Carbonate Depositional Environments Research Articles

Carbonate shoal microfacies characterization using the interdependent of depositional systems and diagenesis of Kais Formation in the Matoa Field, Salawati Basin

Matoa Field in the Salawati Basin, Papua, Indonesia was discovered in 1991. Oil production commenced in 1993, yielding a cumulative production of more than 21 MMBO by 2015. In the latest POD that was approved in 1999 and was valid until 2019, it was recommended to drill six additional wells within three integrated structures. By 2018, a static model was built, incorporating present-day island analogues and the latest interpretation from reprocessed 3D seismic data, paleontological data, and petrographic analyses. The latest static model renews the understanding on reservoir characteristics (geometry and microfacies) for the purpose of further field development. This paper discusses field analogues in correlation with an interplay of carbonate depositional environment with diagenetic evolution in the Matoa Field. Three candidates for possible present-day analogues of Matoa Field were screened i.e., Java Sea’s Thousand Islands, Kapoposang Islands of Spermonde Archipelago, and the Great Barrier Reef of Australia. The objective was to improve understanding of the conceptual geological model in the regional sense. Based on palaeogeomorphology (morpho-arrangements, shelf geometry and sedimentary architecture), the present-day Central Reef Zone of Wet Tropic at the Great Barrier Reef is fit to be utilized as an analogue to the continental, Miocene Salawati carbonate complex. Well-tied 3D seismic data, from which several multiple attributes were analysed, have been reprocessed twice. Attributes such as RMS Amplitude, realized RMS Amplitude, Sweetness and Relative Acoustic Impedance were analysed and extracted to understand the diagenetic features of Matoa Carbonate reservoir. The result shows that RMS Amplitude, in combination with petrographic – paleontological data, exhibits the most likely attribute to characterize sedimentology and top diagenetic picks of the carbonates. It is concluded from four reservoir zones in Matoa’s static model, that present-day analogues and distinct interplay among lithology, depositional and diagenetic systems are key aspects to revise the development strategies. The most significant interdependent factors can be quantitatively measured and modelled, such as the dual porosity system which was synthesized by Phi separate vug from the RFN Lucia porosity model.

Pre-salt carbonate cyclicity and depositional environment: NMR petrophysics and Markov cyclicity of lacustrine acoustic facies (Santos Basin, Brazil)

The Pre-salt strata are idealized as a lake-dominated breakup depositional sequence recorded between the South American and African continental margins to which there is no present-day analog. Most of the complexities of the Pre-salt reservoir rely on addressing carbonate cyclicities at different scales using limited cored intervals where well-log and seismic correlation is still unclear. In this case, the application of borehole image logs (BHI) and nuclear magnetic resonance (NMR) techniques allows not only the scale correlation and extensive data acquisition but also to establish a more robust relationship between acoustic facies and their petrophysical significance. For that, this work combines BHI and NMR data to propose a feasible scheme of depositional acoustic facies for the Barra Velha Formation in the Santos Basin. To verify the statistical significance of the acoustic facies’ vertical organization, this work applied the Markov chain analysis along a c. 500 m stratigraphic interval, deriving four carbonate cycles associated with shallowing and reworking lacustrine cyclicities. Abrupt changes in cyclicity patterns were adopted as stratigraphic markers to subdivide the succession into lowermost, intermediate, and uppermost intervals that record the depositional history of deep, shallow, and emergent lake environments, respectively. By comparing the thickness variation of the Markovian cycles (i.e., accommodation space) with the seismic onlap curve variation (i.e., lake level) throughout the Pre-salt interval, a larger-scale superimposed cycle was unraveled, suggesting that accommodation space of the studied Pre-salt paleolake was likely controlled by fluctuations in the lake level. Finally, this work aimed to explore the applicability of diagnostic criteria for the description and interpretation of depositional acoustic facies, their petrophysical significance, and cyclicity patterns in the Barra Velha Formation.

