Origin of dolomite in the Middle Ordovician peritidal platform carbonates in the northern Ordos Basin, western China

The carbonates in the Middle Ordovician Ma_5～5submember of the Majiagou Formation in the northern Ordos Basin are partially to completely dolomitized.Two types of replacive dolomite are distinguished：（1） type 1dolomite,which is primarily characterized by microcrystalline（〈30 urn）,euhedral to subhedral dolomite crystals,and is generally laminated and associated with gypsumbearing microcrystalline dolomite,and（2） type 2 dolomite,which is composed primarily of finely crystalline（30-100 urn）,regular crystal plane,euhedral to subhedral dolomite.The type 2 dolomite crystals are truncated by stylolites,indicating that the type 2 dolomite most likely predated or developed simultaneously with the formation of the stylolites.Stratigraphic,petrographic,and geochemical data indicate that the type 1 dolomite formed from near-surface,low-temperature,and slightly evaporated seawater and that the dolomitizing fluids may have been driven by density differences and elevation-related hydraulic head.The absence of massive depositional evaporites in the dolomitized intervals indicates that dolomitization was driven by the reflux of slightly evaporated seawater.The δ～（18）O values（-7.5 to-6.1 ‰） of type1 dolomite are slightly lower than those of seawaterderived dolomite,suggesting that the dolomite may be related to the recrystallization of dolomite at higher temperatures during burial.The type 2 dolomite has lowerδ～（18）O values（-8.5 to-6.7 ‰） and Sr～（2＋） concentration and slightly higher Na～＋,Fe～（2＋）,and Mn～（2＋） concentrations and～（87）Sr/～（86）Sr ratios（0.709188-0.709485） than type 1 dolomite,suggesting that the type 2 dolomite precipitated from modified seawater and dolomitic fluids in pore water and that it developed at slightly higher temperatures as a result of shallow burial.

Sedimentary build-ups of pre-salt isolated carbonate platforms and formation of deep-water giant oil fields in Santos Basin,Brazil

In response to the problems of unclear distribution of deep-water pre-salt carbonate reservoirs and formation conditions of large oil fields in the Santos passive continental margin basin,based on comprehensive utilization of geological,seismic,and core data,and reconstruction of Early Cretaceous prototype basin and lithofacies paleogeography,it is proposed for the first time that the construction of pre-salt carbonate build-ups was controlled by two types of isolated platforms:inter-depression fault-uplift and intra-depression fault-high.The inter-depression fault-uplift isolated platforms are distributed on the present-day pre-salt uplifted zones between depressions,and are built on half-and fault-horst blocks that were inherited and developed in the early intra-continental and inter-continental rift stages.The late intra-continental rift coquinas of the ITP Formation and the early inter-continental rift microbial limestones of the BVE Formation are continuously constructed;intra-depression fault-high isolated platforms are distributed in the current pre-salt depression zones,built on the uplifted zones formed by volcanic rock build-ups in the early prototype stage of intra-continental rifts,and only the BVE microbial limestones are developed.Both types of limestones formed into mound-shoal bodies,that have the characteristics of large reservoir thickness and good physical properties.Based on the dissection of large pre-salt oil fields discovered in the Santos Basin,it has been found that both types of platforms could form large-scale combined structural-stratigraphic traps,surrounded by high-quality lacustrine and lagoon source rocks at the periphery,and efficiently sealed by thick high-quality evaporite rocks above,forming the optimal combination of source,reservoir and cap in the form of“lower generation,middle storage,and upper cap”,with a high degree of oil and gas enrichment.It has been found that the large oil fields are all bottom water massive oil fields with a unified pressure system,and they are all filled to the spill-point.The future exploration is recommended to focus on the inter-depression fault-uplift isolated platforms in the western uplift zone and the southern section of eastern uplift zones,as well as intra-depression fault-high isolated platforms in the central depression zone.The result not only provides an important basis for the advanced selection of potential play fairways,bidding of new blocks,and deployment of awarded exploration blocks in the Santos Basin,but also provides a reference for the global selection of deep-water exploration blocks in passive continental margin basins.

