Bioclastic Limestone Research Articles

Application of Novel Differential Injection-production Optimization Strategy in Tight Pioclastic Limestone Reservoir

The permeability of the ultra-low permeability bioclastic limestone reservoir in Middel East A Oilfield is about 5md. The pore structure of the reservoir is in multimodal mode and has strong micro heterogeneity. The single well productivity was low and it was difficult to sustain production after the vertical well development was adopted in the early stage, and it has been gradually transformed into horizontal well pattern at the flank area. However, the reservoir pressure and production declined rapidly, and the development effect became worse. Through the combination of reservoir micro-characteristics and macro-production performance characteristics, this paper clarifies the ultra-low permeability bioclastic limestone oil displacement mechanism and remaining oil distribution law under the micro-macroscopic dual scale, and clarifies the reasonable well spacing of vertical well pattern and horizontal injection-production well pattern. Based on the differential single well performance characteristics and water injection development mechanism, the injection-production connectivity evaluation model based on rapid numerical simulation is established, the regional differential injection-production strategy is formulated, the injection-production optimization adjustment criteria are established, the establishment of effective displacement system for the reservoir is supported, and the adjustment of more than 280 producers have been carried out in 2020-2022. The daily oil production of a single well has been restored from 307 barrels/day in 2019 to 483 barrels/day in 2022, and the water cut is reduced from 27.1% to 22.1%, the reservoir pressure recovery of 496psi provides important experience guidance for the scale benefit production of such tight bioclastic limestone reservoir.

Differential Diagenesis and Pore Evolution Characteristics of Different Sedimentary Microfacies Reservoirs of Bioclastic Limestone: A Case Study of MB2-MC1 Section in X Oilfield in The Middle East

In order to clarify the differential diagenesis of different sedimentary microfacies of bioclastic limestone in Cretaceous M Formation in the Middle East and systematically analyze the characteristics of reservoir pore evolution, the data of regional geology, well logging, core, cast thin section, scanning electron microscope, three-dimensional CT and conventional porosity and permeability test are comprehensively used, taking MB2-MC1 section in X oilfield as an example, starting from the diagenesis and pore evolution of bioclastic-arene shoal, the characteristics of diagenesis of each sedimentary microfacies in different environments are analyzed, and the differential diagenesis, pore evolution process and its influence on physical properties are studied. It is showed that the reservoirs of MB2-MC1 section are mainly developed in the ramp shoal and platform margin shoal. The shoal facies can be further subdivided according to the type of bioclastic debris. The sedimentary microfacies determine the original fabric and bioclastic type of M Formation, and then control the diagenetic sequence and strength. Each sedimentary microfacies mainly experiences different degrees of mud crystallization and burrows in seawater environment, dissolution and cementation in atmospheric freshwater environment, compaction, pressure solution and dolomitization in burial environment. High energy microfacies have the characteristics of non-selective dissolution, mainly developing intergranular pores and intergranular dissolved pores. Medium energy microfacies have the characteristics of selective dissolution, mainly developing mould pores and intragranular pores. The medium-low energy microfacies have the characteristics of moderate cementation, moderate compaction and moderate-weak dolomitization, mainly developing intragranular pores and visceral foramen pores. The low energy microfacies have the characteristics of strong cementation, strong compaction and moderate dolomitization, mainly developing micropores, intercrystalline pores and visceral foramen pores, which are non-reservoirs.

Development characteristics and genetic analysis of dense limestone reservoirs in the Taiyuan Formation of the Ordos Basin, China

