Volcanic Lithofacies Research Articles

The characteristics of volcanic lithofacies and volcanic reservoirs in the Permian Jiamuhe Formation, Southern Zhongguai Rise, Junggar Basin, NW China

The southern Zhongguai Rise has gained wide attentions due to its commercial hydrocarbon discoveries from volcanic deposits in Jinlong Oilfield. The integrated analysis of geological and geophysical data assist in determination and depiction of volcanic lithology and volcanic facie, characterization of volcanic reservoirs as well as discussion of constrains on volcanic lithology/lithofacies. The conclusions suggest that two types of volcanic lithology, including volcaniclastic rocks and silicic moltenlava, can be identified based on cores. In addition, two types of volcanic lithofacies can be delineated according to cores and wireline responses. The explosive facies are mainly developed in early depositional period while the effusive facies are developed through depositional period. Furthermore, the reservoir space predominantly consists of pores and fractures according to thin sections. The physical property varies from time to time. That is, porosity distribution varies slightly. In contrast, permeability distribution varies significantly. The volcanic reservoirs can be generally attributed to high-porosity and low-permeability reservoirs no matter in volcaniclastic rocks or silicic molten lava. The volcanic lithofacies, facies distribution, diagenism and paleogeomorphology exserts controls on physical property and reservoir performance of volcanic reservoirs together.

Volcanic lithofacies control the space in unconventional, rhyolitic hydrocarbon reservoirs: The Hailar Basin, NE China

Volcanic rocks are generally characterized by significant spatial variations of lithofacies. The characterization of volcanic lithofacies is an important tool to achieve accurate reservoir prediction during hydrocarbon exploration because of the type of lithofacies strictly controls the porosity and other physical and chemical properties of reservoir rocks. Studying the lithofacies of rhyolitic volcanic rocks is essential for uncovering the genesis and characteristics of volcanic reservoirs. The Lower Cretaceous Shangkuli Formation is located on the western boundary of the Hailar Basin in NE China. This basin hosts oil-bearing rhyolitic volcanic rocks and represents a natural laboratory for investigating the spatial distribution of different lithologies and volcanic lithofacies. Here, we present detailed field observations, TESCAN Integrated Mineral Analysis (TIMA), and fluorescence imaging to characterize seven types of rhyolitic volcanic rocks belonging to four main lithofacies associations. We examine the relationship between lithofacies associations and reservoir rocks. Field observations reveal the existence of volcanic conduit, pyroclastic, effusive, and extrusive lithofacies associations within the Hailar Basin. These lithofacies associations can be further subdivided into nine distinct subfacies. Among these, the extrusive lithofacies association is dominant and it is followed by pyroclastic, effusive, and volcanic conduit lithofacies associations. Intra-spherulite microfractures and pores, lithophysa cavities in the extrusive lithofacies associations, and inter-clast pores of the volcanic conduit lithofacies associations constitute the primary reservoir spaces of rhyolitic reservoirs. The lithofacies associations exert a control on the reservoir spaces of rhyolitic volcanic rocks by limiting the degree of supercooling (ΔT) and volatile content, which impact the formation of different textures in rhyolitic magma during syn-eruptive processes. We conclude that spherulitic and lithophysa rhyolites in the extrusive lithofacies associations, and the crypto-explosive breccias in the volcanic conduit lithofacies associations are promising targets for oil and gas exploration. The facies model we obtain from the Hailar Basin may be extended to similar rhyolitic rocks in other basins hosting unconventional reservoirs.

Three-dimensional seismic architecture of an Upper Cretaceous volcanic complex and associated carbonate systems; Taylor Group, Elaine field, South Texas, USA

