Facies Control Research Articles

Trace element and stable (C, O) and radiogenic (Sr) isotope geochemistry of stromatolitic carbonate rocks of the Mesoarchaean Pongola Supergroup: Implications for seawater composition

The Mesoarchaean Pongola Supergroup hosts one of the oldest well-preserved stromatolitic carbonate successions. Major- and trace-element geochemistry along with C, O, and Sr isotopic analyses of three stratigraphic sections of stromatolitic carbonate rocks were undertaken in order to infer the depositional environment, and to unravel the chemical characteristics of contemporaneous seawater. The carbonate rocks consist mainly of dolomite and, to a lesser extent, ankerite. Their variable Sr concentrations of ~95 to 450mg/kg are interpreted as resulting from early to late dolomitization of metastable carbonates (aragonite and/or high-Mg calcite), facies control and calcitization during meteoric diagenesis. δ18O values range from −19.6 to −12.0‰ VPDB and reflect recrystallization of carbonates caused by high water-rock interactions during diagenesis and metamorphism. A δ13C value of 2.5‰ VPDB was measured in carbonate samples least affected by alteration, and we suggest that it approximates the composition of dissolved inorganic carbon in coeval seawater. The samples display variable 87Sr/86Sr isotopic ratios, from as low as 0.708 to 0.777. These ratios are considerably higher than those expected in Archaean open-ocean water masses buffered by hydrothermal interaction with the mantle. We interpret the highly radiogenic Sr isotopic ratios in some carbonate samples to reflect isotopic exchange with radiogenic Sr from siliciclastic/volcaniclastic components in the samples and fluids from clay-rich layers intercalated with the carbonates. The least radiogenic Sr isotopic ratio may approximate the mixed isotopic composition of seawater in the epicontinental basin and of riverine freshwater draining the Kaapvaal Craton. The dolomitic carbonate samples show shale-normalized rare earth element and yttrium distribution patterns that resemble those of modern ocean water with positive LaSN, GdSN and YSN anomalies, superchondritic Y/Ho ratios, a small negative CeSN anomaly and depleted light rare earth elements relative to the heavy rare earth elements. These anomalies are not as pronounced as expected for an open marine setting, which together with the Sr isotopic results support a more restricted setting with riverine input and fluctuations in marine ingressions. A convex-upward shale-normalized rare earth element and yttrium pattern is probably a result of several factors, including organic matter complexation, and element fractionation during estuarine mixing. The small negative CeSN anomaly may suggest small amounts of free oxygen in shallow waters due to local oxygenic microbial activity.

Characterization of Eocene lacustrine source rocks and their oils in the Beibuwan Basin, offshore South China Sea

Two hundred twenty five rock samples and thirty seven oil samples from the Beibuwan Basin, South China Sea, were analyzed with geochemical and organic petrological techniques to evaluate the Eocene lacustrine source rocks, and investigate controls on their properties and the distribution of different oil families in the basin. Two types of organic facies are recognized in the Liushagang (LS) Formation. The first organic facies is algal-dominated, and mainly occurs in the organic-rich, laminated mudstones of the middle member of the LS (LS-2) that were deposited in an anoxic, stratified, medium–deep lake environment. It is geochemically identified by its high abundance of C30 4-methylsteranes and heavy δ13C values in the range of -22.4 to -27.5‰. The organic matter in this organic facies comprises type I and II1 kerogens, with its macerals dominated by fluorescent amorphous organic matter (AOM) and exinites, indicating a highly oil-prone character. The second organic facies is of terrestrial algal origin, and is mainly identified in the non-laminated mudstones of the upper (LS-1) and lower (LS-3) members of the Liushagang Formation that were deposited in shallow, dysoxic, weakly stratified, fresh-water environments. Source rocks of the second organic facies mainly contain type II1–II2 kerogens with mixed macerals of AOM, internites and vitrinites. It is geochemically differentiated from the algal-dominated organic facies by its relatively low abundance of C30 4-methylsteranes and lighter δ13C values in the range of -27.20 to -28.67‰. Three oil groups are identified by their biomarkers and stable carbon isotopes. The first two groups (A and B) are probably end-members of two major oil families (A and B) that correspond to the algal-dominated organic facies and algal-terrestrial organic facies, respectively. Most of the discovered oils belong to Group A oils that are characterized by a high abundance of C30 4-methylsteranes and heavy δ13C values, and show a good correlation with the algal-dominated organic facies in LS-2. Group B oils are found only within the LS-1 and LS-3 reservoirs, and they are recognized by their relatively low content of C30 4-methylsteranes and lighter δ13C values, showing a close relation to the algal-terrestrial source facies within the LS-1 and LS-3members, respectively. Group C oils display intermediate biomarker features and stable carbon isotope values, and are interpreted to be a mixture of Group A and B oils. The oil-source correlation reveals a strong control of organic facies on the geographic distribution of oil groups or oil fields in the basin.

