Volcanic Reservoirs Research Articles

Characteristics of Permian volcanism in the western Sichuan Basin and its natural gas exploration potential

It was previously deemed that volcanic rocks in the western Sichuan Basin were not developed, which restricted the exploration of volcanic gas reservoirs in this area. Recently, however, Sinopec's wildcat well, Well YS1 encounters Upper Permian volcanic rocks about 300 m thick in the western Sichuan Basin, the reservoir physical properties of which are good with strong gas shows. In order to evaluate the exploration prospect of volcanic gas reservoirs in western Sichuan Basin, we studied the Permian volcanic rocks in this area from the aspects of formation and development characteristics, reservoir characteristics, gas genesis and plays based on the new understandings on exploration wells, combined with the analysis on tectonic evolution and its controlling effect on volcanism. Then, the volcanic eruption mechanism, the distribution mode of volcanic edifice and the hydrocarbon accumulation pattern of volcanic gas reservoir were figured out. Finally, the exploration potential of natural gas was evaluated. The following research results were obtained: (1) the Permian volcanic rocks in western Sichuan Basin are well developed and its tectonic setting is consistent with that of Emeishan basalt, which is the product of the Emeishan mantle plume. (2) Under the control of vertical faults, a great number of volcanic edifices are developed and there are many eruption cycles. Volcanic rocks are superimposed vertically and connected laterally. (3) The volcanic rocks in western Sichuan Basin are different to some extent from those in southwestern Sichuan Basin in terms of lithology, lithofacies and reservoir characteristics. In the western Sichuan Basin, the tuff of explosive facies is dominant with some basalt of effusive facies, and the reservoir space of volcanic rocks is mainly acted by inter-breccia skeletal pores, dissolved pores and matrix micro-pores. In conclusion, multiple sets of effective source rocks are developed around the volcanic intervals in western Sichuan Basin, and together they constitute effective natural gas plays. Therefore, it has a better natural gas exploration prospect, and it is a new field worthy of attention.

Ca isotopes record rapid crystal growth in volcanic and subvolcanic systems

Kinetic calcium isotope effects can be used as growth-rate proxies for volcanic and subvolcanic minerals. Here, we analyze Ca isotopic compositions in experimental and natural samples and confirm that large kinetic effects (>2‰) can occur during magmatic plagioclase crystallization. Experiments confirm theoretical predictions that disequilibrium isotope effects depend mainly on the rates for crystal growth relative to liquid phase Ca diffusivity (R/D). Plagioclase phenocrysts from the 1915 Mount Lassen rhyodacite eruption, the ∼650-y-old Deadman Creek Dome eruption, and several mafic subvolcanic orbicules and plagioclase comb layers from Northern California have disequilibrium Ca isotopic compositions that suggest rapid crystal growth rates (>1 cm/y to 15 cm/y). The Ca isotope results, combined with complementary crystal-size distribution analyses, suggest that magmatic rejuvenation (and eruption) events, as reflected in crystal growth times, can be as short as ∼10-3 y. Although mafic systems are predicted to have shorter magmatic rejuvenation periods, we find similarly short timescales in both mafic and silicic systems. These results are consistent with a growing body of evidence suggesting that dominantly crystalline volcanic magma reservoirs can be rapidly reactivated by the injection of fresh magma prior to eruption. By focusing on a common mineral such as plagioclase, this approach can be applied across all major magmatic compositions, suggesting that Ca isotopes can be used as a tool for investigating the dynamics and timing of volcanic eruptions.

Fracture Study and Its Applied in Oil and gas Field Development

Fracture, as an important factor affecting the quality of reservoirs, has always been one of the most crucial research contents in the process of oil and gas field development. The basis of research on fault systems in oil and gas field development was introduced from two aspects, namely, data foundation and research methods. The volcanic reservoir of the Yingcheng Formation in the Xudong Area of the Songliao Basin, the glutenite reservoir of the Lower Karamay Formation in a block in the northwestern margin of the Junggar Basin, and the heavy oil reservoir developed by thermal recovery in the Yulou Oil Bearing Sets in an experimental area in the western slope of the western depression of the Liaohe Basin, were taken as examples to elaborate the application of fault research in oil and gas field development from three aspects: design of volcanic gas reservoir development program, architecture analysis of glutenite reservoir, and fault systems in heavy oil reservoirs developed by thermal recovery. Based on these different Chinese examples, the difficulties and hot issues in the research of fault systems in oil and gas field development were discussed. The results show that the fine characterization of location, scale, and distribution law of faults and fractures, analysis of fault architecture of reservoirs, fine interpretation and modeling of faults, and evaluation of fault sealing property are closely related to the effective development of oil and gas fields; therefore, they should be given adequate attention in the process of oil and gas field development.