The seismic architecture of a tuff-dominated volcanic complex and the associated carbonate systems in the Upper Cretaceous Taylor Group (Anacacho Limestone) Elaine field in South Texas was examined using a three-dimensional seismic dataset. The investigation demonstrates that the combination of traditional seismic lithofacies analysis and seismic sedimentology allows the visualization and interpretation of volcanic elements and penecontemporaneous carbonate lithofacies at the scale of tens of meters. A wireline-log based, rock-physics model was analyzed and calibrated using core samples, which link volcanic and carbonate lithofacies to 90°-phase seismic amplitude events. Seismic-velocity effects permit the identification of subtle volcanic lithofacies changes such as low-velocity tuff bodies or high-velocity basalt intrusions. Also, frequency fusion expands seismic visualization of thick lithofacies bodies (e.g., basaltic dykes) to the 100-m range and seismic facies analysis helps understand interrelationships between the components in and adjacent to the volcanic body. Horizontal seismic slices reveal seismic geomorphology at multiple stages of volcanic activity. This investigation reveals three major seismic facies systems (1) a magma-feeder (i.e., plumbing) intrusive system beneath the volcano, including deep faults, dykes and sills, and remnant radial/concentric fault systems; (2) a large (4000–5000 m diameter) volcanic complex consisting of a deep crater (300-m negative feature below the paleo-sea floor), a large mound (200-m positive feature above the paleo-sea floor), multiple secondary craters filled predominantly with tuffs and less common lava flows; and (3) penecontemporaneous carbonate systems including fringing and mound-top shoals and reefs, lagoon and debris fan aprons at the base of the volcanic mounds. The fringing and mound top carbonate systems were probably in water depths less than 20 m whereas for the fan-apron carbonates, water depths are estimated to be about 140–200 m. These carbonate bodies can form hydrocarbon reservoirs.

Main Control Factors and Hydrocarbon Accumulation Model of Volcanic Oil Reservoirs with Complex Oil–Water Relationships: A Case Study of the Carboniferous in the Chepaizi Uplift, the Junggar Basin, China

In order to study the main control factors of volcanic reservoirs with complex oil–water relationships, the Carboniferous in the Chepaizi Uplift of the Junggar Basin was taken as an example and the lithofacies characteristics, main control factors, and hydrocarbon accumulation model of volcanic reservoirs were investigated by combining the petroleum geology with field testing (data of core analysis, well logging, formation testing, and production testing). The results show that the Carboniferous in the Chepaizi Uplift experienced three stages of volcanic activities and developed seven volcanic lithofacies bodies, distributed in a bead-string connected planar form along the Hongche fault. There is no unified oil–water interface across the whole study area and there are multiple oil–water systems within one fault block. The Carboniferous volcanic reservoir experienced two stages of hydrocarbon accumulation from two different source rocks. The distribution of faults penetrating hydrocarbon kitchens and source rocks controls the macro-scale distribution of reservoirs. The physical properties of reservoirs affect the pattern of oil and water differentiation in volcanic rock bodies, while the lithofacies body-controlled hydrocarbon accumulation mode highlighting “one rock body for one reservoir” determines the distribution of reservoirs. The matching between the paleo-structure and hydrocarbon accumulation stage controls the accumulation and adjustment of hydrocarbon distribution. The Permian source rocks in the Shawan Sag serve as the lateral hydrocarbon supply and hydrocarbons accumulate in the Carboniferous structural-lithologic traps, which are summarized as the two stages of hydrocarbon accumulation of newly generated hydrocarbons into older reservoir rocks. This study of the hydrocarbon accumulation pattern in volcanic rocks aims at guiding the development of Carboniferous reservoirs with complex oil and water relationships in this area.

Identification of volcanic lithofacies using box-counting method calculating fractal dimension of logging data

Identification of volcanic lithofacies using box-counting method calculating fractal dimension of logging data

Polymagmatic Glaciovolcanism: Cracked Mountain Tuya, Canadian Cascades

Monogenetic volcanoes are characterized as having no temporal break in eruptive activity and are often assumed to have a simple (singular) magmatic plumbing system. However, recent studies on monogenetic systems have started to recover evidence of complexities within the magma-crustal dynamics. Here we investigate Cracked Mountain (CM), a 401 ± 38 ka glaciovolcanic basaltic landform in southwest, British Columbia, Canada. The volcano covers an area of ∼1.5 km2, has an eruptive volume of ∼0.18 km3, and comprises lapilli tuff, breccia, peperite, pillow and sheet lava, and dykes with no erosional surfaces present between the stratigraphic successions. The paleomagnetic signature of all volcanic lithofacies records a single-pole direction and, in conjunction with stratigraphic evidence, implies a monogenetic eruption. We establish that the Cracked Mountain volcano was fed by two separate crustally-stored magmas (i.e., polymagmatic), each characterized by a unique phenocryst assemblage indicative of different pre-eruptive storage conditions. The first mineral assemblage is an olivine-and-plagioclase phyric (OP) suite, and the second is an olivine-plagioclase-and-augite phyric (OPA) suite. The major-element geochemical compositions of the two petrographic suites vary slightly, with OPA samples higher in SiO2 and total-alkali contents than OP. The two magmas have similar rare earth (REE) trace element signatures, suggesting the same mantle source. We use thermodynamic modeling (Rhyolite-MELTS) to show that the OP suite derives from magma stored at depths <6 km (< 2 kbar) and temperatures of 1240–1155°C. In contrast, the OPA magmas crystallized at depths between 7–9 km (∼2–2.5 kbar) at 1,250–1,150°C prior to eruption. Both magmas are shown to be nearly “dry” having less than 0.5 H2O wt% in their respective systems. We use Pearce Element Ratios (PER) to show that the chemical variations within and between the two CM magmas are controlled solely by the crystal fractionation of two phenocryst assemblages that underwent syn-eruptive mixing. This study concludes that the polymagmatic plumbing system at Cracked Mountain shows similar complexities to other global investigations of monogenetic volcanoes. Lastly, we propose a causal link between the crustal dynamics of magma systems and the impact of crustal loading and unloading during cycles of glaciation.

