石油实验地质

2022, 44(5)

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Abstract:

Evaluation system and exploration optimization for fault-lithologic reservoir on the western slope of Xihu Sag, East China Sea Shelf Basin

ZHOU Liqing, JIANG Donghui, ZHOU Xinghai, LI Kun, ZHUANG Jianjian, LIU Chuang

2022, 44(5): 747-754. doi: 10.11781/sysydz202205747

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With continued progress of exploration degree on the western slope of Xihu Sag of East China Sea Shelf Basin, exploration strategy has gradually changed from structural reservoirs to structural-lithological composite ones. Through the systematic evaluation of the hydrocarbon-generating potential of coal-measure source rocks, it was concluded that coal seams and carbonaceous mudstone are the main source rocks in this area. According to the study of sedimentary system based on the "Source-to-Sink" system, 8 types of structural-lithological composite trap were proposed. Furthermore, the accumulation conditions and distribution characteristics of oil and gas were studied, it is then confirmed that effectiveness of trap is the main controlling factor for oil-and-gas accumulation in the upper and middle members of Pinghu Formation, while the hydrocarbon-supplying capacity and physical properties of reservoir are the main controlling factor in the lower member of Pinghu Formation. Consequently, a comprehensive evaluation system was established. By the means of the comprehensive evaluation of structural-lithological composite reservoirs, the Eocene-Paleocene delta in Yingcuixuan area of the northern segment of the western slope belt, the Eocene tidal flat-delta in Pingnan, Chuyang and Baoshi areas, and the inner zone of Pinghu slope are large-scale reserve areas, which is valuable and favorable for further exploration.

Identification of lithologic trap in coal-bearing stratum of Pingbei area, Xihu Sag, East China Sea Shelf Basin

NIU Huawei, LIU Miao, JIANG Cen, ZHU Lixin, CHEN Maogen

2022, 44(5): 755-760. doi: 10.11781/sysydz202205755

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In the Pingbei area of Xihu Sag, East China Sea Shelf Basin, the sedimentation of the main target Eocene Pinghu Formation, is significantly affected by tidal action. Thin sand bodies are stacked on top of each other, the lateral changes are fast, and coal seams are developed. The target layer has a depth of bury generally deeper than 3 500 m, and the physical properties of reservoir are relatively poor. The existing seismic data have low signal-to-noise ratio, weak energy, small impedance difference of sand and shale in the deep, and it is difficult for the reservoir prediction and description. Based on deep amplitude-preserved and broadband processing, this paper studied the type of lithologic trap development, characterization of sand reservoir, and implementation of lithologic trap targets. The paleogeomorphic background, sand development pattern and the type of litho-logic trap are defined. Under the constraints of this framework, accurate sandstone description is carried out. The sand reservoir is predicted by comprehensive analysis of seismic waveform, seismic reflection characteristics and the elastic properties of pre-stack inversion, the coal seam distribution is predicted by post-stack statistical inversion, and the accuracy and reliability of sand description are improved.

Geophysical identification of river-tide controlled deltaic sedimentation and its implication for petroleum geology: a case study of Pingbei area, Xihu Sag, East China Sea Shelf Basin

YANG Caihong, ZHOU Xinghai, JIN Can, LI Kun, ZHOU Feng

2022, 44(5): 761-770. doi: 10.11781/sysydz202205761

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Understanding of sedimentary environment is the basis of oil exploration and development. Previous research on the characteristics and distinguishing of single sedimentary system by using logging curves and seismic data was mostly carried out on the determined sedimentary system. However, for the river-tide controlled cases, when the main bodies of fluvial and tidal actions are unclear, sedimentary systems are rarely studied. Based on the difference of the oscillation frequency of tide and river water bodies, combined with the correlation between the logging curves of gamma ray and the mud content as well as the influence on seismic facies by hydrodynamic strength and the difference of the size and scale of river-tide controlled delta, a geophysical method on the basis of natural gamma log difference (ΔGR) and seismic facies wave length/height ratio is proposed to distinguish river-tide controlled delta sedimentary systems. A case study was carried out with the sedimentary system of the Pinghu Formation in Pingbei area of the Xihu Sag, East China Sea Shelf Basin, and it was concluded that this method can effectively distinguish river-tide controlled delta system. At the side edge of the underwater low uplift, Baoyunting low uplift, relevant parameters have been effectively picked up to distinguish the river-tide controlled delta system, and to identify the tidal channel development zone and potential targets for channel filling. It is clear that the identification of river-tide controlled system has an indicative significance for the prediction of lithologic traps and favorable reservoir units.

