Seismic Reflection Characteristics Research Articles

Platform margin belt structure and sedimentation characteristics of Changxing Formation reefs on both sides of the Kaijiang-Liangping trough, eastern Sichuan Basin, China

Abstract This article discusses the types and characteristics of platform margin belts on both sides of the Kaijiang-Liangping trough in the Sichuan Basin during the Late-Permian Changhsingian age combined with the seismic reflection characteristics. The results show that there are four types of platform margin belts on both sides of the Kaijiang-Liangping trough: (1) steep-slope, single-platform margin belts controlled by faults; (2) steep-slope, single-platform margin belts controlled by paleogeomorphology; (3) steep-slope, multiple-platform margin belts controlled by paleogeomorphology; and (4) gentle-slope, multiple-platform margin belts controlled by paleogeomorphology. Reefs developed in the different types of platform margin belts exhibit different morphological characteristics and stacking patterns. Among all the reefs that developed in steep-slope, platform marginal belts are mainly characterized by aggradational accretion and different degrees of lateral migration in different directions. In contrast, the reefs developed in gentle-slope platform margin belts show distinctive characteristics of multiple bandings and lateral extension. Research suggests that the differences in the platform margin belts’ structure and reefs’ development rules are jointly controlled by a combination of syn-sedimentary fault activities, paleogeomorphology, and sea-level fluctuations. These results can be used to explore and predict reef reservoirs on carbonate platform margins.

Geologic causatives of the stratigraphic and structural heterogeneties of Baharyia formation, based on their seismic interpretation, Razzak field, Western Desert, Egypt

Evaluation of seismic attributes and seismic reflection characteristics analysis of Razzak Field was undertaken by subdividing the study area rock units. There are four seismo-facies units of varying parameters from top to bottom. These units are comparable to the Apollonia and Khoman Formations; the Abu-Roash Formation; Baharyia Formation and Alamein Formation. The seismo-facies unit 1 of the Apollonia and Khoman Formations reveal parallel to sub-parallel layering, with thickness increases northwards. The lithological distribution consists of limestone with minor clay intercalations with facies varying from middle neritic graded to outer neritic. Unit 2 of the Abu-Roash Formation shows divergent layering, with a thickness increasing gradually northwards and eastwards. The lithological distribution shows predominance of carbonate, sandstone occurring in minor portions and laterally graded into shale with littoral facies ranging to inner and middle neritic facies. Unit 3 of the Baharyia Formation reveals chaotic and oblique layering with a time thickness increases towards northeast and southwest parts, the lithological distribution exhibited sand and shale with minor limestone streaks, with facies varied changed from continental to inner-middle neritic, unit 4 of the Alamein Formation shows variation from parallel-subparallel orientation to oblique and chaotic with thickness increasing towards northwards and eastwards, the lithological distribution exhibited dolomitic limestone with minor streaks of limestone with facies varied from middle neritic facies to outer neritic and bathyal. Finally, the results are integrated to build up a seismo-stratigraphic model of the evaluated area of the northwestern Desert of Egypt. Seismic interpretation involves the construction of structure contour maps, in terms of time and depth, on the tops of the Apollonia, Abu-Roash, Baharyia and Alamein Formations. These maps show three structural closures, due to folding, that are dissected by NW–SE faults, analysis of relevant structural and stratigraphic seismic attributes such as root-mean-square amplitude, local structural dip, variance, ISO frequency component, sweetness and acoustic impedance average energy applied on reservoir tops, to enhance the visibility of faults, geological interpretation and the physical parameters of the subsurface related to lithology and stratigraphy for reservoir characterization. Finally, the results obtained are used to construct a seismic structural model. Integration of seismic and stratigraphic data is used to build a geological model of the northwestern Desert of Egypt.

