Shahezi Formation Research Articles

High-precision geochronology of the Early Cretaceous Yingcheng Formation and its stratigraphic implications for Songliao Basin, China

The Songliao Basin in Northeast Asia is the largest and longest-lived rift basin and preserves a near-continuous continental succession of the most of the Cretaceous period, providing great material to investigate the adaption of the terrestrial systems to the Cretaceous greenhouse climate and tectonic events. However, the paucity of precise and accurate radioisotopic ages from the Early Cretaceous strata of the Songliao Basin has greatly held back the temporal and causal correlation of the continental records to the global Early Cretaceous records. Three tuff layers intercalated in the Yingcheng Formation have been intercepted by the SK-2 borehole, which offer excellent materials for radioisotopic dating and calibration of the chronostratigraphy of the Lower Cretaceous sequence of Songliao Basin. Moreover, the Yingcheng Formation recorded the largest and the last of the two major volcanic events in Songliao Basin, which also represents a turning point in the basin evolution history of Songliao from syn-rift stage to post-rift stage. Here we report high-precision U–Pb zircon geochronology by the CA-ID-TIMS technique on three tuff samples from the Yingcheng Formation of the SK-2 borehole in the Songliao Basin to construct a greatly improved, absolute age framework for the Yingcheng Formation and provide crucial age constraints for the Songliao Lower Cretaceous Strata. The new CA-ID-TIMS geochronology constrained the Yingcheng Formation at 102.571 + 0.320/−2.346 Ma to ca. 113 Ma, correlating to the Albian Stage. Combined with the previous published Songliao geochronology, the Quantou Formation is constrained to between 96.442 + 0.475/−0.086 Ma and 91.923 + 0.475/−0.086 Ma; the Denglouku Formation is constrained to between 102.571 + 0.320/−2.346 Ma and 96.442 + 0.475/−0.086 Ma; the age of the Shahezi Formation is estimated at ca. 113 Ma to ca. 118 Ma, which could extend to ca. 125 Ma in some locations in Songliao Basin. The major unconformity between the Yingcheng Formation and the Denglouku Formation, which represents the transition of the basin from syn-rift to post-rift is thus confined to between 102.571 + 0.320/−2.346 Ma and 96.442 + 0.475/−0.086 Ma. This is roughly contemporaneous with the change in the direction of the paleo-Pacific plate motion from west-southwest to north or northwest in mid-Cretaceous, suggesting their possible connections.

Pyroclastic deposition in the Cretaceous Shahezi Formation (Well SK‐2) Songliao Basin, China: Implications for tectonics and volcanism

Well SK‐2 is located in the Songliao Basin. Core samples from this well indicate that the Early Cretaceous Shahezi Formation has a length of 2,359.01 m and is in direct contact with the basin basement of the Middle Triassic, missing the Huoshiling Formation. This paper is focused on the lowest sequence 1 (S1, at the bottom of the Shahezi Formation, interval of 5,450.72–5,695.00 m, a length of 244.28 m) of the Cretaceous basin fills. It was found that the volcanic components contributed important parts in the sequence of basin fills, and the start of the rifting resulted from the combination of regional extensional tectonics and volcanism. Based on the centimetre‐scale description of the continuous core samples, we propose a model to delineate the interaction between tectonics, volcanism, and pyroclastic deposition. The study shows the tectonic‐volcanism in the earliest rifting stages and its control on sedimentary filling. These results also have common sense for the initial rifting process of a continental half‐graben basin. Sequence 1 of the Shahezi Formation in well SK‐2 shows a typical volcanic‐ sedimentary sequence. During the lowstand stage, the weathering‐depositional system was controlled by extension; during the transgressive and highstand stages, the transitional weathering‐depositional system and the mixed volcanic and weathering depositional system were controlled by transtension.