Distribution of sedimentary system multi-controlled by palaeo-geomorphology, water system and break during the deposition of Pinghu Formation, Baochu slope belt, Xihu Sag, East China Sea Shelf Basin

JIANG Donghui, DU Xuebin, LI Kun, ZHOU Feng

2022, 44(5): 771-779. doi: 10.11781/sysydz202205771

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The Xihu Sag has huge exploration potential and is the main target for oil and gas exploration in the East China Sea Shelf Basin. At present, it has been known that the Xihu Sag has a transitional facies with sedimentary system mainly composed of tidal flat deposits, and the provenance of which is dominated by western and northern uplift area. However, the transport process of sands from source area to deposit area, namely "source-channel-sink" system, has not been well studied. In this study, we take the Pinghu Formation of Baochu slope belt in the Xihu Sag as research object, and use borehole and seismic data to preform high-precision restoration of palaeo-geomorphology. Moreover, on the basis of palaeo-geomorphology restoration, we make qualitative and quantitative analyses of palaeo-water-system and palaeo-break. Results show that the location of sink zone is controlled by palaeo-geomorphology, the distribution of sand transporting channel is controlled by palaeo-water-system, and the development of sedimentary facies is controlled by palaeo-break. Through comprehensive analysis, we suppose that the combination of "palaeo-geomorphology, palaeo-water-system and palaeo-break" restricts the process of sand body deposition from provenance to convergence area through river transportation, and controls the distribution of the sedimentary system of Baochu slope belt.

Distributional signatures of depositional system of Pinghu Formation, Pinghu slope, Xihu Sag, East China Sea Shelf Basin

CHEN Zhongyun, DU Xuebin, LI Shuai

2022, 44(5): 780-789. doi: 10.11781/sysydz202205780

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The Eocene Pinghu Formation on the Pinghu slope of Xihu Sag of East China Sea Shelf Basin is a main confirmed strata for hydrocarbon enrichment. At present, many studies have been carried out on its sedimentary system, but certain controversies are still existed. On the basis of previous studies and newly processed 3D seismic, core and geochemical data, it was re-analyzed in this paper for the sedimentary system of Pinghu Formation. Results show that there are three types of sedimentary system on the slope belt: river-controlled delta, tidal-transformed delta and tidal flat depositional system. These three types of sedimentary modes have obvious zoning characteristics in plane. The northern area is dominated by river-controlled delta deposits. The middle area is dominated by tidal-transformed delta deposits. The southern area is in the mouth of a semi-closed bay, dominated by tidal flat system, and ocean currents control the spatial distribution shape and scale of sand bodies. It is concluded that the influence of seawater on the sedimentary pattern of the Pinghu Formation in the slope belt is strengthening from north to south.

Physical simulation of dynamic accumulation of fault-controlled gas reservoirs and its implications: a case study of typical gas reservoirs in northwestern part of Qaidam Basin

LUO Qun, WANG Shichen, JIA Chun, DAI Bing, ZHANG Hongli, WEN Fan, QIU Zhaoxuan

2022, 44(5): 790-803. doi: 10.11781/sysydz202205790

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The dynamic physical simulation of natural gas accumulation process is one of the important means to reveal the mechanism of natural gas migration and accumulation and to summarize its distribution law. However, due to the characteristics of natural gas itself including "easy to leak, difficult to move and difficult to observe" have become holdbacks for the physical simulation of natural gas migration and accumulation. In order to reveal the migration and accumulation mechanisms of fault-controlled gas reservoirs, and to summarize the formation sequence and distribution pattern of gas reservoirs, typical reservoirs in Dongping, Mahai and Nanbaxian areas in the northwestern part of Tarim Basin were taken as examples to establish a geological evolution model for each reservoir. Aiming at the problems of gas reservoir simulation, an adjustable simulation device for gas migration and accumulation was designed, which could realize the dynamic process of structural change and clarify the phenomenon of gas charging and migration, and the formation process of typical gas reservoirs such as Dongping, and Mahai-Nanbaxian, etc. was successfully simulated. It is clear that the fault not only acts as a channel for gas migration and power transmission, but also controls the evolution sequence and distribution pattern. The formation and evolution sequence models of two different types of natural gas reservoirs, named "late accumulation" and "long-term accumulation", were proposed, the mechanism of natural gas migration and accumulation driven by "fault transmission and high pressure" was revealed, and the vertical preservation sequence featured by "co-existence of natural gas in both deep and shallow formations, and the deeper formations are more favorable for preservation" was concluded. A new concept of natural gas exploration was put forward that "if there are shallow gas reservoirs, there must be deep ones", and we can find natural gas from shallow to deep formations.