Seismic reflection characteristics and controlling factors of shallow gas in the alongshore mud clinoform of the East China Sea

This study aims to investigate seismic reflection characteristics and controlling factors of gas within the East China Sea clinoform based on seismic profiles and borehole core data, including organic geochemistry and lithology. The seismic profiles reveal distinct gas-related seismic reflection characteristics along the coast, extending from the Yangtze Estuary southward to the elongated inner shelf. In the northern region, specifically the northern part off the Yangtze Estuary, widespread acoustic blanking is topped by a shallow gas front reflector parallel to seabed. In the middle region, spanning the southern part off the Yangtze Estuary and the adjacent northern inner shelf, widespread acoustic blanking is topped by a gas front reflector following the clinoform stratigraphy. In the southern region, off the Oujiang Estuary, there are widespread acoustic turbidities that are topped by a severely undulated gas front reflector. Based on the combined seismic-core data, it is suggested that the seismic reflection in the northern region, characterized by the gas front reflector parallel to the seabed, is controlled by the combination of anaerobic oxidation of methane and active gas diffusion triggered by seabed erosion. In the middle region, the obstructive effects of clinoform stratification on gas diffusion contribute to the seismic reflection characterized by the gas front reflector following the clinoform stratigraphy. In the southern region, the seismic reflection of low-concentration gas characterized by widespread acoustic turbidities is primarily influenced by the interplay between abundant gas source supplies due to the clinoform depocenter and elevated hydrostatic pressure in deeper water conditions. Furthermore, local gas seepage is observed in the northern region, likely due to seabed erosion and lower hydrostatic pressure in coastal zones. Additionally, the coastal zones and seaward slope break sites along the rest of the clinoform sections are also identified as having higher potentials for gas seepage.

Azimuthal seismic reflection characteristics with quality factor Q in viscoelastic horizontal transverse isotropic media

Abstract In seismic exploration, a precise description of the seismic reflection property is critical for reservoir prediction and fluid identification. In the study of seismic wave transmission effects, taking into account both the viscoelastic and anisotropic properties of a medium is compatible with the features of the earth. Moreover, it is advantageous to the characterization of complicated reservoirs. According to the elastic medium foundation and the imaginary component with quality factor Q, seismic reflection properties of viscoelastic media are described in the complex domain. The complex wave number is expressed by phase velocity and Q. The attenuation angle is introduced when the complex wave number is represented by a propagation vector and an attenuation vector. The exact velocity and polarization direction of a viscoelastic medium are expressed using a complex stiffness matrix incorporating Q matrix elements. The quasi-Zoeppritz equation for viscoelastic horizontal transverse isotropic (VHTI) media is derived by using boundary conditions based on the wave function of viscoelastic media. The numerical simulation of reflectivity reveals that the reflection coefficient in a viscoelastic medium is clearly different from that of an elastic medium. Moreover, the difference in reflection coefficient in various orientations has distinct characteristics.

Seismic Facies Recognition of Ultra-Deep Carbonate Rocks Based on Convolutional Neural Network

Seismic facies is a seismic reflection unit defined by specific seismic reflection characteristics, that is, the seismic responses of sedimentary facies or geological bodies, whose accuracy will directly affect the reliability of oil and gas exploration results. Currently, seismic facies is generally recognized depending upon the differences between certain single trace seismic attributes (waveform, frequency spectrum, and amplitude, etc.) and adjacent units to conduct cluster analysis. Such methods, however, have ambiguity in identifying special reflective structures with continuous waveforms (e.g. massive carbonate deposits). In order to solve this problem, this paper incorporates artificial intelligence (AI) technology into automatic recognition of seismic facies with special reflection structures. Firstly, a 2D seismic facies classification sample label set is designed and formed. Then, a seismic facies prediction model is designed and constructed using a multi-layer convolutional neural network (CNN). Finally, the trained model is used to automatically track the seismic facies in the study area. This method was applied to seismic facies recognition for the Sinian Dengying Formation in an area of the Sichuan Basin, and the seismic facies recognized were compared with artificially interpreted ones. It is confirmed that the proposed method provides a better effect than artificial interpretation, with greatly improved accuracy and efficiency.

Quantitative prediction of sand bodies in a sparsely explored zone of the Tarim basin: A case study of the aketao area

