Rimmed Platform Research Articles

Sedimentary evolution and sequence stratigraphy of Ediacaran high‐grade phosphorite–dolomite–shale successions of the Bocaina Formation (Corumbá Group), Central Brazil: Implications for the Neoproterozoic phosphogenic event

AbstractExtensive phosphorite deposition is observed in the Neoproterozoic after a prolonged hiatus during most of the Mesoproterozoic era. This event is thought to represent an important record of major palaeoenvironmental, palaeoceanographic and biotic changes that shaped Neoproterozoic ecosystems, suggesting close relationships between phosphogenesis and the preservation of key Ediacaran biotas. However, high‐grade Ediacaran phosphorite deposits are relatively uncommon, diminishing the opportunity to test current phosphate mineralization–deposition models and their implications for Neoproterozoic research. In this scenario, widespread Ediacaran phosphorite–dolomite–shale successions of the Bocaina Formation (Corumbá Group – Central Brazil) are poorly explored in international literature. Nevertheless, recent advances in phosphate exploration gave access to continuous drill core sections and freshly opened mine pits, revealing an unprecedented record of complex phosphatic successions featuring the occurrence of Ediacaran microfossils assigned to the Doushantuo–Pertatataka assemblages. This work seeks to constrain main lithofacies, sequence stratigraphy and depositional settings from these phosphatic successions in order to analyse the sedimentary evolution of the unit under the current Neoproterozoic phosphorite research framework. These results indicate that the Bocaina Formation records secular sustained phosphate deposition. These deposits are related to unprecedented, microbialite reef rim phosphorites deposited during a lower accretionary rimmed platform stage, followed by the deposition of Doushantuo‐like, whole platform phosphorites associated with a later, drowned platform stage, therefore, reinforcing the evidence for the operation of strong allogeneic controls on phosphate mineralization–concentration. In addition, this study concludes that fossiliferous Ediacaran phosphatic deposits such as the Bocaina Formation are important to understanding Neoproterozoic phosphogenic events, because they may record the transition from a Precambrian to Phanerozoic‐like phosphogenesis associated with the instauration of the Ediacaran–Cambrian phosphatic taphonomic window. This evidence hints that the growing dataset from the Bocaina Formation may bring new, exciting perspectives for Neoproterozoic research as a whole.

Carbonate factories and their critical control on the geometry of carbonate platforms (mid-Cretaceous, southern Iran)

Tropical rimmed platforms represent a much more efficient protection from coastal erosion than carbonate ramps. Understanding the controlling factors of platform geometry is therefore crucial to predict the fate of rimmed platforms under global-warming conditions. Here, we present detailed biostratigraphic, sedimentological, geochemical, and high-resolution carbon-isotope analyses of mid-Cretaceous (Albian-Cenomanian) carbonate rocks well exposed in the Khormoj section located in the Zagros Mountains of southern Iran. Sixteen microfacies were identified, allowing us to define the superposition of different platform geometries, a carbonate ramp in the Albian and a rimmed platform in the Cenomanian. Biostratigraphic and carbon-isotope analyses allowed us to identify the OAE1b, OAE1d, and Middle Cenomanian events in the Khormoj section. The paleodepth changes documented by carbonate microfacies testify to two transgressive events that can be correlated with long-term eustatic curves during the ramp stage. The Albian ramp was formed by orbitolinids under high siliciclastic input whereas the Cenomanian rimmed platform was built by rudists during clear water conditions. These two carbonate factories have drastic different carbonate sedimentation modes. The orbitolinid factory is characterized by a low carbonate production rate and fine carbonate grains resulting in ramp geometry, whereas the rudist factory is characterized by a high carbonate production rate and coarse carbonate grains leading to a rimmed platform geometry.

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.

