Tuff Reservoir Research Articles

Multi-Scale Pore Structure Heterogeneity in Tuff Reservoirs Investigated with Multi-Experimental Method and Fractal Dimensions in Chang 7 Formation, Southern Ordos Basin

Multi-Scale Pore Structure Heterogeneity in Tuff Reservoirs Investigated with Multi-Experimental Method and Fractal Dimensions in Chang 7 Formation, Southern Ordos Basin

Characteristics and controlling factors of tuff reservoirs of Huoshiling Formation in Dehui Fault Depression, Songliao Basin, NE China

This study takes the tuff reservoir of the Huoshiling Formation in the Dehui Fault Depression, Songliao Basin, as an example to clarify the characteristics and primary controlling factors of tuff reservoirs. 3D seismic, core, image log, thin section, production dynamic and other data were used. Mineral x‐ray diffraction, core computerized tomography scanning, high‐pressure mercury injection, porosity, permeability, scanning electron microscopy, cathodoluminescence thin section, element testing and other experiments were conducted. The volcanic rocks in the study area are divided into andesite, volcanic breccia, lithic tuff and crystal tuff. The experiment results show that the mineral composition of tuff is primarily quartz, feldspar and clay minerals. The reservoir space comprises secondary pores and fractures, whereas primary pores are not developed. Average pore‐throat radius of most samples ranges from 0.017 to 0.131 μm, and the pore‐throat is primarily fine. The average porosity is 12.21% (a typical medium‐high porosity reservoir), and the average permeability is 0.072 mD (an extra‐low permeability reservoir). Tectonic fractures generated by tectonic movement are one of the reservoir spaces of tuff. Faults and tectonic fractures constitute a crucial migration channel of the fluid in the deep stratum. The tuff formed in the underwater environment, and the reservoir is affected by alteration. Devitrification is a crucial cause of medium‐high porosity and ultra‐low permeability in reservoirs. The alteration caused by hydrothermal fluid in deep fluids creates a considerable number of pores and fractures filled with clay minerals. Furthermore, acid fluid dissolves tuff along faults and fractures. In terms of time, the filling action is earlier than the dissolution action. The weathering indexes of CIA, CIW, PIA and WIP indicate that the tuff has suffered intense weathering, and the reservoir's physical properties have been improved to some extent.

Reservoir Characteristics and Development Model of Subaqueous Pyroclastic Rocks in a Continental Lacustrine Basin: A Case Study of the Chaganhua Subsag in the Changling Fault Depression, Songliao Basin

Industrial oil and gas eruptions underwater have been found in the pyroclastic rocks of the Huoshiling Formation in the continental lacustrine basin of the Changling fault depression, Songliao Basin. This paper investigates the reservoir space characteristics, physical characteristics, and pore structure differences of subaqueous pyroclastic reservoirs in the Huoshiling Formation, and the causes of physical property differences of different types of reservoirs and their formation and evolution processes are analyzed. (1) The content of volcanic glass in tuff is higher, the reservoir space is dominated by devitrification pores and dissolution pores, and the coarser the grain size, the more favorable the physical properties, with larger pore sizes and higher porosities. The content of clay minerals in sedimentary tuff is high, the pores between clay minerals are the main pores, and the physical properties of sedimentary tuff are poor. The content of soluble components such as feldspar, debris, and laumontite is high in tuffaceous sandstone, which is dominated by dissolution pores. (2) Primary pores are not developed in the pyroclastic reservoirs in the study area, and the reservoirs are relatively dense, with an average porosity of 2.43% and an average permeability of 0.076 mD. The coarse-grained tuff has the highest porosity, followed by tuffaceous sandstone and fine-grained tuff, and the sedimentary tuff has the least favorable physical properties. (3) Devitrification was an important cause of the high-porosity and ultralow permeability of tuff reservoirs. Two oil and gas charges in the middle diagenetic stage led to the organic acid dissolution of rocks. In addition, fractures can provide migration channels for organic acids and deep hydrothermal fluids, leading to late dissolution, and can connect various scattered dissolution pores to improve the effectiveness of the reservoir space. (4) Coarse-grained tuff reservoirs that developed in the proximal facies are favorable targets for hydrocarbon exploration.

