Intergranular Volume Research Articles

Sandstone Porosity Evolution and Reservoir Formation Models of the Paleogene Huagang Formation in Yuquan Structure of West Lake Sag, East China Sea Basin

The West Lake Sag is abundant in oil and gas reserves, primarily in the Huagang Formation reservoir which serves as the primary source of production. The level of exploration is rather high, but there are still some unresolved issues, such as an unclear understanding of pore evolution features and reservoir growth mode. To tackle the aforementioned problems, this study employs optical microscopic examination, scanning electron microscope analysis, inclusion analysis, isotope analysis, X-ray diffraction analysis, and other techniques to elucidate the primary factors governing reservoir development and establish an analytical model regarding the cause of the sandstone reservoir. The results are as follows: (1) The sandstone reservoirs of the Huagang Formation of the Yuquan (abbreviated to YQ) Structure are now in the mesomorphic A stage as a whole, and minerals such as 4-phase authigenic quartz, 2-phase illite, 2-phase chlorite, 1-phase kaolinite, 1-phase ammonite mixing layer and 2-phase carbonate were formed during the diagenesis. (2) Feldspar and carbonate solution pores make up the majority of the reservoir space. About 10% of the porosity is made up of carbonate solution pores, which are the most prevalent reservoir space. Carbonate solution pores are primarily made up of metasomatic carbonate solution pores and cemented carbonate solution pores. Feldspar solution pores come next, contributing roughly 6.2% of the porosity. At 1.8%, residual intergranular holes are the least common. (3) The four main processes listed below are responsible for the creation of pores in the sandstone of the Huagang creation. The early carbonate cements resist the destruction of mechanical compaction and effectively preserve intergranular volume. The high content of feldspar provided a material basis for later dissolution. Early chlorite surrounding the edges of particles reduced the damage of authigenic minerals to porosity. The faults and cracks formed by the later structural inversion connected to the acidic water in the atmosphere, causing the dissolution of carbonate minerals and feldspar in the sandstone of the Huagang Formation. (4) Carbonate dissolution + feldspar dissolution type, carbonate dissolution type, and feldspar dissolution type are the three main types of reservoir formation in the Huagang Formation; the first two types mainly develop in the Upper Huagang Formation, while the latter mainly develops in the lower part of the Huagang Formation. The research results are conducive to the establishment of a geological prediction model for high-quality reservoirs of different geneses in the Huagang Formation and promote the exploration process of deep-seated hydrocarbons in the West Lake Sag.

Burial of a sand sea: Heterogeneous compaction and cementation of the jurassic Etjo Formation, Namibia and what it tells

Aeolian sandstones are capable of forming excellent reservoir rock due to their textural and compositional maturity. Often spanning across large areas, such formations may however experience spatial variability in burial and temperature exposure potentially leading to a heterogeneous diagenesis and subsequent reservoir quality. To assess such potential variability, this case study focusses on the diagenesis of the aeolian units of the Lower Jurassic Etjo Formation in Namibia, exposed at three key localities lying >100 km apart from each other. The Etjo Formation has experienced two major burial phases controlled by the emplacement of (I) Jurassic Karoo and (II) Lower Cretaceous Paraná-Etendeka volcanic rocks separated by an intervening exhumation phase. Petrographic analysis reveals very different rock properties between the localities with respect to intergranular volumes (IGV), intensity of quartz cementation, and preserved porosities. Average IGV and quartz cement values are 30.7 % and 24.3 % at Gamsberg, 23.7 % and 6.5 % at Waterberg and 19.7 % and 15.4 % at Mt. Etjo. Point-counted intergranular porosity values are on average 2.6 % (Gamsberg), 10.8 % (Waterberg), and 1.5 % (Mt. Etjo). Waterberg samples had an on average 72 % larger nucleation surface area available for quartz cement growth than samples from Mt. Etjo. The higher quartz cement volumes at Mt. Etjo therefore argue for a higher thermal exposure than at Waterberg. This is supported by kaolinite-to-illite transformation only occurring at Mt. Etjo. We attribute the higher temperature signature to the proximity of Mt. Etjo to the center of lower Cretaceous Paraná-Etendeka volcanism and related elevated thicknesses of the lava pile or heat flow increase. At Gamsberg, there is evidence for a quartz cement generation that formed during the Jurassic. This earlier cement stabilized the grain framework and preserved the IGV during the following Etendeka burial event. The Etjo Formation provides an exceptional example for the wide range of very low to very high degrees of compaction and cementation that can occur within an otherwise homogeneous, texturally and compositionally mature sandstone formation. This study underscores the importance of accounting spatial variations in burial depths and temperatures when predicting the diagenetic properties of reservoir rocks.

