Shihezi Formation Research Articles

Study on Geochemical Characteristics of tight sandstone gas accumulation in Linxing area

Based on the comprehensive analysis of the characteristics of tight sandstone gas composition, carbon isotope, light hydrocarbons and source rocks in Linxing area of Ordos Basin, the reservoir-forming model of tight sandstone gas in this area is discussed. The study shows that methane is the main component of tight sandstone gas, with low contents of heavy hydrocarbons and non-hydrocarbons, mainly belonging to dry gas in the Upper Paleozoic in Linxing area. The values of δ13C1, δ13C2 and δ13C3 of natural gas are in the ranges of -45.6‰ ~ -32.9‰, -28.9‰ ~ -22.3‰ and -26.2‰~ -19.1‰, respectively. The carbon isotopic values of alkane gas show a general trend of positive carbon sequence. δ13C1 value is less than -30‰, with typical characteristics of organic genesis. There is a certain similarity in the composition characteristics of light hydrocarbons. The C7 series show the advantage of methylhexane, while the C5-7 series mainly shows the advantage of isoalkane. The tight sandstone gas in this area is mainly composed of mature coal-derived gas, containing a small amount of coal-derived gas and oil-type gas mixture. According to the mode of hydrocarbon generation, diffusion and migration of source rocks in Linxing area, the tight sandstone gas in the study area can be divided into three types of reservoir-forming assemblages: the upper reservoir type of the far-source type (upper Shihezi formation-shiqianfeng formation sandstone reservoir-forming away from source rocks), the upper reservoir type of the near-source type ( the Lower Shihezi formation sandstone reservoir-outside the source rock), and the self-storage type of the source type (Shanxi formation-Taiyuan formation source rock internal sand reservoir).

Dynamic fracture propagation model for oriented perforation steering fracturing in low permeability reservoir based on microelement method

Oriented perforation steering fracturing technology with the purpose of enlarging fracture extension scale and increasing reservoir reconstruction volume is a common method for stimulating and reforming low permeability oil and gas reservoirs. Accurate prediction of the fracture deflection trajectory is the key to successful implementation of this technology. Based on the relevant theories of elastic fracture mechanics and the microelement method, a prediction model for the fracture deflection trajectory of steering fracturing is established by comprehensively considering the influences of horizontal in situ stress difference, perforation parameters, injection parameters and formation mechanics parameters. The model completes the cyclic iteration calculation between the minor increment and the propagation direction angle of the crack tip using the calculation program compiled by Visual Basic and realizes the simulation of the dynamic propagation of steering fractures. Taking the tight sandstone reservoir of the Shihezi Formation in Linxing block as an example, the differences between the hydraulic fracture trajectory calculated by this model and the extended finite element model and the micro-seismic monitoring values in the fracturing field were compared, and the influence of various factors on the deflection distance of the hydraulic fracture trajectory were studied. The results show that, compared with the XFEM model, the hydraulic fracture trajectory simulated by this model has better matching with the micro-seismic monitoring results. The deflection distance decreases negatively logarithmically with increasing horizontal in situ stress difference and increases linearly with increasing fracturing fluid displacement and viscosity. The larger the perforated angle, the longer the deflection distance, and the length of perforation has little effect on the deflection distance. The model and conclusions are significant to further understand oriented perforation steering fracturing.

Sources of authigenic quartz in the Permian tight sandstones close to Gaoqing Fault, Dongying Sag, Bohai Bay Basin, China

