PSTM Image Research Articles

Seismic models of detachment and faulted detachment folds

Abstract Detachment folds constitute a common structural style in fold-thrust belts and typically form in stratigraphic packages defined by relatively competent cover units underlain by ductile units. PSTM images through these structures contain artifacts resulting from lateral velocity variations between the cover sediments and the core units, steep dips, and complex fold-fault relationships, making them difficult to interpret. Understanding these artifacts and pitfalls in PSTM seismic models enables improved interpretations of PSTM data in natural structures. We conducted 2D seismic modeling for detachment folds models to study the effect of these complexities on PSTM data. Seismic “pull-ups” and “push-downs” of the strata underneath the salt or shale substrate are related to the relative velocities of the cover sediments and the core units. A specific example of an Appalachian Plateau structure demonstrates how the relative velocities can influence the seismic image. Progressive evolution of the structures to disharmonic and lift off folds results in enhanced “pull-ups” and poorer imaging of steep limb segments. For faulted detachment folds, faulted limbs and footwall zones are poorly imaged and marked by wide low-reflectivity bands, making fault interpretation difficult.

An Analysis of the Accuracy of Time Domain 3D Image Geology Model Resulted from PSTM and Depth Domain 3D Image Geology Model Resulted from PSDM in Oil and Gas Exploration

This study aims to obtain a geological model which is close to the truth and compare accuracy between the time domain 3D image of the PSTM results with the depth domain 3D image of PSDM results. There are 3 parameters to determine the accuracy of an interval velocity model in the production of a geology model: depth gathering that is already flat, semblance that has concurred with zero residual move-out axes, and depth image which conforms to the marker (well seismic tie). The analytical method employed is Horizon Based Tomography, which is a method to correct the seismic wave travel time error along the analyzed horizon. Reducing errors in the travel time of the seismic wave will decrease depth errors. This improvement is expected to provide correct information about subsurface geological conditions. The results showed that the depth domain image generated by the PSDM process represents the actual geological model better than time domain image produced by the PSTM process, evidenced by the sharpening of the reflector continuity, reduction of pull-up effect, and high resolution.

Seismic imaging of thrust fault structures in Zagros Iranian oil fields, from surface and well data

The Aghajari field is located in the Dezful Embayment, one of major structural zone in Zagros fold-thrust belt containing most of Iranian oil fields. The late Oligocene-early Miocene Asmari formation is one of the main reservoir rocks of SW Iran with several decades of production history from different oil fields in the region. The objective of the present Ph-D thesis is to improve the structural understanding of the Aghajari field using 3D seismic imaging for the first time, and well seismic. the lack of seismic response of the dipping horizons in the reservoir oil zone constitutes a major identified difficulty. Given the dip values of 30° to 45° of the reflectors in the pay zone, the P-P reflections are expected to appear mainly on the horizontal components of the VSP data, justifying the processing of the 3 components (3C). An innovative method of 3C VSP orientation was developed. The 3C VSP results confirm the presence of dipping reflectors, with dip values in agreement with surface seismic, on the eastern flank of the structure. Several new approaches of seismic imaging have been applied in order to investigate and improve the reservoir illumination. The “Beyond Dix” workflow of CGGVERITAS was applied to land seismic data for the first time; this process enables to build a depth/velocity model from the dips read on the PSTM image (pre-stack time migration), and the residual move out corrections. The depth migrated image is improved in the overburden, unfortunately not at reservoir level, due to low signal to noise, interference with multiples, thus poor dip and velocity determination in the reservoir interval. A new Kirchhoff PSTM prototype technique was developed in IFP, allowing a selection of azimuth sector, offset range and geological dip, with automatic optimization of the local dip. The results from this new technique show clear improvements in dip determination and signal to noise ratio of the migrated image. The additional study of another exploration anticline in Zagros, using surface seismic, well data, VSP and geological information illustrates how an integrated data analysis can improve the understanding of deep structures, and therefore help making more reliable drilling decisions, if achieved timely. In Zagros, Surface seismic is definitely a good tool for the structural exploration, but its application to fine reservoir studies has not been demonstrated, in spite of the efforts achieved in the present study.

Seismic modeling for structure interpretation in Venezuela's Sipororo Field

We used seismic modeling and imaging to verify the structural interpretation of a seismic section from a geologically complex area in an onshore field in Venezuela. By analyzing PSTM image problems and combining with nearby well information, we derived a new structure/velocity model, and reproduced the image problems by prestack time migration of the synthetic seismic data acquired from this new structure model.

Study of the features of PSTM images by means of a perturbative approach

We present a perturbative expression of the scattered near field. A comparison with a rigorous simulation using an integral method allows to determine its range of validity. The numerical results are then compared with near-field data reported by de Fornel et al. and a good qualitative agreement is observed. The perturbative expression of the scattered field provides an insight into the dependence of the near-field measurements upon the plane of incidence and the height of the tip. Finally, we show that it is possible to retrieve the exact profile from the near-field data with a subwavelength resolution.