Effects of Intracratonic Strike-slip Fault on the Differentiation of Carbonate Microfacies: A Case Study of a Permian Platform Margin in the Sichuan Basin(SW China)

In intracratnoic basins, the effect of strike-slip faults on sedimentary microfacies is generally underestimated due to their small scale. Based on the integration of core, well logs, and three-dimensional seismic data, this study presents a comprehensive analysis of the Permian carbonate platform and strike-slip faults in the southwestern Kaijiang-Liangping trough of the Sichuan Basin. The relationship between strike-slip faults and Permian carbonate microfacies is investigated. The results reveals the existence of a NW-trending strike-slip fault zone along the platform margin, exhibiting clear segmentation. The western side of the study area exhibits a rimmed platform margin characterized by type I reefs, which corresponds to the presence of a large-scale strike-slip fault zone. In contrast, the eastern side is characterized by a norimmed and weak rimmed platform margin, accompanied by type II reefs, which align with smaller strike-slip fault zones. It was found that the strike-slip fault had some effects on the platform and reef-shoal complex of the Permain Changxing Formation. First, the platform was divided by strike-slip fault into three segments to show rimmed, week rimmed and norimmed platform. Second, reef-shoal complex devolped along the faulted high position in the strike-slip fault zone, and separated by faulted depression. Third, strike-slip faults can offset or migrated the reef-shoal complex and platform margin. Additionally, the thickness of the platform margin varies across strike-slip fault zone, which is related to the activity of strike-slip faults. The strike-slip faults affect the microfacies by controlling the pre-depositional paleotopography. This case suggests that the strike-slip faults play a crucial role in the diversity and distribution of carbonate microfacies in the intracratonic basin.

Corpus of Carbonate Platforms with Lexical Annotations for Named Entity Recognition

An obviously challenging problem in named entity recognition is the construction of the kind data set of entities.Although some research has been conducted on entity database construction,the majority of them are directed at Wikipedia or the minority at structured entities such as people,locations and organizational nouns in the news.This paper focuses on the identification of scientific entities in carbonate platforms in English literature,using the example of carbonate platforms in sedimentology.Firstly,based on the fact that the reasons for writing literature in key disciplines are likely to be provided by multidisciplinary experts,this paper designs a literature content extraction method that allows dealing with complex text structures.Secondly,based on the literature extraction content,we formalize the entity extraction task(lexicon and lexical-based entity extraction)for entity extraction.Furthermore,for testing the accuracy of entity extraction,three currently popular recognition methods are chosen to perform entity detection in this paper.Experiments show that the entity data set provided by the lexicon and lexical-based entity extraction method is of significant assistance for the named entity recognition task.This study presents a pilot study of entity extraction,which involves the use of a complex structure and specialized literature on carbonate platforms in English.

Depositional architecture of the late Ordovician drowned carbonate platform margin and its responses to sea-level fluctuation in the northern slope of the Tazhong region, Tarim Basin