Quantitative characterization of fracture-pore distribution and effects on production capacity of weathered volcanic crust reservoirs: Insights from volcanic gas reservoirs of the Dixi area, Junggar Basin, Western China

Igneous hydrocarbon reservoirs worldwide are of significant importance to the petroleum industry for oil and gas exploration. The Carboniferous weathered volcanic crust reservoirs of the Dinan salient in the northern Junggar Basin are major exploration targets of PetroChina. The present study analyzes the geological, well logging, and production data to formulate development and distribution methods for quantitative characterization of weathered volcanic crust fractures and examine their fracture-pore distributions. The effects of the fracture-pore distribution on the productivity of various lithologies of weathered volcanic crust reservoirs were investigated. Various volcanic lithofacies demonstrate the lithological differences in their physical properties, fracture development, and distributions due to the conspicuous variations in weathering resistance among different volcanic rock types. The present study revealed the level of physical properties of volcanic rocks, which are ranked as follows: volcanic breccia > mafic lava > tuff > felsic intrusive rock. However, the degree of fracture development ranked differently from most developed to least developed as follows: felsic intrusive rock > mafic lava > volcanic breccia > tuff. The fracture distribution was primarily controlled by lithology, rock stratum thickness, degree of weathering and leaching, along with tectonism. Compared to other rock types, felsic intrusive rock and mafic lava were more prone to fracturing owing to different petrophysical properties. Additionally, fractures typically developed in single lithological bodies with a thickness of less than 15 m and were characterized by the “single-section” developing mode. For lithological bodies of thickness greater than 15 m, fracture development was more limited, primarily concentrated at the top and bottom of the lithological body and characterized by the “bi-section” or “tri-section” developing modes. These differences can be quantitatively characterized by the fracture development thickness, fracture layer density, and fracture development thickness ratio. Moreover, the influence of the degree of fracture development on the productivity of mafic lava and felsic intrusive gas reservoirs was higher than that of the porosity; however, volcanic breccia and tuff exhibited the opposite effect. The production capacity of weathered volcanic crust reservoirs is affected by the following factors in the order of degree of fracture development > porosity > lithology > thickness of a single lithological body > distance from the top surface of the weathered crust. These quantitative characterization methods can provide valuable technical support for optimizing favorable exploration targets.

Lithofacies characteristics and controlling on volcanic reservoirs in the basement: A case study of the offshore Bohai Bay Basin, Eastern China

Volcanic rocks are important, but special reservoirs, for hydrocarbon exploration. Volcanic lithofacies play an important role in reservoir development, but the understanding of the relationship between lithofacies and reservoirs is still limited, especially for volcanic basements. In this paper, we characterized the intermediate-basic lithofacies features and discuss the relationship between lithofacies and reservoirs in the offshore Bohai Bay Basin. The results show that there developed volcanic conduit, eruptive (basic and neutral), explosive, and volcanogenic sedimentary facies in the study area. Among them, the explosive and eruptive facies are dominant. Different lithofacies present diverse well-logging and seismic responses, and the basic-eruptive (basalt), neutral-eruptive (andesite), explosive, and volcanogenic sedimentary facies are distributed from the bottom to top orderly fashion. This vertical superposition pattern represents a volcanic eruption process that has resulted in eruptive facies being more developed than explosive facies. The pores (intershard, dissolution, and devitrified pores) and tectonic fractures are the predominant spaces for explosive and neutral-eruptive facies (andesite), respectively. The lithofacies control the development of the primary spaces, fractures, and the diagenesis processes, leading to explosive and neutral-eruptive facies being the dominant facies for reservoir development. The understanding of high-quality reservoirs in the volcanic buried hills is not only controlled by weathering but also dominant lithofacies that will guide the volcanic exploration globally.