Significance and formation mechanism of abnormally pressured compartments of shale gas in the Ordovician Wufeng-Silurian Longmaxi formations, southeastern Sichuan Basin

LIU Weixin, LU Longfei, YE Deliao, HE Chencheng, LIU Wangwei, YU Lingjie, ZHANG Wentao, SHEN Baojian

2022, 44(5): 804-814. doi: 10.11781/sysydz202205804

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Overpressure is commonly existed in shale gas pools of Jiaoshiba and Dingshan structures in the southeastern part of Sichuan and Huangjinba structure in Changning area of the southern region of Sichuan. From the Upper Ordovician Wufeng Formation to the Lower Silurian Longmaxi Formation, there are two abnormally pressured compartments of shale gas separated by three sealing layers. The lower compartment is in accordance with the superior shale layer in the Wufeng-Longmaxi formations, sealed by tight marl-bearing limestone and nodular limestone in the Upper Ordovician Linxiang and Baota formations at the bottom and the interbeds of volcanic ash-altered clay rock, thin carbonate rock and mudstone in the central segment of the first sub-member of the first member of Longmaxi Formation on the top, which also works as the sealing layer at the bottom of the upper compartment. Between the first and second members of the Longmaxi Formation, sealing layer for the top of the upper compartment exitsed, which is mainly composed of mudstone or mudstone interbedded with thin carbonate rocks.Researches show that the sealing layers have better sealing ability when they contain tight carbonate rock or thin carbonate rock and are interbedded with mudstone. There are differences in logging curves such as acoustic wave, resistivity, natural gamma, etc. between the top and bottom sealing layers and the compartments. In the upper compartment, the microstructure of shale shows strong anisotropy with low quartz content, high clay content, low organic matter content, poor organic pore connectivity, low total hydrocarbon content, and low porosity. In the lower compartment, the microstructure of shale shows isotropy, with high quartz content, low clay content, high organic matter content, interconnected organic matter, interconnected organic pores, good connectivity, high total hydrocarbon content, and high porosity. These compartments have experienced the early stage of rapid burial, the embryonic stage in which organic matter is immature or low-mature and is dominated by normal fluid pressure system, the formation stage in which a large quantity of hydrocarbon is generated after fast and deep burial and is dominated by overpressure system, the formation stage in which a large quantity of gas is generated after deep burial and high-degree thermal evolution and is dominated by overpressure system, and finally the late stage of the formation of abnormally pressured compartments for shale gas, resulting in two isolated and abnormally pressured compartments for shale gas with three sealing layers on the top or at the bottom, lateral dipping of formation, and lateral sealing of reverse fault. The lower compartment with high porosity, high organic matter content, good connectivity and high pressure coefficient is the priority target for the efficient development of shale gas.

Shale gas content evaluation for deep strata and its variation: a case study of Weirong, Yongchuan gas fields in Sichuan Basin

SU Haikun, NIE Haikuan, GUO Shaobin, YANG Zhenheng, LI Donghui, SUN Chuanxiang, LU Ting, LIU Mi

2022, 44(5): 815-824. doi: 10.11781/sysydz202205815

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The exploration and development of shale gas in China are recently focusing on the deep layers. However, the occurring characteristics of shale gas under high temperature and high pressure are not clear, which imposes great restrictions to deep shale gas development on a massive scale. Taking the deep shale in Weirong and Yongchuan regions of Sichuan Basin as research objects, for core samples with different organic carbon content and porosity, the isothermal adsorption experiments and porosity testing at high temperature and high pressure (135 ℃, 80 MPa) were carried out, and the theoretical values of shale adsorption gas, free gas and total gas content and compared with actual values. Results show that: (1) The content of adsorbed gas in deep shale increases gradually with pressure growing up. When the pressure surpasses 40 MPa, the increment flattens out, making 4.46 cm³/g the maximum of adsorbed gas content. (2) The theoretical gas content of shale increases with the growing formation pressure. The total gas content reaches its maximum 11.3 cm³/g under 80 MPa, which is made up with calculated free gas with an average of 6.8 cm³/g and adsorbed gas with an average of 4.5 cm³/g, accounting for about 60% and 40% of the total gas content, [JP]respectively. The ratio of free gas versus adsorbed gas gets larger with a greater buried depth. (3) Based on field desorption experiment, the actual measurement maximum gas content of well Weiye 11-1 is 5.95cm³/g, the minimum value is 3.29cm³/g, and the average content is 4.52 cm³/g. Compared with the theoretical value of 10.3 cm³/g, the result indicates that nearly 50% of gas leaked in the process of formation uplift, displaying the complexity of the deep shale gas preservation conditions. It is recommended to strengthen research on preservation conditions.

