Chemical Compaction Research Articles

Petroleum reservoir quality prediction: overview and contrasting approaches from sandstone and carbonate communities

Abstract The porosity and permeability of sandstone and carbonate reservoirs (known as reservoir quality) are essential inputs for successful oil and gas resource exploration and exploitation. This chapter introduces basic concepts, analytical and modelling techniques and some of the key controversies to be discussed in 20 research papers that were initially presented at a Geological Society conference in 2014 titled ‘Reservoir Quality of Clastic and Carbonate Rocks: Analysis, Modelling and Prediction’. Reservoir quality in both sandstones and carbonates is studied using a wide range of techniques: log analysis and petrophysical core analysis, core description, routine petrographic tools and, ideally, less routine techniques such as stable isotope analysis, fluid inclusion analysis and other geochemical approaches. Sandstone and carbonate reservoirs both benefit from the study of modern analogues to constrain the primary character of sediment before they become a hydrocarbon reservoir. Prediction of sandstone and carbonate reservoir properties also benefits from running constrained experiments to simulate diagenetic processes during burial, compaction and heating. There are many common controls on sandstone and carbonate reservoir quality, including environment of deposition, rate of deposition and rate and magnitude of sea-level change, and many eogenetic processes. Compactional and mesogenetic processes tend to affect sandstone and carbonate somewhat differently but are both influenced by rate of burial, and the thermal and pressure history of a basin. Key differences in sandstone and carbonate reservoir quality include the specific influence of stratigraphic age on seawater composition (calcite v. aragonite oceans), the greater role of compaction in sandstones and the greater reactivity and geochemical openness of carbonate systems. Some of the key controversies in sandstone and carbonate reservoir quality focus on the role of petroleum emplacement on diagenesis and porosity loss, the role of effective stress in chemical compaction (pressure solution) and the degree of geochemical openness of reservoirs during diagenesis and cementation. This collection of papers contains case study-based examples of sandstone and carbonate reservoir quality prediction as well as modern analogue, outcrop analogue, modelling and advanced analytical approaches.

Diagenetic history of late Oligocene-early Miocene carbonates in East Sabah, Malaysia

Limestones are particularly susceptible to drastic early diagenesis modifications, mainly cementation and dissolution. During the early Miocene, a major tectonic deformation has caused a widespread of uplift in Sabah. This has resulted change in depositional environment from deep to shallow marine, which favours the deposition of Gomantong Limestone. This study aims to investigate the diagenetic history of Gomantong Limestone in East Sabah. Thorough understanding of the diagenetic processes may provide data to unravel the tectonic activities which affected the reservoir quality of the carbonates. Combining the data from comprehensive petrographic analysis, and Scanning Electron Microscopy (SEM) of 30 samples, two main cements type were identified. These are microcrystalline cement and Mg-calcite cement of granular and blocky mosaics which are dominantly seen in all samples. The sequence of diagenesis events are determined as (1) micritization; (2) grain scale compaction; (3) cementation (pore-filling); (4) mechanical compaction and cementation infilling fractures and (5) chemical compaction. These diagenetic events are interpreted as reflection of changes in diagenetic environment from shallow marine to deep burial. The massive cementation in the Gomantong Limestone has resulted into a poor reservoir quality.

The forming mechanism and process of tight oil sand reservoirs: A case study of Chang 8 oil layers of the Upper Triassic Yanchang Formation in the western Jiyuan area of the Ordos Basin, China

Upper Triassic Yanchang Formation in the Ordos Basin, northwest China, comprise the typical tight oil reservoirs, which have porosity less than 10% and permeability less than 1 mD. The western Jiyuan area (WJA), at the western margin of the Ordos Basin, has made significant progress in the tight oil exploration of the Yanchang Formation recently. Whereas the lack of recognition of tightening mechanism and process of reservoirs resulting in the misunderstanding of tight oil reservoirs prediction. This study focuses on the petrology, diagenetic processes, as well as forming mechanism and process of the Chang 8 (Ch-8) tight oil reservoirs located in the WJA. Four conclusions were reached. First, Ch-8 tight reservoirs, which are mainly composed of lithic arkose and feldspathic litharenite, experienced diagenesis, such as mechanical and chemical compaction, cementation, and dissolution. The reservoirs are at the late period of the mesogenetic A stage. Second, the tightness of the Ch-8 reservoirs is due to both sedimentary and diagenetic factors. The sedimentary environment where sand bodies are close to the provenance that has a rapidly increasing lacustrine level provides conditions for reservoir tightness. The initial porosity rapidly decreases due to compaction and cementation which produce porosity loss rates of 49.42% and 40.87%, respectively. By contrast, the effect of dissolution is limited, as it only increases porosity by 4.48%. Third, the tightening process of the Ch-8 reservoirs comprise four stages: compaction causes pores to reduce rapidly; followed by early cementation, which causes pores to reduce; dissolution, which causes pores to increase slightly; and late cementation, which causes tightness. Finally, it is likely that the Ch-8 reservoirs were already tight prior to hydrocarbon accumulation, thus demonstrating that they represent typical tight oil reservoirs.

