Platform Margin Research Articles

The Strike-Slip Fault Effects on Tight Ordovician Reef-Shoal Reservoirs in the Central Tarim Basin (NW China)

The largest carbonate condensate field in China has been found in the central Tarim Basin. Ordovician carbonate reservoirs are generally attributed to reef-shoal microfacies along a platform margin. However, recent production success has been achieved along the NE-trending strike-slip fault zones that intersect at the platform margin. For this contribution, we analyzed the strike-slip fault effects on the reef-shoal reservoirs by using new geological, geophysical, and production data. Seismic data shows that some NE-trending strike-slip faults intersected the NW-trending platform margin in multiple segments. The research indicated that the development of strike-slip faults has affected prepositional landforms and the subsequent segmentation of varied microfacies along the platform margin. In addition, the strike-slip fault compartmentalized the reef-shoal reservoirs into multiple segments along the extent of the platform margin. We show that fractured reef-shoal complexes are favorable for the development of dissolution porosity along strike-slip fault damage zones. In the tight matrix reservoirs (porosity < 6%, permeability < 0.5 mD), the porosity and permeability could be increased by more than 2–5 times and to 1–2 orders of magnitude in the fault damage zone, respectively. This suggests that high production wells are correlated with “sweet spots” of fractured reservoirs along the strike-slip fault damage zones, and that the fractured reservoirs in the proximity of strike-slip fault activity might be a major target for commercial exploitation of the deep Ordovician tight carbonates.

Middle–Late Ordovician brachiopods from Ningnan, southern Sichuan Province, Southwest China: Implications for macroevolution and palaeogeography

Middle to Late Ordovician brachiopods from the Huadan Formation (upper Darriwilian–Sandbian) of Ningnan County, southern Sichuan Province, are systematically documented here for the first time. The locality belongs to the western margin of the Upper Yangtze Platform, South China palaeoplate, and the brachiopod fauna includes one new genus and three new species as well as five other constituents: Hingganoleptaena sp., Acculina zhongliangziensis n. sp., Ningnanmena longisepta n. gen. n. sp., Kassinella (Trimurellina) minuta n. sp., Lepidorthis typicalisWang, 1955, Protoskenidioides weixinensisZhan and Jin, 2005, Porambonites transversusXu, Rong and Liu, 1974, and Psilocamerella sp. Taxonomically it is a typical representative of a Middle to early Late Ordovician brachiopod fauna, and, together with some other evidence from other fossil groups like trilobites, conodonts, chitinozoans, a late Darriwilian–Sandbian age could be inferred for the horizon yielding this fauna. According to the richness of each constituent, this fauna is suggested to be called the Acculina-Ningnanmena fauna (ANF). Numerical palaeogeographical analysis shows that two broad palaeobiogeographic provinces could be recognized during this particular time interval, and, although the ANF is grouped into the South China cluster, it shares very little similarity with other representatives of that group except for two cosmopolitans. It further confirms that the Great Ordovician Biodiversification Event (GOBE), in other words the Ordovician radiation, was actually manifested by the strong localization of major marine organisms such as brachiopods, trilobites, graptolites, etc.

Sedimentology and geochemistry of basinal lithofacies in the Mesoarchean (2.93 Ga) Red Lake carbonate platform, northwest Ontario, Canada