Characteristics of Carboniferous volcanic reservoirs in the Chun-feng Oilfield of the Junggar basin, China

As a major petroliferous layer, Carboniferous volcanic reservoirs have developed widely in the Chun-feng Oilfield in the Junggar basin. Volcanic reservoir rocks have been the principal target for exploration and development in the study area, having relatively high porosity and permeability which is favorable for hydrocarbon accumulation. According to observation of thin sections of core samples and logging lithology, the volcanic reservoir rocks of the Chun-feng Oilfield can be divided into 5 types according to their attitudes and composition. These are volcanic fulgurite rocks, clastic volcanic rocks, volcanic clastic rocks, sedimentary volcanic rocks, and volcanic sedimentary rocks. Lithofacies has also been studied according to their lithological distribution and associations. According to observation of thin sections and SEM images of core samples, the pore system of the volcanic reservoir is mainly composed of primary pores, secondary pores, and fractures. Statistical analysis of measured data shows that volcanic reservoirs have poor reservoir properties, with generally low porosity and permeability. Reformed by late diagenesis and tectogenesis, a weathered crust structure has developed in the Carboniferous volcanic reservoirs in the study area, which is composed of five zones. These are soil zone, hydrolytic zone, leaching zone, disintegrating zone, and original zone. Of these, the leaching zone has developed the best reservoirs, with relatively high porosity, followed by the disintegration zone. With low porosity, the reservoirs of the soil and hydrolytic zones act generally as cover layers. There are three main factors which determine the physical properties of volcanic reservoirs. The principal factor is that lithofacies control the primary pore types, which have an influence on subsequent reformation. Faults tend to promote the development of secondary porosity, and paleogeography restricts the distribution of favorable volcanic reservoirs in the area.

Characterization of Volcanic Reservoirs Requires Comprehensive NMR Tests

This article, written by JPT Technology Editor Judy Feder, contains highlights of paper IPTC 19378, “Comprehensive Evaluation of NMR Characteristics of Complex Volcanic Reservoirs With Different Types of Rock Lithology,” by Junchang Sun, Shijie Zhang, Jieming Wang, Hekun Guo, Chun Li, Hongcheng Xu, Sinan Zhu, and Kai Zhao, PetroChina, prepared for the 2019 International Petroleum Technology Conference, Beijing, 26–28 March. The paper has not been peer reviewed. Copyright 2019 International Petroleum Technology Conference. Reproduced by permission. This paper discusses a study undertaken to gain better understanding of nuclear magnetic resonance (NMR) characteristics of volcanic reservoirs with different lithologies. Results of the study confirmed that accurate identification of reservoir-interval lithology, achieved through comprehensive laboratory NMR investigation of the volcanic rock, is a primary prerequisite of correct interpretation of NMR logging of volcanic reservoirs. The research results play an important role in guiding in-depth understanding of NMR characteristics of volcanic reservoirs and the implementation of NMR reservoir evaluation in corresponding areas. Introduction The introduction of NMR to the petroleum industry in the 1990s led to a breakthrough in logging. NMR can distinguish between movable and irreducible fluid, and is currently the only logging technology that can determine reservoir permeability. When used to interpret and evaluate complex reservoirs such as volcanic rocks and low-resistivity oil and gas layers, NMR provides logging analysts with new ideas and techniques to deal with various difficulties. By analyzing the NMR relaxation-time (T2) spectrum of rock plugs at a fully brine-saturated state, the distribution of reservoir pore size can be evaluated, and important reservoir evaluation parameters, including effective porosity, NMR permeability, and movable fluid saturation, can be obtained. Numerous studies have shown that, compared with sandstones and carbonates, volcanic reservoirs are much more complex and heterogeneous because of the special eruption diagenesis mechanism, many types of rock lithology, various mineral compositions, and a broad range of pore sizes. The models currently used for NMR logging interpretation are suitable only for sedimentary reservoirs. Consequently, accurate characterization of volcanic reservoirs using advanced NMR logging requires a comprehensive laboratory NMR investigation of volcanic rock. Materials and Methods For the study described in the complete paper, NMR sample analysis technology was adopted to perform NMR measurements and other related tests on a total of 108 low-permeability rock plugs from the Xushen reservoir of the Daqing field, the Changling reservoir of the Jilin field, and the Dixi reservoir of the Xinjiang volcanic gas field. The rock plugs comprised nine types of lithology representing the main producing formation lithology from the three reservoirs. Tests included CT scans, thin-section petrography, mercury injection, and mineral-composition analysis. Centrifuge tests were conducted with maximum centrifugal forces up to 500 psi to explore the suitable capillary pressure for T2cutoff determination. The paper includes a step-by-step description of the experimental procedure.