Quantitative impact of diagenesis on reservoir quality of the Triassic Chang 6 tight oil sandstones, Zhenjing area, Ordos Basin, China

The Upper Triassic Chang 6 sandstone, an important exploration target in the Ordos Basin, is a typical tight oil reservoir. Reservoir quality is a critical factor for tight oil exploration. Based on thin sections, scanning electron microscopy (SEM), X-ray diffraction (XRD), stable isotopes, and fluid inclusions, the diagenetic processes and their impact on the reservoir quality of the Chang 6 sandstones in the Zhenjing area were quantitatively analysed. The initial porosity of the Chang 6 sandstones is 39.2%, as calculated from point counting and grain size analysis. Mechanical and chemical compaction are the dominant processes for the destruction of pore spaces, leading to a porosity reduction of 14.2%–20.2% during progressive burial. The porosity continually decreased from 4.3% to 12.4% due to carbonate cementation, quartz overgrowth and clay mineral precipitation. Diagenetic processes were influenced by grain size, sorting and mineral compositions. Evaluation of petrographic observations indicates that different extents of compaction and calcite cementation are responsible for the formation of high-porosity and low-porosity reservoirs. Secondary porosity formed due to the burial dissolution of feldspar, rock fragments and laumontite in the Chang 6 sandstones. However, in a relatively closed geochemical system, products of dissolution cannot be transported away over a long distance. As a result, they precipitated in nearby pores and pore throats. In addition, quantitative calculations showed that the dissolution and associated precipitation of products of dissolution were nearly balanced. Consequently, the total porosity of the Chang 6 sandstones increased slightly due to burial dissolution, but the permeability decreased significantly because of the occlusion of pore throats by the dissolution-associated precipitation of authigenic minerals. Therefore, the limited increase in net-porosity from dissolution, combined with intense compaction and cementation, account for the low permeability and strong heterogeneity in the Chang 6 sandstones in the Zhenjing area.

Lateral changes in reservoir properties of the Stø Sandstone in the Snøhvit field, SW Barents Sea

The Sto sandstone being the main reservoir in the Barents Sea area is moderate to well sorted and mineralogically mature. This formation is thickest in the southwestern part of the Snohvit field and gradually thinning eastward. The main objective of this study is to find out this variation using rock physics analysis. Two wells (7120/6-2S and 7121/5-1) from the field were used in this study to investigate lateral rock property variation within the Sto sandstone reservoir. Stress-dependent mechanical compaction varies because of mineralogy and textural difference from east to west despite similar effective stress regime during burial. Chemical compaction also plays a significant role which depends on the dissolution of quartz grains at stylolites and pressure solution of grain-to-grain contact and available specific surface area to precipitate quartz cement. More stylolites generated in shalye sandstone in the east compared to clean sandstone in the west suggested higher cementation in the eastern wells compared to the west. It can be concluded that both mechanical and chemical compaction processes resulted in rock property variations in the same reservoir rock within the field.

Petrographic and geochemical features of the dolostones at the Gölboğazı Formation (Upper Devonian), Southwest Konya, Turkey

This paper describes the occurrence of dolostone and the mechanism of dolomitization of the Upper Devonian Golbogazi Formation in the allochthonous Taurus Mountains Aladag unit in Turkey. The Upper Devonian Golbogazi Formation carbonates, with dominant ostracod-bearing mudstone and wackestone, formed tidal and subtidal environments, and some of these rocks were dolomitized from shallow to deep burial. On the basis of the field, the petrographic and geochemical features, four different replaceable and cement dolostone phases have been recognized. The replacive dolostones contain (1) very fine to fine crystalline planar-s dolostone (df1), (2) medium to coarse crystalline planar-s to planar-e dolostone (df2), (3) coarse to very coarse crystalline non-planar-a dolostone (df3), and (4) coarse to very coarse crystalline planar dolostone cement (df4). The replacive dolostones are disordered to moderate the ordered and calcium-rich. They are non-stoichiometric and have 46–59 mol% CaCO3 and 41–54 mol% MgCO3 total contents. The df1 dolostones have MgCO3 contents of 41–54 mol%, the df2 dolostones have 41–53 mol%, the df3 dolostones have 49 mol%, and the df4 dolostones have 49–50 mol%, respectively. The Golbogazi dolostones have δ18O values of −9.44 to −2.20‰ Vienna Pee Dee Belemnite (VPDB) and δ13C values of −1.58 to +2.52 VPDB. Sr, Na, Mn, and Fe concentrations of replacive dolostones are 74–184, 148–593, below detection level (bdl)–619, and 1049–9233 ppm, respectively. The petrographic and geochemical data demonstrate that the replacive dolostones occurred prior to the chemical compaction at shallow to intermediate burial depths from Late Devonian seawater and/or seawater lightly modified by water–rock interaction process and later recrystallized by basinal brines at increasing burial depths and temperature. The North American Shale Composite-normalized rare earth element values of both limestone and dolostone show very similar rare earth element patterns characterized by slightly or considerably negative cerium (Ce) anomalies and a clear depletion in all rare earth element species. The dedolomitization observed in the Golbogazi Formation is thought to occur by the oxidizing effect of the meteoric water in the shallow burial environment during the telodiagenesis.

