Calcite Cement Research Articles

Reservoir heterogeneity of an Eocene mixed siliciclastic-carbonate succession, northern Pannonian Basin

The quality of any reservoir is strongly controlled by diagenetic processes, especially of those that are comprised of mixed siliciclastic-carbonate sequences. An Eocene mixed siliciclastic-carbonate sequence forms reservoirs with diverse reservoir quality in the Hungarian Palaeogene Basin, which is found within the Pannonian Basin. Late Eocene transgression established extensive shallow-marine depositional environments strongly controlling the early diagenetic processes. Drill cores and cuttings were selected from several boreholes, representing a wide range of lithofacies to reveal the cause for reservoir heterogeneity. Petrographic, stable isotope and fluid inclusion analyses were performed to identify the diagenetic processes that controlled the porosity evolution of the reservoir, The early diagenetic calcite neomorphism caused lateral homogeneity, whereas the early meteoric water incursion enhanced reservoir properties via the development of dissolution pores, which in turn resulted in vertical reservoir heterogeneity. The deterioration of porosity started in the eogenetic realm by kaolinite formation and continued during mesodiagenesis as burial calcite, quartz, dolomite and minor illite cement precipitated in remaining pore spaces. Petrographic evidence, such as kaolinite transformation into dickite and fluid inclusion homogenisation temperatures highlight the role of temperature increase with burial, the effect of changing fluid chemistry during the evolution of the basin is documented by the chemical zonation of dolomite, the variation in salinities of the fluids enclosed fluid inclusions, as well as recurrent and prolonged periods of cementation with calcite, quartz and dolomite. Mesogenetic processes were accompanied by hydrocarbon emplacement and the formation of late-stage pyrite. The propagation and cementation of faults first contributed to increase and later to reduce fluid flow, respectively. These steps collectively resulted in a heterogeneous reservoir, characteristic of the Eocene sequence in the Hungarian Palaeogene Basin.

Quantifying Centimeter- to Microscale Heterogeneities in Sedimentary, Compositional, and Geomechanical Properties of Siltstone Deposits in the Lower Triassic Montney Formation, Northeastern British Columbia, Canada

Abstract Geological and geomechanical heterogeneities exist at multiple scales in fine-grained rocks; however, the complexity of characteristics at the centimeter- to microscale heterogeneities remains poorly understood. In this study, 10 representative samples composed of three centimeter-scale sedimentary fabrics (massive siltstone (F1), stratified siltstone (F2), and bioturbated siltstone (F3)) were analyzed from the Lower Triassic Montney Formation in the Western Canada Sedimentary Basin to describe sedimentological heterogeneity based on sedimentary fabric, compositional, and geomechanical properties. Sedimentary fabric was determined based on grainsize and the distribution of bedforms, which subdivide the facies into four μm- to mm-scale microfacies (massive siltstone (MF1), pinstriped laminated siltstone (MF2), planar- to cross-stratified siltstone (MF3), and bioturbated siltstone (MF4)). Microscale analysis using a scanning electron microscope was used to characterize microfacies and their respective mineralogical makeup (matrix, cement, and framework grains). To quantify heterogeneity, sedimentary fabric was assessed using a CT scan complemented by elemental composition (using X-ray fluorescence), and geomechanical hardness (using Equotip Piccolo Bambino handheld microhardness tool) was collected within a 1 cm by 1 cm grid within each sample. Datasets were compared using a discriminant analysis (DA) to recognize trends between multiple properties and suggest that sedimentary fabric with the highest centimeter-scale aluminum content from XRF (avg. 11%) comprises microfacies that are comparatively matrix-rich consisting of micas, negligible calcite cement, and exhibit the lowest handheld hardness values (<770). Alternatively, sedimentary fabric with a higher elemental calcium component (avg. 18%) comprises microfacies that are matrix-poor, cemented by carbonate (calcite and dolomite) and quartz, and overall exhibit a positive trend with hardness measurement (770–850). Furthermore, to relate the elemental and geomechanical proxies to controls on rock mechanics, natural calcite-filled fractures within the studied core intervals were characterized. Fractures were subdivided into three types—brecciated, bed-parallel, and vertical to subvertical fractures with each type being constrained to a specific sedimentary fabric. Based on centimeter gridding, microscale analysis and the degree of fabric interbedding play a primary role on the variability in mechanical hardness and the geometry and termination of natural fractures. Collectively, this dataset provides insight into the influence that sedimentary fabric and the distribution of elemental composition has on mechanical properties and natural fractures below well log resolution. These findings can be used to better model and predict fine-grained deposit characteristics before undergoing hydraulic stimulation.