Thin sandstone reservoirs of the fan delta front sub-facies occur in the early Neogene (Miocene) series of the Aketao (Akto) structural belt within the Kunlun piedmont zone of the Tarim Basin. Oil and gas reservoirs in this area correspond to stratigraphic traps. However, owing to the low density of the 2D seismic survey grid deployed in the Aketao belt, inferior seismic data quality, and lack of well logging data, reservoir prediction in this area suffers from a multiplicity of problems and it is difficult to effectively identify sand bodies. Here, a new research approach is proposed involving the use of 3D seismic, well logging, and drilling data from a neighboring highly-explored 3D seismic survey area as a reference for the 2D seismic interpretation of the non-drilled Aketao survey area. Moreover, this approach is integrated with forward modeling and the inversion of post-stack seismic data to identify sand bodies. A comparison of the seismic reflection characteristics clarifies that these 3D and 2D seismic survey areas share similar sedimentary environments. Forward modeling confirms their similar reservoir characteristics, while the reservoir distribution in the 2D seismic survey area is effectively mapped via the inversion. The results show that for a 2D seismic survey area characterized by a low degree of hydrocarbon exploration and appraisal, and a lack of well logging data, the proposed approach can confirm the sedimentary characteristics that correspond to the seismic reflection characteristics, and can quantitatively map the reservoir thickness.

West Antarctic Ice Sheet Dynamics in the Amundsen Sea Sector since the Late Miocene—Tying IODP Expedition 379 Results to Seismic Data

Observations of rapid ongoing grounding line retreat, ice shelf thinning and accelerated ice flow from the West Antarctic Ice Sheet (WAIS) may forebode a possible collapse if global temperatures continue to increase. Understanding and reconstructing West Antarctic Ice Sheet dynamics in past warmer-than-present times will inform about its behavior as an analogue for future climate scenarios. International Ocean Discovery Program (IODP) Expedition 379 visited the Amundsen Sea sector of Antarctica to obtain geological records suitable for this purpose. During the expedition, cores from two drill sites at the Resolution Drift on the continental rise returned sediments whose deposition was possibly influenced by West Antarctic Ice Sheet dynamics from late Miocene to Holocene times. To examine the West Antarctic Ice Sheet dynamics, shipboard physical properties and sedimentological data are correlated with seismic data and extrapolated across the Resolution Drift via core-log-seismic integration. An interval with strongly variable physical properties, high diatom abundance and ice-rafted debris occurrence, correlating with partially high amplitude seismic reflection characteristics was identified between 4.2 and 3.2 Ma. Sedimentation during this interval is interpreted as having occurred during an extended warm period with a dynamic West Antarctic Ice Sheet in the Amundsen Sea sector. These records compare to those of other drill sites in the Ross Sea and the Bellingshausen Sea, and thus suggest an almost simultaneous occurrence of extended warm periods in all three locations.

Identifying Submarine Engineering Geologic Hazards in a Potential Gas Hydrate Target Area on the Southern Continental Margin of the South China Sea

The southern continental margin-slope area of the South China Sea is a complex passive continental margin with diverse tectonic structures and movements. This area is rich in gas hydrate resources and is also an area with a high incidence of potential geological hazards. Identifying and understanding the potential submarine geological hazards in this area is very important for disaster prevention and management during the future exploration and development of marine resources. In this paper, five types of potentially hazardous geological bodies are identified in the research area through high-precision two-dimensional seismic processing and interpretation, including submarine mounds, pockmarks, mass transport deposits, submarine collapses and faults. At the same time, the seismic reflection characteristics and the changes in its morphology and surrounding strata are described. In addition to the causes of geological hazards in this region and their influence on exploration and development, the research prospects of geological hazards in this region are also suggested. Special tectonic and sedimentary conditions, fluid activities and hydrate decomposition may be the conditions for geological hazards in this region, which pose a significant threat to the exploration and development of seabed resources and marine engineering construction in this region. Not only does our conclusion provide useful data for the development and utilization of gas hydrate, but it also presents theoretical suggestions for reducing geological hazards in the development process.

Multiple trend integration method based on depositional model and its application in glutenite reservoir modeling

Strong heterogeneity, complicated lithology, and chaotic seismic reflection characteristics are all common features of glutenite reservoirs. It is challenging to pinpoint the interior lithology and quantitatively describe the heterogeneity of the single-stage glutenite. To explore the distribution and superimposition features of subaqueous fans, the upper Es4 in the Y229 region of Dongying sag was used as an example. We have developed a multitrend fusion constraint modeling method based on deposition patterns. First, the truncated Gaussian simulation method is used to establish the sedimentary subfacies model of the subaqueous fan. Second, one lithologic probability volume is generated according to the proportion of various lithology of different sedimentary subfacies. Then, the lobes model is constructed using the object-based simulation method and the quantitative parameters from outcrop and flume sedimentation simulations. In addition, a different lithology probability volume is determined based on how far a particular lobe is from the centerline. The two probability volumes are combined to create the integrated lithology probability volume, which reflects the planar trend and the internal differences of different lobes. To constrain lithologic modeling, the integrated lithology probability volume is used. By comparing the model results with a single-trend constraint, the findings indicate that the multitrend integration constraint modeling method may more accurately depict the internal variability of the glutenite reservoir. In addition, the lithologic model built on this foundation is consistent with the depositional model.