Influence of source-reservoir contact conditions on the enrichment of near-source tight oil: taking Chang 8₁ reservoir in the Longdong area of Ordos Basin as an example

XIAO Zhenglu, LI Yong, ZHU Zhiyong, LU Jungang, LU Zixing, LI Chengshan

2022, 44(5): 825-834. doi: 10.11781/sysydz202205825

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The geological characteristics of tight oil have been relatively well understood, but there are certain questions needed to be answered regarding the distribution of tight oil. With comprehensive analysis of core, well logging and physical properties, and by means of mathematical calculation, the effects of the source-reservoir contact conditions on the enrichment of near-source tight reservoir of the first section of the eighth member of Yanchang Formation (Chang 8₁) in the Longdong area of Ordos Basin were studied in this paper. Results show that the source-reservoir contact conditions can be classified as four types including direct contact, transitional contact, fracture connected, and mudstone barrier. Among them, the direct contact and fracture connected types are favorable for the migration and accumulation of tight oil, while the transitional contact and mudstone barrier types have an obvious barrier effect for the migration of tight oil. The oil wells in the Chang 8₁ member in the Longdong area are mainly located in regions with a thickness of mudstone barrier less than 4 m, while the water or dry wells are mainly located in regions with a mudstone barrier thickness greater than 4 m. Based on the above calculation, it can be concluded that the source-reservoir contact condition determines the enrichment and vertical migration distance of tight oil. For the exploration of near-source tight oil, not only the "sweet zone" of the reservoir is conducive, the type of the source-reservoir contact condition should also be considered as a key factor.

Origin of porphyry dolomite in the 5th sub-member of the 5th member of Ordovician Majiagou Formation (M₅⁵sub-member), Ordos Basin

LIAO Huihong, SU Zhongtang, HUANG Wenming, MA Hui, SHE Wei, LUO Zhenfeng, LIU Guoqing, HU Sunlong

2022, 44(5): 835-844. doi: 10.11781/sysydz202205835

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Large number of heterogeneous porphyry dolomites have been found in the 5th sub-member of the 5th member of Ordovician Majiagou Formation (M₅⁵ sub-member) in the central and eastern parts of Ordos Basin, and many drilled wells have shown good oil and gas potential, which has become the focus for oil and gas exploration, but the genetic mechanism is still undetermined. Based on petrological characteristics and geochemical analysis, the origin of porphyry dolomite in this area is studied. Results show that porphyry dolomite can be classified into three types: unevenly, obviously and slightly porphyry dolomite. According to the morphology characteristics of cloud spots, the structure of cloud spots is supposed to be caused by biological disturbance, and the cathodoluminescence of spots and matrix is the same. Porphyry dolomite has relatively higher contents of Fe, Mn and Na and lower content of Sr, but there is no significant difference between the contents of Fe, Mn and Sr in spots and matrix. The δ¹³C values of porphyry dolomites are highly comparable to those of seawater at the same time, and the δ¹⁸O values are all negative with no significant difference among the three types of dolomites. The ratio of ⁸⁷Sr/⁸⁶Sr is higher than that of seawater in the same period. Petrological and geochemical characteristics show that biodisturbance causes the heterogeneity of rocks, and the degree of disturbance affects porphyry rate. The biodisturbed limestone of porphyry dolomite series is formed by the dolomitization of the osmotic reflux of high salinity seawater. The dolomite underwent recrystallization after re-burial.