The > 200 m thick Red Lake carbonate platform of the 2.940-2.925 Ga Ball Assemblage (Red Lake Greenstone Belt, Superior Province, Canada) is the oldest known carbonate platform preserved on Earth. This study examined surface outcrops and multiple industry cored drill holes comprising a diverse assemblage of chemical and siliciclastic sedimentary rocks of the deeper water lithofacies associated with the shallow water carbonates. The offshore lithologies, underlying, interlayered with, and overlying the mainly stromatolitic carbonates, consist of alternating packages of sandstone, siltstone, carbonaceous slate, pyritic carbonaceous slate, chert, and oxide facies iron formation (OFIF). These interlayer with carbonate in transitional slope environments, which consist in places of carbonate with magnetite laminae above slumped carbonate debris in iron formation and/or carbonaceous slate. Chert and OFIF were deposited further offshore in a suboxic environment, with lithology dependent on concentrations of seawater Fe(II) and silicic acid. The iron sulfides were deposited in a reducing, anoxic, and organic-rich bottom water and muddy pore water environment not conducive to Fe hydroxide or chert precipitation. Carbonaceous mud was the background sediment, accumulating during intervals when the precipitation of chert, Fe hydroxides, or massive layers of Fe sulfides was inhibited. The interlayering of differing assemblages of chemical and siliciclastic sedimentary rocks indicates temporal and spatial fluctuations in the chemistry of local paleo-seawater and variable sedimentation rates on the margins of the Red Lake carbonate platform. Post-Archean Australian Shale (PAAS)- normalized rare earth element (REE) systematics of carbonate and OFIF show positive La and Gd anomalies, super-chondritic Y/Ho ratios, and depleted light rare earth elements (LREEs) relative to heavy rare earth elements (HREEs), similar to modern seawater, while the positive Eu anomalies in the carbonate and OFIF samples reflect hydrothermally influenced Archean seawater. The chemical sedimentary rocks exhibit enrichment of redox-sensitive elements (i.e., Cr, Mo, V, and U) compared to their siliciclastic associates. This, combined with concentrations of MnO up to 6.3 wt% and positive Ce anomalies in the iron formation directly below the carbonate platform, all indicate the likely presence of some O2 ca 2.93 Ga.

Upper Changhsingian to lower Anisian conodont biostratigraphy of the Datuguan section, Nanpanjiang Basin, South China

The Great Bank of Guizhou (GBG) of South China is an important area for the study of Lower Triassic conodont biostratigraphy, but few studies have concerned the clastic facies of this region. The Datuguan section in Luodian County, southern Guizhou Province, is a typical clastic ramp section, preserving a relatively complete conodont succession from the uppermost Permian to lower Middle Triassic. A total of nearly 4000 P1 elements were obtained from this section, and 48 conodont species belonging to 16 genera have been identified, including a new species, Gladigondolella luodianensis sp. nov. 12 conodont biozones have been recognized, namely, the Clarkina yini Zone, the C. meishanensis Zone, the Hindeodus postparvus Zone, the Novispathodus waageni Zone, the Discretella discreta Zone, the Nv. pingdingshanensis Zone, the Icriospathodus collinsoni Zone, the Triassospathodus homeri Zone, the Tr. symmetricus Zone, the Tr. triangularis Zone, the Tr. sosioensis Zone, and the Chiosella timorensis Zone in ascending order. These conodont zones improve the resolution of correlation with other sections in this region, as well as globally. Based on the conodont successions and correlation of lithology, the Permian–Triassic boundary (PTB) and the Griesbachian–Dienerian boundary (GDB) are temporarily placed at the middle of Bed 8, 7 m below the top of the Linghao Formation, and the top of Bed 12, 15 m above the bottom of the Luolou Formation, respectively. The Induan–Olenekian boundary (IOB), the Smithian–Spathian boundary (SSB), and the Olenekian–Anisian boundary (OAB) are drawn at Bed 15–1, Bed 27–1, and Bed 42–2, in terms of the first occurrences (FOs) of Nv. waageni sensu lato, Nv. pingdingshanensis, and Ch. timorensis sensu stricto respectively. The conodonts and ammonoids from the Datuguan section supplement the materials of the Guandao section, which is an important reference section for the definition of OAB. In addition, the distribution of segminiplanate conodonts from the platform margin to the basin center has been determined in the Nanpanjiang Basin, revealing that segminiplanates migrated to equatorial region four times and preferred a narrow mid-depth setting during relatively cooler periods of the Early Triassic, in addition to occupying the more oxygenated deep-water basin at other time.