Characterization of ultra-low permeability tight sandstone reservoir properties and criteria for hydrocarbon accumulation in Chang 6 member, Huaqing area, Ordos basin

How to accurately predict hydrocarbon enrichment and high-yield areas in ultra-low permeability sandstone reservoirs is a hot spot in petroleum geology. Taking the Chang 6 Member of the Yanchang Formation in the Huaqing Area, Ordos Basin as an example, the quantitative characterization and classification criteria of ultra-low permeability sandstone reservoir properties are systematically studied. The research results show that the seismic reflection characteristics of the Chang 6 Member show multi-stage wedge progradation characteristics, reflecting the sedimentary characteristics of the end of the delta front. The maturity of the minerals in the Chang 6 Member sandstones in the study area is very low, it indicates a low degree of transformation of sediments. Three types of sand bodies, namely sandy debris flow, turbidite, and slump rock sand bodies, were identified in the Chang 6 Member of the Yanchang Formation. The pore types of the Chang 6 Member in the study area include intergranular pores, dissolution pores (feldspar, detrital dissolution pores), and intercrystalline pores. Among them, the intergranular pores and feldspar dissolution pores are the most important storage spaces. The study also found that the test oil production is highly correlated with the cumulative thickness of the massive sandstone segments. Therefore, massive sandstone (sand debris flow) is the main contributor to oil well production, and its distribution directly controls the accumulation of oil reservoirs and the high production of oil wells. In addition, geological parameters affecting reservoir enrichment evaluation were analyzed, which include permeability, oil layer thickness, resistivity, and reservoir penetration rate. Finally, the evaluation criteria for hydrocarbon enrichment in the Chang 6 Member were formulated by the analysis of dominant facies, lithologic assemblages and electrical properties.

Analysis of the geometric characteristics of clinothems and the relationship with shelf‐edge trajectories of the Plio‐Pleistocene continental slope in the Qiongdongnan Basin, South China Sea

AbstractBased on high‐quality two‐dimensional seismic profiles and well data, the seismic reflection characteristics from the late Pliocene and Pleistocene succession of the Qiongdongnan Basin in the South China Sea have been analysed. The result will help in understanding the sedimentological development of slope areas. Twenty coeval clinothems are recognized and four major clinothem types are distinguished. In addition, three types of shelf‐edge trajectories are recognized. The various parameters of the clinothems are analysed quantitatively and the relationships between the various clinothem types are studied in combination with the controlling roles. It is concluded that: (i) the shelf‐edge trajectories are related to the type of clinothem; (ii) fluctuations in the relative sea level, in combination with variations in sediment supply, result in different clinothem types and geometric parameters; these parameters show interdependencies; (iii) the alternations of progradation and aggradation of clinothems may provide information of the relative sea level and changes in sediment supply; (iv) some lowstand fans could develop at the foot of the slope in the central–eastern part of the basin when the relative sea level started to fall or began to rise. The results of the present study are consequently significant for further quantitative analyses of the characteristics and development of the continental slope, not only in the Qiongdongnan Basin, but also in other basins.

Sinian tectonic evolution and distribution of source rocks in northwest Tarim Basin, China