Differential development and maintenance mechanism of reservoir space for marine shale gas in South China's deep strata

YANG Zhenheng, TAO Guoliang, BAO Yunjie, LU Longfei, SUN Yongge, LIU Weixin, SHEN Baojian, NIE Haikuan

2022, 44(5): 845-853. doi: 10.11781/sysydz202205845

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The deep marine strata with burial depth greater than 3 500 m in South China have good geological conditions and huge potential for shale gas resource. However, due to the burial depth and relative poor understanding, the productive difference between horizontal wells in different positions is obvious, and exploration and development are still challenging. On the basis of investigating the current situation and development trends of global deep shale gas, this paper focuses on the differential development of reservoir of marine deep shale gas in South China, the pore preservation by pressure resistance of rigid minerals such as quartz, and the pore preservation of reservoir fluid overpressure. The differential development of reservoir space and effectiveness is one of the main factors for the controlling of gas content, productivity and EUR per well of deep shale gas of the Wufeng-Longmaxi formations in the southeastern part of Sichuan Basin, but the characteristics of differential development and maintenance mechanism is unclear, and the differentiated response process under in-situ conditions of near-deep reservoirs and the mechanism still need to be studied. Although it is recognized that rigid minerals such as quartz play an important role for the pressure resistance and pore preservation, its varied effect on different types of storage spaces is still unclear. It has been shown that the effect of reservoir fluid overpressure is of great significance to the preservation of deep shale gas storage space, it is still limited understanding of how reservoir fluid overpressure affects deep shale reservoir space. The key factor to answer the problems above is to study the characteristics and mechanism of differential development, response and maintenance of different types of reservoir space in a typical organic-rich shale in a near-deep in-situ state with multiple stress loading and high temperature.

Molecular characteristics of source rocks in Upper Triassic Xujiahe Formation, Western Sichuan Depression, Sichuan Basin

WU Xiaoqi, ZHOU Xiaojin, CHEN Yingbin, WANG Ping, WANG Yanqing, YANG Jun, ZENG Huasheng

2022, 44(5): 854-865. doi: 10.11781/sysydz202205854

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The Upper Triassic Xujiahe Formation (T₃x) is the most important terrigenous stratum in the Western Sichuan Depression; however, it is still insufficient for the understanding of molecular compositional characteristics of its source rocks. Based on the analysis of both saturated and aromatic fractions of extracts of source rocks from different members of the Xujiahe Formation, the indication is then discussed for sedimentary environment and thermal evolution of source rocks. The distribution of tricyclic terpanes (TTs) and dibenzothiophene/phenanthrene (DBT/P) ratio of the argillaceous source rocks in 2nd to 4th members of the Xujiahe Formation mainly display the signatures of typical lacustrine source rocks, and the relatively high gammacerane/C₃₀ hopane (0.16-0.23), as well as 9-/1-methylphenanthrene (1.16-1.41) ratios suggest relatively high salinity of water body. The parameters such as methylphenanthrene index (MPI₁), methylphenanthrene distribution fraction (F₁), and methyldibenzothiophene distribution index (MDBI) are effective for the evaluation of thermal maturity, and they show positively and negatively linear correlation with maturity degree before and after R_o=1.35%. The distribution pattern of C₂₇-C₂₉ regular steranes, relative composition of three fluorine series, and methyldibenzothiophene/methyldibenzofuran ratios for high to over-maturity T₃x source rocks are distorted due to the effect of thermal maturation.

A comparison of geochemical features of source rocks of Eocene Wenchang Formation in the deep and shallow water zones of Pearl River Mouth Basin, SE China

YUAN Liping, JIANG Wenmin, LI Yun, ZHANG Lin, WANG Jianfeng, WANG Wei, XIONG Yongqiang