Study on the relationship between lithology and reservoir characteristics of Yingcheng Formation in the southern Songliao Basin

In order to deeply understand the relationship between the microstructural characteristics of volcanic reservoirs and the lithology of volcanic rocks, it provides important theoretical significance for the effective development of volcanic reservoir oil and gas fields. The microscopic characteristics of the volcanic reservoirs of the Yingcheng Formation in the Songliao Basin were analyzed by means of rock section determination, pore structure parameters analysis and casting thin section analysis. The results show that the target layer has low porosity and permeability, and the volcanic pore structure of different lithologies is different. Volcanic lava generally develops original gas pores, phenocryst dissolved pores, matrix dissolved pores, and tectoclase, while volcanic clastic rocks generally have developmental dissolution pores as the main feature. The micro-heterogeneity of volcanic reservoirs is generally strong, and the development of tectonic fractures is an important factor in changing the reservoir performance of volcanic rocks.

A major discovery in Permian volcanic rock gas reservoir exploration in the Sichuan Basin and its implications

Recently, a high-yield gas flow of 22.5 × 104 m3/d was achieved in the wildcat well Yongtan 1, deployed by PetroChina Southwest Oil and Gasfield Company, and an extrusive facies volcaniclastic gas reservoir was first discovered in the Sichuan Basin, which is a major breakthrough in the exploration of volcanic rock gas reservoirs. In order to further evaluate the exploration potential in Permian volcanic rock gas reservoirs in this basin, we systematically reviewed and summarized the exploration process of volcanic rocks in this study area, as well as the characteristics of gas reservoirs of well Yongtan 1, the controlling factors and modes of hydrocarbon accumulation. The following findings were obtained. (1) Over the past five decades of gas exploration of volcanic rocks, the change of exploration thought and progress in seismic prediction contribute to a major discovery in this study area, i.e., the well location deployment of Yongtan 1 was determined in the Jianyan area, where the volcanic anomalies of the Permian developed outbursts are well developed and the gas reservoir conditions are favorable. This also proves that another new gas exploration area in this basin was found. (2) From the data of Yongtan 1, we found high-quality volcanic porous reservoirs with a thickness of over 100 m developed in the Permian strata, the effusive volcanic rock types dominated by breccia and tuffaceous breccia, reservoir space composed of dissolution pores, interbreccia dissolution pores, and other pores, the porosity of which is 6.68%–13.22% and 10.26% on average, the permeability 0.01–4.43 mD and 2.35 mD on average. (3) This abnormally high pressure gas reservoir was buried 4500–6000 m deep underground with the middle formation pressure being 125.625 MPa and the pressure coefficient being up to 2.22. Methane content in the minor H2S-bearing produced gas flow reached up to 99.03%. (4) Volcanic rocks in the Jianyang–Zhongjiang–Santai area are developed, and they are adjacent to the Cambrian high-quality source rocks in the Deyang–Anyue rifting. The early extensional deep faults play a role in connecting source and storage. There are many sets of caprocks, such as the Upper Permian Longtan Formation and Lower Triassic layered thick paste rocks, providing conditions for forming large-scale tectonic–lithologic composite trap gas reservoirs, and the favorable exploration area reaches up to 6000 km2. In conclusion, this major discovery not only proves that Permian volcanic rocks in the Sichuan Basin has large potential for oil and gas exploration, but reaffirms the confidence in changing our exploration thought, speeding up new exploration zones and areas, and discovering more and bigger gas fields.

