Late Diagenetic Stage Research Articles

Diagenesis and reservoir quality evolution of the paleogene sokor1 sandstones in the agadem block, termit basin, eastern Niger

The Paleogene Sokor1 Formation in Termit Basin is recognized as the most important hydrocarbon reservoir. However, in spite of its reservoir importance, published studies on its diagenetic process and their effects on reservoir quality are absent or limited. Petrographic analysis, scanning electron microscopy and X-ray diffraction were used to assess diagenetic characteristics, controls on reservoir and reservoir quality of Sokor1 Formation. The Sokor1 sandstones are mostly quartz sandstone, lithic quartz-arenite and rarely lithic fedspathic-quartz-arenite, with an average mass fraction of quartz 95%, feldspar 1.6% and rock fragments 3.4% (Q95F1.6R3.4). Diagenetic processes in Sokor1 sandstones include mechanical compaction, cementation, dissolution and replacement. The main authigenic minerals are quartz overgrowth and clay minerals, which occur as pore-filling and pore-lining cements. Sokor1 sandstone has undergone stages A and B of eodiagenesis and now, it is experiencing stage A of mesodiagenesis. The widespread occurrences of quartz overgrowth suggest that Sokor1 sandstones lost a significant amount of primary porosity during its diagenetic history. Secondary porosity occurred due to partial and complete dissolution of feldspar, quartz grains and rock fragments, so increasing reservoir quality. The latter is predominantly controlled by depositional environment controls on grains size, sorting and matrix. Thus, reservoirs of best quality were deposited in braided river channel environments. In addition, oil accumulation has no discernable effects on porosity and oil probably entered the reservoir at late diagenetic stage, after quartz overgrowth and authigenic cements had already occurred.

Origin of carbonate cements and reservoir evolution of tight sandstone in the Upper Triassic Yanchang Formation, Ordos Basin, China

The Upper Triassic Chang 8 Member, the eighth member of the Yanchang Formation, is a key reservoir interval in the Jiyuan area of the Ordos Basin. The reservoir quality of the Chang 8 Member tight sandstones is extremely heterogeneous owing to the widespread distribution of carbonate cements. The carbonate cements commonly develop near sandstone–mudstone interfaces and gradually decrease away from the interfaces to the centres of the sand bodies. However, the content of carbonate cements (≤6%) has a positive correlation with the visual porosity in the Chang 8 Member sandstone, revealing that the carbonate cements contribute to the compaction resistance and the residual primary pores of reservoirs during the diagenetic process. Three main types of carbonate cement are identified: type I (calcite), type II (calcite and ferrocalcite), and type III (dolomite and ankerite). The type I calcite is characterised by enriched δ13C (mean –3.41‰) and δ18O (mean –15.17‰) values compared with the type II (mean δ13C = –7.33‰, δ18O = –18.90‰) and type III (mean δ13C = –10.0‰, δ18O = –20.2‰) cements. Furthermore, the mean δ18O value (–4.7‰) of the type I pore fluids is 1.5‰ and 0.9‰ lower than the type II (mean –3.2‰) and type III (mean –3.8‰) pore fluids, respectively. This indicates that the evolving pore fluids experienced some relative strong water–rock interactions that provided the original materials (e.g. Ca2+, Fe3+, and Mg2+) for the carbonate cements during the diagenetic process. The highly saline lake water directly provided the primary material for the type I calcite precipitation, which also provided the material necessary for the precipitation of the type II and type III carbonate cements, causing enriched δ18O values of the pore fluids during the precipitation of the type II and type III carbonate cements. Although the earlier dissolved pores were filled with ferrocalcite, dolomite and ankerite in the middle–late diagenetic stages, some residual pores and fractures remained to become the potential reservoir storage spaces for the oil and gas exploration in the Jiyuan area.

Diagenesis and rock properties of sandstones from the Stormberg Group, Karoo Supergroup in the Eastern Cape Province of South Africa