Rift-uplift-depression accumulation-controlling models of the large gas province in Cambrian pre-salt, Tarim Basin, China

The Cambrian pre-salt has the accumulation conditions for forming a compound large gas province and was the strategic replacement area with the lowest exploration level and the largest exploration potential in the Tarim Basin. However, no significant discoveries have been found in the exploration for many years. The key problem is that the unclear controlling factors and spatial distribution of the hydrocarbon generation center and scale beach reservoir result in the ambiguity of favorable exploration directions and zones. Given this, a comprehensive application of seismic, drilling, well-logging, mud-logging, and geochemical data has been carried out to study the source-controlling, reservoir-controlling, caprock-controlling, and accumulation-controlling of the Cambrian pre-salt area, constructing the accumulation-controlling model and defining the favorable zones and exploration breakthrough direction. The following points have been gained: (1) Paleo-rifts controlled hydrocarbon generation center. The rift-depression structure formed in the Nanhua System to Sinian System provided the structural background for the development of the Lower Cambrian and controlled the deposition of the Cambrian Yuertusi Formation. In the negative tectonic area controlled by rifting, the thickness of the Yuertusi Formation was large and formed the hydrocarbon generation center. (2) The paleo-uplifts controlled the development of beach facies dolomite reservoir. The paleo-uplifts formed simultaneously with the Nanhua System paleo-rifts controlled the development and distribution of high-energy beaches in the Xiaerbulake Formation, and large-scale dolomitic beach facies reservoirs were formed on both sides of the Tanan paleo-uplift. (3) The paleo-depression controlled the development of gypsum-salt caprock. In the Middle and Late Cambrian, the Taxi platform has closed and formed a strong evaporation environment, while a wide-covering gypsum-salt caprock was deposited in the unified depression within the platform due to the development of the Luntai-Gucheng near the north-south rimmed platform margin. (4) The time-space matching of rift-uplift-depression controlled the formation and distribution of oil and gas. Three types of accumulation-controlling models, the stable rift-uplift-depression, the active rift-uplift-depression, and the transitional rift-uplift-depression, were established, indicating the stable and transitional areas as the main exploration directions, and the Aman low uplift between Awati Sag and Manxi Sag as the preferred target area for the large gas province in Cambrian pre-salt.

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.

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.

Depositional evolution, sequence stratigraphic framework and its response to relative sea-level change, the Middle and Lower Ordovician carbonate system on outcrops, north-western margin of Tarim Basin

Middle and Lower Ordovician series carbonate platform deposits develop well in the north-western Tarim Basin, Kalpin and Bachu outcrop. An integrated analysis of the outcrop, hand specimen, thin slice and geochemical data allows for interpreting the sequence framework, depositional architecture and the response to sea-level changes. The Middle and Lower Ordovician Yingshan and Yijianfang Formations comprise 2 composite sequences (CS1 and CS2) bounded by regional or major unconformities, 5 third-order sequences (Sq1–Sq5) defined by subaerial exposure unconformities or facies changing and 13–15 fourth-order sequences classified by the outcrop depositional cycle change. Twelve microfacies and seven microfacies associations are identified using hand specimens and thin sections. The primary depositional facies include platform margin reef–bank complexes, open platform intraclast or bioclast shoal, intra-shoal and restricted platforms shoal-dolomitic or intra-clastic tidal flats. The research indicates that the carbonate platform evolved from arid and semiarid ramps, restricted platform and dolomitic tidal flat (CS1) to the open and rimmed platform margins (CS2) and eventually was drowned at the end of the Middle Ordovician. The sea-level changes have been studied using the Fischer plot method, showing two sets of long-term shallow–deep–shallow trends. Large-scale transgressions occurred at the lower parts of Sq4, with a significant negative shift of δ13C and low-energy mudstone or wackestone depositional architectures. The Fischer plot, lithofacies cycle and the geochemical indicators correlate well and are comparable with Haq's global sea-level changes. Sea-level changes dominate the sequence framework and depositional evolution, and dual factors of sea-level change and tectonic subsidence constrain local Sq3-HST. The high-frequent fluctuation of sea-level changes has a noticeable control effect on the development of fourth-order depositional cycles of carbonate deposits.