Reservoir Prediction Under Control of Sedimentary Facies

Acoustic impedance (AI) from seismic inversion can indicate rock properties and can be used, when combined with rock physics, to predict reservoir parameters, such as porosity. Solutions to seismic inversion problem are almost nonunique due to the limited bandwidth of seismic data. Additional constraints from well log data and geology are needed to arrive at a reasonable solution. In this paper, sedimentary facies is used to reduce the uncertainty in inversion and rock physics modeling; the results not only agree with seismic data, but also conform to geology. A reservoir prediction method, which incorporates seismic data, well logs, rock physics and sedimentary facies, is proposed. AI was first derived by constrained sparse spike inversion (CSSI) using a sedimentary facies dependent low-frequency model, and then was transformed to reservoir parameters by sequential simulation, statistical rock physics and [Formula: see text]-model. Two numerical experiments using synthetic model and real data indicated that the sedimentary facies information may help to obtain a more reasonable prediction.

Treptichnus pedumand the Ediacaran–Cambrian boundary: significance and caveats

AbstractThe Ediacaran–Cambrian (E-C) boundary is based on the first appearance of the ichnofossilTreptichnus pedum. Investing an ichnotaxon with such biostratigraphic pre-eminence has been the focus of criticism. Points of contention have revolved around four main issues: (1) ichnotaxonomy, (2) behavioural significance, (3) facies controls and (4) stratigraphic occurrence. First, confusion results from the fact thatTreptichnus pedumwas originally referred to asPhycodes pedumand, more recently, some authors have placed it inTrichophycusorManykodes. However, the overall geometry of these burrows indicates they belong inTreptichnus.Second, regardless of its precise mode of feeding, the behaviour involved is iconic of the Cambrian explosion. Third, objections are based on the idea that trace fossils show a closer link to facies than body fossils. Notably, in contrast to common assumptions,T. pedumis not only present in the low-energy offshore of wave-dominated marine settings, but it occurs at considerably shallower water in intertidal and shallow-subtidal zones of tide-dominated systems, as well as in mouth bars of deltaic systems and lower shoreface to offshore transition zones of wave-dominated marine settings. Its broad environmental tolerance supports evolutionary innovations rather than facies controls as the main mechanism underlying the observed vertical pattern of distribution ofT. pedumin most E-C successions comprising shallow-marine deposits. Fourth, although treptichnids have been documented below the E-C boundary,T. pedumis not known from Ediacaran rocks. The delayed appearance ofT. pedumin E-C successions should be analysed on a case-by-case basis.

Distribution of remaining oil based on fine 3-D geological modelling and numerical reservoir simulation: a case of the northern block in Xingshugang Oilfield, China

As the Xingshugang Oilfield is in the late stage of development, a conventional geological model could not meet the needs of further enhancing oil recovery, and the establishment of a fine 3-D geological model, namely the 3-D reservoir architecture model, is urgently required. The 3-D reservoir architecture model has a strong advantage in the detailed characterization of the distribution of various architectural elements and flow baffles and barriers in 3-D space. Based on the abundant data from close well spacing, in combination with the understanding of sedimentary facies and reservoir architecture, this study builds the 3-D reservoir architecture model to show the spatial distribution of different architectural elements and intercalations (mud drapes) under the control of third-, fourth- and fifth-order bounding surfaces. The study then establishes the property model under the control of sedimentary facies (architectural elements). Subsequently, based on the fine 3-D geological model, the distribution of remaining oil is obtained after the numerical reservoir simulation. The remaining oil primarily lies in the port of channel bifurcation, the parts blocked by intercalations and abandoned channels, and the edges of different facies. This observation provides a theoretical basis for further development and adjustment.