2022, 44(5): 866-876. doi: 10.11781/sysydz202205866

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The Pearl River Mouth Basin (PRMB) is one of the important offshore targets for oil and gas exploration in China and has great potential of oil and gas resources. The Eocene Wenchang Formation is one of the potential source rocks in the PRMB, but its geochemical characteristics are insufficiently studied, particularly for the Wenchang Formation source rocks in deep-water area of the PRMB. Due to the deep burial condition, core samples can rarely be obtained, and most samples are drilling cuttings which are easily to be contaminated by drilling mud, which makes it difficult to directly access the geochemical characteristics of Wenchang Formation source rocks. This paper aims to compare the geochemistry of the Wenchang Formation source rocks in the deep-water and shallow-water areas of the PRMB with an improved catalytic hydropyrolysis method to release the bound hydrocarbons from kerogen macromolecules. Based on the biomarkers and carbon isotopic compositions of individual n-alkanes released, the geochemical characteristics of Wenchang Formation source rocks in deep-water (mainly from Zhu Ⅱ Depression) and shallow water areas (mainly from Zhu Ⅰ Depression) from PRMB are compared. Results indicate that the source rocks of Wenchang Formation in PRMB can be classified into three types including semi-deep to deep lacustrine (WC-Ⅰ), shallow lacustrine (WC-Ⅱ), and semi-deep to deep lacustrine with algal blooming (WC-Ⅲ). The WC-Ⅰ are characterized by low C₃₀ 4-methylsteranes (4-Me/C₂₉ < 0.2) and bicadinanes contents, and relatively light carbon isotopic composition of individual n-alkanes (-33‰ to -31‰). The WC-Ⅱ are characterized by low 4-Me/C₂₉ ratios (average=0.06), low to high T/C₃₀H ratios (average=1.04), and relatively heavier carbon isotope composition of individual n-alkanes (-30‰ to -27‰). The WC-Ⅲ are characterized by high content of C₃₀ 4-methylsteranes (4-Me/C₂₉=0.66) and heavy carbon isotope compositions of individual n-alkanes (-25‰ to -23‰). The Wenchang Formation source rocks in deep-water and shallow-water areas have semi-deep to deep lacustrine facies (WC-Ⅰ) and shallow lacustrine facies (WC-Ⅱ). In addition, the semi-deep to deep lacustrine source rocks with algal blooming (WC-Ⅲ) also developed in shallow-water areas, but no such type of source rocks were found in deep-water area. This study compared the geochemical characteristics of bound hydrocarbons in source rocks of Wenchang Formation in different regions of PRMB, and found that the semi-deep to deep lacustrine source rocks of Wenchang Formation in shallow-water area can be divided into two types. It provides a scientific basis for a better understanding of the geochemical features of source rocks in different areas of Wenchang Formation in PRMB, and is helpful for oil-source correlations in this area.

Re-Os chronology and REE analysis of paleo-oil reservoir in high evolution areas: a case study of Banjie paleo-oil reservoir, Nanpanjiang Basin

TAN Xiaolin, HU Yuzhao, ZHOU Liang, CHENG Yong, LI Piyou

2022, 44(5): 877-886. doi: 10.11781/sysydz202205877

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The traditional organic index can be greatly affected in the areas with high thermal evolution degree whilst combination of REE for oil-source correlation will improve the results. In addition, Re-Os dating is a quantitative method of geochronology, but the significance of the test results is quite controversial. In this paper, REE geochemical analysis and Re-Os dating of bitumen in the Banjie paleo-oil reservoir is carried out. The REE distribution pattern of bitumen was compared with that of the main source rocks in the area using the method of grey correlation. Results show that the ΣREE of bitumen in the Banjie paleo-oil reservoir is low, and the fractionation of light and heavy REEs is obvious, showing a weak negative Ce anomaly and an obvious negative Eu anomaly, which indicates that the bitumen was formed in a reducing environment and has not experienced any strong weathering or leaching. The REE distribution mode belongs to the right-leaning mode of LREE enrichment. The REE distribution pattern curve is similar to that of Devonian source rocks in this area, indicating a genetic relationship. The results of Re-Os dating show that the Re-Os model age of bitumen in the Banjie paleo-oil reservoir ranges mainly from (203.6±1.1) Ma to (238.5±4.2) Ma, with an average of (224.8±3.3) Ma (n=7). The last factor which disturbed the Re-Os system of the bitumen is the process of bitumen generation. The model age represents the age of bitumen formation in the Banjie paleo-oil reservoir, when oil was cracked into gas.

Maturity indication of 17α(H)-diahopane in expelled and retained oils from artificial maturation experiments of mud shale