The Tazhong Uplift of the late Ordovician is a drowned rimmed carbonate platform. The carbonate rock of the late Ordovician Lianglitage Formation in the northern slope of the Tazhong region is one of the significant petroliferous intervals. Based on petrofacies, depositional cycles, natural gammaray spectrometry and carbon/oxygen isotope data from the Lianglitage Formation, one 2nd-order, three 3rd-order and several 4th-order sequences have been recognized, and the late Ordovician relative sealevel fluctuation curve has been established. The sequences O3 1-1 and O3 1-2 on the platform are composed of highstand and transgressive systems tracts, but lack the lowstand systems tract. The sequence O3 1-3 is a drowning sequence. The sequence O3 1-1 overlapped the eroded slope and pinched out to the northwest and landward. The highstand systems tract in the sequence O3 1-2 consists of low-angle sigmoid and high-angle shingled progradation configuration. Major sedimentary facies of the Lianglitage Formation include reef and shoal in the platform margin and lagoon, which can be subdivided into coral-sponge-stromatoporoid reef complex, sand shoal, lime mud mound, and intershoal sea. Reefs, sand shoals and their complex are potential reservoir facies. The reefs and sand shoals in the sequence O3 1-1 developed in the upper of its highstand systems tract. In the sequence O3 1-2, the highstand systems tract with an internal prograding configuration is a response to the lateral shifting of the complex of reef and sand shoal. The transgressive systems tract, in particular the sand shoals, developed widely on the slope of the platform margin and interior. The reefs in the sequence O3 1-3 migrated towards high positions and formed retrograding reefs in the western platform and low relief in the platform interior. Basinward lateral migration of the reefs and pure carbonate rock both characterize highstand systems tract and show that the rise of the relative sea-level was very slow. Shingled prograding stacking pattern of the 4th-order sequences and reefs grow horizontally, which represents the late stage of highstand systems tract and implies relative sealevel stillstand. Reefs migrating towards high land and impure carbonate rock both indicate transgressive systems tract and suggest that the relative sea-level rose fast. Erosional truncation and epidiagenetic karstification represent a falling relative sea-level. The relative sea-level fluctuation and antecedent palaeotopography control the development and distribution of reef complexes and unconformity karst zones. Currently, the composite zone of epidiagenetic karstic intervals and high-energy complexes of reefs and sand shoals with prograding configuration is an important oil and gas reservoir in the northern slope of the Tazhong carbonate platform.

Comparison of basic features and origins of oolitic shoal reservoirs between carbonate platform interior and platform margin locations in the Lower Triassic Feixianguan Formation of the Sichuan Basin,southwest China

The oolitic shoal reservoirs of the Lower Triassic Feixianguan Formation carbonates in the Sichuan Basin of southwest China are an important target for gas exploration in the basin.Their occurrence,like other cases worldwide,can be divided into two locations in general,i.e.,platform interior and platform margin locations.Their differences of reservoir features and origins,however,have not been investigated comprehensively due to different exploration degrees.This issue is addressed in this paper,to provide basic data and information for the basin＇s hydrocarbon exploration and for the study of carbonate platform sedimentology and reservoir geology worldwide.We compared the features of these two types of reservoirs in detail,including the depositional and diagenetic features,pore types and petrophysical features.Based on the comparison,the origin of the reservoirs was further discussed.It is shown that the reservoirs in platform interior and platform margin locations differ significantly.The interior carbonates were deposited in moderate to high energy settings and the dominant lithologic type was limestone,which was weakly compacted and intensely cemented and has undergone meteoric dissolution.Pore types include intragranular dissolution and moldic pores,with low porosities（6%） and low permeabilities（0.1 mD）.By contrast,the platform margin carbonates were deposited in relatively high energy settings and mainly consisted of dolostones with some limestones.The rocks were strongly compacted but incompletely cemented.As a result,some primary intergranular pores were preserved.Both meteoric solution and burial solution have taken place.There are various types of pore spaces including intergranular and intercrystalline solution pores and residual intergranular pores.This type of reservoir generally has better petrophysical properties（9% porosity and 0.1 mD permeability） and pore-throat structures than the interior reservoirs.These differences were influenced by both primary depositional features and secondary diagenesis.For the interior carbonate reservoirs,early meteoric dissolution,weak compaction and strong cementation are important controlling factors.By contrast,the factors controlling the formation of the margin carbonate reservoirs mainly include dolomitization,preservation of primary pores and burial dissolution.

Seismic Characteristics and Development Patterns of Miocene Carbonate Platform in the Beikang Basin,Southern South China Sea