The upper and lower limits and grading evaluation of the Shahezi tight gas reservoirs in the Xujiaweizi Rift, northern Songliao Basin: Implications from microscopic pore structures

The upper and lower limits and grading evaluation scheme of tight gas reservoirs (TGRs) at a microscopic pore structure level were explored by conducting high-pressure mercury intrusion porosimetry (HMIP), rate-controlled mercury intrusion porosimetry, nuclear magnetic resonance (NMR), and wettability experiments on tight rock samples from the Lower Cretaceous Shahezi (SHZ) Formation in the Xujiaweizi Rift, northern Songliao Basin. The results showed that the upper throat radius limit of the SHZ TGRs corresponds to 1.86 μm, derived from the microscopic equilibrium state of buoyance and capillary forces. The above value is closely related to several geological factors, including a pore-to-throat radius ratio, the density difference between formation water and gas, formation dip angle, gas–water interfacial tension, and wettability. The SHZ Formation enters a tight reservoir stage when the burial depth is deeper than 3460 m. A new method integrating HMIP and NMR experiments was proposed to estimate the thickness of the adsorbed water film with a value of ∼21.56 nm. The theoretical lower throat radius limit of the SHZ TGRs was determined as ∼22 nm, considering the individual methane dimension. Natural gas can be scarcely injected into tight reservoirs below the theoretical lower limit, serving as theoretically invalid tight reservoirs in the study area. From the configuration relationship between pores and throats, two TGR types were identified in the SHZ Formation (types I and II). Type-I TGRs are characterized by relatively excellent seepage capacity, with the maximum connected throat radius and permeability of 0.4 μm–1.86 μm and (0.015–0.27) × 10−3 μm2, respectively, and their storage space mainly comprises abundant dissolution and residual intergranular pores. Furthermore, an adequate storage space (3.6% to more than 12%) and an appropriate burial depth (<5000 m) promote type-I TGRs to be the most favorable targets for current SHZ tight gas accumulation development in the Xujiaweizi Rift.

Characteristics of fluid inclusions and hydrocarbon accumulation period of Huoshiling -Yingcheng Formations in Wangfu fault depression, Songliao Basin, China

With the advancement of technology and the improvement of the level of understanding, deep hydrocarbon exploration in the Songliao basin has become one of the key objects. Wangfu fault depression is a typical oil-gas fault depression with a high degree of deep hydrocarbon exploration in the southeast of the Songliao basin. Finding out the oil-gas accumulation period of the deep hydrocarbon exploration is of great practical significance. The aim of the study is to find out the period and time of deep hydrocarbon accumulation in Wangfu fault depression, so as to provide an important theoretical basis for deep hydrocarbon exploration. The objective is to reveal the process and track of deep hydrocarbon accumulation of Huoshiling -Yingcheng Formations based on the lithofacies characteristics and the homogenization temperature distribution characteristics of fluid inclusions, combined with the burial history and thermal history simulation of strata in Wangfu fault depression. The results show that the reservoir of Huoshiling-Yingcheng Formations in Wangfu fault depression have experienced at least two periods of oil and gas filling events and two periods of oil filling events. The first hydrocarbon filling event was formed in the middle of Quantou period (about 95.4–97.5 Ma). Being influenced by the hydrocarbon expulsion from the source rocks of Huoshiling Formation, a large-scale oil and gas migration was formed. The second filling event occurred in the early Qingshankou period (about 93.6-92.0 Ma). During this period, the lake of the Songliao Basin expanded extensively, and the oil migrated and recharged through the opening fault on a large scale. The third filling event occurred in the late Qingshankou period (about 88.9–90.8 Ma), during which the source rocks of Shahezi Formation were in the peak of hydrocarbon generation and a large number of hydrocarbon migration occurred. The last large-scale filling event occurred in the early Nenjiang period (about 85.8-84.0 Ma). Affected by the rapid expansion of the lake basin in this period, the fault fractures reopened, resulting in the secondary migration and filling of oil.