Characteristics and Influencing Factors of Tight Carboniferous Tuff Reservoir in Malang Sag

Characteristics and Influencing Factors of Tight Carboniferous Tuff Reservoir in Malang Sag

A Unique Volcanic Jatibarang Reservoir: Facies Characterization and Fracture Calculation

Jatibarang structure, situated in Jatibarang low, one of the prolific sub-basin in the northwest Java basin, belongs to PT. Pertamina EP working area. One hundred seventy wells have been drilled, from which there were 56 production wells. Forty production wells were sunk on a volcanic reservoir. Since 1970, volcanic Jatibarang has faced disputed facies concepts, porosities heterogeneity, and permeability system. A fractured reservoir model was proposed. It was due to unique high-performance hydrocarbon in the early years, and then steeply depleted in the early next 3 years, and flat over 20 years. A unique petrophysical method was developed with various explosives and effusive facies. This study proposes a new facies determination concept related to unavailable default petrophysical plots usually used in the clastic-carbonate reservoir. The new concept of formation evaluation attempts to distinguish the fractured vitric tuff reservoir and non-reservoir zone, which is both effusive lava and explosive product. Reservoirs differed from common clastic-carbonate, typically high gamma-ray reservoir due to potassium feldspar composition. The fracture existence is reflected by a sonic overrunning density log aligned with a deep resistivity pattern. The width of the fracture apertures was measured, and spacing intensities were calculated. Finally, fracture permeability was estimated comprehensively to describe reservoir performance. Property models built with only sonic and triple combo have been debatable; however, advanced logging of JTB-211 has proven this accuracy method.

Study on Influencing Factors of Water Huff-n-Puff Oil Recovery in Matrix-Fracture Systems of the Tight Sedimentary Tuff Reservoirs.

Tight sedimentary tuff reservoirs (TSTRs) are a new type of tight oil reservoirs, which are mainly developed by water huff-n-puff (WHP). However, there is no quantitative study on the effect of water injection pressure (WIP) and fracture density (FD) on the oil recovery effect of WHP, and the reasons for the low flow-back rate (FR) of the injected water are also not fully explained. In this study, the real cores of TSTRs were used to simulate the seepage state of the matrix-fracture systems of the reservoir, the effects of WIP and FD on the WHP were quantitatively studied, and the reasons for the low FR of the injected water were comprehensively analyzed. The result shows that in five cycles of WHP, the recovery factor (RF) of the core only increases from 8.72 to 10.91% with the WIP increasing from 25 to 30 MPa. However, when the WIP is 40 MPa (rock breakdown pressure), the RF of the core reaches 16.47%, indicating that overfracture-pressure water injection has an obvious improvement effect on the oil recovery effect of WHP in TSTRs. Increasing the FD can also significantly improve the RF and oil recovery efficiency (ORE) of WHP in TSTRs. When the FD of the core increases from 0.34 to 0.44 cm–1, the RF of five cycles of WHP increases by 9.26%, the ORE increases by 8.61%, and the FR of the injected water decreases by 0.56%. The reasons for the low FR of the injected water in WHP in tight oil reservoirs are matrix water locking, fracture water locking, and reservoir nonconstant-volume water locking. The study can provide an important reference for the efficient development of the WHP in TSTRs.

Characteristics and formation mechanism of the tight tuff reservoirs of the Upper Triassic Chang 7 member in the southern Ordos Basin, China

Organic-rich shales are commonly associated with tuff layers. Previous studies have mainly focused on the influence of water-laid tuffs on the primary productivity and enrichment of organic matter. Tuffs can also act as tight or low permeability reservoirs or as sweet spots in shales. However, the characteristics and formation mechanism of tight tuff reservoirs are poorly explored. The tuffs in the study area are divided into five lithologies: vitric tuff, crystal-vitric tuff, crystal tuff, tuffite, and tuffaceous sandstone. The pores in these tuffs have various types and sizes, and mainly include (1) intergranular pores, (2) intragranular pores, (3) dissolution pores, (4) seams around grains, (5) organic matter related pores, and (6) fractures. The tuffs form tight reservoir rocks with low porosity and permeability, ranging from 2.33% to 12.62% (average 7.2%) and from 0.000518 mD to 0.281mD (average 0.0783 mD), respectively. The average porosity and permeability of the tuffs are better than those of shales, and the pores in the tuff reservoirs are often filled with oil residues, which indicate that the tuff reservoirs may be favorable for oil and gas accumulations.The tuff reservoir qualities have strong heterogeneity, which are significantly controlled by lithology, grain size, components, devitrification, dissolution, and fluid pathway systems. Tuffs with large grain sizes commonly have high primary porosity with relatively more cracks and seams within and around grains, which can be conducive to the entry of late diagenetic fluids, which cause tuff reservoirs to be more prone to devitrification and dissolution. The devitrification of volcanic glass can produce abundant microcrystalline quartz, feldspars and intergranular pores, and can greatly improve the reservoir quality. Devitrification has generally occurred in the vitric tuffs and crystal-vitric tuffs, and the porosities have a positive covariation with the quartz contents in the two lithologies. The devitrification pores can be further enlarged by dissolution. Devitrification and dissolution were controlled by the tuff compositions and fluid actions. The fluid pathway systems, including scouring surfaces, bedding, fractures, and well-developed primary pores, controlled the fluid movement. Tuffs that contain high volcanic glass contents and coarse particles with abundant beddings and fractures are prone to intense devitrification and dissolution, which results in high reservoir quality.

