Low-angle Fractures Research Articles

Characteristics of hydrothermal-structure-controlled fracture-vug dolomite reservoir and its influence on oil-water distribution: Lower Cretaceous, Baiyinchagan sag, Erlian Basin, North China

Fractures and vugs were used as the primary dolomite reservoir space in the study area, which improved the reservoir’s physical properties and influenced the distribution pattern of oil and water that were developed in the Upper Tenger Formation, Lower Cretaceous, Baiyinchagan sag, Erlian Basin. Based on the core observation data, thin slice analysis, and image logging, the cave-fractured reservoir was primarily developed in the actic region near the side of a depositional fault. Tectonic and diagenetic fractures were the primary types of fractures. High-angle tectonic fractures in the closure and open status were developed well in dolomitic mudstone, while a low-angle fracture and reticulate fractures were developed in the tectonic-hydrothermal hybrid process and were filled with hydrothermal minerals. The reservoir was located in low-lying land; therefore, detrital grain pressolution fractures were formed by burial diagenesis. Intergranular and intrangranular dissolved pores, which were formed by hydrothermal activity, were developed primarily in the highly dolomite lithology. According to the diagenesis temperature of dolomite from a carbon-oxygen isotope analysis, multiple hydrothermal fluid injection events occurred, and dolomitization ensued before massive hydrocarbon migration. The dissolution of organic acids formed by hydrocarbon charges after dolomitization and inorganic acid produced by burial di agenesis for soluble minerals effectively promoted the reservoir properties. Strong heterogeneity due to lithology and fracture-vug systems led to intricate oil-water relationships. Four statuses occurred, including cave-fracture oil, isolated oil, stored oil, isolated water, and stored water. The oil-water distribution patterns were established based on the core observation.

Identification and distribution of fractures in the Zhangjiatan shale of the Mesozoic Yanchang Formation in Ordos Basin

The natural fractures in mud or shale directly affect the quality and efficiency of shale gas reservoirs, and fracture identification and prediction play an important role in drilling shale gas wells and making plans for reservoir stimulation. We adopted ant tracking technology for 3D poststack reflective seismic waves to identify the size and distribution of high-angle structural fractures in the Zhangjiatan shale of the Yanchang Formation in the Ordos Basin, which is a typical continental shale. The parameters for ant tracking fractures are extracted from the investigation on outcrop, cores, and image logs. The prestack seismic diffractive wave imaging technique for the super-resolution identification of mid- and small-scale breakpoints can be used as the constraint conditions for ant tracking. The identified result of high-angle fractures was validated by the image logging and drilling gas logging results. The geologic and logging data indicate that the Zhangjiatan shale is mainly characterized by high-angle fractures and a smaller number of low-angle fractures. The fractures mainly trend in the near east–west direction, followed by the near north–south direction, and a small amount of fractures in the north–northeast and northwest–west directions. The average density of structural fractures is relatively low, but the cemented rate is only 15.7%, and most structural fractures maintain an open state. The identified and predicted structural fractures are mainly distributed in the southeast of well LP180 and south of well LP179. The higher gas shows from actual well drilling in shale directly correspond to the density and intensity of high-angle fractures rather than the matrix gas abundance in shale, which indicates that the sweet spot of gas production in shale is clearly controlled by structural fractures.

Formation and destruction mechanism as well as major controlling factors of the Silurian shale gas overpressure in the Sichuan Basin, China

Taking the Well JY1 and Well PY1 in the Eastern Sichuan Basin as examples, the formation mechanism of shale gas overpressure was studied by using the cross plot of acoustic versus density logging data. During the processes of hydrocarbon generation and the uplifting, the pressure evolution of fluids in shale gas layers was reconstructed by fluid inclusions and PVTSIM software. The major factors controlling the evolution of shale gas overpressure were established according to the study of fracture, the timing of the uplifting, and episodes of tectonic deformation. Our results showed that the main mechanism of overpressure in the Silurian shale gas reservoirs in the Sichuan Basin was the fluid expansion, which was caused by hydrocarbon generation. Since the Yanshanian, the strata were uplifted and fluid pressure generally showed a decreasing trend. However, due to the low compression rebound ratio of shale gas reservoir rocks, poor connectivity of reservoir rocks, and low content of formation water and so on, such factors made fluid pressure decrease, but these would not be enough to make up the effects of strata erosion resulting in a further increase in fluid pressure in shale gas reservoirs during the whole uplifting processes. Since the Yanshanian, the Well PY1 zone had been reconstructed by at least three episodes of tectonic movement. The initial timing of the uplifting is 130 Ma. Compared to the former, the Well JY1 zone was firstly uplifted at 90 Ma, which was weakly reconstructed. As a result, low-angle fractures and few high resistance fractures developed in the Well JY1, while high-angle fractures and many high resistance fractures developed in the Well PY1. In totality, the factors controlling the overpressure preservation in shale gas reservoirs during the late periods include timing of late uplifting, superposition and reconstruction of stress fields, and development of high-angle fractures.