Continental shale gas dynamic enrichment and evolution over geological time

Shale gas content alters with geological time, which indicates that its formation has evolved and is dynamically enriched. In this paper, a numerical model is proposed to describe the dynamic enrichment and evolution process of shale gas over geological time. The model involves the coupled process of geological conditions (temperature and pressure), source rock quality (abundance, type and maturity), and reservoir characteristics (mineral composition and pore characteristics). By comprehensively considering the gas occurrence state, pore water content and overpressure, we divide the shale gas dynamic evolution process into six stages: dissolved gas stage, adsorbed gas stage, free gas stage, overpressure formation stage, overpressure maintenance stage, and overpressure reduction stage. As a case study of continental shale gas from the Shahezi Formation in the Xujiaweizi Fault Depression, the dissolved and adsorbed gas stages are of less significance to shale gas accumulation, despite their long life. In contrast, the free gas and overpressure formation stages with short geological times are of decisive significance for the efficient accumulation of shale gas, whereas the overpressure maintenance and reduction stage are of great significance to shale gas preservation. Moreover, the gas-in-place content simulated by this method is close to the estimated value of the process analysis method, which is higher than that estimated by the United States Bureau of Mine method and lower than that estimated by the Amoco curve fitting method.

Mineralogical Evolution of the Cretaceous Strata in the Songliao Basin, Northeastern China: Implications for Thermal History and Paleoenvironmental Evolution

The Songliao Basin in northeastern China is one of the largest and longest-lived Cretaceous sedimentary basins enriched in petroleum and geothermal resources worldwide. Although the modern Songliao Basin has a high geothermal gradient, the geological thermal history of the basin has not been well constrained. The SK-2 drilling program, as the second stage of the International Continental Drilling Project of Cretaceous Songliao Basin, is for recovering extensive Early Cretaceous terrestrial strata and providing valuable materials for decoding the mineralogical evolution and the paleoenvironmental changes. Here, we present whole-rock and clay mineralogical analysis on 72 core samples covering 3346–5705 m of the Shahezi Formation in the SK-2 borehole. The whole-rock minerals mainly include clay minerals, quartz, plagioclase, as well as some calcite, K-feldspar, siderite, and pyrite. The clay mineral assemblages include illite, chlorite, and illite–smectite interlayer minerals. Above 4500 m, clay minerals are dominated by illite and illite–smectite interlayers. Below 4500 m, more plagioclase, K-feldspar, and calcite are present, while illite–smectite interlayers are completely replaced by illite. The whole-rock and clay mineralogical evolution of the Shahezi Formation is primarily controlled by thermal diagenesis, although paleoenvironmental change may act as a minor contribution. Combined with published data from the Upper Cretaceous in SK-1 cores, we infer that Cretaceous greenhouse climatic and environmental changes left fingerprints on whole-rock and clay mineralogical assemblages and that the Songliao Basin reached a maximum burial depth and a peak of thermal evolution at the end of the Cretaceous.

Research and Application of Globally Optimized Sequence Stratigraphic Seismic Interpretation Technology: Taking the Lower Cretaceous Shahezi Formation of Xujiaweizi Fault Depression as an Example

In the study of sequence stratigraphy in continental rift basins, the use of seismic data to track different levels of sequence stratigraphic boundaries laterally is the key to the division of sequence stratigraphic units at all levels and the establishment of an isochronous sequence stratigraphic framework. Traditional seismic interpretation and the establishment of a 3D sequence stratigraphic structure model are a difficult research work. This paper introduces the concept of cost function minimization and performs global stratigraphic scanning on 3D seismic data to interpret horizons and faults in a large grid. Constrained by the results, human-computer interactive intelligent interpretation, by adding iterative interpretation of geological knowledge, established a global stratigraphic model with a relative geological age. The application in the Lower Cretaceous Shahezi Formation of Xujiaweizi fault depression shows that this technology has improved the accuracy and efficiency of sequence stratigraphic interpretation, and the application of this technology has achieved the interpretation of each event horizon under the current seismic data resolution conditions. In this way, a continuous sequence stratigraphic model is established. From this stratigraphic model, any high-frequency sequence-interpreted seismic horizon can be extracted, which provides a basis for the combination of lateral resolution and longitudinal resolution of subsequent reservoir prediction.