Tight limestone reservoirs in the Permian Taiyuan Formation of the Ordos Basin have garnered increasing attention in recent years, emerging as a pivotal domain in the quest for natural gas exploration. This research delves into the comprehensive investigation of petrology, reservoir space, reservoir physical properties, and microscopic characteristics through the examination of field outcrops, core observations, thin section identification, scanning electron microscopy, stable carbon and oxygen isotope analysis, and testing. The systematic analysis focused on elucidating the development characteristics of these reservoirs and identifying the controlling factors governing favorable reservoir conditions. Based on our research analysis, the specific rock types with the potential to serve as excellent reservoirs, includes bioclastic silty limestone, bioclastic micrite limestone, and algal-rich limestone. The predominant reservoir spaces within these formations were found to consist of dissolved pores, residual bioclastic cavity pores, intercrystalline pores, and microfractures. These reservoirs exhibit an average porosity of 2.1%, and an average permeability of 0.22 × 10−3 μm2, indicating their classification as low porosity and low permeability reservoirs. The formation of favorable reservoirs in the Taiyuan Formation limestone was determined to be influenced by many factors; notably, the favorable sedimentary microfacies associated with bioclastic shoals and bioherms provided the foundational material basis for the formation of reservoirs, influencing the type of reservoir space and its extensive planar distribution. Penecontemporaneous karstification, guided by high-frequency cycles, was favorable for the formation of dissolution holes, effectively improving reservoir performance and facilitating the development of thick limestone reservoirs. Furthermore, fractures were identified as crucial agents in improving the seepage capacity of these tight limestone reservoirs. Drawing from our research results, this study offers valuable guidance for the future exploration of limestone formations in the Taiyuan Formation in the Ordos Basin. Additionally, these findings hold considerable significance as a reference point for research and exploration endeavors focused on bioclastic limestone reservoirs in the North China Platform.

Carbon Isotope Stratigraphy across the Devonian–Carboniferous Boundary in the East Paleo-Tethys Realm, Tibet, China

The Devonian–Carboniferous boundary is one of the most important turning points in geological history, marked by the Late Paleozoic Ice Age and Late Devonian extinction. This study investigates the carbon isotope stratigraphy across the Devonian–Carboniferous boundary in Lhasa block, Tibet, China, which was part of the Gondwana continent during that time. The carbon isotope curves show a significant negative excursion trend, consistent with those of the South China block and other regions on the Laurentia continent. This global negative shift may be attributed to the burial of significant amounts of 12C-rich organic matter in strata, a consequence of the Late Devonian extinction event. Based on the carbon isotope curve and stratigraphic data of the Lhasa block, this study determined, for the first time, the specific horizon of the Devonian–Carboniferous boundary in Tibet, which is located between grayish white bioclastic limestone and yellowish-brown sandy limestone in the upper part of the Chaguoluoma Formation (D1C1ĉ). These findings provide a new reference for the international stratigraphic community to reconsider the position of GSSP on the Devonian–Carboniferous boundary, as well as study the Late Devonian mass extinction and Late Paleozoic glaciation.

Petrography and geochemistry of the Permian-Triassic boundary interval, Kangjiaping section, South China: Implication for the drastic marine environment change

The Permian-Triassic transitional interval exposed well at Kangjiaping section, northwestern Hunan, South China, contains microbialites, oolitic and oncolitic limestones intercalated with vermiculate micritic limestones in the Lower Triassic overlying the uppermost Permian bioclastic limestones, which provides an excellent opportunity to study the environmental changes after the end-Permian mass extinction (EPME) and the origins and environmental significance of these various anomalous sediments. In this paper, we describe in detail the petrographic and geochemical characteristics of these anomalous sediments, together with the reconstruction of the marine environment after the EPME. The continental margin weathering was significantly enhanced after the EPME, indicated by the higher concentrations of terrigenous minerals, Th, Sc, Sr, and rare earth elements in the Lower Triassic carbonates than in the uppermost Permian bioclastic limestones. Indicators such as V/(V + Ni) and Ni/Co ratios suggest that the oxygen content of shallow seawater significantly decreased after the EPME. While, most samples from the Kangjiaping section preserve a negative cerium anomaly, indicating the shallow seawater was still oxygenated both before and after the EPME. In addition, the abnormally rapid carbonate precipitation rate indicates a supersaturation state with respect to calcium carbonate in the shallow seawater after the EPME, but the depth and salinity of seawater and climate indicated by different microfacies are still different. Under the setting of a rapid transgression after the EPME, the microbialites first deposited on the platform with the exclusion of grazers at the beginning of the Early Triassic, followed by the oncolites formed when the climate turned humid with a relative sea-level rise. The oolites were primarily deposited in an arid climate with a relative sea-level fall, and the salinity and hydrodynamics of seawater controlled the ooids’ grain sizes. In contrast, the (vermiculate) micritic limestones were mainly formed in a humid climate with a relative sea-level rising and more terrigenous inputs. Therefore, the periodic relative sea-level fluctuation and arid-humid climate alternations play a vital role in the evolution and repeat stacking of various microfacies in the Kangjiaping section.