The Beikang Basin is located in the southern part of the South China Sea(SCS),which is one of most tectonically complex sea areas.It is a deepwater sedimentary basin that was mainly deposited during the Cenozoic era.Owing to data restrictions,the research on carbonate platforms of this area is still in its infancy.High-resolution seismic data are analyzed to identify the Miocene carbonate platforms and reconstruct the architecture and growth history.The carbonate platforms of Beikang Basin began to develop in the Late Oligocene-Early Miocene,were extended in the Middle Miocene,and declined in the Late Miocene.The carbonate platform mainly developed during two periods:the Oligocene to the Early Miocene,and the Middle Miocene.The carbonate platforms that developed in the Middle Miocene were the most prosperous.The Middle Miocene carbonate platform in the Beikang Basin can be divided into three stages.In the first stage,the platforms had wide range which were thin.During the second stage,the platforms had a smaller range that was controlled by faults.In the third stage,the platforms were gradually submerged.The platform structure developed in the Middle Miocene at the Beikang Basin was controlled by the rate of rising/falling of the sea level and the carbonate growth rate.Based on an analysis of these changes and relationship,the platform can be divided into several patterns:retrogradation,submerged,aggradation,progradation,outward with up-stepping,outward with down-stepping,and down-stepping platforms.At the top of the carbonate platforms in the Beikang Basin a set of carbonate wings or mushrooms usually appeared.These were formed during a period of relative sea-level decline.It is believed that the Miocene carbonate platforms in the Beikang Basin are mainly controlled by tectonic and sedimentary environments,and are also affected by terrestrial detritus.

Distribution of Chancelloriids in a Middle Cambrian Carbonate Platform Deposit, Taebaek Group, Korea

The onset of the Cambrian witnessed the diversification of ＂small shelly fossils （SSF）＂, which affected carbonate depositional system. One of the problematic SSF, chancelloriids, are common components of the early to middle Cambrian carbonate and shale, and their contributions toward Cambrian carbonates are not yet fully understood. This study assesses distribution patterns of chancelloriid sclerites in the Cambrian Series 3 Daegi Formation based on microfacies analysis and discusses their sedimentologic implications. In the lower part of the formation, partially articulated chancelloriid sclerites occur mainly in bioclastic packstone and grainstone facies, with isolated sclerite rays in nodular packstone to grainstone facies. In the middle part of the formation, chancelloriid fragments occur only sporadically in bioclastic wackestone to packstone, bioclastic grainstone and oolitic packstone to grainstone facies, whereas boundstone facies are nearly devoid of their fragments. There are no chancelloriid fragments in the upper part of the formation, which consists of oolitic packstone to grainstone facies. Chancelloriids are interpreted to have primarily occupied platform margin shoal environments, shedding their sclerites to surrounding areas, and thus contributed as sediment producers. The distribution of Daegi chancelloriid sclerites is similar to other Cambrian examples, with the exception of common chancelloriids in Cambrian Series 2 reefs and their apparent near absence in the Daegi and other Cambrian Series 3 reefs. This disparity resulted from changes in the ecologic niche of chancelloriids after the end-Cambrian Series 2 reef crisis, coupled with an overall decline of chancelloriids in the middle Cambrian.

Characteristics and distribution patterns of reef complexes on the carbonate platform margin in deep water areas:the western South China Sea

As a potential oil and gas reservoir, reef complexes have been a research focus from petroleum geologists for a long time. There are favorable conditions for the development of reef complexes in the South China Sea; however, their internal structures, evolution and distribution are still poorly understood. Based on 2D and 3D seismic data, the internal structures and evolution patterns of the reef complexes on the carbonate platform margin in the deep water areas over the western South China Sea were studied in detail. The result shows that two types of reef complexes, i.e., fault controlling platform margin reef complexes and ramp reef complexes have been developed in the study area. The reef complexes have independent or continuous mound or lenticular seismic reflections, with three internal structures （i.e., aggrading, prograding and retrograding structures）. There are different growth rates during the evolution of the reef complexes, resulting in the formation of catch-up reefs, keep-up reefs and quick step reefs. The study also reveals that different platform margin reef complexes have different internal structures and distributions, because of the different platform types. These results may be applied to the exploration and prediction of carbonate platform margin reef complexes in other areas that are similar to the study area.

