浅层地震技术在济南地下空间探测中的应用

浅层地震作为城市地下空间探测中必不可少的一种手段,其勘探效果受到很多方面的影响,如激发、接收因素、采集参数以及处理方法等,因此需要根据地质任务进行系统科学的论证、选取。笔者以济南地下空间探测项目为例,探讨提高浅层地震勘探分辨率的方法,通过综合分析研究区的资料建立合理的地震地质模型,基于该模型进行地震采集参数波动方程正演分析与照明分析,获得高密度二维地震观测方案;通过对叠前去噪、反褶积和速度场优化等针对性数据处理技术的研究,获取高分辨率的反射成像。项目取得了很好的勘探效果,为济南智慧城市建设提供了可靠的物探资料参考,对今后该地区浅层地震勘探工作的开展有一定的借鉴作用。

Petrogenesis of the Late Triassic shoshonitic Shadegai pluton from the northern North China Craton: Implications for crust-mantle interaction and post-collisional extension

Latest Permian to Triassic plutons are widespread in the northern North China Craton(NCC); most of them show calc-alkaline, high-K calc-alkaline, or alkaline geochemical features. The Shadegai pluton in the Wulashan area has shoshonitic affinity and I-type character, and is composed of syenogranites containing abundant mafic microgranular enclaves(MMEs). LA-MC-ICP-MS U-Pb data yield weighted mean 206 Pb/238 U ages of 222 ± 1 Ma and 221 ± 1 Ma for the syenogranites and MMEs, respectively, suggesting their coeval formation during the Late Triassic. The syenogranites have high SiO_2(70.42-72.30 wt%),K_2O(4.58-5.22 wt.%) and Na_2O(4.19-4.43 wt.%) contents but lower concentrations of P_2O_5(0.073-0.096 wt.%) and TiO_2(0.27-0.37 wt.%), and are categorized as I-type granites, rather than A-type granites, as previously thought. These syenogranites exhibit lower(^(87)Sr/^(86)Sr)i ratios(0.70532-0.70547) and strongly negative whole-rock εNd(t) values(-12.54 to-11.86) and zircon εHf(t) values(-17.81 to-10.77),as well as old Nd(1962-2017 Ma) and Hf(2023-2092 Ma) model ages, indicating that they were derived from the lower crust.Field and petrological observations reveal that the MMEs within the pluton probably represent magmatic globules commingled with their host magmas. Geochemically, these MMEs have low SiO_2(53.46-55.91 wt.%)but high FeOt(7.27-8.79 wt.%) contents. They are enriched in light rare earth elements(LREEs) and large ion lithophile elements(LILEs), and are depleted in heavy rare earth elements(HREEs) and high field strength elements(HFSEs). They have whole-rock(^(87)Sr/^(86)Sr)i ratios varying from 0.70551 to 0.70564, εNd(t) values of -10.63 to -9.82, and zircon εHf(t) values of -9.89 to 0.19. Their geochemical and isotopic features indicate that they were derived from the subcontinental lithospheric mantle mainly metasomatized by slab-derived fluids, with minor involvement of melts generated from the ascending asthenospheric mantle. Petrology integrated with elemental and isotopic geochemistry suggest that the Shadegai pluton was produced by crust-mantle interactions, i.e., partial melting of the lower continental crust induced by underplating of mantle-derived mafic magmas(including the subcontinental lithospheric mantle and asthenospheric mantle), and subsequent mixing of the mantle-and crust-derived magmas. In combination with existing geological data, it is inferred that the Shadegai pluton formed in a post-collisional extensional regime related to lithospheric delamination following the collision between the NCC and Mongolia arc terranes.

Gas in place and its controlling factors of deep shale of the Wufeng–Longmaxi Formations in the Dingshan area, Sichuan Basin

Recently, deeply-buried shale (depth > 3500 m) has become an attractive target for shale gas exploration and development in China. Gas-in-place (GIP) is critical to shale gas evaluation, but the GIP content of deep shale and its controlling factors have rarely been investigated. To clarify this issue, an integrated investigation of deep gas shale (3740–3820 m depth) of the Lower Paleozoic Wufeng–Longmaxi Formations (WF–LMX) in the Dingshan area, Sichuan Basin had been carried out. Our results show that the GIP content of the studied WF–LMX shale in the Dingshan area ranges from 0.85 to 12.7 m^(3)/t, with an average of 3.5 m^(3)/t. Various types of pores, including organic matter (OM) pore and inorganic pore, are widely developed in the deep shale, with total porosity of 2.2 to 7.3% (average = 4.5%). The OM pore and clay-hosted pore are the dominant pore types of siliceous shale and clay-rich shale, respectively. Authigenic quartz plays a critical role in the protection of organic pores in organic-rich shales from compaction. The TOC content controls the porosity of shale samples, which is the major factor controlling the GIP content of the deep shale. Clay minerals generally play a negative role in the GIP content. In the Sichuan Basin, the deep and ultra-deep WF–LMX shales display the relatively high porosity and GIP contents probably due to the widespread of organic pores and better preservation, revealing great potentials of deep and ultra-deep shale gas. From the perspective of rock mechanical properties, deep shale is the favorable exploration target in the Sichuan Basin at present. However, ultra-deep shale is also a potential exploration target although there remain great challenges.

