Subsalt Prospect Research Articles

Critical reflection illumination analysis

Poor imaging is frequently observed in many subsalt regions, making the subsalt stratigraphy interpretation and prospect evaluation challenging. We propose a critical reflection illumination analysis to evaluate subsalt illumination in areas where high-velocity contrasts create illumination and imaging shadows. Critical reflection often occurs at the base or flank of salt bodies. If critical reflection occurred, continued iterations of processing and imaging would generate little, if any, improvement in imaging results. Similarly, increasing the offset/azimuth of the acquisition would offer limited or no advantage. We introduce the critical reflection illumination map and illumination rose diagram to efficiently and effectively evaluate the probability of critical reflection for the target. This analysis can help avoid expensive processing, imaging, and acquisition efforts for areas that are in the critical/postcritical reflection regime. Critical reflection illumination analysis can also be applied to other high-velocity contrast scenarios.

Visibility analysis for optimal imaging of target areas and its application to a Gulf of Mexico deep-water data set

Marine wide-azimuth data in the Gulf of Mexico, reverse time migration (RTM) and anisotropic velocity models have led to significant improvement in subsalt imaging. However, imaging of some steeply dipping subsalt targets such as three-way closures against salt is still difficult. This can be attributed to poor illumination and noise contaminations from various shot records. We apply the visibility analysis method that quantitatively determines which shot records contribute most energy on a specific subsalt prospect area. As a result we selectively migrate only those shot records thereby reducing noise contamination from low energy contributing shot records, improving signal continuity and better trap definition in the target area. Like conventional illumination analysis, the computation takes into account the overburden velocity distribution, acquisition geometry, target reflectivity and dip angle. We used 2D and 3D synthetic data examples to test the concepts and applicability of the method. A Gulf of Mexico case study example using wide-azimuth data demonstrated its use in an industry scale project. It is shown that for the particular 60°–65° subsalt target of interest only 30% of the wide-azimuth shot records are sufficient for the imaging. By reducing noise, the image results show significant improvement in the subsalt area compared to the full shot record RTM volume.

Inversion of P-wave VSP data for local anisotropy: Theory and case study

We discuss, improve, and apply the slowness-polarization method for estimating local anisotropy from VSP data. Although the idea of fitting a given anisotropic model to the apparent slownesses measured along a well and polarization vectors recorded by three-component downhole geophones is hardly new, we extend the area of applicability of the technique and make the anisotropic inversion more robust by eliminating the most operationally difficult and noisy portion of the data, the shear waves. We show that the shear-wave velocity is actually unnecessary for fitting the slowness-of-polarization dependence of P-wave VSP data. For the most common geometry of a vertical borehole in a vertically transversely isotropic subsurface, such data are governed by the P-wave vertical velocity [Formula: see text] and two quantities, [Formula: see text] and [Formula: see text], that describe the influence of anisotropy. These quantities depend on conventional anisotropic coefficients [Formula: see text] and [Formula: see text] and absorb the S-wave velocity. We apply the developed theory to a 2D walkaway VSP acquired over a subsalt prospect in the Gulf of Mexico. Our data set contains geophones placed both inside the salt and beneath it, allowing us to estimate the anisotropy of different rock formations. We find the salt to be nearly isotropic in the examined [Formula: see text] [Formula: see text] depth interval. In contrast, the sediments below the salt exhibit substantial anisotropy. While the physical origins of subsalt anisotropy are still to be fully understood, we observe a clear correlation between lithology and the values of [Formula: see text] and [Formula: see text]: both anisotropic coefficients are greater in shales and smaller in the sandier portion of the well.

3D wave equation PSDM optimizes and improves imaging of subsalt prospects

The convergence of advanced computing technologies and innovative algorithm designs has made it possible to provide 3D wave equation depth migration methods to many explorationists. It is well known that the wave equation method can provide the full wavefield solution to most imaging problems. On the other hand, the Kirchhoff technique, which has become the industry's standard prestack depth migration (preSDM) algorithm, has some inherent limitations, such as diverging raypaths in steeply dipping areas where high lateral velocity contrasts exist. More accurate imaging in such challenging geologic conditions can often be addressed by a wave equation (WE) depth migration.

ABSTRACT: Hickory Field: Integrated Technologies Revitalize a Sub-salt Prospect

Abstract The integration of improved seismic technologies and new geologic structural and stratigraphic models can often revitalize a prospect. This is a case history of the Hickory subsalt prospect in offshore Louisiana where 3-D raytrace modeling, new geologic structural and depositional models, and improved seismic processing techniques were employed. They developed a prospect which when drilled became Hickory Field. In 1994, Anadarko and Phillips drilled Teak Prospect, which was a non-commercial subsalt discovery in South Timbalier Block 260. Initial mapping showed the Hickory Prospect in Grand Isle Block 116 to be a synclinally-separated look-alike with little potential for better sand development. Improvements in seismic imaging, notably a 3-D prestack depth migration, allowed the base of salt to be accurately re-interpreted and showed the local minibasin to be open to the northeast, toward potential sand sources. A new depositional model was developed in which deepwater turbidite sands were transported into the basin from the northeast, making Hickory Prospect potentially more sand-prone. Empirical models supported the likelihood of increased sand presence as compared to the Teak area. The improved seismic imaging also revealed several amplitude anomalies. A raytrace modeling study was undertaken to better understand these amplitude anomalies and thus reduce exploration risk. Comparing modeled results to the existing 3-D prestack depth-migrated seismic dataset enabled us to high-grade the initial location and make the exploratory well a success. End_of_Record - Last_Page 528-------

Depth Imaging Sub-salt Structures: A Case Study in the Midyan Peninsula (Red Sea)