Improved reservoir quality assessment by evaluating illite grain coatings, quartz cementation, and compaction – Case study from the Buntsandstein, Upper Rhine Graben, Germany

Reservoir quality (RQ) in the Buntsandstein of the Upper Rhine Graben is in the center of attention as it is a possible target formation for geothermal energy production and hydrocarbon exploration surrounding existing fields. An understanding of properties affecting reservoir quality of the target lithology is still fairly poor and the success of accurately targeting high RQ intervals is limited. This is due to the fact, that the effect of compaction and the interplay between enhanced chemical compaction (i.e. pressure dissolution of quartz grains), illitic contact coatings and quartz cementation in the lithology has been underestimated. The understanding and quantification of controlling factors on reservoir qualities in fluvio-eolian sedimentary rocks, deposited in a (semi-)arid climate has been improved in recent years and this case study highlights the benefits of detailed petrographic analyses in understanding diagenetic and compactional processes in this lithology. This is especially relevant in the observation of grain coating clay minerals, whose effect depends on their specific location, i.e. either at grain contacts between quartz grains (GTG coating) or at the interface between detrital quartz grains and the intergranular volume (GTI coating).Illitic GTI coatings affect syntaxial quartz overgrowth precipitation, as precipitation sites are locally blocked. The negative correlation between the grain coating coverage and quartz cement volumes support these findings across multiple sample sets, and they may locally preserve intergranular porosity. Illitic GTG coatings on the other hand enhance chemical compaction (i.e. pressure dissolution) and will reduce the IGV. The negative correlation between these two properties again underlines the negative effect of this process on reservoir properties. Studied samples from a deep Buntsandstein well in the central URG show low reservoir quality due to either intense quartz cementation (0.7–31.7%) or a high degree of mechanical and chemical compaction (IGV: 2.3–38.0%). Higher illitic GTG and GTI coating coverages play a substantial role in controlling reservoir quality development, as demonstrated by comparing data from other fluvio-eolian lithologies (Triassic Buntsandstein and Permian Rotliegendes) to results of this study. In relation to their respective burial histories, higher illitic GTI coating coverages always correlate with smaller syntaxial quartz cement volumes. Similarly, higher illitic GTG coating coverages always correlate with lower IGV values in the three compared sample series.As both, the precipitation of quartz cements and compaction, are a function of the burial history, i.e. effective stresses and experienced temperatures, understanding the interaction of both these processes may enable the prediction of reservoir properties in undrilled areas.

The interplay of detrital modes and depositional facies on diagenesis and petrophysics of deep-marine Miocene sandstones (Cilento Group), Southern Apennines, Italy

The Cilento Group (Langhian-to-Tortonian) is a thick turbiditic succession of the Southern Apennines foreland region that unconformably overlain the Lucanian oceanic terranes. The San Mauro Formation (SMF) form the uppermost portions of the Cilento Group, and consists of 1400–1600 m thick turbiditic succession including quartzolithic, volcanolithic, and quartzofeldspathic sandstones with several carbonatoclastic layers interbedded, passing upward from distal-to proximal-facies associations. Here, based on the vertical changes of texture and petrology in the succession, the SMF was divided in two main sections. The key depositional features and the main post-depositional processes, of lower- and upper-part of SMF, are compared with the most significant petrophysical properties with the purpose to elucidate the major controls on the porosity and permeability evolution during burial. With this aim, fifty-four thin sections were petrographically investigated, while mercury intrusion analysis was performed on twenty-four samples. Compaction, cementation, dissolution and fracturing occurred in the SMF, altering porosity and permeability. The main authigenic minerals are: 1) carbonates, mainly calcite and less dolomite, dominantly occurring as replacement of framework grains; 2) Phyllosilicates, mainly developing in the upper part of SMF as pore-filling cement or as small and incomplete grain coatings; 3) Fe-oxides occurring as coatings or localized crystals. The relationship between the compactional porosity loss (COPL) and the cementational porosity loss (CEPL) testifies the primary role of compaction in decreasing porosity, reducing the intergranular volume (IGV) to 11.1% in the lower SMF and to 16.7 % in the upper SMF. The coarse upper SMF samples exhibit lower mean porosity and permeability (6 % and 215 mD, respectively) than the fine lower SMF samples (9.2 % and 774.2 mD, respectively). In the upper section of SMF, a comparatively more widespread pore-filling cementation and the abundant detrital matrix affect depositional intergranular porosity, reducing the pore size and interconnectivity. Moreover, the higher amount of rigid grains (petrofacies dependent) and their brittle deformation produce an intricate fragmentation and micro-fracturing system, altering pores geometry and affecting permeability. On the contrary in the lower SMF, ductile components suffer compaction more than rigid ones, leading the lowest IGV. In addition, a higher volume of authigenic carbonates occur as replacement on framework components, hence playing a minor occluding role and less affecting the pore system. Composition and tectonics rule the effects of diagenetic processes in the San Mauro turbiditic succession, following the vertical evolution of depositional facies association and petrology.