Lower Berriasian ammonites from Dedina (Golubac Mountains, eastern Serbia) and their biostratigraphic implication

A rich ammonite fauna was collected from the Dedina section, which is part of the Getic nappe system of the Serbian Dacia Megaunit. More than three hundred ammonites were collected from the light grey to reddish bioclastic limestones of the lower part and light grey marlstones of the upper part. From this fauna, 10 genera with 17 species were described, and 4 aptychi species were recorded. The ammonites belong to the suborders Haploceratina, Perisphinctina, Olcostephanina, and Ancyloceratina. Perisphinctids belong to the genera Pseudosubplanites, Hegaratella, Delphinella, Substeueroceras, Malbosiceras, and Strambergella. Based on the occurrence of Pseudosubplanites grandis, the studied section can be considered to belong to the Grandis ammonite Subzone of the upper part of the lower Berriasian. The association described here appears to be close to the coeval ammonite association in Crimea.

First Report of Small Skeletal Fossils from the Upper Guojiaba Formation (Series 2, Cambrian), Southern Shaanxi, South China.

A small skeletal fossil assemblage is described for the first time from the bioclastic limestone interbeds of the siltstone-dominated Guojiaba Formation, southern Shaanxi, China. The carbonate-hosted fossils include brachiopods (Eohadrotreta zhujiahensis, Eohadrotreta zhenbaensis, Spinobolus sp., Kuangshanotreta malungensis, Kyrshabaktella sp., Lingulellotreta yuanshanensis, Eoobolus incipiens, and Eoobolus sp.), sphenothallids (Sphenothallus sp.), archaeocyaths (Robustocyathus sp. and Yukonocyathus sp.), bradoriids (Kunmingella douvillei), chancelloriids sclerites (Onychia sp., Allonnia sp., Diminia sp., Archiasterella pentactina, and Chancelloria cf. eros), echinoderm plates, fragments of trilobites (Eoredlichia sp.), and hyolithelminths. The discovery of archaeocyaths in the Guojiaba Formation significantly extends their stratigraphic range in South China from the early Tsanglangpuian at least to the late Chiungchussuan. Thus, the Guojiaba Formation now represents the lowest known stratigraphic horizon where archaeocyath fossils have been found in the southern Shaanxi area. The overall assemblage is most comparable, in terms of composition, to Small skeletal fossil (SSF) assemblages from the early Cambrian Chengjiang fauna recovered from the Yu'anshan Formation in eastern Yunnan Province. The existing position that the Guojiaba Formation is correlated with Stage 3 in Cambrian Series 2 is strongly upheld based on the fossil assemblage recovered in this study.

Creep crack energy dissipation and instability prediction in micritic bioclastic limestone

Time-dependent deformation and long-term stability of rocks are important issues in water conservancy and geotechnical engineering. Currently, there are no well-accepted theoretical criteria with which to predict stability and damage considering time-dependent deformation. In-depth research is still needed. Multilevel creep experiments were performed on micritic bioclastic limestone obtained from a continuously deforming tunnel in Xinjiang, China. Based on crack strain theory, the axial crack strain evolution characteristics during loading and creep processes were investigated. The evolutionary characteristics of the crack dissipation energy density, which was obtained via integration of the crack strain during loading and creep, were revealed. The energy dissipation leading to rock fracture in each multilevel creep experiment was identified and used to calculate the total energy dissipated at the point at which the rock loses strength. A rock instability index based on the creep crack dissipation energy density was proposed. An instability index evolution model was proposed, expressed as a surface for the change in instability index with the bearing state (Rd) and time. Based on the instability index surface, the stress threshold creep-sensitive stress σcs was defined. The time-dependent instability index surface was divided into a long-term stability zone (Z1), an initial time-dependent instability zone (Z2), a time-dependent instability stable evolution zone (Z3) and a time-dependent instability sensitive zone (Z4) by the crack initial stress, crack damage stress, and creep sensitive stress. The instability index model was established as an energy instability evolution criterion to evaluate and predict the rock instability and lifespan under specific stress states.