Tarim Basin experienced several tectonic events during Precambrian, including the Neoproterozoic Rodinia supercontinent assembly/breakup event and Snowball Earth, and has great potential for oil generation in the ultra-deep deposits. The Precambrian strata in the northwest margin of the Tarim Basin (abbreviated as the northwest Tarim) are well exposed, which play a key role for understanding the tectonic evolution of Neoproterozoic. The northwest Tarim underwent the transformation that tectonic setting changed from convergence to expansion in the Neoproterozoic, and finally formed a continental rift until 572 Ma. During this period, thick layer of black mudstone was deposited, which probably was the important petroleum source rock. The TOC of Sinian mudstones is between 0.32 and 1.25%, with an average value of 0.53%. The average of rock pyrolysis parameters (S1+S2) is 0.17 mg/g, Ro is between 1.61% and 2.52%, and the average of chloroform asphalt "A" is 0.0076%. These geochemical characteristics indicate that the source rocks are now highly mature. The carbon isotopes of kerogen are between −27.2‰ and −30.2‰ and biomarkers show that algae and bacteria are the main source input, therefore, the Sinian source rock mainly contains marine type I kerogen from a single source. The above data show that the Sinian source rock is a set of effective source rocks. The tectonic evolution of northwest Tarim was reconstructed based on the age characteristics of detrital zircons, and it indicated that the Sinian source rocks were controlled by continental rift deepening and large-scale transgression. Combined with seismic reflection characteristics, the distribution of Sinian source rocks are shown, which provides an important basis for Precambrian oil and gas exploration.

Natural Gas Migration Pathways and Their Influence on Gas Hydrate Enrichment in the Qiongdongnan Basin, South China Sea

2D and 3D seismic data and basin simulation were used to investigate the gas hydrate distribution and natural gas migration pathways in the Qiongdongnan Basin (QDNB). Hydrate-related amplitude anomalies and extensive bottom simulating reflectors (BSRs) were mapped within the uppermost part. Based on their seismic reflection characteristics, the three main types of natural gas migration pathways and their distributions in the QDNB were identified through high-resolution seismic data. Basin modeling was carried out to document the migration efficiency of different migration pathways and their effects on hydrate enrichment. The basin modeling results show the following: (1) Diapirs, fault structures, and fractures constitute the three main types of natural gas migration pathways that transport the thermogenic gas from the deep to shallow layers in the QDNB. (2) The three migration pathways impact hydrate enrichment in different ways. Diapirs and faults contribute significantly to hydrate enrichment due to their higher migration efficiency. In comparison, the migration efficiency of the fracture systems is lower, with minimal benefit to hydrate enrichment. (3) The natural gas hydrate in the QDNB is mainly distributed along the diapirs and deep faults and generally scattered around the fracture system. These conclusions indicate that the migration pathways in the QDNB are regionally distributed and are closely related to hydrate accumulation.

Approximate equations of PP-, PS1- and PS2-wave reflection coefficients in fluid-filled monoclinic media

SUMMARY Studying the seismic reflection characteristics of fractured strata can provide a theoretical basis for the exploration of unconventional oil and gas reservoirs. When several (more than 1) sets of oblique vertical fractures that are rotationally non-invariant are embedded in an isotropic or vertical transversely isotropic (VTI) background, the stratum exhibits equivalent monoclinic anisotropy. However, the exact algorithm of the reflection and transmission coefficients of monoclinic media is algebraically complicated and difficult to use in theoretical analysis and application. In this paper, under the assumption that fractures are filled with fluid, based on the linear-slip theory, we solve the phase velocities of monoclinic media from the Christoffel equations by using the first-order perturbation method and then derive the recursive equations of the PP-, PS1-, and PS2-wave reflection coefficients. The theoretical analysis results show that for an interface of monoclinic media, when setting a series of fracture densities for the lower medium, the relative error of the proposed approximate reflection coefficient of the PS2 wave is less than 5.7 per cent, and those of the PP and PS1 waves are generally less than 2.5 per cent for a wide range of anisotropy strength and impedance contrast; when setting a series of fracture densities for the upper medium, the relative error of the proposed approximate reflection coefficients of the PP, PS1 and PS2 waves are generally less than 6.4, 5.1 and 4.5 per cent, respectively. When degenerated to the horizontal transversely isotropic model, compared with Rüger's approximation, at incidence angles of 1–30°, the two results are almost coincident, but the proposed approximations are closer to the exact solutions.