Characteristics and Formation Mechanism of the Tight Tuff Reservoirs of the Upper Triassic Chang 7 Member in the Southern Ordos Basin, China

Characteristics and Formation Mechanism of the Tight Tuff Reservoirs of the Upper Triassic Chang 7 Member in the Southern Ordos Basin, China

Formation Mechanism and Prediction Method for the Permian Fused Breccia Tuff Reservoir, Wuxia Region, Junggar Basin

Fused breccia tuff occurs globally, but its formation mechanism is very controversial. Volcanic reservoirs have developed at the bottom of the Permian Fengcheng Formation in the Wuxia region of the Junggar Basin, and here, the lithology is fused breccia tuff. The reservoir porosity is mainly vesicles, but the development and relative filling of the vesicles vary spatially, resulting in strong reservoir heterogeneity. Through core and thin section observations and structural analysis, and combined with reconstructions of the paleosedimentary environment, we discussed in detail the formation mechanism of the fused breccia tuff reservoir. Our conclusions are as follows. In the high-temperature and high-pressure environment of the deep crust, intermediate acidic lava containing volatile components rapidly rose to the earth’s surface along a fault. The volatile components in the lava foamed strongly and then exploded due to the sharp decline of pressure and temperature. A small part of the volcanic dust and pyroclastic material was erupted into the upper atmosphere. Most of the magma became magmatic pyroclast, vitric pyroclast, rock debris, dust, and other matter. This material was in a semimolten state and overflowed into a nearby low-lying lake. The extremely high-temperature pyroclastic flow quickly vaporized the water into high-pressure water vapor, which was squeezed into the pyroclastic flow and became mixed with other volatiles in the foam. On cooling, the pyroclastic material solidified into rock, and the vesicles were preserved. In a later period, due to strong tectonic movement, faults and fractures developed, surface water penetrated into the vesicles along the faults and fractures, and silica and other substances were deposited, filling the primary vesicles. To quantify the development and relative filling of vesicles, drilling parameters were used to establish different geologic models, and wave equation forward modeling was used to obtain a relationship between the development and filling of vesicles, and the seismic amplitude. The 3D seismic amplitude attributes were then extracted to predict the extent of the reservoir, yielding prediction results consistent with the drilling observations.

Characteristics and controlling factors of tuff tight reservoir in Tiaohu formation of Malang sag, Santanghu basin, China

Santanghu basin in Northwest China is a petroliferous basin. In recent years, researchers have discovered tuff tight reservoirs in the Tiaohu formation of Malang sag. Thin section, scanning electron microscopy, electron probe, X-ray diffraction, and CT scanning analysis are applied to reveal the petrological characteristics, physical properties, and pore texture of tuff tight reservoirs. Major controlling factors of the formation of high-quality tuff tight reservoirs are also discussed. The results show that the tight reservoir is intermediate-acidic crystal-vitric tuff. The main component of tuff reservoir is felsic vitroclastic, followed by crystal fragment and a small amount of bioclast, and trace clay content. Mercury injection and CT scanning show that the tight oil reservoir is characteristic of medium-high porosity, ultra-low permeability, and high oil saturation, which is controlled by its pore texture. There are four types of micropores in tuff reservoir, among which the principal reservoir space is devitrified micropores and crystal fragment dissolution micropores. Micropores are distributed universally in the tight oil reservoir, and the throat is nano-scale throat. It is considered that the tuff in the study area is formed by the deposition of volcanic ash in stable water after long-distance drifting in the air. The formation of high-quality tuff tight oil reservoir derives from 3 key factors: (1) shallow-semi-deep lacustrine environment, which is less affected by terrigenous input; (2) intermediate-acidic volcanic ash which forms the tuff reservoir; (3) devitrification and dissolution during the diagenesis.