Carbonate Facies and Depositional Environment of Pre-Parigi Carbonate in Ardjuna Sub-basin: Application of Core-facies Coding and Electro-facies

The Pre-Parigi Formation is one of the formations in the West Java Basin North as a potential reservoir. However, the carbonate rocks of the Pre-Parigi Formation are considered to have high heterogeneity due to rock and depositional processes its diagenesis, so an appropriate approach is needed to understand its reservoir distribution. Therefore, in this study, interpretation of the facies distribution of the Pre-Parigi Formation was carried out using the integration of core data and well logs. On the core data, facies coding and clustering is performed to interpret the facies association. Then, the identified facies associations are integrated with the characters so that facies interpretation can be carried out for all intervals and all wells. The end result is an interpretation of the distribution of the facies of Pre-Parigi Carbonate. Based on the analysis results, the Pre-Parigi Carbonate in the study area is composed of five facies associations, such as deep shelf (FA 1), fore-slope (FA 2), near-reef lagoon (FA 3), lagoon (FA 4), and restricted circulation shelf – tidal flat (FA 5). FA 1 is characterized by a very high value, slightly serrated gamma-ray character. FA 2 is characterized by a high gamma-ray, serrated, intercalated with a fining-upward low gamma-ray layer, FA 3 is characterized by a low value and blocky gamma-ray, FA 4 is also characterized by a gamma-ray with a fairly low value, but has a pattern that tends to be serrated, and then FA 5 is characterized by a serrated gamma-ray character with a slightly higher value than FA 4. Based on the integration of all interpretations, the Pre-Parigi Carbonate in the study area can be divided into three depositional sequences (DS), where DS-1 develops in all area as a reef-initiation, DS-2 developed in the MVA-1 and MVA-3 wells, and did not develop in the MVA-6 well (had a give-up phase), and then DS-3 only developed in the MVA-3 well. In general, facies interpretation using the core-facies coding and electro-facies characterizing methods tends to be effective and provides more confidence interpretation. Keywords: Pre-Parigi Carbonate, Carbonate facies, Core-facies coding, Arjuna Sub-basin

Impacts of Particle Shape, Skeletal Porosity and Density on the Settling Velocity of Gravel‐Size Coral Debris

AbstractKnowledge of the hydrodynamic behavior of carbonate bioclasts is essential for solving coastal morphodynamic problems in marine carbonate depositional environments. Marine carbonate grains are produced within the sedimentary system itself and are deformed by subsequent hydrodynamics, resulting in particle densities, sizes and shapes that are distinct from those of terrigenous sediments. In this study, we investigated the shapes and settling characteristics of gravel‐size natural coral debris collected from the coasts of Okinawa Prefecture, Japan. Based on the shape parameters and settling velocities obtained using image analysis techniques and settling tube experiments, respectively, a correlation model for predicting the settling velocity of coral gravel particles was developed by extending the previous formula of Dietrich (1982, https://doi.org/10.1029/wr018i006p01615) over particle Reynolds number values ranging from 384.23 to 4439.76. The model determines the settling velocity as a function of particle nominal diameter, form, roundness, and surface texture parameters that were selected after examining several other shape descriptors. Compared with other models, the proposed model showed better settling velocity predictive skills for coral gravel particles classified into four shape types. The proposed model also determines how the drag coefficient of coral gravel particles should be modified from that for spherical particles, by accounting for the abovementioned coral gravel particle shape parameters. The obtained data set of settling velocity and modification of the drag coefficient can be used for quantitative interpretation of suspension, transport, and sedimentation processes in marine carbonate depositional environments.

Flooding of a carbonate platform: The Sian Kaʼan Wetlands, Yucatán, Mexico—A model for the formation and evolution of palustrine carbonate factories around the modern Caribbean Sea and in the depositional record

AbstractThe dynamic inter‐relationships between marine and freshwater carbonate depositional environments are illustrated in the Sian Ka'an Wetlands, a 5 280 km2 complex of groundwater‐fed freshwater marshes, lakes and brackish coastal lagoons in the South‐East Yucatán Peninsula (Mexico). The Yucatán Platform was subaerially emergent and extensively karstified during the last glacial maximum at 18, 000 yr bp. The Late Holocene transgression has caused progressive reflooding of the continental margin, backstepping of the MesoAmerican Reef and encroachment of coastal environments into the platform interior as rising groundwaters flood an interconnected cave and sinkhole system and feed seasonal marshes above. The Sian Ka'an Wetlands form a vast palustrine carbonate factory which is directly juxtaposed and dynamically linked with the marine carbonate factory to seaward. Continuing sea‐level rise has caused synchronous landward migration of marginal marine and freshwater environments as beach barriers were breached and palustrine sloughs flooded to form marginal marine seagrass lagoons. The Rio Hondo Fault conditions fluid inflow while the sub‐environments of the Sian Ka'an Wetlands reflect tectonic controls on microtopography and hydroperiod. Modern analogues for the Sian Ka'an Wetlands include the Florida Everglades, formed during transgression of the Florida Platform, and relict marsh environments preserved on leeward shores of Andros, Abaco and other Bahama islands. A wide range of ancient examples deposited in coastal and continental interior settings similarly reflect seasonal aquifer rise in response to marine transgression and/or onlap of late‐stage basin fill onto a karstified pediment. Freshwater palustrine carbonate factories on carbonate platforms are transient deposystems, controlled by subtle water depth, climate, vegetation and hydrological factors while being critically sensitive to sea‐level changes and tectonics. The preservation potential of palustrine carbonates may be relatively low in coastal settings due to erosion or shallow marine overprinting, while greater further inland where marine flooding is rarer and in tectonically subsident continental interior basins where accommodation space is continuously created.