Quantitative evaluation of fracture development in Triassic Yanchang Formation, Ordos Basin, NW China

Abstract To uniformly evaluate the fracture development of the Triassic Yanchang Formation in Ordos Basin, a set of evaluation methods for fracture development degree is established by observation and account of fractures developed in the sandstone cores of the Yanchang Formation in the five blocks of Maling, Huaqing, Anbian, Wuqi and Yanhewan in the Shaanbei Slope. The methods include four aspects of qualitative and quantitative indices, i.e., fracture type, fracture strength, fracture grade and fracture assemblage, which are applicable to the fracture assorting, single well evaluation, block evaluation and belt evaluation, respectively. Quantitative evaluation plates of block fracture development degree have been built and applied to evaluate the fracture development degree of the five blocks. The Chang 7 1 Member in Maling block has high angle fractures of three grades (weak, moderate and strong) developed simultaneously and low angle fractures in moderate-strong development degree; the Chang 7 2 Member in Maling block has weakly to moderately developed high angle fractures and moderately to strongly developed low angle fractures; both the Chang 7 1 and Chang 7 2 members in Anbian block have strongly developed high angle fractures, but undeveloped low angle fractures; the Chang 6 3 Member in Huaqing block has undeveloped high angle fractures and moderately to strongly developed low angle fractures locally; the Chang 6 members in Wuqi block and Yanhewan block have undeveloped high angle fractures and low angle fractures.

Numerical simulation of high-resolution azimuthal resistivity laterolog response in fractured reservoirs

The high-resolution azimuthal resistivity laterolog response in a fractured formation was numerically simulated using a three-dimensional finite element method. Simulation results show that the azimuthal resistivity is determined by fracture dipping as well as dipping direction, while the amplitude differences between deep and shallow laterolog resistivities are mainly controlled by the former. A linear relationship exists between the corrected apparent conductivities and fracture aperture. With the same fracture aperture, the deep and shallow laterolog resistivities present small values with negative separations for low-angle fractures, while azimuthal resistivities have large variations with positive separations for high-angle fractures that intersect the borehole. For dipping fractures, the variation of the azimuthal resistivity becomes larger when the fracture aperture increases. In addition, for high-angle fractures far from the borehole, a negative separation between the deep and shallow resistivities exists when fracture aperture is large as well as high resistivity contrast exists between bedrock and fracture fluid. The decreasing amplitude of dual laterolog resistivity can indicate the aperture of low-angle fractures, and the variation of the deep azimuthal resistivity can give information of the aperture of high-angle fractures and their position relative to the borehole.

Identification of fractures in carbonates using sonic imaging logs Example from the central of East European plain

Compared with sandstone, the structure of carbonate rocks is much more complex for its heterogeneity and anisotropy. The Republic of Tatarstan of Russia lies in the central of East European Plain, where oil-gas resource is very rich, and formations are mainly carbonates. In order to detect the underground geology of the region, array acoustic logging tool, micro-electrical scanning tool, and conventional logging tools were applied. We processed and analyzed the array acoustic logging data. The results demonstrated that monopole shear wave attenuation, Stoneley wave attenuation and monopole variable-density waveforms were all sensitive to the boundary where wave impedance contrast was high. The boundary may be a fracture, or a bed interface between shale and limestone in the region. In the micro-electrical scanning images, both the low-angle fracture and the bed interface showed one sine curve. The sine curve of the low-angle fracture was relative rough and the curve width changed with different azimuth, compared with that of the bed interface. The direction of fracture and stress can be obtained by processing cross-dipole waveforms of array acoustic logging data. Micro-electrical scanning tool provided three caliper curves of different azimuth that can determine borehole expanding direction, further getting the stress direction.