LI Shanshan, BAI Bin, YAN Gang, XU Yaohui, LIU Yan

2022, 44(5): 887-895. doi: 10.11781/sysydz202205887

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With the comparable sedimentary organic phase, the relative abundance of rearranged hopanes can be correlated to maturity degree. By the approach of artificial maturation experiments of low-maturity (R_o=0.58%) and organic-rich (TOC=3.87%) mud shale collected from the Ordos Basin, 17α(H)-diahopanes in expelled and retained oils were examined by gas chromatography-mass spectrometry (GC-MS). The thermal evolution characteristics of parameters related to 17α(H)-diahopane were discussed, and the maturity range of these parameters as maturity indicators was proposed, combined with the random mean vitrinite reflectance (R_o) of maturated rock samples of each step of pyrolysis. Results show that the parameters of C₂₉*/(C₂₉*+C₂₉H) and C₃₀*/(C₃₀*+C₃₀H) have similar three-stage variations. As experimental temperature increased, the values of the two parameters decreased firstly, then increased and finally decreased again, indicating that they may be good indicators for oil-source correlation. Before 325 ℃, i.e. R_o < 1.01%, both parameters showed insignificant changes. However, they both showed an obvious upward trend with increasing temperature from 325 to 385 ℃, indicating that they were valid indicators for maturity from the moderate to the early stages of high maturity (i.e. 1.01% < R_o < 1.48%).

Molecular microbial prospection for oil and gas and its preliminary application

XU Kewei, ZHENG Xuying, GU Lei, GUO Jiaqi, TANG Yuping

2022, 44(5): 896-903. doi: 10.11781/sysydz202205896

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Due to the advantages such as low multi-solution, high signal-to-noise ratio, low environmental impact, high efficiency and low cost, microbial prospection for oil and gas has been accepted for petroleum exploreration. With the rapid development of high-throughput sequencing, molecular biology and bioinformatics, describing the changes of specific oil and gas indicator micro-organisms from the perspective of microbial community as a whole has become an valuable progress of microbial exploration method for the future. In this paper, soil sampling, DNA extraction and analysis, discriminant model development and favorable area prediction are introduced, and soil DNA extraction as well as oil/gas indicator micro-organism detection are emphasized. With the study of industrial-grade DNA extraction technology, Griffth method has been considered as the best effect for the extraction and purification of soil microbial DNA. For projects with a large sample number, high-throughput pipetting workstations and commercial multi-well plate DNA extraction kits can greatly improve working efficiency. Integrated stable isotope probes and the molecular detection of high-throughput sequencing showed that the main hydrocarbon indicator microorganism strains typically imply the expected distribution. High frequency oil indicator bacteria (AMNR) are Arthrobacter, Mycobacterium, Nocardia, and Rhodospirillum, while high frequency gas indicator bacteria include Methylococcus, Methylobacterium, and Methylocystis. The preliminary application in the Hangjinqi area shows that the microbial molecular exploration technology can significantly improve the oil and gas identification accuracy in loess tableland regions, and has a good effect on the evaluation of oil and gas bearing property, which provides a reference for the selection of favorable tight gas regions in the northern part of Ordos Basin.

Evaluation method for fault safety and its application based on ANSYS: a case study of Baiju aquifer gas storage in Dongtai Depression, Subei Basin

LEI Ming, WANG Dandan, QIU Xiaosong, LU Shiwei, DENG Qingjie, LI Pan, MIN Wenmao, LIU Ningwei

2022, 44(5): 904-913. doi: 10.11781/sysydz202205904

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Aquifer gas storage uses porous media reservoir to store natural gas. The location of aquifer gas storage has a wide geographical range and does not depend on large salt mines or depleted oil and gas reservoirs, which is of great significance to China's natural gas strategy. Aquifer gas storage realizes periodic injection-production through natural fault blocking natural gas and cementing technology, so as to achieve the purpose of adjusting seasonal peak value. Affected by the change of injection-production pressure, faults are easy to be opened or broken during operation, leading to the leakage of natural gas stored. Therefore, it is very important to evaluate the fault safety of aquifer gas storage, but the common fault evaluation and numerical simulation methods do not involve the porous media reservoir, and the study of its expansion mechanism is insufficient, which does not satisfy the fault safety evaluation of water-bearing structure. Based on the basic characteristics of aquifer gas storage, this paper summarizes a set of complete fault safety evaluation methods by ANSYS finite element simulation and static mechanics analysis. A case study was carried out in Baiju aquifer gas storage in the Dongtai Depression of the Subei Basin. A 3D entity model of formation and fault was established. According to the initial stress equilibrium analysis and the axial compression experiments of fault rock samples, the validity of the model can be demonstrated. With displacement constraint method, the stress state of target faults at different operating pressure was simulated. The maximum value of predicted operating pressure is 29.50 MPa, and tensile failure may occur above this value.