Control of facies, maturation and primary migration on biomarkers in the Barnett Shale sequence in the Marathon 1 Mesquite well, Texas

A great deal is known about the genetic relationships between biomarkers and their biogenic precursors in organic rich rocks. The same is true of the way in which biomarker compound ratios change during maturation. On the other hand, very little is known about whether a crude oil can fully retain its inherent compositional ancestry during expulsion from a source rock. Thanks to shales being characterized in great detail for their unconventional resource potential, new information is gradually coming to light. Here we report on observations in biomarker geochemistry of a thermally mature core of the Barnett Shale, in which organofacies and maturity are essentially the same, but where intraformational sources and reservoirs have already been reported.Our results indicate that most biomarkers are not fractionated as the primary migration of petroleum within source rocks takes place. The 20S/(20S + 20R) ratio of C27 steranes is uniform in the whole source-rock sequence, while the 20S/(20S + 20R) ratio of C29 steranes shows indistinctly high values in the reservoir unit. The 20S/(20S + 20R) ratio of diasteranes and the 22S/(22S + 22R) ratio of C31 17α-hopanes do not appear to have been fractionated, which may be a result of the thermal isomerization reactions predominating over and masking out the possible fractionation effects. Diasteranes/steranes ratios do not exhibit features that suggest an association with fractionation, but rather are broadly correlated with lithology. However, compared to the diasteranes/steranes ratios, the Ts/(Ts + Tm) ratio is much more sensitive to changes in mineral compositions. Variations in the Ts/(Ts + Tm) ratio show a positive correlation (R2 = 0.73) with mixed-layer illite-smectite content. Fractionation in the Ts/(Ts + Tm) ratio, if it has so occurred, may be subsequently overprinted by in-situ clay-catalyzed reactions.

Control of coal facies to adsorption-desorption divergence of coals: A case from the Xiqu Drainage Area, Gujiao CBM Block, North China

Adsorption and desorption of coal to methane are substantially related to coal facies, referring to the production potential of a coalbed methane (CBM) well. Based on analyses of the stratified channel samples from coal seams No. 2, No. 4 and No. 9 in the Xiqu Drainage Area, Gujiao CBM Block, North China, the relationship between coal facies and adsorption-desorption behaviors was first proposed and discussed. Using the coal facies diagrams suggested by previous investigators, four types of coal facies were distinguished in the coal seams, including the upper delta plain wet forest swamp (I), the lower delta plain fen (II), the lower delta plain marsh (III) and the back barrier low moor (IV). The upper delta plain wet forest swamp occurred dominantly, developing at the bottom and top of all the coal seams. Both the ash yield and groundwater index (GWI) tend to increase from coal facies I to IV, whereas the volatile compounds, fixed carbon, organic sulfur, total sulfur, micropore volume and specific surface area, tissue preservation index (TPI) and vegetation index (VI) decreased, which indicates changes in the vegetation types, water supplements and disturbance, acid-base properties and redox conditions in the paleomires. Paleomire evolution accompanied the rise and drop of the sea level, with a transgression or regression process that could be represented by the coal facies sequence. Coal seams No. 2, No. 4 and No. 9 were discriminated into two, three and two evolving stages, respectively, and these stages were characterized by complete cycles, except stage 4-I from the lower delta plain marsh. Mudstone partings in the coal seams deposited during the highest sea level. Collectively, paleomire environments, especially the variation of vegetation types and mire water condition, result in adsorption and desorption divergences among different coal facies. Arborescent plants decrease from coal facies I to IV as the hydrodynamic intensity increases, leading to corresponding changes in botanical tissue preservation, medium oxidizability, mineral content and, ultimately, adsorption and desorption behaviors. In view of the gas content and desorption capacity of the coal layers in the same coal seam, the layers from the upper delta plain wet forest swamp may have the best potential for CBM drainage, followed by those from the lower delta plain fen and marsh, while those from the back barrier low moor may have the worst potential.

