carbonate cementation had the poorest reservoir quality. It hardly possessed fluid storage capability. After comparing diagenetic facies indexes, absorption strength and remaining oil saturability, the perfect corresponding relation between quantitative evaluation results and reservoir quality could be verified.

Diagenetic differences of the Zhenzhuchong Member of Ziliujing Formation in the Jiulongshan–Jiange area, Sichuan Basin

The rocks of the Zhenzhuchong Member of the Ziliujing Formation, Jiulongshan–Jiange area in the Sichuan Basin, were analyzed by petrography, XRD, and SEM techniques to investigate their diagenetic history and properties, such as authigenic mineral types, evolution of mixed-layer illite–smectite minerals, the clay assembly, and the fraction of mixed-layer clay minerals. The results revealed that the Zhenzhuchong Member has experienced several important episodes of diagenetic alteration since the deposition, including compaction (pressure-solution), cementation, metasomatism, dissolution, fracturing, and infilling of caves and cracks. It was also observed that diagenetic properties of the Jiulongshan area were significantly different from those of the Jiange area. The rock samples from the Jiulongshan area were characterized by the composition of siliceous and calcareous cements, varying amounts of detrital grains, clay matrix and kaolinite replaced by calcites, a certain amount of rarely dissolved early-stage kaolinite, dickite, and infillings by late-stage calcite. On the other hand, for the rock samples from the Jiange area, the dissolution is a common phenomenon with features of abundant aluminosilicates-dissolution pores or components, but the replacement phenomenon has rarely been seen. These rock samples were characterized by the presence of clay mineral cements, quartz, and dolomite infillings. It indicated that there was a great difference of diagenesis between the two areas in the types, phase, and temperature of diagenetic fluids. Revealing the difference would provide theoretical and practical implications for the exploration of high quality oil and gas reservoirs.

Fracture development and diagenesis of Torridon Group Applecross Formation, near An Teallach, NW Scotland: millennia of brittle deformation resilience?

The Late Proterozoic Torridon Group Applecross Formation in the foreland of the Moine Thrust Belt, NW Scotland, contains deformation bands, three fracture sets (from oldest to youngest A, B, and L) defined by orientation, crosscutting relations, and progressively less quartz cement in younger sets, and joints. Set A crosscuts deformation bands and strikes north–south. Set B has trimodal orientation defining three linked subsets that formed concurrently. Set L strike ranges from NE–SW to ENE–WSW, in parent crack–wing crack arrays that formed progressively; these are more abundant near small-displacement, oblique-slip faults that offset the overlying Cambrian Eriboll Formation and the Moine Thrust Belt. Applecross sandstones have low fracture abundance, possibly a consequence of low elastic moduli (Young’s modulus 2.3–17.0 GPa, most values &lt;6.9 GPa) and moderate to high subcritical crack index (45–78), resulting from compacted soft lithic grains and clay-mineral cements. Low abundance contradicts models that postulate persistent incipient failure by subsurface fracture. The fracture sequence resembles that found in the overlying Cambrian Eriboll Formation quartzarenites, implying that no widespread late Proterozoic fracture sets exist in this part of the Applecross Formation, an uneventful record for a rock profoundly resistant to brittle deformation.

Quantitative research of diagenesis: its effect on pore evolution of the Fuyu oil reservoir in the north Qijia region

From observing cores of 18 wells, identifying 175 ordinary thin sections and 61 thin casting sections, energy spectrum analyses of 37 samples, homogenization temperature measurement of fluid inclusions of 11 samples, we determine the types of diagenesis and pores of the Fuyu oil reservoir in the north Qijia region. We classified the pores and measured their plane porosity using CIA 2000, the software of rock image analysis, calculated the effect of different kinds of diagenesis on porosity, studied the controlling actions of diagenesis to pore evolution quantitatively, combined with burial history, thermal history and their diagenetic environments. Our results show that mechanical compaction and carbonate cementation are the major destructive diagenesed, developed during early diagenesis stages. The reduction in porosity by mechanical compaction and carbonate cementation are about 25% and 8%, while the destructive intensity of siliceous cementation and clay mineral cementation is relatively much smaller, i.e., the reduction of porosity is about 2% and 0.2% Dissolution is constructive diagenesis, the increment of porosity is about 6%. There are four diagenesis evolution stages, during which the porosity reduced from 30%∼38% to 2%∼20%. Mechanical compaction and early cementation are the main diageneses in the early diagenesis stages, when porosity was reduced to 2%∼10%. Dissolution is the main diagenesis of an A I substage of the middle diagenesis stage, when porosity increased 1%∼8%. The dissolution of the A II substage of the middle diagenesis stage affected by late cementation, raised porosity 1%∼5%. The porosity varied slightly during the middle stage B.

