Restricted Platform Facies Research Articles

Discovery of Late Carboniferous high-grade carbonate-hosted manganese mineralization in the Maerkansu area of the Western Kunlun Orogen, Northwest China

The Aoertuokanashi deposit located in the Western Kunlun Orogen is the first known Late Carboniferous large high-grade carbonate-hosted Mn deposit in the world and it has been mined since 2018. The deposit is considered to have formed in a back-arc basin developed during the northward subduction of the Paleo-Tethys Ocean beneath the Tarim Block. The Re-Os isotope isochron age of Mn ore at the Aoertuokanashi deposit is 302 ± 9 Ma. The minimum initial 187Os/188Os values obtained from the deposit is 0.28, which indicates the presence of a mantle source for the mineralization. The Aoertuokanashi deposit is similar in age to the 301 ± 5 Ma Wajilitag kimberlite in the Tarim Block, which is considered to be the initial magmatic pulse triggered by the Tarim mantle plume. The Late Carboniferous Aoertuokanashi Mn deposit was likely promoted by igneous activity from the Tarim mantle plume.The Aoertuokanashi deposit shows geochemical characteristics of hot water activity in a back-arc basin. The Mn mineralization is hosted in the restricted platform facies. Framboidal pyrite, typical biomat, and authigenic quartz are observed in the Mn deposit, indicating the influence of microbial processes. The sedimentary facies show a starving basin feature that can enhance microbial mediation. The δ13CV-PDB values of mineralized rocks (from −19.5 to −8.2 ‰) are more negative than those of wall rocks (from −5.3 to + 4.2 ‰), whereas the δ13CV-PDB values (around −29 ‰) of kerogen for the Mn ore are significantly lower than those of the whole rock. These compositions indicate that the reducing function of organic matter played an important role in Mn mineralization. In the inferred extensional tectonic environment, under the influence of a mantle plume, the stratum with strong hot water activities and a large amount of organic matter becomes the focal point for the formation of carbonate type manganese ore deposit.

Lithofacies analysis and paleosedimentary evolution of Taiyuan Formation in Southern North China Basin

The Permian Taiyuan Formation contains alternate shale and coal lithologies which are vital for the exploration of shale gas reservoirs. However, previous studies have neglected the influence of paleosedimentary environment on the formation and distribution of marine-continental shale. A better understanding of the evolution characteristics of the paleosedimentary environment of the marine-continental strata is particularly critical to the accurate prediction of high-quality shale reservoirs. In this study, the geochemical and mineralogical characteristics of Taiyuan Formation, marine-continental strata in the Southern North China Basin, were carried out using lithology identification, element analysis, and X-ray diffraction analysis. These interpretations were helpful to develop a sedimentary depositional model by principles of sedimentology and sequence stratigraphy. The evolution of the paleosedimentary environment and the control of the paleosedimentary environment on the organic matter abundance, mineral composition, lithofacies, and gas enrichment of shale were analyzed. The results show that the vertical variation of the paleosedimentary environment had obvious stages during the Taiyuan Stage of the Early Permian. Based on sequence identification and division, a multi-stage superimposed marine-continental sedimentary model of transgressive and regressive assemblages was established. From tidal flat facies to lagoon facies and then to restricted platform facies, the contents of felsic and clay minerals decreased, while the contents of TOC and carbonate minerals increased sequentially. The lithofacies types of limited platform shale and tidal flat shale were different and relatively single, while the lagoon contained the shale lithofacies types of the above two sedimentary microfacies due to its sedimentary microfacies development location. The lagoon facies shale of highstand system tract had high-quality paleo-productivity, and was distributed in a stretch on the plane and superimposed in multiple stages vertically of the basin, which could be selected as a favorable facies belt of marine-continental facies shale gas.