Sequence-paleogeography and coal accumulation of the Late Carboniferous – Early Permian paralic successions in western Shandong Province, northern China

The coal-bearing strata of the Late Carboniferous to the Early Permian in western Shandong Province, northern China, were deposited within a range of paralic facies associations spanning from marine offshore carbonate shelf to fluvial and delta, which constituted a general paralic cratonic setting. In this paper, the sedimentary characteristics, sequence stratigraphy, paleogeography, and coal accumulation of the Benxi, Taiyuan and Shanxi formations of the Moscovian to Sakmarian age are studied by using data of outcrops, borehole cores, and geophysical loggings. A total of 14 different lithofacies are identified, with lithologies including sandstones, siltstones, mudstones, and bioclastic limestones, and grouped into fluvial, delta, tidal bar-lagoon, and offshore shelf facies associations. Furthermore, the key sequence boundaries represented by regional unconformity, incised valley erosional surface, switching surface of transgressive direction, stratigraphic color change surface marking the abrupt change of environment, and paleosol horizon have been recognized. Based on these sequence boundaries, three third-order sequences (S1, S2, S3 in ascending order) and corresponding lowstand, transgressive, and highstand systems tracts are subdivided. S1 corresponds to the Benxi Formation and the lower part of Taiyuan Formation, S2 corresponds to the middle and upper parts of Taiyuan Formation, and S3 corresponds to the Shanxi Formation. The lithofacies paleogeography maps of three sequences are reconstructed based on the contours of a variety of lithological parameters including strata thickness, limestone thickness, limestone percentage content, coarse (sandstone)/fine (siltstone and mudstone) ratio, and total coal thickness. S1 and S2 were developed with the offshore carbonate shelf paleogeographic units and the tidal bar – lagoon paleogeographic units, and S3 was developed with delta and fluvial units. In general, the western Shandong experienced transgression in the S1, transgression direction switching between S1 and S2, transgression again in S2, and then regression in the S3 during the Late Carboniferous to the Early Permian. Most significant coal accumulation occurred in the S3, followed by S1 and then S2. The coal accumulation centers of S1 were mainly distributed in Liaocheng, Feicheng, Jining regions in the west of the western Shandong where the tidal bar and lagoon were developed, those of S2 were mainly distributed in Tai'an, Jining and Tengzhou regions in the central part of the western Shandong, where the lagoon and tidal flat were developed, and those of S3 were mainly distributed in Heze, Jining, Tengzhou and Zaozhuang regions of the southern part of the western Shandong where the delta environment was developed. Overall, the high accommodation setting of S1 and S2, represented by the Benxi and Taiyuan formations, favors thick coal accumulation in the early TST, while the low accommodation setting S3, represented by the Shanxi Formation, favors thick coal accumulation in the middle and late TST. The research results of this study can be used in the evaluation of the favorable areas for the exploration and development of the deep-buried coal and CBM resources in western Shandong Province.

Characteristics of High Flow Zones and a Balanced Development Strategy of a Thick Bioclastic Limestone Reservoir in the Mishrif Formation in X Oilfield, Iraq

