Strong Seismic Reflectors Research Articles

Sub-basalt hydrocarbon prospect assessment in Peninsular India using seaward dipping reflectors

Conventionally, volcanic margins have been considered devoid of hydrocarbon, but many discoveries and research in recent past have proved the presence of hydrocarbon prospects within them. However, hydrocarbon exploration within volcanic margin is constrained by seismic imaging. Further, identification of continent to oceanic boundary (COB) is critical to hydrocarbon search as hydrocarbon is found mostly over continental crust. Seaward dipping reflectors, associated with volcano-sedimentary sequences, located along rifted continental margins and represented by highly dipping strong amplitude seismic reflectors, play an important role to study volcanic margins. Keeping in view the problems of hydrocarbon exploration within volcanics, seaward dipping reflectors (SDRs) in Indian context have been studied in detail in this paper. It facilitates mapping of continent-oceanic boundary (COB). It has impact on hydrocarbon prospectivity in volcanic margins as it can provide formidable seal and secondary induced maturity. In India, several discoveries have been made in volcanic margins along west coast, from weathered, fractured basalt and sub-basalt sediments. In the present work, long offset regional seismic data (18 dip lines and 1 cross line), along east coast, have been interpreted. SDRs help in hydrocarbon prospectivity assessment for both discrimination of interbedded sediments and preferential accumulation of hydrocarbon. Mapping of linear features not only helped to demarcate COB but also their presence at 6–10 km depth. For the first time, such comprehensive study on SDRs has been done along Indian peninsular region with indications for hydrocarbon prospectivity at deeper levels along Indian peninsular region.

100,000 years climatic cycles recorded on very high-resolution seismic data from the Santos Basin’s upper slope during the last 800 ka

This study proposes to combine seismic stratigraphic interpretation of very high-resolution sub-bottom chirp profiles acquired along the Santos Basin slope on the Southern Brazilian Margin during the SANBA project to recently published results from five continuous long cores acquired in the Santos Basin (sampling sediments as old as 772 ka with detailed biostratigraphic and chronostratigraphic data). The seismic-core correlation is consistent over the area and shows that 100,000 years cycles’ terminations (G/IG transitions, identified on long sediment cores) do correlate to strong seismic reflectors identified regionally on chirp profiles.In particular, the last eight 100,000 years cycles (down to MIS19) are well-identified throughout the area. Additional higher-resolution sea-level drops or cooling events (e.g., MIS4, MIS7b, MIS9b) can sometimes be distinguished on the chirp profiles but only in areas with sufficiently high sedimentation rates (around 10 cm/ka). At MIS12, we observe a change in seismic architecture with more numerous, more closely spaced seismic reflectors but with smaller amplitudes. According to the available data, this change also corresponds to a change in sedimentation rates (smaller rates before MIS12), which may be related to the mid-Brunhes Transition (at 430 ka) and/or the imprint of 400 ka Milankovitch cycles. Two cores show a specific Nanno Foram Ooze at this transition towards the following Super-interglacial MIS11. This study demonstrates the ability and the reliability of very high-resolution chirp profiles to record continuously the stacking of G/IG climatic cycles on the slope of the Santos Basin despite relatively low subsidence rates on this old passive margin and small direct input from rivers. Therefore, the study points to the continuous feeding of the margin by sediment brought up laterally by currents, most probably mainly Brazilian Current (BC) that follows the shelfbreak down to the South.

Characteristics and Seismic Identification Mode of Ultra-Deep Carbonate Fault-Controlled Reservoir in Northwest China