Far-field brittle deformation record in the eastern Paris Basin (France)

AbstractJurassic carbonate strata in the eastern Paris Basin exhibit several generations of faults, tension gashes and stylolites. Although their relative chronology can sometimes be determined according to cross-cutting relationships, the duration of major deformation phases and their influence on fluid flow and carbonate cementation are still uncertain. This contribution aims to clarify the timing of brittle deformation and associated calcite cementation. Tension gashes filled by calcite in Jurassic carbonates were sampled in outcrops and boreholes and dated through U–Pb geochronology. Almost all the sampled fractures were cemented during the Cenozoic period. Continuous deformation spread from c. 50 to 30 Ma. Tension gashes oriented N10° to N20° dated at 48–43 Ma show the main Pyrenean contractional stage. A second set of calcites were dated at c. 35–33 Ma and document a Late Eocene – Oligocene extension. A transition from the compressional to the extensional regime is expressed by tension gashes dated between 43 and 35 Ma. Finally, tension gashes oriented N150° to N175°, dated between 32 and 18 Ma, may result from the propagation of the horizontal stress generated by the Alpine orogen or by late Pyrenean deformation. Clumped isotope thermometry on five samples revealed both low crystallization temperatures (from 27 to 53 °C) and the meteoric origin of calcite-precipitating fluids. Our research therefore documents a continuous fracturing from Ypresian to Rupelian times, and less expressed brittle deformation during the Miocene period.

Diagenetic characterization and the influence on the porosity properties of a mixed bioclastic-siliciclastic rift lacustrine deposit of the Sergipe-Alagoas Basin, northeastern Brazil

Bioclastic and siliciclastic deposits of the Morro do Chaves Formation, Early Cretaceous of the Sergipe-Alagoas Basin, northeastern Brazil, show depositional and diagenetic analogies with the Brazilian Pre-salt rift reservoirs. A detailed petrographic study carried out on 221 thin sections from four well-cores were used to define the stratigraphic and sedimentological controls of the diagenetic events on coquinas supported by cathodoluminescence and EDS-MEV analysis. The study of cement distribution allowed identification in high- and low-frequency cycles of the subaerial exposure zones and intervals with porosity and permeability enhancement. The eodiagenetic processes and products show a good correlation with the maximum regressive events marked in the lower carbonate-dominated sequence, by the occurrence of nodules, micritization, pseudomicrokarst, mechanical compaction, three types of calcite fringes, coarse calcite cement, and three dissolution events. This sequence shows less than 25% of terrigenous constituents, thus favoring a pervasive early calcite cement precipitation and consequently an occlusion of the porosity. The upper sequence displays a predominance of poikilotopic and blocky calcite cement related to the mesogenetic stage. This sequence shows a dominance of mixed carbonate and siliciclastic constituents, which reflects directly the reduced early cementation, and the higher porosity and permeability relatively to the lower sequence. This study on cement occurrence and distribution on mixed deposits and their relationships with sedimentological and stratigraphic controls adds new and useful data for helping in the understanding of the complex coquina successions of the analogous lacustrine bioclastic reservoirs of the Pre-salt interval. The diagenetic model can support further studies regarding to predictive models of porosity and permeability distribution in other bioclastic lacustrine reservoirs.

Determining the gas accumulation period using fluid inclusion observations: Xiang Zhong Basin, China

Abstract There are a large number of natural gas shows in the Upper Paleozoic of the central Hunan Basin. However, the hydrocarbon accumulation process was complicated due to the strong tectonic reformation in this area. A study of the natural gas accumulation period is of significance for determining gas accumulation models. In this research, the fluid inclusion samples were tested, the fluid inclusion occurrence characteristics, uniform temperature data, and reservoir evolution were analyzed, and four diagenetic periods and four diagenetic periods and three hydrocarbon accumulation periods were identified. (i) For the syntaxial stage, the filling is dark and nonluminous, and the fluid captured by the overgrowing cement around the particles is mainly seawater; (ii) for the (Early) Mesogenetic burial stage, the calcite cement grows around the particles or micritic calcite matrix, and the fluid captured is mainly brine; (iii) for (Late) Mesogenetic burial stage, two sets of decomposed fissures developed in massive calcite colloids with non-luminous gas-rich hydrocarbon inclusions within the crystals; and (iv) for Telogenetic burial stage, weakly fluorescent pure gaseous hydrocarbon inclusions are developed in luminescent calcite veins. The result indicates that the organic inclusions in the Lower Carboniferous of the central Hunan Basin are dominated by weakly fluorescent and nonfluorescent rich gas inclusions and pure gas-phase inclusions, with three stages of hydrocarbon filling. By integrating the average homogenization temperatures on burial curves, the three charging orders correspond to 260–250 Ma during the Late Permian, 249–245 Ma during the Early Triassic, and 208–170 Ma during the Late Triassic. Dense reservoir characteristics reveal deep basin gas reservoir in deep basin.