Sequence sedimentary evolution and reservoir distribution in the third and fourth members of Sinian Dengying Formation, Gaomo area, Sichuan Basin, SW China

Based on comprehensive analysis of cores, thin sections, logging and seismic data, the sequence stratigraphy and sedimentary evolution of the third and fourth members of Sinian Dengying Formation (Deng 3 and Deng 4 members for short) in the Gaomo area of Sichuan Basin were investigated, and the favorable zones for reservoir development in the Deng 3 Member and Deng 4 Member were predicted. (1) Two Type I and one Type II sequence boundaries are identified in the Deng 3 and Deng 4 members. Based on the identified sequence boundaries, the Deng 3 and Deng 4 members can be divided into two third order sequences SQ3 and SQ4, which are well-developed, isochronal and traceable in this area; the SQ3 thins from west to the east, and the SQ4 thins from northwest to southeast. (2) The sedimentary environment from the depositional period of SQ3 to SQ4 has experienced the evolution from mixed platform to rimmed platform, and the platform rimmed system on the west side is characterized by the development of platform margin microbial mound and grain shoal assemblages. The intraplatform area is a restricted platform facies composed of a variety of dolomites, and there are local micro-geomorphic highlands of different scales and scattered intraplatform mounds and shoals. (3) The Deng 4 Member reservoirs, with obvious facies-controlled characteristic, are mainly distributed in the upper part of high-frequency upward shallow cycle and the high-stand systems tract of the third-order sequence vertically, and are more developed in the platform margin belt than in the intraplatform belt, and more developed in the Gaoshiti platform margin belt than in the west Suining platform margin belt on the plane. (4) Three types of favorable reservoir zones of Deng 4 Member have been finely delineated with 3D seismic data; among them, the mound and shoal facies zones developed in the ancient highlands of the intraplatform are the first choice for the next exploration and development of the Deng 4 Member.

Lithofacies paleogeography and characteristics of large-scale reservoirs of the Middle Triassic Leikoupo Formation in Sichuan Basin, China

Material exploration discoveries have been achieved in the Middle Triassic Leikoupo Formation in the Sichuan Basin in recent years, and a large-scale reservoir is one of the key factors controlling reservoir formation. In this paper, the lithofacies paleogeographic map of the Leikoupo Formation is prepared based on field outcrops, exploratory wells, well logs, and seismic data. The characteristics of large-scale reservoirs have been analyzed, which resulted in three new summarized understandings: (1) At the Leikoupo stage, the Sichuan Basin was relatively closed in the south, east and north, and the seawater entered through the west side, so it was a relatively closed carbonate platform; during the sedimentary stage of Lei I and Lei II members, it was a weakly fringed carbonate platform. In the sedimentary period of Lei III (T2l3) and Lei IV (T2l4) members, it was a rimmed carbonate platform. (2) The sedimentary facies including the platform margin, open platform, restricted platform, and evaporative platform were developed in the Leikoupo Formation; these were mainly the restricted platform facies covering the subfacies of the lagoon, intra-platform shoal, and tidal flat. Two types of hydrocarbon reservoirs, namely the platform-margin shoal and intra-platform shoal reservoirs, were developed in this period. (3) Two sets of large-scale reef beach facies reservoirs were developed in the Leikoupo Formation, one is the platform margin reef beach facies of Lei III (T2l3) and Lei IV (T2l4) members in northwestern Sichuan with a large thickness (generally larger than 50 m); the other one is the intra-platform shoal facies in the lower part of Lei I Member (T2l1) in Central Sichuan with a small thickness, stable distribution, and a large area (larger than 20,000 km2) forming large-scale reservoirs. The two types of large-scale reservoirs experienced multi-stage karstification transformation to become reservoirs with a good quality, laying the material basis for large-scale oil & gas reservoir formation. The research results have important theoretical and practical significance to improve the understanding on the sedimentation and reservoirs of the Leikoupo Formation in the Sichuan Basin, and to provide guidance for the oil and gas exploration in this region.