Abstract Diagenesis is one of the factors that affect reservoir quality in sandstones. Knowledge of diagenetic transformation and how it impacts the development of porosity in reservoirs rocks is thus key to successful mineral exploration. To date, little is known about the diagenesis of the uranium-hosted sandstones of the Stormberg Group, Karoo Basin, South Africa. Petrographic study, scanning electron microscopy (SEM) aided with energy dispersive X-ray (EDX), and X-ray diffraction (XRD) analyses were employed for the diagenetic study of the sandstones. This study aims to provide an account of the diagenesis of these rock units and how it impacts their reservoir quality. The diagenetic processes that have affected the sandstones are cementation, compaction, recrystallization, replacement, mineral overgrowth and dissolution. These processes have passed through early, late and uplift-related diagenesis. Formation of authigenic minerals and precipitation of the mineral cements occurred during different diagenetic phase but mostly during the early diagenetic stage. This stage was followed by lithification, which resulted in increased in grain packing, loss of pore spaces and thinning of bedding. Quartz and feldspar overgrowths, chloritization, muscovitization, concave-convex contacts, recrystallization, albitization and suture contacts are present in the late diagenetic stage. The uplift-related diagenetic stage was affected by calcitization, grain deformation and fracturing, dissolution and saussuritization. All these diagenetic processes largely affected the porosity and permeability of the sandstones. Primary and secondary porosities were both observed in the sandstones. In general, there is no single diagenetic process that is controlling the pattern of porosity evolution in the sandstones. On the other hand, the presence of fractured and dissolution pores tend to increase the porosity, thus enhancing the reservoir quality. This study shows that diagenesis has significantly altered the original petrologic characteristics of the sandstones which have induced considerable deterioration and heterogeneity in the reservoir quality of the Stormberg Group sandstones in the Main Karoo Basin of South Africa.

Geotechnical properties of Precambrian carbonate, Saudi Arabia

The effect of post depositional processes (e.g., diagenesis) on geotechnical properties of the studied Fatima carbonates has been evaluated on 32 oriented samples of Precambrian carbonate of Fatima Group, Jeddah, Saudi Arabia where Fatima carbonate considered as the oldest sedimentary rock unit. Fatima carbonate sequence represents the middle part of Fatima Group and attains 90–130 m thick. Fatima carbonate composes mainly of alternating cycles of highly deformed, folded, and faulted stromatolitic carbonates. Algal laminates with scattered dolomite pseudomorphs are recorded within these carbonates and are shown clearly in cut-slabs. The laminae are millimetric in thickness. The Fatima carbonate has a relative abundance of nearly uniform-sized dolomite crystals. Dolomite crystal are represented by small-sized rhombs of euhedral, subhedral, and non-rhombic. The larger euhedral rhomb-shaped dolomite crystals are occurred as cavity filling dolomite. The silicification features are recognized as patches of cryptocrystalline quartz replacing laminated carbonates. It is believed that this type of silicification represents late diagenetic stage and not influenced by the depositional environment or by physicochemical conditions of water. XRD charts indicate that the Fatima carbonate is consisted mainly of dolomite, quartz, and minor constitutes of gypsum. These carbonates are mostly barren in marine fossils and it is clear that these carbonates were deposited within restricted (lagoonal) marine environment. The rock unit weight values of the studied Fatima carbonate range from 29.429 to 25.993 kN/m3. The total porosity varies from 9.147 to 6.112%. The uniaxial compressive strength (UCS, measured by Schmidt Hammer) values of Fatima carbonate range from 22.6 to 110 MPa for the horizontal direction and from 19.7 to 71.8 MPa for the vertical direction). The abovementioned results implicate that the studied Fatima carbonate rock samples are classified as medium to strong rocks.

An experimental study simulating the dissolution of gypsum rock

The dissolution of gypsum rock is of significance to study because it affects the formation of hydrocarbon reservoirs, cap rocks and evaporite deposits. However, the characteristics and mechanism of the dissolution process remain unclear. Here, we present data from experiments performed to address this issue. The experiments simulate various geological conditions, including different diagenetic stages of burial under different fluid types. The diagenetic stages include: 30°C and 0.3 MPa for the epidiagenetic stage; 60°C and 13 MPa for the early diagenetic stage; 100°C and 27 MPa for the middle diagenetic stage; and 150°C and 43 MPa for the late diagenetic stage. The different fluid types include pure water representing continental water, seawater, 0.3 wt.% CO2 solution representing meteoric water, and a 0.2 wt.% acetic acid solution representing organic fluid. We also carried out the experiments on limestones and dolomites, because these rocks also occur in saline water sedimentary systems with gypsum rocks. Experimental results show that lithology, fluid type and temperature–pressure conditions can all affect dissolution. In terms of lithology, gypsum rocks dissolve more easily than limestones and dolomites. Fluid type has little effect on the dissolution of gypsum rock, and gypsum is soluble in all four types of fluids. In contrast, limestones and dolomites are almost insoluble in pure water and seawater, but show clear dissolution in CO2 and acetic acid solutions. The data indicate that gypsum rock has a dissolution peak close to the early diagenetic stage. In contrast, limestones and dolomites have dissolution peaks in the CO2 solution at the early–middle diagenetic stage, and do not show a peak in the acetic acid solution under surficial temperature–pressure conditions. The dissolution rates of limestone and dolomite show different trends with increasing temperature and pressure: limestone dissolution rates decline whereas dolomite dissolution rates increase. Therefore, we infer that the physicochemical properties of a rock are important drivers of dissolution.

