--> Abstract: Technology and Multidiscipline Integration for Deepwater Reservoir Characterization, by R. Nurmi; #90923 (1999)

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NURMI, ROY, Borehole Geoscience Advisor & Consultant, Houston,TX

Abstract: Technology and Multidiscipline Integration for Deepwater Reservoir Characterization

Summary

Deep-water exploration targets are becoming easier to find but the reservoir characteristics remain a challenge. Fortunately there is a rapid evolution of technology and new techniques to resolve these very thin-bedded reservoirs which can be a complex mixture of sand and shale. Some of the reservoir beds can be homogeneous but the mixtures of bedding types and lithologies make these reservoirs heterogeneous. Syndepositional faulting and slumping further complicate the reservoir possibilities. The solution is to use experienced teams of geoscientists with other disciplines and technology experts who together can optimally assess and develop these reservoirs.

Introduction

Deep-water depositional sequences have become the leading exploration play around the world for a variety of reasons. As explorationists progressively search deeper and deeper waters they are finding more deepwater depositional systems and reservoirs each year. As wells probe deeper targets below the prolific Tertiary deltaic sequences still more deep water depositional sequences are being encountered. The insights gained from the new offshore deeper water plays explorationists are returning to older basins with deepwater deposits to find more oil and gas. An important key to, or reason for many of the deepwater successes is new technology, better techniques and geoscientists who know when and where to use them.

Discussion

Without question 3-D seismic acquisition and new processing techniques, such as 'coherency', lead the way in finding more reservoirs with less dry holes than ever before. In addition, there are many other seismic and sonic technologies to consider for complicated stratigraphic and/or structural cases that require still better data. Multicomponent seismic, 3-D vertical seismic profiling around the well, seismic while drilling, logging sonic anisotropy to confirm seismic characteristics. Moreover, each of these acoustic technologies is continuing to evolve, as have LWD sonic tools. In fact, one of the newer techniques, acoustic imagery that reveals formation boundaries, tops and sometimes faults10s of feet away from a well holds much promise. A number new acoustic logging tools are providing better fault characterization even when the borehole may not intersect the fault or it is unrecognizable with BHTV type acoustic imagery. Rotation of the stress field as defined by sonic waveforms (Endo et al., 1997) and also imaging away from the well with sonic imagery has shown to be very promising in field tests. However, for detail fault characterization by borehole resistivity imagery cannot be matched in dip, strike and fault type characterization as well as other fault attributes relevant to migration/sealing or compartmentalization (Nurmi et al., 1998).

Accurate structural and depositional analysis of deep-water deposits even beyond 3-D seismic resolution can be critical before development because of the very large expenses involved.As there is often an interaction between tectonism, syndepositional deformation and depositional processes in controlling fan and channel geometry and orientation an integration.of high resolution VSP profiles with optimal borehole imagery and high-resolution dip data are critical. Routine borehole imagery interpretation is beginning to include an integration of different logging measurements to enhance the geological and formation evaluation interpretation beyond resistivity.

The lithofacies of deep-water deposits can be quite complex and result in low-resistivity zones that can be complex mixtures of very thin beds of sand, silt, and shale or the presence of heterogeneous conglomerates or slumped sequences generally frustrate the standard evaluation of cuttings, sidewall cores and standard well logs. Very thin-bed alternations are usually present as either channel fills, channel adjacent levee deposits and/or as distal fan lobes. In other cases it is important to define the presence of deformation of bedding and terrigeneous mixtures in large-scale slumps or moderate burrowing all of which can adversely affect permeability. Thick homogeneous slumped beds can be interbedded with classic turbidites with graded beds as well as beds thinner than standard well logging resolutions. Borehole resistivity imagery provides the reservoir sand thickness as well as the presence of interbedded nonreservoir materials while setting the coarse grain texture and bedding fabrics of the various lithology mixtures (Trouiller et al., 1989; Lambertini et al., 1998; Brackett and Tingey, in press).

High-resolution core analysis is indicating that one centimeter resolution is often needed yet almost no well logging tool approaches such a vertical resolution other than borehole resistivity imagery (Shirley, 1998). Unfortunately, borehole resistivity imagery is somewhat subject a measurement and needs to be calibrated with other measurements to quantitatively contribute to high-resolution formation evaluation beyond net sand counting. Some of the high-resolution measurements used in quality control of formation evaluation logging measurements can be used to quantify borehole imagery if the quality of the hole is in very good condition.

A relatively new commercial measurement that serves as an excellent compliment to imagery is nuclear magnetic resonance. In addition to providing porosity data relatively independent of lithology variations pore size distribution and/or fluid type is generally revealed. In that the measurement is of individual pores the presence of thin beds can be over common by data sampling at a high rate or by some new but minor acquisition and processing changes (Sezginer et al., in press).

However, an integration of different types of imagery with nuclear magnetic resonance and other new high-resolution measurements, such as density allow for the routine detection geometrical analysis of terrigeneous mixtures even at a centimeter scale, especially in the presence of unknown fluids.

Conclusions

The commercial possibilities of the future (next10 years) are already in testing and evaluation and showing promise of still finer resolution and better geological and reservoir evaluations. Resistivity imagery interpretation methodology needs to become more quantitative by cooperation of geologists, log analysts and petrophysicists working thin bed and other deep-water facies. Sand counts as well as structure and sand/shale fabrics from borehole resistivity imagery and permeability from NMR and reasonable lithology plots at the well site all help in designing well testing and completion of complex deep water reservoirs.

The capability of seeing the structure and lithofacies while drilling the well, such as with resistivity or density, is now possible but limited by tool availability and geologists experience and awareness. The full contribution of LWD and Geosteering (or Pay Zone Steering) will be a function of still better cooperation between drilling and geoscience. The detection of seismic imagery around a well bore, including detection of reservoir boundaries, faults and channels10s of feet away is now possible, but awaits geologists knowing why and when to look away from the well.

Chronostratigraphic magnetic reversals hold great promise for deep-water and deltaic depositional systems, but await the geologic community to recognize the need and to create the demand. Such surveys have just been completed as part of the Ocean Drilling Program for a few years now but almost no geologists know of these successes. The results of the most recent cruise leg, along the west coast of Africa, about to be unveiled may lead to the break-through needed.

AAPG Search and Discovery Article #90923@1999 International Conference and Exhibition, Birmingham, England