Lessons Learnt Reprocessing a Noisy Onshore 3D Seismic Survey

Abstract An 1,100 km2 400-fold seismic survey was acquired over some of the largest sand dunes in UAE in 2007. Large sand dunes generate many challenges for seismic processing due to the irregular data acquisition, large statics caused by the significant difference between the sand and sabkha velocities, and a massive amount of reverberation noise that hides the signal in the data. Occidental and ADNOC Sour Gas reprocessed this survey from January 2019 to August 2020 to overcome the challenges of the strong sand dune noise. For the first time, it was processed through prestack depth migration (PSDM). The primary objectives of the reprocessing project were to get an accurate PSDM volume that tied all of the available well control data; and to derive as accurate seismic amplitudes as possible over the target reservoir interval from near to far offsets to enable elastic inversion for reservoir porosity and net-thickness prediction. Whilst the reprocessing project achieved the project objectives and generated good subsurface images, it did not run as smoothly as hoped, despite being processed by one of the premier multinational processing companies. The extremely large sand dunes, which are present across most of the survey area, created major imaging problems. Key technical lessons learnt during reprocessing included: (1) CRS errors occurred sporadically during acquisition, requiring correction; (2) the sand curve (Liner, 2008) worked well for sand dune static corrections for this data set; (3) near surface statics changed whilst the survey was acquired by up to 6 ms - each shot station needed to be corrected for these statics changes because the shot stations were acquired twice with a symmetric split recording spread; and (4) the contractor's standard post-migration processing sequence (gather flattening, radon, noise attenuation, stack) did not work well for this very noisy data set. Next time we work with similar data and require a high quality result, we know to double the estimated project timeline as every step in the processing sequence takes much longer than expected when the signal-to-noise ratio of the data is very low. The novelty of this work was that we obtained large improvements in the seismic stack by applying offline gather conditioning before calculating trim statics to optimally flatten the very noisy migrated offset vector tile (OVT) gathers, prior to running the final noise attenuation and stacking workflows. Without this offline gather conditioning, the trim statics workflow mostly aligned the noise and damaged the stack.

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The Role of Legacy Seismic in Enhancing the Image in the Current Seismic Data – Abu Dhabi: Case Study

10.2118/207734-ms ◽

2021 ◽

Author(s):

Ramy Elasrag ◽

Thuraya Al Ghafri ◽

Faaeza Al Katheer ◽

Yousuf Al-Aufi ◽

Ivica Mihaljevic ◽

...

Keyword(s):

Seismic Data ◽

Seismic Survey ◽

Surface Model ◽

Noise Attenuation ◽

Near Surface ◽

Velocity Models ◽

Multiple Attenuation ◽

Seismic Image ◽

Seismic Acquisition

Abstract Acquiring surface seismic data can be challenging in areas of intense human activities, due to presence of infrastructures (roads, houses, rigs), often leaving large gaps in the fold of coverage that can span over several kilometers. Modern interpolation algorithms can interpolate up to a certain extent, but quality of reconstructed seismic data diminishes as the acquisition gap increases. This is where vintage seismic acquisition can aid processing and imaging, especially if previous acquisition did not face the same surface obstacles. In this paper we will present how the legacy seismic survey has helped to fill in the data gaps of the new acquisition and produced improved seismic image. The new acquisition survey is part of the Mega 3D onshore effort undertaken by ADNOC, characterized by dense shot and receiver spacing with focus on full azimuth and broadband. Due to surface infrastructures, data could not be completely acquired leaving sizable gap in the target area. However, a legacy seismic acquisition undertaken in 2014 had access to such gap zones, as infrastructures were not present at the time. Legacy seismic data has been previously processed and imaged, however simple post-imaging merge would not be adequate as two datasets were processed using different workflows and imaging was done using different velocity models. In order to synchronize the two datasets, we have processed them in parallel. Data matching and merging were done before regularization. It has been regularized to radial geometry using 5D Matching Pursuit with Fourier Interpolation (MPFI). This has provided 12 well sampled azimuth sectors that went through surface consistent processing, multiple attenuation, and residual noise attenuation. Near surface model was built using data-driven image-based static (DIBS) while reflection tomography was used to build the anisotropic velocity model. Imaging was done using Pre-Stack Kirchhoff Depth Migration. Processing legacy survey from the beginning has helped to improve signal to noise ratio which assisted with data merging to not degrade the quality of the end image. Building one near surface model allowed both datasets to match well in time domain. Bringing datasets to the same level was an important condition before matching and merging. Amplitude and phase analysis have shown that both surveys are aligned quite well with minimal difference. Only the portion of the legacy survey that covers the gap was used in the regularization, allowing MPFI to reconstruct missing data. Regularized data went through surface multiple attenuation and further noise attenuation as preconditioning for migration. Final image that is created using both datasets has allowed target to be imaged better.