Palaeocommunities, diversity and sea-level change from middle Eocene shell beds of the Paris Basin

The middle Eocene interval at some Paris Basin localities was studied through high-resolution stratigraphy. Abundance data (499 species, 37 719 individuals) on the distribution of molluscs, collected at 12 shell beds of the middle Lutetian and lower Bartonian, formed the basis for a palaeoecological study. The middle Lutetian succession is subdivided into several elementary depositional sequences (EDS) interpreted as the product of relative sea-level change. Species-abundance distributions are better correlated with EDS than with geographical locality, suggesting that sea level played an important role in the distribution of palaeocommunities. Diversities were compared with analogous data from modern subtropical and warm-temperate intertidal and subtidal communities. We found that sea-level variation is responsible for a major change in the upper part of the middle Lutetian succession, leading from high- to low-diversity palaeocommunities. From base to top sampled palaeocommunities indicate a transition from high-energy and mesotrophic (EDS 2) to oligotrophic low-energy conditions of a sandy lower shoreface (EDS 4) to an upper shoreface (EDS 5 and lower Bartonian), the last with mangroves and a seagrass cover. Notwithstanding the Lutetian cooling, we found that subtropical conditions reached as far north as the Paris Basin. Our study suggests that climatic fluctuations might be obscured by facies control.

Facies controls on the distribution of diagenetic alterations in fan delta deposits: a case study from the Lower Cretaceous sandstone, Chagan Sag, Inner Mongolia, China

AbstractBy integrating diagenesis and sedimentary facies, the distribution of diagenetic alterations and their impacts on reservoir quality were investigated within a lacustrine fan delta depositional environment in the Lower Cretaceous of the Western sub‐sag in the Chagan Sag, Yin‐E Basin, northern China. Core observation and analyses of petrography and geochemistry of the sandstones revealed that the eogenetic alterations display spatial and temporal distribution patterns associated with sedimentary facies, including distributary channels of a fan delta front (DC), distributary bays (DB) and sheet sands (SS) and mouth bar (MB) (SS and MB are collectively referred to as S). Percolation of meteoric waters occurring in high permeability DC sandstones resulted in leaching of feldspar, the formation of kaolinite, as well as mechanical infiltrated clays around detrital grains. Conversely, the DB deposits containing abundant ductile lithic fragments were subjected to mechanical compaction and thus the development of a pseudomatrix. During a long burial residence time (the Early Cretaceous period) in a lacustrine environment, abundant early carbonate cements were precipitated, especially in the distal S sandstones. The sandstones lacking eogenetic cements were subjected to stronger mechanical compaction. Additionally, eogenetic alterations have an important impact on the distribution of the mesogenetic alterations. Sandstones containing few eogenetic cements or thin or discontinuous infiltrated clay rims around the detrital grains were subjected to quartz cementation. However, during the hydrocarbon generation stage, the most efficient percolation of meteoric waters and organic acid dissolved feldspar minerals and carbonate cements, resulted in DC sandstones having more intragranular and intergranular porosity. Owing to high matrix contents and early calcite cement contents, the DB and S sandstones had lower permeability and were rarely dissolved. During the late mesogenetic stage, late carbonate cements occurred in all sedimentary facies and iron and magnesium ions were released from the transformation from kaolinite to illite or to chlorite. The results from this study show possible diagenetic evolutionary pathways in the reservoir sandstones within the fan delta depositional environment, which in turn provides some insights into the controls on reservoir potential. Copyright © 2016 John Wiley & Sons, Ltd.

Sedimentary features and exploration targets of Middle Permian reservoirs in the SW Sichuan Basin

The exploration direction and targets for the large-scale Middle Permian gas reservoirs in the Sichuan Basin are hot spots and challenges in current exploration researches. The exploration successes of large gas field of Cambrian Longwangmiao Formation in Gaoshiti-Moxi region, Central Sichuan Basin, indicated that prospective sedimentary facies belt was the basis for the formation of large gas fields. In this paper, based on seismic data, outcrop data and drilling data, the tectonic framework and sedimentary features of the Middle Permian in the SW Sichuan Basin were comprehensively studied. The following conclusions were reached from the perspective of sedimentary facies control: (1) during the Middle Permian, this region was in shallow water gentle slope belts with high energy, where thick reef flat facies were deposited; (2) the basement was uplifted during Middle Permian, resulting in the unconformity weathering crust at the top of Maokou Formation due to erosion; the SW Sichuan Basin was located in the karst slope belt, where epigenic karstification was intense; and (3) reef flat deposits superimposed by karst weathering crust was favorable for the formation of large-scale reef flat karst reservoirs. Based on the combination of the resources conditions and hydrocarbon accumulation conditions in this region, it was pointed out that the Middle Permian has great potential of large-scale reef flat karst gas reservoir due to its advantageous geological conditions; the Middle Permian traps with good hydrocarbon accumulation conditions were developed in the Longmen Mountain front closed structural belt in the SW Sichuan Basin and Western Sichuan Basin depression slope belt, which are favorable targets for large-scale reef flat karst reservoirs.