The Mishrif Formation in X Oilfield in Iraq is heterogeneous and has prominent development contradictions, and the development plan required urgent adjustment. Based on data regarding the core, cast thin sections, physical property, mercury injection experiments, and development performance, the main geological factors causing the unbalanced development of the Mishrif Formation are identified, and the corresponding development strategy is proposed. The results show that the High Flow Zones (HFZs) are the main geological factors causing unbalanced production in the thick bioclastic limestone reservoir. There are three kinds of HFZs in MA, MB1, and MB2 intervals, namely, the point shoal type, the tidal channel type, and the platform margin shoal type. All HFZs have different scales and distribution patterns. HFZs have ultra-high permeability and large permeability differences with the surrounding reservoir. During development, the oil mainly comes from HFZs, and the considerable reserves in the low permeability reservoir surrounding the HFZs are difficult to develop. The size of the pore throat of the HFZs greatly varies, and permeability is mainly dominated by the mega-pore throat (>10 μm) and the macro-pore throat (2.5~10 μm). In water flood development, the injected water rapidly advances along the mega-pore throat and the macro-pore throat, and the oil in the micro-pore or medium-pore throats are difficult to be displace. It can be concluded that the Mishrif Formation is vertically heterogeneous. The connectivity of HFZs in different intervals greatly varies. As a result, the Mishrif Formation is divided into three development units, MA, MB1, and MB2 + MC, and production wells are deployed in HFZs. The MA adopts a reverse nine-point injection-production pattern, for which the well spacing is 900 m using a vertical well, and the injection well should avoid the HFZs near the faults. The MB1 adopts an irregular five-point injection-production pattern using a vertical well, and the injection wells are deployed at the edge of the tidal channel or in the lagoon. MB2_1 deploys horizontal production wells, for which the well spacing is 900 m. Horizontal production wells, for which the well spacing is 300 m, are deployed in the lower MB2, and the lateral horizontal production wells are converted into injection wells after water breakthrough, and the horizontal wells deployed in the lower part of MC should moderately inject water.

Lower Devonian conodonts from the Xainza area, central Tibet and the Lower Devonian stage boundaries in China

We report conodont assemblages from the Dardong Formation of the Xainza area, Lhasa Block, yielding several chronostratigraphically significant conodont elements discovered from the Lhasa Block for the first time, including Gondwania profunda, Polygnathus pannonicus, P. kitabicus, P. cf. sokolovi and P. excavatus excavatus. Two conodont zones are identified, namely, the middle Pragian profunda Zone and the lower Emsian excavatus Zone. The Pragian–Emsian boundary is presumably located within the interval between bioclastic limestone beds 4 and 6 with an interbed of siltstones. Although the sparse conodont occurrences in this study precludes more precise chronostratigraphic subdivision, the Xainza area of the Lhasa Block is potentially important for recognition of Lower Devonian stage boundaries in China and deserves further conodont investigations.

Audio Magnetotellurics Study of the Geoelectric Structure across the Zhugongtang Giant Lead–Zinc Deposit, NW Guizhou Province, China

Non-invasive geophysical exploration methods a play key role in the exploration of ore deposits. In the present study, the audio-frequency magnetotelluric (AMT) method was applied to metallic mineral exploration. The metallic mineral deposit targeted was the recently discovered super large lead–zinc deposit of the Zhugongtang mining area of Hezhang County in the northwestern Guizhou province in China. The main objectives of this study were to estimate the geoelectric strike and generate geoelectric models that estimate both the depth and distribution of resistivity structures across the deposit. To achieve the objectives, we deployed sixty-one (61) AMT survey sites with an interstation separation of 20 m on a 1280 m survey track perpendicular to the geological strike across the Zhugongtang deposit. We operated in fifty-three (53) frequencies in the range 1–10,400 Hz to record the resistivity distribution of subsurface to a depth of more than 1200 m. The results from the AMT data computations estimated the geoelectric strike that varies between NE285° and NE315°. This range of strikes suggested that structures across the deposit are oriented in the NW–SE direction. Obtained two-dimensional (2D) models elucidated a remarkably low resistivity body (<15 Ωm) at an elevation of less than 1600 m above sea level (>0.50 km depth), thus extending to great depth and were interpreted as lead–zinc mineralization. Furthermore, low resistivity (<63 Ωm) features were imaged both in superficial and deeper depths and interpreted as shale, sandstone, claystone, and silty mudstone units. Dolomite and limestone lithologies were found widely distributed with high resistivity (>1000 Ωm). Bioclastic limestone and dolomite limestone were inferred and characterized by moderate-high resistivity (>250 Ωm) and were not widely distributed. A unit of basalts was found with moderate resistivity (>63 Ωm). In addition, it was also found that regions with high number of faults tend to have low resistivity values compared to regions with a low fault number. In summary, this case study presents the results of applying an AMT approach to explore the conductivity characteristics of structures across the Zhugongtang deposit. The findings may contribute to the literature about this deposit.