Paleogeothermal Reconstruction and Thermal Evolution Modeling of Source Rocks in the Puguang Gas Field, Northeastern Sichuan Basin

The thermal history and organic matter maturity evolution of the source rocks of boreholes in the Puguang gas field were reconstructed. An integrated approach based on vitrinite reflectance and apatite fission track data was used in the reconstruction. Accordingly, the geothermal conditions of gas accumulation were discussed in terms of the geological features of reservoirs in the northeastern Sichuan Basin. The strata reached their maximum burial depth in the Late Cretaceous era and were then uplifted and denuded continuously to the present day. The geothermal gradient and heat flow in the Late Cretaceous era were approximately 30.0 °C/km and 66 mW/m2, respectively, which were both higher than those at present. The tectonothermal evolution from the Late Cretaceous era to the present is characterized by denudation and cooling processes with an erosion thickness of2.7 km. In addition to the Triassic era, the Jurassic era represents an important hydrocarbon generation period for both Silurian and Permian source rocks, and the organic matter maturity of these source rocks entered into a dry gas period after oil generation. The thermal conditions are advantageous to the accumulation of conventional and unconventional gas because the hydrocarbon generation process of the source rocks occurs after the formation of an effective reservoir cap. In particular, the high geothermal gradient and increasing temperature before the denudation in the Late Cretaceous era facilitated the generation of hydrocarbons, and the subsequent cooling process favored its storage.

Carbon isotope reversal of desorbed gas in Longmaxi shale of Jiaoshiba area,Sicuhan Basin

Through analysis of components and carbon isotope compositions of gas desorbed from shale cores,the carbon isotope reversal phenomenon in the shale gas from the Silurian Longmaxi Formation of Jiaoshiba area in Sichuan Basin were well studied.Results showed that compared with the wellhead gas,the desorbed gas from Longmaxi shale had significantly more wet components and more heavy carbon isotope values;carbon isotope values of each component became heavier with the desorption time,δ^(13)C_(1)values of different samples had maximum positive variations of 12.3-23.9‰,butδ^(13)C_(2)values only had maximum positive variations of 0.8e2.3‰,indicating carbon isotope values of methane changed more obviously than that of heavy hydrocarbon.The above results were consistent with previous results of shale core desorption experiments carried out by other researchers.Shale gas in strata might have no carbon isotope reversal,and the phenomenon thatδ^(13)C_(1)values changed more significantly thanδ^(13)C_(2)values during the core desorption was not caused by diffusion rate differences among different components,but mainly due to different desorption stages of methane and ethane,i.e.,the ethane was in its early desorption stage while the methane was in its later desorption stage;during the production process,phase differences among different components of alkane gas and differences in the desorption stages induced by adsorption,could be the major cause for total reversal of carbon isotopes of shale gas in Longmaxi Formation,but it also could not excluded that mixture of kerogen cracking gas and crude oil cracking gas probably had a partial or more major contribution to the carbon isotope reversal.

Detrital Zircon U-Pb Geochronologyof Sinian–Cambrian Strata in the Eastern Guangxi Area, China

The Eastern Guangxi area locates in the southwestern part of the transition zone between Yangtze and Cathaysia blocks, which is an important region because the boundary between two blocks probablycrosses there. We determined LA-ICPMS U-Pb ages for detrital zircons extracted from three sandstone samples in the Sinian-Cambrian strata in this region. The resulting ages are in the range of the Archeozoic and Neoproterozoic, with three notable concentrates at 991 Ma, 974 Ma, and 964 Ma, all of which are coeval to the Grenvillian magmatic activity. The new age distribution is similar to the data reported in the Precambrian strata of the adjacent southwestern Cathaysia Block, suggesting that most of our detrital zircons are likely derived from the Cathaysia Block. Combined with others＇ research, we are more inclined to accept the opinion that there was not an ocean basin between the two blocks during the Sinian-Cambrian period in Eastern Guangxi area if the timing of collision is the Early Neoproterzoic. But if the timing of collision is the Early Paleozoic, we conclude that Luzhai uplift（i.e., the uplift between Guilin-Yongfu faultand Lipu fault） beyond the west of Dayaoshan regoin might be one part of southwestern sedimentation boundary of Cathaysia Block and Yangtze Block. We also get a few of detrital zircons with ages of ~590 Ma which probably sourced from northeastern Gondwana and 13 detrital zircons with over 3 000 Ma U-Pb ages which record the early formation of the earth.

Discovery of Filling Shale Gas Field and its prospects

A series of breakthroughs have been made in the understanding, evaluation, and exploration of shale gas from discovery, environmental protection to efficient exploration in the discovering of Fuling Gas Field. By revealing the positive correlation between organic carbon content and siliceous mineral content of shale deposited in deep shelf, dynamic preservation mechanism of “early retention and late deformation,”it is clarified that the shales deposited in deep shelf are the most favorable for shale gas generation, storage and fracturing. The preser-ving conditions determine the levels of shale gas accumulation, thus the evaluation concept of taking the quality of the shale as the base and the preserving conditions as key is proposed, the evaluation system for strategic selection of favorable zones is established for marine shale gas exploration in Southern China. Moreover, the “sweet point” seismic forecasting technologies for marine shale gas, the “six properties” logging technologies for evaluating shale gas layers, the technologies for quick and efficient drilling of horizontal well groups, and the fracturing technologies for composite fractures for hor-izontal wells are invented. The paper discussed the exploration prospect of shale gas in the shales of Wufeng-Longmaxi Formation in great depth in Sichuan Basin, shale gas exploration in the outer region of the south, and continental shale gas exploration in China.