Distribution,Seismic Characteristics and Controlling Factors of Carbonate Platforms on the Northwestern Margin of the South China Sea

Newly-acquired seismic data reveal widespread carbonate deposits covering a large part of the northwestern South China Sea margin.Three carbonate platforms are identified to have developed on the topographic highs inherited from tectonic deformation and volcanic accretion.Across the carbonate platforms,the Miocene strata are characterized by high-amplitude seismic reflections and distinct platform architecture that overlaps older strata.The Guangle and Xisha carbonate platforms grew on faulted blocks due to South China Sea continental rifting,while the Zhongjian carbonate platform developed on a structural high induced by early Miocene volcanism.During the late Miocene,partial drowning resulted in the inhibition of platform growth,eventual platform drowning and termination of most carbonate deposition.The drowning of the Guangle and Zhongjian carbonate platforms is shown by the supply of siliciclastic sediments during the late Miocene and seems to be closely linked to late Neogene volcanic activity,whilst the drowning of the Xisha carbonate platform is primarily related to relative eustatic changes.Our results imply that tectonic activity,volcanism and eustasy are the dominant controls on the evolution of carbonate platforms on the northwestern margin of the South China Sea.

Development of the Miocene Guangle Carbonate Platform in the South China Sea:Architecture and Controlling Factors

This study investigates the evolution of the Miocene Guangle carbonate platform(or Triton Horst)of the northwestern South China Sea margin.The platform is located at a junction area surrounded by Yinggehai basin,Qiongdongnan basin and Zhongjiannan basin.Well and regional geophysical data allow the identification of the morphologic and stratigraphic patterns.The Guangle carbonate platform was initiated on a tectonic uplift during the Early Miocene.The early platform was limited at Mesozoic granitic basement,pre-Paleogene sediments localized tectonic uplift and was small extension at the beginning stage.While during the Middle Miocene,the carbonate buildup flourished,and grow a thrived and thick carbonate succession overlining the whole Guangle Uplift.The isolated platforms then united afterward and covered an extensive area of several tens of thousands of square kilometers.However,it terminated in the Late Miocene.What are the control factors on the initiation,growth and demise of the Guangle carbonate platform?The onset of widespread carbonate deposits largely reflected the Early Miocene transgression linked with early post-rift subsidence and the opening of the South China Sea.Stressed carbonate growth conditions on the Guangle carbonate platform probably resulted from increased inorganic nutrient input derived from the adjacent uplifted mainland,possibly enhanced by deteriorated climatic conditions promoting platform drowning.Therefore,tectonics and terrigenous input could be two main controlling factors on the development of the Guangle carbonate platforms and main evolution stages.

Types,structural evolution difference and petroleum geological significance of Cambrian-Ordovician carbonate platforms in Gucheng-Xiaotang area,Tarim Basin,NW China

Based on the latest drilling cores,thin sections,and 3D seismic data,types,features,and evolution processes of Cambrian-Ordovician carbonate platforms in the Gucheng and Xiaotang areas are studied,and favorable exploration zones in this area are also discussed.There are two types of carbonate platforms developed in the Cambrian-Ordovician in the Gucheng-Xiaotang area,namely,carbonate ramp and rimmed platforms,and the evolution process of the platform in the Gucheng area is different from that in Xiaotang area.In the Early Cambrian,the study area was a homoclinal carbonate ramp.During the Middle to Late Cambrian,it evolved into a rimmed platform,with 5 phases of mound shoals developed.In the southern area,mound shoals were in progradational arrangement and the third and fourth stages of mound shoals suffering exposure and further developed abundant karst vugs.In the northern area,the mound shoals appeared in a superposition of aggradation-weak progradation,the third,fourth,and fifth stages of mound shoals suffered exposure and dissolution,and the platform slope developed gravity flow deposits.In the Early to Middle Ordovician,the southern area gradually evolved into a distally steepened carbonate ramp,where retrogradational dolomitic shoal developed;while the northern part experienced an evolution process from a weakly rimmed platform to a distally steepened carbonate ramp,and developed two or three stages of retrogradational mound shoals.The high-frequency oscillation of sea level and local exposure and dissolution were beneficial to the formation of mound or shoal reservoirs in platform margin and ramp,and the configuration of these reservoirs with low energy slope-basin facies source rocks could form good oil-gas enrichment zones.The dolomitic shoal in the Ordovician platform ramp is the practical exploration field for increasing reserve and production in the Gucheng area.The mound shoal at the Cambrian rimmed platform margin is the key exploration object in the Xiaotang area.In addition,the Cambrian slope gravity flow deposits can be taken as the favorable exploration fields in the study area.