Sedimentological and engineering geological features of the Sarmatian limestones along the Black Sea Coast between the capes of Kaliakra and Shabla

This article aims to present the results of geotechnical studies of the main rock complexes that compose the periphery of the Dobrudzha Plateau in the area from Cape Kaliakra to Cape Shabla (Bulgarian Black Sea Coast), directly related to the development of hazardous geological processes. Sediments of the Sarmatian age were studied: carbonate tempestites, Mactra limestones (Karvuna Formation), nubecular (foraminiferal), reef and bioclastic limestones (Odartsi Formation). The corresponding microscopic and petrographic analyses of the studied sediments are also presented. Studies of collapses in caves near Cape Kaliakra show that they occur in friable peloidal limestones, with disturbances following stratification. This is confirmed by laboratory tests of samples showing lower strength indicators, namely in the direction to the layering. The results of laboratory tests of specimens from the sections between the village of Tyulenovo and Cape Shabla, including foraminiferal, reef and bioclastic limestones, show higher values of physical and mechanical parameters compared to limestones at Cape Kaliakra. In the case of the bioclastic limestones, higher strengths are found crosswise to the layering, and in the case of compressive strength, the ratio is double. The obtained data provide valuable information about the engineering geological parameters of the main varieties in which the rock deformations are developed, which can be used in the analysis of slope stability.

New insight into the organogenus Ioanella Granier & Berthou, 2002, with description of Ioanella dobrogiaca organosp. nov. from Triassic limestones of the Dobrogea (Romania)

ABSTRACT Triassic deposits are widespread in the Măcin Unit of Northern Dobrogea (SE Romania). At the Enisala Fortress hill, near the village of Enisala, they are predominantly represented by grey bioclastic limestones of Wetterstein-type facies of the latest Ladinian to Carnian age. Samples collected from the lower part of this unit, i.e. from strata cropping out on the right side of the road from Sarichioi to Enisala, yield an association of calcareous algae, among which one is described in this paper as a new representative of the organogenus Ioanella. Ioanella dobrogiaca organospecies nov. differs from the type-species of the genus, Ioanella lusitanica, by its morphological and dimensional characteristics. Its features allow us to refine the diagnosis of the organogenus Ioanella. Additionally, comparisons with other dasycladalean organogenera demonstrate that Ioanella is a discrete genus.

Unified transient creep constitutive model based on the crack evolution of micritic bioclastic limestone.

The surrounding rock at the exit of the No. 1 drainage tunnel of the Artashi Water Conservancy Project is micritic bioclastic limestone with 55% bioclastic material. This rock underwent unpredictable large and time-dependent deformation during excavation. To date, the mechanical behaviour of this kind of rock has rarely been studied. In this study, traditional triaxial compression tests and multilevel creep tests were conducted on micritic bioclastic limestone, and the results clarified the instantaneous and time-dependent mechanical properties of the rock. Considering that the essence of rock failure is crack growth, the crack strain evolution properties were revealed in rock triaxial compression tests and multilevel creep tests. Based on triaxial compression tests, the evolution of axial cracks with increasing deviatoric stress ratio Rd (ratio of deviatoric stress to peak deviatoric stress) was observed, and an axial crack closure element and new crack growth element were proposed. To simulate the creep behaviour of a rock specimen, the relationship of the rock creep crack strain rate with Rd was studied. A creep crack element was created, and the creep crack strain evolution equation was obtained, which closely fit the experimental data. Combining the 4 element types (elastic element, crack closure element, crack growth element, and creep crack element), a unified transient creep constitutive model (Mo’s model) was proposed, which represented both the transient and time-dependent mechanical properties of the micritic bioclastic limestone.