Abstract Authigenic quartz is a significant cause of reduced porosity and permeability in the cement of the Permian Upper Shihezi Formation (P2s) sandstones in the Gaoqing area, Dongying Sag, Bohai Bay Basin, China. The sandstones are mainly quartzarenite with minor amounts of clay minerals. Authigenic quartz is generally the most commonly occurring cement and occurs as quartz overgrowth, microfracture-healing quartz, pore-filling quartz, and microquartz in the sandstones. Three distinct, separate phases of authigenic quartz (I, II, and III) were distinguished by using cathodoluminescence microscopy (CL). In the scanned CL images, phase I quartz generally appears with light brown luminescence in the 450–670 nm range, phase II quartz is non-luminescent in the 420–480 nm range, and phase III quartz appears with dark brown luminescence in the 470–650 nm range. Phase I quartz, originating mainly from pressure solution, has a composition similar to that of detrital quartz grains and was formed at approximately 79.3–104 °C. In the phase II quartz, Li+, Na+, Mg2+, Al3+, and Ba2+ are enriched, and the homogenisation temperature of some aqueous inclusions is significantly higher than its maximum burial temperature of 130 °C. The δEu of phase II quartz is up to 1.42. The silica sources for phase II quartz are dominated by hydrothermal fluids; pressure solution, feldspar dissolution, and illitisation of smectite also provided a small amount of silica. In the phase III quartz, all of the aqueous inclusions are almost higher than its maximum burial temperature. Al3+, Mg2+, and Rb+ are enriched in most of this quartz, which was sourced mainly from hydrothermal fluids; pressure solution also provided a very small amount of silica. The hydrothermal fluids providing the silica source for phases II and III quartz migrated into the P2s sandstones along the adjacent Gaoqing Fault.

A synthesis of late Paleozoic and early Mesozoic sedimentary provenances and constraints on the tectonic evolution of the northern North China Craton

The North China Craton (NCC) and its adjacent areas experienced a series of complex tectonic events during the late Paleozoic–early Mesozoic, the closure of the Paleo-Asian Ocean (PAO), the collision between the Central Asian Orogenic Belt (CAOB) and the NCC, and the subduction of the Yangtze Plate. This paper provides an overview of the provenances and tectonic implications of late Paleozoic and early Mesozoic sediments in the northern NCC based on a comprehensive analysis of the available geological and geochronological data together with our new clast compositions, paleocurrents, detrital zircon ages and Hf isotope data. Detrital zircons from the middle Permian Shihezi Formation in the Yungang basin have age distributions of 442–264 Ma, 2179–1606 Ma and 2665–2249 Ma and are characterized by negative εHf(t) values that match the age population and Hf isotope peculiarities of the Yinshan–Yanshan Orogenic Belt (YYOB). These observations suggest that these zircons were derived from the YYOB, which is consistent with most regions of the NCC. The CAOB became a minor source of sediments in the northern NCC no earlier than the late Permian, and the ultimate closure time of the PAO can be constrained to 267–261 Ma by the youngest detrital zircon ages. The tectonic evolution of the northern NCC in the late Paleozoic was closely related to the southward subduction of the PAO and the amalgamation of the CAOB and the NCC. The angular unconformity underlying the Early Jurassic strata and the absence of Triassic strata in the Yungang basin might be ascribed to the continuous collision of the CAOB and the NCC during the Late Triassic.

Effect of tectonic evolution on hydrocarbon charging time: A case study from Lower Shihezi Formation (Guadalupian), the Hangjinqi area, northern Ordos, China

A giant gas field has been discovered in the Lower Shihezi Formation in the Hangjinqi area, northern Ordos Basin, China. Coal seams and mudstone in the Taiyuan and Shanxi Formation are considered as source rocks and sandstone in the Lower Shihezi Formation is treated as reservoirs for gas. The charging time in the sandstone reservoirs was studied based on the homogenization temperature of aqueous inclusions, thermal and burial history simulation and K-Ar dating of authigenic illite. Two episodes of gas charging in the northern part and one episode in the southern part were delineated in the reservoirs of the Hangjinqi area, evidenced by microscopic observation of the occurrence of fluid inclusions and the analysis of salinity in aqueous inclusions. In the southern part of the study area gas charging occurred between circa 171-162 Ma, whereas in the northern part gas charging was between 158-144 Ma and 120-109 Ma, respectively. The gas charging events appeared to be influenced by the tectonic evolution of northern Ordos Basin. Paleo topography caused reduction in thickness of source rocks northwards, and contributed to differences in hydrocarbon generation potential and thermal evolution between the northern and the southern part of the study area. Therefore, gas charging occurred in the southern part prior to that in the northern part. Furthermore, tilting occurred in Middle Jurassic and provided the driver and pathways for gas, leading to a first charging event in the northern part of the study area.