Controls on diagenesis and dolomitization of peritidal facies, Early Cretaceous Lower Edwards Group, central Texas, USA

The Early Cretaceous Fort Terrett Formation of Mason County, central Texas, is a succession of subtidal to peritidal mud-dominated facies with minor intervals of bioclastic packstone–grainstone, rudist floatstone, and interbedded chert nodules. The strata conformably overlie the Hensel Formation, which was deposited unconformably on Precambrian basement. The Hensel Formation also contains a significant percentage of dolomite, precipitated within a fine-grained clayey matrix. The Hensel and Fort Terrett Formations were deposited during a transgressive episode, which provided the conditions for the extensive shallow-water Comanche carbonate platform. Siliciclastic and carbonate sediments were deposited along the coastal margin in subtidal, intertidal to supratidal areas. Previous dolomitization models have suggested that high permeability layers are required for dolomitizing brines to flow through a carbonate succession. Although, interparticle porosity in muddy tidal-flat successions can be significant, it has a limited flow capacity. However, interconnected fenestral porosity can allow sufficient fluid flow to move dolomitizing fluids more efficiently through the succession. Thus, it is hypothesized that interconnected fenestral porosity could have had a significant impact on permeability within this muddy succession and provided the pathways and conduits for Mg-rich brines. Four types of dolomite are recognized in the Fort Terrett succession. Three of these dolomite types formed largely by replacement and they occur throughout the succession. Features such as crystal size, crystal face geometry and zonation reflect the progressive development and recrystallization of the dolomite types. Only type 4 dolomite formed as a cement in void spaces during a late diagenetic stage. The direction of the dolomitizing fluid movement is difficult to determine, but it was likely downward in this case, controlled by a density-head driving-mechanism generated by dense hypersaline fluids from an evaporating lagoon.

Characteristics and controlling factors of shale gas reservoir in the Fuling area, Sichuan Basin

Statistical analysis of massive laboratory analytical data shows that for intervals in the Upper Ordovician Wufeng Formation and the Lower Silurian Longmaxi Formation in the Fuling area, Sichuan Basin, the total organic carbon content is 0.55%–5.89% with an average of 2.54%, presenting a gradual increase trend from top to bottom. Based on whole-rock X-ray diffraction data, the content of clay minerals of the shale is 16.6%–62.8% with an average of 40.9%, gradually decreasing from top to bottom. The content of brittle minerals gradually increases from top to bottom, ranging from 37.2% to 83.4% with an average of 59.1%. Porosity is 1.17%–7.98% with an average of 4.61% through the helium injection method. The horizontal permeability is 0.002–335.209 mD with an average of 23.785 mD by the steady-state method. Massive analytical data from the high-pressure mercury injection method show the micropores are well developed in the shale, and mainly provide the primary specific surface area of pores, while the micropores, transitional pores and mesopores primarily contribute to permeability. The pore types were observed using SEM in combination with argon ion polishing technology. From top to bottom of the shale interval, the organic pores increase and the inorganic pores decrease. Gas desorption data show that the total gas content is 0.44–5.19 m3/t with an average of 1.97 m3/t, and gradually increase from top to bottom of the shale interval. The development of marine shale gas reservoir in Well A is controlled by mineral compositions, the development characteristics of organic matter. TOC is not only a major intrinsic factor controlling the lower reservoir interval, but also an important material to provide the reservoir space for the shale gas. In the late diagenetic stage, the clay mineral assemblages changed from montmorillonite to illite, forming new micropores and leading to an increase of the porosity and great influences on the upper reservoir interval. The content of brittle minerals is more than 50%, facilitating formation of fractures, and the permeability performance can be enhanced significantly. In general, the Wufeng Formation and the bottom interval of the Longmaxi Formation are both high-quality reservoirs and primary pay intervals.

Implications of controlling factors in evolving reservoir quality of the Khatatba Formation, Western Desert, Egypt

This paper sheds light on the role of petrophysical properties, framework grains and its textural properties, capillary pressure, diagenetic constitutions and events as controlling factors in evolving sandstone quality of the Khatatba Formation in the Western Desert, Egypt. Petrophysical analyses coupled with petrographic observations and diagentic studies have been carried out for many core samples in order to infer the controlling factors of reservoir quality. It has been observed that the sandstone quality of the Khatatba Formation have been adversely affected from well to well in the study area and from zone to zone in the same well and formation. Good- quality sandstones have been found with the porosity ranges from 10–17% and permeability ranges from 100 –1000 mD. The petrographic description indicates the presence of many open hydraulic fractures and dissolution phenomena which all took place in multiple phases during the late diagenetic stage, leading to improvement in the reservoir quality. Dramatic reservoir quality deterioration has been recorded in many zones. The reduction of permeability and fluid pathways in addition to the mechanical compaction, solid bitumen occupying many pore spaces and fractures are observed in many zones. These cause major destroy in reservoir quality that leading to lowering the oil recovery compare to the expected outcome