Early Late Triassic retro-foreland basin in response to flat subduction of the Paleo-Tethyan oceanic plate, SE Tibet

Syn-subduction basins bear significant implications to understand tectonic evolution of any fossil subduction zone. The late Paleozoic to early Mesozoic (Paleo-Tethyan) tectonics of the eastern and southeastern Tibetan Plateau (i.e., the Sanjiang Orogenic Belt) is featured by ocean-continent subduction systems. A huge pile of volcanic-absent sedimentary succession developed in the middle segment of the Sanjiang orogenic belt, its age and tectonic nature remain unclear. Detailed geological mapping and zircon U-Pb dating results demonstrate that the early Late Triassic volcanic-absent succession comprises the nonmarine Maichuqing Formation in the lower part and the shallow marine Sanhedong Formation in the upper part. The Maichuqing Formation consists of coarse to fine-grained sandstone, siltstone and mudstone with abundant basal erosional surfaces, trough and planar cross-beddings, ripples, mudcracks, and plant fragments. The Sanhedong Formation comprises predominantly bioclastic limestones interlayered with marl, calcareous-muddy siltstone, and calcareous sandstone with abundant bivalve fossils. Syn-sedimentation deformation structures, such as slump folds and associated normal faults are common, suggesting intense tectonism during deposition. Synthesizing sedimentary data, paleocurrent and provenance results, combined with other available data, demonstrate that the volcanic-absent succession deposited within a retro-foreland basin along the rear part of the Permian-Triassic Jomda-Weixi-Yunxian arc in response to flat-subduction of the Paleo-Tethyan Ocean during the early Late Triassic time.

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.

Heavy δ26Mg values in carbonate indicate a magmatic-hydrothermal origin of Carlin-type Au deposit

The Carlin-type Au deposits in Youjiang Basin, SW China, (referred to as Guizhou Carlin-type Au deposits) are the second largest Carlin-type Au province in the world after Nevada, USA. To date, the source of ore fluids that formed the Guizhou deposits remains controversial, hampering the formulation of a genetic model. Compared to Nevada Carlin-type Au deposits, a significant difference is that the Guizhou Carlin-type Au deposits contain abundant ore-stage dolomite in the ore. Herein, we present carbonate Mg isotopes combined with C-O isotopes from the giant Shuiyindong deposit to provide new insights into the source of ore fluids and to constrain the ore genesis of the Guizhou Carlin-type Au deposits.Petrographic observation shows that from least altered bioclastic limestone to high-grade ore, dolomite increased significantly, suggesting that dolomite formed as part of the Au mineralization process. Chemical analyses reflect a small amount of Mg in the ore fluids was variably added to ore during Au mineralization, and this added Mg was fixed in the dolomite. Although most ore-stage dolomite formed from the sulfidation of Fe in Fe-dolomite, some ore-stage dolomite formed by the combination of host rock calcite with Mg from ore fluids.The δ26Mg values of carbonate, primarily dolomite, range from −3.49‰ to −0.07‰, with a median value of −1.01‰. The δ18O values range from 21.7‰ to 27.6‰, with a median value of 23.2‰. The δ13C values vary from −14.6‰ to 1.2‰, with a median value of −0.7‰. The δ26Mg values exhibit a positive correlation trend with MgO, S, SiO2, and Carlin-suite elements (Au, As, Hg, Sb, Tl, and Cu), and a negative correlation trend with CaO. Comparison of Mg isotopes data from this study with the major Mg reservoirs indicates that the Shuiyindong samples contain heavier Mg isotopes that drifts towards the heavy Mg reservoirs (magmatic and metamorphic rocks) compared with normal carbonate rocks. Two mechanisms, namely heavy Mg addition from ore fluids and isotopic exchange reaction with heavy Mg-rich ore fluids, could have attributed to heavy Mg in carbonates in the Shuiyindong deposit.The Mg isotopes results, combined with other isotopes (e.g., S, Hg, and He-Ar) and geologic evidence, support a magmatic-hydrothermal origin for the ore fluids. The thick Devonian-Triassic sedimentary sequence and weak extension following sedimentation prevented igneous activity from reaching the surface. However, buoyant ore fluids released from the deep-seated intrusions are interpreted to have ascended along basement-penetrating faults and eventually produced the Guizhou Carlin-type Au deposits at shallow levels. This study implies that Mg isotopes are a novel proxy to infer ore fluids source and evolution, and that they can provide significant constraints on the genesis of hydrothermal deposit.