Ordovician carbonate reservoirs in the Shunbei area of the Tarim Basin are characterized by large burial depth, small vertical fault displacement of the strike-slip fault zone, small karst development scale, strong reservoir heterogeneity, complex and diverse seismic reflection types, which lead to unclear seismic response characteristics of fault zone and reservoir, and further restrict the fine description of ultra-deep (>7300 m) fault controlled reservoirs. On the basis of high-precision 3D seismic data analysis, combined with drilling, logging, and comprehensive geological interpretation data in the Shunbei area, seismic geological models of different types of reservoirs and fault zones are established. The changes in seismic response characteristics of fault zones and reservoirs are simulated and analyzed by changing reservoir size, scale, fault and fracture cavity combination modes, as well as other factors. On this basis, two seismic identification models of fault-controlled reservoirs in the Shunbei area are summarized and established. The seismic identification mode of seismic reflection marker wave of fault-controlled reservoir is mainly “fault seismic response characteristics & weak reflection” and red trough anomaly under T74. The internal seismic identification modes are mainly “fault seismic response characteristics & random reflection”, “fault seismic response characteristics & beading”, “fault seismic response characteristics & linear weak reflection”, and “fault seismic response characteristics & random reflection”. Among them, except for “fault seismic response characteristics & random reflection”, the coincidence rate of reservoir prediction of other three types of seismic response characteristics is more than 70%. The coincidence rate of the other three types of seismic response characteristics of a reservoir is more than 70%. The research results provide a reference for the prediction and description of ultra-deep carbonate fault-controlled reservoirs.

Formation of SDRs-Ocean transition at magma-rich rifted margins: Significance of a mantle seismic reflector at the western Demerara margin

At distal magma-rich rifted margins, seismic reflections several seconds below the ∼10 s TWT expected base of petrological crust for thermally equilibrated lithosphere, are commonly observed. These deep seismic reflectors are often interpreted as the petrological Moho as they distally rise and merge into the imaged oceanic Moho, however, the origin of these deep seismic reflections beneath the expected base of the petrological crust is not understood.We investigate the example of the Jurassic Demerara Plateau located offshore north-east South America, which shows a strong seismic reflection at ∼15 s TWT, beneath 20 km thick SDRs (Seaward Dipping Reflectors). We use combined inversion of gravity and seismic reflection data, together with seismic interpretation, to investigate the depth of the deep reflector, its origin, formation as well as the composition and structure of the overlying material. Results from quantitative techniques show that the deep reflector is located below the SDRs at least 10 km within the mantle at a depth of ∼42 km. These results, together with seismic interpretation, suggest that the deep reflector corresponds to an intra-mantle detachment limiting semi-ductile and sheared magmatic-infiltrated mantle above from depleted ductile asthenospheric mantle below. Distally, the semi-ductile mantle is removed when the asthenospheric mantle focuses to form a stable mid-oceanic ridge. This lateral evolution results from a progressive coupling between the magmatic SDRs above and the asthenospheric mantle below. Therefore, we propose that the intra-mantle detachment, corresponding to a rheological contrast, plays a key role in the transition from distributed lithosphere extension to focused mid-ocean ridge accretion at distal magma-rich rifted margins. In addition, it may represent an indicator to determine the thermal structure of a magma-rich rifted margin.

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.

Borehole Seismic Observations From the Chicxulub Impact Drilling: Implications for Seismic Reflectivity and Impact Damage

AbstractWe conducted a vertical seismic profile (VSP) in the borehole of International Ocean Discovery Program/International Continental Scientific Drilling Program Expedition 364 Site M0077 to better understand the nature of the seismic reflectivity and the in situ seismic properties associated with the Chicxulub impact structure peak ring. Extraction of the up‐going wavefield from the VSP shows that a strong seismic reflection event imaged in seismic reflection data results from discontinuities in the elastic impedanceZ(the product of density and wave speed) at the top and bottom of a zone of hydrothermally altered melt‐bearing polymict breccia (suevite) that are characterized by anomalously lowZ. Below this strong carbonate/suevite reflection event, the upgoing seismic wavefield is chaotic, indicating high levels of scattering from the suevites and underlying melt rocks and shocked granitoids of the peak ring, in contrast to the clear coherent reflections throughout the overlying Cenozoic sediments. We extract shear wave speeds, which, together with those provided from the complementary sonic log and densities from core scanning, allowed determination ofVP/VSand Poisson's ratiov. These values are anomalously high relative to comparable terrestrial lithologies. We also calculate a variety of damage parameters for the disrupted peak ring granitoids. These values may assist in linking seismic observations to shock levels that are necessary to calibrate current impact models and may also be useful in assessing levels of fracturing within major fault zones.