Analysis of sedimentary facies characteristics and source reservoir allocation of fault depressed Lake Basin—Taking Xu-Xi area of Songliao Basin as an example

At present, Xu-Xi area is in the primary stage of exploration, and its sedimentary characteristics and distribution law are not clear. In this study, based on the analysis of regional structural background, through core observation and description, grain size analysis, well logging and seismic and other technical means, the sedimentary facies types and spatial distribution characteristics of Shahezi Formation in Xu-Xi area are studied. It is believed that there are three types of sedimentary systems in Shahezi Formation in the study area: Fan Delta, braided river delta and lake. The fan delta is mainly developed in the steep slope belt in the west of Xu-Xi area, while the braided river delta is mainly developed in the gentle slope belt in the East and the axial belt in the north, and the lake facies is developed in the depression belt between them. On the whole, the sedimentary pattern shows the obvious characteristics of “North-South block, east-west belt”. From the West steep slope to the East gentle slope, the sedimentary facies type transits from fan delta to deep lake semi deep lake and then to Braided River Delta. Combined with the study of reservoir forming conditions, it is found that the sand bodies of fan delta front and braided river delta front have good reservoir physical properties and are often adjacent to large-scale high-quality source rocks in the upper and lower or lateral directions. The reservoir forming conditions are superior and are favorable for oil and gas accumulation facies belt.

Quantitative Characterization of Organic Pore Structure from Gas Adsorption in Lower Cretaceous Lacustrine Shales in the Songliao Basin, NE China

Abstract Organic matter (OM) pores are widely considered to be important for gas storage and transportation. In this work, we quantitatively analyze the pore structure of OM and its controlling factors through geochemical and petrologic analyses, optical microscope, OM isolation, and adsorption isotherms. These analyses were carried out on lacustrine shale samples from the Lower Cretaceous Shahezi Formation, which is located in the Changling Fault Depression in Songliao Basin. The results show that the content of soluble OM (SOM) is low, accounting for 0.26%-3.75% of total OM. The contribution of pore development from SOM itself is limited. After extraction of SOM by chloroform, pore volume (PV), specific surface area (SSA), and average pore diameter (APD) exposed to gas molecules greatly increase. The existence of SOM has an obvious effect on pores of &amp;gt;10 nm, especially the clay mineral-related pores that contribute the most to the total PV. The content of kerogen is higher than SOM and accounts for 9.9%-65.5% (averaging 24.0%) of total PV in bulk shale, only second to minerals. More importantly, kerogen is the dominant contributor to organic PV, accounting for 95.8%-99.7% (averaging 98.3%) of the total organic PV. The pores developed in the kerogen peak at 0.4-0.7 nm and 10-30 nm. The solid bitumen (SB) followed by vitrinite and inertinite in kerogen contributes the most to the total kerogen PV.

Fine evaluation and favorable area prediction of deep source rocks in northern Songliao Basin

Focusing on Xujiaweizi fault depression, the geological conditions and geochemical characteristics of deep natural gas formation in the north of Songliao basin are evaluated, the natural gas resources are estimated, and the favorable areas are optimized. Shahezi Formation shale is a set of coal bearing sediments with high organic matter abundance (TOC is 1%～12%), high over maturity (Ro is 1%～4%) and shore shallow lake facies, which are mainly distributed in Xujiaweizi fault depression, Gulong-Lindian fault depression and Yingshan fault depression. The thickness, TOC, Ro and hydrocarbon generation of four thirdorder sequences with different lithology (mudstone and coal) are carefully evaluated for the Shahezi Formation shale in the deep layer of Songbei. The comprehensive evaluation shows that the mudstone thickness of Es4 member in Anda and Xuzhong areas of Xujiaweizi fault depression is large (150 ~ 525m), TOC is high (1% ~ 4%), thermal evolution degree is high (Ro is 1.2% ~ 3.4%), and gas generation intensity is high (20 ~ 815) × 108m3 / t), moderate buried depth (3000~4500m) and overlapping area of 756km2. It is a favorable exploration area for natural gas and shale gas in Daqing Oilfield.