Elemental geochemical characteristics of Lower—Middle Permian mudstones in Taikang Uplift, southern North China Basin: implications for the FOUR-PALEO conditions

The paleo-sedimentary environment can be restored qualitatively or semiqualitatively using elements or combinations of elements that are sensitive to the depositional environment. Through the analysis and testing of the main elements, trace elements and rare earth elements of twenty-one Lower—Middle Permian coal-bearing strata mudstone samples in Taikang Uplift, southern North China Basin, this study explored the FOUR-PALEO conditions (paleo-water depth, paleo-redox conditions, paleoclimate and paleo-provenance). The La and Co contents indicate that the water depth of the northern margin of the southern North China Basin became gradually shallower during the Lower—Middle Permian period. According to the U/Th, Uau, V/Cr, Ni/Co and V/Sc ratios, it is known that this section of the basin is in an oxidizing environment, and the oxidation is gradually enhanced from the Taiyuan Formation to the Lower Shihezi Formation. Paleoclimate from bivariate plot diagrams of C-value and Sr/Cu versus Ga/Rb shows that the Taikang Uplift was characterized by a warm and arid climate during the depositional period of Permian Taiyuan Formation and a warm and humid climate during the Lower Shihezi Formation and shows that the depositional period of Permian Shanxi Formation was in a transitional period of humid and arid. CIA, PIA, ICV, Th/U and K/Rb reflect the tectonic settings of the source area. From the Taiyuan Formation to the Lower Shihezi Formation, the CIA and PIA gradually increase, and the ICV gradually decreases, indicating enhanced chemical weathering and the transition from arid to humid in the source area, the arid and humid frequent change in the Taiyuan Formation, and a sudden climate change in the Shanxi Formation. Additionally, the provenance discriminant function, Zr/Sc-Th/Sr, REEs ratios, Th versus Hf-Co, Hf versus La/Th and ZREE vs. La/Yb indicate that the type of provenance is associated with magmatic differentiation. Moreover, discriminant diagrams, consisting of K2O/Na2O versus SiO2, La-Th-Sc, Th-Sc-Zr/10 and Th-Co-Zr/10, suggest that the mudstones of Lower-Middle Permian were deposited in active continental margin tectonic settings.

Formation of the primary petroleum reservoir in Wumaying inner buried-hill of Huanghua Depression, Bohai Bay Basin, China

Well Yinggu 1 drilled on the tectonic belt of the Wumaying buried-hill in Huanghua Depression obtained non-H2S high-yield oil and gas flow from the Permian Lower Shihezi Formation sandstone. The oil and gas are derived from the Upper Paleozoic coal source rock, the petroleum reservoir is an inner buried-hill primary oil and gas accumulation, showing a good prospect of the Paleozoic inner buried-hill primary reservoir exploration. The formation and accumulation of the primary petroleum reservoir in the Wumaying inner buried-hill are discussed by studying the primary source conditions, the inner buried-hill reservoir-cap combinations and the hydrocarbon accumulation period. The primary petroleum reservoir has three preponderant characteristics of accumulation: secondary large-scale gas generation of coal source rock, multi reservoir-cap combinations and mainly late hydrocarbon charging, which formed the compound hydrocarbon accumulation of the above-source sandstone and under-source carbonate rock in the Paleozoic inner buried-hill. Along with the Mesozoic and Cenozoic tectonic activities, the formation of the primary reservoir in Wumaying inner buried-hill is characterized by “mixed oil and gas charge in local parts in early stage, adjustment accumulation due to structural high migration in middle stage, and large-scale natural gas charge and compound accumulation in late stage”.