Tectono-sedimentary characteristics and controlling factors of the lower-middle Cambrian gypsum-salt rocks in the Tarim Basin, Northwest China

Due to their excellent sealing capacity, gypsum-salt rocks are often associated with hydrocarbon accumulations. In the Tarim Basin (Northwest China), exploration activity has demonstrated that the Cambrian gypsum-salt rocks played a key role in the development of hydrocarbon plays, but there is still a lack of research on them. This study utilizes geological data coming from outcrops, well logs, rock cores, and seismic reflection profiles to quantify the distribution and tectono-sedimentary characteristics of the Cambrian gypsum-salt rocks in the Tarim Basin, and decipher the controlling factors governing their deposition. The gypsum-salt rocks are present in the Lower Cambrian Wusonger and the Middle Cambrian Awatage Formations, which are distributed around the Aman and Awati depressions, two structures inherited from the Ediacaran intracontinental rift, and the Bachu uplift. These Formations are mainly composed of salt, gypsum, gypsum halite, gypsum dolomite and gypsum mudstone, and are usually deposited in medium-thin beds. The total and single layer thicknesses of gypsum-salt rocks in the north of Bachu uplift are greater than that in the south. These lithofacies can be associated with different environments of deposition, with the Bachu uplift interpreted as supratidal sabkhas and evaporation lagoons and the Aman-Awati depressions as large-scale evaporation salt basins with a bull's eye architecture of halite-gypsum-dolomite. Carbon and oxygen isotopes indicate an overall fall in relative sea level from the late Early Cambrian to the Middle Cambrian in the Tarim Basin, accompanied by higher-frequency oscillations. Influenced by this continuous decline of sea level, the eastern platform margin transformed from gentle ramp to progradational rimmed margin and blocked the water exchange between the open sea and the platform. The southwest side of the platform was characterized by a stable gentle ramp margin, which was flooded by seawater during rise of sea level. In addition, the development of the intraplatform shoals further provided morphological barriers for the lagoons in the Bachu-Tazhong uplift belt. The Aman-Awati depressions continued to subside during the Early and Middle Cambrian and became the main loci of salt deposition, whereas the depressions and half grabens formed under the tensile tectonic regime in the Bachu-Tazhong uplift belt controlled the development of evaporation lagoons and gypsum-salt flats. This study constrained the distribution and evolution of gypsum-salt caprocks, and thus provide new insights for the sub-salt and inter-salt hydrocarbon exploration in the Tarim Basin.

The Fractured Permian Reservoir and Its Significance in the Gas Exploitation in the Sichuan Basin, China

Large gas reserves have been found in the Permian platform margin of the Kaijiang-Liangping area of the Sichuan Basin in SW China. They are assumed to be a widely developed reef–shoal reservoir. However, the tight matrix reservoir cannot support high gas production using conventional development technology at deep subsurface. In this contribution, we analyze the fractured reservoirs along the strike-slip fault zones using the compiled data of cores, well logging, and production data, and provide a seismic description. It was shown that the fractures and their dissolution developed along the strike-slip fault zones. The porosity and permeability of the fractured reservoir could increase by more than one and 1–2 orders of magnitude, respectively. The seismic anisotropic energy found in the steerable pyramid process suggests that fractured reservoirs have a strong heterogeneity, with a localized fault damage zone. This fracturing has both positive and negative effects, showing varied reservoir parameters in the fault damage zone. The development pattern should adopt a non-uniform well pattern, in order to target the localized “sweet spot” of the fractures in these deep tight matrix reservoirs.

Terminal Ediacaran microbialite lithofacies associations with paleo-environmental constraints in a high-frequency sequence stratigraphic framework of Sichuan Basin, SW China