Depositional environment, sand provenance, and diagenesis of the Basal Salina Formation (lower Eocene), northwestern Peru

The Basal Salina Formation is a lower Eocene transgressive sequence consisting of interbedded shales, siltstones, and conglomeratic sandstones. This formation occurs in the Talara basin of northwestern Peru and is one of a series of complexly faulted hydrocarbon-producing formations within this extensional forearc basin. These sediments were probably deposited in a fan-delta complex that developed along the ancestral Amotape Mountains during the early Eocene. Most of the sediment was derived from the low-grade metamorphic and plutonic rocks that comprise the Amotape Mountains, and their sedimentary cover. Detrital modes of these sandstones reflect the complex tectonic history of the area, rather than the overall forearc setting. Unlike most forearc sediments, these are highly quartzose, with only minor percentages of volcanic detritus. This sand is variably indurated and cemented by chlorite, quartz, calcite, and kaolinite. Clay-mineral matrix assemblages show gradational changes with depth, from primarily detrital kaolinite to diagenetic chlorite and mixed-layered illite/smectite. Basal Salina sandstones exhibit a paragenetic sequence that may be tied to early meteoric influx or late-stage influx of thermally driven brines associated with hydrocarbon migration. Much of the porosity is secondary, resulting from a first-stage dissolution of silicic constituents (volcanic lithic fragments, feldspar, and fibrous quartz) and a later dissolution of surrounding carbonate cement. Types of pores include skeletal grains, grain molds, elongate pores, and fracture porosity. Measured porosity values range up to 24% and coarser samples tend to be more porous. Permeability is enhanced by fractures and deterred by clay-mineral cements and alteration residues.

Diagenesis of the Gassum Formation Rhaetian-Lower Jurassic, Danish Subbasin

The Upper Triassic-Lower Jurassic Gassum Formation is a shallow marine or deltaic sandstone body with intercalated mudstones. The sandstones are primarily arkoses and subarkoses, but the composition is strongly influenced by diagenesis. Depths of burial vary between ea. 550 m and 3350 m, and the diagenetic modifications are strongly dependent on depth. The diagenetic pattern is also related to primary lithology. At shallow depth of burial, the most important diagenetic changes are compaction, calcite cementation, formation of kaolinite, and dissolution of feldspar. In deeply buried parts of the formation, the most important diagenetic changes are reduction of primary and secondary porosity by strong cementation by quartz, albitization of partly dissolved feldspar and ankerite cementation. In chlorite-cemented parts, quartz cementation was inhibited, and here pressure solution occurred in deep parts of the formation. The porosity in these deeply buried samples may be reduced to secondary intragranular porosity in partly dissolved feldspar and intercrystalline porosity in clay mineral cements. Other diagenetic minerals are siderite, pyrite, anhydrite, potassium feldspar and illite. The diagenesis evolved through the eodiagenesis and the immature and semimature stages of mesodiagenesis, whereas mature stages have not been reached.

Comparisons Between the Diagenesis of Dioctahedral and Trioctahedral Smectite, Brazilian Offshore Basins

AbstractBurial diagenetic reactions of di- and trioctahedral clay minerals were compared in Brazilian offshore, basinal sediment sequences of Cretaceous age. Originally dioctahedral smectite-rich shales of three basins—Potiguar,Ceara, and Ilha de Santana—exhibited the classical smectite-to-illite burial pattern. Trioctahedral clay-rich shales and trioctahedral clay-mineral cements in sandstones, however, showed a burial sequence of saponite to mixed-layer chlorite/saponite with progressive increase in the percentage of chlorite layers with increasing burial depth.The change from disordered to ordered interstratifications of chlorite/saponite occurred in the temperature range 60°-70°C at a vitrinite reflectance value of about 0.65. These values are lower than those found for the ordering of illite/smectite clays. Increasing substitution of Al for Si in tetrahedral sites, followed by fixation of interlayer hydroxide sheets was found to be the major chemical change promoting transformation of saponite to chlorite via corrensite.