Climatic, depositional and environmental controls on early carbonate cementation in fluvial and shallow marine sandstones

Sandstones with carbonate cements are found in a variety of depositional settings (e.g. fluvial, lacustrine, deltaic, shallow marine) with abundances ranging from just a few volume percent to pervasive pore-filling cements and are of major interest due to their impact on reservoir quality. Despite the widespread occurrence of such cements, predictive models for eogenetic carbonate cementation have yet to be developed. Hence, investigations on the controls and distribution of early carbonate precipitation are necessary to improve the accuracy of predictions of the nature and distribution of diagenetic alterations. We conducted a petrological and petrophysical analysis of early near-surface carbonate cementation in siliciclastic and mixed siliciclastic-carbonate sandstones from fluvial and shallow marine environments to reconstruct eogenetic processes and their associated controls. Furthermore, sediment burial scenarios up to a loading of 50 MPa were simulated with the reservoir quality prediction software Touchstone to evaluate the impact of carbonate cements on the compaction behaviour of the rocks. Results highlight the importance of climate, depositional environment and detrital composition and texture in regards to the precipitation processes of near-surfaces carbonate cements and burial development. Our data shows that the presence and distribution of early carbonate cements reflects a complex interaction of several parameters, such as the composition and texture of detrital material in the source area, the availability and infiltration of meteoric or marine waters and the presence of precursor carbonate during deposition. Burial simulations demonstrate the strengthening effect of early carbonate cementation and its influence on the development of intergranular volume and porosity during burial with up to five times less porosity loss due to compaction than without near-surface cementation.

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

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

Depositional and diagenetic controls on porosity evolution in sandstone reservoirs of the Stuttgart Formation (North German Basin)

The Mesozoic succession of the North German Basin comprises permeable sandstones operated by ‘conventional’ open doublet systems for geothermal heat production. The high permeability, exceeding 6 Darcy at individual localities, is the result of undercompacted grain fabrics, low abundance of authigenic minerals and large secondary porosity volumes. The discrepancy of undercompacted grain fabrics and porosity volumes of >20% at depth of up to 2500 m gave rise to a basin-scale study, in which classical petrographic methods were employed to reconstruct the diagenesis of sandstones on the example of the Stuttgart Formation (Schilfsandstein, Late Triassic). Associated to the deposition of sandstones in distinct fluvio-deltaic environments, four diagenetic pathways are indicated by systematic variations of detrital and authigenic assemblages. These four pathways are herein introduced as Delta Channel, Delta Plain, Fluvial Channel, and Floodplain Diagenesis Types. In response to the history of the North German Basin, high permeabilities of sandstones of the Delta Channel and Fluvial Channel Diagenesis Types are related to the subsequent control of depositional and diagenetic regimes on porosity evolution. Pervasive eogenetic calcite cementations contributed to preservation of pre-burial intergranular volumes and effectively prevented the mechanical compaction of grain fabrics during late Triassic–late Jurassic burial diagenesis. The uplift from deep to shallow burial depth, associated to latest Jurassic–Cenozoic structural differentiation and inversion of the North German Basin, triggered substantial dissolution processes that mainly affected carbonate cementations. This opened pores and contributed to the evolution of secondary porosity, in particular in sandstones of the Delta Channel and Fluvial Channel Diagenesis Types. Moderate post-inversion reburial rates resulted only in negligible mechanical compaction and reduction of secondary porosity. The improved knowledge of diagenetic pathways, herein exemplified by sandstones of the Stuttgart Formation, will significantly contribute to improved predictions of Mesozoic hydrothermal reservoir in the North German Basin.