The terminal Ediacaran shallow marine microbialites are significant for paleo-environment reconstruction and petroleum exploration. Their depositional and paleo-environmental characteristics during high-frequency sea-level fluctuations were paid less attention, which limited the understanding of their evolution and related reservoir characterization. Microbialites from the fourth member of the upper Ediacaran Dengying Formation, Gaoshiti-Moxi area, Sichuan Basin, SW China are important records for studying their lithofacies associations and paleo-environmental constraints in high-frequency depositional sequences. Petrographic and geochemical analyses (in-situ major and trace elements and carbon and oxygen isotopes) indicate microbialites depositional and paleo-environmental evolution on a rimmed platform within a fourth-order depositional sequence. From late TST (transgressive systems tract) to early HST (highstand systems tract), thick-bedded dark-grey dolomudstone-thin-bedded spotted thrombolite dominates the platform margin; while thick-bedded dark-grey dolomudstone-thin bedded dark-grey wavy stromatolite dominates the platform interior. From late HST to early TST of the next fourth-order sequence, the platform margin is dominated by thick-bedded spotted thrombolite—dolograinstone, while the carbonate interior is dominated by thin-medium-bedded layered thrombolite—light-grey wavy stromatolite. Salinity correlates well with microbialite lithofacies associations: the platform interior layered thrombolite and wavy stromatolite enriched lithofacies associations are characterized by higher salinity than that of the platform margin spotted thrombolite enriched lithofacies associations. The microbialite lithofacies associations are less constrained by redox: platform interior layered thrombolite and wavy stromatolite enriched lithofacies associations are slightly more reduced than the platform interior spotted thrombolite enriched lithofacies associations. The redox analysis of the microbialites also indicates low oxygen levels of shallow marine in the study area during terminal Ediacaran, with possible oxygenation at the Ediacaran-Cambrian boundary. The study suggests that the terminal Ediacaran microbialite development was controlled by both high-frequency sea-level fluctuations and paleo-environmental factors such as salinity and redox. Platform margin spotted thrombolitic reefs from late fourth-order HST to next early fourth-order TST serve as favorable microbialite reservoirs. Platform interior layered thrombolites and wavy stromatolites are regarded as potential reservoirs, which deserve more detailed depositional-paleo-environmental research.

Main controlling factors and genetic mechanism for the development of high-quality reservoirs in the mound-shoal complexes on the platform margin of the intra-cratonic rift: A case study of the fourth member of the Dengying Formation in the eastern limb of Deyang-Anyue intra-cratonic rift, Sichuan Basin, China

Reservoirs of large platform margin mound-shoal complexes of the fourth member of Dengying Formation (Deng 4 Member) are developed in the margin of the Deyang-Anyue intra-cratonic rift in the Sichuan Basin and it is the main pay horizon of the Anyue gas field. A clear understanding of the reservoir genetic mechanism of the mound-shoal complexes is the key to predicting the distribution of high-quality reservoirs and guiding the deployment of exploration. Based on the data of drilling, seismic, outcrop, and analytical data, this paper analyzes the reservoir characteristics and genetic mechanism of the mound-shoal complexes at the margin of the Deng 4 Member and obtains three new understandings: (1) Platform margin mound-shoal reservoirs are developed on the margin of Deyang-Anyue intra-cratonic rift in Sichuan Basin. The mound-shoal complexes are mainly composed of algal mounds and bioclastic shoals in multiple stages. The reservoir space is mainly dissolution pores, caverns, and fractures, with low porosity and low permeability in general. (2) The reservoir can be divided into three types, i.e., the fracture-dissolution pore type, the dissolution pore type, and the matrix pore type, and the reservoirs of fracture-dissolution pore type are high-quality reservoirs; the development of reservoirs is mainly controlled by the platform margin mound-shoal complexes, the penecontemporaneous interstratal karst, and two-stage weathering crust karstification as well as multi-stage disruptive actions; the upper part of the reservoir in the same stage is good, and the reservoir at the top of Deng 4 Member is good. (3) Before the Himalayan movement, the reservoir forming and the environments of the two platform margin mound-shoal complexes were the same, and the characteristics of the formation of the reservoir were similar. The Himalayan movement led to a great difference in the current buried depth of the reservoir, resulting in a host of fractures and retaining a host of dissolution pores and caverns. The whole platform margin mound-shoal complexes have large-scale reservoirs developed and have a good exploration prospect. The results enrich the theory of ancient and deep carbonate reservoir forming and its genetic mechanism and provide the geological basis for the deployment of exploration.

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.

Fragmentation, rafting, and drowning of a carbonate platform margin in a rift-basin setting