Reducing Drilling Hazard Risk in a Carbonate Environment Using Seismic Processing, Diffraction Imaging and Interpretation

Buried structural anomalies within carbonate depositional environments, such as karst sinkholes, impose significant dangers for borehole integrity and drilling equipment due to rapid changes in rock properties (from tight to the porous) potentially resulting in extensive mud loss, drill bit jamming and other complications. This often leads to delays in the well construction planning and sometimes to subsequent sidetrack drilling and a dramatic increase in costs. Thus, Paleo-karst localization, prior to well planning and construction, becomes a critical task for economic optimization. However, this is a non-trivial exercise due to the relatively small scale and size of karst type features and anomalies, which are at the limit of lateral resolution of most standard surface geophysical methods. In this paper, we introduce a methodology of karst prediction based on a combination of targeted 3D seismic processing and integration of the diffracted wavefield anomalies in addition to use of conventional seismic attributes. The presented approach has been proven to be efficient to detect karsts in a real oil field and can be deployed in other regions.

Characteristics of Carbonate Facies and Depositional Environment of Tapak Formation in the Ajibarang Area, Central Java, Indonesia

The limestone of Tapak Formation is well exposed in the Ajibarang area and its surroundings, Central Java, Indonesia. Very intensive mining activity is a very serious threat to this limestone. Hitherto, the geological study on this limestone is still very limited. The objective of this study was to investigate the characteristics of these limestone facies and their sedimentation. This research consists of field and laboratory (petrography and micropaleontology) analysis. The results showed that the limestone of the Tapak Formation consisted of three facies: planktonic packstone, algal foraminifera packstone, and foraminifera algal packstone. Planktonic packstone facies are composed of bioclastic fragments in a micro matrix. The fragments were dominated by planktonic foraminifera consisting of a genus of Globigerina, Globigerinoides, Globorotalia, and Orbulina. Benthic foraminifera and radiolarians were also identified. Sedimentation from planktonic packstone facies was estimated to occur in the deep marine environment. Algal foraminiferal packstone facies are composed of bioclastic fragments in a micro matrix. Large algae and large foraminifera dominated the fragments. Algae were from the type of red algae, consisting of Lithophyllum, Corallina, and Rhodolite. Foraminiferal algal packstone and algal foraminiferal packstone that were observed in the studied area were characterized by the abundance of large foraminifera and interpreted to be deposited in the upper reef slope. We interpreted that the limestone in the study area was deposited in the upper reef slope to the lower reef slope environments. We hope that the information we provided here is useful for petroleum system study in Central Java, Indonesia.

Facies Mapping of the Holocene Carbonate Complexes in Kepulauan Seribu Java Basin, Indonesia Using Satellite-Derived Data Set

The analog study is common in understanding buried reservoirs and the relationship between architectural complexity and heterogeneity of carbonate deposits. This study processed satellite and false-color images using single-band images and classified them using a supervised classification technique to generate an environmental facies map. Thus, the study’s objectives are to map facies distribution in selected carbonate depositional environment and investigate oceanographic parameters that influence the development and evolution of modern carbonates in Holocene Kepulauan Seribu patch reef complexes, Java Basin. The main sub-environments are reef sand apron, subtidal reefal margin, and shallower subtidal lagoon. Annual wind patterns in the Java Basin have influenced the development of carbonate sediment in the Kepulauan Seribu archipelago, resulting in the formation of an isolated carbonate platform pattern with a single crest and asymmetrical dipping flanks. Meanwhile, the salinity of seawater influences the production of modern carbonate deposits as the Java basin is situated at the equator line, where the salinity of the seawater is moderately salty (35‰) and contributes to the favorable conditions for carbonate growth. The analysis of oceanographic elements with the integration of quantified environmental facies distribution is conducted to monitor the deposition of carbonate sediments which gives insight into carbonate distribution on the studied platforms. Satellite-derived facies maps provide an accurate overview of depositional facies patterns at the field scale in the oil and gas industry, enabling geologists to assess the potential of an oil and gas reservoir.