Controls of normal diagenesis on poroperm parameters in red beds: examples in Miocene Muddy Creek Formation, Mesquite basin, USA and upper Devonian Old Red Sandstone, Orcadian basin, Scotland

General diagenetic patterns in Miocene Muddy Creek Formation and upper Devonian Old Red Sandstone (ORS) can be characterized as follows: i) early diagenesis characterized by the formation of early hematite, carbonate, and clay cement; ii) burial diagenesis followed by the formation of quartz and feldspar overgrowths, poikilotopic calcite and pore-filling clays; iii) late diagenesis characterized by the formation of late hematite replacing previous poikilotopic carbonate cement. Normal diagenesis has a significant impact on poroperm parameters as indicated by the destruction of pore spaces from cementation and intergranular pressure solution. Early cementation in unburied sandstones of the Muddy Creek Formation reduces sample porosities to 2÷20% of the total rock volume. Cementation destroyed the original porosity of studied sandstones through pore occlusion due to the formation of equant and meniscus calcite cement. Point-count data indicate that the intergranular cement of studied samples ranges between 22 and 44%; in contrast, the intergranular porosities range from 2÷20% of the total rock volume. These consequently indicate that the intergranular volumes that are considered to represent the original porosities of studied samples, ranged from 35÷50% of the total rock volume. Completely pore-occluding poikilotopic calcite cement in the upper Old Red Sandstone reduces poroperm values to as low as 5% porosity and 0.003 mD permeability. Late reddening is caused by replacive hematite cement in the calcite. In addition, normal diagenesis also has an impact on porosity enhancement due to dissolution in the studied red beds. This improves porosity and permeability by as much as 14% and 254 mD in the upper ORS.

Depositional and diagenetic controls on sandstone compactional fabrics in an Andean broken foreland basin, the Miocene Vinchina formation, northwestern Argentina

The very thick (up to 6000 m thick) Miocene Vinchina Formation was deposited between 16 and 8 Ma ago and later covered by more than 2500 m of Pliocene-Pleistocene sediments in a broken foreland basin. The very thick sedimentary pile accumulated in a medium to low-gradient basin must have suffered intense mechanical compaction during burial.Seventy thin sections of unaltered sandstones collected in three sections along the depositional strike of the Vinchina Formation at the Sierra de Los Colorados, La Rioja Province, northwestern Argentina, were analyzed under the microscope for determining compactional paths and controls.The role of compaction in sandstone diagenesis was firstly assessed using the change in intergranular volume (IGV), which is the sum of all cement and remaining primary porosity. It is clear from the IGV analysis that most of the studied sandstones lose more porosity by compaction (COPL) than by cementation (CEPL). In general compaction increased from the upper to the lower member of the Vinchina Formation (i.e with depth) and there is a clear trend of the compaction index ICOMPACT increase from the northern, shorter Los Pozuelos to the central, thicker La Troya sections. Surprisingly, the thickest southern El Yeso section shows anomalous results possibly due to the extensive development of cements as indicated by the highest CEPL values. Cements are related to the depositional environment (gypsum, calcite) and clast composition (zeolites). Early cements may have prevented further compaction while late cements may have produced open fabrics by dissolution of matrix and framework clasts. Both cases show that diagenetic processes are not homogeneous basin-wide and that the diagenetic pathways are not only controlled by the increase in burial depth but also by characteristics associated with depositional environments and framework clast composition.We also attempted to relate compaction to burial depth based on the nature (point, long, concave-convex and sutured contacts) and the number (contact and packing indexes) of grain-to-grain contacts in the studied sandstones. The sandstones present predominance of concave-convex grain-to-grain contacts and IC indexes mostly between 3 and 4. Sutured contacts are uncommon, and stylolites were not observed at all. Persistence of open fabrics, floating textures and point grain-to-grain contacts at different depths are the result of dissolution-cementation processes. A clear trend of increasing mechanical compaction with depth is only seen along the northern coarser-grained but thinner Los Pozuelos section. Although compaction seems to have increased with depth and also from north to south, the thickest but overall finer-grained El Yeso (southern) section do not show a clear pattern of change with depth. This behavior seems to be the result of at least four main factors: 1) mean sandstone grain-size, 2) thickness of muddy intervals, 3) development of early diagenetic cements, and 4) depth of burial.Comparison with comparable examples suggests that similar compactional characteristics are achieved between 3.5 and 6 km of maximum burial depths. However, cumulative thickness of the Vinchina basin-fill indicate that the base of the Vinchina Formation may have been buried at 8.5 km deep but preliminary backstripping models suggest that by 5 Ma this surface may have reached up to 10 km depth. Alternatively, recorded repetitive episodes of deformation, uplift, and erosion (progressive unconformities) in the Vinchina basin may have prevented the sedimentary pile to be deeply buried and therefore allow to reconcile the observed compactional textures with depth of burial.The results obtained in this work show that the analysis of mechanical compaction in sandstones constitutes a complex task and in occasions there is not a linear relationship between the contact indexes and depth of burial. Sandstone compaction is controlled not only by the depth of burial but also by other factors such as, time, geothermal flow, matrix content, the development of early cements, the ratio between ductile and rigid lithic fragments and, in tectonically active basins the development of progressive unconformities. Therefore, in most cases, it is very speculative to extend diagenetic conditions to basin-wide scales. In particular, the porosity-depth relationship must be based on primary porosity only.