Implications of lithofacies and diagenetic evolution for reservoir quality: A case study of the Upper Triassic chang 6 tight sandstone, southeastern Ordos Basin, China

The Chang 6 tight sandstone reservoir of the Upper Triassic, developed in a large lacustrine shallow-water delta depositional environment, is the most important target for oil exploration and development in the basin. This study is undertaken with the purpose of investigating the influences of lithofacies, diagenetic fluid activity and diagenetic process on quality of the Chang 6 tight sandstone reservoir situated in the southeastern Ordos Basin, based on core observation and description, petrographic investigations, measurement of electron probe microanalysis (EMPA), homogenization temperature and laser Raman spectroscopy of fluid inclusions, laser microzone sampling carbon and oxygen isotope of carbonate cement, the X-ray diffraction (XRD), back scattered (BSE) image observation. Research result shows that three lithofacies are recognized. Each lithofacies has its initial petrological components that slightly differ from other ones, and this is in turn restricted the diagenetic evolution route and imposed diagenetic process and oil-charging intensity, which led to a different reservoir quality.Three carbonate cementation of microcrystalline calcite, sparry calcite I, sparry calcite II and ankerite cements enriched near the mudstone-sandstone interface, occurred in the Chang 6 reservoir which are unfavorable for the reservoir quality. The Chang 6 reservoir experienced three phases of diagenetic fluid activities. The fluid activity of the meteoric water occurred in the eodiagenetic stage offered a carbon source for the cementation of microcrystalline calcite (δ18OPDB: 13.3‰∼-10.0‰; δ13CPDB: 0.4‰∼2.1‰) and sparry calcite I (δ18OPDB: 19.8‰∼-13.4‰; δ13CPDB: 0.4‰∼0.8‰). The latter two fluids released by the thermal decarboxylation of organic matter during the two oil-charging stages, occurred in the mesodiagenetic stage, providing a carbon source for the formation of sparry calcite II (δ18OPDB: 21.8‰∼-18.7‰; δ13CPDB: 5.1‰∼-0.4‰) and ankerite (δ18OPDB: 21.5‰∼-16.8‰; δ13CPDB: 4.6‰∼-3.9‰). The dominant impact on reservoir quality is mechanical compaction, which destroys more intergranular pore than cementation, especially in the sandstone of lithofacies III (the porosity loss by compaction and cementation is 30.9% and 7.3%, respectively), compared to that of the lithofacies I sandstone (the porosity loss by compaction and cementation is 28.7% and 6.6%%, respectively) and the lithofacies II sandstone (the porosity loss by compaction and cementation is 28.8% and 8.5%, respectively). Laumontite cement formed during the eodiagenetic stage effectively resisted the intensity of mechanical compaction. The authigenic clay minerals in the three lithofacies are dominated by chlorite and illite, and most abundant in the lithofacies III. Primary intergranular pore can be effectively protected when the chlorite cement is less than 4%. The dissolution pores with average of 3.0%, 2.4% and 2.3% in the lithofacies I, lithofacies II and lithofacies III sandstone, respectively, formed during the mesodiagenetic stage improved the reservoir quality to a certain degree. Four models of diagenesis and pore evolution of the Chang 6 reservoir are summarized. The pattern I has the best reservoir quality due its fewer ductile component and more coarser grains, the weaker mechanical compaction and weaker cementation of laumontite and carbonate, as well as a stronger dissolution attributed to the intensified oil-charging during mesodiagenesis.

Nature and evolution of diagenetic fluids in the deeply buried Cambrian Xiaoerbulake Formation, Tarim Basin, China

Significant amounts of petroleum are found in the deeply buried Cambrian Xiaoerbulake Formation in the Tarim Basin. However, limited understanding of origin of diagenetic fluids hampers petroleum exploration. In this study, petrological and high-resolution geochemical methods have been performed to understand the origin and evolution of diagenetic fluids, and their effects on the formation of dolostone reservoir. The Xiaoerbulake Formation is characterised by microbialites. The matrix is characterised by modern seawater-like rare earth element and yttrium (REY) patterns and δ18O values of −6.02 ± 2.11‰ Vienna Pee Dee Belemnite (VPDB). Fine-crystalline dolomite cements have similar δ13C values and heavy rare earth element enrichment patterns to that of matrix dolomite, yet with lower δ18O values of −7.47 ± 1.2‰ VPDB. In the deep-burial regimes, medium- to coarse-crystalline dolomite cements and saddle dolomites were precipitated at temperatures of 90–120 and 130–190 °C, respectively, lower δ18O values, distinct middle rare earth element enrichment, and high Fe and Mn contents compared with matrix and fine-crystalline dolomite cements. These imply thermochemical reduction of Fe- or Mn-rich oxides and/or hydrothermal fluids. Compared with other types of dolomite cements, calcite cements are characterised by low δ13C values (as low as −6‰ VPDB), light rare earth elements enrichment and significantly elevated Y/Ho ratios (86 ± 59.3), likely related to the oxidation of organic matter during thermochemical sulfate reduction (TSR). Consequently, diagenetic fluid may have evolved from: Cambrian seawater, meteoric water, shallow-buried seawater-dominated pore water, medium-deep-buried ferrous-rich pore water, lately high-salinity hydrothermal fluid-dominated pore water and finally to acidic fluids related to TSR dissolution. Reservoir quality is enhanced by hydrothermal pulses and TSR in deep-burial settings. This study is significant for those concerned with the formation of deep-buried carbonate reservoirs.