AbstractHigh-resolution bathymetric and shallow seismic data along the northeast Red Sea margin reveal a previously disregarded mechanism for carbonate platform drowning at a steep-flanked rift basin. At the seafloor, salt extrusions highlight the influence of extensional salt tectonics, with a salt flow from the southern flank of the Al Wajh carbonate platform that likely originates from below. Salt-flow direction, morphology, and kilometer-sized slumps and rotated blocks indicate platform-margin disintegration and rafting of platform blocks toward the southwest. The outlines of several smaller detached or semi-detached carbonate platforms to the south of the main platform can be refitted to the larger platform margin by counter-moving the direction of mass wasting. Several platforms, reaching heights above the seafloor of up to 650 m, are partially or fully submerged in the mesophotic zone and appear to be in danger of drowning. We conclude that the southern outer rim of the Al Wajh platform is breaking apart owing to salt withdrawal, which indicates that carbonate platforms on top of salt sequences grow on mobile ground, leading to platform disintegration, basinward rafts, and the demise of broken-off, smaller pieces of platform. Salt displacement also controls the growth geometries of individual platform rafts, with keep-up reef growth (growth rate equal to sea-level rise) and drowning occurring in close spatial proximity. Therefore, the interplay between salt diapirism and platform growth is not limited to platforms growing on the apexes of diapirs and is more complex than previously thought.

Seismic architecture of Yongle isolated carbonate platform in Xisha Archipelago, South China Sea

This study presented recently reprocessed multi-channel seismic data and multi-beam bathymetric map to reveal the geomorphology and stratigraphic architecture of the Yongle isolated carbonate platform in the Xisha Archipelago, northwestern South China Sea. Our results show that the upper slope angles of Yongle carbonate platform exceed 10° and even reach to ∼32.5° whereas the lower slope angles vary from .5° to 5.3°. The variations of slope angles show that margins of Yongle Atoll belong to escarpment (bypass) margins to erosional (escarpment) margins. The interior of carbonate platform is characterized by sub-parallel to parallel, semi-continuous to continuous reflectors with medium-to high-amplitude and low-to medium-frequency. The platform shows a sub-flat to flat-topped shape in its geometry with aggradation and backstepping occurring on the platform margins. According to our seismic-well correlation, the isolated carbonate platform started forming in Early Miocene, grew during Early to Middle Miocene, and subsequently underwent drowning in Late Miocene, Pliocene and Quaternary. Large-scale submarine mass transport deposits are observed in the southeastern and southern slopes of Yongle Atoll to reshape the slopes since Late Miocene. The magmatism and hydrothermal fluid flow pipes around the Yongle Atoll have been active during 10.5–2.6 Ma. Their activity might intensify dolomitization of the Xisha isolated carbonate platforms during Late Miocene to Pliocene. Our results further suggest that the Yongle carbonate platform is situated upon a pre-existing fault-bounded block with a flat pre-Cenozoic basement rather than a large scale volcano as previously known and the depth of the basement likely reached to 1400 m, which is deeper than the well CK-2 suggested.

Sedimentary facies, lithofacies paleogeography, and an evaluation of the Ordovician sequences in the Sichuan Basin, southwest China

Ordovician strata in the Sichuan Basin occur in formations of varying thicknesses. Deposited in intracratonic and craton margin depressions of the Yangtze Block, these strata are rarely exposed in the basin with poorly described facies types and multiple controversies in sequence stratigraphy, which can lead to inaccurate interpretations of depositional settings and paleogeography. We studied outcrops, samples, and thin sections, and conducted interpretations of the environment of deposition and sequence stratigraphy as well as the lithofacies paleogeographic characteristics of different depositional stages of Ordovician units. Macroscopic facies (08) and their associated microfacies (14) were identified from outcrops and thin sections. The depositional environments of the carbonate rocks were interpreted using the standard microfacies types (SMF) of Flügel (2010) as a reference. The Ordovician units in the Sichuan Basin were divided into three 2nd-order sequences, including two complete cycles (SSQ1 and SSQ2) and one semi-cycle (SSQ3), and further subdivided into ten 3rd-order sequences (SQ1-SQ10). Ordovician units were partitioned into seven formations: the Tongzi, Honghuayuan, Meitan, Shizipu, Baota-Linxiang, and Wufeng formations. Based on the variation characteristics of the carbon-13 isotope (δ13C) curve, two large cycles of sea-level rise and fall were detected in the study area. An extensional rifting event occurred in the basin during the first cycle (Lower Tongzi Formation to Early/Middle Meitan Formation). The basin was in a compressive stage during the second cycle (Late Meitan Formation to Late Wufeng Formation). The Early Ordovician Tongzi and Honghuayuan formations were deposited in shallow water. During the deposition of these units, shoal facies occurred in the platform margin. Afterward, rimmed carbonate platforms occurred in the platform margin with a fauna mainly composed of brachiopods, echinoderms, trilobites, bryozoans, and algae. The Middle Ordovician Meitan Formation is characterized by increased clastic components deposited in continental shelf environments with graptolite and trilobite fossils. The Late Ordovician Baota Formation is marked by the development of slope facies deposited in carbonate ramp environments with cephalopod and trilobite fragments. The Late Ordovician Wufeng Formation was deposited in deep water shelf settings characterized by graptolite fossils. Tectonic activities and relative sea-level change mainly controlled the paleogeographic pattern and the evolution of the Sichuan Basin. The results presented in this study provide valuable insights and fill a significant gap in facies analysis, sequence stratigraphic division, and paleogeography of Ordovician units in the Sichuan Basin.