Build-ups and hydrocarbon accumulation of the isolated carbonate platforms in the eastern Mediterranean

Based on regional geology,2D seismic and ocean drilling data,the formation of the Eratosthenes Seamount(ESM)and its surrounding isolated platforms,types of organic reefs and hydrocarbon accumulation conditions in the eastern Mediterranean were analyzed through fine tectonic interpretation and seismic facies study,and the future exploration targets were pointed out.The formation and evolution of the ESM and its peripheral isolated platforms are highly related to the open and close of the Neotethyan ocean.The precursors of the ESM and its peripheral isolated platforms are both horst-type fault blocks formed in the Middle Triassic-Early Jurassic intracontinental rift stage.The ESM and its peripheral isolated platforms underwent continued and inherited carbonate build-ups during the Middle Jurassic intercontinental rift stage,the Late Jurassic-Late Cretaceous Turonian passive drift stage,and Late Cretaceous Senonian-Miocene subduction stage,as well as medium-slight inversion transformation beginning in the Late Miocene Messinian caused by the closure of the Neotethyan ocean.Three types of isolated platforms formed controlled by variant paleo-tectonic settings:the first type is composed of a single patch-like reef controlled by a small-scale and narrow horst-type fault block,the second type consists of a single atoll controlled by a middle-scale and wide horst-type fault block,and the third type is comprised of multiple reef-beach complexes controlled by a large-scale and broad paleo-high.The first two types universally developed in the highs of the alternate sag-uplift structural zones on the south and west of Eratosthenes,and the third type only developed in the ESM.As a result of fluctuation of sea level,two sequences of reef build-ups,i.e.the Middle Jurassic Bajocian-Upper Cretaceous Turonian and the Miocene,developed in the ESM as well as the highs in the alternate sag-uplift structural zones on the south and west of Eratosthenes.Drillings have confirmed that the first two types of isolated platforms with a single patch-like reef and a single circle-like reef have good conditions for natural gas accumulations.The isolated platform of reef-beach complexes in the ESM also has accumulation potentials of natural gas and is worth prospecting.

Carbonate Platform Margin Slope Characteristics of Cambrian Chefu Age in Baojing-Fenghuang and Adjacent Areas,Western Hunan,China

The Cambrian paleogeography in south Ch ina can be divided into Yangtze platform, platform margin slope (including upper slo pe and lower slope) and basin. The carbonate gravity deposits are well developed there, particularly in the Yangtze platform adjacent areas. This paper mainly d eals with declivity fan characters of the Chefu age in Fenghuang adjacent areas, western Hunan Province. According to their distribution and thickness, three ma in declivity fans have been divided in the study areas, namely, Dama (∈c dsf), Machong (∈c msf) and Huangheyuan (∈c hsf) declivity fans. Each fan's charac ters are described in detail in this paper. Based on their distribution range, s cale and fossiliferous layers, two peak periods (fair developmental period) are distinguished: (1) Linguagnostus reconditus Zone (time), and (2) Glyptagno stus reticulatus Zone (time). They were characterized by huge thick limestone breccia layers (single layer 3-4 m thick) and closely spaced spread. Besides, th ere are also two developmental periods: (1) Lejopyge laevigata Proagnostus b ulbus Zone (time), and (2) Agnostus inexpectans Proceratopyge protracta Z one (time) to Erixanium Zone (time) to Corynexochus plumula Sinoprocerato pyge cf. kiangshanensis Zone (time), while Glyptagnostus stolidotus Zo ne (time) was an interstitial period (without or rare limestone breccias). All t hese features may verify the relative movement of the earth crust and paleoclima te variety of the Chefu age in the study areas.