Reservoir diagenesis research of Silurian Longmaxi Formation in Sichuan Basin, China

The reservoir diagenesis of Silurian Longmaxi Formation in Sichuan Basin was studied based on a large number of thin section identification, scanning electron microscopy analysis, X-ray diffraction tests, and some other experiments. Seven diagenetic processes were identified, including compaction, cementation, clay mineral transformation, replacement, dissolution, organic matter thermal maturation, and tectonic disruption. Three kinds of cements (quartz, carbonate and sulfide) were recognized, while the source material of quartz cements and the main factor of forming abundant carbonate cements were summed up. According to the single well analysis of the Well N3, it shows that the best, the suboptimal and the none shale reservoir sections were subjected to different diagenetic transformations. As to best shale reservoir, except for compaction, all the main inorganic diagenesis were significantly related to organic matter maturation. Through comprehensive analysis of diagenetic indicators, it is observed that the reservoir has already been in period B of middle diagenetic stage to late diagenetic stage. The inorganic diagenesis has a significant impact on shale reservoir, because it not only controls the conservation, development, and evolution of porosity, but also the mechanical property and the adsorption capacity of rocks. The organic diagenesis is the source material of shale gas, and it generates a large number of nanoporosity in organic matter, which increases the total porosity and the adsorption capacity of the reservoir.

Study of Diagenesis and Pore Evolution of Triassic Jialingjiang Formation in Southern Puguang Gasfield

The second member of Jialingjiang formation is considered to be an important gas reservoir with good exploration prospects in the southern Puguang gasfield. The diagenesis types are analyzed and different diagenetic stages are divided by analyzing carbon and oxygen isotopes as well as observing the slices. The widespread forms of diagenesis in the research area principally include compaction, cementation, pressure solution, dolomitization, recrystallization, dissolution, and tectonic disruption, among which cementation, dissolution, dolomitization, and recrystallization contribute greatly to the development of porosity in the reservoir. The reservoir has experienced four diagenetic stages: syndiagenetic stage, early stage of diagenesis, diagenetic stage, and late diagenetic stage. Most of the primary pores were destroyed in the diagenetic evolution stages of reservoir; the present porosity mainly belongs to the secondary pores.

Analysis of developmental characteristics and dominant factors of fractures in Lower Cambrian marine shale reservoirs: A case study of Niutitang formation in Cen’gong block, southern China

Abstract Fractures are important to shale gas reservoirs because they can increase the storage spaces and seepage pathways. In addition, natural fracture systems are important to hydraulic-fracture stimulation to form fracture networks. Currently, China has achieved a breakthrough of the Lower Silurian Longmaxi formation in Sichuan basin and its surrounding areas. Compared with the Longmaxi shale, the Lower Cambrian Niutitang shale has greater TOC content, depositional thickness and wider distribution areas; thus, it is another significant stratum for China's shale gas. This paper discusses the characteristics and dominant factors of fractures and their relationship to the gas content in the Niutitang shale based on several types of data: observations and descriptions of fracture systems in outcrops, drilling cores, casting thin sections and field-emission scanning electron microscopy (FE-SEM); detailed fracture characteristics and parameters; and analyzes and tests of corresponding fracture segment samples. The results indicate that the Niutitang shale has similar brittle mineral and TOC content as Barnett shale, except that it has a higher maturity (equivalent Ro>2.2%). Multiple types of fractures are abundant and have suffered multi-stage activation and modification; these are mainly structural fractures due to the late diagenetic stage and multi-stage tectonic movements in southern China. The primary types of micro-fractures are interlayer, inter-particle and intra-particle fractures. The fractures with scratches have abundant pores in mylonitic minerals, and unsealed fractures (inter-particle, intra-particle fractures, and cleavages) and pores (inter-particle, intra-particle and dissolved) in the cement of the fractures and have improved the effectiveness of sealed fractures. The development of fractures is controlled by structural factors, mineral composition, TOC, reservoir heterogeneity, lithology and rock mechanical properties, and the structural factor is the uppermost. Within the same tectonic setting, the content of the brittle minerals, brittleness (rock mechanical properties), and heterogeneity are positively correlated with the fracture density. When the TOC is less than 6.5 wt%, the fracture density, porosity and brittleness display positive correlations with TOC; however, when the TOC is greater than 6.5 wt%, the positive correlations become negative. The high quartz content (rigid particles), which is usually accompanied with high TOC, favors the development and preservation of micro-pores and fractures. Fractures have played important roles in seepage pathways, storage spaces, and interconnections of isolated micro-pores, which leads to positive correlations between the fracture density, total and desorbed gas content. Knowledge regarding this area will guide the exploration and development of shale gas and is helpful in optimizing the intervals of drilling and fracturing stimulation.