Bio-stabilisation of soft soil using cattle manure

Environmentally friendly materials are being explored as alternatives to lime and cement binders in ground improvement. Biological treatment methods are environmentally friendly as they rely on the natural activity of ubiquitous microbes to produce extracellular polymeric saccharides (EPS) and calcite cements. EPS extruded by microbes exhibit strong adhesion from growth of fibrous bridges and bonds between surfaces. In situ polysaccharide secretion in soils to improve their engineering properties has not been examined. In this study, pulverised dried cattle manure (CM) are is used for in situ EPS production in a synthetic soft soil specimen. The indigenous facultative anaerobes and aerobes in animal waste produce negatively charged EPS molecules in the soil specimen. Bridging of sand particles and embedment of CM fibres in kaolinite aggregates caused an immediate increase in unconfined compressive strength (UCS) of the stabilised specimen. EPS cementation improved soil strength by filling pores and engulfing kaolinite aggregates into bonded mass. Fibre reinforcement and bio-stabilisation by CM increased the UCS of the soil by 740%. Replacing C-intensive lime/cement binders by CM as a stabilising agent has the potential to prevent release of 1 t of carbon dioxide to the atmosphere per tonne of lime/cement consumed.

Comparison of diagenesis and reservoir quality of microporous lime mudstones (Aptian) between anticline crest and flanks: Abu Dhabi, United Arab Emirates

The impact of in-situ and migrated oil on diagenesis and reservoir quality of microporous (porosity ≤ nil to 26%; permeability ≤ nil to 3 mD) intra-shelf basinal lime mudstones (Aptian) is constrained by comparing samples from anticline crest (i.e. oil zone) versus flanks (i.e. water zone), Abu Dhabi, United Arab Emirates. Higher porosity and permeability and larger pore-throat radii in the crest than in the flank mudstones is attributed to retardation of calcite cementation in the crest due to oil emplacement. Calcite cement has precipitated as syntaxial micro-overgrowths around micrite particles and as equant microspar in moldic pores and small vugs. Deviation of a large number of samples from this crest versus flanks porosity and permeability trend reflects the impact of other parameters, including: (i) subtle grain-size variations (ii) proportion of intact (i.e. with intragranular pores), versus broken coccolith tests, and (ii) degree of calcite cementation. Calcite cement was sourced by stylolitization of the limestones, which is more frequent and extensive in the flanks than in the crest. Similar bulk δ18OVPDB values of calcite in the crest (−7.3‰ to −4.6‰) and flanks (−6.9‰ to −5.5‰) is attributed to closed diagenetic system in the microporous mudstones, which masks the role of burial temperature in fractionation of oxygen isotopes during continued cementation of the water-saturated flank mudstones. The presence of miscellaneous saddle dolomite in the crest and flanks is attributed to the field-wide flow of hot basinal fluids along fractures and stylolites. A conceptual model unravelling the diagenetic and related reservoir-quality evolution pathways of mudstones across an oilfield anticline has been developed. This model has important implications for exploration of tight hydrocarbon reservoirs and improves our understanding of oil migration and emplacement in source rocks.

Episodic reactivation of carbonate fault zones with implications for permeability – An example from Provence, Southeast France

Exposed in large, continuous outcrops in Provence, southern France, the Castellas fault formed as a normal fault in the Upper Cretaceous and was reactivated as a left-lateral fault in the Eocene. Structural, petrographical, isotopic, and geochronological analyses shed light on the sequence of deformational-diagenetic stages of the fault zone and help identify properties that controlled fluid-flow behavior through time. Abrupt contrasts in fracture abundance compared to the undeformed host rock define a damage zone containing fractures arranged in regularly spaced clusters. We identified eleven episodes of calcite cementation within opening-mode fractures and host rock primary pores. Cement fabrics, sediment fills, and geochemistry show evidence of shallow burial environments. U–Pb geochronology of calcite cements indicate two main deformational phases of the fault zone during the Albo-Cenomanian Durancian uplift and Eocene Pyrenean orogeny at ca. 90 Ma and 50-40 Ma, respectively. Deformation created poro-permeability, but cementation followed shortly after, pervasively occluding most of the fault zone porosity by the end of the Eocene.