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Regularization and datuming of seismic data by weighted, damped least squares

Geophysics ◽

10.1190/1.2235616 ◽

2006 ◽

Vol 71 (5) ◽

pp. U67-U76 ◽

Cited By ~ 7

Author(s):

Robert J. Ferguson

Keyword(s):

Least Squares ◽

Seismic Data ◽

Synthetic Data ◽

Real Data ◽

Structural Data ◽

Irregular Sampling ◽

Data Set ◽

Near Surface ◽

Damped Least Squares ◽

Wavefield Extrapolation

The possibility of improving regularization/datuming of seismic data is investigated by treating wavefield extrapolation as an inversion problem. Weighted, damped least squares is then used to produce the regularized/datumed wavefield. Regularization/datuming is extremely costly because of computing the Hessian, so an efficient approximation is introduced. Approximation is achieved by computing a limited number of diagonals in the operators involved. Real and synthetic data examples demonstrate the utility of this approach. For synthetic data, regularization/datuming is demonstrated for large extrapolation distances using a highly irregular recording array. Without approximation, regularization/datuming returns a regularized wavefield with reduced operator artifacts when compared to a nonregularizing method such as generalized phase shift plus interpolation (PSPI). Approximate regularization/datuming returns a regularized wavefield for approximately two orders of magnitude less in cost; but it is dip limited, though in a controllable way, compared to the full method. The Foothills structural data set, a freely available data set from the Rocky Mountains of Canada, demonstrates application to real data. The data have highly irregular sampling along the shot coordinate, and they suffer from significant near-surface effects. Approximate regularization/datuming returns common receiver data that are superior in appearance compared to conventional datuming.

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MAX-DOAS observations of aerosols, formaldehyde and nitrogen dioxide in the Beijing area: comparison of two profile retrieval approaches

Atmospheric Measurement Techniques ◽

10.5194/amt-8-941-2015 ◽

2015 ◽

Vol 8 (2) ◽

pp. 941-963 ◽

Cited By ~ 47

Author(s):

T. Vlemmix ◽

F. Hendrick ◽

G. Pinardi ◽

I. De Smedt ◽

C. Fayt ◽

...

Keyword(s):

Standard Deviation ◽

A Priori ◽

Systematic Bias ◽

Aerosol Extinction ◽

Data Set ◽

Near Surface ◽

Beijing Area ◽

Least Squares Fit ◽

Relative Differences ◽

Good Agreement

Abstract. A 4-year data set of MAX-DOAS observations in the Beijing area (2008–2012) is analysed with a focus on NO2, HCHO and aerosols. Two very different retrieval methods are applied. Method A describes the tropospheric profile with 13 layers and makes use of the optimal estimation method. Method B uses 2–4 parameters to describe the tropospheric profile and an inversion based on a least-squares fit. For each constituent (NO2, HCHO and aerosols) the retrieval outcomes are compared in terms of tropospheric column densities, surface concentrations and "characteristic profile heights" (i.e. the height below which 75% of the vertically integrated tropospheric column density resides). We find best agreement between the two methods for tropospheric NO2 column densities, with a standard deviation of relative differences below 10%, a correlation of 0.99 and a linear regression with a slope of 1.03. For tropospheric HCHO column densities we find a similar slope, but also a systematic bias of almost 10% which is likely related to differences in profile height. Aerosol optical depths (AODs) retrieved with method B are 20% high compared to method A. They are more in agreement with AERONET measurements, which are on average only 5% lower, however with considerable relative differences (standard deviation ~ 25%). With respect to near-surface volume mixing ratios and aerosol extinction we find considerably larger relative differences: 10 ± 30, −23 ± 28 and −8 ± 33% for aerosols, HCHO and NO2 respectively. The frequency distributions of these near-surface concentrations show however a quite good agreement, and this indicates that near-surface concentrations derived from MAX-DOAS are certainly useful in a climatological sense. A major difference between the two methods is the dynamic range of retrieved characteristic profile heights which is larger for method B than for method A. This effect is most pronounced for HCHO, where retrieved profile shapes with method A are very close to the a priori, and moderate for NO2 and aerosol extinction which on average show quite good agreement for characteristic profile heights below 1.5 km. One of the main advantages of method A is the stability, even under suboptimal conditions (e.g. in the presence of clouds). Method B is generally more unstable and this explains probably a substantial part of the quite large relative differences between the two methods. However, despite a relatively low precision for individual profile retrievals it appears as if seasonally averaged profile heights retrieved with method B are less biased towards a priori assumptions than those retrieved with method A. This gives confidence in the result obtained with method B, namely that aerosol extinction profiles tend on average to be higher than NO2 profiles in spring and summer, whereas they seem on average to be of the same height in winter, a result which is especially relevant in relation to the validation of satellite retrievals.