Shoal-to-Strata Ratio Technique and Its Use in Describing Carbonate Rock Sedimentary Facies in the Eastern Sichuan Basin

The requirements for the accurate characterization of shoal sediments have increased in view of the fact that strata are eroded due to uplift and it is difficult to calculate the real thickness of granular shoal. To solve this problem, with the shoal-to-strata ratio, the type and distribution range of carbonate rock shoal facies are described, and the characterization of sedimentary facies is subsequently presented. Taking the P2m2a in Eastern Sichuan as an example, the following conclusions were obtained. (1) The primary rock types of the shoal facies of the P2m2a are bioclastic limestone and micrite limestone. The logging facies of platform margin shoals, intra-platform high-energy shoals, intra-platform low-energy shoals, and sloping low-energy shoals are large funnel-type, small funnel-type, box-type, and finger-like, respectively. The platform margin shoals and intra-platform high-energy shoals are imbricate seismic facies, and the intra-platform low-energy shoals and sloping low-energy shoals are S-type progradation seismic facies and oblique progradation. (2) The shoal-to-strata ratio of platform margin shoals is the highest, followed by intra-platform high-energy shoals, intra-platform low-energy shoals, and sloping low-energy shoals. (3) Shoal facies deposits have a certain controlling effect on gas production, and platform margin shoals have greater exploration potential. (4) This example shows that in the areas where carbonate rock strata are eroded, compared with the method of characterizing the sedimentary facies of carbonate rock using the shoal thickness, the shoal-to-strata ratios are more accurate. (5) The planar distribution diagram of the shoal-to-strata ratio can be used as a supporting map for compiling sedimentary facies maps. This method is worth applying in related research fields.

Cretaceous uplift history of the Tibetan Plateau: Insights from the transition of marine to terrestrial facies in central Tibet

The uplift of the Tibetan Plateau has been widely associated with India-Asia continental collision during the Cenozoic. However, recent studies have proposed that a proto-Tibetan Plateau in central Tibet was elevated during the late Mesozoic. This paper improves that understanding by focusing on the Cretaceous terrestrial strata in the Gerze area of central Tibet. Our studies suggest that the Qushenla Formation in the Nare area contains alluvial fan conglomerate intercalated with rhyolitic ignimbrite and vesicular rhyolite. Zircon UPb dating of rhyolitic ignimbrite (103.9 ± 1.4 Ma) and two granitoid boulders of conglomerate (106.0 ± 1.6 and 106.1 ± 1.5) indicate that the Qushenla Formation was deposited during the late Albian. The compositions of the Qushenla Formation conglomerate were dominated by hypabyssal intrusions on the southern Qiangtang terrane, which recorded a rapid uplift and denudation process in the late Albian, accompanied by intense volcanic eruptions. The second set of terrestrial strata is the Upper Cretaceous Jingzhushan Formation alluvial fan-braided river deposits, which can be divided into a lower conglomerate member (Member 1) and a upper sandstone member (Member 2). The large amount of bioclastic limestone fragments in the Member 1 show that they come from the Upper Cretaceous Langshan Formation in the central and northern Lhasa subterranes. While in the Member 2, abundant limestone debris, detrital zircon yielding Jurassic–Cretaceous ages with εHf(t) values between −18.6 and + 8.4, and occurrence of volcanic and ancient zircon grains indicate a volcanic-sedimentary source located in the central and northern Lhasa subterranes, with some input from the Bangong-Nujiang suture zone. This suggests that the central and northern Lhasa subterranes experienced rapid crustal uplift in the Late Cretaceous, thus ending the marine sedimentary environment since the Jurassic. Based on the temporal and spatial sedimentary characteristics of the Qushenla and Jingzhushan formations as well as the regional data, our integrated study suggests that the central plateau (Lhasa and southern Qiangtang terranes) experienced a complex uplift history during the Aptian-Turonian, with southwards stepwise growth in time and space. Furthermore, the uplift mechanism of the central plateau is complex, which was controlled by the diachronous closure of the Bangong-Nujiang Tethyan Ocean from east to west during the Early Cretaceous, and by the far-field forces associated with northwards subduction of the Neo-Tethys Ocean during the Late Cretaceous.