Influence of horizontal control holes on fracture height containment in low permeability layered coal measure gas reservoirs

In view of the difficulty of fracture height containment during fracturing in low permeability layered or thin-layered coal measure gas reservoirs, this paper proposes a method of excavating horizontal control holes in the barriers to contain the fracture height. Considering the influence of excavating control hole on the ground stress at hydraulic fracture tips, a calculation model of hydraulic fracture height in layered strata is established. Taking the coal measures tight sandstone reservoir of Shihezi Formation in Linxing block as an example, the effect of horizontal control holes on fracture height was simulated, and the influence of various factors on the containment effect was studied. The results showed that: the excavation of horizontal control holes can effectively prevent the hydraulic fracture propagation in its height within a certain range (about 0-10r from the center of the hole, r is radius of the control hole) and reduce the penetration depth of the barrier. The greater the ground stress difference between reservoirs and barriers, the stronger the containment effect of the control holes on the fracture height. When the ground stress difference is greater than 2 MPa, the fracture height can be effectively controlled. The fracture toughness difference between reservoirs and barriers has little effect on the control holes' containment. The greater the control hole radius is, the stronger the containment effect on the fracture height is, and the greater the effective containment range will be. The optimal height of the control hole is within the stratum which the ground stress is larger, and near the interlayer interface. The research results can provide new ideas and theoretical support for fracture height containment in low permeability layered or thin layered coal measures stratum.

Quartz cement origins and impact on storage performance in Permian Upper Shihezi Formation tight sandstone reservoirs in the northern Ordos Basin, China

Authigenic quartz is an important cementing material that can degrade storage capacity, whereas the effect of quartz cement on microscale pores and throats in tight sandstone is controversial. Therefore, it is crucial to verify the sources of quartz cement as well as the controls on microscopic storage. Toward this end, a variety of tests were performed on Permian Upper Shihezi Formation sandstones, and they are the vital exploration and development interval for gas in the northern Ordos basin. We found that quartz cement is the most abundant interstitial mineral in the Upper Shihezi Formation tight sandstones, which forms at approximately 55 °C to 188 °C with a continuous process. All the samples can be divided into pore dominated and throat dominated types, and there is an increase in micropores and a decrease in macropores as the quartz cement content decreases. Chemical compaction and transformation of clay minerals were the main sources of silica. This work shows that although quartz cement would occupy the void space and lead to a loss of porosity, it could retard the compaction and preserve the pores, which is attributed to the limited compressibility of quartz cement supported rocks. Thus, the difference in the pore radius and throat radius is diminished, resulting in the decrease in pore-throat size heterogeneity in tight sandstones.

Channel identification using 3D seismic attributes and well logging in lower Shihezi Formation of Hangjinqi area, northern Ordos Basin, China

Channel identification in oil and gas exploration is a vital task due to the importance of fluvial reservoirs. The fluvial sandstone of Lower Shihezi Formation of Ordos Basin is the primary target layer of gas exploration and development which has the maximum potential of producing tight gas sand reservoir in China. Fluvial paleo-channels can be identified easily by core, outcrop and logs data, but due to development of the several faults and limited seismic resolution in the study area, it is difficult to identify them by conventional seismic methods. The study was conducted in Hangjinqi area on the regional level of about 4537.87 km2. The radical objective of this research is to recognize the horizontal distribution and geometry of the multiple channels. Based on the study of integrated analyses of geological cores, geophysical well-logs, and seismic attributes it is confirmed that multiple braided channels are present in the study area. Channel distribution based on seismic attributes and its overlapping on the paleogeographic map showed that the channels are coming from the North and are merging towards South direction.

Cementation and porosity evolution of tight sandstone reservoirs in the Permian Sulige gas field, Ordos Basin (central China)