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Structural interpretation using refraction velocities from marine seismic surveys

Geophysics ◽

10.1190/1.1442026 ◽

1986 ◽

Vol 51 (1) ◽

pp. 12-19 ◽

Cited By ~ 3

Author(s):

James F. Mitchell ◽

Richard J. Bolander

Keyword(s):

Sedimentary Rock ◽

Seismic Energy ◽

Seismic Survey ◽

Subsurface Structure ◽

Seismic Reflection Data ◽

Near Surface ◽

Reflection Data ◽

Wide Range ◽

Streamer Length ◽

Set Up

Subsurface structure can be mapped using refraction information from marine multichannel seismic data. The method uses velocities and thicknesses of shallow sedimentary rock layers computed from refraction first arrivals recorded along the streamer. A two‐step exploration scheme is described which can be set up on a personal computer and used routinely in any office. It is straightforward and requires only a basic understanding of refraction principles. Two case histories from offshore Peru exploration demonstrate the scheme. The basic scheme is: step (1) shallow sedimentary rock velocities are computed and mapped over an area. Step (2) structure is interpreted from the contoured velocity patterns. Structural highs, for instance, exhibit relatively high velocities, “retained” by buried, compacted, sedimentary rocks that are uplifted to the near‐surface. This method requires that subsurface structure be relatively shallow because the refracted waves probe to depths of one hundred to over one thousand meters, depending upon the seismic energy source, streamer length, and the subsurface velocity distribution. With this one requirement met, we used the refraction method over a wide range of sedimentary rock velocities, water depths, and seismic survey types. The method is particularly valuable because it works well in areas with poor seismic reflection data.

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Seedling desiccation tolerance of Leymus racemosus (Poaceae) (wild rye), a perennial sand-dune grass inhabiting the Junggar Basin of Xinjiang, China

Seed Science Research ◽

10.1079/ssr2004172 ◽

2004 ◽

Vol 14 (2) ◽

pp. 233-239 ◽

Cited By ~ 6

Author(s):

Zhenying Huang ◽

Yitzchak Gutterman

Keyword(s):

Desiccation Tolerance ◽

Sand Dune ◽

Sand Dunes ◽

Perennial Grass ◽

Junggar Basin ◽

Seedling Development ◽

Ecological Implications ◽

Leymus Racemosus ◽

Seedling Desiccation Tolerance ◽

Dune Grass

Leymus Racemosus, The Mammoth Wild Rye, Is A Rhizomatous Perennial Grass, Mainly Distributed In The Moving Or Semi-Stabilized Sand Dunes In Deserts Of The Junggar Basin In Xinjiang, China. The Revival Ability Of The Young Seedling After Periods Of Desiccation Can Be Influenced By Several Factors: (1) The Stage Of Seedling Development – The Later The Stage At Dehydration, The Longer The Root Length And The Lower Is The Percentage Of Seedlings That Survive; (2) The Length Of The Period Of Desiccation – The Longer The Period That The Seedlings Are Under Desiccation, The Lower Is The Percentage Of Seedlings That Survive; (3) Endosperm Size – The Smaller The Proportion Of Endosperm That Remains In The Caryopses, The Lower Is The Percentage Of Seedlings That Revive, Determined By (A) The Stage Of Seedling Development, And (B) The Proportion Of The Endosperm That Is Removed By Cutting; And (4) Caryopsis Size – The Larger The Polymorphic Caryopses, The Higher Is The Percentage Of Young Seedlings That Revive From Periods Of Desiccation. The Physiological And Ecological Implications Of L. Racemosus Seedling Desiccation Tolerance Are That Under Extreme Desert And Unpredictable Environmental Conditions, The Chances Of Seedling Establishment Are Increased.