ABSTRACT In the Midyan Peninsula (onshore northern Red Sea, Saudi Arabia), the current prospective oil and gas exploration targets are sub-salt structures. In this region, conventional time-migrated seismic sections are distorted due to the presence of salt diapirs, faults, and related lateral velocity variations. As demonstrated in other sub-salt prospects (North Sea, Gulf of Suez, and Gulf of Mexico), pre-stack depth migration can remove these distortions and accurately focus the structural image. Depth migration, however, requires a model which includes both lateral and vertical velocity variations to compensate for ray bending. Building such a velocity model is an iterative process which involves integration of various time/depth processing and interpretation skills. A 2-D seismic line, crossing various extensional structures in the dip direction, is used to illustrate these depth-imaging techniques. At the location of the sub-salt prospect, the depth image is improved and the lateral position of the main fault is shifted by 345 meters. The resulting structural model has refined the target definition and well position. This imaging approach is compared with the different steps of the seismic processing/interpretation flow.

Effects of salt‐related mode conversions on subsalt prospecting

Mode conversion of waves during seismic reflection surveys has generally been considered a small phenomenon that could be neglected in data processing and interpretation. However, in subsalt prospecting, the contrast in material properties at the salt/sediment interface is often great enough that significant P‐to‐S and/or S‐to‐P conversion occurs. The resulting converted waves can be both a help and a hinderance for subsalt prospecting. A case history from the Mississippi Canyon area of the Gulf of Mexico demonstrates strong converted‐wave reflections from the base‐of‐salt that complicate the evaluation of a subsalt prospect using 3-D seismic data. Before and after stack, the converted‐wave reflections are evident in 2-D and 3-D surveys across the prospect. Ray‐tracing synthetic common midpoint (CMP) gathers provides some useful insights about the occurrence of these waves, but elastic‐wave‐equation modeling is even more useful. While the latter is more time‐consuming, even in 2-D, it also provides a more realistic simulated seismic survey across the prospect, which helps to reveal how some converted waves survive the processes of CMP stack and migration, and thereby present possible pitfalls to an unwary interpreter. The insights gained from the synthetic‐data study suggest some simple techniques that can assist an interpreter in the 3-D interpretation of subsalt events.

The Use of Sequence Stratigraphy in Evaluating the Prospectivity of Neogene Turbidite Systems, Northern Green Canyon and Ewing Bank, Northern Gulf of Mexico: ABSTRACT

The Neogene turbidite systems of northern Green Canyon and Ewing Bank lease areas in the northern Gulf of Mexico are a major exploration play. The regional sequence stratigraphic framework for this area has been interpreted to help define potential areas for future exploration. The study area includes 17 fields/discoveries primarily in turbidite-related reservoirs. Once the sequence stratigraphic framework was established, the existing fields and potential prospects can be quickly evaluated in terms of sequence stratigraphy. Good potential for future exploration exists both in sub-salt prospects and in the deeper water portions of the area. For example, two discoveries (GC 200/244, 506) have been made during the past year. Two main potential reservoir intervals have been identified, which reflect different Winds of turbidite systems based upon lithologies and seismic facies. Lower Pliocene sediments (5.5 to 3.0 Ma) consist of sand-rich turbidite systems (up to 50 % sand in some sequences). Sand bodies are interpreted to be a really widespread sheet and amalgamated sheet sands. The lower Pleistocene interval (1.9 to 0.7 Ma) is predominantly shale-rich with localized thick sands developing in channel-levee systems and unchannelized sands (basin-floor fans). Most of the reservoirs in the area occur within this interval.

A Subsalt Prospect in Offshore Louisiana - Evolution of a Prospect from Lease Sale to Present: ABSTRACT

A Subsalt Prospect in Offshore Louisiana - Evolution of a Prospect from Lease Sale to Present: ABSTRACT

A High Resolution 3-D Marine Gravity Gradiometry Survey Over a Gulf of Mexico Deepwater Subsalt Prospect in the Mississippi Canyon Area: ABSTRACT

A High Resolution 3-D Marine Gravity Gradiometry Survey Over a Gulf of Mexico Deepwater Subsalt Prospect in the Mississippi Canyon Area: ABSTRACT

Understanding subsalt overpressure may reduce drilling risks : J. O'Brien & I. Lerche, Oil & Gas Journal, 92(4), 1994, pp 28–34

An analysis of the sub-salt physical environment can help engineers and geologists understand the potential for over-pressure, which may lead to drilling problems during exploration near salt sheets. Subsalt prospect evaluations must consider the probable pressure and temperature distribution through and below the salt sheets. Although subsalt targets share many characteristics with nonsalt targets, the presence of the salt body can influence the physical properties of the target in a number of ways. Some of these factors may be critical to the success of a well and need to be addressed in the prospect evaluation and during well planning. The paper describes the physical properties of salt deposits, salt emplacement, temperature regime, overpressure, and data from the Gulf of Mexico wells.

New technologies improve seismic images of salt bodies : Ratcliff, D W Oil Gas JV91, N39, Sept 1993, P41–49

Seismic technology is revealing new exploratory possibilities around and beneath salt. Several new seismic tools give geologists and geophysicists clearer, more-accurate images of salt formations than were available just a few years ago. The result is a new way of thinking about salt. The shape of salt formations is proving to have greater variety than was assumed until recently. And with shapes and thicknesses depicted with greater accuracy than ever, reliable mapping of subsalt prospects becomes possible. The image from 3D prestack depth migration correctly positions the bottom of salt. This is crucial to imaging features below salt because of the great velocity contrast between salt and sediments. With salt properly imaged and its velocity accurately accounted for, depth migration correctly positions structures below salt. This article describes seismic tools that Amoco Production Co. has used successfully to image salt bodies and target drilling in the Gulf of Mexico. The technologies have application where salt occurs or, more generally, where significant lateral velocity variations are present.