Genesis of Paleocene and Lower Eocene shallow-water nodular limestone of South Tibet (China)

This paper examines the genesis of the Paleocene to Lower Eocene shallow-water nodular limestones in South Tibet. The negative carbon isotope excursion representative for the Paleocene–Eocene boundary is located in one nodular limestone bed of the Zhepure Shan Formation showing an extraordinary thickness of about 9 m, inspiring the question under which conditions these nodular limestones were formed. Based on field appearance, the shallow-water nodular limestones of Tingri and Gamba can be classified into five nodular limestone categories (Stylonodular Rock I, Nodular Rock I, Nodular Rock II, stylobedded rock and stylomottled rock) and some transitional members (stylobedded rock transitional to Stylobedded Rock II). Clay variations are assumed to be responsible for these various types of nodular limestones within the sediments. Observations of nodular limestones in South Tibet suggest that those sediments were mostly formed due to autochthonous rather than allochthonous processes. Differential diagenesis resulted in an early selective cementation of limestone nodules due to carbonate supply, while the marls were not cemented but provide the carbonate for the nodule cementation. Additionally, cemented and carbonate-rich nodules are resistant to chemical compaction, while the uncemented and clay-rich marl layers are affected by pressure solution processes due to an overburden of sediments. Additionally, a model is presented, illustrating the origin of different nodular limestones described here.

Diagenesis and Reservoir Properties of the Permian Ecca Group Sandstones and Mudrocks in the Eastern Cape Province, South Africa

Diagenesis is one of the most important factors that affects reservoir rock property. Despite the fact that published data gives a vast amount of information on the geology, sedimentology, and lithostratigraphy of the Ecca Group in the Karoo Basin of South Africa, little is known about the diagenesis of the potentially feasible or economically viable sandstones and mudrocks of the Ecca Group. This study aims to provide an account of the diagenesis of sandstones and mudstones from the Ecca Group. Twenty-five diagenetic textures and structures were identified and grouped into three stages that include early diagenesis, burial diagenesis and uplift-related diagenesis. Clay minerals are the most common cementing materials in the sandstones. Smectite, kaolinite, and illite are the major clay minerals that act as pore lining rims and pore-filling materials. A part of the clay minerals and detrital grains was strongly replaced by calcite. Calcite precipitates locally in the pore spaces and partially or completely replaced clay matrix, feldspar, and quartz grains at or around their margins. Precipitation of cements and formation of pyrite and authigenic minerals occurred during the early diagenetic stage. This process was followed by lithification and compaction which brought about an increase in tightness of grain packing, loss of pore spaces, and thinning of bedding thickness due to overloading of sediments and selective dissolution of the framework grains. Mineral overgrowths, mineral replacement, clay-mineral transformation, dissolution, deformation, and pressure solution occurred during burial diagenetic stage. After rocks were uplifted, weathered and unroofed by erosion, this resulted in decementation and oxidation of iron-rich minerals. The rocks of the Ecca Group were subjected to moderate-intense mechanical and chemical compaction during their progressive burial. Intergranular pores, secondary dissolution, and fractured pores are well developed in the sediments of the Ecca Group. The presence of fractured and dissolution pores tend to enhance reservoir quality. However, the isolated nature of the pore linkage makes them unfavorable producers of hydrocarbons, which at best would require stimulation. The understanding of the space and time distribution of diagenetic processes in these rocks will allow the development of predictive models of their reservoir quality, which may contribute to the reduction of risks involved in hydrocarbon (oil and gas) exploration.