Callovian-Oxfordian sedimentary microfacies in the middle of Block B on the right bank of the Amu Darya Basin, Turkmenistan

The right bank of the Amu Darya Basin enjoys abundant natural gas resources, on which the Callovian-Oxfordian strata in the middle of Block B serve as the major horizons for natural gas production. However, the characteristics and distribution patterns of the sedimentary microfacies in these strata are yet to be further explored. Based on the analysis of data on drilling, logging, cores, and thin sections from 29 typical wells, as well as the regional sedimentary background, this study inferred that the middle of Block B evolved from the Callovian ramp platform into the Oxfordian rimmed platform. Moreover, this study determined that the inner-ramp intertidal-subtidal shallow-water subfacies mainly developed during the Callovian and transitioned into the shallow shelf subfacies during the Oxfordian. This study identified eight sedimentary microfacies, namely reef knoll, reef-shoal complex, bioclastic shoal, psammitic shoal, bioherm, lime mud mound, intershoal (intermound), and static-water mud. Based on research into the high-precision sequence-sedimentary microfacies framework, this study built a geological model for the development of sedimentary microfacies in the study area. According to this geological model, the sedimentary microfacies in the study area are characterized by vertical alternation of reef-shoal complex, bioclastic (psammitic) shoal, bioherm, and intershoal microfacies. Moreover, they show the development of reef knoll, reef-shoal complex, bioclastic (psammitic) shoal, and bioherm (or lime mud mound) laterally from west to east, with the physical properties of the reservoirs deteriorating from west to east accordingly. The microfacies of reef-shoal complex and the bioclastic (psammitic) shoal predominate in the study area, and their deposition and development are controlled by sequence boundaries and are also affected by paleo-landforms. The Oxfordian reef-shoal complexes were largely inherited from the Callovian uplifts and show lateral seaward progradation.

Tectonic-lithofacies paleogeography, large-scale source-reservoir distribution and exploration zones of Cambrian subsalt formation, Tarim Basin, NW China

To predict the large-scale source rock and reservoir distribution in Cambrian subsalt formations (Upper Sinian to Mid-Cambrian) in the Tarim Basin, more than 50 2D seismic lines and 3D data volume acquired latest, 22 wells and some outcrops around this area were used to study the tectonic-lithofacies paleogeography, define the distribution of large scale source rocks and reservoirs, and sort out favorable zones. (1) The basin experienced evolution from ramp to deep-water mud-rich ramp, low-gradient slope, weak rimmed to strong rimmed platform, and the differentiation in the platform was controlled by pre-Cambrian palaeo-rifts. (2) The Luonan-Yubei ancient rift controlled the distribution of source rocks in the Lower Cambrian Yuertusi Formation, and this rift together with the northern depression are the main source rock area. (3) There are three sets of large-scale reservoirs, the Upper Sinian Qigebulake Formation, the Lower Cambrian Xiaorbulake Formation and the Wusongger Formation, and shoal-mounds, early dolomitization and multi-levels of unconformities controlled reservoir development. (4) Four favorable zones, the north slope of Tazhong, upper slope of Maigaiti area, Keping-Wensu periphery, and south slope of Tabei, were evaluated. Of them, the north slope of Tazhong is most likely to have exploration breakthrough; the south slope of Tabei is the best area for exploring the Upper Sinian large scale weathering crust dolomite; Maigaiti slope and Keping-Wensu periphery area improved in exploration potentials significantly, and are worth prospecting faster.

Contribution to the sedimentology of the Messinian carbonates of the Chelif Basin (Boukadir, Algeria)

The Messinian (Upper Miocene) is characterized at the level of its marginal basins by the development of numerous carbonate platforms. This study concerns the Messinian platform of the Boukadir region in the south of the Chelif Basin in Algeria. It is composed of a lower prograding rimmed platform and an upper aggradational homoclinal ramp resting upon the Tortonian–Lower Messinian Blue Marl Formation, and its thickness reaches ~280 m in the Chelif Basin. The upper red-algae unit is uniform and subhorizontal with a minimum thickness of 90 m. Petrographic analysis of the upper ramp reveals three different microfacies, characterized by Lithothamnium, foraminifera, high porosity, and a microsparitic matrix. MF1 is a packstone, MF2 a packstone/bindstone deposited above the fair-weather wave base and MF3 is a wackestone to packstone deposited below this level. The upper unit is made up entirely of autochthonous biogenic elements without significant external fluvial contribution. It was formed in a shallow marine environment, with very high productivity and a significant export of the sediments produced. This aggradation was followed by a rapid exhumation (regression) transforming all the aragonite into calcite. The platforms correspond to the T2 complex reef formation (6.7–5.95 Ma) documented on the other Messinian carbonate platforms in the South of the Alboran Sea that formed just before the Messinian Salinity Crisis.