Function of aromatic compounds as indicators in laboratory experiments of heavy oil with fire flooding

YAN Hongxing, YANG Junyin, JIANG Wenrui, ZHANG Yanfang, LI Peixin

2022, 44(5): 914-921. doi: 10.11781/sysydz202205914

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Abstract:
Identifying whether high-temperature oxidation is realized in fire flooding has become one of the technical difficulties in evaluating the effect of fire flooding for heavy oil reservoirs. In order to study how the chemical properties of crude oil varied during fire flooding, a series of heavy oil fire flooding laboratory experiments were carried out using a three-dimensional physical model, and the gas chromatography-mass spectrometry of aromatics were performed for the crude oil before and after high-temperature oxidation during fire flooding. Results show that after fire flooding, the relative contents of naphthalenes, phenanthrenes and polycyclic aromatic hydrocarbons in the crude oil sample increased, while the relative content of tri-aromatic steroids decreased. During fire flooding, naphthalenes and phenanthrenes were all prone to demethylation, methyl migration and methyl substitution, showing significantly higher thermal stabilities in β-naphthalenes and β-phenanthrenes relative to α-naphthalenes and α-phenanthrenes. Due to the difference in thermal stability between 4-methyldibenzothiophene and 1-methyldibenzothiophene, the high-temperature oxidation in fire flooding could be identified according to the changes in their relative contents and the distribution characteristics of their spectra. Anthracene in polycyclic aromatic hydrocarbons could be used as a marker of high-temperature oxidation in fire flooding, while the changes in perylene/benzo(e)pyrene, fluoranthene/pyrene and anthracene/phenanthrene ratios were also good indicators for high-temperature oxidation in fire flooding. As observed from these findings, it can thus be concluded that the variation characteristics and characteristic markers of aromatic compounds in crude oil are good indicators for combustion state during fire flooding, providing favorable support for the identification of combustion state in heavy oil fire flooding.

Quantitative assessment for the formation fractures in source rock strata and its application

HAO Muge, ZHANG Jingong, GAO Yi, LI Jian, MA Shilei

2022, 44(5): 922-929. doi: 10.11781/sysydz202205922

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Abstract:
Quantitative evaluation of formation fractures and research on the promotion of fractured rocks on formation transport capacity are of certain importance to the exploration of unconventional oil and gas reservoirs. By comparing the rock fracturing pressure under the same confining pressure condition with the confining pressure range of the source rock series, the rock fracturing situation of the source rock series under the action of static rock pressure in the Bonan-Sikou area of Jiyang Depression, Bohai Bay Basin was analyzed. The effect of source rock fractures to the transporting capacity was quantitatively measured. On this basis, a method of comprehensive comparison of confining pressure, fracture pressure and post-fracture conductivity was proposed to study formation fracture. In the upper and middle sections of the third member of Paleogene Shahejie Formation in Bonan-Sikou area, the sandstones and shale rocks in the deeper strata are generally ruptured, and the sandstone layers in the middle strata are regionally ruptured. In the lower section of the third member of Shahejie Formation, the middle and deep strata are generally ruptured. In the uppersection of the third member of Shahejie Formation, the middle and deep strata are generally ruptured, whilst only part of sandstone layer in the upper part is ruptured. The fractured formation has greatly increased permeability and can be used as an effective conduction channel. The methods described above provide a powerful complement to existing "sweet spot" evaluations.

NMR technology in reservoir evaluation for shale oil and gas

SUN Zhongliang, LI Zhiming, SHEN Baojian, ZHU Qingmin, LI Chuxiong

2022, 44(5): 930-940. doi: 10.11781/sysydz202205930

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Abstract:
Since the development of unconventional oil and gas business, Nuclear Magnetic Resonance (NMR) technology has been gradually applied in the evaluation for unconventional reservoirs due to the merits such as nondestructive, sensitive and fast, this technology has become one of the important methods in shale oil and gas reservoir evaluation. Therefore, based on the experimental principle of NMR technology, this paper focuses on the applications of NMR technology in the full-scale integrated characterization of pore and fracture distribution, characterization of shale porosity, pore wettability, fluid mobility and fluid classification, etc. In addition, the applications of NMR in describing water migration, methane adsorption and desorption, carbon dioxide displacement and other fluid behaviors, obtaining organic matter information, oil shale interface area, determining organic pores and inorganic pores, analyzing pore connectivity, and obtaining information about high-viscosity asphalt and kerogen are also briefly reviewed. Finally, the shortcomings of NMR and the development trend of NMR in shale reservoir evaluation are analyzed.

2022, 44(5): 941-941.

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Abstract:

2022 Vol. 44, No. 5

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