The Nuheting deposit, Erlian Basin, NE China: Synsedimentary to diagenetic uranium mineralization

Sedimentary units deposited during the post-rift stage of the Erlian Basin located in northeast China present an alternation of sandstone and mudstone layers. This sedimentological architecture is at the origin of confined permeable reservoirs hosting sandstone-type uranium deposits. The study of the Nuheting deposit offered the opportunity to identify synsedimentary/early diagenetic uranium concentrations and diagenetic mineralization hosted in mudstone-dominated layers of the Erlian Formation, indicating that a stock of uranium was present in the basin prior to the genesis of sandstone-hosted uranium deposits. Therefore, this pre-existing stock may constitute a significant source of uranium for the formation of roll front deposits present in other parts of the Erlian Basin.Detailed petrographic and geochemical study of drill-core samples from the Nuheting deposit led to the characterization of different stages related to the formation of the uranium ore bodies and allowed to propose a new metallogenic model. Uranium mineralization of the Nuheting deposit is mainly hosted in dark gray silty mudstone of wetland depositional environment of the Late Cretaceous Erlian Formation. Petrographic observations and EMP analyses evidenced that a significant amount of uranium was associated with clay minerals (interstratified clays, smectite, chlorite, palygorskyte, illite and kaolinite), either adsorbed on mineral surfaces as U (VI) ions or reduced mainly as UO2 nano to microcrystals disseminated in the clayey matrix, which corresponds to synsedimentary/early diagenetic concentrations. Trace elements on pyrite analyzed by LA-ICPMS, petrographic observations and whole-rock geochemical data led to the characterization of a diagenetic uranium mineralization. High As (1–50ppm), Mo (10–500ppm) and Se concentrations in the whole rock and the incorporation of these elements in pyrite highlight reducing conditions within the host-rocks during the diagenesis of the Erlian Formation. During the early diagenetic stage, uranium was either desorbed from clay minerals and organic materials to be reduced or directly reduced and precipitated as P-rich coffinite and pitchblende on pyrite crystals. During the late diagenetic stage, uranium was redistributed in situ and locally deposited mainly as coffinite on pyrites. Finally, an epigenetic stage of cementation was identified with sulfate and carbonate minerals, which may enclose some uranium minerals. This epigenetic stage of fluid circulation may be responsible for a minor uranium remobilization. Therefore, the Nuheting deposit experienced three main stages: (i) a synsedimentary/early diagenetic uranium concentration and mineralization, (ii) a late diagenetic in situ uranium remobilization and deposition on pyrite and (iii) an epigenetic cementation. Rock-Eval pyrolysis indicates that the organic matter contained in host-rocks of the Nuheting deposit is of type IV, inherited from land plant, and do not contain free hydrocarbons (very low S1). Therefore, our results do not support that migrated hydrocarbons were involved as a reducing agent for uranium mineralization.

Paragenetic sequences of carbonate and sulphide minerals of the Mamfe Basin (Cameroon): Indicators of palaeo-fluids, palaeo-oxygen levels and diagenetic zones

The occurrence of evaporite minerals in the Mamfe Basin (Albian–Cenomanian) is considered to be an indicator of aridity and palaeo-saline waters. A detailed lithostratigraphic analysis of the Mamfe sedimentary basin-fill indicates the occurrence of carbonate and sulphide minerals in close association with halite and gypsum within the Manyu, Mamfe and Bagba Members of the Nfaitok Formation. The composition and textures of these carbonate and sulphide minerals were studied using the Scanning Electron Microscope (SEM), attached to Energy Dispersive Atomic X-ray analyser (EDAX) that generates Backscattered and Secondary Electron Imaging (BSEI). Two paragenetic suites were encountered in this study; calcite/dolomite/ankerite/siderite and pyrite/sphalerite/galena respectively. The compositional and textural variability of different suite members appear to be conditioned by the competition for CO32- and SO42- anions by metal cations with respect to redox conditions and diagenetic stages. Early formed authigenic minerals exhibit euhedral textures, while late diagenetic minerals exhibit pseudomorphic textures, as cations with higher ionic radii integrate into earlier structures. Calcite is a precursor for dolomite, ankerite and siderite at late diagenetic stages. Pyrite, sphalerite and galena indicate deposition in anoxic and sulphate reducing environments. Micron-sized euhedral pyrite framboids are syn-depositional, while Pb–Zn mineralization in voids and cavities are formed at late diagenetic stages below the sulphate reduction zone (SRZ), where siderite forms encrustations around skeletal pyrite. The pristine source of saline water, SO42- and Mg2+ could be related to sea water splay into a lagoon. The source of Fe2+ is detrital, while Zn2+ and Pb2+ indicate metalliferous fluids with high heat flux, probably of hydrothermal origin.