Genesis and petroleum geological significance of calcareous mudstone in Beier Sag

Beier sag is one of the most oil-rich sags in the south of Hailaer Basin in China. The purpose of the study is to clarify the identification characteristics and geological origin of calcareous mudstone strata in Nantun Formation of Beier sag, so as to provide new ideas for the identification and evaluation of high-quality source rocks, and to provide effective support for efficient exploration of oil and gas in the basin. The comparative analysis of logging curves and seismic reflection characteristics shows that the calcareous mudstone layer has the “three high and one low” logging response with high GR (Natural Gamma Ray), high LLD (Deep Investigate Induction Log), high AC (Acoustic) and low DEN (Rock Density), showing continuous and strong seismic reflection characteristics. Through the comparative analysis of experimental data of rock thin slice and microscopic image and organic geochemistry, the layer lithology is mainly gray-black mudstone, locally mixed with oil shale and marl, rich in calcium and algae. The layer has the high abundance of organic matter, good organic matter type ( II1-I ), high hydrocarbon-generating potential and high hydrocarbon-exhausting efficiency. It is the highest quality source rock found in Beier sag, and is the material base to form the large scale oil and gas accumulation. The formation of calcareous mudstone formed in the brackish water and wide lake basin by a wide range, represents the anoxic event early in Cretaceous period and is the significant stratigraphic correlation marker bed of the sag. In addition, this layer and the fan-delta front sand widely distributed in the lower part of Nantun Formation form the most favorable reservoir combination, which controls the plane distribution of oil and gas reservoirs. Above and below the calcareous mudstone strata, oil and gas are very rich.

The Abra sedimentary-hosted Pb-Ag-Cu-Au deposit, Western Australia: A geophysical case study

Abra is a high-grade sedimentary-hosted Pb deposit located in the Paleoproterozoic Edmund Basin in Western Australia. Mineralization is blind, with the top of the deposit occurring 250 m beneath the land surface. The deposit consists of a stratiform apron of Pb-Ag-Ba mineralization in laminated iron-oxide- and barite-altered dolomite and siltstone, which overlies a feeder zone of chlorite-altered, brecciated, and veined carbonatic siltstone that contains Pb-Ag mineralization in the core that transitions to Pb-Cu and Cu-Au at depth. Abra is characterized by discrete geophysical anomaly responses in magnetic, gravity, time-domain electromagnetic (TDEM), and induced polarization survey data. A +450 nT magnetic anomaly is caused by magnetite in the lower stratiform zone. Dense galena, barite, dolomite, and iron-oxide mineralization in the apron and galena in the feeder zone is surrounded by lower-density sedimentary host rocks, which results in a +1 mGal gravity anomaly. Airborne, ground, and downhole TDEM surveying resolved known mineralization as weak electromagnetic conductor responses, and petrophysical testing on core samples shows that this is caused by galena. Pole-dipole-induced polarization surveying resolved a +20 ms chargeability anomaly on the southern flank of the deposit. This chargeable anomaly response is related to disseminated galena, pyrite, chalcopyrite, and alteration. Joint audiomagnetotelluric-magnetotelluric 2D inverted data sections resolved Abra as a broad weakly conductive anomaly. Weak conductor responses associated with Abra were also resolved in 2D and 3D inversion modeling of airborne Z-axis tipper electromagnetic data. 2D seismic reflection surveying resolved Abra as strong flat-lying seismic reflectors, which are bounded and offset by faults and surrounded by a seismically bland zone. The seismic reflections are related to significant density contrasts between high-density stratiform mineralization that is in contact with low-density sedimentary host rocks, as the mineralization and host rocks have similar seismic velocities. Passive seismic horizontal to vertical spectral ratio surveying resolved the top of the deposit as a subtle layer sitting below a flat impedance contrast horizon that is interpreted as weathered siltstone on top of diagenetically cemented siltstone.