Assessing the Karst Groundwater Quality and Hydrogeochemical Characteristics of a Prominent Dolomite Aquifer in Guizhou, China

Karst groundwater is one of the primary water resources in most provinces of Southwestern China where karst topography is strongly featured. In Guizhou Province, a prominent geologic sedimentary formation is the dolomite stratum which exists as the restricted platform facies and potentially provides a large reservoir for drinking water. A proper understanding and evaluation of its hydrogeochemical characteristics and water quality is the key to ensuring the drinking water safety. In the present study, groundwater samples were collected from 25 locations of the dolomite aquifer across Guizhou to determine their major chemical compounds, including the cations (K+, Na+, Ca2+, Mg2+) and the anions (HCO3−, F−, Cl−, NO3−, SO42−), as well as the pH, total hardness, and total dissolved solids. HCO3− and Ca2+ were found to be the dominant anion and cation, respectively, which is characteristic of typical karst groundwater and supports the overall observation of a slightly weak acid to weak alkaline environment in the studied groundwater, as the pH measurements ranged from 6.80 to 8.37. Fuzzy comprehensive evaluation method is used to evaluate the groundwater quality based on typical drinking water safety standard. The results show that the groundwater in most of the studied aquifers is of reasonably good quality. However, in some aquifers, concentrations of NO3− and/or SO42− were found to be excessively high. Overall, the studied dolomite aquifer in its natural environment as investigated in the present study can be considered as a potential geological stratum for water resources exploitation in Guizhou.

Sedimentary characteristics and formation mechanisms of the Ordovician carbonate shoals in the Tarim Basin, NW China

The Ordovician carbonate shoals are extensively developed in the Tarim Basin, NW China, and are taken as important petroleum reservoirs. The sedimentary characteristics and controlling factors of these shoals are poorly studied. Multiple means, including field geological investigations, core observations, thin section identifications by scanning electron microscope and cathodoluminescence microscope, granulometric analyses, and geochemical analyses, are conducted. The classification of limestones and shoals in the Ordovician of the Tarim Basin is established. Four types of shoals are developed, namely, intraclastic shoals, bioclastic shoals, oolitic shoals, and algal shoals. It is demonstrated that these four shoal types are deposited in progradation sequences, among which, the low energy bioclastic shoals are in the sequence of retrogradation. Different sedimentary environments develop various sedimentary combination patterns: the platform margin facies mainly develop reef–shoal combinations followed by shoal–shoal combinations; the open platform facies mainly develop shoal–shoal combinations and then reef–shoal combinations; the restricted platform facies develops restricted shoal combinations. Intraclastic shoals are the most developed shoals, which are directly controlled by hydrodynamic force that caused by sea level changes and paleogeography that dominated by tectonic movement. In the platform margin facies, shoals are well-developed with large thickness and limited horizontal extent, and the main shoal type is bioclastic shoal, followed by algae shoal. In the open platform facies, Shoals usually present thin-layered with large horizontal extent. The main shoal type is intraclastic shoal that shows a gently moundy or sill-like structure, the followings are bioclastic shoals and oolitic shoals that developed in the highland near the platform margin facies. In the restricted platform facies, a small number of shoals are developed, which distributed in striped shape paralleling to the shoreline. This study provides great guidance for predicting shoals in different sedimentary facies and build good foundation for future shoal-related reservoirs assessment and prediction.

New Findings On an Orbitolinid Foraminifer Coskinolinella Bariensis (luperto-sinni & Reina, 1992) from the Albian Shallow-water Carbonate Sequence of the Bey Dağlari (s Turkey)

AbstractThe orbitolinid foraminifer Heterocoskinolina bariensis Luperto-Sinni & Reina, 1992, which was first described from the Cenomanian (lower?) of the Apulian Carbonate Platform (southern Italy), has also been found in a Cretaceous shallow-water carbonate sequence of the Bey Dağları Carbonate Platform (Western Taurides, southern Turkey). The morphological analysis, based on numerous successive acetate-peels, revealed annular chambers subdivided into wedge-like chamberlets ( = cupulae) by apparent “radial partitions” produced by the strong infolding of the chamber floor; these are characteristic of the genus Coskinolinella Delmas & Deloffre, 1961. Specimens were compared with descriptions and images of closely related taxa. The stratigraphic position of C. bariensis was determined as upper Albian, based on its occurrence 50 m below the lower–middle Cenomanian Sellialveolina gr. viallii Zone and the presence of Protochrysalidina elongata. Because of its restricted stratigraphic range, this species may have age-diagnostic value for Cretaceous biostratigraphy in restricted platform facies. Other benthic foraminifera, some poorly known, are also documented and illustrated from this same shallow-water carbonate sequence.