Abstract Tight sandstone gas reservoirs constitute an important type of unconventional gas resources and may be of crucial significance as environment-friendly source of energy in the world. However, the dynamics of densification and evolution of the low-porosity and low-permeability sandstone reservoirs are intensively debated in literature. Recently, the question of sequence of reservoir densification and hydrocarbon accumulation has arisen. A typical tight sandstone reservoir (the first member of the Shanxi Formation and the eighth member of Shihezi Formation) in the Permian Sulige gas field in Central China provides an excellent case study to probe these questions. By using a variety of different methodologies such as scanning electron microscopy (SEM), X-ray diffraction (XRD), cathodoluminescence, fluid inclusion analysis and confocal Raman spectrometer analysis, we identified different authigenic minerals such as illite, kaolinite, dickite, chlorite and smectite illitization coexisting with siliceous and calcitic cements. These authigenic minerals precipitated at different diagenetic stages, related to the changing conditions during compaction and lithification, and therefore, changed the porosities of the host rocks during each of these stages. The types of pores in the Sulige gas field are dominated by illite and kaolinite intercrystalline pores, matrix solution pores, and primary intergranular pores. The results of this study suggest that compaction and cementation are the reasons for the most distinct loss in porosity. On the other hand, kaolinite and illite preserve their intercrystalline pores, which could be identified as the main reserve space of the tight gas reservoirs. Therefore, the secondary porosity of the sandstone reservoirs forms the most important contributor to the gas reservoir potential in the Sulige gas field. In addition, it could be inferred that the tight gas accumulation took place during the densification of the reservoirs. The implications of the presented results may also help to understand the formation of other tight reservoirs world-wide.

Brittleness Logging Evaluation of Shihezi Formation in Yanchang Gas Field in Ordos Basin Based on Rock Mechanical Characteristics

Brittleness determines the effect of reservoir fracturing and is important for increasing the production of unconventional oil and gas. The reservoir of Yanchang gas field is characterized by low porosity, low permeability and strong heterogeneity, which make it difficult to produce industrial gas flow under natural conditions. Triaxial compression tests in sandstone samples of Shihezi formation in Yanchang gas field show that the main fracturing mode is multiple horizontal splitting, the main cracks are completely fractured, and the brittleness index ranges from 46.1% to 51.4%. After S-wave velocity model in the study area is established, the conversion relation of dynamic and static mechanical parameters could be obtained, that is,Es=0.4142Ed+10.769(R2=0.8387),and μs=1.1076 μd-0.068(R2=0.7366), according to which, the dynamic mechanics parameters calculated by well logging can be adjusted, and then the reservoir brittleness could be quantitatively evaluated. Combining with the brittleness index calculated by mineral component method, it can be concluded that the brittleness calculated by well logging is reliable and the brittleness of Shihezi formation in Yanchang gas field is high, which is beneficial for reservoir fracturing.

Detrital zircon U–Pb ages from the Middle to Late Permian strata of the Yiyang area, southern North China Craton: Implications for the Mianlue oceanic crust subduction

Basin formation and sedimentary development in the southern North China Craton (NCC) are closely related to the tectonic evolution of the Qinling Orogenic Belt (QOB). With the aim of revealing the coupling evolution between the QOB and southern NCC during the Late Palaeozoic, an investigation based on provenance analysis was performed in this study. We present new detrital zircon U–Pb data from the Middle–Late Permian sediments in the Yiyang area, southern NCC. The results show that detrital zircons, from the Middle Permian Shihezi Formation, have three major peaks at 2,447, 1,905, and 296 Ma. In contrast, the lower and upper parts of the Late Permian Sunjiagou Formation yield major peaks at ~1.85 Ga and ~260 Ma, with weak peaks at ~2.5 Ga, ~450 Ma, and ~315 Ma, and the middle part of the Sunjiagou Formation displays a major peak at 429 Ma. Combined with published palaeocurrent data and lithofacies palaeogeography, the sediments of the Middle Permian Shihezi Formation were sourced from the Inner Mongolia Palaeo‐Uplift along the northern margin of the NCC, whereas the Upper Permian Sunjiagou Formation originated mainly from the southern margin of the NCC including the North Qinling Belt. This provenance shift indicates that the North Qinling Belt and southern margin of the NCC initiated the uplift and exhumation during the Late Permian. On the basis of the evolutionary history of the QOB, we attribute the uplift geodynamics to the Mianlue oceanic crust subduction. This study provides important sedimentological constraints for the tectonic conversion during the Late Palaeozoic along the southern margin of the NCC, which also restricts the start‐up time of the Mianlue oceanic crust subduction from the perspective of the basin–mountain system.