Controls on Timing of Hydrothermal Fluid Flow in South-Central Kansas, North-Central Oklahoma, and the Tri-State Mineral District

Paleozoic sedimentary rocks in the southern midcontinent of the United States have been affected by multiple events of deformation and fluid flow, resulting in petroleum migration, thermal alteration, Mississippi Valley-type mineralization, and a complex diagenetic history. This record is a hidden history of how cratonal settings respond to tectonic and non-tectonic drivers. The aim of this contribution is to better understand the controls on fluid migration in Paleozoic strata to evaluate whether hydrothermal activity is forced by tectonic or non-tectonic processes. This paper summarizes and vets the distribution of published dates related to thermal events in the southern midcontinent. In addition, we present new U-Pb dates obtained by laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) on calcite cements that were formed from hydrothermal fluids. These are from three samples from the Berexco Wellington KGS 1-32 core in Sumner County, Kansas; an ore sample from the Tri-State Mineral District, Neck City, Missouri; and a core sample from the Blackbird 4-33 well in Osage County, Oklahoma. Previous studies of these calcite samples provided evidence for hydrothermal fluid flow, with one of the Wellington samples possibly recording vertical hydrothermal fluid flow out of the basement. The sample from the Tri-State Mineral District (Missouri) yields a mid-Cretaceous age of 115.6±3.1 Ma. This age falls into the timing of the Sevier Orogeny along the west coast and the development of its foreland basin in the midcontinent. Calcites from the Mississippian interval in the Wellington KGS 1-32 core yield dates of 305±10.5 Ma and 305.1±9.1 Ma. Calcite in Mississippian strata from the Blackbird 4-33 core yields a date of 308.6±2.5 Ma. These dates from Mississippian calcite cements indicate hydrothermal fluid flow in the Late Pennsylvanian that coincides with the timing of the Marathon-Ouachita Orogeny or the Ancestral Rocky Mountains Orogeny. A calcite sample from the Ordovician Arbuckle Group from the Berexco Wellington KGS 1-32 core yielded an age of 5.6±1.6 Ma, coinciding with a time after high elevation uplift of the Rocky Mountains was already far advanced. We propose that this hydrothermal fluid flow may have been associated with increased meteoric recharge and increased regional fluid pressure in a basement aquifer that activated local seismic events far into the continental interior. The distribution of ages of hydrothermal fluid flow confirms a syntectonic driver during the Ouachita Orogeny and Ancestral Rocky Mountains Orogeny deformation. Continuation of hydrothermal fluid flow well into the Permian and tailing off early in the Triassic indicates a post-tectonic driver, where uplifted areas continued to provide the recharge from gravity-driven fluid flow, until the mountains were mostly beveled by the early part of the Triassic. A dearth of Triassic and Jurassic hydrothermal events suggests Gulf of Mexico rifting and extension were less important. Rejuvenation of hydrothermal fluid flow in the Cretaceous and continuing into the Paleogene indicates that elevation and regional flexure from both the Sevier and Laramide events continued to drive hydrothermal fluid flow far from the main sites of mountainous uplift and deformation. Finally, hydrothermal fluid flow associated with more recent uplift of the Rocky Mountains may have been activated by recharge events that pressurized a regional basement aquifer and triggered seismic activity.

Depositional Facies Model and Reservoir Quality of Paleogene Limestone in Labengki Island, Southeast Sulawesi

Eastern Indonesia has become an attractive venture for hydrocarbon exploration since 10 years ago. The discovery of hydrocarbon from Miocene carbonate of Tondo Formation has opened a new opportunity and hopes in the Southeast Sulawesi region. Can we find other potential reservoirs in Southeast Sulawesi? In this study, we assess and reconstruct the depositional model and the reservoir quality of the Paleogene Tampakura Formation in Labengki Island, Southeast Sulawesi. This field observation and petrographical study revealed that: (1) Tampakura Formation comprises mainly of grainstone, boundstone and floatstone with minor packstone and dolomitic wackestone/mudstone, (2) Tampakura Formation was deposited mainly in wide carbonate sand shoals and reef margin belt of rimmed carbonate shelf, (3) Boundstone and floatstone facies could be the best reservoir candidate in the region since they show extensive porosities development of cavernous and fracture porosity, (4) Dolomite cementation has deteriorated the reservoir quality of packstone which was deposited in platform interior-restricted marine, (5) Extensive calcite cementation in grainstone facies has reduced the reservoir quality of Tampakura Formation. However, locally, solution enlarged fracture porosity may have enhanced it. We suggest that post collision event of Late Oligocene - Early Miocene between Australian-originated microcontinent and ophiolite complex was highly responsible to create cavernous porosity. The collision resulted in the folding and uplifting of Tampakura Formation to the subaerial exposure. The carbonate strata were exposed to the surface developing a cavernous porosity and potentially becoming the best reservoir candidate for the next exploration target.Keywords: Carbonate facies, Kendari Basin, reservoir quality, Tampakura Formation.