Development characteristics and hydrocarbon accumulation significance of fractures in Lower Silurian Kalpintag Formation, northern slope of central Tarim Basin

Abstract Based on the data of core, microscope thin section, scanning electron microscope, cathode luminescence, and conventional and dipole acoustic logging, the development characteristics and hydrocarbon accumulation significance of fractures in the Lower Silurian Kalpintag Formation on the northern slope of the central Tarim Basin were studied. A large number of mainly high-angle or vertical fractures developed in the study area. They feature low density, long length, medium width and low filling extent. More fractures were found in the relatively weak bands of rock, such as the margins and interlayers of boulder clays. Conventional logging curves showed two peaks and one order of magnitude difference. The anisotropy of fast shear wave in dipole acoustic logging showed that fracture development was influenced by NE trending strike-slip faults and the NE-SW trending maximum horizontal principal stress. The fractures in the Kalpintag Formation played important roles in the formation of tight sand-stone reservoirs. They improved the porosity and permeability of tight sandstones, and the large-scale, high-angle or vertical fractures served as pathways for vertical migration of hydrocarbons. For the small-scale adjustment of hydrocarbon migration, the small-scale, low-angle or horizontal fractures were important.

The ‘Jiaodong type’ gold deposits: Characteristics, origin and prospecting

The Jiaodong type comprises a unique class of gold mineralisation which includes the Linglong type quartz-vein hosted and the Jiaojia type fracture-disseminated style deposits. Although the type area is in the Jiaodong Peninsula in NE China, where some of the world's richest gold reserves occur, we identify similar occurrences in other parts of China and elsewhere in the world under this category. The ore fluids migrated upwards forming quartz vein type of gold ore (Linglong type) wherever the ore-controlling fractures are at high angles. Fluid infiltration into the surrounding wall rocks and generation of the Jiaojia type disseminated style deposits resulted where the ore-bearing fluids encountered low angle fractures. On a regional scale, the Jiaodong type gold mineralisation occurs along the margins of reactivated cratons, within the interior of cratons along paleosuture zones, or along the junctions of micro-blocks, and is distinctly different from the classic orogenic gold in terms of their tectonic setting, ore characteristics and genetic history. Unlike skarn and porphyry type of deposits, the Jiaodong type gold deposits do not always show close spatial relationship with magmatic intrusives and some of the deposits occur distal to plutons (>10km). In this paper, we evaluate the salient geological, geochronological, geochemical and isotopic features of the Jiaodong type gold deposits and propose some general guidelines for gold prospecting. Based on case studies, we predict that the gold lodes in the Jiaodong type mineralised zones might extent vertically up to 3000m with only minor mineralogical and geochemical variations.

Atomistic view of thin Ni/Ni3Al (0 0 1) under uniaxial tension of twist grain boundaries

Atomic motion and the structure response of grain boundaries (GBs) are essential to the plastic deformation of small-volume polycrystal systems, especially for thin materials that exhibit some dramatic characteristics. Here, the microstructure and properties of thin Ni/Ni3Al (0 0 1) with the uniaxial tension of twist GBs are investigated using molecular dynamics (MD) simulations with an embedded atom (EAM) potential. We find that low angle twist GB dislocations propagate mainly from the corners to the center of the interface, and for high angle twist GBs they originate from the edge of the interface but do not extend to the center. In the process of plastic deformation, both low angle and high angle twist GB fractures occur in the center of the interface, which is completely different from the situation of thick materials reported by other references. More interestingly, the fracture patterns between low angle and high angle twist GBs exhibit extremely different performance due to the different characteristics of the initial dislocation structures in the interfaces.

Features of the carboniferous volcanic rocks fracture reservoirs in Hongshanzui oilfield, Junggar Basin