Investigation of fluid effects on seismic responses through a physical modeling experiment

We have investigated fluid effects on seismic responses using seismic data from a physical modeling experiment. Eight cubic samples with cavities quantitatively filled with air, oil, and water and 16 nonfluid samples were set within a physical model. The pre- and poststack seismic responses of the samples were analyzed to quantitatively investigate the fluid effect on the seismic response. We determined that fluids could cause detectable changes in pre- and poststack seismic responses for tight rocks. At first, fluids filled within samples caused changes in prestack seismic responses. Visible differences could be detected between angle gathers of the samples filled with air, oil, and water. For the base reflections, the amplitudes at large angles of the air- and oil-filled samples are obviously stronger than those of the water-filled sample. In addition, the presence of fluids within samples led to significant changes in poststack seismic reflections. For samples with similar P-wave impedances to the background, we found strong seismic reflections for the fluid samples and weak or even no reflections for the nonfluid samples. There was notable interference between the top and base reflections for the fluid samples, whereas there was none for the nonfluid samples. Seismic velocities were estimated using the two-way traveltimes between the top and base reflections. The estimated seismic velocity gently declined with increasing water saturation until 90%. When the water saturation was more than 90%, the seismic velocity showed a steep increase.

Late Quaternary depositional and glacial history of the Arliss Plateau off the East Siberian margin in the western Arctic Ocean

Sedimentary stratigraphy and facies analysis along with seismostratigraphic and multibeam bathymetry data are used to reconstruct the last glacial impact on the Arliss Plateau (AP) and attendant sedimentation in the adjacent Chukchi Basin (CB) in the western Arctic Ocean off the East Siberian margin. Sediment core ARA02B/16B-GC from the AP lower slope captures glacier-related depositional history during the last estimated ca. 100 ka (Marine Isotope Stage, MIS 1 to 5c) based on regional lithostratigraphic correlation. The sedimentary record shows distinguishable interglacial (interstadial) and glacial (stadial) patterns. The identified sedimentary facies reflect several modes of glaciogenic deposition by drifting icebergs, suspension settling from turbid meltwater plumes and/or detached underflows, and turbidity currents. Based on strong seismic reflectors related to lithological boundaries, a downslope subbottom profile from AP to CB is divided into seismostratigraphic units (SSU) 1 and 2 corresponding in the core record to MIS 1–3 and MIS 3-5c, respectively. An acoustically transparent lens within SSU 2 correlates on the upper slope to debris lobes downslope from the AP top covered by megascale glacial lineations. This geomorphic/sedimentary pattern indicates a glacial erosional impact on the AP and proglacial deposition of eroded sediments on the slope and in the basin. Based on the developed sediment stratigraphy and facies analysis, the last debris lobe horizon was deposited in glacial/deglacial environments during late MIS 4 to early MIS 3. The absence of similar glaciogenic debris lobes within SSU 1 indicates no direct glacial impact on the AP during the Last Glacial Maximum (LGM). These results suggest that the last glacial erosion of the AP occurred during or immediately after MIS 4, possibly related to major glaciation in northern Siberia at ∼50–70 ka.

Lithospheric structure of the xuefengshan belt, South China: Evidence from a seismic reflection profile

In this study, we analyse and interpret a new seismic reflection profile across the Xuefengshan Belt (XFSB), South China, to determine the deep structure of this belt and provide insights into collisional orogenic architecture. We identify a deep regional fault located beneath the XFSB, which cuts through the reflection Moho below the belt and is interpreted as representing the collisional boundary between the Yangtze and Cathaysia blocks along the southeastern margin of the belt. We identify structural evidence for collision between the two blocks, including two groups of layered narrow slip belts (∼30 km) in the middle crust, one distributed in the Yangtze Block and the other in the Cathaysia Block. A narrow velocity-gradient belt in the upper mantle at a depth of ∼90 km is revealed by numerous strong seismic reflectors, some of which are layered. The study provides important new information on the lithospheric structure of the South China Block at the interface between the Yangtze and Cathaysia blocks.

A Strong Seismic Reflector within the Mantle Wedge above the Ryukyu Subduction of Northern Taiwan

AbstractMajor structures within the mantle wedge are often revealed from seismic velocity anomalies, such as low‐velocity zones at magma reservoirs, partial melting regions, or the upwelling asthenosphere. However, no significant seismic boundaries have been reported in the shallow mantle wedge beneath volcanic arcs. Here, we present evidence for a strong seismic reflector dipping in the opposite direction of the subducting slab in the mantle wedge beneath northern Taiwan in the western end of the Ryukyu subduction system. We find that two unambiguous P waves generated by a deep earthquake (ML 5.1) at a depth of 132.5 km were clearly recorded by the dense seismic array (Formosa Array), composed of 140 broadband seismic stations with a station spacing of approximately 5 km in northern Taiwan. Forward modeling using both raytracing and travel times shows that a seismic reflector exists beneath the Tatun volcano group (TVG) around depths of 80–110 km. The reflector dips in the opposite direction of the subducting slab and is unlikely to be associated with mantle wedge corner flow. Instead, it probably belonged to parts of possible structures such as the asthenospheric flow, the mantle diapir, or other undiscovered structures above the subducting slab. No matter what the seismic boundary is exactly, it might be associated with the active volcanism in the TVG. The detailed geometry and mechanism of the seismic boundary in the mantle wedge will be obtained as the Formosa Array collects more seismic data in the near future.