Variscan structures and their control on latest to post-Variscan basin architecture: insights from the westernmost Bohemian Massif and southeastern Germany

Abstract. The Bohemian Massif exposes structures and metamorphic rocks remnant from the Variscan orogeny in central Europe and is bordered by the Franconian Fault System (FFS) to the west. Across the FFS, Variscan units and structures are buried by Permo-Mesozoic sedimentary rocks. We integrate existing DEKORP 2D seismic reflection, well, and surface geological data with the newly acquired FRANKEN 2D seismic survey to investigate the possible westward continuation of Variscan tectonostratigraphic units and structures and their influence on latest to post-Variscan basin development. Subsurface Permo-Mesozoic stratigraphy is obtained from available wells and tied to seismic reflection profiles using a synthetic seismogram calculated from density and velocity logs. Below the sedimentary cover, three main basement units are identified using seismic facies descriptions that are compared with seismic reflection characteristics of exposed Variscan units east of the FFS. Our results show upper Paleozoic low-grade metasedimentary rocks and possible Variscan nappes bounded and transported by Variscan shear zones ca. 65 km west of the FFS. Basement seismic facies in the footwall of the Variscan shear zones are interpreted as Cadomian basement and overlaying Paleozoic sequences. We show that the location of normal fault-bounded latest to post-Variscan late Carboniferous–Permian basins are controlled by the geometry of underlying Variscan shear zones. Some of these late Carboniferous–Permian normal faults reactivated as steep reverse faults during the regional Upper Cretaceous inversion. Our results also highlight that reverse reactivation of normal faults gradually decreases west of the FFS.

Spatial distribution prediction of pre-Sinian volcanic and metamorphic rocks in Xinhenan-Sandaoqiao area, Northern Tarim Basin

The main type of oil and gas reservoir in the Xinhenan-Sandaoqiao area is buried hills. The distribution pattern and scale of reservoirs in this area are obviously controlled by pre-Sinian basement strata. However, the lithologic combination and spatial distribution pattern of the pre-Sinian basement in this area are still unclear. We have used the method of multifactor comprehensive analysis to study the spatial distribution of pre-Sinian basement volcanic and metamorphic rocks. First, we determine the lithology and lithologic association of volcanic and metamorphic rocks according to cores and thin sections. Guided by the seismic reflection characteristics of different lithologic combinations, different lithologic combinations are identified on the profile by combining the seismic reflection characteristics of single well and multiwell. Second, using cluster analysis technology, we select three seismic attributes that are sensitive to lithology from 10 attributes, make crossplots of the three seismic attribute values, and ensure the distribution range of attribute values corresponding to different lithologic combinations for plane lithologic identification. Finally, we combine the recognition results of the plane and the profile to summarize the plane lithology distribution of the pre-Sinian basement surface. Six distribution types were identified deep metamorphic bedrock area in Kuqu depression, dynamic mixed metamorphic rock and intermediate-acidic intrusive rock area, metamorphic bedrock in thrust napple slopes area, thermal contact metamorphic rock area, intermediate-acidic intrusive rock area, and dynamic metamorphic rock area and gneiss area in faulted uplift core.

Crustal Structure of the Hikurangi Margin From SHIRE Seismic Data and the Relationship Between Forearc Structure and Shallow Megathrust Slip Behavior

AbstractMarine multichannel and wide‐angle seismic data constrain crustal structure along a 530 km margin‐parallel transect of the Hikurangi subduction zone. The subducting Hikurangi Plateau crust (VP 5.0–7.4 km/s) is ∼1 km thicker (11 ± 1 km) and mantle velocities are ∼0.2 km/s higher (VP 8.3–8.5 km/s) beneath south/central Hikurangi relative to north Hikurangi. In the overthrusting plate, an abrupt 0.5 km/s south‐to‐north reduction in forearc wavespeeds occurs in concert with a change in seismic reflection character. We analyze legacy seismic data to show that the forearc transition likely reflects lateral variability in the updip extent of the Torlesse Backstop. Furthermore, we map this unit along‐strike and note a broad correlation between the backstop and down‐dip extent of shallow slow‐slip. We propose that the geological architecture of the overthrusting plate contributes to spatial variability in the location of shallow frictional transitions along the Hikurangi margin, impacting both seismic and tsunami hazard.