Hydrocarbon accumulation conditions and exploration targets of the Sinian Dengying Formation in the Southeastern Sichuan Basin

No breakthrough has been made in petroleum exploration of the Sinian Dengying Formation in the southeastern Sichuan Basin as the major factors controlling the hydrocarbon accumulation of the Dengying Formation have not been well established, including the development period and distribution of the platform margin, the effectiveness of the source-reservoir combination and the reliability of the Cambrian pre-salt structural trap. Based on the interpretation of seismic data and hydrocarbon-source correlation, the distribution of the platform margin of the Sinian Dengying Formation in the southeastern Sichuan Basin was mapped. Moreover, the gas source rocks of the Dengying Formation were clarified according to geochemical characteristics of fluid inclusions, and the pre-salt structures were further confirmed by seismic forward modeling and experiments of prestack depth migration. Results showed that the sedimentary facies of the 4th member of Dengying Formation in the southeastern Sichuan Basin was platform margin of gentle slope type. The platform margin reservoir of the Dengying Formation and the Lower Cambrian source rocks constitute a favorable source - reservoir combination. The carbon isotope characteristics of gas in fluid inclusions indicate that some of the oil/gas in the Dengying Formation were contributed by the Doushantuo Formation source rocks. As the difference in seismic velocities between carbonate rocks and gypsum/salt beds may cause a reduction in the amplitude of pre-salt structures, the low-amplitude structures identified on current seismic profiles are underestimated instead of overestimated. Accordingly, the Sinian structural traps are confirmed to develop in the deep pre-salt sequences in the low-steep anticline belt, southeastern Sichuan Basin. On the basis of the above new understanding, we propose two types of hydrocarbon migration modes for the Sinian Dengying Formation in southeastern Sichuan Basin, including lateral migration of hydrocarbons generated from the Cambrian Qiongzusi Formation and vertical migration of hydrocarbons derived from the Sinian Doushantuo Formation. Lithologic traps at the platform margin and pre-salt structural traps within the platform are favorable targets for natural gas exploration.

A glimpse of the lost Upper Triassic to Middle Jurassic architecture of the Dinaric Carbonate Platform margin and slope

In the southernmost outcrops of the Slovenian Basin the Middle Jurassic coarse-grained limestone breccia (mega)beds are interstratified within a succession that is otherwise dominated by hemipelagites and distal turbidites. In this paper, these beds are described as the Ponikve Breccia Member of the Tolmin Formation. We provide descriptions of the studied sections with detailed geological maps and analysis of the breccia lithoclasts. From the latter, a non-outcropping margin of the Dinaric Carbonate Platform is reconstructed. In the Late Triassic the platform margin was characterized by a Dachstein-type marginal reef. After the end-Triassic extinction event, the platform architecture remained, but the reefs were replaced by sand shoals characterized by ooids. In the late Early Jurassic and/or early Middle Jurassic a slope area might have been dissected by normal faults and a step-like paleotopography was formed. In the Bajocian, during a period of major regional geodynamic perturbations, extensional or transtensional tectonic activity intensified and triggered the large-scale collapses of the Dinaric Carbonate Platform margin producing the limestone breccias described herein. This may in turn have caused a backstepping of the platform margin, as is evident from the occurrence of Late Jurassic marginal reefs that are installed directly above the Upper Triassic and Lower Jurassic inner platform successions.