Study on optimal selection of porosity logging interpretation methods for Chang 73 segment of the Yanchang Formation in the southwestern Ordos Basin, China

The physical properties of tight shale reservoirs have always been a hot topic of discussion, and the tuff reservoirs have also attracted more and more attention. Log predicting of the porosity is also important for the exploration and development of tight oil. Tight oil resources have been found in tight shale and tuff reservoirs in the Chang 7 Member of the Upper Triassic Yanchang Formation in the Ordos Basin, China. However, the logging method that effectively predicting porosity of the Chang 7 Member is lacking, and few people have discussed how to optimize the log interpretation methods. In this study, the characteristics of shale and tuff reservoirs were summarized by microscope and scanning electron microscope observation, X-ray diffraction, pore and permeability experiment, and the logging data. In order to find the optimal method of porosity prediction, four methods were performed: multiple regression fitting, multi-component volume model, porosity logging formula, and back propagation neural network method.The “Zhangjiatan Shale” of the Chang 7 Member is characterized by abundant sandy lamina and tuff lamina, compared with the tuff reservoir, the enrichment of pyrite (average 22.7 wt%) and total organic matter (>6%) increase the content of intercrystalline pores and organic matter related pores, the logging response is characterized by high natural gamma ray, low spontaneous potential and high acoustic. The tuff reservoir is mainly composed of vitric tuff and crystal-vitric tuff, hence, the rich in intergranular pore and micro-fracture can be explained by the dissolution and devitrification of vitric and crystal fragments, and the logging characteristic is opposite to that of the “Zhangjiatan Shale”. Four methods for predicting porosity are established. First, appropriate logging parameters are selected for regression fitting in the “Zhangjiatan Shale” (e.g., gamma ray, deep lateral logging resistivity, density and spontaneous potential) and tuff reservoir (e.g., acoustic, density, compensated neutron logging). Then, the rock multi-component volume model was established as rock skeleton, clay or tuff matter, organic matter, and pores. The logging parameters of the skeleton, clay and organic matter were obtained by intersection graphs, which were applied in the improved porosity logging formulas. The effect of organic matter on logging was considered into the traditional porosity prediction formula, a third prediction method was established. Finally, the BP method was introduced into the porosity prediction work, and the neural network parameters were adjusted according to the log and porosity data. The learning rate and the learning accuracy were set as 0.1 and 10−6 in the study area. The error analysis between the result predicted and measured shows that the multi-component volume model is suitable for the “Zhangjiatan Shale” (the correlative coefficient R>80%), while the back propagation neural network is the best method for the tuff reservoir (the correlative coefficient R>90%).

Research on well logging evaluation method of igneous reservoir in Nanpu No.5 structure

ABSTRACT Igneous gas reservoir in Nanpu No. 5 structure which located in the east coast of China is one of the important areas for deep natural gas exploration. Compared with conventional sandstone reservoirs, the well logging evaluation of igneous reservoirs is more complex and difficult. This research provides a practical workflow for log interpretation of igneous reservoir, including curves reconstruction, reservoir parameters settlement, and fluid identification. The target reservoirs (basalt and basaltic tuff reservoirs) are identified through the lithology identification chart, and then the logging data of the borehole enlargement intervals are reconstructed by BP neural network, the average absolute error between reconstructed data and core experimental data is 0.021 g/cm3 and the reservoir parameters are calculated based on the reconstructed curves and ECS logging data. Finally, the time-frequency analysis method is used to identify the gas and water layers. Through the above method, we solve some difficult problems of logging interpretation in igneous reservoir. It could provide a very practical method for well logging evaluation of igneous reservoirs.