Examining the interplay of climate and low amplitude sea‐level change on the distribution and volume of massive dolomitization: Zebbag Formation, Cretaceous, Southern Tunisia

AbstractDuring the Cretaceous, a humid global climate, calcitic seas, high relative sea‐level and low amplitude changes in relative sea‐level largely prevented large‐scale dolomitization in many carbonate successions. However, the well‐exposed shallow‐water carbonate sediments of the Upper Albian–Lower Turonian Zebbag Formation on the Jeffara Escarpment, southern Tunisia, are pervasively dolomitized. This study considers why dolomitization was so widespread in this region during a period of Earth history when platform‐scale dolomitization is rare. Marine conditions were established in the Upper Albian, evidenced by stacked upward‐shallowing packages of shallow subtidal to peritidal carbonate sediments in the basal Rhadouane Member. A gradual increase in the volume of subtidal sediments in the Cenomanian Kerker Member, culminated in deposition of laterally extensive marls, during maximum flooding of the platform in the Lower Turonian. The overlying Gattar Member was then deposited in shallower water as relative sea‐level fell. The entire Zebbag Formation is pervasively replaced by stratabound, fabric‐retentive, dolomite, except within the marl at the top of the Kerker Member, which is only partially dolomitized. Petrographic textures indicate dolomitization largely post‐dated marine cementation and platform emergence but pre‐dated chemical compaction. Slightly more positive oxygen isotope signatures, slightly elevated concentrations of Sr and a near‐absence of evaporites are consistent with dolomitization by reflux of mesohaline sea water. An upward‐decrease in major element concentrations and higher 87Sr/86Sr compared to Upper Cretaceous sea water suggest that basal, Albian siliciclastic beds acted as aquifers facilitating dolomitization by fluxing fluids offshore. Dolomitization is interpreted to have resulted from multiple fluxes of sea water over periods of 0·5 to 2·5 Ma. The unusually high volume of dolostone for a platform of this age most probably reflects deposition within an arid climate belt, where an efficient reflux system was facilitated by basal, permeable siliciclastic strata.

Az Endrődi Formáció kőzettani és palinológiai vizsgálata a Hódmezővásárhely–I fúrásban (Makói-árok) — őskörnyezeti és diagenezis-történeti értékelés

Munkánkban a Hódmezővásárhely–I fúrás 35. és 40. magfúrási szakaszaiból (5167,0–5183,0 m, illetve 5468,0–5486,0 m) kiválasztott, az Endrődi Formációba sorolt kőzeteket (Tótkomlósi Tagozat, mészmárga és homokkő) vizsgáltuk. A minták általános jellemzője, hogy mikrites–mikropátos kalcitot, agyagásványokat, továbbá kőzetliszt–homok méretű, szögletes törmelékszemcséket (monokristályos és polikristályos kvarc, muszkovit, biotit, klorit; karbonát törmelék, metamorf kőzettörmelék) tartalmaznak. A polimikt, éretlen üledékanyag lokális forrást jelez, ami a környező aljzatmagaslatok kőzeteinek eróziójára hívja fel a figyelmet. A fúrás válogatott mintáin palinológiai vizsgálatok készültek az egyes magszakaszok biosztratigráfiai és őskörnyezeti értékelése céljából. A vizsgált mintákból két biozónát: az idősebb Spiniferites bentorii oblongus és a fiatalabb Spiniferites paradoxus Zónát lehetett kimutatni. A Spiniferitesbentorii oblongus Zónára jellemző dinociszta-együttes egy a parthoz közelebbi környezetet és/vagy nagyobb tápanyagbeáramlást jelez, míg a fiatalabb együttesben több olyan alak található, ami disztális környezetre, a vízszint meg növekedésére és/vagy a szárazföldi eredetű törmelékbehordódás csökkenésére utalhat. A membrános dinociszták jelenléte a felszíni vizek ingadozó sótartalmára és bizonyos dinoflagellaták esetén a holoplankton életmódra való áttérésre utal, amit a vízoszlopban bekövetkező ideiglenes oxigényszegény állapotok tehettek szükségessé. Mindkét zónaegyüttes regionális, az egész Pannon-medencére kiterjedő előfordulása alapján a planktonközösségekben lejátszódó regionális folyamatokra és a víztestek kommunikációjára lehet következtetni. A palinomorfák falának színe és megtartása a bete -metődés során bekövetkező jelentős hőmérsékleti hatásra, valamint az üledékes szerves anyag nagyfokú érettségére utal. Az intenzív diagenetikus átalakulási folyamatokat mind az ásványtani, mind a petrográfiai vizsgálatok meg erő sítették. A korai diagenezis során bakteriális szulfátredukcióra utaló pirit, majd vasmentes kalcitcement vált ki, amit Fe-dolomit/ankerit továbbnövekedési cement követett. A laza szemcseilleszkedés a mechanikai kompakció alárendelt szerepét jelzi, a vázszemcsék korai mechanikai átrendeződésével, rotációjával párhuzamosan a karbonátos cementáció a rendelkezésre álló pórustérfogatot szinte teljes egészében redukálta (a vizsgált kőzetek makro- és mikroporozitása elhanyagolható). A mély betemetődési diagenezis során elkülönített események a kémiai kompakció (szerves anyagos/pirites nyomási oldódási filmek megjelenése), az agyagásványok illitesedése, dolomithelyettesítés a kalcitcementben, valamint késői pirit megjelenése.