New understanding of the sedimentary model of Lower Cambrian Longwangmiao Formation reservoirs in the Sichuan Basin

Natural gas in the gas reservoirs of Lower Cambrian Longwangmiao Fm is mainly concentrated in the granular shoal facies in central Sichuan Basin, therefore the study on lithofacies paleogeography and the prediction on granular shoal distribution are the key to improving the success ratio of natural gas exploration in this area. In this paper, outcrop, drilling, seismic and logging data were studied comprehensively. Subsequently, the sedimentary facies and sedimentary model of Longwangmiao Formation in the Sichuan Basin and its adjacent areas were studied, and lithofacies paleogeographic maps were prepared based on previous research results. Besides, “three-shoal” sedimentary model of Longwangmiao Fm was established, the distribution range of three shoal belts was predicted and their development characteristics and main controls were analyzed, thereby pointing out the direction and field of natural gas exploration in the next stage. It is indicated from the research results that: first, the paleogeographic pattern of the Sichuan Basin in the Longwangmiao period was represented as a rimed carbonate rock platform, with the developments of sedimentary facies such as mixed tidal flat, restricted platform, open platform, platform margin and slope facies. Second, the “three-shoal” sedimentary model of Longwangmiao Fm was established. One of them is the granular shoal in platform margin facies and is distributed along the eastern margin of the Sichuan Basin. The other two are intra-platform granular shoals which were distributed on the east and west sides of lagoon subfacies, belonging to restricted platform facies and open platform facies, respectively. Third, the granular shoal of restricted platform facies is the main reservoir of Longwangmiao Fm gas reservoir in the Anyue Gas Field. It is distributed in central Sichuan, and its development is controlled by lagoons and Gaoshiti–Moxi Paleo-uplift. Fourth, the platform margin granular shoal is mainly distributed along the northern and eastern margins of the basin, and its development is controlled by the slope break of sedimentary paleogeomorphy. In conclusion, the granular shoal of restricted platform facies and shoal of platform margin facies are good reservoirs that they are the important fields of natural gas exploration. Furthermore, the new understanding to “three-shoal” sedimentary model and granular shoal distribution of Longwangmiao Fm is of great theoretical significance to the study on sedimentary systems of ancient strata.

Sedimentary facies and sequence stratigraphy in the Lower Triassic Jialingjiang Formation, Sichuan Basin, China

The Lower Triassic Jialingjiang Formation is an important oil and gas exploration target in the Sichuan Basin. The lithofacies mainly consists of clastic rocks, carbonates, and evaporates, which represents a transition from continental to marine depositional environment. The Jialingjiang Formation of the Sichuan Basin is a typical epeiric carbonate platforms. The study area contains four types of facies, including mixed tidal flat facies, open platform facies, restricted platform facies, and evaporitic platform facies. Based on the analysis of cores and logging data, two types of type II sequence boundaries were identified, which are, respectively, local exposure unconformity boundary and lithology and lithofacies transformation boundary. The Jialingjiang Formation is divided into three third-order sequences and six system tracts. Thus, we establish a connected multi-well sequence stratigraphic framework based on typical single-well sequence stratigraphic division. The sedimentary evolution of the Jialingjiang Formation was divided into three major sedimentary periods and six stages, with rising and falling of the sea level, mixed tidal flat, open platform, and restricted platform prograded first, and then restricted platform and evaporitic platform retrograded. It also shows that the open platform shoal and restricted platform tidal flat have oolitic limestone and algal dolomite with good-quality physical properties and are the favorable reservoir facies belt for hydrocarbon exploration and development.