Background fractures in carbonates: inference on control of sedimentary facies, diagenesis and petrophysics on rock mechanical behavior. Example of the Murge Plateau (southern Italy)

Characterizing fracture networks in Naturally Fractured Reservoirs (NFR) is a major challenge for hydrocarbon exploration and production. The fracture organization is: i) a key parameter for fluid flow understanding in carbonates (in a general sense) and ii) the key parameter in tight, non-porous carbonates. However, at present-day, the fracture parameters from subsurface data are not sufficient to define the 3D distribution of a fracture network. Moreover, in carbonate reservoirs, fracture occurrence strongly depends on the mechanical properties of the host rock such that characterizing fractures is only possible through the close study of the host rock heterogeneities. Studying fractures in outcrop analogues allows accessing to 3D fracture patterns, determining the host rock mechanical properties and their impact on fracture distribution. In this paper, we trace the conditions for the fracturing process through the geodynamic history of the reservoir. We aim at determining the sedimentary and diagenetic controls on fracture patterns and the genetic correlation with tectonic and burial history.To this purpose, we have targeted two formations made of platform carbonates from the Murge Plateau. We analyzed the fracture patterns and sedimentary facies as well as the tectonic, diagenetic and burial history for each studied site. Fracture patterns are characterized from geometrical, kinematic and diagenetic criteria based on field and laboratory measurements. Fracture sequences are defined based on cross-cutting and abutting relationships and compared with geodynamic history.Fractures are bed-perpendicular joints and veins, and numerous bed-parallel stylolites. We observe a stage of fracturing synchronous with shallow burial and prior to major tectonic events. Carbonates have undergone early diagenesis during fast and shallow burial, conferring early brittle behavior. Fracture development, mechanical and petrophysical properties are acquired during early burial.

Diagenesis and Diagenetic Evolution of Deltaic and Neritic Gas‐Bearing Sandstones in the Lower Mingyuefeng Formation of Paleogene, Lishui Sag, East China Sea Shelf Basin: Implications for Depositional Environments and Sequence Stratigraphy Controls

AbstractGas‐bearing deposits in the Lower Mingyuefeng Formation of Paleogene, Lishui Sag, East China Sea Shelf Basin consist of shoreface sandstones of the highstand systems tract (HST) and transgressive systems tract (TST), and deltaic sandstones of the lowstand systems tract (LST) and falling stage systems tract (FSST). Detailed petrographic observations suggest that the diagenetic features and related evolution of these deposits cannot be simply characterized and demonstrated in the depth domain. However, the occurrence of diagenetic minerals systematically depends on the studied interval within the HST, TST, LST, and FSST; therefore, diagenesis in this region can be better constrained when studied in the context of the depositional environments and sequence stratigraphic framework. The eogenetic processes in such settings include: (1) microcrystalline siderite precipitated as concretions in almost all environments and systems tracts, which inhibited further mechanical compaction; (2) grain dissolution and kaolinitization occurred in shoreface HST sandstones and deltaic LST and FSST sandstones; (3) glaucony was locally observed, which did not clearly reflect the controls of facies or sequence stratigraphy; and (4) cementation by pyrite aggregates occurred in the shoreface HST sandstones and deltaic LST sandstones. The mesogenetic diagenesis includes: (1) partial conversion of kaolinite into dickite in deltaic LST sandstones, and minor chlorite cementation in deltaic FSST sandstones; (2) transformation of kaolinite into illite and quartz cementation in deltaic LST and FSST sandstones; (3) frequent precipitation of ankerite and ferroan calcite in shoreface TST sandstones and early HST sandstones, forming baffles and barriers for fluid flow, with common calcite in shoreface HST sandstones as a late diagenetic cement; and (4) formation of dawsonite in the deltaic LST and FSST sandstones, which is interpreted to be a product of the invasion of a CO2‐rich fluid, and acts as a good indicator of CO2‐bearing reservoirs. This study has thus constructed a reliable conceptual model to describe the spatial and temporal distribution of diagenetic alterations. The results may provide an entirely new conceptual framework and methodology for successful gas exploration in the continental margins of offshore China, thus allowing us to predict and unravel the distribution and quality evolution of clastic reservoirs at a more detailed and reliable scale.