Geochronology and geochemistry of Early Cretaceous volcanic sequences in Northwestern Peru: implications for Farallon Plate subduction

ABSTRACT The Early Cretaceous geology of Northwestern (NW) Peru was characterized by emplacement of voluminous felsic volcanic rocks (e.g. the Oyotún Fm.), which have important implications for the understanding of Early Cretaceous crust-mantle interactions in the Central Andes. We present U-Pb-Hf isotopic compositions and whole-rock and trace elemental compositions of a series of Early Cretaceous volcanic rocks from the Western Cordillera and the Eastern Cordillera of the Central Andes. The volcanic rocks outcrop sporadically with limited thickness and uniform volcanic lithofacies in the western region, while the eastern region has distributions of larger scale volcanic rocks with more complex volcanic lithofacies. LA-ICP-MS zircon U-Pb dating defines the age of the Oyotún Fm. range from 138.8 Ma to 131.0 Ma in the Western Cordillera, and from 139.7 Ma to 132.3 Ma in the Eastern Cordillera. In situ Hf isotope analysis on dated zircon yield an ε Hf(t) range of +3.37 to +8.42 and a T DM2 (Hf) range from 983 Ma to 650 Ma with a peak at ca. 840 Ma. The Early Cretaceous felsic magmas are predominantly dacites and rhyolites with minor components of basalt and andesite, and show relatively homogeneous Hf isotopic compositions of zircon in single samples. Both the ε Hf(t) of zircons and the magma temperature increase from the bottom to the top of the Early Cretaceous volcanic rocks in each studied area. All these facts imply an increasing contribution of contemporaneous underplated hot and dry mantle-derived magmas with time, both as a heat source inducing crustal melting and a source of material (melt) that variably mixed with the ancient continental crustal materials.

Volcanic facies architecture of early bimodal volcanism of the NW Deccan Traps: Volcanic reservoirs of the Raageshwari Deep Gas Field, Barmer Basin, India

AbstractThe Deccan Traps large igneous province (LIP) comprises one of the largest continental flood basalt provinces on Earth with the main phase of volcanism spanning the Cretaceous‐Palaeogene boundary. The oldest volcanism of the province is encountered in the northwest of modern‐day India where Deccan stratigraphy is often buried beneath thick Cenozoic sedimentary sequences. The Raageshwari Deep Gas (RDG) Field, located onshore in the central Barmer Basin, NW India, produces gas from the early Deccan Raageshwari Volcanics which are subdivided into two members, the Agni Member and the overlying Prithvi Member. The RDG comprises a globally important example of a producing volcanic reservoir whilst also offering unique insights into the early volcanism of the Deccan with the aid of extensive high quality sub‐surface data. Within this study, the volcanic facies of the RDG sequences are investigated from five cored intervals (total 160 m). Core‐based facies determinations are compared with geochemical analyses, petrophysical analyses of the cores (density, porosity and permeability), and wireline data including micro‐resistivity borehole images (FMI) and Nuclear Magnetic Resonance (NMR) data. A wireline based volcanic lithofacies scheme is developed and applied to the uncored parts of the sequence which in turn is compared to 3D seismic data. Results of the study reveal the Agni Member to comprise a compositionally bimodal (basalt through to trachyte), dominantly alkaline series with mixed volcanic facies including spectacular felsic ignimbrites, basic‐intermediate simple lava flows, volcaniclastic units and newly identified shallow intrusions. The Prithvi Member in contrast is dominated by tholeiitic basalt compositions with less common basic‐intermediate alkaline compositions and comprises a sequence dominated by classic tabular lava flow facies inter‐digitated with boles, volcaniclastic units, rare compound braided lava facies and evolved tuffaceous ash layers. In one interval of the Prithvi Member, evidence for agglutinated spatter is recorded inferring potential proximity to a palaeo‐eruption site within the area. Comparison between core data and volcanic facies reveals a first order control of volcanic facies on reservoir properties highlighting the importance of volcanic facies appraisal in the development of volcanic reservoirs.