Origin of carbonate cements in Ben Nevis sandstones of Jeanne d’Arc Basin: Insights from geochemistry

The Ben Nevis sandstones in the Jeanne d’Arc Basin in offshore eastern Newfoundland have abundant carbonate cements, and are the primary hydrocarbon reservoirs of the White Rose Oilfield. A multiapproach of petrographic, carbon and oxygen isotopic, and in situ geochemical analyses were applied to investigate the origin of the carbonate cements in the Ben Nevis sandstones (quartzose to litho-quartzose, Well F-04). Petrographic examinations reveal five generations of cements that occur in concretions, which are (from earliest to latest), siderite, early calcite cement (C1), recrystallized fossil (Crf), sparry fossil-filling calcite (Cf), and late calcite cement (C2). The comparable carbon (3.6‰ VPDB) and oxygen isotopic (−1.3‰ VPDB) compositions of siderite to those of the bivalve shells (δ13C = 3.6 ± 0.6‰ VPDB and δ18O = −1.7 ± 0.5‰ VPDB, respectively) indicate an early precipitation from suboxic water near the sediment-seawater interface. The well-preserved intergranular volumes (IGVs up to 41%) and low δ18O (−3.3 ± 1.1‰ VPDB) as well as comparable δ13C values (3.6 ± 0.6‰ VPDB) of the early poikilotopic calcite (C1) argue for a pre-compaction precipitation at a shallow burial setting with carbon derived mainly from the dissolution of bioclasts. The later calcite cement (C2) precipitated during progressive burial with involvement of organic carbon as indicated by its depleted δ18O (−6.7 ± 2.0‰ VPDB) and δ13C (1.8 ± 1.3‰ VPDB) values, and the reduced IGVs (28.2%), as well as the consistent depletion of Sr with enrichment of Mn. The dissolution of aragonitic bioclasts, was likely the main source of Ca for calcite, which is consistent with the high Sr content of C1 (1369 ± 597 ppm), Crf (2005 ± 472 ppm), Cf (1354 ± 48 ppm) and C2 (1831 ± 696 ppm). The parallel shale normalized (REESN) patterns of the cements and the way they mimic, to some extent, that both of seawater and meteoric water, suggest that the diagenetic fluid is bracketed by those of the early Cretaceous marine and meteoric waters, as reflected by the estimated δ18OSW composition of parent fluid for C1. The consistent Eu anomalies of carbonate cements and the increasing δ18OSW composition of pore water during C2 precipitation reflect the evolution of the diagenetic fluids in siliciclastic intervals during progressive burial.The secondary porosity development as the main contributor to the present reservoir porosity seem to be closely related to the dissolution of calcite cements, especially in the lower part of the Ben Nevis interval. The meteoric water influx from the overlying unconformities may have induced the dissolution of calcite concretions.

Diagenetic controls on the reservoir quality of Neogene sandstones in the offshore area of the Bengal Basin, Bangladesh

The Neogene sandstones offer hydrocarbon reservoirs in the offshore area of the Bay of Bengal (Bangladesh), but their reservoir quality is not well known. Understanding the impact of depositional and diagenetic controls on reservoir quality is crucial to hydrocarbon exploration and production in reservoir sandstones. This study aims to investigate the depositional and diagenetic controls on reservoir quality of the Neogene Surma Group from the Sangu Gas Field (the only offshore gas field), in the Bay of Bengal, Bangladesh using a combination of petrographical, petrophysical, X‐ray diffraction, scanning electron microscope fitted with energy dispersive spectra and core analysis data. Diagenetic modifications to the sandstones include mechanical and ductile grain compaction followed by clay cementation (dominantly chlorite, illite–smectite and minor kaolinite), poikilotopic calcite cement and subordinate amounts of quartz cement, as well as partial dissolution of feldspars and lithic fragments. Sandstones with primary porosities ≥20% have average intergranular volume value of 33%, although permeability in most sections of the reservoir falls below 8–10 mD. There has also been localized creation of secondary porosity due to dissolution. The average intergranular porosity lost by compaction is 14% (ranges from 3% to 37%), and the average porosity lost by cementation is 10% (ranges from 3% to 37%). This study shows that the reservoir quality is predominantly controlled by depositional environment (ductile grain supply, clay content), a diagenetic processes (compaction followed by clay cement and early to intermediate poikilotopic calcite cement) and will provide insights into the reservoir quality evaluation in the offshore areas of the Bengal Basin.