Possible Tsunami-Induced Sediment Transport From Coral Reef to Deep Sea Through Submarine Canyons on the Southern Ryukyu Forearc, Japan

Submarine canyons are efficient sediment transport pathways from shallow marine areas to deep sea. Along active margins, large tsunamis are a trigger for sediment transport to deep sea. However, sediment transport through submarine canyons by such extreme wave events in the carbonate depositional environments has not been fully understood. Large tsunamis have repeatedly struck the coral reef islands of the southern Ryukyu Islands and have transported large boulders composed of coral from the reef to shore. In this study, we examined sediment cores collected near the mouths of submarine canyons and basin floor on the southern Ryukyu arc’s forearc. The presence of coarse calciturbidites containing coral, molluskan fragments, and coral reef benthic foraminifera was limited on the submarine fan formed at the mouth of the reef-connected and shelf-incised submarine canyon. In cores collected near the mouth of shelf-incised submarine canyons with no reef connection and slope-confined canyons, no coarse calciturbidite is observed. Few calciturbidites were found in more downslope cores, implying that most calcareous grains derived from shallow marine areas were deposited on this fan. Depositional intervals of turbidites were calculated to be several hundred–a few thousand years, which agree with the recurrence intervals of large tsunamis estimated from onshore tsunami deposits, based on radiocarbon dates in hemipelagic mud intervals. No temporal change in the depositional intervals of calciturbidites in the cores from the submarine fan since the last glacial maximum. Therefore, the tsunamis may be an important mechanism for surface sediment reworking in coral reefs, and shelf-incised and reef-connected submarine canyons can play an important role in the efficient transport of shallow marine calcareous grains to the deep sea.

Extreme continental weathering in the northwestern Tethys during the end-Triassic mass extinction

The end-Triassic mass extinction (ETE) is thought to have been triggered by widespread eruption and emissions of volcanic and contact metamorphic carbon and sulfur gases from the Central Atlantic Magmatic Province (CAMP) flood basalts. Although palynological studies of shallow marine deposits in the basins of Denmark and Germany suggest that deforestation and catastrophic soil loss occurred during the CAMP volcanism and ETE interval, the intensity and timing of continental weathering in the region of northwestern Tethys Ocean remain unclear. Here we present Sr, C, and O isotope data, as well as multivariate statistical analyses of major element contents, for carbonate–clastic deposits in the Kardolína section, Slovakia, to develop a continental weathering record in the NW Tethys during the ETE event. This section consists of a shallow marine carbonate sequence in the Rhaetian Fatra Formation and the overlying Hettangian Kopieniec Formation. The Fatra Formation represents ramp facies deposits that formed in a restricted pull-apart basin of the Fatric Zone along the southern margin of the Bohemian Massif in the region of northwestern Tethys. Carbon isotope analysis of the limestones revealed two negative carbon isotope excursions (NCIEs) in the uppermost Fatra Formation. These two NCIEs occurred in the latest Rhaetian and can be compared to the “precursor” and “initial” NCIEs reported for the northwestern Tethys. Strontium isotope analysis of the limestones revealed an abrupt increase in Sr isotope ratios between the precursor and initial NCIEs, which indicates that continental weathering of the Bohemian Massif increased rapidly in the latest Rhaetian. Multivariate analysis of major element contents in the carbonate rocks also shows that intense chemical weathering of the hinterland was accelerated after the precursor NCIE, supporting the inferences drawn from Sr isotopes. Furthermore, our study reveals that the carbonate depositional environments of the Fatra Formation changed to the formation of specific Fe-rich oolites with increased continental weathering after the precursor NCIE. The concentrations of redox-sensitive major elements (e.g., Mn and Fe) and multivariate analysis of the major element data suggest that the Fe-rich oolites were formed by an influx of O-depleted water into the shallow water depositional environment of the Fatra Formation. A possible origin for this O-depleted water is the oxygen minimum zone (OMZ) that formed at intermediate water depths in European basins (e.g., the German and Eiberg basins) during the latest Rhaetian. Our results suggest that the marine environment in the European basins may have developed an OMZ due to the increase in continental weathering during the latest Rhaetian, and these environmental changes may have had an important role in the marine ETE event. • Sr, C, and O isotope data of the Rhaetian succession in Slovakia were determined. • Two negative carbon isotope excursions (NCIEs) were detected in latest Rhaetian. • Abrupt increase in Sr isotope ratios recognized between these two NCIEs. • Continental weathering was temporarily enhanced during the end-Triassic extinction. • Intense weathering led to development of oxygen minimum zone in NW Tethys.