Full-size pore structure characterization of deep-buried coals and its impact on methane adsorption capacity: A case study of the Shihezi Formation coals from the Panji Deep Area in Huainan Coalfield, Southern North China

The characteristics and influence on methane adsorption capacity of the pore structure of coals was investigated through an approach that integrates mercury intrusion porosimetry, low pressure N2/CO2 adsorption, and field emission scanning electron microscopy. Shihezi Formation coal samples from the Panji Deep Area in Huainan Coalfield in Southern North China were adopted as the study subjects. The fractal features of heterogeneous coal pore structures were quantified on the basis of N2 adsorption isotherms by using the Frenkel−Halesy−Hill (FHH) model. Pore structure and morphology characterizations confirm that Shihezi Formation coals developed with multiple pore types and heterogeneous pore structures, and the slit-shaped and ink-bottle-shaped pores are mainly distributed at the pore size interval of 3.3–10 nm. Our coal samples present uni-modal and multi-modal pore size distributions, wherein multiple peaks are concentrated in the intervals of 0.45–0.55 nm and 2–20 μm. The major volumes from mesopores (2 nm < pore diameter < 50 nm) and macropores (pore diameter > 50 nm) occur in the full-scale pore size distributions, revealing Shihezi Formation coals are mesopores and macropores rich. Micropores (pore diameter < 2 nm) provide the majority of pore specific surface area, whereas pore volume is mostly contributed by mesopores and macropores. The volumes and specific surface areas of mesopores and macropores are positively correlated with ash yield and inertinite contents but are negatively correlated with maximum vitrinite reflectance (Ro,max). The methane adsorption capacity of Shihezi Formation coals is largely independent of mesopores and macropores but is positively correlated with the pore volume and specific surface area of micropores. This correlation suggests that micropores provide the predominant sites for methane adsorption. The calculated values of pore surface fractal dimension D1 and spatial fractal dimension D2 are 2.440–2.610 (avg. 2.515) and 2.542–2.761 (avg. 2.650), respectively, suggesting that the pore structures of Shihezi Formation coals are highly complex and heterogeneous. The positive correlation between D1 and methane adsorption capacity reveals that high surface roughness of the pore structure is associated with high methane adsorption capacity. However, the irregular pore structure does not influence methane adsorption capacity.

Gas accumulation conditions and key technologies for exploration & development of Sulige gasfield

Up to now, the Sulige area in Ordos Basin has the favorable exploration area of 55 × 103 km2, the total reserve of natural gas of nearly 6 × 1012 m3 and the proven reserve (including basic proven reserve) of 4.77 × 1012 m3, where the annual production of natural gas reaches 23 × 109 m3, and the Sulige gasfield is the largest onshore natural gas field in China. The pay zone of the Sulige gasfield mainly is Member 8 of Shihezi Formation and Member 1 of Shanxi Formation of Permian which belong to the typical tight sandstone gas reservoir. The coal measure strata in Carboniferous Benxi Formation, Permian Taiyuan Formation and Shanxi Formation provide abundant gas sources for the Gulige gas reservoirs. An open-flow sedimentary model of lacustrine delta is developed, the gentle bottom, sand supply from multi-source, strong hydrodynamic force and multi-period superposition control the distribution of large-area reservoir sand body. Lithology of the reservoir is the sandstone of the fluvial-delta facies, the physical property is poor and the heterogeneity is strong, the average porosity ranges from 4% to 12% and the average permeability varies from 0.01 to 1 mD. The gas reservoir is characterized by wide hydrocarbon generation, pervasive hydrocarbon charging, short-range migration and massive accumulation. The pressure coefficient of the gas reservoir ranges from 0.62 to 0.90, indicating the low-pressure gas reservoir, and the single-well yield is low. Full digital seismic technique in the desert area, non-longitudinal seismic technique in the loess plateau, accurate logging evaluation technique, tight sand reservoir stimulation technology and horizontal well development technology are key technologies for exploration and development of Sulige gasfield.