Charging of Carboniferous volcanic reservoirs in the eastern Chepaizi uplift, Junggar Basin (northwestern China) constrained by oil geochemistry and fluid inclusion

Substantial amounts of petroleum were recently discovered in the Carboniferous andesite, tuff, breccia, and basalt reservoirs of the Chepaizi uplift in the western Junggar Basin. However, the charging history of the Carboniferous petroleum reservoir is poorly understood. Oil–oil correlation studies indicate that all of the oils were mainly derived from the middle Permian Wuerhe Formation source rocks, possibly mixed with a small contribution from Carboniferous Baogutu Formation source rocks in the neighboring Changji sag. Based on the petrographic and microthermometry of fluid inclusions, two hydrocarbon charging episodes are defined; these episodes were characterized by a low-peak-range homogenization temperature (Th) distribution (80°C–90°C) and high salinity (13.22–13.42 wt. % NaCl) and a high-peak-range Th distribution (120°C–130°C) and low salinity (4.89–11.72 wt. % NaCl), respectively. Through one-dimensional basin modeling and pressure–volume–temperature–composition simulation, the burial-thermal histories for wells P61, P66, P668, and P663 were reconstructed, and their trapping temperatures of the hydrocarbon inclusions were calculated to be higher than their corresponding highest paleotemperature (i.e., 56.8°C, 53.7°C, 60.9°C, and 58.1°C, respectively), implying fast hydrocarbon charging processes promoted by deep hydrothermal fluids. Associated with the hydrocarbon generation history, sealing process of the Hongche fault, and regional tectonic evolution, these two hydrocarbon charging events were deduced as the adjustments of oils previously accumulated along the Hongche fault zone, because of the tectonic extension in the Paleogene and regional tilting in the Neogene, respectively. The general direction of oil charging was traced from south to north and from east to west, as indicated by the molecular parameters of nitrogen-bearing compounds and C20 + C21 triaromatic steroids/C20 + C21 + C26–C28 triaromatic steroids (TA(I)/TA(I+II)), which roughly coincided with the active fracturing.

Three-dimensional imaging of Miocene volcanic effusive and conduit facies: Implications for the magmatism and seafloor spreading of the South China Sea

Abstract An integrated and practical seismic-based three-dimensional (3D) visualization technique of volcanic effusive and conduit facies is proposed in the Miocene Lower Zhujiang Formation of the Baiyun Sag, Pearl River Mouth Basin (PRMB), South China Sea (SCS). The technique is based mainly on the 3D visualization to separately described the strong amplitude reflection volcanic effusive facies and the weak amplitude chaotic reflection volcanic conduit facies with different seismic-cube attributes (instantaneous amplitude attribute and variance cube attribute). The detail work-flow of the method has been proposed with eight steps, including the original data type, seismic attributes selection, the visualizing skills and the final validation with borehole and thin sections. It is convenient to operate and describe the spatial distribution and interrelation of various volcanic facies effectively and intuitively. The 3D imaging result of different volcanic facies described in the Baiyun Sag can be viewed from any perspectives and better understand the related genesis mechanism which associated with the NW–SE trend normal fault system during the breakup stage. The two volcanic facies indicate that the spatial distribution of the volcanic rocks is parallel to the normal faults strike and the magma probably flows along the normal fault plane and then feds into the volcanic mounds with eruption. The volcanic mounds are mostly belonged to the central-fissure eruption types, and mainly correspond to the second magmatic episode (∼23.8 Ma) of Early Miocene in the PRMB and were induced associated with the seafloor spreading of the northern SCS. We may easier to find effective volcanic reservoirs or other volcano-related metallic minerals based on the proposed new method. This significant work can also serve as a useful reference for other continental margins or sedimentary basins with typical volcanic influence.