Permian sedimentary tuff tight reservoirs in the Santanghu Basin, NW China

Sedimentary tuffs comprise important tight oil reservoirs in the Middle Permian Tiaohu Formation in the Santanghu Basin. This paper describes three research advances made on these reservoirs. (1) There are many sets of sedimentary tuff in the second member of the Tiaohu Formation and they are the “sweet section” for tight oil exploration. Vitric, crystal-vitric, and argillaceous tuffs are the main reservoir types, of which the first two are more favorable. (2) A model was established to show how the volcanic edifice controls the type and distribution of the volcanic lacustrine tuff deposition. The crystal-vitric tuff is distributed in the lacustrine deposits nearest to the volcanic crater, and the vitric tuff is distributed on both sides of the active volcanic belt a certain distance from the crater, and this is the most favorable distribution area for sedimentary tuff. Later devitrification resulted in formation of a large number of micro-pores, and this is the main reason for the presence of favorable reservoirs. (3) The sedimentary tuff reservoir has undergone successive geological processes from formation of secondary dissolution pores, then hydrocarbon charge and accumulation, to compaction. The tight oil accumulation is the result of the coupling of the diagenetic evolution of the tuff in the Tiaohu Formation and the hydrocarbon generation and charging from the shale in the Lucaogou Formation.

Tight tuff reservoir characteristics and its controlling factors: A comparative study of the Permian Tiaohu Formation and Carboniferous Haerjiawu Formation in the Santanghu Basin, NW China

Tight oil resources have been found in tight tuff reservoirs in many oil and gas bearing basins. The tight tuff reservoir developed in the Santanghu Basin (NW China) has provided an excellent opportunity for further research to enhance our cognition and understanding of tuff reservoir characteristics. This study focuses on the differences in the basic characteristics in petrologic features, reservoir space, and physical properties between the Tiaohu and Haerjiawu formations tuff reservoirs, then determines the formation mechanism causing these differences. (1) The tuff samples of the Tiaohu Formation mainly consist of vitric fragments, and quartz and albite are the major mineralogical components. However, the crystal fragment content is much higher in the Haerjiawu Formation tuff, and albite and anorthite are the major components. (2) Primary pores and the interparticle pores among authigenic minerals formed during alteration are the main pore types for both the Tiaohu Formation and Haerjiawu Formation tuffS. (3) The Tiaohu Formation tuff reservoir is characterized by high porosity (ranging from 10% to 25%) and relatively low permeability (on the order of 0.01–0.50 mD), while that of the Haerjiawu Formation is characterized by lower porosity (<10%) and lower permeability (mainly <0.1 mD). (4) Alteration and compaction processes contributed most to the pore formation in both the Tiaohu and Haerjiawu tuff reservoirs. Hence, the changes in total tuff reservoir porosity can be explained by the evaluation of compaction residual porosity and alteration porosity. The vitric fragments in the Tiaohu Formation originated from acidic or intermediate-acidic volcanic ashes, which are more liable to be altered than the basic volcanic ashes that formed the crystal fragments in the Haerjiawu Formation. Moreover, the Haerjiawu Formation tuff reservoir is buried hundreds of metres deeper than the Tiaohu Formation in general, and thus, experienced stronger diagenetic compaction, which indicates fewer residual pores for Haerjiawu Formation tuff samples. The nature of tuff and the deeper burial depth are the major reasons for the worse physical properties of the Haerjiawu Formation tuff reservoir.

Mechanism of hydrocarbon accumulation and enrichment of tuffaceous tight oil with separate reservoir and source rock: A case study of tuff reservoir from the Permian Tiaohu Formation in the Santanghu Basin, northwest China

Different from most tight oil reservoirs, the tuffaceous tight oil of the Tiaohu Formation is not in situ oil, and no close contact exists between the source rock and reservoir in the Malang sag (Santanghu Basin, China). This study determined the mechanism of hydrocarbon accumulation of this tuffaceous tight oil reservoir through an integrated analysis of oil–source rock correlation, reservoir characteristics, and rock wettability combined with a comprehensive analysis of geological conditions. An oil–source rock correlation using biomarkers and stable carbon isotopes shows that the crude oil originated from underlying source rocks in the Lucaogou Formation. The oil in the tuffaceous tight reservoir is not indigenous but has migrated over a long distance to accumulate in these reservoirs. Faults and fractures that developed at the end of the Cretaceous are the oil migration pathways. Vitric and crystal-vitric tuffs constitute the main rock types of the tuffaceous tight reservoir. Matrix-related pores in the tuffs mainly comprise interparticle pores between minerals and dissolution intraparticle pores formed by devitrification. The adsorption of polar components of the oil generated from original organic matter in the tuff leads to wettability of lipophilicity, which is the main reason for hydrocarbon charging and accumulation. To our knowledge, this is the first comprehensive study reporting this finding.