Diagenesis in the Middle Jurassic Khatatba Formation sandstones in the Shoushan Basin, northern Western Desert, Egypt

The Middle Jurassic Khatatba Formation acts as a hydrocarbon reservoir in the subsurface in the Western Desert, Egypt. This study, which is based on core samples from two exploration boreholes, describes the lithological and diagenetic characteristics of the Khatatba Formation sandstones. The sandstones are fine- to coarse-grained, moderately to well-sorted quartz arenites, deposited in fluvial channels and in a shallow-marine setting. Diagenetic components include mechanical and chemical compaction, cementation (calcite, clay minerals, quartz overgrowths, and a minor amount of pyrite), and dissolution of calcite cements and feldspar grains. The widespread occurrence of an early calcite cement suggests that the Khatatba sandstones lost a significant amount of primary porosity at an early stage of its diagenetic history. In addition to calcite, several different cements including kaolinite and syntaxial quartz overgrowth occur as pore-filling and pore-lining cements. Kaolinite (largely vermicular) fills pore spaces and causes reduction in the permeability of the reservoir. Based on framework grain–cement relationships, precipitation of the early calcite cement was either accompanied by or followed the development of part of the pore-lining and pore-filling cements. Secondary porosity development occurred due to partial to complete dissolution of early calcite cements and feldspar. Late kaolinite clay cement occurs due to dissolved feldspar and has an impact on the reservoir quality of the Khatatba sandstones. Open hydraulic fractures also generated significant secondary porosity in sandstone reservoirs, where both fractures and dissolution took place in multiple phases during late diagenetic stages. The diagenesis and sedimentary facies help control the reservoir quality of the Khatatba sandstones. Fluvial channel sandstones have the highest porosities and permeabilities, in part because of calcite cementation, which inhibited authigenic clays or was later dissolved, creating intergranular secondary porosity. Fluvial crevasse-splay and marine sandstones have the lowest reservoir quality because of an abundance of depositional kaolinite matrix and pervasive, shallow-burial calcite and quartz overgrowth cements, respectively. Copyright © 2013 John Wiley & Sons, Ltd.

The Impact of carbonate cements on the reservoir quality in the Napo Fm sandstones (Cretaceous Oriente Basin, Ecuador)

The Napo Formation of Lower-Middle Cretaceous age in the Oriente basin, Ecuador, is an important sandstone reservoir. The formation is buried at a depth of 1,500 m in the eastern part of the basin and down to 3,100 m in the western part. The sandstones display higher porosity values (av. 20%) than other reservoirs in the region. These sandstones were deposited in fluvial, transitional and marine environments, and they are fine to medium grained quartzarenites and subarkoses. The principal cements are carbonates, quartz overgrowth and kaolin, with scarce amounts of pyrite-pyrrhotite and chlorite. Carbonate cements include: Eogenetic siderite (S1), mesogenetic and post-compactional calcite, Fe-dolomite, ankerite and siderite (S2). Early siderite and chlorite helped to retain porosity by supporting the sandstone framework against compaction. Dissolution of feldspars and carbonate cements are the main mechanism for secondary porosity development during mesodiagenesis. The high intergranular volume (IGV) of the sandstones indicates that cementation is the predominant contributor to porosity loss in the reservoir and that the precipitation of the carbonate cement occurred in early and late diagenetic stages. The stable–isotope composition of the S1 siderite is consistent with precipitation from meteoric waters in fluvial sandstones, and from mixed meteoric and marine waters in transitional sandstones. The low � 18 O‰ values of some of these carbonate phases reflect the replacement and recristalization from S1 to S2 siderite at deep burial and high temperature. Textural evidence, together with a low Sr content, also suggests that siderite (S1) in fluvial environment is an early cement that precipitated from meteoric waters, near the sediment/water interphase, followed by the generation of calcite with a higher Fe and Mg content. However, due to this higher Mg content, siderite S2 could have precipitated as a result of the thermal descarboxilation of the Mg rich organic matter. The progressive decrease in � 18 O values in all carbonate cements could be related to the continued precipitation at different temperatures and burial depth.