Magnetotelluric transect of Unzen graben, Japan: conductors associated with normal faults

We conducted a broadband magnetotelluric (MT) survey along a north–south transect across Unzen graben, Japan. The MT survey line is located ~ 2 km west of the most recent lava dome and consisted of 27 stations along a 9-km profile. We estimated the 3-D resistivity structure and correlated it with the seismic reflection structure obtained by the same survey line as in the present study. The best-fit resistivity structure shows an upper resistive layer underlain by a moderately conductive layer. The resistive layer, which is interpreted as a cold groundwater zone, is cut by four faults marked by their relatively high conductivity. The underlying layer, which is interpreted as a hydrothermal-water-rich layer, also shows relatively conductive values near the faults. By assuming that the faults are imaged as relatively conductive zones, we infer the dip and depth extent of fracture zones around the faults. Beneath the Chijiwa Fault, which is the longest and most active fault of Unzen graben, the dominant conductor (C1) has a width of 2 km and extends down to below 4 km depth. C1 corresponds to a zone of strong seismic reflection and is located close to one of the pressure sources causing surface deformation. In this study, we interpret C1 as a network of fractures generated by the Chijiwa Fault to which magmatic volatiles are supplied from a deeper pressure source. Given that C1 extends to a greater depth and its resistivity is lower than other conductive zones, it is possible that earthquakes have occurred repeatedly on the Chijiwa Fault. In the center of the study area, we identify a vertically oriented body of high resistivity (R1) that corresponds to a zone of low seismic reflectivity. We interpret R1 as a cooled dyke complex that may have acted as a volcanic conduit.

Gas hydrate occurrence and distribution controlled by regional geological structure off eastern India: Estimates from logging-while-drilling in Area-B, National Gas Hydrate Program Expedition 02 (NGHP-02)

Abstract The National Gas Hydrate Program Expedition 02 (NGHP-02) was designed to examine gas hydrate occurrences off the eastern margin of the Indian Peninsula. Twelve holes were logged with logging-while-drilling (LWD) tools and nine holes at five sites were cored in Area-B of NGHP-02. Five logging units were defined based on the characteristics in logging data responses. Distinctive Logging Units R1 and R2 characterized by high gamma ray log values and composed of quartz and biotite-rich medium sized sand grain units that act as gas hydrate resevoirs and correlated to strong seismic reflectors. Hydrate-bearing intervals can be identified by resistivity and sonic velocity logs and classified into pore-filling type, fracture-filling type, and a mixture of pore- and fracture-filling types based on borehole image characteristics. Pore-filling type gas hydrate is typically developed in the case where the depths of R1 and/or R2 is close to the seismic inferred bottom simulating reflector (BSR). Fracture-filling type hydrates associated the R1 section occur above the BSR associated with the anticline in Area-B. The occurrence of pore-filling and fracture-filling gas hydrate represent different processes of hydrate formation related to the regional geological structure. The stratigraphic layered pore-filling gas hydrate occurrences are generated along often dipping sand layers, which are also conduits for methane-bearing fluid migrating from below the BSR in many settings. Fracture-filling type gas hydrate is an alternative mechanism for the occurrence of gas hydrate in mudstone-rich sediments above the BSR and found associated with the anticline in Area-B. This structural constraint of the hydrate-bearing fractures in the Area-B anticline suggest that the fracture dominated reservoirs possibly formed by anticlinal flexure, and the upward flow of methane-bearing fluids along porous sedimentary layers and the subsequent formation of pore-filling gas hydrate occurrences account for the range of gas hydrate accumulations discovered during NGHP-02.