Stratigraphic Framework and Depositional Controls on Reservoir Occurrence, Big Valley Formation, Southern Alberta

Abstract Within the past decade, the Big Valley and Exshaw formations in southern Alberta have received significant interest as a potential unconventional oil play because of the overwhelming exploration/exploitation success in the stratigraphically-equivalent Three Forks and Bakken formations in Saskatchewan, Manitoba and North Dakota. Because of continuous industry hype, this upper Devonian-lower Mississippian succession came to be known as the “Alberta Bakken”. Such a term suggests potentially prolific oil production from a reservoir similar to the Bakken in Saskatchewan. In fact the term “Alberta Bakken” is misleading since to date there has been only moderate oil production from localized carbonate reservoirs in the Big Valley Formation, and from very fine-grained sandstones and siltstones of the Mississippian Lower Banff Formation. Reservoir development in the Big Valley Formation is stratigraphically and areally restricted by both depositional facies controls and post-depositional early diagenetic, subaerial and structural-collapse processes. Detailed lithofacies analysis indicates that the Big Valley Formation is divisible into two units: an upper shallow marine limestone and a lower hydrocarbon-bearing, partially dolomitized, peritidal carbonate. The peritidal unit, in turn, is divisible into four lithofacies: peloidal packstone-grainstone (the primary oil-bearing lithofacies), microbial laminite, laminated dolomudstone and intraclastic breccia-laminite. Regionally, each lithofacies is discontinuous and in the order of 0.5 to 2.0 m thick. Locally, however, the peloidal packstone-grainstone attains thicknesses of up to 8 m, forming isolated oil-producing reservoirs. These over-thickened zones correspond to specific areas of Big Valley Formation ‘thicks’, which tend to align with a NNW-SSE trending ‘basement’ lineament that underlies the study area. It is equivocal whether this structural trend is basement-controlled, or reflects dissolution of salt beds along the margin of an underlying evaporite basin. Successful exploration in the Big Valley Formation appears to depend on whether the over-thickened areas can be located.

The trace fossil Gyrophyllites in deep-sea siliciclastic deposits of the Istebna Formation (Upper Cretaceous–Palaeocene) of the Carpathians: An example of biologically controlled distribution

Gyrophyllites kwassizensis Glocker occurs abundantly in deep-sea, density current siliciclastics of the Istebna Formation (Silesian Nappe, Outer Western Carpathians; Poland, Slovakia and Czech Republic), close to the Cretaceous–Palaeogene boundary. It is composed of petal-shaped lobes radiating from a thin shaft, forming rosettes on a few levels in the sediment. The trace fossil is interpreted as a feeding structure produced by a soft-body “worm”, which actively filled the lobes with mud transported from the underlying layer, probably in response to migration of redox boundary in a generally poorly oxygenated environment. Probably due to microbial decomposition of cellulose and other organic particles, the mud from below was more nutritional than the more fresh mud from the sea floor. The number of lobes in a rosette (from 7 to 15) is referred to ontogenetic development of the tracemaker and indirectly to time of burrow occupation. The number varies between localities and may indicate a more hospitable habitat in the areas where the number is higher. G. kwassizensis occurs only in a narrow stratigraphic interval after a significant drop of sedimentation rate caused by non-eustatic reasons, in facies conditions, which are not unique during the long history (Late Jurassic–Miocene) of the external Carpathian subbasins. This suggests that facies control was less important for distribution of this trace fossil than some undetermined biological factors, which enabled successful, but incidental invasion of the tracemaker to deep-sea floor of one of the subbasins – the Silesian Basin. • Gyrophyllites kwassizensis is a burrow of a worm following migrating redox boundary. • It is filled with mud from underlying layer, more nutritional than surface sediments. • Tracemaker incidentally invaded slope-apron sediments, partly irrespective of facies.