Based on comprehensive analysis of the drilling core, slice analysis, FMI image well logging and other well logging data, the features of the Carboniferous volcanic rocks fracture reservoirs in Hongshanzui (红山嘴) oilfield of Junggar Basin have been studied. According to contributing factor, the fracture reservoirs were divided into diagenetic fractures, tectonic fractures, weathered fractures and dissolving fractures. And the diagenetic fractures were further divided into three subclasses of condensation contraction fractures, intergravel fractures and intercrystal fractures. The fracture reservoirs were also divided into 4 categories, such as high-angle fractures, oblique fractures, mesh fractures and low-angle fractures according to fracture attitude. With the FMI image logging and other logging information, the scale logging by the core method was adopted, our study shows that the fracture of tuff was the most developed, and the fracture density can be reached 11.46 bar/m, followed by the lava, and the sedimentary rock fracture was the lowest level. The fractures thickness in the region of 300 m apart from weather crust was accounted for 80.3% of the total thickness. Because of image logging data is limited, the method of drilling wells trajectory to calculate the fracture trend was proposed. The fracture trend was divided into two groups NE and NW. The secondary pore-fracture forming and distribution were controlled by fracture system, formed the beneficial passage system of the oil and gas, the fracture system was the key factor in restricting oil and gas accumulation, and was crucial for the volcanic rocks reservoirs.

Integrated petrophysical log evaluation for coalbed methane in the Hancheng area, China

In order to investigate coalbed physical parameters and gas bearing properties, based on coal core experiments, a nonlinear comprehensive model estimating coal properties and gas content is established. Three-dimensional numerical simulation of the dual laterolog response of a coalbed fracture is carried out, and a dual laterolog fast computation model for predicting fracture porosity is established. The results show that the study area mainly developed lean coal and meagre coal. The coalbed is rich in gas. Ash content, volatile dry ash-free basis, true density and fixed carbon show good correlation. The logging responses of the coalbed have distinct characteristics; however, the responses highly depend on coal composition and gas content. The simulated annealing differential evolution (SADE) neural network is adopted to predict coal composition and gas content. Numerical analysis of the dual laterolog responses of the coal fractures shows that as coal matrix resistivity increases, the dual laterolog apparent resistivity increases accordingly. For a coalbed with low-angle fractures, deep and shallow resistivity show little difference; however, for a coalbed with high-angle fractures, its dual laterolog shows an obvious positive difference. Fracture porosity and dual laterolog apparent conductivity presents a linear relationship. According to coal matrix resistivity distribution and fracture development, a fast computation model to predict fracture porosity is established based on the dual laterolog. Field data show that the predicted coal composition and gas content from the SADE algorithm agree with that of coal core analysis, and the calculated fracture porosity based on the dual laterolog matches the coal core's macroscopic description.

Characteristics of Fracture and Fractured Reservoir of Hailaer Basin Budart Group

The output of fracture pool is over half of the entire outout of oil and gas,and fracture pool is one of the important fields of oil inhancing yield in 21st Century. Fractured reservoir evaluation is always a huge challenge for the oil exploration and development. Budart Group of Sudert district in Hailaer Basin is a reservoir that has very low porosity、very low permeability、double pore system and it is rich of fracture. The paper summarized Hailaer Basin Budart Group reservoir’s characteristics, especially fractures’s characteristics in conventional logs,fracture’s parameters,such as fracture density,dip,width and filling and illustrate the response of low angle fracture and high angle fracture in logs.

Numerical simulation of the dual laterolog response to fractures and caves in fractured-cavernous formation

Abstract To study the response characteristics of dual laterolog in fractured-cavernous reservoirs, a model of fractures and caves was built, and the 3-D finite element method was utilized to simulate its logging responses. Caves of certain size have influence on the response of dual laterolog within certain space. With the cave size increasing, the logging apparent resistivity becomes lower, and the apparent resistivity ratio of deep laterolog to shallow laterolog decreases. Within less than the one-cave radius scope, the dual laterolog is more sensitive to the cave to borehole wall. The logging response of fractured-cavernous formation is mainly affected by fractures, and low angle fractures result in a negative difference of dual laterolog which is contrary to high angle fractures, but the existing caves do not change the matching relationship between the shallow and deep resistivity and fracture dip. The apparent conductivity of dual laterolog in fracture-cave formation presents a linear relationship with fracture aperture.