New data from ICDP borehole SK2 and its constraint on the beginning of the Lower Cretaceous Shahezi Formation in the Songliao Basin, NE China

New data from ICDP borehole SK2 and its constraint on the beginning of the Lower Cretaceous Shahezi Formation in the Songliao Basin, NE China

Study on characteristics and source of tight gas in the Shahezi formation, Xujiaweizi Fault Depression, Songliao Basin, China

The Xujiaweizi fault depression is rich in natural gas resources in Lower Cretaceous and a major fault depression of the deep strata in northern Songliao Basin. In this study, the tight gas is coaliferous gas in the K1sh Formation of the Xujiaweizi Fault Depression. The methane carbon isotope value of tight gas is controlled by mature evolution. The cause of reversed sequence and partially reversed sequence of gas components carbon isotope is carbon isotope fractionation with hydrocarbon generation evolution of high-overmature stage source rock. The carbon isotope values of the low content components are obviously lighter with the increase of buried depth at the late stage of mature evolution. The carbon isotope values of adsorbed gas show similar characteristics, whether mudstone, carbargilite or coal in the same well. The tight gas reservoir is self-generation and self-accumulation gas reservoir by vertical diffusion and short distance migration in the K1sh Formation of the Xujiaweizi Fault Depression. It is important to study on the geochemical characteristics and the genesis of tight gas for guiding the exploration and deployment in the future.

Physical Simulation of Hydraulic Fracturing of Large-Sized Tight Sandstone Outcrops

Summary Hydraulic fracturing is an indispensable technology in developing tight oil and gas resources. However, the development of tight oil and gas is not consistently satisfactory. Further understanding of hydraulic fracturing of tight sandstone is required, which increases the production of tight oil and gas reservoirs, particularly in China. Currently, there are a few true triaxial hydraulic fracturing physical simulations of large tight sandstone outcrops. To weaken the boundary effect, this study performed simulations using large tight sandstone outcrops (500 × 500 × 500 mm and 500 × 500 × 800 mm) in the Shahezi Formation (Fm.), Jilin Province, China. The effect of natural fracture (NF) development degree, in-situ stress conditions, fracturing treatment parameters, and temporary plugging on fracture propagation were investigated. Furthermore, fracture propagation was investigated based on post-fracturing fine reconstruction, high-energy computed tomography (CT) scan, acoustic emission monitoring (AEM), and analysis of a fracturing pressure curve. Finally, suggestions on fracturing treatment were proposed. The results show that the NF is a key factor in determining the hydraulic fracture (HF) morphology in the tight sandstone reservoir. Further, the number, approaching angle, and cementation strength of the preexisting NF affect the HF propagation path; these are the key factors for forming complex fractures. In the tight sandstone reservoir with well-developed NFs, the fracture morphology is dominated by the NF under horizontal differential stress ≤ 9 MPa. A single fracture is more likely to occur under horizontal differential stress ≥ 12 MPa, which is less affected by the NF. In the fracturing at variable injection rates, a low rate facilitates fluid penetration into the NF, while a high rate facilitates deep HF propagation. A low-viscosity fracturing fluid at a high rate facilitates further propagation of the temporary plugging agent (TPA), thus achieving deep temporary plugging and fracture diversion. A high-viscosity fluid does not facilitate accumulation and plugging of particulate TPA. Higher horizontal differential stress leads to a smaller diversion radius of new HF, which is closer to the original HF, leading to poorer stimulation effect. The results provide a reference for the fracturing design of the tight sandstone.

Natural gas accumulation processes of tight sandstone reservoirs in deep formations of Songliao Basin, NE China

The deep formations (underlying strata of the 3rd Member of Quantou Formation, Upper Cretaceous) in Songliao Basin possess a large natural gas potential, with a proven reserve of more than 200 × 109 m3 (706.3bcf). However, due to deep burial depth, multiple tectonic evolutions, and sets of source rocks, complex gas accumulations and different gas reservoir types developed, which may seriously restrict the natural gas exploration and target selection. This study focuses on the hydrocarbon generation, porosity evolution, and hydrocarbon accumulation processes of the tight sandstone gas (TSG) reservoir in the Cretaceous Shahezi Formation. 1) The timing and cumulative amounts of hydrocarbon generation were ascertained, with the Huoshiling being 143–10 Ma and 108.00 mg/g TOC and the Shahezi being 122–44 Ma and 30.00 mg/g TOC, respectively. 2) The mean original porosity of the Shahezi sandstone was 23.11%, and experienced the porosity decreasing of compaction (avg. = 12.57%) and cementation (avg. = 7.07%), and porosity increasing of dissolution (avg. = 3.25%), and the average present porosity is 6.72%. The sandstone densification (porosity <10%) occurred in the late period of Early Cretaceous (~115–105 Ma). 3) Four main stages of hydrocarbon accumulation in the K1sh occurred: I) Early oil accumulation stage (128–114 Ma)，corresponding to the 1st episode of charging; II) Middle oil and gas accumulation stage (111–90 Ma), corresponding to the 2nd episode of charging; III) Late gas accumulation stage (77–67 Ma), corresponding to the 3rd episode of charging; and IV) Adjustment stage (63 Ma–present), corresponding to the uplift stage. The 2nd and 3rd stages were the main gas accumulation stages. 4) Two types of tight sandstone gas reservoirs developed in the K1sh: I) tight conventional gas, and II) tight deep gas. The tight deep gas reservoirs, with favorable potentials, are the main exploration targets in the Shahezi Formation of Songliao Basin.