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A hybrid regularization scheme for the inversion of magnetotelluric data from natural and controlled sources to layer and distortion parameters

Geophysics ◽

10.1190/geo2012-0018.1 ◽

2012 ◽

Vol 77 (4) ◽

pp. E301-E315 ◽

Cited By ~ 10

Author(s):

Thomas Kalscheuer ◽

Juliane Hübert ◽

Alexey Kuvshinov ◽

Tobias Lochbühler ◽

Laust B. Pedersen

Keyword(s):

Transfer Functions ◽

Homogeneous Distribution ◽

Stable Configuration ◽

Inversion Algorithm ◽

Magnetotelluric Data ◽

Data Set ◽

Near Surface ◽

Minimum Solution ◽

Magnetic Transfer ◽

Distortion Parameters

Magnetotelluric (MT), radiomagnetotelluric (RMT), and, in particular, controlled-source audiomagnetotelluric (CSAMT) data are often heavily distorted by near-surface inhomogeneities. We developed a novel scheme to invert MT, RMT, and CSAMT data in the form of scalar or tensorial impedances and vertical magnetic transfer functions simultaneously for layer resistivities and electric and magnetic galvanic distortion parameters. The inversion scheme uses smoothness constraints to regularize layer resistivities and either Marquardt-Levenberg damping or the minimum-solution length criterion to regularize distortion parameters. A depth of investigation range is estimated by comparing layered model sections derived from first- and second-order smoothness constraints. Synthetic examples demonstrate that earth models are reconstructed properly for distorted and undistorted tensorial CSAMT data. In the inversion of scalar CSAMT data, such as the determinant impedance or individual tensor elements, the reduced number of transfer functions inevitably leads to increased ambiguity for distortion parameters. As a consequence of this ambiguity for scalar data, distortion parameters often grow over the iterations to unrealistic absolute values when regularized with the Marquardt-Levenberg scheme. Essentially, compensating relationships between terms containing electric and/or magnetic distortion are used in this growth. In a regularization with the minimum solution length criterion, the distortion parameters converge into a stable configuration after several iterations and attain reasonable values. The inversion algorithm was applied to a CSAMT field data set collected along a profile over a tunnel construction site at Hallandsåsen, Sweden. To avoid erroneous inverse models from strong anthropogenic effects on the data, two scalar transfer functions (one scalar impedance and one scalar vertical magnetic transfer function) were selected for inversion. Compared with a regularization of distortion parameters with the Marquardt-Levenberg method, the minimum-solution length criterion yielded smaller absolute values of distortion parameters and a horizontally more homogeneous distribution of electrical conductivity.

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Decennial multi-approach monitoring of thermo-hydro-mechanical processes, Kammstollen outdoor laboratory, Zugspitze (Germany)

10.5194/egusphere-egu21-13815 ◽

2021 ◽

Author(s):

Riccardo Scandroglio ◽

Till Rehm ◽

Jonas K. Limbrock ◽

Andreas Kemna ◽

Markus Heinze ◽

...

Keyword(s):