Evidence of early Miocene synextensional volcanism and deposition in the northern Calico Mountains, central Mojave metamorphic core complex, southern California, USA

The spatiotemporal link between large-scale continental crustal extension and magmatic activity has been identified by numerous past studies, yet commonly the details of these associations remain unresolved. This study in the central Mojave metamorphic core complex (CMMCC) of southern California (USA) presents new geologic mapping, stratigraphic interpretations, and 40Ar/39Ar geochronology of the Jackhammer and Pickhandle Formations in the northern Calico Mountains to provide additional age constraints on the relative timing of early Miocene volcanism, deposition, and extension. The Jackham­mer Formation, the oldest Tertiary stratigraphic unit, is nonconformable with pre-Cenozoic nonmylonitic metasedimentary and plutonic basement rocks and consists primarily of alluvial deposits and primary to reworked silicic tuffs, interbedded locally with basement-derived avalanche megabreccia, lacustrine limestone, and mafic lava; in addition, the “Mammut ignimbrite”, an ~130-m-thick crystal-rich welded lapilli tuff, is exposed only in the eastern part of the study area and appears to transition laterally into thinner, nonwelded lapilli tuff ~6 km to the west. The Pickhandle Formation conformably overlies the Jackhammer Formation and consists of: (1) a lower assemblage composed of reddish monomictic debris-flow breccias with porphyritic rhyodacitic clasts and silicic block-and-ash-flow deposits of sim­ilar composition; and (2) an upper assemblage of polymictic (metaplutonic basement and rhyodacite) alluvial deposits, primary to reworked lapilli tuff, and local rhyodacitic lava and block-and-ash flows. Rhyodacitic lava domes were emplaced during the final stages of Pickhandle Formation deposition, primarily intruded along preexisting normal fault zones. Sedimentary and volcanic lithofacies suggest that the Jackhammer and Pickhandle Formations were deposited in a volcanic vent–proximal alluvial fan system that formed within a half-graben basin bounded on the east by the southwest-dipping “Amphitheatre fault”. Growth strata within the hanging-wall deposits, primarily southwest-directed paleocurrents, and interbedded alluvial debris-flow, basement-derived megabreccia, and lacustrine deposits adjacent to this fault suggest synextensional deposition in an intra-hanging-wall basin that developed during upper-plate extension in the CMMCC. New 40Ar/39Ar ages for six samples of silicic pyroclastic flows and a lava dome from the synextensional Pickhandle and Jackhammer Formations in the Calico Mountains have a mean age of 20.10 ± 0.06 Ma. This age is 3–4 m.y. younger than the maximum age of initial extension determined by previous studies in other areas of the central Mojave, suggesting that CMMCC extension was not a synchronous large-magnitude regional event. Rather, extension and contemporaneous volcanism was more localized and asynchronous across the region, occurring in many smaller extensional basins that eventually culminated in exposure of the CMMCC mylonitic footwall rocks.

The characteristics of Carboniferous volcanic lithofacies and volcanic reservoirs of Ke‐Bai Fault Belt, northwestern Junggar Basin

The Ke‐Bai Fault Belt in northwestern Junggar Basin has gained robust insights and wide attention on large‐scale hydrocarbon discoveries in the Carboniferous volcanic rocks. The integrated analysis of wireline logs, image logs, core photographs, thin sections, and scanning electron microscope allows the description and interpretation of volcanic facies and volcanic reservoirs, as well as their responses to controlling factors. The results indicate that three types of lithology are identified based on cores, including volcanic rocks, followed by sedimentary rocks and metamorphic rocks. In addition, explosive facies, effusive facies, volcanic sedimentary facies, and sub‐volcanic facies are delineated according to spatial distribution, occurrence condition, and appearance characteristics of volcanic products. Furthermore, the characteristics of reservoir space (including pores and fractures) are analysed and described. Finally, the controls on the characteristics of volcanic reservoirs are revealed, including volcanic lithofacies, tectonic movements, and diagenesis.

Chapter 5.4a Marie Byrd Land and Ellsworth Land: volcanology

Abstract Nineteen large (2348–4285 m above sea level) central polygenetic alkaline shield-like composite volcanoes and numerous smaller volcanoes in Marie Byrd Land (MBL) and western Ellsworth Land rise above the West Antarctic Ice Sheet (WAIS) and comprise the MBL Volcanic Group (MBLVG). Earliest MBLVG volcanism dates to the latest Eocene (36.6 Ma). Polygenetic volcanism began by the middle Miocene (13.4 Ma) and has continued into the Holocene without major interruptions, producing the central volcanoes with 24 large (2–10 km-diameter) summit calderas and abundant evidence for explosive eruptions in caldera-rim deposits. Rock lithofacies are dominated by basanite and trachyte/phonolite lava and breccia, deposited in both subaerial and ice-contact environments. The chronology of MBLVG volcanism is well constrained by 330 age analyses, including 52 new 40 Ar/ 39 Ar ages. A volcanic lithofacies record of glaciation provides evidence of local ice-cap glaciation at 29–27 Ma and of widespread WAIS glaciation by 9 Ma. Late Quaternary glaciovolcanic records document WAIS expansions that correlate to eustatic sea-level lowstands (MIS 16, 4 and 2): the WAIS was +500 m at 609 ka at coastal Mount Murphy, and +400 m at 64.7 ka, +400 m at 21.2 ka and +575 m at 17.5 ka at inland Mount Takahe.