Compositional and Thermodynamic Variability in a Stratified Magma Chamber: Evidence from the Green Tuff Ignimbrite (Pantelleria, Italy)

The Green Tuff Ignimbrite, Pantelleria, is compositionally zoned from pantellerite at the base to comenditic trachyte at the top, the variation apparently representing an inverted vertical zonation in the pre-eruptive reservoir. The main phenocryst assemblages are alkali feldspar + olivine + clinopyroxene + ilmenite + apatite in the trachytes and alkali feldspar + aenigmatite + clinopyroxene ± quartz in the rhyolites. Thermodynamic modelling indicates that the temperature range was ∼900–700°C, fO2 FMQ –1·5 to FMQ –0·5 (where FMQ is fayalite–magnetite–quartz buffer) and aSiO2 (relative to quartz saturation) 0·74–1·00. Melt water contents ranged from ∼1 wt % in the trachytes to ∼4 wt % in the pantellerites. Matrix glass analyses in the more evolved rocks are highly variable, showing that compositional layers in the upper parts of the reservoir, formed by fractional crystallization, were mixed during eruption, the proportion of rhyolitic to trachytic melts increasing towards the top of the reservoir. Some areas of glass have low Al2O3 contents (5·16–5·46 wt %) and high FeO* contents (9·66–10·02 wt %), making them the most evolved melts yet reported from Pantelleria. The new glass data reveal how whole-rock analyses do not truly reflect the complete range of melt compositions in the pre-eruptive reservoir. The trachytes contain >40% modal phenocrysts, which with relatively high Ba contents and positive Eu anomalies are considered to have originated in a feldspar-accumulitic layer. Phenocrysts in the trachytes are commonly heavily resorbed, inferred to be a result of heating by influxes of intermediate composition magma, which, however, were not erupted. It is argued that magmas of intermediate composition were present in the Green Tuff reservoir but were efficiently trapped in a crystal-rich layer below the Green Tuff magmas, which was eventually erupted during a resurgent phase as the Montagna Grande Trachyte.

Formation and distribution of tuffaceous tight reservoirs in the Permian Tiaohu Formation in the Malang sag, Santanghu Basin, NW China

Based on analysis of major and trace elements, observation of thin sections, cathodoluminescence and scanning electron microscope of tuff samples, in combination with the restoration of paleotopography in the Malang sag, the formation and distribution characteristics of the Permian Tiaohu Formation tuff tight reservoirs in the Santanghu Basin are examined. The analysis shows that the tuff samples of the Tiaohu Formation are acid-intermediate, and they are the product of the end of the volcanic eruption cycle. The tuff reservoir rock includes vitric tuff, crystal-vitric tuff, and pelitic tuff, among which, vitric tuff has the best physical properties, followed by crystal-vitric tuff, and pelitic tuff is the poorest. Vitric tuff is usually distributed further away from the crater zone laterally and vertically in the central-lower part of the tuff. Crystal-vitric tuff, mostly interbedded with vitric tuff, is usually distributed near the crater zone laterally and also vertically in the central-lower part of the tuff. Pelitic tuff is generally distributed far away from the volcanic crater on the plane, and in the upper part of the tuff vertically. Types of tuffs are affected by the distance from the volcanic activity belt, and the thickness of tuff is controlled by both volcanic activity belt and sedimentary paleotopography. Thus, depositional depressions on both sides of volcanic activity belt are the favorable locations for the development of tuff. It is predicted that there are five thickness centers of tuff in the Tiaohu Formation of Malang sag (Wellarea M1, L1, M56, M7 and southwest of M7), of which the biggest thickness of tuff is up to approximately 40 m.