Evolution of formation pressure and accumulation of natural gas in the Upper Palaeozoic, eastern‐central Ordos Basin, Central China

The exploration achievements in the eastern‐central Ordos Basin have attracted tremendous attention in recent years. However, only a few studies were carried out on the relationship between formation pressure and distribution of natural gas in this area. The formation pressure of 293 wells, logging data of 154 wells, and fluid inclusion samples of 53 wells were analysed in this paper, and the results show that the mudstones from Permian Upper Shihezi Formation to Permian Taiyuan Formation are mostly undercompacted. Four gas‐bearing layers with significant differences in pressure profile can be further divided into Permian negative pressure system and Carboniferous normal pressure system. The distribution of current formation pressure is high in the east and north areas but low in the west and south areas. The sedimentary facies and distribution of formation pressure in Permian have a good consistency. The vertical development (zonation) of gas reservoirs is controlled by the undercompacted mudstone layers, and the reservoirs can be found in the “low pressure” areas of negative pressure strata in the Permian and “high pressure” regions of normal pressure strata in the Carboniferous. The pressure evolution and accumulation process can be divided into 3 stages: hydrocarbon‐generation supercharging accumulation during Late Jurassic—end of Early Cretaceous, sandstone resilience discharging accumulation during end of Early Cretaceous—Paleogene, and adjusting accumulation during Paleogene—Quaternary. The discovery of the relationship between formation pressure and gas reservoirs is significant to the future natural gas exploration in this area.

Ternary analytic porosity-reduction model of sandstone compaction trend and its significance in petroleum geology: A case study of tight sandstones in Permian Lower Shihezi Formation of Shilijiahan area, Ordos Basin, China

The tight sandstones in the Permian Lower Shihezi Formation of Shilijiahan area in the Ordos Basin was taken as study object in this research to quantitatively determine the effects of burial depth, burial time and compaction strength on porosity during densification of reservoir. Firstly, sandstone compaction profiles were analyzed in detail. Secondly, the theoretical study was performed based on visco-elasto-plastic stress–strain model. Thirdly, multiple regression and iterative algorithm were used respectively to ascertain the variation trends of Young's modulus and equivalent viscosity coefficient with burial depth and burial time. Accordingly, the ternary analytic porosity-reduction model of sandstone compaction trend was established. Eventually, the reasonability of improved model was tested by comparing with thin-section statistics under microscope and the models in common use. The study shows that the new model can divide the porosity reduction into three parts, namely, elastic porosity loss, visco-plastic porosity loss and porosity loss from cementation. And the results calculated by the new model of litharenite in He 2 Member are close to the average value from the thin-section statistics on Houseknecht chart, which approximately reveals the relative magnitudes of compaction and cementation in the normal evolution trend of sandstone porosity. Furthermore, the model can more exactly depict the compaction trend of sandstone affected little by dissolution than previous compaction models, and evaluate sandstone compaction degree and its contribution to reservoir densification during different burial and uplift processes.

PORE-THROAT STRUCTURE AND FRACTAL CHARACTERISTICS OF SHIHEZI FORMATION TIGHT GAS SANDSTONE IN THE ORDOS BASIN, CHINA

In order to better understand the impact of fractal features of pore-throat structures on effective physical properties of tight gas sandstones, this paper carried out constant-rate mercury injection tests, gas–water permeability analyses, helium-based porosity and nitrogen pulse decay permeability measurements, and SEM and casting thin-section analyses on 20 sandstone samples of the Shihezi Formation from 16 wells of the Sulige Gas Field in the Ordos Basin, China. The fractal dimensions of pores corresponding to two pore radius ranges, namely fractal dimensions [Formula: see text] and [Formula: see text], and those of throats also corresponding to two throat radius ranges, namely fractal dimensions [Formula: see text] and [Formula: see text], were then calculated using the constant-rate mercury injection data. Fractal dimensions [Formula: see text] and [Formula: see text] are negatively correlated with the average pore radius and pore volume as well as quartz content, and they are positively related to contents of clay minerals and lithic fragments. Compared with pore fractal dimensions, throat fractal dimensions have lower correlations with throat structural parameters, and show vague relations with mineral compositions. Although the effects imposed by mineral compositions upon throat structures are similar to those upon pore structures, the flake-like and curved shapes of throats result in irregularity of fractal dimensions. Tight gas sandstones, with smaller fractal dimensions [Formula: see text] and [Formula: see text], have larger effective porosity, indicating that more gas volumes can be replaced by liquids. As for tight gas sandstones with lower fractal dimensions [Formula: see text] and [Formula: see text], the effective gas permeability is relatively high. In such cases, gas can flow more easily through the reservoir.