Process Simulation of Carboniferous Volcanic in Jinlong Area, Junggar Basin

The Jinlong area is located in the northwestern margin of Junggar Basin. Volcanic is widely distributed in Carboniferous layers, showing well petroleum resources prospects. The distribution of volcanic reservoirs is obviously controlled by the distribution characteristics of volcanic lithofacies. Therefore, through the analysis of core observation, thin section identification, data of well logging and mud logging, and interpretation of seismic data, this paper studies process simulation of Carboniferous volcanic in the Jinlong area, which is located in the northwestern margin of Junggar Basin. Based on the characteristics of volcanic materials and comparison of profile facies, the paper restores distribution characteristics of lithofacies of Carboniferous in the study area: The study area mainly develops volcanic lithofacies such as effusive phase and explosive phase. The thickness between wells varies greatly, which is affected by eruption mode and fault development. The vertical and horizontal changes of lithofacies are discontinuous and the phase transformation is fast. The volcanic activity of Carboniferous gradually weakened. There are multiple volcanic eruption centers in each period, which are controlled by crack and distributed in beads. In addition, an intermediate-mafic composite volcanic-crack eruption edifice was developed in the southern zone of the study area. Because of strong tectonism in the area near the crater, there are many dissolvable components in volcanic, which can form favorable hydrocarbon reservoir bodies and become the most favorable target zone in the Jinlong area.

Ferroan alkalic volcanism associated with Calymmian rifting in the Paramirim aulacogen, São Francisco craton, Brazil: New insights from lithofacies analysis and evidence of mantle-derived alkaline H2O-rich metasomatic fluids affecting ancient crustal materials

Rift-related volcanism is widely used in models of supercontinent reconstruction, while the associated metasomatic fluids provide information about the mantle-crust process during intracrustal melting events. The Paleoproterozoic/Mesoproterozoic Paramirim aulacogen corresponds to multiple intracratonic rift cycles that register many basin developing events. Intense volcanic activity is recorded in some rift events, but the volcanic lithofacies and their petrogenesis are poorly constrained. Herein, we present lithofaciological characterization, whole-rock and mineral chemistry, and U-Pb and Lu-Hf isotope data from the 1.58 Ga volcanic rocks. Trachytic and trachy-andesitic lava flows and primary to secondary volcaniclastic lithotypes characterize this volcanism. The primary volcaniclastic rocks resulted from direct volcanic activity and can be divided into pyroclastic and hydroclastic lithofacies associations. The pyroclastic lithotypes correspond to air-fall, density currents and block-and-ash flow deposits formed in explosive eruptions; hydroclastic lithotypes are represented by hyaloclastites and peperites, reflecting effusive eruptions and interactions between magma, water and sediments. The secondary volcaniclastic lithotypes resulted from late-stage reworking processes and consist of debris flows, ephemeral streams associated with high energy and gravitational flow deposits. The lava flows show well-preserved primary volcanic features, such as magma flow textures and euhedral phenocrysts. Based on petrographic and chemical characteristics, associated with negative ɛHf(t) values, we infer that high melt rate of an ancient crust, previously metasomatized by H2O-rich alkaline mantle-derived fluids, generated the alkalic ferroan lavas. These fluids promoted potassic and sodic metasomatism that affected all the volcanic rocks, modifying the chemistry of the feldspars to pure orthoclase (Or96-98) and albite (Ab98-100). In some lavas, a higher concentration of Na-rich fluids synthesized aegirine at a minimum temperature of 200 °C and promoted its peralkaline character. The potassic-sodic metasomatism influenced the volcano-sedimentary sequence of the Pajeú Group and the oldest rocks of the basement but did not affect the overlying sedimentary sequences, indicating that fluids ceased with the end of volcanism. We suggest that the H2O-rich alkaline fluids arose by focused degassing in response to a rising asthenospheric mantle during the Calymmian rift of the Paramirim aulacogen.