The evolution and significance of microfracturing within feldspars in low-grade granitic mylonites: A case study from the Eastern Ghats Mobile Belt, India

Patterns of microfracturing are investigated in plagioclase and K-feldspar porphyroclasts formed within granitic mylonites along the boundary of the Eastern Ghats Mobile Belt, India. The mineral assemblage comprising quartz, feldspar, biotite and hornblende suggests lower greenschist facies conditions during mylonitisation, with the contrasting ductile behaviour of quartz and brittle fracturing of feldspars restricting the temperature range during deformation to 300–350 °C. Microfracturing of feldspars takes place by concentration of pure shear within the feldspar-rich layers. This may reflect strain partitioning into dominantly pure and simple shear due to the competency contrasts between the two major constituent minerals (quartz and feldspar). The microfractures occur in conjugates (here designated T 1 and T 2) with T 1 inclined in the same direction as the S-fabric and showing an antithetic sense to the NW verging shear, while T 2 is inclined in an opposite sense to the S-fabric and displays synthetic shear. The direction of maximum compression occurs at high angles to the C-fabric, and the T 1 and T 2 fractures are the result of pure shear localized into brittle layers within the mylonite. With progressive shear, the fractures along with their host feldspar grains are rotated. Theoretical graphs are plotted between bulk shear ( γ) and the angle of initiation ( α) of T 1 and T 2 with respect to C-planes, for fractures hosted in a circular or elliptical objects. The kinematics of these fractures are also analyzed with regard to variations in shear strain and sense of shear along them. The sense of shear may vary or remain stable within fractures, depending on their initial angle of inclination with respect to the C-fabric. As T 1 is inclined at low angles to the XY plane and in the same direction as the S-fabric, it undergoes maximum shear strain compared to T 2 and may even exceed the bulk shear. This facilitates breakdown of feldspar porphyroclasts during progressive deformation. The T 1 set maintains an antithetic sense of shear even at low angles with the C-fabric, while T 2 displays a synthetic sense in spite of being at high angles to C. The T 1 and T 2 fractures therefore differ significantly from the classic Riedel, P and Y shears where low angle fractures display synthetic shear and high angle ones ( R′) show antithetic displacement with respect to the bulk shear sense.

Mineral deformation and subglacial processes on ice-bedrock interface of Hailuogou Glacier

Hailuogou Glacier is located in a warm and humid maritime environment. It is large and moves very fast. The bottom of the glacier slides intensively and the temperature at the bottom approaches the pressure melting point. Therefore, there are abundant melting water and debris which act as effective “grinding tools” for glacial abrasion. Polarizing microscope is used to observe the mineral deformation characteristics on the ice-bedrock interface. It is found that feldspar, quartz, hornblende and biotite are exposed to deformation, fracture and chemical alteration to various extents. Bending deformation is common for biotite, due to their lattice characteristics, and the bending orientations are mostly the same as the glacier flow. Bending deformation also occurs in a few hornblendes. High-angle tension fracture and low-angle shear fracture are common for quartz and feldspar, some of them are totally crushed (mylonizations) due to their rigidity. Thus, all the abrasion, quarrying, subglacial water action and subglacial dissolution processes at the bottom of the glacier are verified at the micro-scale level. Mineral deformation and fracture are the basic subglacial erosion mechanisms. The abrasion thickness is 30–90 μm for each time and the average is 50 μm. Most of the debris are silt produced by glacial abrasion. The extent of mineral deformation and fracture decreases drastically downwards beneath the bedrock surface. The estimated erosion rate is about 2.2–11.4 mm/a, which is similar to that of other maritime alpine glaciers, smaller than that of large-scale piedmont glaciers in Alaska (10–30 mm/a), and larger than that of continental glaciers (0.1–1.0 mm/a). The type and size of a glacier are the main factors that influence its erosion rate.

花崗岩体上部に発達する低角度亀裂の空間分布特性と地質学的解釈―瑞浪超深地層研究所周辺の土岐花崗岩からの知見―

The spatial distribution of hydrogeologically-relevant geological structures such as fracture and fault is usually very heterogeneous, varying, in general, according to geological history, scale of observation and rock type. With the objective of improving analytical methods to better understand structural heterogeneities, this paper describes the results of investigations of the spatial relationship between regional geological structures (Tsukiyoshi fault and Tsukiyoshi paleo-channel) and fracture density with special emphasis on low-angle fractures in the Toki granite, central Japan. As a result of the analysis, positive correlations between average of fracture spacing data and the horizontal distances from two regional geological structures have been observed. Because the positive correlations can be approximated with a polynomial function of the response surface, we have obtained an improved understanding of the spatial distribution of low-angle fractures with respect to the location of regional geological structures. We have interpreted that the observed spatial distribution of low-angle fracture density conforms to the tectonic setting, on the assumption that the Tsukiyoshi fault and Tsukiyoshi paleo-channel are probably associated with the pull-apart basin by strike-slip faulting from the past investigations and geophysical surveys for this research area. In addition, the proposed spatial modeling techniques and geological interpretations are able to evaluate certainty of interpreted faults and fractures distributions, quantify general trend in structural heterogeneities and detect unknown faults.