Compaction and cementation control on bleaching in Triassic fluvial red beds, S-Germany

This study focusses on bleaching phenomena on the pore scale in an Olenekian (Upper Buntsandstein) sandstone quarry. The study area exposes a 10 m thick red sandstone body with up to cm-sized, greyish-white laminae in sandstones. Analyses focus on bleached and unbleached zones in the same cm-sized samples. Bleached zones show a larger grain size (by 27 μm), less compaction and a higher porosity (by 3 %). They also exhibit stronger cementation by all observed authigenic phases of quartz, illite, K-feldspar and to a minor extent dolomite. Calculated intergranular volumes and cementational porosity loss also correlate positively with bleaching. Meanwhile unbleached zones contain more ductile grains (e.g. micas, detrital clay, rock fragments like phyllites and plutonic rock fragments) and are affected by major porosity loss via compaction. Bleaching is related to primary features like grain size-selective lamination and associated higher permeability in coarse-grained laminae. It is also reliant onto an early framework stabilising cement phase, which keeps pathways open for uplift-related leaching of the detritus and few dolomite cements.

Controls on compactional behavior and reservoir quality in a Triassic Buntsandstein reservoir, Upper Rhine Graben, SW Germany

The main reservoir quality controls in Triassic Buntsandstein deposits in the Upper Rhine Graben and surrounding outcrops in Germany and France are discussed to be authigenic mineral precipitates and the vicinity to structural elements (faults and fractures). To better understand reservoir quality controls in this lithology, a detailed diagenetic study based on petrographic methods (optical and SEM) in combination with petrophysical measurements (porosity and permeability) on core material (1133–1591 m TVD) was initiated. This case study highlights that large detrital grain sizes and low compactional porosity loss in association with blocky authigenic cements such as quartz and dolomite are beneficial to reservoir quality, whereas variable illite textures have a dual, but opposite effect on reservoir qualities. Grain coating illite (tangential and radial) may preserve intergranular porosity by inhibition of syntaxial quartz precipitation, while at the same time, if present at grain contacts, tangential illite enhances chemical compaction and reduces the available intergranular volume. The presence of early diagenetic carbonate cemented nodules is also beneficial for reservoir quality in stabilizing the grain framework against compactive porosity loss. Overall, detrital grain size and the intergranular volume, as a result of compaction and cementation, are derived as the main reservoir quality controls. The formation of hydrothermally mineralized veins prior to hydrocarbon charging does not show a distinct impact on porosity and permeability in the sandstone matrix. Fractures and interstices between vein minerals such as barite, galena, tetrahedrite, and siderite are equally covered in hydrocarbon residue, as are all present detrital and authigenic minerals in the sandstones indicating that vein precipitates, if not pervasive, do not alter the overall reservoir quality. This study may indicate additional potential to identify high reservoir quality intervals in the studied succession.

Whether the diagram of intergranular volume versus cementation volume is normative? —Take “depositional and compositional controls on diagenesis of the mixed siliciclastic-volcaniclastic sandstones: A case study of the Lower Cretaceous in Erennaoer Sag, Erlian Basin, NE China” for example

The diagrams of intergranular volume versus cementation volume brought up by Houseknecht (1987) and Ehrenberg (1989), are extensively used around the globe to assess the relative significance of compaction and cementation to porosity reduction in sandstones. However, Houseknecht (1987) overlooked the fact that the bulk sediment volume is dynamically compressed during burial, which directly leads to deviation in evaluating original porosity loss. The present discussion is based on the theoretical and contrastive analysis. These analyses indicate the compactional porosity loss calculated by Ehrenberg's (1989) method is always greater than Houseknecht's (1987) method with a maximum difference of 12.7%. In contrast, the cementation porosity loss using Ehrenberg's (1989) method is always less than Houseknecht's (1987). Besides, the demonstration based on the data of Wei et al. (2020) also supports Ehrenberg (1989). Therefore, the method proposed by Ehrenberg (1989) should be highly recommended to be granted as a reference far and wide.

Integrated sedimentological and petrophysical rock-typing of the Messinian Abu Madi formation in South Batra gas field, onshore Nile Delta, Egypt