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.

An integrated petrographical, petrophysical and organic geochemical characterization of the Lower Turonian Abu Roash-F carbonates, Abu Gharadig field, Egypt – Inferences on self-sourced unconventional reservoir potential

Marine carbonate source rocks are ubiquitous worldwide and many of these can act as self-sourced unconventional reservoirs. This work focuses on the Lower Turonian Abu Roash-F Member from the prolific Abu Gharadig field, Western Desert, Egypt which consists of argillaceous and bioclastic limestones. Thin section petrographic analysis indicated the presence of various microfacies assemblages dominated by planktonic foraminifera in association with abundant calcispheres indicating an open-marine, deep-shelf depositional setting. The X-Ray Fluorescence data exhibits overall poor detrital influx throughout the AR-F while the Mo and V enrichment in the lower part represents Late Cenomanian/Early Turonian (C/T) oceanic anoxic event (OAE2) associated with the global sea-level rise. Geochemical analysis suggests that the studied AR-F has good to very good total organic carbon (TOC) content (up to 3.57 wt%) consists of Type-II kerogen (HI > 400 mgHC/gTOC), thermally mature (Tmax ∼ 438.5 °C) and presently within the oil generation window. The TOC-rich (>2.5 wt%), highly anoxic lower AR-F showed higher oil production probability which is inferred as a sweet spot zone. Petrophysically AR-F is very tight and impervious with 0.9–2.6% porosity and negligible permeability (<0.0033 mD), dominated by nano- and microporosities. Micritization is found to be the most dominant diagenetic process reducing the AR-F storage capacity, followed by calcite cementation filling the bioclast tests and fractures, chemical compaction (stylolites), and minor dolomitization. 2D CT scanning revealed some minor vuggy or fracture pores but those were isolated and lacks in connectivity, which therefore did not improve the flow capacity of AR-F. Therefore, stimulation is necessary to bring this tight carbonate reservoir into production. This study offers critical insights on the AR-F Member which is an excellent source rock and at the same time holds potential as a promising unconventional reservoir.

Multiphase Diagenetic Processes and Their Impact on Reservoir Character of the Late Triassic (Rhaetian) Kingriali Formation, Upper Indus Basin, Pakistan

Multiple episodes of dolomitization of the shallow marine carbonates of the Late Kingriali Formation resulted in regional scale mappable dolostone geobodies in the Kohat and Potwar sub-basins. With the exception of few unaltered patches of the host limestone, more than 90% of the carbonates of the studied formation are diagenetically altered by replacive dolomites with associated dolomite cementation. Petrographical and geochemical data interpretation reveals that during the initial stage of dolomitization, the precursor limestone was significantly modified by the fabric-retentive replacive dolomite (RD-I) and produced bulk dolostones with non-planar-a to planar-s crystals. Neomorphic recrystallization (RD-II) was observed as an overgrowth of the already formed RD-I dolomite crystals during progressive dolomitization. The seawater at shallow depths is enriched with Fe-ions due to its interaction with Fe-rich beds within the studied formation. The modified seawater actively participated in the formation of ferroan replacive dolomites (RD-III). Stable isotopic composition of the unaltered Echinoderm plates, calcite cement (CC-I), and RD-I demonstrates signatures of δ18O and δ13C within the limit of late Triassic marine seawater or modified seawater. Depletion in the stable oxygen isotopic composition (from −0.99‰ to −3.75‰ V-PDB) demonstrates that RD-II and RD-III were formed in a sequence with progressively higher temperature fluids than normal seawater. Precipitation of dolomite cements as cavity filling rhombs (DC-I) and crystal overgrowth (DC-II) with highly depleted δ18O values (−5.44‰ to −7.45‰ V-PDB) illustrates dolomite cementation at higher temperatures and greater depths. The highly depleted values of δ18O (up to −9.16‰ V-PDB) and (up to 0.42‰ V-PDB) for δ13C of saddle dolomite (SD-I) indicate the precipitation of SD-I as a cavity filling dolomite at considerable depth. Calcite cementation and calcitization actively participated in the early, middle, and late diagenetic modifications as interpreted from their petrographic and stable isotopic studies. Porosity enhancement is clearly demonstrated by dissolution, stylolization, fracturing, and replacement dolomitization. Dolomite and calcite cementation had a negative impact on the reservoir character and occluded the dolostone porosity to a greater extent.