Characteristics of carbonate cementation in clastic rocks from the Chang 6 sandbody of Yanchang Formation, southern Ordos Basin

Densification of reservoir is an important factor that restricts oil and gas exploration from low porosity and extra-low permeability reservoirs. Carbonate cementation was heavily developed in Chang 6 sandbody, a facies of underwater distributary channel in delta front, of Upper Triassic Yanchang Formation in Fuxian area, southern Ordos Basin, and the cementation is one of the major factors that affect quality of reservoir. Based on the macro-microcosmic petrology and geochemistry features, the genesis of densification of carbonate-cemented reservoir was systematically discussed. The carbonate cementation can be classified into endogenous and exogenous, and the essential differences between them are that they were formed in different fluids and in different diagenesis periods. With the aid of identification of thin sections, analyses on electron probe, trace and rare-earth elements, carbon and oxygen isotope, we propose that the endogenous fluid for cementation came from the rock itself during early diagenetic stage. The minerals related to endogenous fluid had good shapes. The reservoir property was enhanced with porosity increasing by 3%–8% because of later dissolution by endogenous fluid. The exogenous fluid might be water combining with CO2, likely released from organic matter-rich mudstone. Calcite cement, in form of substrate cementation, was precipitated from the fluid and filled in the remaining pores of sandstones in late diagenetic stage as variations of physical and chemical conditions. The exogenous cement reduced rock porosity, damaged reservoir property, affected some oil enrichment, and seriously caused Chang 6 reservoir densification. Some of the dense layers that formed on top of sandbody could have served as diagenetic traps, and thus the exogenous cementation area could be favorable for oil exploration.

Controls on reservoir quality in the paleogene Kalatar Formation of the southwestern region of the Tarim Basin, China

The Paleogene Kalatar Formation is a main target for petroleum ex125 ploration in the southwestern region of the Tarim Basin (SWRTB) and a systematical summary of controls on reservoirs of high quality has important implications for this area. According to outcrop analysis, as well as core, and laboratory experiments based on theories of sedimentology, it can be inferred that the main pore types in the Kalatar Formation are moldic, vuggy, intercrystalline and interparticle pores, fractures, and mudstone microporosity, respectively. The foreshore in the shore-shelf depositional model is the most favorable target, wherein grainstone and dolomitized grainstone are characterized by high quality in a middle ramp subfacies of the carbonate ramp model. Diagenetic factors, such as micritization, cementation, compaction, neomorphism, silicification, and pyritization, are detrimental for reservoirs. At the same time, leaching and dolomitization can improve the quality of reservoirs. Permeability of reservoirs can be greatly improved by fractures, through which quality of reservoirs can be effectively enhanced. Concentration of CO2, temperature, and humidity are affected by changes in climate, which can have an influence on depositional setting, composition and diagenesis of sediments, and eventually the properties of reservoirs. The hot and arid climate of the Paleogene was harmful to development of reservoirs, which is demonstrated in the contemporaneous diagenetic and epigenetic stages. Shallow and deep-burial diagenesis are closely related to the formation and the connate water, however these are rarely affected by paleo-climatic variation. There is possible erosion that mainly resulted from organic acid in relative well intervals according to indications of hydrocarbons in the early diagenetic stage. Fracture systems can be established when acidic fluids are emplaced during periods of uplift, by which pores can be effectively enlarged during the late diagenetic stages.

The Transfiguration continental red-bed Cu–Pb–Zn–Ag deposit, Quebec Appalachians, Canada

The Transfiguration Cu–Pb–Zn–Ag deposit, enclosed within reduced grey sandstone, is associated with continental red beds of the Lower Silurian Robitaille Formation in the Quebec Appalachians, Canada. The Robitaille Formation rests unconformably on foliated Cambro-Ordovician rocks. The unconformity is locally cut by barite veins. The basal unit of the Robitaille Formation comprises green wacke and pebble conglomerate, which locally contain calcite nodules. The latter have microstructures characteristic of alpha-type calcretes, such as “floating” fabrics, calcite-filled fractures (crystallaria) and circumgranular cracks. Massive, grey sandstone overlies the basal green wacke and pebble conglomerate unit, which is overlain, in turn, by red, fine-grained sandstone. Mineralisation occurred underneath the red sandstone unit, chiefly in the grey sandstone unit, as disseminated and veinlet sulphides. Chalcopyrite, the most abundant Cu sulphide, replaced early pyrite. Calcrete, disseminated carbonate and vein carbonate have stable isotope ratios varying from −7.5‰ to −1.1‰ δ13C and from 14.7‰ to 21.3‰ δ18O. The negative δ13C values indicate the oxidation of organic matter in a continental environment. Sulphur isotope ratios for pyrite, chalcopyrite and galena vary from −19‰ to 25‰ δ34S, as measured on mineral concentrates by a conventional SO2 technique. Laser-assisted microanalyses (by fluorination) of S isotopes in pyrite show an analogous range in δ34S values, from −21‰ to 25‰. Negative and positive δ34S values are compatible with bacterial sulphate reduction (BSR) in systems open and closed with respect to sulphate. We interpret similarly high δ34S values for sulphide concentrates (25.1‰) and for vein barite (26.2‰) to result from rapid and complete thermochemical reduction of pore-water sulphate. Two early to late diagenetic stages of mineralisation best explain the origin of the Transfiguration deposit. The first stage was characterised by the ponding of groundwater over the Taconian unconformity, recorded by calcrete and early pyrite formation via BSR in grey sandstone. Early pyrite contains up to 2 wt.% Pb, which is consistent with Pb fixation by sulphate-reducing bacteria. The second stage (II) is defined by the replacement of early pyrite by chalcopyrite, as well as by sulphide precipitation via either BSR or thermochemical sulphate reduction (TSR) in grey sandstone. This event resulted from the synsedimentary fault-controlled percolation and mixing of (1) an oxidising, sulphate-bearing cupriferous fluid migrating per descensum from the red-bed sequence and (2) a hydrocarbon-bearing fluid migrating per ascensum from the Cambro-Ordovician basement. Mixing between the two fluids led to sulphate reduction, causing Cu sulphide precipitation. The positive correlation between Cu and Fe3+/Fe2+ bulk rock values suggests that Fe acted as a redox agent during sulphate reduction. Stage II diagenetic fluid migration is tentatively attributed to the Late Silurian Salinic extensional event.