Effects of the coalification jump on the petrophysical properties of lignite, subbituminous and high-volatile bituminous coals

The evolution of the petrophysical properties (porosity/permeability) of lignite, subbituminous and high-volatile bituminous coals (LSBC) and the effects of the first coalification jump on the pore-cleat physical properties are still poorly understood. We investigated the pore-cleat heterogeneity, volume, aperture, connectivity and pore-contributed porosity/permeability with increasing coal ranks based on measurements obtained by photometer microscopy, mercury intrusion porosimetry and nuclear magnetic resonance. The results show that the values of adsorption-pore fractals (DA)<seepage-pore fractals (DS)<cleat fractals (DC), which indicates that the heterogeneity is in the order adsorption-pores<seepage-pores<cleats. The first coalification jump makes abrupt changes in the pore volume, efficiency of mercury withdrawal and coal cleat/total porosity, and results from the generation of hydrocarbons, asphalt and cleats in vitrinite. Additionally, coal porosity displays a “U” shaped trend with increasing coal rank. Cleat-contributed porosity also exhibits a “U” shaped trend, with ∼95% for 0.3 Ro, m%, ∼80% for 0.6 Ro, m% and ∼95% for 0.9 Ro, m%. The pore-contributed porosity decreases with increasing coal rank, which may result from pore space evolution by mechanical and chemical compaction. It is found that coal permeability has no obvious trend with increasing coal ranks, possibly related to the degree of cleat development. The cleat characteristics, including the aperture, length, frequency and connectivity, have a significant influence on LSBC porosity and permeability. However, certain subbituminous coals have pore-contributed permeabilities of 35–79%, which demonstrates that the influence on coal permeability of seepage-pores (providing methane flow pathway) should not be ignored.

The Transformation of Sediment Into Rock: Insights From IODP Site U1352, Canterbury Basin, New Zealand

Abstract: At Integrated Ocean Drilling Program (IODP) Expedition 317 Site U1352, east of the South Island New Zealand, we continuously cored a 1927-m-thick Holocene-to-Eocene section where we can uniquely document downhole changes in induration and lithification in siliciclastic to calcareous fine-grained sediment using a wide range of petrological, physical-property, and geochemical data sets. Porosity decreases from around 50% at the surface to 5–10% at the base of the deepest hole, with a corresponding increase in density from ∼ 2 to ∼ 2.5 g cm 3 . There are progressive bulk mineral changes with depth, including an increase in carbonate and decrease in quartz and clay content. Grain compaction is first seen in thin section at 347 m below sea floor and intensifies downhole. Pressure solution (chemical compaction) begins at 380 m and is common below 1440 m, with stylolite development below 1600 m, and sediment injection features below 1680 m. Porewater geochemistry and petrographic observations document two active zones of cementation, one shallow (eogenetic) down to ∼ 50 m, as evidenced by micritic nodules and pore-water geochemistry driven by methane oxidation by sulfate, and another burial-related cementation zone (mesogenetic) starting at ∼ 300 m. A transitional zone occurs between 50 and 300 m. Our results quantify downhole diagenetic changes and verify depth estimates for these processes inferred from outcrop studies, and provide an actualistic example of cementation and compaction trends in a slope setting.

Rock typing using geological and petrophysical data in the Asmari reservoir, Aghajari Oilfield, SW Iran

The Oligo-Miocene Asmari Formation, which extends over the entire Zagros foreland basin, is one of the most important hydrocarbon reservoirs in the Middle East. The Aghajari Field is one of the largest oilfields in the Zagros basin with the Asmari Formation being the major reservoir rock. We used petrophysical and geological data, by means of the neural network clustering technique, to identify rock types in the Asmari reservoir. Using petrophysical logs, such as gamma ray, sonic, density and neutron, along with calculated reservoir data (water saturation and effective porosity) from 20 wells (5 cored and 15 uncored wells) and their correlation with capillary pressure (Pc) curves, led to recognition of 5 electrofacies (EF1-EF5). These electrofacies were used to extend our interpretations to 60 uncored wells. The link between electrofacies and geological data (obtained from 1100m cores and 2500 thin sections) indicate that both sedimentary and diagenetic processes controlled the reservoir quality of the Asmari Formation. Porosity, permeability, water saturation, and reservoir quality index (RQI) were used to estimate reservoir quality of each electrofacies. EFs 1 and 2 with low porosity and permeability, high water saturation and low RQI are considered as non-reservoir that are related to sedimentary textures (argillaceous nature of wackestone-packstones) and effect of destructive diagenetic processes (cementation, physical and chemical compaction). EFs 3 and 4 show high values of porosity, permeability, RQI, and low percent of water saturations, which characterizes them as good reservoirs rocks. Vuggy, growth framework and interparticle porosities are major in EF-3, while the intercrystaline porosity is the major type in EF-4. Finally, EF-5 is the only electorfacies in the siliciclastic parts of the Asmari Reservoir, which is composed of rounded and well-sorted quartz grains that are slightly cemented. The latter is the best reservoir electrofacies in the Asmari Formation of the Aghajari oilfield due to its’ high values of porosity, permeability, and RQI and low percent of water saturation. Based on vertical distribution of EFs in the Asmari Formation, zones 1, 2, 3 and 4 are the best zones for drilling new productive wells around the oilfield. The results of this research can be useful in the development of the field, taking a decision and proper strategy in the connection of hydrocarbon production and recovery in the future of this field and similar fields.