The Controlling Effect of Fault Structure on the Development of Shahezi Formation in Xujiaweizi Fault Depression

Xujiaweizi fault depression is a dustpan-like fault depression controlled by Xuxi and Songxi faults. Several synsedimentary faults are developed in the fault depression. The different patterns of synsedimentary faults and their related structural characteristics in different periods play an important role in controlling the sequence development and sedimentary filling of the whole Shahezi Formation in the fault depression. Specifically, the lower strata are relatively flat and the sedimentation rate is slow; in the middle, the sedimentation rate increases, while in the periphery of the sedimentary center, the topography gradually sags, and the degree of sag is fast in the early stage and slow in the later stage.

A study of sequence stratigraphy of the Early Cretaceous coal-bearing series in the southeastern Songliao Basin, NE China

The Early Cretaceous is an important coal accumulation period in China. Abundant coal resources were formed in the southeastern margin of Songliao Basin. Sequence stratigraphy and coal accumulation of the Early Cretaceous in the southeastern margin of Songliao Basin have been studied on the basis of extensive outcrop and borehole data. Based on regional unconformity, basal erosional surfaces of incised valley fills, and abrupt depositional facies-reversal surface, six sequence boundaries have been identified, which subdivide the Early Cretaceous coal-bearing series into 5 third-order sequences. Sequence I corresponds to Huoshiling Formation, sequences II and III correspond to member I and member II of Shahezi Formation. Sequences IV and V correspond to member I and member II of Yingcheng Formation. Sequence I developed at the initial subsiding stage, sequence II and III developed during the stable subsiding stage, while sequence IV and V developed during the basin shrinkage stage. The major coal seams were developed in the sequence II and III, which ensured the relative balance between the increase rate of the accommodation space and the rate of peat accumulation for a longer period, thus forming a thick coal seam.

Pore structure and physical properties of sandy conglomerate reservoirs in the Xujiaweizi depression, northern Songliao Basin, China

Pore structure analysis is the basis of reservoir classification and exploitation, especially for tight reservoirs with complex pore throat systems. The sandy conglomerates in the Lower Cretaceous strata in northeastern China have attracted little attention from explorers in the past 50 years, but prospecting of promising tight gas reservoirs has recently begun in this area. Here, we report on the pore structure and physical properties of samples from the Shahezi Formation in the Xujiaweizi depression. Pore throat structure was investigated using thin section observations, scanning electron microscopy (SEM), high-pressure mercury injection (HPMI), and nuclear magnetic resonance (NMR). Regression analysis was used to analyze the correlation between the various pore structure parameters and the reservoir's physical properties. The results show that the pore types in the Shahezi sandy conglomerates are primarily dissolution pores with few inter-particle pores and micro-cracks. The pores are mostly less than 1 μm in radius, and the large pores are generally connected by tiny throats with radii of less than 0.1 μm. The Porapex and Porem values, which reveal the connected pore volume, were derived from the HPMI capillary pressure curve and the NMR T2 spectrum, respectively. The Porapex and Porem values are more strongly correlated with the reservoir's permeability than the other pore structure parameters, including the max throat radius (Rmax), the median radius (R50), the homogeneity coefficient (ϕ), the sorting parameter (Sp), the displacement pressure (Pd), and the mercury removal efficiency (Ew). Porapex and Porem account for only a small portion of the total pore volume (average of 37.1%) but contribute (average of 81.7%) to the permeability. By studying the microscopic and macroscopic characteristics, three typical pore structures were identified mainly based on Porapex and Porem. Type I reservoirs are characterized by relatively well connected inter-dissolution pores and high Porapex(>2.0), Porem(>2.0), and total porosity(>5.5%). Type III reservoirs show poor physical properties with some intercrystalline pores and small Porapex(<1.0), Porem(<1.0), and total porosity(<3.5%). Type II reservoirs are between type I and type III reservoirs. Favorable reservoirs (Type I and Type II) are located in the strata shallower than 4200 m.