Geological Characteristics of Oil and Gas Reservoirs in the Songpan‐Aba Area, SW China

Abstract: The Songpan‐Aba area, similar to those basins on the Yangtze block, following the rifting and separation of the Yangtze block, gradually developed into a passive marginal basin on a passive continent margin in Early Paleozoic, and later, with the Qinling‐Qilian oceanic crust subduction and ocean closure, the Caledonides were formed and the foreland basin was superimposed upon. Being influenced by the Paleo‐Tethyan extension, intra‐continental rifting‐margin basins were formed in Late Paleozoic. Following the formation of peripheral orogenic belt, the Upper Triassic again superimposed the foreland basin. The Mesozoic and Cenozoic overprinted the faulted basin, forming the Qinghai‐Tibet Plateau domes. Hydrocarbon source rock in the Early Paleozoic passive basin, the Upper Paleozoic platform carbonates and the Triassic mudstones comprise the main source‐reservoircap combination. Each layer of this area is at the advanced stage of diagenetic evolution, being entered the middle and late diagenetic stages, and anadiagenetic stage. Besides the highly matured Triassic and Permian in Zoigê and Hongyuan, almost all the area is at the early stage of over maturatation, generating much methane. This area has the potential for oil and gas.

Sandstone diagenesis of the Lower Cretaceous Sindong Group, Gyeongsang Basin, southeastern Korea: Implications for compositional and paleoenvironmental controls

Abstract The Gyeongsang Basin is a non‐marine sedimentary basin formed by extensional tectonism during the Early Cretaceous in the southeastern Korean Peninsula. The sediment fill starts with the Sindong Group distributed along the western margin of the basin. It consists of three lithostratigraphic units: the Nakdong (alluvial fan), Hasandong (fluvial) and Jinju (lacustrine) formations with decreasing age. Sindong Group sandstones are classified into four petrofacies (PF) based on their detrital composition: PF‐A consists of the lower Nakdong Formation with average Q73F12R15; PF‐B the upper Nakdong and lower Hasandong formations with Q66F15R18; PF‐C the middle Hasandong to middle Jinju formations with Q49F29R22; and PF‐D the upper Jinju Formation with Q26F34R41. The variations of detrital composition influenced the diagenetic mineral assemblage in the Sindong Group sandstones. Illite and dolomite/ankerite are important diagenetic minerals in PF‐A and PF‐B, whereas calcite and chlorite are dominant diagenetic minerals in PF‐C and PF‐D. Most of the diagenetic minerals can be divided into early and late diagenetic stages of formation. Early diagenetic calcites occur mostly in PF‐C, probably controlled by arid to semiarid climatic conditions during the sandstone deposition, no early calcite being found in PF‐A and PF‐B. Late‐stage calcites are present in all Sindong Group sandstones. The calcium ions may have been derived from shale diagenesis and dissolution of early stage calcites in the Hasandong and Jinju sandstones. Illite, the only diagenetic clay mineral in PF‐A and lower PF‐B, is inferred to be a product of kaolinite transformation during deep burial, and the former presence of kaolinite is inferred from the humid paleoclimatic conditions during the deposition of the Nakdong Formation. Chlorites in PF‐C and PF‐D are interpreted to be the products of transformation of smectitic clay or of precipitation from alkaline pore water under arid to semiarid climatic conditions. The occurrence of late‐stage diagenetic minerals largely depended on the distribution of early diagenetic minerals, which was controlled initially by the sediment composition and paleoclimate.