Sequence stratigraphy, depositional setting and evolution of the Fahliyan carbonate platform (Zagros fold-thrust belt, SW Iran) in the Early Cretaceous

A regional sequence-stratigraphic framework is proposed for the Berriasian-Barremian shallow platform succession in the Zagros Fold-Thrust Belt (FTB), based on outcrop sections, in combination with subsurface data. The time framework is supported by new biostratigraphic results, using benthic foraminifera, dasycladalean algae and tintinnids, calibrated by Sr-isotope data. We identified and described four large-scale sequences and sixteen small-scale sequences throughout the studied succession. Large-scale sequences occur in the Berriasian-earliest Valanginian, Valanginian, Hauterivian and early to late Barremian. Based on facies associations, a relative water depth has been assigned to distinct depositional environments. Four successive architectures were observed in the platform growth: rimmed platform during the early-middle Berriasian, isolated rimmed platform in the middle Berriasian-earliest Valanginian, non-rimmed open platform to ramp in the Valanginian, and finally a mixed siliciclastic-carbonate platform to ramp in the Hauterivian-Barremian.

Early Cambrian syndepositional structure of the northern Tarim Basin and a discussion of Cambrian subsalt and deep exploration

Abstract Using field geological survey, drilling and seismic data, combined with the study of regional tectonic evolution and structural deformation, as well as lithological and sedimentary analysis, we reconstructed the basin filling process and paleo-geography of north Tarim Basin in Early Cambrian, aiming to analyze the factors controlling the distribution and spatial architecture of the subsalt reservoir and source units and to define the favorable exploration direction. The Late Sinian tectonic activities in the northern Tarim Basin were characterized by different patterns in different areas, which controlled the sedimentary pattern in the Early Cambrian. The boundary faults of Nanhuaian rift basin in the south slope of Tabei uplift and the north slope of Tazhong uplift became reactivated in the Early Cambrian, forming two NEE and EW striking subsidence centers and depocenters, where the predicted thickness of the Yurtusi Formation could reach 250 meters. In the Xiaoerbulake period, the weak rimmed platform was developed in the hanging wall of syndepositional fault. Whereas the Nanhuaian rift system in the Tadong and Manxi areas were uplifted and destroyed in the Late Sinian, and appeared as gently slope transiting toward the subsidence center in the Early Cambrian. The former had the sedimentary features of hybrid facies platform and the latter had the sedimentary features of ramp platform. The black shale of the Yurtus Formation in the footwall of syndepositional fault and the reef bank of Xiaoerbulake Formation platform margin in the hanging wall in Early Cambrian constitute a predicable source-reservoir combination. The activity intensity of syndepositional fault controlled the thickness of black shale and the scale of the reef bank. It is suggested carrying out high accuracy seismic exploration to determine the location of Early Cambrian syndepositional faults, on this basis, to search the reef bank of Xiaoerbulake Formation along the faults westward, and then drill risk exploration wells at sites where traps are shallow in buried depth.