Occurrence and Origin of H2S from Volcanic Reservoirs in Niudong Area of the Santanghu Basin, NW China

A series of samples including natural gas, formation water, and rocks were collected from volcanic rock reservoirs in the Niudong area of the Santanghu Oilfields and analyzed for their mineral and/or chemical compositions and sulfur and carbon isotopes in order to investigate the occurrence and origin of hydrogen sulfide (H2S). H2S was mostly dissolved in the formation water along with petroleum production in the study area. The δ34S values of on-well H2S samples varied in a range of 9.2‰ to 20.5‰, probably indicating thermochemical sulfate reduction (TSR) and/or thermal decomposition of organic sulfur-bearing compounds (TDS) as the genetic process for H2S. However, the chemical composition of formation waters from the Kalagang Formation (C2k) and their coefficient of desulfurization also revealed that TSR could be the main principle for H2S formation. Considering the regional geological background, especially the tectonic structures and thermal evolution features of the basin, it was concluded that H2S in the study area was dominantly produced by thermal genesis with TSR as a domain through interactions between hydrocarbons and aqueous sulfate dissolved from sulfate minerals.

Charging history and fluid evolution for the Carboniferous volcanic reservoirs in the western Chepaizi Uplift of Junggar Basin as determined by fluid inclusions and basin modelling

Fluid inclusions are rich in the Carboniferous volcanic reservoirs in the western Chepaizi Uplift, occurring in liquid, gaseous, and gas–liquid phases. In this study, the fluid inclusions were investigated in terms of petrographic, optical, and microthermometric characteristics, coupled with pressure–volume–temperature–composition (PVTX) simulations. Two periods of hydrothermal activities in the Carboniferous volcanic reservoirs were suggested by the bimodal distributions of Th (homogenization temperatures) of fluid inclusions with 120–140°C and 160–190°C as the main peaks. The early charged fluids were characterized by a low temperature, low salinity, relatively higher density, and low G/L (gas–liquid volumetric ratio) of aqueous inclusions, whilst the late charged fluids featured a high temperature, moderate salinity, moderate density, and high G/L of aqueous inclusions. The oil inclusions hosted in calcite or quartz veins exhibited light yellow and yellowish‐green fluorescence, whereas those in the microcracks cutting or within quartz or calcite veins showed blue‐white fluorescence. The burial and geothermal histories imply an existence of abnormal hydrothermal activity under overpressure and high temperature, which is evidenced by the simulated trapping pressure that is higher than the lithostatic pressure and the measured Th values that are higher than the formation paleo‐temperatures. The Th range is widely and continuously distributed and, in combination with the burial history, indicates that the hydrocarbon accumulation in the research area has been a continuous process since the Paleogene. Combined with the reconstruction of tectonic evolution and hydrocarbon generation and expulsion, two oil‐charging episodes were defined, that is, the Late Oligocene–Early Miocene (23–16 Ma) and the Late Miocene–Holocene (6 Ma–). The western section of the Aika Fault Belt (F2), together with the sheet sandbodies of the Toutunhe Formation (J2t), and the unconformities between Carboniferous–Cretaceous may have served as the migration pathways.

The application of cracking coefficient method to the study on fractures in volcanic of Anda area

According to the actual situation of volcanic reservoirs in Anda area, this paper uses the comprehensive analysis of drilling and logging data to predict the fracture development of volcanic reservoirs in the study area. Among them, the ratio of longitudinal wave velocity, “crack coefficient” method, is used to analyze and calculate the crack development frequency. The research shows that the fractures of volcanic reservoirs in Anda area are relatively developed, and the characteristics of fracture parameters will inevitably affect the exploration and production of natural gas in this area.