Description technology for unconformity trap reservoirs with strong seismic reflections: a case of the Mesozoic unconformity reservoir in Changdi area, Jiyang depression

Description technology for unconformity trap reservoirs with strong seismic reflections: a case of the Mesozoic unconformity reservoir in Changdi area, Jiyang depression

Reconstruction of repeated Quaternary slope failures in the northern South China Sea

A series of Quaternary mass transport deposits (MTDs) with a total areal extent of ~11,000 km2 and a conservative volume estimate of ~1035 km3 are observed in the Pearl River Mouth Basin (PRMB), northern South China Sea (SCS), based on integrated analysis of extensive 2-D/3-D seismic data, multibeam bathymetric data and ODP/IODP wells. Unlike other large MTDs worldwide, the MTDs in this study are not directly sourced from slopes built on major deltaic depositional systems. Moreover, they are remarkable in that the whole PRMB is a giant debris flow corridor that has transected a broad subsided continental margin. The MTDs in this study have a large evacuation zone on the slope and a depositional zone in the ocean basin connected by a narrow pathway. Six events of MTDs (MTDa - MTDd on the slope and MTDo1 - MTDo2 in the ocean basin) have been identified as being separated by basal erosional surfaces defined by strong negative seismic reflections. Similar ages and volumes (depleted volume of MTDb on the slope and the depositional volume of MTDo2) suggest that MTDo1 and MTDo2 correlate with MTDa and MTDb, respectively. The major factor controlling the occurrence of slope failures is argued to be the presence of a key reflection package. This package is interpreted to consist of gas-charged, coarse-grained sediments deposited during a relative lowstand of sea level, overlain by a unit of fine-grained sediments. Other contributing factors that influenced the morphology and position of the MTDs include the presence of free gas, normal faulting in the slope sediments, and seafloor relief related to igneous activity. From MTDd to MTDb, slope failures are shown to have retrogressed upslope with a decreasing time interval between events. The events, dimensions, susceptible factors and formation processes of slope failures in the study area are demonstrated for the first time in the petroliferous basin of northern SCS. Their recognition is important for geo-hazard assessment (e.g. tsunami) in this area. Moreover, this study highlights that the lithostratigraphy of the slope succession, especially the coarse-grained sediments in the deepwater area, can be an important component for the occurrence of slope failures, and may be a key factor in slope stability on other continental margins too.

Seismic attributes for characterization of a heavy-oil shaly-sand reservoir in the Muglad Basin of South Sudan

Seismic attributes are often used to identify lithology and evaluate reservoir properties. However, interpretation based only on structural attributes and without knowledge of the Vp/Vs ratio can limit the ability to evaluate changes in heavy oil reservoirs. These limitations are often due to less obvious impedance differences. In order to investigate pieces of evidence of a heavy-oil shaly-sand reservoir from seismic data, besides geochemistry, we studied seismic attributes and characterized the reservoir using seismic stack data and well logging data. The study area was the Muglad rift basin in South Sudan. We conducted a seismic complex analysis to evaluate the target reservoir. To delineate the frequency responses of the different lithological units, we applied the spectral decomposition method to the target reservoir. The most unexpected result was continuous bands of strong seismic reflectors in the target reservoir, which extended across the borehole. Spectral decomposition analysis showed that the low-frequency zone of 25 Hz dominant frequency was consistent with instantaneous attributes. This approach can identify lithology, reveal frequency anomalies, and filter the stacked section into low- and high-frequency bands. The heavy-oil reservoir zones exhibited velocity attenuation and the amplitude was strongly frequency dependent.

Model identification and control of development of deeply buried paleokarst reservoir in the central Tarim Basin, northwest China