Pore structure and fractal characteristics of a tight gas sandstone: A case study of Sulige area in the Ordos Basin, China

To understand the pore structure and fractal characteristics of tight gas reservoirs, thin sections, nuclear magnetic resonance, rate-controlled mercury injection, microcomputed tomography scanning, and field emission scanning electron microscopy investigations under laboratory conditions were conducted on a suite of core samples from the Middle Permian Shihezi Formation of Sulige area in the Ordos Basin, China. The investigated tight gas sandstones comprise three types of pores, i.e. residual intergranular pore, secondary dissolution pore, and micropore. The pore–throat size distribution is extremely wide and multiscale (10 nm–400 μm) co-existing in tight gas reservoirs. The submicron- and micron-scale pore–throats with radius above 0.05 μm, which are characterized by combining rate-controlled mercury injection with nuclear magnetic resonance, are considered to be the effective pores and throats that dominated the reservoirs flow capacity. Tight gas sandstones have stage fractal characteristics, and the intrusion pressure of approximately 1 MPa is regarded as an inflection point. Fractal dimension is negatively correlated with permeability, average throat radius and mainstream throat radius, positively correlated with heterogeneous coefficient, while there are no obvious relationships with porosity and average pore radius. Additionally, the percolation characteristics of tight gas reservoirs can be characterized by fractal structure. When the pore structure does not follow the fractal structure (i.e. intrusion pressure is lower than 1 MPa), the mercury intrusion saturation is dominated by pores; in contrast, the mercury intrusion saturation is almost solely dominated by throats. This research sheds light on the pore–throat size distribution of tight gas reservoirs by identifying the role of multiple techniques and the relationships between the pore structure parameters and percolation characteristics of tight gas reservoirs and fractal dimension.

Geochemical characteristics of n-alkanes and isoprenoids in coal seams from Zhuji coal mine, Huainan coalfield, China, and their relationship with coal-forming environment.

Ten coal seams in Upper Shihezi Formation, Lower Shihezi Formation, and Shanxi Formation from the Zhuji mine, Huainan coalfield, China, were analyzed for n-alkanes and isoprenoids (pristine and phytane) using gas chromatography-mass spectrometry (GC-MS), with an aim of reconstructing the coal-forming plants and depositional environments along with organic carbon isotope analyses. The total n-alkane concentrations ranged from 34.1 to 481mg/kg. Values of organic carbon isotope (δ13Corg) ranged from - 24.6 to - 23.7‰. The calorific value (Qb,d), maximum vitrinite reflectance (Romax), proximate, and ultimate analysis were also determined but showed no correlation with n-alkane concentrations. Carbon Preference Index (CPI) values ranged from 0.945 to 1.30, suggesting no obvious odd/even predominance of n-alkane. The predominance of C11 and C17 n-alkanes implied that the coal may be deposited in the fresh and mildly brackish environment. According to the contrary changing trend of pristine/phytane (Pr/Ph) ratio and boron concentrations, Pr/Ph can be used as an indicator to reconstruct the marine transgression-regression in sedimentary environment of coal formation. The influence of marine transgression may lead to the enrichment of pyrite sulfur in the coal seam 4-2. C3 plants (- 32 to - 21‰) and marine algae (- 23 to - 16‰) were probably the main coal-forming plants in the studied coal seams. No correlation of the n-alkane concentration and redox condition of the depositional environment with organic carbon isotope composition were found.