Reservoir-forming conditions and key exploration & development techniques for Xushen gas field in Northeast China

Exploration and development of volcanic gas reservoirs is a world-class problem, and less experience can be learned at home and abroad. Through study of reservoir-forming conditions of Xushen gas field, four sets of hydrocarbon source rocks, mainly mudstone and coal seam of Shahezi Formation, are identified in this gas field; its cap rocks are mudstone in Member 2 of Denglouku Formation, and Member 1 and Member 2 of Quantou Formation. By simulating reservoir-forming history, four filling periods of the Xushen gas field are determined, and the most favorable exploration targets are crater areas with structural highs, large thickness and good physical properties. On the basis of diagenetic mechanism study, the criteria of sequence series, lithology identification and reservoir types of gas reservoirs have been established; the volcanic rock bodies have been carefully depicted and the distribution laws of effective reservoirs of gas pools have been figured out. Based on evaluating reservoir-permeability characteristics of reservoirs, the gas well productivity characteristics (mainly type II, III and IV wells) have been identified, and the optimum design technology for volcanic gas reservoir development has been developed. The gas reservoirs in all blocks of Xushen gas field are classified into three types: single-body gas reservoir model, disconnected multi-body gas reservoir model and interconnected multi-body gas reservoir model. Stimulation technologies, including multi-layer fracturing of vertical well, staged fracturing of open-hole horizontal well and volume fracturing, have been developed. According to gas well production performance, pressure change, well-controlled reserves and spatial distribution of volcanic bodies, combined with numerical simulation and prediction of gas reservoirs, the evaluation method of residual potential of volcanic gas reservoirs has been formed. Key technologies for effective exploration and development of complex volcanic gas reservoirs have been established and perfected, and bring good development effect. The research results can provide guidance and reference for exploration and development of similar gas reservoirs.

Controls on hydrocarbon accumulation in clastic reservoirs of the Tarim Craton, NW China

The Tarim Craton of NW China hosts abundant hydrocarbon resources within clastic reservoirs characterized by a complex entrapment and fill history. In this paper, we review the processes of hydrocarbon accumulation in typical clastic reservoirs from the literature, and combine this with new research from six hydrocarbon-bearing regions (the Yakela Faulted-Uplift, the southern slope of the Tabei Uplift, the Tazhong Uplift, the northern slope of the Tazhong Uplift, the southern margin of the Bachu Uplift, and volcanic reservoirs) to identify the key controls on hydrocarbon accumulation and distribution in reservoirs of the Tarim Craton. Favorable reservoir–trap configurations exist on structural highs created during the evolution of paleo-uplifts and paleo-slopes in the region, with hydrocarbons accumulating in both structural (fault, salt dome, and thrust-fault anticline) and combined structural-primary stratigraphic traps. Vertical hydrocarbon migration occurred via deep–shallow superposed faults, and (rarely interpreted) fault textures in volcanic conduits. Unconformity surfaces and sand bodies provided pathways for both limited lateral redistribution and significant long-distance (>100 km) migration of hydrocarbons. The prospectivity of upper Paleozoic sandstone facies and paleogeography was controlled by denudation of the paleo-uplifts, and these reservoirs are most likely to host giant oil and gas fields in the region.

Using orthorhombic depth imaging to image fractures in tight reservoirs of the RDG Field, Barmer Basin, India

Raageshwari Deep Gas (RDG) Field is situated in the southern part of the Barmer Basin in Rajasthan, India, at a depth of 3000 m. With both clastic and volcanic lithologies, the main reservoirs are tight, and hydraulic fracturing is required to enhance productivity, especially to improve permeability through interaction of induced fractures with natural fractures. Therefore, optimal development of the RDG Field reservoirs requires characterization of faults and natural fractures. To address this challenge, a wide-azimuth 3D seismic data set over the RDG Field was processed to sharply define faults and capture anisotropy related to open natural fractures. Anisotropy was indicated by the characteristic sinusoidal nature of gather reflection events processed using conventional tilted transverse imaging (TTI); accordingly, we used orthorhombic imaging to correct for these, to quantify fracture-related anisotropy, and to yield a more correct subsurface image. During prestack depth migration (PSDM) processing of the RDG data, TTI and orthorhombic velocity modeling was done with azimuthal sectoring of reflection arrivals. The resultant PSDM data using this velocity model show substantial improvement in image quality and vertical resolution at the reservoir level compared to vintage seismic data. The improved data quality enabled analysis of specialized seismic attributes like curvature and thinned fault likelihood for more reliable characterization of faults and fractures. These attributes delineate the location and distribution of probable fracture networks within the volcanic reservoirs. Interpreted subtle faults, associated with fracture zones, were validated with microseismic, production, and image log data.