Upper Ediacaran fibrous dolomite versus Ordovician fibrous calcite cement: Origin and significance as a paleoenvironmental archive

Fibrous carbonate cement is a common phase in marine phreatic environments — but its origin and diagenetic history are underexplored. This paper compares two fibrous fabrics that are different in terms of their peculiarities but share important similarities regarding their essentials: upper Ediacaran fibrous dolomite cement and Upper Ordovician fibrous calcite cement, both from the Tarim Basin, China. The focus is on the significance of these fabrics as archives of their marine paleoenvironment. Upper Ediacaran fibrous dolomite cement is present as (i) a fascicular-optic and (ii) a radial-fibrous phase. Judging from the optical length-slow character and luminescent zones, radial-fibrous dolomite cement precipitated directly from early diagenetic marine porewater. Fascicular-optic dolomite cement has a length fast optical character and hence likely formed as a high-Mg calcite precursor penecontemporaneously in a reefal, shallow-marine diagenetic environment. Similarly, two types of Upper Ordovician fibrous cement are present: (i) radiaxial-fibrous and (ii) fascicular-optic calcite cement. Fascicular-optic calcite precipitated in a marine phreatic platform environment, whereas radiaxial-fibrous calcite cement formed in an early marine diagenetic porewater environment. Both fabrics arguably replace a magnesium calcite precursor. For the case of the Tarim Basin, cathodoluminescence and redox-sensitive elements indicate predominantly anoxic, upper Ediacaran marine waters (Ce anomaly = 0.8 ± 0.2), and suboxic, Upper Ordovician shallow-marine seawater (0.7 ± 0.2). Based on PHREEQC modeling, the shallow burial, upper Ediacaran porewater (5.1) yields a higher dolomite saturation index than upper Ediacaran (4.8) and Upper Ordovician seawater (3.7). The direct precipitation of radial-fibrous dolomite from Ediacaran porewaters was likely induced due to organic matter decomposition and related seawater oxygen depletion, elevated Mg2+/Ca2+ ratios, and high alkalinity values. Modeling and proxy data show significant hydrogeochemical differences between seawater and porewater in the upper meters of the sediment column. The upper Ediacaran seawater apparently favored fibrous cement formation with magnesium calcite precursors. In contrast, fibrous dolomite cement was directly precipitated in an early-diagenetic marine porewater regime. The comparability of proxy data reflecting seawater and such recording early diagenetic marine porewater is critically discussed.

Tectonic-fluid evolution of an ultra-deep carbonate reservoir in the southern Halahatang Oilfield area, Tarim Basin, NW China

Several giant oilfileds have been discovered during recent exploration in the Ordovician carbonates in zone between the Awati and Manjiaer depressions, Tarim Basin, China. These oilfields are deeply buried (>7000 m), and most productive wells are found adjacent to fault zones, suggesting faults may play a crucial role in reservoir development. Here, we use petrographic, microanalytical and geochemical techniques on different stages of void-filling calcite cements to decipher the tectonic-fluid evolution history. Vugs are generally occluded by V1 and V2 calcite cements, which precipitated from marine and burial diagenetic fluids. Two major episodes of fluid invasion are evident in each set of fractures from Middle Ordovician strike-slip faults (F1) to Late Devonian to the Permian strike-slip faults (F2). The sub-horizontal fracture-filling F1 calcite cements show the involvement of near-surface marine to marine-evolved brines, given that δ18Ocarb, restored δ18Ow values and rare earth element and yttrium (REY) profiles of F1-1 calcite cements are comparable to Ordovician seawater. The F1-2 calcite cements are characterized by negative δ18Ocarb, positive restored δ18Ow, and apparent Ce positive anomalies. The sub-vertical fracture-filling ferroan F2-1 calcite cements with fluorescence show the highest restored T(Δ47) (up to 125 °C), high restored δ18Ow values (up to +7.0‰ VSMOW), and an enrichment in MREE, which is consistent with an origin from hot hydrocarbon-bearing basinal brines. The subsequent F2-2 calcite cements may have precipitated during the Late Permian from mixing of basinal brines and deeply circulated meteoric water, as suggested by significantly negative δ18Ocarb values, cooler restored T(Δ47), negative restored δ18Ow values (down to −5.9‰ VSMOW), and flat REY patterns. The 87Sr/86Sr ratios of calcite cements are similar to coeval seawater. This study highlights the importance of fault-related fluid circulation in the development of a hydrocarbon reservoir, and may be applicable to other deep carbonate-hosted reservoirs.