Carbonate platform architecture and carbon isotope stratigraphy from the neoproterozoic Narji limestone formation, Kurnool Group, India: Decoding clues of relative sea level fluctuations

From a detail process-based facies and facies association analysis in a spatio-temporal framework, a rimmed shelf platform geometry is proposed for the Neoproterozoic Narji Limestone Formation, Kurnool Group, India. Ten carbonate facies types were delineated and clubbed under seven facies associations belonging to four different paleogeographic settings viz. Forereef (intertidal to shallow subtidal and shallow to deep subtidal), peritidal reef and back-reef lagoon. Whereas crinkly microbial laminites with subordinate domal stromatolites constitute the reef part, the fore slope deposition in the platform largely remained tide-and storm-dominated. Grey to black colored pyrite bearing bedded limestone-shale alternation without any current or wave imprint represents the back-reef lagoon deposition. A high carbonate saturation and productivity in the Narji inner and mid shelf is suggested from an overall dominance of microalgal laminite and microdigitate stromatolites over columnar stromatolites. From documentation of decimeter-to meter-thick shallowing-upward depositional hemicycles, embedded within an overall aggradational Narji lithosuccession, a combined control of sea level and carbonate productivity is inferred in course of Narji sedimentation. A short-duration medium-to large-scale rise in the base level led to the aggradation in shallow water rim of the Narji platform margin; interruption of this overall motif by short thickening- and shallowing-upward progradational hemicycles bear testimony for higher productivity in the Narji mid-to inner shelf areas. The present study intends to correlate shallowing-upward hemicycles with keep-up to catch-up depositional cycles with signature of apparent sea level fall. The control of relative base level variation in Narji depositional history is strengthened with multiple signatures including secular δ13C composition, total organic carbon (TOC) content and siliciclastic content documented from the entire Narji litho-succession.

Ediacaran stratigraphy and paleogeography in the north Yangtze Block, South China

The Ediacaran gas exploration of the Sichuan Basin in the northwest Yangtze Block, South China, indicates that the paleogeography and lithofacies are of great significance for understanding sedimentary environments and further exploration. There are several contrarian views, however, regarding the Ediacaran paleogeographic configuration and lithofacies distribution in the north Yangtze Block. In this study, we systematically analyzed the Ediacaran stratigraphic, paleogeographic, and facies distributions by integrating outcrop observation, well correlation and seismic interpretation. The Ediacaran in the Micangshan-Hannan terrane is dominated by progressive onlap successions from west to east. We suggest that during the Ediacaran an overall paleogeographic configuration evolved from a shallow-water platform to deeper slope-basin and finally into an open ocean (i.e., the Proto-Tethys ocean) in the north Yangtze Block. The platform margin is roughly distributed along the north margin of the Micangshan-Hannan terrane and the present-day Chengkou fault. During the Doushantuo time, the paleogeographic configuration was characterized by a distally steepened ramp passing seaward into slope-basin facies. During the Dengying time, a rimmed carbonate platform developed on the Doushantuo Formation ramp. The platform margin facies predominantly contained microbially laminated dolomites and stromatolite reef complex. The slope-basin facies were dominated by thin-bedded and slump dolomites. The Ediacaran of the north Yangtze Block experienced a sedimentary evolution from a mixed carbonate-siliciclastic ramp of the Doushantuo Formation to a rimmed carbonate platform of the Dengying Formation, primarily controlled by tectonic subsidence differences and sea level changes.

Mechanism of formation and Characteristics of deposition of Cambrian gravity flow in the eastern gucheng area, Tarim Basin

The Gucheng Cambrian margin and Luoxi Cambrian margin is on both sides of eastern Tarim basin. Base on the analysis of cores,thin sections and seismic data of Luoxi slope areas,this paper systematically studies the formation condition, reservoir characteristics and favorable accumulation factors of gravity flow. The gravity flow sedimentary types include debris flow, high density flow and low density flow, debris flow and high density flow reservoir is better. Hydrothermal reconstruction has active effect on reservoir. There are six phases mound-beaches develop in platform margin, reservoir characteristics are algae framework dolomite and granular dolomite. Phases of mound-beaches develop to steep slope. Seismic and geochemical data indicate that the mound-beaches are exposed with corrosion characteristics,the platform margin-slope have the requirements of geomorphologie antique and quality reservoir source.