Oil Family Typing Using a Hybrid Model of Self-Organizing Maps and Artificial Neural Networks.

Identifying the number of oil families in petroleum basins provides practical and valuable information in petroleum geochemistry studies from exploration to development. Oil family grouping helps us track migration pathways, identify the number of active source rock(s), and examine the reservoir continuity. To date, almost in all oil family typing studies, common statistical methods such as principal component analysis (PCA) and hierarchical clustering analysis (HCA) have been used. However, there is no publication regarding using artificial neural networks (ANNs) for examining the oil families in petroleum basins. Hence, oil family typing requires novel and not overused and common techniques. This paper is the first report of oil family typing using ANNs as robust computational methods. To this end, a self-organization map (SOM) neural network associated with three clustering validity indexes was employed on oil samples belonging to the Iranian part of the Persian Gulf oilfields. For the SOM network, at first, 10 default clusters were selected. Afterward, three effective clustering validity coefficients, namely, Calinski–Harabasz (CH), Silhouette (SH), and Davies–Bouldin (DB), were studied to find the optimum number of clusters. Accordingly, among 10 default clusters, the maximum CH (62) and SH (0.58) were acquired for 4 clusters. Similarly, the lowest DB (0.8) was obtained for four clusters. Thus, all three validation coefficients introduced four clusters as the optimum number of clusters or oil families. According to the geochemical parameters, it can be deduced that the corresponding source rocks of four oil families have been deposited in a marine carbonate depositional environment under dysoxic–anoxic conditions. However, oil families show some differences based on geochemical data. The number of oil families identified in the present report is consistent with those previously reported by other researchers in the same study area. However, the techniques used in the present paper, which have not been implemented so far, can be introduced as more straightforward for clustering purposes in oil family typing than those of common and overused methods of PCA and HCA.

Carbonate Facies and Depositional Environment On Baturaja Formation, “Mk” Field, Jatibarang Sub-Basin.

Carbonate Facies and Depositional Environment On Baturaja Formation, “Mk” Field, Jatibarang Sub-Basin.

Pushing the boundary: A calibrated Ediacaran-Cambrian stratigraphic record from the Nama Group in northwestern Republic of South Africa

The Nama Group exposed on the Neint Nababeep Plateau along the Orange River in northwestern Republic of South Africa is now recognized as an expanded record of the Ediacaran-Cambrian transition that provides opportunity for an integrated stratigraphic approach in examining the geochemical and biologic evolution across this fundamental geologic boundary at unprecedented resolution. U-Pb zircon geochronology by the CA-ID-TIMS method on six intercalated volcanic ash beds in the Nama Group (from the Huns Member to the Nomtsas Formation) at this locality is used to construct a high-resolution, Bayesian, age-stratigraphic model, which allows a direct temporal calibration of the biostratigraphy and carbon isotope record from 539.63 ± 0.15 Ma to 537.95 ± 0.28 Ma (2σ internal errors). Across the border in the Witputs subbasin of southern Namibia, ash beds at the base of Nudaus Formation and within the Nasep Member yielded new U-Pb ages of 545.27 ± 0.11 Ma and 542.65 ± 0.15 Ma, respectively. Our combined geochronology reveals the detailed depositional history of the Nama Group at a regional scale, suggesting that a relatively low sediment accumulation rate in the Kuibis Subgroup and the lower Schwarzrand Subgroup was followed by accelerated sedimentation in the upper Schwarzrand Subgroup. This is consistent with a pattern of exponential increase in subsidence typical of foreland basins. Some of the observed chemostratigraphic trends throughout the Nama Group could relate to a shift from a seawater-buffered to a sediment-buffered regime of early marine diagenesis driven by this increase in sedimentation rate.Occurrences of soft-bodied erniettomorphs, calcified body fossils, and trace fossils within the Neint Nababeep Plateau are broadly consistent with known global biostratigraphic ranges. However, we document the youngest radioisotopically calibrated occurrences of Ediacaran-type fossils, which stratigraphically overlap with large and complex bilaterian ichnofossils, between 539.18+0.17/−0.26 Ma and 538.30+0.14/−0.14 Ma. Yet, the index fossil Treptichnus pedum remains undocumented from this section, and we suggest that its first regional occurrence may be younger than these strata. Despite relatively continuous and high rates of carbonate sedimentation across the Ediacaran-Cambrian boundary (as currently recognized), the upper Nama Group of the Neint Nababeep Plateau does not preserve the characteristic negative carbon isotope excursion observed within other basal Cambrian successions. One possible explanation for its absence is that this chemostratigraphic marker is not ubiquitous in all carbonate depositional environments. Alternatively, the basal Cambrian carbon isotope excursion, and perhaps the Ediacaran-Cambrian boundary as defined by the first appearance of Treptichnus pedum, might be >1 m.y. younger than currently recognized, postdating 538 Ma and, thus, suggesting a more condensed early Cambrian radiation. Difficulties in determining with confidence the first appearance datum of the index fossil Treptichnus pedum in the Nama Group highlight the challenge of a global biostratigraphic definition for the base of Cambrian and underscore the necessity of an integrated stratigraphic and radioisotope geochronologic approach to understand the tempo and patterns of environmental and biologic evolution across the Ediacaran-Cambrian boundary.

