Characteristics and impacts on favorable reservoirs of carbonate ramp microfacies: A case study of the Middle-Lower Ordovician in Gucheng area,Tarim Basin,NW China

Based on the latest drilling core, thin section, 3D seismic, well logging data as well as exploration results, the sequence stratigraphy and sedimentary microfacies of the Middle-Lower Ordovician carbonates in Gucheng area, and their controlling effects on the development of reservoir were examined by the theories and methods of fine carbonate sedimentology.The results show that the Middle-Lower Ordovician in Gucheng area is a set of typical carbonate ramp deposits, which can be divided into 10 microfacies in 4 subfacies as follows: back ramp, inner shallow ramp, outer shallow ramp, and deep ramp. The back ramp subfacies consists of muddy-dolomitic flat and dolomitic lagoon microfacies;it is dominated by lamellar micrite dolomite tight in lithology. The inner shallow ramp subfacies includes dolomitic shoal and dolomitic flat in shoal top and dolomitic flat between shoals microfacies;it is mainly composed of crystal dolomite with metasomatic residual structure, as well as abundant karst vugs and intercrystalline pores. The crystal dolomite has an average porosity of 4.36%. The outer ramp subfacies includes medium-high and low energy grain shoal and inter-shoal microfacies;it is dominated by sandy limestone,oolitic limestone, and micrite limestone with few pores. The deep ramp subfacies is dominated by low-energy argillaceous deposits, with local presences of storm shoal microfacies. The Lower-Middle Ordovician has six third-order sequences from bottom to top, among which sequence(SQ3) represents the third member of Yingying Formation. The main reservoir has three fourth-order high-frequency sequences. Apparently, the favorable reservoir in the study area is jointly controlled by sedimentary microfacies and high frequency sequence in the carbonate ramp. The former controls the primary pore structure, and the latter controls the intensities of karstification exposure and dolomitization in the penecontemporaneous period. The dolomitic shoals and top dolomitic flats of different stages, superimposed and connected into favorable reservoirs of considerable scale like "platform margin", are favorable exploration facies in the carbonate ramp.

Platform Evolution in an Oligo-Miocene Back-arc Basin:An Example from the Central Iran Basin

The Qom Formation is the most important hydrocarbon reservoir target in Central Iran.The Qom platform developed in a back-arc basin during the Oligo-Miocene due to the closing of the Tethyan Seaway.This formation consists of a variety of carbonate and non-carbonate facies deposited on a platform ranging from supratidal to basin.A combination of tectonic and eustatic events led to some lateral and vertical facies variations in the study area.Six third-order depositional sequences and related surfaces were identified regarding vertical facies changes in the studied sections of this Oligo-Miocene succession.According to all results and data,this succession was initially deposited during the Chattian upon a distally steepened ramp of siliciclastic-carbonate composition,including the Bouma sequence.Then,from the late Chattian to the Aquitanian,the platform changed into a homoclinal carbonate ramp with a gentle profile.With respect to tectonic activity,this phase was a calm period during the deposition of the Qom Formation.Finally,a drowned carbonate platform and a rimmed shelf emerged during the Burdigalian,terminated by the continental deposits of the Upper Red Formation.Regarding all geological characteristics,three main tectono-eustatic evolutionary phases have been recognized in the Qom back-arcbasin.

Ramp facies in an intracratonic basin:A case study from the Upper Devonian and Lower Carboniferous in central Hunan,southern China

Detailed studies on Late Devonian to Early Carboniferous carbonate rocks in central Hunan, southern China have led to the recognition of 25 lithofacies which can be grouped into： （I） inner ramp peritidal platform, （2） inner ramp organic hank and mound, （3） mid ramp, （4） outer ramp, and （5） shelf basin facies associations. The peritidal platform facies association dominates the Zimenqiao Formation （Namurian A or late Datangian） and is characterized by gypsum and dolostone-containing sequences, indicating a peritidal platform environment. The other four facies associations dominate the Menggongao Formation （late Famennian）, Liujiatang Formation （Tournaisian or Yangruanian）, Shidengzi Formations （early Visean or early Datangian）. Five upward-shallowing cycles were distinguished in these three Formations. The predominant facies associations developed in each Formation demonstrate an overall transgression-regression cycle in the Late Devonian to Early Carboniferous in central Hunan. The overall transgressive sequence was preserved in the Shaodong, Menggongao, and Liujiatang Formations, and the overall regressive sequence was preserved in the Liujiatang, Shidengzi, Ceshui and Zimenqiao Formations.