Joint inversion of gravity with cosmic ray muon data at a well-characterized site for shallow subsurface density prediction

SUMMARYEstimating subsurface density is important for imaging various geologic structures such as volcanic edifices, reservoirs and aquifers. Muon tomography has recently been used to complement traditional gravity measurements as a powerful method for probing shallow subsurface density structure beneath volcanoes. Gravity and muon data have markedly different spatial sensitivities and, as a result, the combination is useful for imaging structures on spatial scales that are larger than the area encompassed by crossing muon trajectories. Here we explore and test a joint inversion of gravity and muon data in a study area where there is an independently characterized target anomaly: a regionally extensive, high-density layer beneath Los Alamos, New Mexico, USA. We resolve the nearly flat-lying structure using a unique experimental set-up wherein surface and subsurface gravity and muon measurements are obtained above and below the target volume. Our results show that with minimal geologic (prior) constraints, the joint inversion correctly recovers salient features of the expected density structure. The results of our study illustrate the potential of combining surface and subsurface (e.g. borehole) gravity and muon measurements to invert for shallow geologic structures.

Fracture Fillings and Implication of Fluid Activities in Volcanic Rocks: Dixi Area in Kelameili Gas Field, Junggar Basin, Northwestern China

The Carboniferous Batamayineishan Formation of the Kelameili Gas Field is a specific weathered crust-related volcanic reservoir that has a significant production rate in the Junggar Basin, Northwestern China, attributed to debatable processes of fluid evolution. The results suggest that various types of fluids occurring in volcanic rocks lead to the filling of quartz and calcite in fractures and their associated alteration haloes. The silica that formed quartz veins was mainly derived from deep hydrothermal fluids, while the carbon dioxide that formed calcite veins originated from sources characterized by mixing and alteration of deep hydrothermal and hydrocarbon fluids. Siliceous hydrothermal fluids rich in sulphur dioxide and other volatile components were driven by a pressure gradient and buoyancy, and circulated both laterally and vertically along the fractures, forming quartz veins and tension fractures under different temperature conditions. Moreover, changes in salinity, pressure, and carbon dioxide of deep fluids, varying from acidic to weakly alkaline, resulted in earlier calcite precipitation in contraction fractures and weathered fractures. Tectonic uplift resulted in the long-term exposure of volcanic rocks, where fresh water mixed with the partially alkaline fluid escaping the basin to form calcite cements, thus retaining the characteristics of a seepage environment in the weathered fractures. Structural fractures occurred due to tectonic movements during the burial period. Filling and leakage of hydrocarbons caused pore fluids to convert from acidic to alkaline, precipitating late sparry calcite in dissolution fractures. Late hydrothermal fluid metasomatism, brought about by infiltration into the permeable zone, caused partial dissolution of local calcite along cleavage cracks.

Joint inversion of gravity and surface wave data constrained by magnetotelluric: Application to deep geothermal exploration of crustal fault zone in felsic basement

Geophysical exploration in unconventional geothermal reservoir is challenging because of the lack of structures with specific geophysical signature such as a clay cap above volcanic geothermal reservoirs. The combination of several geophysical methods such as magnetotelluric (MT), gravity and seismic methods provides complementary insight into the geothermal reservoir. We present a joint inversion approach of ambient noise surface wave and gravity constrained by the 3D distribution of electrical resistivity. The method is applied to analyze the potential of unconventional deep geothermal resources in the French Massif Central (FMC). We coupled the resistivity, density and shear wave velocity with a linear correlation. We characterized the common properties in relation to the geological domains down to about 10–15 km. We validated the proposed approach on a synthetic model derived from models obtained from each independent geophysical method. The joint inversion limited the non-uniqueness of the gravity and the ambient noise tomography inversion and allow to recover the features of the synthetic models. We applied the method to real field data acquired in the Sioule Valley, Massif Central, France. The joint features observed in the geophysical models suggest a deep heat source associated with a possible shallow (∼6–8 km) brittle–ductile transition. A large fault zone, the Pongibaud Fault zone, seems to become listric and connect the surface to this transition zone to possibly constitute a permeable path for water circulation.