The Permian–Triassic boundary section at Baghuk Mountain, Central Iran: carbonate microfacies and depositional environment

Sections at Baghuk Mountain, 45 km NNW of Abadeh (Central Iran), have excellent exposures of fossiliferous marine Late Permian to Early Triassic sedimentary successions. Detailed bed-by-bed sampling enables the analysis of microfacies changes of three successive rock units across the Permian–Triassic boundary. The Late Permian Hambast Formation is mainly the result of biogenic carbonate production. Its carbonate microfacies is dominated by biogen-rich and bioturbated nodular limestones, indicating a well-oxygenated aphotic to dysphotic environment. The biogen-dominated carbonate factory in the Permian ceased simultaneously with the main mass extinction pulse, which is marked by a sharp contact between the Hambast-Formation and the overlaying Baghuk Member (= ‘Boundary Clay’). The clay and silt deposits of the Baghuk Member with some carbonate beds show only a few signs of bioturbation or relics of benthic communities. The Early Triassic Claraia Beds are characterised by a partly microbially induced carbonate production, which is indicated by frequent microbialite structures. The depositional environment does not provide evidence of large amplitude changes of sea level or subaerial exposure during the Permian–Triassic boundary interval. The deposition of the Baghuk Mountain sediments took place in a deep shelf environment, most of the time below the storm wave base.

Geochemical characterization and origin of the Cretaceous Sa’di, Khasib, Mishrif, and Nahr Umr Crude Oils in Halfaya Oilfield, Southern Mesopotamian Basin, Iraq

A suite of crude oil samples from the Cretaceous reservoirs in Halfaya Oilfield, Southern Iraq were analyzed using organic geochemistry analysis to study their geochemical characterization and origin. High sulfur content (3.74 ± 1 wt.%), C22/C21, C29/C30 hopane, ratios, and low C19/C23 and C24/C23 for all oil samples are indicators of marine carbonate environment source. Low pr/ph, Pr/nC17, Ph/nC18, and highest C35S/C34S ratio are consistent with a more reducing carbonate depositional environment. The C28/C29 sterane ratio and stable carbon isotope analysis values of crude oils are suggested Upper Jurassic to Middle Cretaceous anoxic marine carbonate source origin.

Botryoidal and Spherulitic Aragonite in Carbonates Associated with Microbial Mats: Precipitation or Diagenetic Replacement Product?