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.

Microfacies Analysis and Paleoecology of the Oligocene Succession in a Central Tethyan Carbonate Platform,Zagros Basin,SW Iran

Two stratigraphic sections(Arjooieh and Firoozabad)of the Mymand anticline,located in the Interior Fars subbasin of the Zagros Mountains,were measured and sampled,in order to document sedimentological characteristics,microfacies types and paleo-seagrasses indicators of the Oligocene succession(Asmari Fm.).Planktonic and benthic foraminifera and coralline red algae are the principal fossils from these strata.Foraminifera are represented by the following families:Soritidae,Peneroplidae,Austrotrillinidae,Alveolinidae,Planorbulinidae,Discorbidae,Lepidocyclinidae,Amphisteginidae,Rotaliidae,Nummulitidae and Globigerinidae.Nine microfacies types were recognized,namely planktonic foraminifera-peloidal packstone(MF1),bioclast nummulitid/Nerorotalia/Amphistegina packstone-grainstonerudstone-floatstone(MF2),Neorotalia-echinoid coralline red algae packstone-grainstone(MF3),coral boundstone(MF4),coral/coralline red algae rudstone-floatstone-packstone-grainstone(MF5),diverse imperforate foraminifera bioclast packstone-grainstone(MF6),peloid wackestone-packstone-grainstone(MF7),fenestrated mudstone and microbial mats(MF8)and anhydrite(MF9).MF1 indicates an outer ramp,MFs 2-4 represent a mid-ramp and MFs 5-9 are interpreted as inner ramp environment.Paleo-seagrass indicators consisting of foraminifera,hooked and tabular forms of coralline red algae and corals.They were identified in MFs 5 and 6,reflecting the presence of vegetated environments within the mid/inner ramp setting.The Mymand anticline was dominated by the outer ramp environment at the start of the Rupelian.Mid to inner ramp environments prevailed during the Rupelian.The Chattian corresponds to the spread of the inner ramp setting over the Mymand anticline.

Facies analysis and depositional environments of the OligoceneeMiocene Asmari Formation, Zagros Basin, Iran

The Asmari Formation （a giant hydrocarbon reservoir） is a thick carbonate sequence of the Oligocene Miocene in the Zagros Basin, southwest of h＇an. This formation is exposed at Tang-e-Lendeh in the Fars interior zone with a thickness of 190 m comprising medium and thick to massive bedded carbonates. The age of the Asmari Formation in the study area is the late Oligocene （Chattian）-early Miocene （Burdigalian）. Ten microfacies are defined, characterizing a gradual shallowing upward trend; the related environments are as follows： open marine （MF 8-10）, restricted lagoon （MF 6-7）, shoal （MF 3 5）, lagoon （MF 2）, and tidal fiat （MF 1 ）. Based on the environmental interpretations, a homoclinal ramp consisting of inner and middle parts prevails. MF 3-7 are characterized by the occmrence of large and small porcelaneous benthic foraminifera representing a shallow-water setting of an inner ramp, influenced by wave and tidal pro- cesses. MF 8-10, with large particles of coral and algae, represent a deeper fair weather wave base of a middle ramp setting.

Microbiostratigraphy and depositional environment of Eocene Shahbazan deposits at Chenareh section,southwest of Iran

The Shahbazan Formation was deposited in the foreland basin in southwestern Iran( Lorestan Basin). In this research,microbiostratigraphy and depositional environmental implications related to the Shahbazan Formation at the northern flank of Chenaerh anticline are discussed. Carbonate sequences of the Shahbazan Formation consist mainly of large benthic foraminifera along with other skeletal and non-skeletal components.Two biozones have been recognized by distribution of large foraminifera in the studied area that indicate middle Eocene age( Lutetian). Based on analysis of large benthic foraminiferal assemblages and microfacies features,7 different microfacies have been recognized,which can be grouped into three depositional environments: inner,middle and outer ramps.

The marine Triassic sequence stratigraphy of Lower Yangtze

Based on detailed field observation and multidisciplinary studies of integrated stratigraphy, the marine Lower and Middle Triassic of Lower Yangtze region is divided into five third order sequences, the general approach of the outcrop sequence stratigraphical study of carbonate ramp is proposed, the pattern in the development of the Early and Middle Triassic sequence under the major regression is summarized, and the sequence stratigraphical and chronostratigraphical frameworks across various paleogeographical facies zones on the marginal platform are established.