Tight-reservoir micropore formation and evolution in sedimentary organic-matter-bearing tuff: a case study from the Permian Tiaohu Formation in the Santanghu Basin, NW China

ABSTRACTRecently, a tuffaceous tight oil reservoir with considerable reserves has been found in the Santanghu Basin, northwestern China. The physical properties of the tuff are characterised by high porosity (5–25%) and low permeability (0.01–0.50 mD). This reservoir has the peculiar property of being a sedimentary organic-matter-bearing tuff formed by air-fall volcanic ash in a lake. Its discovery offered an excellent opportunity to further our knowledge of the formation and evolution of tuffaceous tight-reservoir micropores. This study integrates analyses of organic geochemistry, thin-sections, scanning electron microscopy, CT scanning, mineral and element compositions, mercury injections and measurements of porosity and permeability to determine the characteristics of the formation and evolution of the tuff micropores. (1) The tuff reservoir comprises vitric, crystal-vitric, pelitic and silicified tuffs, which are characterised by high porosity and big throat, moderate porosity and thin throat, low porosity and thin throat, and very low porosity and moderate throat pore structures, respectively. (2) Of the four types, vitric tuff has the properties best suited to a reservoir, and devitrification of volcanic glasses is the principal mechanism of reservoir micropore formation. (3) The tuff of the Permian Tiaohu Formation contains sedimentary organic matter, and organic acids produced during kerogen maturation facilitate the process of devitrification. (4) Evolution of tuff porosity depends mainly on the original composition and burial depth. The porosity of vitric tuff is higher than crystal-vitric tuff at the same depth, but both their porosities initially decrease and then increase with depth. At depths >3000 m, vitric tuff retains the trend towards higher porosity, whereas the porosity of crystal-vitric tuff remains largely unchanged or even decreases. The results of this investigation could be considered a reference regarding the distribution and prediction of tuffaceous reservoirs in similar situations in other parts of the world.

Geochemical and tight reservoir characteristics of sedimentary organic-matter-bearing tuff from the Permian Tiaohu Formation in the Santanghu Basin, Northwest China

Tuff reservoirs have been found in petroliferous basins but have not been sufficiently studied. The discovery of the Permian Tiaohu Formation reservoir in the Santanghu Basin in Northwest China has offered an excellent opportunity to further enhance our knowledge of the tuff's geochemical and reservoir characteristics. The tuff reservoir has the peculiar property of being a sedimentary organic-matter-bearing tuff. In this study, an integrated analysis of the organic geochemistry, elemental and mineral compositions, quartz crystallinity, pore types, and tight reservoir characteristics was conducted based on samples from the 20–30 m-thick tuffs. The extracted tuff samples have total organic carbon (TOC) values of 0.5–1.0 wt.%, total hydrocarbon yield values of 2–6 mg/g, and hydrogen index values of 20–336 mg HC/g TOC. The organic matter consists predominantly of Type III and II2 kerogens, and the temperature of the maximum yield of pyrolysate varies from 420 to 450 °C, which reflects the oil-generating capacity of the rock. The physical properties of the tuff are characterised by high porosity (varying from 10% to 25%) and low permeability. The air permeability mainly ranges from 0.01 to 0.50 mD. The devitrification of vitreous textures within the tuff primarily dictates the reservoir's characteristics, i.e., the greater the degree of devitrification, the higher the porosity of the reservoir. Vitric tuffs have reservoirs with higher porosity than crystal-vitric tuffs, and the degree of devitrification is generally controlled by the burial depth (temperature) and organic matter (organic acid) content. Vitric tuffs with high porosity and permeability are usually located far from the crater and within the central–lower part of the reservoir. This research may influence geologists to pay more attention to the exploration value of this special type of reservoir in other regions.

Geological features and exploration potential of Permian Tiaohu Formation tight oil, Santanghu Basin, NW China

Abstract Based on seismic data, logging, mud logging and lab testing data, the geological features and exploration potential of Permian Tiaohu Formation tight oil were studied in Santanghu Basin, Hami Area, Xinjiang. The tight sedimentary tuff characterized as high porosity and low permeability is developed in the second member of Tiaohu Formation in Santanghu Basin, and its granularity is fine, nano-scale pores and throats account for 88.3%, its permeability is lower than 0.5×10−3 μm2. Because of high brittleness, the reservoir rocks are easy to be fractured, and its oil saturation and crude oil density are high. The oil in tight sedimentary tuff is mainly from the underlying Permian Lucaogou Formation high-quality source rocks with a 100−500 m interval between source rocks and reservoirs. The crude oil generated in source rocks migrated up to the second member of Tiaohu Formation along faults and fractures characterized as other-sourced charging and source-reservoir separating. Tight oil distribution is mainly controlled by Lucaogou Formation source rocks and second member of Tiaohu Formation sedimentary tuff reservoirs. The area of “sweet spots” in the second member of Tiaohu Formation of Malang-Tiaohu Sag is about 203 km2, and the oil resource is predicted to be 2.4×108 t.