The paleokarst reservoirs of the Ordovician Yingshan formation, rich in oil and gas, are deeply buried in the central Tarim Basin, northwest China. Dozens of imaging well-logs in this region reveal five typical paleokarst features, including solution vugs, solution-enlarged fractures, filled caves, unfilled caves and collapsed caves, as well as two typical paleokarst structures located in different paleotopographic sites, including paleokarst vadose and phreatic zones. For seismic data, the large wave impedance contrast between the paleocave system and the surrounding rocks leads to a strong seismic reflection, which is highlighted as a bead-like 'bright spot' in a seismic section. By quantitatively estimating the seismic resolution limits of deep seismic reflections, a single paleocave cannot be identified from a seismic profile, and the bead-like reflection represents an entire paleocave complex. The spectral decomposition technique was employed to depict the planar shape and semi-quantitatively measure the size of the paleocave complexes. The results indicate that the sizes of the paleokarst caves are all small, and most of the karst caves are nearly completely filled by clay and calcite. The small cave size and the effective support of cave fills for the overlying strata mean that some individual paleocaves in a paleocave complex are preserved at a burial depth of more than 6000 m. Paleotopography and faults strongly impact the distribution of paleokarst reservoirs. Well-developed paleokarst reservoirs are generally located in paleotopographic highlands and on slopes, and for a specific paleotopographic site, the distribution of paleokarst reservoirs is obviously controlled by NW-SE trending faults. The most favorable area for paleokarst development is the Tazhong No. 10 fault zone, a faulted anticline bounded by two NW-SE trending back thrusts.

Internal seismic stratigraphy of the Messinian evaporites across the northern sector of the eastern Mediterranean Sea

A dense grid of high-resolutions multichannel seismic reflection profiles from the northern basins of the eastern Mediterranean (Rhodes, Finike, Antalya, Cilicia, Adana and Latakia basin) allow documenting a Messinian evaporite succession of variable thickness, bounded at its base and top by two strong seismic reflectors named respectively BS/BES (or the N-reflector) and TS/TES (or the M-reflector). Messinian evaporite succession is thickest in the deep Antalya Basin (2000–2400 ms twt), but is notably thinner in across the Rhodes and Finike basins in the west and the Adana, Cilicia, Latakia basins in the east. Five acoustically distinctive Messinian subunits are identified in the Antalya Basin (2e–2a): three of these subunits (2e, 2c, 2a) are acoustically stratified with strong laterally continuous reflections while two (2d, 2b) are acoustically transparent with few coherent reflections. Interval velocities and the internal acoustic character of subunits are used in conjunction with correlations with borehole data to suggest the lithological composition of the subunits. Subunit 2e is possibly composed of detrital sediment and resedimented primary lower gypsum, associated with the intensive subaerial erosion immediately after the onset of the Messinian drawdown. Correlations with the DSDP Sites 375 and 376 suggest that the acoustically transparent subunits 2d and 2b are likely composed of massive halite with interbeds of anhydrite, magnesium and potassium salts and minor siliciclastics. Subunit 2c is probably composed of interbedded siliciclastic and evaporites and/or trapped fluids. Subunit 2a probably corresponds to the gypsum horizons interbedded with carbonates and siliciclastics. Comparison between the Messinian subunits in the eastern and western Mediterranean shows remarkable acoustic similarities. In both regions there is a stratified lower subunit (2e), sequentially overlain by a thick transparent subunit (2d), thin stratified subunit (2c) and thick transparent subunit (2b), which are all capped by a mainly stratified subunit (2a). These similarities strongly suggest similarities in the sedimentary evolution of basins in the eastern and western Mediterranean.

The syn‐depositional fault control of the evolution of axial meandering river delta‐sublacustrine fan deposits during the Ed2x deposition in the northern Liaodong Bay, Bohai Bay Basin, NE China

This research study presents the results of an integrated study employing core samples, wireline logs, and seismic dataset collected in the study area for analysing the syn‐depositional fault's control of the evolution of the axial meandering river delta‐sublacustrine fan deposits during the Ed2x depositional period. The results indicate that the delta deposits that had developed during a retrogressive period, with the characteristics of retrogressive stacking patterns, had resulted from a high subsidence rate caused by the syn‐depositional faults with large fault throw. These appeared to be characterized by strong amplitude, low frequency, and continuous seismic reflections on the seismic profiles, and high‐amplitude serrated cylinder‐shaped stacking patterns in the wireline logs, with no obvious sedimentary structures. In contrast, the delta deposits developed during a progressive period and characterized by progressive stacking patterns were determined to have resulted from a low subsidence rate caused by the syn‐depositional faults with small fault throw. They were found to be characterized by weak‐to‐medium amplitude vermicule‐shaped seismic reflections, as well as low‐amplitude bell‐shaped or funnel‐shaped stacking patterns, with soft‐sediment deformation structures.