3-D geological modeling for tight sand gas reservoir of braided river facies

Abstract Considering the poor applicability of conventional geological modeling to tight sand gas reservoir in braided river facies, a modeling method of “multi-stage constraints, hierarchical facies control and multi-step modeling” was put forward taking Sulige gas field in Ordos Basin as the study object. The method obtains the GR field by seismic inversion constrained by logging data, and GR model is built under the control of the prior geological knowledge; the relation regression is realized between the GR model and the sandstone probability, sandstone probability model is built, and rock facies model is obtained by multi-point geostatistics theory; sedimentary microfacies model controlled by rock facies and braided-river-system is made; and eventually an effective sand body model is built by integrating sedimentary microfacies, effective sand body scale and reservoir properties distribution. The research method discussed in this paper has put geological constraints into the model as far as possible, enhanced the inter-well sand body predictability and improved the precision rate, thus it can provide a more reliable geological basis for gas reservoir development.

Geometry, amount, and sequence of thrusting in the Subalpine Molasse of western Austria and southern Germany, European Alps

In this paper we review the structure of the most external thrust belt of the Alps between the Rhein valley and Salzburg based on a new tectonic map and the (re)interpretation of seismic sections. Specifically we address the correlation between deformation in the Subalpine Molasse and the Alpine thrust belt in general and focus on the control of sedimentary facies on the structural style. A dramatic change in architecture from a ramp-flat structure to buckle folding is related to a change from coarse-grained fans to fine-grained deposits within the Subalpine Molasse. Additionally the interaction of escape tectonics with postcollisional shortening controls the decrease of late Early Miocene and younger shortening within the Subalpine Molasse from 50 km near the Rhine valley to almost zero near Salzburg. Transfer of shortening into the hinterland, which is the zone of lateral escape, ended foreland propagation of the Alpine thrusts and initiated a general break-back sequence of thrusting. Throughout this time the thrusts remained active. In such a scenario, tectonic units on top of the Subalpine Molasse are expected to undergo clockwise rotation around vertical axes. As thrusting in the Subalpine Molasse is closely related to contemporaneous transport and shortening within the tectonically higher Helvetic thrust sheets, amounts of Miocene differential shortening and related clockwise vertical axis rotation are minimum amounts. True clockwise vertical axis rotation is probably larger than the 12° deduced from the Subalpine Molasse thrust belt.

THE OCCURRENCE OF VERTEBRATE AND INVERTEBRATE FOSSILS IN A SEQUENCE STRATIGRAPHIC CONTEXT: THE JURASSIC SUNDANCE FORMATION, BIGHORN BASIN, WYOMING, U.S.A

Previous studies of the sequence stratigraphic distribution of fossils have focused on the record of relatively abundant marine invertebrates. Only a handful of studies have examined how sequence stratigraphic architecture influences the occurrence of vertebrates, particularly large and rare tetrapods. The Jurassic Sundance Formation of the Bighorn Basin, Wyoming, USA, contains a rich suite of invertebrate and vertebrate fossils, including large and rare marine reptiles, and this allows the sequence stratigraphic controls on the distribution of these groups to be compared. The Sundance Formation consists of four depositional sequences, with the lower two being carbonate dominated and the upper two siliciclastic dominated. Two incised valley fills are also present. The presence of multiple depositional sequences and strongly erosional sequence boundaries is the likely cause of the complicated lithostratigraphic nomenclature of the Sundance. Invertebrates (mollusks and echinoderms) in the Sundance conform to well-established patterns of occurrences, including strong facies control and fossil concentrations at maximum flooding surfaces, in the upper portion of parasequences, and within lags overlying sequence boundaries. As expected from their rarity, marine reptiles (ichthyosaurs, plesiosaurs, and pliosaurs) show a weaker connection to sequence stratigraphic architecture. Nonetheless, they do display facies control and are found primarily in offshore mudstone, rather than shoreface and estuarine sandstone. They are also more common at hiatal surfaces, including a zone of concretions at the maximum flooding surface and in lag deposits overlying sequence boundaries. These associations suggest that sequence stratigraphic architecture may be a useful approach for discovery of marine vertebrates and that sequence stratigraphic context should be considered when making paleobiological interpretations of marine vertebrates as well as invertebrates.