Palaeobiogeographic dynamics of brachiopod faunas during the Frasnian-Famennian biotic crisis in South China

The Late Devonian biotic crisis has been studied for a long time mainly on the basis of its taxonomic severity, but with little attention paid on the relationship between biogeography and evolutionary dynamics. In this study, brachiopod faunas across the biotic crisis are surveyed based on revised data compiled from 15 sections in South China. Data of brachiopod faunas are plotted using a network method to detect biogeographic variation and dynamics through the upper Frasnian, lower and middle Famennian. In addition, frequency analysis is performed on the occurrence of brachiopod faunas within the different localities and depositional settings through time. These data show significant taxonomic loss in the five depositional zones in South China, particularly in the littoral clastic and restricted-platform facies. Refuges during the survival interval were probably located in deeper water zones, especially in the inter-platform basin, but also in the peritidal and the open carbonate platform. A decrease of biogeographic connections within the brachiopod occurrence network, accompanied with an increase of low frequency taxa in the survival and recovery intervals, supports biogeographic divergence of ecological preferences of survival and recovery faunas in South China, as well as a turnover of brachiopod faunas through the biotic crisis. Reasons for variation of biogeographic distribution and connections are less known, besides the faunal colonisation that facilitated by sea-level fluctuation through the biotic crisis.

Distinguishing coral reef facies from coral-bearing open platform facies: Examples from Ordovician Ordos Basin, Northwest China

Fossil coral communities are often simply considered as “coral reefs.” In fact, coral-bearing communities are not necessarily coral reefs. The key criterion is whether the corals are in situ and if they are abundant enough to form topographic relief. Two outcropped profiles in the western margin of the Ordos Basin, northwestern China, drill cores of the Lianglitag Formation from 4 wells in the Tarim Basin and 500 large thin sections were examined in this study to reveal the difference between coral reefs and coral-bearing carbonate open platform facies in Upper Ordivician carbonate succession. The second layer of the Shijiezigou section is massive in internal structure, mound-like in shape, 5.0m in height and 33.6m in width and is interpreted as a patch reef in an open platform. This layer has a variety of corals that are preserved in situ. Reef-dwelling organisms include dasycladaceans, crinoids and gastropods. Other layers of the section are bedded limestones, many of which bear corals and crinoids, while others bear dasycladaceans or only the Dasycladacea genus Vermiporella. Unlike in the reef, the corals in the open platform facies are sparse and generally occur as fragments, most of which are not preserved in situ. According to the dominant species, four communities are distinguished from the Shijiezigou section, namely, the Vermiporella community, coral community, crinoid-coral community and dasycladacean-crinoid community, each of which adapted to some special environmental conditions. The patch reef of the Shijiezigou section contain abundant and diverse tabulate corals, which form bafflestones, while stromatoporoids are the primary builders in the platform margin reefs of well TZ 822 and the Lijiapo section. The common biotic components of the open platform of the Shijiezigou section and well TZ 822 are crinoids and dasycladaceans. The restricted platform of the Shijiezigou section is dominated by the dasycladacean Vermiporella, while the organisms of the restricted platform facies in well TZ 35 are mainly composed of cyanobacteria, ostracods and Tetradium-like organisms.

Formation mechanism of dolomite reservoir controlled by fourth-order sequence in an evaporated marine environment – An example from the Lower Ordovician Tongzi Formation in the Sichuan Basin

The high-porosity dolomite reservoirs of the Lower Ordovician Tongzi Formation (Fm.) were widely developed in the Sichuan Basin of southern China. The characteristics and developing mechanisms of the high-porosity dolomite reservoirs under the control of fourth-order sequence boundaries are discussed. In the Tongzi stage of the Early Ordovician, the Sichuan Basin was in a restricted platform facies in an evaporated shallow seawater environment. From the western to eastern regions of the basin, the Tongzi Fm. was serially developed in a tidal flat-lagoon-high-energy shoal depositional system. The evaporated seawater consequently led to dolomitization by way of the refluxing model. The Tongzi Fm. dolomites were subdivided into four coarsening-upward fourth-order sequences. Many tiny dissolution pores were formed in the dolomite beneath the four fourth-order sequence boundaries due to syn-sedimentation meteoric water erosion. Exposure above the seawater due to the short-term fall of the relative sea level consequently led to contemporaneous meteoric erosion. The Tongzi Fm. dolomites in the belt surrounding the Central Paleo-uplift were further subaerially dissolved by meteoric water due to tectonic uplift in the Guangxi Movement since the end of the Silurian period. Therefore, dolomitization, syn-sedimentation meteoric erosion under the fourth-order sequence boundaries, and meteoric karst during the Guangxi tectonic uplift jointly controlled the development of the Tongzi Formation high-porosity dolomite reservoirs. In the eastern and southeastern Sichuan Basin, the favourable reservoirs are the high-energy shoal dolomites that were eroded by meteoric water under fourth-order sequence boundaries. Around the Central Paleo-uplift, the favourable reservoirs are the dolomites dissolved by subaerial meteoric karst during the Guangxi Movement.