Mechanisms controlling the localisation of fault‐controlled hydrothermal dolomitisation, Derbyshire Platform, UK

AbstractThe Derbyshire Platform is a Mississippian aged flat‐topped, steep sided platform that forms the westernmost expression of the Derbyshire‐East Midlands Platform. On the south‐east platform margin, 60 km2 of Visean limestone has been dolomitised, forming two distinct bodies. One of these bodies forms along a major NW–SE trending basement fault and smaller, associated, N–S trending faults and fractures. This study uses outcrop, petrographic and geochemical analysis to better constrain the timing and mechanism for this fault‐controlled dolomitisation. Field relationships demonstrate dolomitisation was multi‐phase and initiated after the main phase of matrix pore‐occluding calcite cementation on the Derbyshire Platform and terminated prior to the main phase of mineralisation. Fluids are interpreted to have fluxed from adjacent basins, primarily along strike‐slip crustal faults that were reactivated during basin inversion at the onset of the Variscan Orogeny. Fluid supply was episodic and progressively confined to fractures as matrix porosity became occluded. The study demonstrates the complex interplay between basin kinematics, host rock permeability and timing of fluid supply through seismic valving along faults that connect the carbonate platform to basin compartments. This ultimately controlled the position of dolomite geobodies along faults and provides a record of fluid flow during the transition from thermal subsidence to post‐rift basin inversion. The findings have implications for the exploration of both minerals and hydrocarbon within dolomitised host rocks and can inform studies of fluid transfer and reaction on carbonate platforms within the burial realm.

Carbonate microfacies classification model based on dual neural network: a case study on the fourth member of the upper Ediacaran Dengying Formation in the Moxi gas field, Central Sichuan Basin

The dual neural network (DNNW) model combines two neural networks to imitate artificial sedimentary facies division by learning the characteristics of multi-type logging curves corresponding to the sedimentary microfacies of a coring section. The model predicts many non-coring wells in the research zone through logging data. After comparing the classification performance of a fully connected neural network (FCNWW) and AutoML when dealing with three FNDA, EALS, and HRA datasets, the DNNW shows high stability and assists exploration in the Moxi gas field. Four sedimentary microfacies are identified through thin section observation, including thrombolite boundstone (MF1), laminated stromatolite boundstone (MF2), siliceous laminar boundstone (MF3), and micritic dolostone (MF4). Results suggest the fourth member of Dengying Formation in the Moxi gas field is carbonate platform facies deposition: specifically, restricted platform and platform margin.

The soft-sediment deformation structures and the siliceous concretion presence as indicators of the depositional processes during Middle Paleocene to Middle Eocene in the Apulian Platform Margins in Kefalonia Island, Greece

Kefalonia Island is situated in western Greece and geologically corresponds to the Apulian Platform Margins (APM), with the Apulian Platform lying to the west and the Ionian Basin to the east. The studied deposits, with a thickness up to 164.5, extend along the 650 m long Agia Efimia bay, ranging from the Middle Paleocene (Selandian) to the uppermost Eocene (Priabonian), during the latest part of the rift stage. Fieldwork measurements of soft-sediment deformation structures (SSDS), paleocurrent directions, the size and abundance of siliceous concretions (SC) and siliceous beds (SB), and shale horizons were correlated with the age of selected samples, and thus, the relation of the tectonic regime with the depositional conditions was presented. As SSDS could represent an indicator of tectonic activity and instability of the basin floor instability, the size and the abundance of the SSDS whereas used to identify the parameters that produced the basin floor instability like the intensity of tectonic activity. The studied section, with a general NE-SW orientation, was subdivided into five (5) different units. The changes of these units in paleocurrent directions, the size, the abundance, and the thickness of SSDS, SC, and SB and the presence of shale horizons in the upper unit, characterize a gradual change of the depositional conditions in the APM, from the Upper Paleocene (Selandian) to the Upper Eocene (Priabonian). The factor that influenced the abovementioned changes was related with the presence and the activity of normal faults that produced instability of the basin floor. Therefore, a reduction in tectonic activity from the Paleocene to the Eocene is suggested.