Sedimentological and petrographic evolution of a fluvio‐lacustrine environment during the onset of volcanism: Volcanically‐induced forcing of sedimentation and environmental responses

AbstractThis work focuses on the transition from the Las Leoneras to Lonco Trapial Formations, in the lower part of the Early Jurassic succession of the incipient rift phase of the Cañadon Asfalto Basin (western part of Chubut Province – Patagonia, Argentina). Twenty lithofacies have been identified and grouped into seven facies associations on the basis of field characteristics (sedimentological and lithological) and optical microscope analysis, from two localities representing proximal and distal locations in the basin. The spatial relationship between all the lithofacies provided a four‐dimensional reconstruction of the palaeoenvironmental evolution, showing how the original, clastic sedimentation in alluvial/fluvial and lake environments was modified by short‐lived volcanic events during three volcanic cycles, and how the environment reacted after the input of huge amounts of volcaniclastics. Progradation of small deltas and subaqueous lobes, retrogradation caused by rising lake levels, and frequent erosion of valleys were typical processes in this environment. When explosive volcanism began, the original tectono‐climatic control on sedimentation was replaced by the volcanic control, and the volcanically‐forced sedimentation broke the equilibrium among production, delivery and accumulation of sediments. The nature of the volcanic eruptions and the different propensity of volcanic lithofacies to produce particles of different size and types (lithics, crystals and glass) are also analyzed. The role of volcanism in the production and transport of great volumes of sediments across sedimentary systems needs to be carefully re‐examined, and the analysis on the variability in the composition of volcaniclastic deposits must take into account that volcaniclastic particle types may not simply reflect a linear deepening in the dissection of magmatic arcs through time but are often controlled by the style of the eruptions and the lithological variation of the volcanic products.

The Upper Permian volcanic-sedimentary succession in northern Qamdo Block, central Qinghai-Tibet Plateau and its sedimentary, paleogeographic and tectonic significance

A combined sedimentological and geochemical study has been carried out on a volcanic-sedimentary succession from a borehole newly drilled in the northern Qamdo Block (QMB, in central Qinghai-Tibet Plateau) in order to improve the knowledge of the sedimentary and tectonic environment. The lithologic data allow to separate four main lithofacies, including massive volcanic lithofacies (LF1), tuffaceous lithofacies (LF2), sandstone lithofacies (LF3) and mudstone lithofacies (LF4). LF1 likely reflects the products of felsic lava flows that were formed during the process of magma ascent onto the seafloor at an earlier stage. LF2 contains two sublifhofacies that respectively represents the proximal and distal accumulation of tuffaceous turbidity flows. LF3 could be the arc-sourced submarine fan deposits, and the sublithofacies LF3a and LF3b represent mid-fan distributaries and interchannel deposits in fan lobes. LF4 is mainly composed of suspension deposits, which represents the sedimentation of post-volcanic activities. Furthermore, the lithofacy associations reveal the evolution of a volcano-sourced submarine fan in the back-arc basin. Geochemically, LF1 and LF2 are characterized by LREE-enriched patterns with negative Eu anomalies, enrichment in large ion lithophile elements (LILEs) and depletion in high-field-strength elements (HFSEs), which correspond with the previously published island arc environment. However, LF2 shows more distinctive patterns, which represents a later evolution stage than that of LF1. This volcanic-sedimentary succession reveals that the sedimentary-tectonic setting could be consistent with the subduction of Jinshajiang oceanic slab in the late Permian period.

Lithofacies and reservoir characteristics of Permian volcanic rocks in the Sichuan Basin