Similar carbonate fabrics may result from different pathways of precipitation and diagenetic replacement. Distinguishing the underlying mechanisms leading to a given carbonate fabric is relevant, both in terms of an environmental and diagenetic interpretation. Prominent among carbonate fabrics are aragonite botryoids and spherulites, typically interpreted as direct seawater precipitates and used as proxies for fluid properties and depositional environments. This study investigated μm to mm-scale Holocene botryoidal and spherulitic aragonite from marine and non-marine carbonate settings associated with microbial mats, and reports two distinct formation mechanisms: 1) early diagenetic replacement, and 2) primary precipitation via nanocrystal aggregation. In the intertidal microbial mats of Khawr Qantur (Abu Dhabi), botryoidal and spherulitic aragonite are replacement products of heavily micritized bioclasts. To form the botryoidal and spherulitic aragonite, skeletal rods and needles, resulting from disintegration of micritized bioclasts, recrystallize into nanocrystals during early marine diagenesis. These nanocrystals then grow into fibrous crystals, forming botryoidal and spherulitic aragonite. In the lacustrine microbial bioherms of the hypersaline Great Salt Lake (United States) and in the hydrothermal travertines of Bagni San Filippo (Italy), botryoidal and spherulitic aragonite evolve from nanocrystals via precipitation. The nanocrystals are closely associated with extracellular polymeric substances in microbial biofilms and aggregate to form fibrous crystals of botryoidal and spherulitic aragonite. The studied fabrics form a portion of the bulk sediment and show differences in terms of their formation processes and petrological features compared to the often larger (few mm to over 1 m) botryoidal and spherulitic aragonite described from open-marine reefal cavities. Features shown here may represent modern analogues for ancient examples of carbonate depositional environments associated with microbialites. The implication of this research is that botryoidal and spherulitic aragonite associated with microbial mats are relevant in paleoenvironmental interpretations, but must be combined with a detailed evaluation of their formation process. Care must be taken as the term “botryoidal and spherulitic aragonite” may in fact include, from the viewpoint of their nucleation and formation mechanism, similar fabrics originated from different pathways. At present, it seems unclear to which degree the μm to mm-scale botryoids and spherulites described here are comparable to their cm-to dm-size counterparts precipitated as cements in the open pore space of reefal environments. However, it is clear that the investigation of ancient botryoidal and spherulitic aragonite must consider the possibility of an early diagenetic replacement origin of these precipitates.

Sequence stratigraphy and carbon isotopes from the Trenton and Black River Groups near Union Furnace, PA: Constraining the role of land plants in the Ordovician world

Sea-level change influences carbon isotopic trends in both modern and ancient carbonate depositional environments. Generally, this relationship is manifested as a positive carbon isotopic excursion where the rising limb of the excursion is associated with transgression. These excursions exist because sea level can influence 1) local/regional/global carbon cycling, 2) basin restriction and meteoric influence, and 3) carbonate sedimentation. The first two processes are influenced, in part, by the large carbon reservoir represented by the terrestrial biosphere. Our goal is to explore the relationship between carbon isotopes and sea level prior to the evolution of vascular plants at a time when the terrestrial carbon reservoir was far smaller.This study focuses on the Upper Ordovician Union Furnace section in Pennsylvania, where a previously documented ~1‰ increase in δ13C values in the upper Sandbian (458.4–453.0 Ma; referred to as the “baseline shift”) has been interpreted to reflect a perturbation to the global carbon cycle due to the proliferation of non-vascular plants. By focusing on the interval deposited before the “baseline shift”, we were able to examine the relationship between sea level, carbon isotopes, and some of the earliest terrestrial ecosystems. We present a high-resolution δ13C record directly tied to the sedimentological and sequence stratigraphic framework. By sampling genetically related packages of rock we were able to document a relationship between carbon isotopes and sea level at the sequence and parasequence level. Our results show that carbon isotopic values decreased during transgressions and increased during regressions. This seemingly inverse relationship between δ13C values and sea level is interpreted to reflect increased terrestrial fluxes during regression in a world devoid of vascular plants. Our results suggest that the early colonization of land by non-vascular plants impacted regional carbon cycling in the Appalachian Basin and eventually the global carbon cycle in the Late Ordovician.

Microbial carbonate sequence architecture and depositional environments of Member IV of the Late Ediacaran Dengying Formation, Gaoshiti–Moxi area, Sichuan Basin, Southwest China