Sequence framework, depositional evolution and controlling processes, the Early Carboniferous carbonate system, Chu-Sarysu Basin, southern Kazakhstan

Abstract The Lower Carboniferous carbonate system comprises the important oil/gas interval in the Chu-Sarysu Basin, Kazakhstan. Based on comprehensive analysis of seismic, well logging, core and thin section data, sequence architecture and depositional evolution of the carbonate platform and their controlling processes are documented. Fourteen lithofacies and five lithofacies associations of the Lower Carboniferous carbonate systems are identified according to thin section, core and logging data. The main depositional facies include platform margin reef-shoal, platform margin and open platform shoal, restricted platform shoal, intrashoal-tidal flat and lagoon-evaporate platform deposits. The Lower Carboniferous consists of four composite sequences (CS1-4) mainly bounded by regional unconformities, nine sequences confined by local unconformities and eighteen fourth-order sequences defined by instantaneous exposed discontinuities. Sequences are generally composed of the transgressive systems tracts (TST) and the highstand systems tracts (HST). TST consists mainly of medium-low energy intershoal or tidal flat deposits, while HST is characterized by evaporative lagoon, open platform or platform margin reef-shoal complexes. The research indicates that the carbonate platform experienced the evolution from ramp-non rimmed platform (CS1-2) to rimmed platform (CS3-4) in the study area. Large-scale transgressions occurred at the lower-middle parts of CS2 and CS3, resulting in the short-period submergence of the platform. The sedimentary architecture from the transgressive marlstone, wackestone in the bottom to gypsum in the top in CS4 and CS1 is suggested to be attributed to the interplay of climate and sea level changes. The sea level changes reflected by sequence architectures of the Lower Carboniferous in the study area appear to be comparable with Haq's global sea-level changes. The high-frequent fluctuation of sea-level changes (CS3-4) during the Icehouse in the Early Carboniferous may have an important influence on the development of the third to fourth order depositional cycles of the carbonate deposits.

High-resolution sequence divisions and stratigraphic models of the Amu Darya right bank

The Amu Darya Basin in Turkmenistan is a large petroliferous sedimentary basin. We analyzed the Callovian-Oxfordian succession of the Amu Darya right bank area based on Vail’s classic sequence stratigraphy and drill core, logging, seismic, thin-section petrographic, and geochemical data. We found that the Callovian-Oxfordian sequence can be divided into five third-order sequences (SQ1–SQ5) and 15 fourth-order sequences (sq1–sq15). A stratigraphic framework for the study area was newly established, based on these high-resolution sequence divisions. Two types of carbonate platform models were developed: (i) Callovian, in which the region was likely a ramp platform with a gradual slope to the east. It is mainly developedouter ramp and mid-ramp facies deposition which consists of micritic limestones, shoal grainstones, and reef limestones; (ii) Oxfordian, during which the region likely evolved into a rimmed platform with highly disparate sedimentation rates (low in the east) that were sensitive to sea level change, and a starkly different sedimentary landform that was high in the west and low in the east.

The Cambrian-Ordovician boundary in the Tarim Basin, NW China: Geochemistry and geophysics data anomalies

This paper presents outcrop, petrographic, geochemical, well log and seismic data which together characterise the T80 unconformity that occurs between the carbonate-dominated Ordovician and Cambrian successions in the Tarim Basin, NW China. In the region the T80 unconformity represents the Cambrian-Ordovician transition surface controlled by both large-scale tectonic movements and sea level changes. The unconformity had a major influence on platform-margin architectures and depositional environments in the Palaeozoic carbonates below and above the unconformity. Seismic profiles show that a progradational ramp platform margin developed in the Upper Cambrian below the T80 unconformity, while a retrogradational rimmed platform margin developed in the Lower Ordovician above the T80 unconformity. The Upper Cambrian carbonates are dolomitized whereas limestones dominate in the Lower Ordovician strata. At four measured outcrop sections near the northwestern margin of the basin (Yingshan, Yonganba, Penglaiba and Shuinichang), extensive dissolution porosity from karstification is developed below the T80 unconformity surface. The unconformity is associated with various geochemical anomalies. δ13C and δ18O ratios in the dolomitized Upper Cambrian strata immediately below the unconformity surface display negative excursions, paired with significantly reduced concentrations of major and trace elements such as, K, Ti, Sr, Fe, Ca, Si, Cl and S, while Mg and Ba are enriched relative to the Ordovician strata above. Furthermore, it is proposed that changes in the natural gamma radiation and Sr element concentrations can be used as a new tool for the identification and correlation of the Cambrian-Ordovician boundary, especially where diagnostic fossil markers are missing.