Evaluation of pore structures in volcanic reservoirs: a case study of the Lower Cretaceous Yingcheng Formation in the Southern Songliao Basin, NE China

Different pore types and variable pore structures can determine reservoir potentials of volcanic rocks. Therefore, it is of great importance to understand the pore structures of volcanic reservoirs. With the Lower Cretaceous Yingcheng volcanic rocks, this study attempts to investigate and evaluate the micro pore structures of a volcanic reservoir through: (1) measuring porosity and permeability of core samples; (2) identifying pore types within thin sections; (3) characterizing pore structures with pressure-controlled mercury injection. The Lower Cretaceous Yingcheng volcanic rocks exhibits low porosities (on average 7.39%) and very low permeability (on average 0.283 × 10−6 m2). Three major pore types were identified from the Lower Cretaceous Yingcheng volcanic reservoir, including primary pores, secondary pores and fractures. Additionally, the pore structures of the Lower Cretaceous Yingcheng volcanic reservoir can be classified into three types. Type I pore structure primarily occurs in the volcanic breccia, crystal tuff and tuffites. The corresponding pore system is dominated by primary intergranular pores, intragranular dissolved pores and devitrified micropores with porosity and permeability higher than 5.4% and 0.05 × 10−6 µm2, respectively. Corresponding mercury intrusion curves are characterized by low displacement pressure, high maximum mercury saturation and mercury withdrawal efficiency. Type II pore structure is commonly observed in the tuff and tuffites, while primary intergranular pores and secondary micropores dominate pore system with porosity of 2.5–5.4% and permeability of 0.012–0.05 × 10−6 m2. The mercury intrusion curves are similar to the type I. Type III pore structure is also pervasively developed in the crystal tuff and tuffites. Intragranular dissolved micropores are most important ones with porosity and permeability less than 2.5% and 0.01 × 10−6 m2, respectively. Different from type I and II pore structure, the mercury intrusion curves of the type III have high-displacement pressure, low maximum mercury saturation and mercury withdrawal efficiency, indicating poor reservoir quality.

The influence of fracture development on quality and distribution of volcanic reservoirs: a case study from the carboniferous volcanic reservoirs in the Xiquan area, eastern Junggar Basin

The distribution pattern of fractures in the Carboniferous volcanic rocks controls the oil and gas distribution in the Xiquan area located in the eastern Junggar Basin. This paper describes the types, characteristics, distribution of fractures in the volcanic rocks, and the evolutionary history of fractures, as well as the influence of fractures on the reservoir distribution in this area, based on analyzing cores, casting thin sections, imaging logging data, the measurement of the physical properties and integrated with the production testing. The lithology of these Carboniferous volcanic rocks is primarily andesite and volcanic breccia. Basalt, dacite, tuff, breccia lava, tuffaceous sandstone, and tuffite were also present. The fracture types are mainly shrinkage fractures, weathered fractures, dissolution fractures, and structural fractures, the majority of which are structural fractures and weathered fractures. The distribution of fractures has obvious zonation characteristics in the vertical direction. Fractures are mainly distributed at four intervals under the top surface of the Carboniferous unconformity: 0–50 m, 65–115 m, 125–155 m, and 170–210 m. There are mostly weathered fractures and dissolution fractures in the 0–50-m interval. The physical properties and oil production of andesite are the highest, and weathering fractures and dissolution fractures play a major role in controlling the secondary pores of volcanic lava and the formation of favorable reservoirs in this interval. The structural fractures were developed in the intervals of 65–115 m, 125–155 m, and 170–210 m. The physical properties and oil production of volcanic breccia are the highest, and therefore, structural fractures mainly control the formation of favorable reservoir in volcanic breccia in these three intervals. The evolution of fractures in the Carboniferous volcanic rocks experienced four phases: the condensation and shrinkage fracture stage, which developed the primary pores in the middle-late Carboniferous period; the weathered fracture and dissolution fracture stage, which greatly increased the porosity in the late Carboniferous period to the early-middle Permian period; the structural fracture and weathered fracture stage, which greatly increased the porosity and permeability of the reservoir from the end of the Triassic to the early Cretaceous period; and the filled fracture and dissolution fracture stage, which can also greatly increase the porosity of the volcanic reservoir and occurred wholly within the late Cretaceous period until the present.