Prediction of diagenetic facies using well logs – A case study from the upper Triassic Yanchang Formation, Ordos Basin, China

Understanding diagenetic heterogeneity in tight sandstone reservoirs is vital for hydrocarbon exploration. As a typical tight sandstone reservoir, the seventh unit of the Upper Triassic Yanchang Formation in the Ordos Basin (Chang 7 unit), central China, is an important oil-producing interval. Results of helium porosity and permeability and petrographic assessment from thin sections, X-ray diffraction, scanning electron microscopy and cathodoluminescence analysis demonstrate that the sandstones have encountered various diagenetic processes encompassing mechanical and chemical compaction, cementation by carbonate, quartz, clay minerals, and dissolution of feldspar and lithic fragments. The sandstones comprise silt-to medium-grained lithic arkoses to feldspathic litharenites and litharenites, which have low porosity (0.5%–13.6%, with an average of 6.8%) and low permeability (0.009 × 10−3 μm2 to 1.818 × 10−3 μm2, with an average of 0.106 × 10−3 μm2).This study suggests that diagenetic facies identified from petrographic observations can be up-scaled by correlation with wire-line log responses, which can facilitate prediction of reservoir quality at a field-scale. Four diagenetic facies are determined based on petrographic features including intensity of compaction, cement types and amounts, and degree of dissolution. Unstable and labile components of sandstones can be identified by low bulk density and low gamma ray log values, and those sandstones show the highest reservoir quality. Tightly compacted sandstones/siltstones, which tend to have high gamma ray readings and relatively high bulk density values, show the poorest reservoir quality. A model based on principal component analysis (PCA) is built and show better prediction of diagenetic facies than biplots of well logs. The model is validated by blind testing log-predicted diagenetic facies against petrographic features from core samples of the Upper Triassic Yanchang Formation in the Ordos Basin, which indicates it is a helpful predictive model.

Microfacies and Diagenetic Analysis of Lockhart Limestone, Shah Alla Ditta Area Islamabad, Pakistan

The Larger Foraminifera made a major contribution to Paleocene and Eocene biota. The microfacies and diagenetic setting of the Paleocene Lockhart Formation were analyzed in Shah Alla Ditta area, Islamabad. The Lockhart Formation is highly fossiliferous, comprising of thin nodular limestone with interbedded shale/marl. The lower contact is unconformable with Jurassic Samansuk formation while the upper contact is conformable with Margalla Hill Limestone. Rock samples were collected from the field and thin sections were prepared and analyzed under the microscope and on the basis of detailed petrographic study three types of microfacies were identified and interpreted based on Dunham’s textural classification, allochem type, fossil content and sedimentary structures. The microfacies include: LH-MF 1: Foraminiferal Wackestone/Biomicrite Microfacies, LH-MF 2: Algal Foraminiferal Wackestone Microfacies, LH-MF 3: Mixed Bioclastic Mudstone Microfacies. A number of larger benthic foraminifera, and the lack of Planktonic foraminifera in the Lockhart Limestone, indicate shallow to deep water of restricted inner to middle shelf environment of deposition. Larger foraminifera and dasycladacena algae are the two major fossil contents present in the Lockhart Formation. Larger foraminiferal species like Lockhartia hameii and Lockhartia conditi present in the formation confirm the age of Lockhart Limestone as Paleocene. The main diagenetic fabric recognized in the limestone reveals mechanical and chemical compaction, deep burial water pressure, pressure solution and tectonics-related fracturing. The nodularity in the limestone is credited to pressure solution.