伊拉克M油田Asmari组碎屑岩与碳酸盐岩混合沉积特征

结合薄片、岩心、测井等及相关生产资料，对伊拉克M油田Asmari组B段储层特征及成因进行了分析。Asmari组B段岩性复杂，碳酸盐岩与陆源碎屑岩呈现互层和成分的混积，储层非均质性强，研究认为沉积相以三角洲、混积台地和局限台地的交替变化为特点。构造运动和海平面频繁震荡共同控制碎屑岩和碳酸盐岩的分布，形成了两种不同岩性储层的混积现象。 Based on thin section, core, well logging and production data, the characteristics and genesis of B reservoir in Asmari formation in Iraq were studied. The lithology of B reservoir is complex: carbonate and terrigenous clastic rock present interbeding and mixed composition; reservoir heterogeneity is strong. The sedimentary facies alternate among delta, mixed platform facies and restricted platform facies. Tectonic movement and frequent change of sea level control the distribution of clastic rock and carbonate rock, which results in two different lithologic reservoir of mixed phenomenon.

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).

Geologic features and genesis of the barriers and intercalations in carbonates: A case study of the Cretaceous Mishrif Formation, West Qurna oil field, Iraq

Abstract The type, poroperm relationship, log response characteristics and identification criteria of the barriers and intercalations in the carbonates of the Cretaceous Mishrif Formation in the West Qurna oil field of Iraq are studied through comprehensive analysis of cores, thin sections and well logs. The genesis and distribution of the barriers and intercalations are analyzed from the perspectives of sequence stratigraphy, depositional facies and diagenesis. The barriers and intercalations can be classified into three types: grainstone, packstone and wackestone. The barriers and intercalations generally exist in restricted platform facies and evaporative platform facies, some in open platform. They are common in transgressive cycles and early regressive cycles, forming continuously, extensive barriers and intercalations near the sequence boundaries. Penecontemporaneous cementation, burial compaction, and burial cementation led to the decrease of the porosity, damaged the space of the reservoir pores and became the important factors for the genesis of the barriers and intercalations. In the epidiagenetic phase, a large number of CaCO3 precipitated in the phreatic water zone below free-water table, leading to the formation of packstone and grainstone barriers and intercalations in a large scale. Through comprehensive log analysis, log identification criteria and the distribution of the barriers and intercalations are determined. The barriers are mainly distributed in the sections of CRI and CRII, and the intercalations are concentrated in the section of mB1. For different types of barriers and intercalations, packstones are the most in quantity, followed by grainstones and wackestones.

Facies and early dolomitization in Upper Albian shallow-water carbonates of the southern Apennines (Italy): paleotectonic and paleoclimatic implications

Thick successions of Cretaceous carbonates in the southern Apennines of Italy are of great economic interest since they host important aquifers and huge hydrocarbon accumulations. The reservoir of the Val d’Agri and Tempa Rossa oilfields (in the subsurface of Basilicata) consists of Upper Cretaceous rudist-rich limestones passing downward into mid-Cretaceous dolomitized limestones of restricted platform facies. Upper Albian-Lower Cenomanian dolomitized carbonates exposed in the Sorrento Peninsula and in the Cilento Promontory, part of the Apennine Carbonate Platform, represent a good surface analogue for the lower part of the reservoir. They are composed of meter-thick beds of stratabound dolomite and shallowing-upward cycles of restricted platform limestones capped by silicified evaporites and marly levels. Field relations, petrography, and geochemistry implicate the reflux of penesaline waters as the most probable dolomitization process. High-frequency climatic variability between dry and wet phases can explain the formation of evaporites, which are coeval with karstic bauxites in other sectors of the southern Apennines. The dolomitized carbonates of the Sorrento Peninsula pass laterally into dolomitized breccias, which were the result of local tectonic collapse of the platform. This is further evidence of mid-Cretaceous syn-sedimentary tectonics that in other areas of the Adria passive margin contributed to the formation of intraplatform basins where source rocks accumulated.