Electrical Resistivity ◽

Electrical Resistivity Tomography ◽

Environmental Research ◽

Permafrost Degradation ◽

Earth Surface ◽

Data Set ◽

Resistivity Tomography ◽

Near Surface ◽

Permafrost Rock ◽

Austrian Alps

The warming of alpine bedrock permafrost in the last three decades and consequent reduction of frozen areas has been well documented. Its consequences like slope stability reduction put humans and infrastructures at high risk. 2020 in particular was the warmest year on record at 3000m a.s.l. embedded in the warmest decade.Recently, the development of electrical resistivity tomography (ERT) as standard technique for quantitative permafrost investigation allows extended monitoring of this hazard even allowing including quantitative 4D monitoring strategies (Scandroglio et al., in review). Nevertheless thermo-hydro-mechanical dynamics of steep bedrock slopes cannot be totally explained by a single measurement technique and therefore multi-approach setups are necessary in the field to record external forcing and improve the deciphering of internal responses.The Zugspitze Kammstollen is a 850m long tunnel located between 2660 and 2780m a.s.l., a few decameters under the mountain ridge. First ERT monitoring was conducted in 2007 (Krautblatter et al., 2010)&#160;and has been followed by more than one decade of intensive field work. This has led to the collection of a unique multi-approach data set of still unpublished data. Continuous logging of environmental parameters such as rock/air temperatures and water infiltration through joints as well as a dedicated thermal model (Schr&#246;der and Krautblatter, in review)&#160;provide important additional knowledge on bedrock internal dynamics. Summer ERT and seismic refraction tomography surveys with manual and automated joints&#8217; displacement measurements on the ridge offer information on external controls, complemented by three weather stations and a 44m long borehole within 1km from the tunnel.Year-round access to the area enables uninterrupted monitoring and maintenance of instruments for reliable data collection. &#8220;Precisely controlled natural conditions&#8221;, restricted access for researchers only and logistical support by Environmental Research Station Schneefernerhaus, make this tunnel particularly attractive for developing benchmark experiments. Some examples are the design of induced polarization monitoring, the analysis of tunnel spring water for isotopes investigation, and the multi-annual mass monitoring by means of relative gravimetry.Here, we present the recently modernized layout of the outdoor laboratory with the latest monitoring results, opening a discussion on further possible approaches of this extensive multi-approach data set, aiming at understanding not only permafrost thermal evolution but also the connected thermo-hydro-mechanical processes.&#160;&#160;Krautblatter, M. et al.&#160;(2010) &#8216;Temperature-calibrated imaging of seasonal changes in permafrost rock walls by quantitative electrical resistivity tomography (Zugspitze, German/Austrian Alps)&#8217;, Journal of Geophysical Research: Earth Surface, 115(2), pp. 1&#8211;15. doi: 10.1029/2008JF001209.Scandroglio, R. et al.&#160;(in review) &#8216;4D-Quantification of alpine permafrost degradation in steep rock walls using a laboratory-calibrated ERT approach (in review)&#8217;, Near Surface Geophysics.Schr&#246;der, T. and Krautblatter, M. (in review) &#8216;A high-resolution multi-phase thermo-geophysical model to verify long-term electrical resistivity tomography monitoring in alpine permafrost rock walls (Zugspitze, German/Austrian Alps) (submitted)&#8217;, Earth Surface Processes and Landforms.

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Simulation of a seismic survey with combined surface and in-mine data acquisition for imaging steeply dipping ore veins

10.5194/egusphere-egu21-15350 ◽

2021 ◽

Author(s):

Olaf Hellwig ◽

Stefan Buske

Keyword(s):

Finite Difference ◽

Message Passing Interface ◽

Seismic Survey ◽

Difference Operators ◽

Mining District ◽

Prestack Depth Migration ◽

Data Set ◽

Depth Migration ◽

Difference Grid ◽

Triangulated Surfaces

The polymetallic, hydrothermal deposit of the Freiberg mining district in the southeastern part of Germany is characterised by ore veins that are framed by Proterozoic orthogneiss. The ore veins consist mainly of quarz, sulfides, carbonates, barite and flourite, which are associated with silver, lead and tin. Today the Freiberg University of Mining and Technology is operating the shafts Reiche Zeche and Alte Elisabeth for research and teaching purposes with altogether 14 km of accessible underground galleries. The mine together with the most prominent geological structures of the central mining district are included in a 3D digital model, which is used in this study to study seismic acquisition geometries that can help to image the shallow as well as the deeper parts of the ore-bearing veins. These veins with dip angles between 40&#176; and 85&#176; are represented by triangulated surfaces in the digital geological model. In order to import these surfaces into our seismic finite-difference simulation code, they have to be converted into bodies with a certain thickness and specific elastic properties in a first step. In a second step, these bodies with their properties have to be discretized on a hexahedral finite-difference grid with dimensions of 1000 m by 1000 m in the horizontal direction and 500 m in the vertical direction. Sources and receiver lines are placed on the surface along roads near the mine. A Ricker wavelet with a central frequency of 50 Hz is used as the source signature at all excitation points. Beside the surface receivers, additional receivers are situated in accessible galleries of the mine at three different depth levels of 100 m, 150 m and 220 m below the surface. Since previous mining activities followed primarily the ore veins, there are only few pilot-headings that cut through longer gneiss sections. Only these positions surrounded by gneiss are suitable for imaging the ore veins. Based on this geometry, a synthetic seismic data set is generated with our explicit finite-difference time-stepping scheme, which solves the acoustic wave equation with second order accurate finite-difference operators in space and time. The scheme is parallelised using a decomposition of the spatial finite-difference grid into subdomains and Message Passing Interface for the exchange of the wavefields between neighbouring subdomains. The resulting synthetic seismic shot gathers are used as input for Kirchhoff prestack depth migration as well as Fresnel volume migration in order to image the ore veins. Only a top mute to remove the direct waves and a time-dependent gain to correct the amplitude decay due to the geometrical spreading are applied to the data before the migration. The combination of surface and in-mine acquisition helps to improve the image of the deeper parts of the dipping ore veins. Considering the limitations for placing receivers in the mine, Fresnel volume migration as a focusing version of Kirchhoff prestack depth migration helps to avoid migration artefacts caused by this sparse and limited acquisition geometry.