Fluid flow through carbonate platforms as evidence for deep-seated reservoirs in Northwest Australia

Fluid flow features in carbonate platforms of the outer Browse Basin, Northwest Australia, are investigated using 3D seismic and borehole data. During the Cenozoic, the study area evolved from a carbonate ramp to a rimmed platform with isolated carbonate build-ups; as such it played a key role in focusing fluid on their buoyant flow to the surface. Statistical analyses of direct hydrocarbon indicators show fluid flow to be focused in elevated areas i.e., the greater is the focusing of fluid, the larger is the carbonate platform or isolated build-up. Locally, karst systems created regions of enhanced permeability through the evolving carbonate stratigraphy. Karstified horizons are located on the top of carbonate build-ups and platform clinoforms as fluid buoyantly concentrates and diffuses up into topographic highs. In turn, the putative migration of gas and fluid generated hypogenic karst systems, enhancing permeability in otherwise lower permeability rock. Based on the interpreted data, we suggest a pivotal relationship between the presence of carbonate build-ups (and karsts) in the Browse Basin and hydrocarbon accumulations at depth. Areas of elevated topography focus sub-surface fluid, enhancing the permeability of carbonate successions. In turn, focused fluid flow can lead to the generation of methanogenic carbonates, promoting the growth of isolated build-ups and leading to further generation of elevated features. This grants the identification of similar features on seismic data from Northwest Australia, and other Equatorial margins in the world, as a valid proxy for the recognition of hydrocarbon accumulations below thick carbonate successions.

Lithofacies paleogeography and favorable gas exploration zones of Qixia and Maokou Fms in the Sichuan Basin

In the Sichuan Basin, the Middle Permian marine carbonate rocks are important natural gas pay zones with immense exploration potential, but the facies belts and distribution situations of layered dolomite reservoirs are not clear, which hinders the progress in natural gas exploration. In view of the sedimentary background of the Sichuan Basin, field outcrop section, drilling data and seismic data were comprehensively analyzed with the favorable reservoir intervals in the framework of sequence stratigraphy as the key research units. Research results about its lithofacies paleogeography were obtained as follows. First, a gentle slope SW high and NE low was presented during the sedimentation of the Qixia Fm in the Middle Permian. In the Maokou Fm of the Middle Permian, however, a series of the N–W trending intra-platform rifts were developed in this setting, and eventually a paleogeographic pattern of NE-dipping alternative uplift and depression was evolved. Second, in the Qixia Fm, the transgressive tract was in an open platform environment and the highstand system tract evolved into a rimmed platform. And the platform margin beach in the area of Jiange–Ya'an is the favorable reservoir facies belt. And third, in the Maokou Fm, the transgressive tract was in the carbonate shelf environment and the highstand system tract evolved into a rimmed platform. And the platform margin reef flat in the area of Jiange–Ya'an and the syneclise margin beach in the area of Yanting–Guang'an are favorable reservoir facies belts. It is concluded that the two grain beach facies belts in the areas of Jiange–Ya'an and Yanting–Guang'an were the favorable zones for the large-scale development of Middle Permian layered dolomite reservoirs and they are the main target of subsequent natural gas exploration.

Stratigraphic architecture and platform evolution of the Changxing Formation (Upper Permian) in the Yuanba Gas Field, northeastern Sichuan Basin, China

During the late Permian, the Yuanba area of the northeastern Sichuan Basin, China, was predominantly occupied by a carbonate platform in an extensional setting. On the basis of wireline log, core sample, and cutting fragment, two depositional systems are recognized in the Changxing Formation, including a homoclinal ramp and a rimmed carbonate platform system. During deposition, the platform was evolved from a homoclinal ramp through a poorly rimmed platform into a rimmed platform. High-energy deposits of shoals and reefs preferentially formed on the pre-existing palaeohighs and grew upward to develop an aggradational architecture. Two depositional (third-order) sequences, consisting of several parasequences, are recognized in the Changxing Formation, the development of which was mainly controlled by eustatic fluctuations, with minor influence of tectonism. Individual sequences were defined by conformable surfaces along the depositional dip section. The distribution of facies and parasequence composition are variable both laterally across the platform-basin profile and vertically through time due to a combination of relative sea-level changes, palaeogeographic architecture, and tectonism. The sedimentological and stratigraphic characterization will aid further future exploration and production of hydrocarbon from the Changxing Formation.