Geophysical Evidence for Carbonate Platform Periphery Gravity Flows in the Xisha Islands,South China Sea

High-resolution bathymetric and partial seismic data were utilized to determine the characteristics and distribution of mass-transport deposits(MTDs)in the Xisha area,North of the South China Sea.Many isolated carbonate platforms are found in this area,providing mass-wasting materials as a sediment source for MTDs.The MTDs in the study area were divided into three types(slide,slump,and debris flow)based on their rheological properties.The MTDs were mostly distributed at the toe of the carbonate platform slope,where a large amount of carbonate debris accumulated.A total of 32 MTDs were identified on the carbonate platform slopes.A systematic assessment of the MTD morphometric parameters showed that their total area covered 2225.2 km2,with the largest MTD covering 354.1 km2.These MTDs differ largely from the MTDs of other areas,especially in terms of sediment compositions and deposit processes.Gullies and channels on the slope and reefs on the top of the platform act as sediment conduits and carbonate-producing sources,respectively,playing vital roles in the distribution and generation of gravity flow deposits.The development model proposed in this work states that reefs disintegrate into carbonate debris and mix with seawater to form gravity flows,which scour the seafloor and accumulate at the downslope,eventually resulting in failure.Sediments from these failures turn into gravity flows,flowing into submarine canyons and channels.The results of this research further our understanding of the development pattern of MTDs in a carbonate setting.

The Role of Bottom Currents on the Morphological Development Around a Drowned Carbonate Platform,NW South China Sea

The seafloor around carbonate platforms is largely shaped and modified by downslope processes.However,the role of alongslope processes,including bottom currents,on the morphological development of carbonate platforms remains poorly understood.Here,we use high-resolution multibeam bathymetric data and two-dimensional seismic profiles to investigate the detailed sea-floor morphology around the Zhongjianbei carbonate platform(ZCP)in the northwest South China Sea.A series of depositional bodies and erosional channels are identified to the south of the ZCP and are interpreted as contourite drifts and channels resulted from the interaction between bottom currents and bathymetric features.In addition,active fluid seepages have led to the formation of widespread pockmarks on the seafloor.Importantly,the contourite channels and widespread pockmarks also show a close relationship in their distribution.We propose that the contourite channels around the ZCP are evolved from the coalescence of pockmarks under the persistent erosion of bottom currents.Based on the morphological analysis,we reconstruct the past bottom-current pathways around the ZCP that are parallel to the platform slopes and heading to the south.This study provides new insights into the formation of complex bathymetry and helps understanding how bottom currents and active fluid seepages can influence the morphological development around carbonate platforms.

Thalassinoides Ichnofabrics of Lower Cambrian Longwangmiao Formation(Stage 4,Toyonian)on the Yangtze Platform,South China:Improving Paleoenvironmental Interpretations

An analysis of Thalassinoides ichnofabrics in the Cambrian Stage 4 Longwangmiao Formation from six sections near Chonqing in the Middle and Upper Yangtze Block of South China was conducted to promote the understanding of the paleoenvironment of this period.Thalassinoides ichnofabrics are divided into three types according to their morphology,bioturbation index,abundance and related parameters:banded,mottled and grid.The completeness of the ichnofabrics gradually increases from banded to grid,the scale of the burrows expands,and the structure transitions from twodimensional to three-dimensional.Malacostracans,phyllocarids and enteropneusts are the most likely burrowers.The findings reveal that frequent changes in paleoenvironmental conditions are the dominant factors for generating the various Thalassinoides ichnofabrics.The changes in depositional rate,substrate properties and nutrient levels affect the behavior and building patterns of the burrowers,thus leading to the diversity of Thalassinoides ichnofabrics.In the lower Longwangmiao Formation beds,many incomplete Thalassinoides burrow systems were built in the substrates with terrigenous detrital minerals,revealing significant environmental pressure on the burrowers.Later,the Thalassinoides burrows went into complete three-dimensional boxworks.At the end of the stage,bioturbation almost disappeared,which indicates that burrowers were finding it difficult to survive in the harsh environment.