The recent discovery of the Anyue gas field, with huge reserves, has great potential for ultradeep gas production in carbonate reservoirs. Previous exploration and exploitation of Member IV of the Dengying Formation in the Gaoshiti–Moxi area deposited during the Late Ediacaran have provided useful insight into palaeo‐gas reservoirs, and geological significance of the Precambrian microbial carbonate strata. In this study, the depositional sequence architecture has been established and typical environments from Member IV of the Dengying Formation in the Gaoshiti–Moxi area have been reconstructed through detailed sedimentological analysis of cores, petrophysical data, well‐logging data and seismic interpretation. One third‐order sequence was identified, which is subdivided into seven fourth‐order sequences and 18–19 fifth‐order sequences. The third‐order sequence is bounded by denudation surfaces or onlap surfaces, and composed of transgressive system tracts and highstand system tracts. The fourth‐order sequences are bounded by instantaneously exposed punctuated surfaces or lithology and lithofacies transition surfaces, featured by specific lithological cycles, such as ‘lower microcrystalline dolomite, middle intraclastic dolomite and sparry thrombolite, and upper intraclastic dolomite’, ‘middle–lower sparry thrombolite and upper microcrystalline dolomite and sparry thrombolite’, ‘lower micritic thrombolite, middle micritic dolomite, and upper sparry thrombolite and laminites’, and ‘lower microcrystalline dolomite, middle intraclastic dolomite and argillaceous or micritic dolomite, and upper intraclastic dolomite’. The sequence characteristics reveal that periodic high‐frequency sea‐level change and palaeogeomorphology are controlling factors affecting sequence development, which lay a significant foundation for reservoir development. During the depositional period, a tidal flat on a restricted carbonate platform is well‐developed, allowing the identification of 16 microfacies, 13 microfacies associations, and 6 seismic facies. Inter‐lower tidal flats are widely developed with band‐shaped mound–bank complex, microbial mound, and shoal on the platform margin. On the inner platform, patchy microbial mounds, mound–flat combinations, and inter‐shoal/microbial mounds are developed in the Gaoshiti area, whereas patchy shoals and shoal–flat combinations are developed in the Moxi area. During the early depositional period of Member IV of the Dengying Formation, the microbial mounds and shoals were developed on the platform margin. The dolomitic flat and inter‐shoal/microbial mounds are mainly developed on the inner platform with little microbial mound and shoal deposition. During the late depositional period of Member IV of the Dengying Formation, large‐scale microbial mounds and shoals are sustainably and stably developed on the platform margin; enlarged dolomitic flat, patchy microbial mounds, and patchy shoals are predominantly developed in the platform interior with inter‐shoal/microbial mounds dying out.

The impact of diagenesis precipitation on fracture permeability in naturally fractured carbonate reservoirs

Naturally fractured carbonate reservoirs are often described as very heterogeneous systems due to the carbonate depositional environment and to the subsequent diagenesis processes such as mineral precipitation in fractures. Fluid flow behaviour in fractures is highly influenced by the fracture aperture size and its morphology. Mineral precipitation can alter the fracture effectiveness to the fluid flow and cause a partial or entire blockage of fractures. Therefore, accurate characterisation of the fracture morphology can help to enhance the prediction of the fluid flow behaviour in naturally fractured carbonate reservoirs. Mineral precipitation on fracture walls can reduce the fracture aperture significantly. As a result, the fracture permeability affected notably, which reduces the flow potential through fractures as well as changes the flow pattern. The objective of this work is to predict the flow behaviour in fractures under various levels of mineral precipitation to mimic reality. We have approached this objective by using outcrop-based models supported by a set of rock and fluid properties of a nearby fractured formation. Then, the model tested for the gas flow using the derivative plot technique of the synthetic well testing data. The simulation results have shown that mineral cementation can cause a partial blockage in fractures, hence a reduction in their flow capacity, as fractures become a less conductive medium. Nevertheless, the matrix medium can enhance the fluid flow in fractures by providing a bypass path to the fluid to overcome the sealed fractures. In this work, a formula has concluded to estimate the reduction in fracture permeability based on the fraction of the precipitated cement. In the studied formation, the core description has shown that 34% of fractures were blocked, which can lead to a reduction in the permeability by 29–64% and by 37–83% with and without matrix contribution, respectively. Thus, including the fracture morphology in the simulation model enables us to predict the performance of fractured carbonate reservoirs accurately.

Dating lacustrine carbonate strata with detrital zircon U-Pb geochronology

Abstract Carbonate lacustrine strata in nonmarine systems hold great potential for refining depositional ages through U-Pb dating of detrital zircons. The low clastic sediment flux in carbonate depositional environments may increase the relative proportion of zircons deposited by volcanic air fall, potentially increasing the chances of observing detrital ages near the true depositional age. We present U-Pb geochronology of detrital zircons from lacustrine carbonate strata that provides proof of concept for the effectiveness of both acid-digestion recovery and resolving depositional ages of nonmarine strata. Samples were collected from Early Cretaceous foreland basin fluvial sandstone and lacustrine carbonate in southwestern Montana (USA). Late Aptian–early Albian (ca. 115–110 Ma) maximum depositional ages young upsection and agree with biostratigraphic ages. Lacustrine carbonate is an important component in many types of tectonic basins, and application of detrital zircon U-Pb geochronology holds considerable potential for dating critical chemical and climatic events recorded in their stratigraphy. It could also reveal new information for the persistent question about whether the stratigraphic record is dominated by longer periods of background fine-grained sedimentation versus short-duration coarse-grained events. In tectonically active basins, lacustrine carbonates may be valuable for dating the beginning of tectonic subsidence, especially during periods of finer-grained deposition dominated by mudrocks and carbonates.