Stratigraphical Study of the Taft Formation in the Bohruk Area from Yazd Block, Central Iran

The Taft Formation is exposed at the Bohruk area from Yazd Block in central Iran with 136 m thickness comprising three informal members which are named, in ascending stratigraphical order named Lower, Middle and Upper members. The Lower and Upper members consist of bioclastic carbonate rocks, whereas the Middle Member contains two sub-members: carbonate and siliciclastic carbonate rocks. Four patterns of facies are recognized on the basis of field observation and microscopic study. They are grainstone facies, siliciclastic-carbonate facies, intermediate facies (grainstone-wackestone/packstone facies) and foraminiferal-clastic wackestone/packstone facies. Based on the facies associations, three facies belts are presented for the first time. Therefore, this formation was deposited on a shallow carbonate platform from the inner platform to rimmed platform. The main fauna of the Taft Formation include Istriloculina eliptica (Iovcheva), Palorbitolina lenticularis (Blumenbach), Charentia cuvillieri Neumann, Everticyclammina hedbergi (Maync), Pseudocyclammina litus (Yokoyama), Melathrokerion valserinensis Bronnimann & Conrad, Nautiloculina oolithica Mohler, Glomospira urgoniana Arnaud Vanneau, Dicyclina schlumbergeri Munier-Chalmas, Derventina filipescui Neagu, Praechrysalidina infracretacea Luperto-Sinni, Pseudochoffatella cuvillieri Deloffre, Paracoskinolina sp., Textularia sp., and Lenticulina sp. Based on the foraminiferal distribution throughout the Taft Formation, two biozones like Istriloculina elliptica-Palorbitolina lenticularis zone and Glomospira urgoniana-Dicyclina schlumbergeri zone are recognized they and range from the Late Barremian to Early Albian in age.

Carbonate platform-margin architecture and its influence on Cambrian-Ordovician reef-shoal development, Tarim Basin, NW China

Six types of carbonate platform marginal slope have been identified in the Cambrian-Ordovician of Tarim Basin. These include a progradational carbonate ramp platform, a progradational carbonate rimmed platform, an aggradational rimmed carbonate platform, a retrogradational rimmed carbonate platform, an aggradational margin on an isolated carbonate platform and a retrogradational margin on a submerged carbonate platform. Various platform margin architectures and reef-shoal characteristics occur in different areas of the Tarim basin. A progradational carbonate ramp platform margin began in the Early Cambrian in the Tarim basin. A progradational rimmed platform margin developed in the Middle Cambrian time in the northern Tarim basin, whereas contemporaneously a progradational ramp platform margin developed in the southwestern and central Tarim basin. A retrogradational rimmed carbonate platform developed in the northern Tarim basin, whereas an aggradational rimmed carbonate platform occupied the central Tarim basin in the Early Ordovician time. An aggradational margin on an isolated platform and a retrogradational margin on a submerged platform developed mainly during the Late Ordovician in the central Tarim basin. Platform margin-slopes migrated continually throughout the Cambro-Ordovician. The broad platform area (western Tarim Platform) that occupied most of the northwestern and central Tarim Basin in the Early Cambrian time was split by an east-west oriented shelf seaway that began to develop along the eastern and western margins of the platform in the Early Ordovician time and completely transected the platform by Late Ordovician time. The resultant central Tarim Basin platform area was much reduced, elongate east-west around the Central Tarim paleo-uplift, and terminated eastward towards region of the Late Ordovician basinal deposition across the eastern Tarim Basin. Affected by platform margin structure, Climate and paleodepth, algal reefs and shoals developed mainly in the Cambrian, and intraclastic shoals developed mainly in the Early Ordovician, followed by reef-building of framestone or bafflestone in the Late Ordovician.