Low-angle fracture zones in rapakivi granite at Hästholmen, southern Finland

Geological and geophysical borehole data from the site investigation programme for spent nuclear fuel in Finland are integrated to analyse the main engineering geological structures of the rock mass. At the Loviisa nuclear power plant site at Hästholmen, there are 100–150-m-long altered sections with anomalies in geophysical logs along the holes drilled in the orthogonally fractured rapakivi granite. In detail, the high water conductivity is concentrated in narrow sections as observed in the tube wave logs and hydraulic measurements. The TV image locates sporadic subhorizontal fractures that are most probably the main hydraulic conduits in the rock mass. The mean length of the measured fracture traces is 8 m. However, geophysical cross-hole investigations and seismic surveys indicate the continuity of a few fracture patterns between shallow boreholes drilled with spacing of the order of 100 m. Furthermore, the continuity of five gently dipping fractured structures that contain a few sections of broken rock between the 1-km-deep holes drilled approximately 0.5 km apart is suggested according to similarities in the acoustic character in the full waveform sonic borehole logs.

Seismic reflector imaging by prestack time migration in the Kakkonda geothermal field, Japan

We show that a prestack migration method improves the S/N ratio of seismic reflection profiling in the Kakkonda geothermal field where seismic reflection data are of poor quality. We use non-iterative prestack time migration (PSTM), which does not require multiple iterations to determine the velocity structure for prestack time migration. The optimum constant migration velocity can be determined at each image point from a migration velocity analysis based on primary diffraction patterns. Our results delineate a strong reflector beneath a zone of high seismicity. According to the correspondence between the fracture distribution, the distribution of microearthquakes, and geothermal structure, this reflector is interpreted to be a zone of low-angle fractures saturated with hydrothermal fluids, and to be strongly controlled by the geothermal structure.

Glaciofluvial crevasse and conduit fills as indicators of supraglacial dewatering during a surge, Skeiðarárjökull, Iceland

AbstractThis paper documents the glaciological structures associated with the surge of Skeiðarárjökull, Iceland, in 1991. These structures are interpreted as units of stratified ice, low-angle fractures, vertical and sub-vertical fractures (crevasse traces) and thrusts. The inferred thrusts are debris-rich and, unusually, have both down-glacier and up-glacier dips close to the ice margin. Sediment infills consist of either massive sand or horizontally stratified sand units. The most significant debris-rich structures on the glacier surface, however, are supraglacial crevasse and conduit fills, which contain either massive or horizontally stratified silts, sands and granule-gravels. These sediments infill both vertical fractures (relict crevasses) and englacial conduits. At the stratigraphic base of these sediment fills there is evidence of syn-sedimentary deformation, suggesting that sedimentation occurred during crevasse closure and continued thereafter. We argue that these structures relate to an episode of supraglacial meltwater flow during the 1991 surge, caused by the build-up of subglacial water pressure in a linked-cavity system or some similar distributed drainage system beneath the glacier. The development of this high-level drainage route may have helped regulate basal water pressures and therefore the active phase of the surge. The idea that the supraglacial leakage of subglacial water may have played a role in terminating the surge is explored.

Fracture systematics in and around well WD-1, kakkonda geothermal field, japan

Interpretation of FMI (Schlumbergers Fullbore Formation MicroImager) logs from well WD-1 and observation of spot cores from well WD-1 in the Kakkonda geothermal field have provided important clues about the nature and configuration of the fracture networks controlling circulation of high-temperature hydrothermal fluid. The fractures comprise four groups: (1) fractures striking NW, (2) high-angle fractures striking NE to E, (3) low-angle fractures striking N, and (4) low-angle fractures striking E. Most permeable fractures in the shallow (<1500 m depth) and deep reservoirs are of group 3, and likely reflect reverse faulting in the modern stress field. Low-angle, group 4 fractures are also permeable, and may have formed in the stress field that existed prior to 0.2±0.1 Ma. Distributions and densities of fractures are comparable in the shallow and deep reservoirs; the temperature difference between the reservoirs is independent of these variables.