In order to provide basis for further exploration and deployment of volcanic gas reservoirs in the Sichuan Basin, we classified Permian volcanic rocks in this basin using drilling, mud logging, assay, logging and seismic data comprehensively. Then, the lithological characteristics, lithofacies characteristics, lithofacies distribution and reservoir characteristics of different types of volcanic rocks were studied. Besides, the main factors controlling the development of volcanic reservoirs were determined. And the following research results were obtained. First, volcanic rocks in the Sichuan Basin can be comprehensively classified into 4 categories (i.e., volcanic lava, volcaniclastic lava, volcaniclastic rocks and sedimentary volcaniclastic rocks), and basalt, breccia lava, tuffaceous breccia lava, tuff, sedimentary tuff are mainly developed. Second, 3 types of volcanic lithofacies are extensively developed in the Sichuan Basin, including overflow facies, effusive facies and volcanic sedimentary facies. Overflow facies is mainly dominated by basalt, effusive facies is mainly composed of volcaniclastic lava, and volcanic sedimentary facies is mainly composed of tuff and sedimentary tuff. Third, volcanic reservoirs are lithologically composed of breccia lava and basalt. Breccia lava reservoirs are pore-type reservoirs of moderate porosity and low permeability with devitrified micropores and dissolution pores as main reservoir space. In basalt, fractures and dissolution pores are mainly developed, and they are porous-fractured reservoirs with lower matrix porosity. Fourth, the factors controlling volcanic reservoirs mainly include lithology, lithofacies, later fluid alteration and tectonism. Volcaniclastic lava is devitrified in the process of rapid condensation, and a great number of dispersion-shape micropores are formed to provide favorable conditions for further dissolution and pore enhancement in the later stage. Post-diagenesis effect (e.g. dissolution) and fracture development make the reservoir properties better. In conclusion, the areal distribution of volcanic reservoirs in the Sichuan Basin is mainly controlled by the lithofacies. Porous-fractured basalt reservoirs are mainly developed in the Ya'an–Leshan–Pingshan area in the southwestern Sichuan Basin, and pore-type volcaniclastic lava reservoirs are mainly developed in the Chengdu–Jianyang–Santai area.

Evaluation of volcanic gas reservoir in Yingcheng formation of Block Shengshen2-1 of Northern Part of Songliao Basin

According to the thin sections of volcanic rocks, chemical composition analysis and log interpretation, and well data, it is determined that the volcanic rocks of Yingcheng Formation in Shengshen 2-1 block are mainly intermediate-acid volcanic lava and pyroclastic rocks. Through physical analysis of core samples, it is considered that Shengshen 2-1 block belongs to low-porosity and low-permeability reservoir. Microfracture-fracture is relatively developed in the compact section, and fracture is not developed in the porous section. Microfracture and reticular fracture can be seen in the thin section. Statistical analysis of porosity and density of volcanic reservoir shows that volcanic rock density below 2.53 can be used as reservoir. According to the relationship between reservoir space and gas content, the characteristics of volcanic lithology, lithofacies and reservoir space, the reservoir in this block can be divided into four types.

Subaqueous strombolian eruptions, and eruption-fed pyroclastic deposits in a Paleoproterozoic rifted-arc: Hidden formation, Flin Flon, Canada

The Hidden formation of the Paleoproterozoic Flin Flon arc assemblage comprises the stratigraphic hanging wall to the volcanogenic massive sulfide (VMS) deposits at Flin Flon, Manitoba, Canada. It represents a basaltic volcanic edifice erupted within a large subsidence structure developed in a rifted-arc environment. It is predominately pillow lavas with subordinated amounts volcaniclastic rocks.Rocks that stratigraphically overly the Schist and Mandy VMS deposits ∼5 km southeast of the main Flin Flon deposits have been correlated through litho- and chemo-stratigraphy with the Hidden formation. They display evidence of pyroclastic volcanism and concomitant subsidence. Pillow lavas, and hyaloclastite are unequivocal evidence of subaqueous eruption.The Hidden formation in the Schist – Mandy area comprises mafic tuff, mafic megabreccia, basalt lava, fluidal-clast breccia, and plagioclase crystal-rich volcaniclastic lithofacies, intruded by synvolcanic dikes and sills. The megabreccia lithofacies is characterized by angular basalt clasts (1–20 m) in a matrix of finer basalt clasts and is interpreted to have been deposited by mass flows due to slumping of the caldera wall. A within flow vertical transition in the lavas from massive to pillowed flows suggests a slowing of emplacement velocity, decrease in temperature, and an increase in the viscosity during eruption. Based on the monolithic nature of the fluidal-clast breccias, and the fluidal shapes of clasts, with delicate tails and intact glassy margins, they are interpreted to be the result of strombolian eruptions. An increase in the ingestion of water in the vent(s) lead to a change from magmatic to phreatomagmatic fragmentation. Planar and cross laminations, graded beds and the abundance of broken plagioclase crystals, in combination with scoria and lithic clasts suggest the mafic tuff and plagioclase crystal-rich volcaniclastic lithofacies were deposited by subaqueous, eruption-fed density currents derived from a phreatomagmatic eruption.Detailed mapping of the volcanic lithofacies in the Schist – Mandy area has facilitated the identification of synvolcanic faults defining inner caldera margins and volcanic vents. These structures when traced downwards through stratigraphy define structural corridors corresponding with the Schist and Mandy VMS deposits, indicating they represent long-lived structures that also acted as magmas and fluid pathways.