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Allelopathic potential of Artemisia campestris ssp. caudata on Lake Huron sand dunes

Canadian Journal of Botany ◽

10.1139/b97-902 ◽

1997 ◽

Vol 75 (11) ◽

pp. 1903-1912 ◽

Cited By ~ 5

Author(s):

Kyeong W. Yun ◽

M. A. Maun

Keyword(s):

Seed Germination ◽

Seedling Growth ◽

Mycorrhizal Fungi ◽

Sand Dune ◽

Sand Dunes ◽

Dry Weight ◽

Fungal Colonization ◽

Test Species ◽

Allelopathic Potential ◽

Artemisia Campestris

Greenhouse studies were conducted to test allelopathic effects of Artemisia campestris ssp. caudata on seed germination and seedling growth of several sand-dune species and colonization by mycorrhizal fungi. The aqueous extracts of A. campestris showed no inhibitory effect on seed germination, seedling elongation, or dry-weight growth of plants at lower concentrations (10 and 50%), but 100% concentration of the extracts caused varying degrees of inhibition depending on the test species. The mixing of dry leaves of seedlings of A. campestris to the sand showed severe inhibition of Elymus canadensis seedlings. The percent germination of test species in soil from the rhizosphere of A. campestris was significantly lower than that of the control. The leaf area and dry weight were also lower but the differences were not significant. The aqueous extract inhibited mycorrhizal fungal colonization in roots of three sand-dune grasses. Key words: allelopathy, Artemisia campestris ssp. caudata, seed germination, seedling growth, mycorrhizal fungi.

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ADAPTATION OF THE METHOD OF CHROMATOGRAPHIC ANALYSIS OF GASOLINE TO ACHIEVE PURPOSES OF GEOCHEMICAL PROSPECTING FOR OIL AND GAS

Oil and Gas Studies ◽

10.31660/0445-0108-2018-6-16-23 ◽

2019 ◽

pp. 16-23

Author(s):

A. R. Kurchikov ◽

R. I. Timshanov ◽

E. A. Ustimenko

Keyword(s):

Chromatographic Analysis ◽

Oil And Gas ◽

Seismic Survey ◽

Near Surface ◽

Petroleum Potential ◽

Geological Interpretation ◽

Gasoline Hydrocarbons ◽

Snow Water ◽

Individual Hydrocarbons ◽

Background Content

Geochemical survey is commonly applied during geological exploration to predict petroleum potential of large areas and to estimate the content of traps identified by the results of seismic survey. C1-C6 hydrocarbon concentrations in samples of surface and subsurface air, soil, snow, water, etc. are used as predictive indicators. At the exploration stage the capabilities of geochemical methods can be significantly expanded by comparing the content of gasoline hydrocarbons in samples of formation fluids and in samples of near-surface sediments. The method of chromatographic analysis of gasolines Chromatec Gazolin has been adapted for sample analysis. The taken measures to increase the sensitivity allowed us to register individual hydrocarbons C1-C10 in concentrations up to 0,01 ppb, which is obviously lower than their background content in the oil prospect areas. The revealed patterns are used in the geological interpretation of geochemical distributions based on theoretical ideas about the subvertical migration of hydrocarbons from the reservoir to the surface.

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