Paleozoic suture and Mesozoic tectonic evolution of the lithosphere between the northern section of the Xing'an Block and the Songnen Block: Evidence from three-dimensional magnetotelluric detection

The Xing'an and Songnen Blocks are microcontinents in the eastern Central Asian Orogenic Belt (CAOB). The Heihe–Hegenshan suture has been interpreted to represent the boundary between the blocks. However, the lack of ophiolite exposure and deformation towards the northern extent of the suture mean that its precise nature and location remain controversial. To better understand the position and structure of the suture, magnetotelluric data were acquired at 104 stations along five profiles in the northern Songnen and Xing'an Blocks. A three-dimensional nonlinear conjugate gradient algorithm was used to image the resistivity structure of the lithosphere. Our resistivity models indicate that the lithospheres of both the Xing'an and Songnen Blocks are characterized by overall high resistivity and have heterogeneous electrical structures. We define a highly conductive anomaly (C2) to the east of the Duobaoshan Island Arc that divides lithosphere of high resistivity and controls the distribution of Carboniferous magmatic rocks related to collision. We suggest that C2 is the northern extension of the Heihe–Hegenshan suture. The highly conductive anomaly (C1) is located in the Xing'an Block on the west side of the island arc. While the exposed lithology on the surface is Jurassic intrusions, usually with high resistivity. We suggest that C1 may be an early structure, which was transformed by the closure of the Mongolia–Okhotsk Ocean. No large-scale Mesozoic intrusive bodies occur above C3, a highly conductive anomalous region in the Songnen Block. And the seismic reflection characteristics have changed significantly in the region C3. Therefore, we interpret C3 as a shear zone in the crust related to a change in the subduction direction of the paleo-Pacific plate. The resistivity model provides new geophysical evidence for determining the suture and the evolutionary traces of the Mongolia-Okhotsk Ocean and the Paleo-Pacific Ocean.

Using 4-D Seismic Data for Detecting Gob Areas of Coal Mines: A Case Study From the Zhangji Coal Mine

Subsidence of the gob area is an important geological problem in coal mines. Such events not only pose potential safety risks, but also can seriously impair the subsequent mining of coal mines. Therefore, detection of gob subsidence is important to ensure sustainable coal mining. Due to the complex seismic and geological conditions in the gob subsidence area, it is difficult to accurately identify the gob area by using traditional three-dimensional (3D) seismic detection method. Herein, a new, model-driven gob detection method based on four-dimensional (4D) seismic data is proposed. According to the proposed strategy, geological models before and after coal seam mining are firstly established, and synthetic data are generated through forward modeling. Then, the prestack time migration (PSTM) sections of synthetic data are used to retrospectively analyze the seismic reflection characteristics of the gob area, and the gob area is identified according to the variations of reflection characteristics and velocity fields caused by the gobs. Next, the characteristics of gob areas in synthetic data are used to guide the identification of the gob area in real seismic data. Meanwhile, a 3D stratum stripping technique is used to finely depict the gob area. According to the method, the scope of the caving zone, the height of the overburden fractured zone in the gob area, and the geological structure of the lower coal seam in the gob area can be ascertained. This paper presents the successful use of the proposed method in detecting the gob area from a real coal mine.

Evaluation Method of the Gas Hydrate and Free Gas System and Its Application in the Shenhu Area, South China Sea

As a new alternative energy source, gas hydrate has attracted wide attention all over the world. Since gas hydrate is always associated with free gas, the evaluation of the gas hydrate and free gas system is an important aspect of hydrate reservoir exploration and development. In this study, based on identifying gas hydrate and free gas by well logging, the seismic reflection characteristics of gas hydrate and free gas are determined by an accurate well-to-seismic calibration method. On account of seismic reflection characteristics, AVO attributes are used to identify gas hydrate and free gas qualitatively. Using prestack and poststack inversion to get the ratio ofP-wave impedance andP-wave-to-S-wave velocities, we determine the three-dimensional space distribution of gas hydrate and free gas, predict their effective porosity and saturation, and eventually achieve the meticulous depiction of gas hydrate and free gas in the body, which is necessary in subsequent estimation of gas hydrate and free gas resources. Results show that according to logging interpretation, gas hydrate of the B-well is located in the depth range of 1460–1510 mbsl and free gas is in 1510–1542 mbsl. Moreover, gas hydrate of the A-well is located in the depth range of 1425–1512 mbsl, and no obvious free gas is identified. Gas hydrate is located above free gas and distributed continuously. In plane form, gas hydrate and free gas both present subelliptical distribution in the NW-SE direction. Gas hydrate has an effective porosity of 0.30–0.40, an average saturation of 0.33–0.40, and an effective thickness of 3.0–10.5 m, whereas free gas possesses an effective porosity of 0.35–0.40, a saturation of 0.24–0.32, and an effective thickness of 2.0–5.0 m.