The Messinian incised-valley fill (IVF) sediments of the Abu Madi Formation host the most prolific gas-bearing sandstones in the onshore Nile Delta, Egypt. This study provides the first integrated geological, geophysical and petrophysical approach in order to link the Abu Madi sandstones’ reservoir properties with their geological framework, and thereby optimize the predictability of the most prospective reservoir rock types (RRT). Sandstone-bearing reservoir levels are mostly associated with the fluvial-estuarine valley-fill sediments (Abu Madi levels II and III). The intergranular volume of some sandstones (e.g. arkoses) is occupied by compacted ductile argillaceous rock fragments (pseudomatrix), and therefore have dominantly microporosity with anisotropic, poorly-connected pore system.Three RRT were recognized in Abu Madi reservoir levels. RRTI has the best poro-perm characteristics with excellent capillary properties (e.g. megapores, lowest initial displacement pressure ~10 psi air/mercury and irreducible water saturation Swir below 20%). Pore system heterogeneity increases in RRTII and RRTIII. The latter contains dominant micropores and exhibits the highest initial displacement pressure ~500 psi air/mercury and Swir exceeding 50%. RRTI often comprises subarkoses associated with the cross-bedded, massive and cross-laminated lithofacies of the fluvial channel-fill and fluvial channel sand bars facies. Whilst RRTII and III mainly comprise the sandstone lithofacies deposited in the abandoned fluvial, estuarine and restricted marine successions. Excellent reservoir quality facies follows the depositional trend of the valley-fill fluvial channels and pinches-out where repeated avulsion and channel abandonment processes dominate. The spatial distribution of Abu Madi RRT highlights the paramount impact of syn-depositional attributes and canyon-floor relief on the sediments’ reservoir heterogeneity and productivity. Moreover, the present integrated RRT approach could serve as a model for exploration in analogues settings where autogenic processes induce widespread sedimentary heterogeneity, which largely impacts the reservoir quality.

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

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

Origin of calcite cements and their impact on reservoir heterogeneity in the Triassic Yanchang Formation, Ordos Basin, China: A combined petrological and geochemical study

Abstract Calcite cement is a typical product of fluid–rock interaction in petroleum reservoirs, and its migration has a major impact on reservoir heterogeneity. A case study of a closed diagenetic system was carried out on the Upper Triassic Yanchang Formation of Ordos Basin, China—the most successful exploration region for tight sandstone oil in China. The investigated sandstones are mainly lithic arkoses (Q31F42L27). Three occurrences of calcite cement are the most important diagenetic minerals affecting reservoir quality: early calcite completely filling intergranular volumes; late calcite filling pores after chlorite coating, distributed in oil layers, or locally replacing feldspar; and crystal-stock calcite typically distributed in dry layers. δ13C values of the sandstones are in the range −9.69‰ to −2.05‰, indicating that calcite is derived mainly from thermodynamic decarboxylation of organic matter. Dark mudstone interbedded with sandstone and hydrocarbon emplacement are the two crucial sources of organic fluid, providing sufficient Ca2+, Mn2+, Fe2+, and Mg2+ for calcite precipitation through interaction with feldspar and other silicates, such as dissolution and replacement together with transformation of smectite to illite. Manganese was continuously enriched in calcite during burial diagenesis. Driven by compaction of interbedded mudstone and fluid overpressure, calcite migrated widely into calcite-cemented zones (CCZs) where fluid–rock interaction was strong, such as in the margins of sandstone bodies, intrastratal sandstones with original high-permeability zones, and thin sandstones, thereby forming an important organic CO2 sink. Potential high-quality reservoirs are distributed mainly in the central part of the closed system, restricted by calcite-cemented zones, and are an exploration target. Our data suggest that frequently alternating sandstone and mudstone sequences comprise a complete organic CO2 source/sink system, controlling the migration of calcite cement in a closed diagenetic system and reservoir heterogeneity by fluid–rock interaction.

Quartz types in the Upper Pennsylvanian organic‐rich Cline Shale (Wolfcamp D), Midland Basin, Texas: Implications for silica diagenesis, porosity evolution and rock mechanical properties

AbstractThe origin and form of quartz in mudrocks has significant implications for interpretation of depositional environments, diagenetic pathways, mechanisms of porosity reduction and rock mechanical‐property evolution. Quartz types in the Upper Pennsylvanian Cline Shale, Midland Basin, Texas, were examined using a combination of field‐emission scanning electron microscopy‐based energy‐dispersive spectroscopy elemental mapping (to determine mineralogy) and scanning electron microscopy‐based cathodoluminescence imaging (to determine quartz types) with the goal of elucidating a high‐resolution imaging protocol at the micrometre scale for shale petrology. Also, the unconfined compressive rock strength of shale samples with contrasting proportions of different quartz types was measured using Equotip Bambino analyses. The results suggest that extrabasinal detrital quartz, which accounts for an average of 26 vol.% of the rock in all analyzed samples, is the dominant form of quartz in the Cline Shale. The intergranular clay‐size microquartz, which accounts for an average of 10 vol.% of the rock in all analyzed samples, is the dominant form of authigenic quartz. Dissolved radiolarians and sponge spicules are likely sources of silica for clay‐size microquartz and other authigenic quartz showing pale‐mauve to dark greyish cathodoluminescence colour. Some authigenic quartz in the form of intragranular pore filling and mollusc skeletal replacement displays bright‐reddish cathodoluminescence colour, which may be associated with silica released at a different time in the rock's diagenetic history, such as during smectite illitization. Porosity reduction in the Cline Shale predominantly resulted from compaction because of extremely low intergranular volume and the general lack of early cementation. Quartz form significantly impacts rock mechanical properties in the Cline Shale: widely distributed intergranular clay‐size microquartz cement is a major factor controlling rock strength. This correlation also applies to other mudrock successions of various geological ages, tectonic histories and lithologies.