Petrology, Diagenesis and Reservoir Quality in the Hawkesbury Sandstone, Southern Sydney Basin, Australia

Petrography of the Hawkesbury Sandstone was described by thin section, scanning electron microscope and Xray diffraction techniques. Quartz is dominant in the Hawkesbury Sandstone which contains very minor feldspar, lithic grains, mica and heavy minerals. The Hawkesbury Sandstone is quartzarenite to sublitharenite. Quartz includes monocrystalline and polycrystalline grains while the feldspar includes both K-feldspar and plagioclase. Volcanic, sedimentary and chert rock fragments are present. Thin section porosity occurs in the Hawkesbury Sandstone, particularly the coarse-grained deposits. Primary porosity is more common than secondary porosity in the Hawkesbury Sandstone. Thin sections and scanning electron microscopy were used to describe diagenetic alterations and their influence on porosity in Hawkesbury Sandstone. These diagenetic alterations include compaction, quartz overgrowths, authigenicclay minerals and carbonate cement. Compaction occurred in the Hawkesbury Sandstoneduring throughout diagenesis. The influence of chemical compaction on thin section porosity was greater in the Hawkesbury Sandstone. Quartz overgrowths are common in the Hawkesbury Sandstone where they have a strong influence on porosity. Authigenic clay minerals are the widespread in the Hawkesbury Sandstone filling pores and occurring as grain-coatings on detrital and authigenic grains. Where they coat quartz grains they preserve porosity by preventing growth of quartz overgrowths. Pore-filling carbonate cement reduced porosity whereas dissolution of carbonate resulted in secondary porosity. Dissolution of unstable feldspar and lithic grains is absent in the Hawkesbury Sandstone. The Hawkesbury Sandstone is characterized by primary and secondary porosity, thus it has good groundwater storage and flow potential. Medium- and coarse-grained sandstone beds are common in the Hawkesbury Sandstone but it shows vertical variations in porosity. A few impermeable shale and siltstone units occur in the Hawkesbury Sandstone forming local confining layers.

A new stylolite classification scheme to estimate compaction and local permeability variations

We modeled the geometrical roughening of bedding-parallel, mainly layer-dominated stylolites in order to understand their structural evolution, to present an advanced classification of stylolite shapes and to relate this classification to chemical compaction and permeability variations at stylolites. Stylolites are rough dissolution seams that develop in sedimentary basins during chemical compaction. In the Zechstein 2 carbonate units, an important lean gas reservoir in the southern Permian Zechstein basin in Germany, stylolites influence local fluid flow, mineral replacement reactions and hence the permeability of the reservoir. Our simulations demonstrate that layer-dominated stylolites can grow in three distinct stages: an initial slow nucleation phase, a fast layer-pinning phase and a final freezing phase if the layer is completely dissolved during growth. Dissolution of the pinning layer and thus destruction of the stylolite's compaction tracking capabilities is a function of the background noise in the rock and the dissolution rate of the layer itself. Low background noise needs a slower dissolving layer for pinning to be successful but produces flatter teeth than higher background noise. We present an advanced classification based on our simulations and separate stylolites into four classes: (1) rectangular layer type, (2) seismogram pinning type, (3) suture/sharp peak type and (4) simple wave-like type. Rectangular layer type stylolites are the most appropriate for chemical compaction estimates because they grow linearly and record most of the actual compaction (up to 40mm in the Zechstein example). Seismogram pinning type stylolites also provide good tracking capabilities, with the largest teeth tracking most of the compaction. Suture/sharp peak type stylolites grow in a non-linear fashion and thus do not record most of the actual compaction. However, when a non-linear growth law is used, the compaction estimates are similar to those making use of the rectangular layer type stylolites. Simple wave-like stylolites are not useful for compaction estimates, since their growth is highly non-linear with a very low growth exponent. In the case where sealing material is collected at the tooth during dissolution, stylolites can act as barriers for local fluid flow as they intensify sealing capabilities of pinning layers. However, the development of teeth and spikes offsets and thus destroys continuous stylolite seams so that the permeability across the stylolite becomes very heterogeneous and they are no continuous barriers. This behavior is best shown in rectangular layer and seismogram pinning type stylolites that develop efficient fluid barriers at teeth tips but destroy sealing capabilities of layers by offsetting them at the flank, leading to a permeability anisotropy along 2-D stylolite planes. Suture/sharp peak stylolites can create fluid barriers if they collect enough sealing material. However, if the collecting material does not seal or if spikes offset the sealing material the stylolite leaks. We propose that our classification can be used to realistically estimate chemical compaction in reservoirs and gives an indication on how heterogeneous the permeability of stylolites can be.

Overpressure preventing quartz cementation? - A reply

Chemical compaction and the relative importance of the pressure dissolution and illite-mica induced dissolution (IMID) models have remained a contentious issue, as is the role played by stress in chemical compaction. This paper offers further support and evidence as discussed in Stricker et al. (2016b), focusing on the reservoir quality of the Triassic Skagerrak Formation sandstones in the high pressure high temperature (HPHT) Central Graben, North Sea. The reply discusses alterative reservoir quality interpretations and comments as raised by Maast (2016). A series of theoretical and experimental studies, as well as field based evidence is presented providing strong support to the important role of stress (e.g. vertical effective stress) during chemical compaction. The evidence leads to the conclusion that the process of chemical compaction is stress and temperature driven and significantly enhanced by clay minerals, playing a catalytic role by increasing the width of diffusion pathway or by modifying the kinetics of the dissolution process.