Sedimentary Facies and Distribution of Reservoir Rocks from the Feixianguan Formation in the Daxian-Xuanhan Region, NE Sichuan

Abstract The Lower Triassic Feixianguan Formation in the Daxian-Xuanhan region, northeastern Sichuan is interpreted to be a carbonate platform intermediate between the West Hubei-East Chongqing marine basin and Guangyuan-Wangcang marine basin. Outcrops, well logs and seismic data have disclosed that three sedimentary facies can be identified for the formation: open platform, platform exposed shoal and restricted platform facies. During the early stage of deposition of the Feixianguan Formation, there existed a sedimentary framework all the same as the Late Permian one: open platform in the west, and platform exposed shoal, restricted platform and open platform eastwards. The gradual increase of the shoal area permitted eastward migration of the shoal facies. During the deposition of the third member of the Feixianguan Formation, the study area was invaded by temporary transgressions. Till the deposition of the fourth member of the formation, the study area was prevailed over the unified restricted platform deposits, as indicated by the sedimentary model for the typical carbonate platform shoal deposits. The reservoir rocks in the Feixianguan Formation consist mostly of solution opening dolostone, and the rock types are assembled by oolitic dolostone, residual oolitic dolostone and sucrosic residual oolitic medium- to coarse-grained dolostone. The deep and shallow dual laterologs show relatively high resistivity, clear amplitude differences and time difference saltation of acoustic waves in some intervals. The seismic responses are indicated by low-frequency and highly variable amplitudes and chaotic reflection configurations. The reservoir rocks are characterized by high porosity-high permeability and medium porosity-medium permeability, showing a marked correlation between porosity and permeability. Solution openings are extremely developed in the reservoir rocks, including cast pores, intergranular solution openings, intercrystal pores, intercrystal solution openings, solution openings (caves) and minute fissures. Macropores and coarse pore throats are common. The reservoir rocks are apparently controlled by sedimentary facies in distribution patterns. Vertically, they occur dominantly in the second and first members of the Feixianguan Formation, and laterally in the platform exposed shoals and restricted platforms, where good natural gas potential is expected.

Distribution of Biomicrofacies and Cement Types in the Changxing Formation (Upper Permian), Western Hubei-eastern Sichuan Provinces, South China

The Changxing formation is subdivided into lower, middle and upper parts. Four types of microfacies were recognized in the Changxing formation. They include SMF-3, SMF-6, SMF-7 and SMF-19. The Changxing Formation is composed of deep basin margin, platform margin and open restricted platform environments. The lower, middle and upper parts are dominated by deep basin margin facies, platform margin facies and open restricted platform facies, respectively. The lower part contains foraminifera, radiolarian and mollusk organisms. The middle part is characterized by presence of crinoids, sponge, rugose, ostracods, trilobite, brachiopods, abundant bryozoans, mollusks and foraminifera organisms. The upper part is marked by dolomite, pyrite and abundant mollusks. Most of these organisms were concentrated within the middle part. The types of cements in the Changxing Formation consist of coarse grain sparite, micrite, microspar, silica and radiaxial cements. The lower part is dominated by silica cement. The middle part shows presence of abundant coarse grain sparite, micrite, microspar and radiaxial cements. The upper part is dominated by micrite, microspar cements.

Study of the factors affecting the abundance of organic matter in Jurassic carbonate rocks in Qiangtang Basin, Tibet

Field and laboratory analyses of carbonate rock samples from the Qiangtang Basin, Tibet, indicate that carbonate source rocks are mainly developed in the Middle Jurassic Xiali Formation and Upper Jurassic Suowa Formation. Comprehensive studies showed that the Suowa Formation carbonate source rocks have a favorable hydrocarbon-generating potential. The abundance of organic matter in the carbonate rocks is controlled mainly by sedimentary environment and inorganic compounds in the rocks, which is higher in the restricted platform facies than in the open platform facies. Organic carbon contents decrease with increasing CaO contents in the source rocks.