ОПЫТ ПРИМЕНЕНИЯ ДИАГРАММЫ Д. У. ХАУСКНЕХТА ДЛЯ ОЦЕНКИ ВЛИЯНИЯ УПЛОТНЕНИЯ И ЦЕМЕНТАЦИИ НА ПОРИСТОСТЬ НИЖНЕ-СРЕДНЕЮРСКИХ ПЕСЧАНИКОВ НЮРОЛЬСКОЙ ВПАДИНЫ И КОЛТОГОРСКО-УРЕНГОЙСКОГО МЕГАПРОГИБА

Using D. U. Hausknecht diagram, the degree of impact of authigenic cementation, pressure solution and mechanical consolidation on the reduction of porosity and intergranular volume of the Lower - Middle Jurassic sandstones in the territory of the Tomsk region was determined. Sandstones of the Nadoyakha (Lower Toarcian – Lower Aalenian, bed Yu15), Vymskoye (Lower Bajocian, beds Yu11–Yu14), Leontievskiy (Lower-Upper Bajocian, beds Yu8–Yu10) and Malyshevka (Upper Bajocian – Bathonian, beds Yu3–Yu6) were analyzed horizons penetrated by a number of wells in western regions of the Nyurolka depression and the southern part of the Koltogor-Urengoy megatrough in the depth interval 2702–3326 m. It was found that the reduction in the intergranular volume of studied rocks is associated not only with their genesis, material composition, and structural and textural features, but also with the amount, type and composition of cement. It was revealed that the main factor influencing the reduction of the primary volume of the pore space of terrigenous rocks to a depth of ~ 2900–3000 m is the authigenic cementation, and below 3000 m the leading role is played by mechanical consolidation and dissolution of detrital grains under pressure. For Late Toarсian-Aalenian sandstones in the studied areas, the contribution of the factor of mechanical consolidation and pressure solution on the reduction of the primary intergranular volume was 38–53%, and for sandstones of the Bajocian – 20–30%. For the latter, the share of authigenic cementation was 24–56%. The work also considers circumstances that limit this method and possibilities of its application for other types of rocks.

Rock typing of diagenetically induced heterogeneities – A case study from a deeply-buried clastic Rotliegend reservoir of the Northern German Basin

Abstract Reservoir quality of sandstones is mainly derived from their permeability and porosity. As a result, porosity-reducing processes need to be understood in order to evaluate and model reservoir quality in sandstones. This case study from a Rotliegend gas reservoir in the Northern German Basin utilizes petrophysical measurements in conjunction with petrography in order to assess reservoir qualities and define rock types. The most significant diagenetic factors influencing the development of the IGV (intergranular volume) are quartz cementation due to low illite grain coating coverages on grain to IGV interfaces and chemical compaction due to pronounced illite grain coating coverages on grain to grain interfaces. Where large proportions of the interface between adjacent grains are coated by illite, stronger chemical compaction (pressure dissolution) was observed to occur. This chemical compaction reduces the IGV, and thus open pore space. Permeabilities measured under decreasing confining pressures from 50 to 2 MPa were used to determine the pressure sensitivities of permeability (David et al., 1994), which ranged from 0.005 to 0.22 MPa−1. The pressure sensitivity of permeability, porosity and permeability were linked to the petrographic texture, implying three different major rock types: Type A is characterized by an uncemented petrographic texture with high porosities (avg.: 9.8%), high permeabilities (avg.: 126 mD), and low pressure sensitivities of permeability (avg.: 0.019 MPa−1). Type B is intensely cemented with reduced porosities (avg.: 4.0%), reduced permeabilities (avg.: 0.59 mD), and increased pressure sensitivities of permeability (avg.: 0.073 MPa−1). Type C is characterized by intense chemical compaction leading to the lowest porosities (avg.: 1.8%) and permeabilities (avg.: 0.037 mD) in concert with the highest pressure sensitivity of permeability (avg.: 0.12 MPa−1). The heterogeneity induced by diagenesis will have an impact on recoverable resources and flow rates in both hydrocarbon and geothermal projects in similar siliciclastic reservoirs.