New SIMS U-Pb geochronology for the Shahezi Formation from CCSD-SK-IIe borehole in the Songliao Basin, NE China

New SIMS U-Pb geochronology for the Shahezi Formation from CCSD-SK-IIe borehole in the Songliao Basin, NE China

Geochemical characteristics and genesis of tight gas in Shahezi Formation, Xujiaweizi Fault Depression, North Songliao Basin, China

Along with exploration of unconventional natural gas in North Songliao Basin, the tight gas in Shahezi Formation has become a new hotspot, followed by continual breakthroughs in terms of gas exploration. In order to understand the generation and distribution of tight gas in deep-layer of North Songliao Basin, this study focuses on the geochemical characteristics and accumulation period of tight gas, based on analysis of geochemical data such as gas compositions, carbon isotopes and homogenization temperature. According to the result of these analysis, gas in Shahezi Formation is dominated by methane with a light portion of heavy hydrocarbons and generally non-hydrocarbon, such as carbon dioxide and nitrogen. There is a widely distribution of δ13C1 which is ranges from −33.2‰ to −27.8‰, while δ13C2 is higher and have a relatively intensive distribution from −29.2‰ to −22.4‰. Carbon isotopic distribution of tight gas is characterised by a normal pattern, some partial reversal of carbon are observed. The identification of tight gas shows that the main type of tight gas in Shaheizi Formation are coal-formed gas and there are two accumulation periods including Nengjiang period and Qingshankou period. The mix of high maturity coal-formed gas in Nengjiang period and relatively low maturity coal-formed gas in Qingshankou Period is the major cause of carbon isotope reversal. In addition, it is notable that carbon dioxide in study area are inorganic origins, resulted from magma activity in Yingcheng Period.

Transition metal catalysis in natural gas generation: Evidence from nonhydrous pyrolysis experiment

Abstract Transition metal catalysis has been advocated as a critical mechanism for natural gas generation. But its presence and significance in geological settings have long been debated due to conflicting results of laboratory experiments and the lack of reliable field indicator. In this study, a low rank coal sample from the Shahezi Formation of the Songliao Basin was subjected to nonhydrous pyrolysis with and without transition metal. The chemical and isotopic compositions of gaseous products in experiments were analyzed to investigate the mechanism and potential indicator of transition metal catalysis in sedimentary basins. The chemical and carbon isotopic compositions of hydrocarbon gases generated in pyrolysis experiments revealed that transition metal could catalyze both decomposition of organic matters and methane synthesis such as Fischer-Tropsch Type reactions during maturation of source rocks under experimental conditions resembling geological settings. Catalytic methane has uniform δD values unrelated to source rock maturity, which is helpful in identification of catalytic natural gases in sedimentary basins. Transition metal catalysis is probably not a common mechanism for natural gas generation in sedimentary basins due to the positive correlation between δDCH4 of natural gases and maturities of source rocks observed in most natural cases. However, transition metal catalysis could well interpret the restricted range of δDCH4 and carbon isotopic reversal phenomenon of natural gases in the Yingcheng Formation of the Xujiaweizi fault-depression whose origin has long been debated. So we inferred that the transition metal catalysis may have occurred in deep strata of the Xujiaweizi fault-depression and played a significant role in natural gas generation. Our study revealed that restricted range of δDCH4 of natural gases with different maturities may serve as an effective indicator of transition metal catalysis in sedimentary basins. It provides a new perspective to investigate the significance and universality of transition metal catalysis in different geological settings.