Pore-pressure prediction challenges in chemical compaction regimes: An alternative VP/VS-based approach

Understanding pressure mechanisms and their role in porosity-effective stress relationship is crucial in pore-pressure prediction estimation, particularly in complex geologic and high-temperature regimes. Overpressures are commonly associated with undercompaction and/or unloading mechanisms; those associated with undercompaction generally possess a direct relationship between effective stress and porosity, whereas those associated with unloading do not provide such direct indications from porosity trends. The type of associated unloading mechanism can be correlated when the effective stress and velocity become distorted with the onset of unloading. In the Ravva field, the pore-pressure distribution and overpressure mechanism in the Miocene and below it is a classic example of the unloading mechanism related to chemical compaction, thereby making it difficult to resolve the magnitude and trend of pore pressures. Here, the ratio of P- and S-wave velocities ([Formula: see text]) is analyzed from the drilled locations to understand the effects of lithology, pressure, and fluids on formation velocities and indicates a distinct decreasing trend across the overpressure formations, which I have corresponded to excess pressure resulting from chemical compaction. Across the high-pressured zones, [Formula: see text] ratios show low values compared with normally pressured zones possibly due to the presence of hydrocarbon and/or overpressures. A velocity correction coefficient ranging 0.83–0.71 is resolved for overpressure zones by normalizing the [Formula: see text] values across the normally pressured formations, and thereby assuring that a pore-pressure estimation using corrected velocity from [Formula: see text] analysis shows a high degree of accuracy on prediction trends. Pore-pressure predictions based on [Formula: see text] are a more effective and valid approach in high-temperature settings, in which numerous factors can contribute to pressure generation and a direct effective stress-porosity relationship deviates from the trend.

Stylolites and Porosity In A Lower Cretaceous Limestone Reservoir, Onshore Abu Dhabi, U.A.E.

Abstract Abstract The vertical distribution of bedding-parallel stylolites in cores from the approximately 50-m-thick Thamama-B reservoir zone of the Lower Cretaceous Kharaib Formation is compared between two wells on the water-filled flanks and two wells on the crest of a giant oilfield where oil has previously been interpreted as having preserved higher porosity on the crest. Stylolite abundances indicate division of the reservoir zone into three intervals: The top 12 ft (3.7 m) of the zone contains abundant low-amplitude stylolites in both crest and flank locations. The main, central part of the zone has abundant high-amplitude stylolites in the flank wells but nearly no stylolites in the crest. The basal 17 ft (5.2 m) of the zone contains high-amplitude stylolites in all wells, but these are somewhat more abundant in the cores from the crest of the field. The contrast in stylolite abundance in the center interval is consistent with oil having inhibited chemical compaction on the crest of the field. Minor dissolution along wispy seams in the center interval of the crest is interpreted as representing an incipient stage of stylolite development. The similarity in stylolite abundance in the top interval between crest and flank wells is interpreted as reflecting stylolite formation pre-dating oil emplacement because of greater depositional concentration of clay in thin laminations resembling flaser bedding. Stylolite growth in the basal interval may also partly predate oil filling, but the evidence for timing is unclear because stylolites in the heavily bitumen-stained basal interval of the crestal wells have thick bitumen coatings that may be residues from dissolution of the surrounding limestone. Negative correlation between porosity and both amplitude and proximity of stylolites supports the model of porosity loss by calcite cementation derived from stylolites. The overall porosity difference between crest (22%) and flanks (12–16%) approximately matches the volume of cement that would have been supplied by the thickness of strata dissolved on the flanks. The match between porosity difference and thickness variation supports a diagenetic system closed to significant calcite import or export. Profiles of bulk-chemical analyses reveal how clay (proportional to aluminum) and dolomite (proportional to magnesium) vary both vertically in the formation and laterally between the crest and flank locations. Higher clay values, suggested to facilitate stylolite development, occur mainly in the top and basal parts of the zone, reflecting episodic deposition of siliciclastic fines. Higher dolomite contents throughout the wackestone-dominated lower two thirds of the reservoir zone (around 5–10 wt. %) than in the grain-supported upper one third (little or no dolomite) are suggested to reflect differences in early dolomitization by seawater attending slower sedimentation rates in the lower part of the zone. Like the variations in chemical compaction, dolomitization is thus a manifestation of the layer-cake geometry of this reservoir. Detailed core descriptions from each well and tables of bulk-rock chemical analyses and stylolite data are available as supplemental material.