Primary dolomite in the Late Triassic Travenanzes Formation, Dolomites, Northern Italy: Facies control and possible bacterial influence

AbstractIn the late Carnian (Late Triassic), a carbonate‐clastic depositional system including a distal alluvial plain, flood basin and sabkha, tidal flat and shallow carbonate lagoon was established in the Dolomites (Northern Italy). The flood basin was a muddy supratidal environment where marine carbonates and continental siliciclastics interfingered. A dolomite phase made of sub‐micrometre euhedral crystals with a mosaic microstructure of nanometre‐scale domains was identified in stromatolitic laminae of the flood basin embedded in clay. This dolomite is interpreted here as primary and has a nearly stoichiometric composition, as opposed to younger early diagenetic (not primary) dolomite phases, which are commonly calcian. This primary dolomite was shielded from later diagenetic transformation by the clay. The stable isotopic composition of dolomite was analyzed along a depositional transect. The δ13C values range between ca −6‰ and +4‰, with the most 13C‐depleted values in dolomites of the distal alluvial plain and flood basin, and the most 13C‐enriched in dolomites of the tidal flat and lagoon. Uniform δ18O values ranging between 0‰ and +3‰ were found in all sedimentary facies. It is hypothesized that the primary dolomite with mosaic microstructure nucleated on extracellular polymeric substances secreted by sulphate reducing bacteria. A multi‐step process involving sabkha and reflux dolomitization led to partial replacement and overgrowth of the primary dolomite, but replacement and overgrowth were facies‐dependent. Dolomites of the landward, clay‐rich portion of the sedimentary system were only moderately overgrown during late dolomitization steps, and partly retain an isotopic signature consistent with bacterial sulphate reduction with δ13C as low as −6‰. In contrast, dolomites of the marine, clay‐free part of the system were probably transformed through sabkha and reflux diagenetic processes into calcian varieties, and exhibit δ13C values of ca +3‰. Major shifts of δ13C values strictly follow the lateral migration of facies and thus mark transgressions and regressions.

Influence and control of coal facies on physical properties of the coal reservoirs in Western Guizhou and Eastern Yunnan, China

Western Guizhou and Eastern Yunnan have abundant coal resources, and they are considered as the most prospective areas for coalbed methane (CBM) development in Southern China. To explore the potential of CBM production in these areas, this paper identifies the plane distribution of coal facies, and analyses its control on the physical properties of coal reservoirs. Four types of coal facies including a wet forest swamp, an intergradation forest swamp, a drained forest swamp, and a fresh-water peat swamp were distinguished. The results show that coal facies play a significant role in the control of the coal reservoir properties for coals with similar rank.

Abrupt early Eocene global climatic change as a control of carbonate facies and diagenesis: a new record of the Palaeocene–Eocene Thermal Maximum in the Umm er Radhuma Formation, Saudi Arabia and Kuwait

Terra Nova, 24, 487–498, 2012AbstractRecords of the Palaeocene–Eocene Thermal Maximum (PETM) and associated Carbon Isotope Excursion (CIE) are relatively rare in shallow‐ marine carbonates when compared with those from other regimes, and it remains unclear exactly what effect this global climatic event had upon the widespread epeiric carbonate platforms of the Palaeogene. In this study, we demonstrate that the Umm er Radhuma Formation in the Wafra field, Kuwait and Saudi Arabia, shows a carbon isotope profile consistent with the CIE that can be correlated globally and further, that global climate change during and subsequent to the PETM exerted a fundamental control on primary depositional facies and subsequent diagenesis. The Palaeocene–Eocene Thermal Maximum was a brief hyper‐thermal event marking the onset of sustained high global temperatures during the Early Eocene. This warm period increased aridity in epeiric carbonate platform interiors of what is now the Middle East, affecting composition of depositional facies and the precipitation of early dolomite. This is demonstrated through the use of new data from the Umm er Radhuma Formation, Wafra field, Partitioned Zone, Saudi Arabia and Kuwait. Biostratigraphy and carbon isotope profiles have been used to constrain the position of the PETM and Carbon Isotope Excursion; a distinct ∼2–3‰ negative spike. The observed excursion is demonstrated to correlate globally and provides age dating of this regionally important succession. Core and thin‐section data show marked restriction of facies across the PETM, evidence of increasing aridity reflected in proportion of evaporite precipitates and a stratigraphically restricted non‐mimetic dolomite coeval to the CIE that is interpreted to relate to a change in the rate of dolomite precipitation. This work provides an example of how a global environmental event is manifest in depositional and early diagenetic changes in marine carbonates.