Reservoir quality evaluation using sedimentological and petrophysical characterization of deep-water turbidites: A case study of Tariki Sandstone Member, Taranaki Basin, New Zealand

The current study aims to ascertain the reservoir characteristics of the Tariki Sandstone Member of the Otaraoa Formation, Taranaki Basin, New Zealand. This study was carried out by integrating the comprehensive petrophysical evaluation, sedimentological and petrographic studies, as well as well log analysis by using data from six wells. The porosity-permeability relationship is used to divide the samples of the Tariki Sandstone Member into reservoir and non-reservoir facies. A thorough petrophysical analysis shows that the maximum porosity values fluctuate between 16.6% and 22.1%, while permeability ranges from 102 mD to 574 mD, which indicates fair to good reservoir quality. Moreover, the Tariki sandstone represents six hydraulic flow units with a high reservoir quality index and flow zone indicator representing good reservoir characteristics. The pore size varies between nano and megapores with dominant macropores. Based on the sedimentological and petrographic analysis, the Tariki Sandstone Member is classified as a combination of subarkose, arkose, and lithic arkose with fine to medium and moderately to moderately well-sorted grains. The main diagenetic factor affecting the reservoir quality is cementation, which occupied all the pores with calcite. On the bright side, the secondary pores are developed due to the dissolution of calcite cement and few grains. The well log analysis demonstrates the presence of low clay volume ranging from 0.3% to 3.1%, fair to good effective porosity values between 13.6% and 15.9%, net pay thickness from 18.29 m to 91.44 m, and hydrocarbon saturation from 56% to 77.9%. The findings from this study revealed that the Tariki Sandstone Member possesses fair to good reservoir quality and hydrocarbon potential, which indicate submarine fans as appealing hydrocarbon reservoirs. This study can be used in similar depositional environments elsewhere in the world.

Characteristics and Consequences of Red Bed Bleaching by Hydrocarbon Migration: A Natural Example From the Entrada Sandstone, Southern Utah

AbstractExtensive regions of yellow and white (“bleached”) sandstones within the terrestrial Jurassic red bed deposits of the Colorado Plateau reflect widespread interaction with subsurface reduced fluids which resulted in the dissolution of iron‐oxide grain coatings. Reduced fluids such as hydrocarbons, CO2, and organic acids have been proposed as bleaching agents. In this study, we characterize an altered section of the Slick Rock member of the Jurassic Entrada Sandstone that exposes bleached sandstone with bitumen‐saturated pore spaces. We observe differences in texture, porosity, mineralogy, and geochemistry between red, pink, yellow, and gray facies. In the bleached yellow facies we observe quartz overgrowths, partially dissolved K‐feldspar, calcite cement, fine‐grained illite, TiO2‐minerals, and pyrite concretions. Clay mineral content is highest at the margins of the bleached section. Fe2O3 concentrations are reduced up to 3× from the red to gray facies but enriched up to 50× in iron‐oxide concretions. Metals such as Zn, Pb, and rare‐earth elements are significantly enriched in the concretions. Supported by a batch geochemical model, we conclude the interaction of red sandstones with reduced hydrocarbon‐bearing fluids caused iron‐oxide and K‐feldspar dissolution, and precipitation of quartz, calcite, clay, and pyrite. Localized redistribution of iron into concretions can account for most of the iron removed during bleaching. Pyrite and carbonate stable isotopic data suggest the hydrocarbons were sourced from the Pennsylvanian Paradox Formation. Bitumen in pore spaces and pyrite precipitation formed a reductant trap required to produce Cu, U, and V enrichment in all altered facies by younger, oxidized saline brines.

The Role of Seal and Source Rocks in Fluid‐rock Interactions and their Reservoir Effects within the Carbonate Deep Burial Realm

AbstractDiscoveries of deep high‐quality carbonate reservoirs challenged the general understanding on the evolution of porosity decreasing with depth. New mechanisms of pore generation and preservation in the deep realm require to be proposed. Dolostones in the Feixianguan and Dengying Formations experienced maximum depths in excess of 8000 m, but still retained high porosity. Petrographic observation and homogenization temperatures help to identify products of deep fluid‐rock interactions, visual and experimental porosity were used to quantify reservoir effects, the distribution of products finally being plotted to unravel the mechanisms. Th data reveal that thermochemical sulfate reduction (TSR), burial dissolution and quartz cementation are typical deep fluid‐rock interactions. The SO42– of residual porewater sourced from the evaporative dolomitizing fluid was supplied for TSR in the hydrocarbon column, the TSR‐inducing calcite cements were homogeneously dispersed in the hydrocarbon column. Quartz cementation was caused by the increasing acidity and Si‐rich residual porewater in the oil column. Burial dissolution is forced by organic acid and limited in oil–water contact. This study suggests that seal and source rocks not only play important roles in hydrocarbon accumulation, but also have a general control on the deep fluid‐rock interactions and porosity evolution in the deep burial realm.