Combination and distribution of reservoir space in complex carbonate rocks

This paper discusses the reservoir space in carbonate rocks in terms of types,combination features,distribution regularity,and controlling factors,based on core observations and tests of the North Truva Oilfield,Caspian Basin.According to the reservoir space combinations,carbonate reservoirs can be divided into four types,i.e.,pore,fracture-pore,pore-cavity-fracture,and pore-cavity.Formation and distribution of these reservoirs is strongly controlled by deposition,diagenesis,and tectonism.In evaporated platform and restricted platform facies,the reservoirs are predominately affected by meteoric fresh water leaching in the supergene-para-syngenetic period and by uplifting and erosion in the late stage,making both platform facies contain all the above-mentioned four types of reservoirs,with various pores,such as dissolved cavities and dissolved fractures,or structural fractures occasionally in favorable structural locations.In open platform facies,the reservoirs deposited continuously in deeper water,in an environment of alternative high-energy shoals（where pore-fracture-type reservoirs are dominant） and low-energy shoals（where pore reservoirs are dominant）.

Sedimentary characteristics and depositional evolution of carbonate platform during the Cambrian and Ordovician in eastern Tarim Basin,NW China

The eastern Tarim Basin(Tadong Area)has gained wide attentions on large-scale marine carbonate reservoirs in Cambrian-Ordovician due to significant hydrocarbon discoveries.A systematic analysis combining thin sections,cores,wireline logs,and seismic data is conducted on Cambrian-Ordovician carbonate platform in the whole eastern Tarim Basin,including Gucheng area,Majiaer area,and western Luobopo rise(Luoxi area).The results show that 8 sub-facies and more than 10 microfacies are developed including open platform,restricted/semi-restricted platform,reef-shoal around platform margin,drowned platform,foreslope,neritic platform,and deep-water basin.As both key areas for hosting petroleum reserves during the Cambrian and Ordovician,the Luoxi area is dominated by deep-water basin facies,while the Gucheng area is dominated by neritic platform facies and deep-water basin facies during the Lower Cambrian.The deposition evolution during the whole Cambrian is dominated by slope facies and deep-water facies,platform margin facies,and platform facies.In contrast,it is dominated by open platform facies during the whole Ordovician.The depositional evolution of carbonate platform is mainly controlled by paleo-geomorphology and sea-level changes.The distribution of paleo-geomorphologic units plays an important role in controlling types and distributions of carbonate platform facies.The transgression assists in growth of reef-shoal complex and lime mud mound in the Early Ordovician.However,with neritic platform and slope being to disappeared,in the Middle Ordovician,platform margin facies are well developed in Gucheng Area.Platform facies and deepwater basin facies are widely distributed.Finally,carbonate platform is drowned due to sea level rising in the Late Ordovician.The depositional evolution of carbonate platform coinciding falling and rising of sea-level changes can be beneficial for appropriate carbonate reservoirs identification and petroleum exploration.

Analysis and Characterization of Pore System and Grain Sizes of Carbonate Rocks from Southern Lebanon

Carbonate rocks are common in many parts of the world, including the Eastern Mediterranean, where they host significant groundwater supplies and are widely used as building and ornamental stones. The porosity of carbonate rocks plays a critical role in fluid storage and retrieval. The pore structure connectivity, in particular, controls many properties of geological formations, as well as the relationships between the properties of individual minerals and the bulk properties of the rock. To study the relationships between porosity, rock properties, pore structure, pore size, and their impact on reservoir characteristics, 46 carbonate rock samples were collected from four stratigraphic sections exposed near Sidon, South Lebanon. The studied carbonate rocks are related to marine deposits of different ages(e.g., Upper Cretaceous, Eocene, and Upper Miocene). In order to understand the pore connectivity, the MICP(mercury injection capillary pressure) technique was conducted on ten representative samples. Results from the SEM analysis indicate the dominance of very fine and fine pore sizes, with various categories ranging in diameter from 0.1 to10 μm. The MICP data revealed that the pore throat radii vary widely from 0.001 to 1.4 μm, and that all samples are dominated by micropore throats. The grain size analysis indicated that the studied rocks have significant amounts of silt-and clay-size grains with respect to the coarser ‘sand-size’ particles, suggesting a high proportion of microporosity. Obtained results, such as the poorly-sorted nature of grains, high microporosity, and the high percentage of micropore throats, justify the observed low mean hydraulic radius, the high entry pressure, and the very low permeability of the studied samples. These results suggest that the carbonate rocks near Sidon(south of Lebanon) are possibly classified as non-reservoir facies.