Robert J. Graebner

High-resolution multicomponent seismic imaging of deepwater gas-hydrate systems

Multicomponent seismic data have unique value for studying near-seafloor geology in deepwater environments. When properly processed, PP (compressional) and PS (converted-shear) images made from multicomponent seismic data acquired in deepwater with seafloor sensors show near-seafloor geology with impressive detail. These high-resolution images are invaluable for studying deepwater gas-hydrate systems.

Evaluation of deepwater gas-hydrate systems

The worlds offshore continental margins contain vast reserves of gas hydrate, a frozen form of natural gas that is embedded in cold, near-seafloor strata. Published estimates suggest that the energy represented by gas hydrate may exceed the energy available from conventional fossil fuel by a factor of 2 or more. Understanding marine hydrate systems has become critical for long-term worldwide energy planning. Groups in several nations are attempting to evaluate the resource and to define seafloor stability problems across hydrate accumulations.

Field Development with Three-Dimensional Seismic Methods in the Gulf of Thailand — A Case History

A three-dimensional (3-D) marine seismic survey was conducted in the Gulf of Thailand to aid in the development of a gas field indicated by three wildcat wells which had been located by seismic reconnaissance programs shot over a period of several years. The key to successful exploration in the area, basically a hinge line play, was a detailed understanding of the complex faulting controlling the hydrocarbon traps. Since the prospect lies 160-220 km offshore, some specialized surveying techniques were employed to achieve the required positioning accuracy. About 1280 km of seismic data were recorded at 100-m line spacing over a roughly rectangular block covering about 120 km 2 . The 48-fold data were processed using a 3-D wave equation migration algorithm yielding a set of seismic traces representing the data vertically below a grid of depth points spaced at 33 1/3 m by 100 m.The results of the 3-D program showed greater fault resolution and structural delineation. The interpretation developed from a series of horizontal slices provided by the 3-D processing further improved fault resolution. Five wells, drilled on the basis of the 3-D survey, are productive and closely tie the seismic data. Initial studies of amplitude patterns of key reflectors, combined with interval velocities from seismic derived logs, appear to offer the potential of direct detection of productive gas zones thicker than 25 to 30 ft. The 3-D seismic data are being utilized for planning additional development wells and potential platform locations.

OBC Sensor Response and Calibrated Reflectivity

Summary Given deep-water OBC data, we can combine hydrophone (P) and vertical-geophone (Z) data to separate upgoing and downgoing acoustic wavefields. By using the downgoing wavefield to estimate the seismic wavelet, we can recover compressional-wave reflectivity and provide an acoustic impedance log for the near-seafloor sedimentary section. This procedure requires the calibration of geophone response to hydrophone response with an appropriate matching filter (z2p). P and Z data containing only upgoing energy can be provided in several ways to estimate z2p and p2z filters. Alternatively, the multiple/primary ratio (in the frequency domain) can be used to obtain calibrated reflectivity from the hydrophone alone or from the geophone alone, with no sensor calibration required. We apply these methods to a deep-water OBC line. We obtain consistent results by all of the methods. By combining low-frequency (5–70 Hz) z2p filters based on energy arriving before the direct arrival, with high-frequency (25–180 Hz) z2p filters based on near-offset reflection data, we can provide broadband (5–180 Hz) calibrated reflectivity and impedance of shallow seafloor sediments.

Case History: 3-D Shear-wave Processing And Interpretation In Radial-transverse (SV-SH) Coordinates

Summary A 9-C 3-D seismic reflection data set from Clark County, Kansas is processed to produce SH and SV data volumes. Data processing is quite straightforward; rotation of the data from field coordinates to radial-transverse coordinates, static corrections (elevation, shot, and receiver) derived from the SH data, NMO correction with a single velocity function, inside-trace mute, and stack. Super gathers monitor the convergence of the shear-wave statics, which are found to be relatively small (+/- 40 ms), and simple to estimate from the dominantly 12 Hz SH data. Single velocity function NMO (velocities derived from SH) show the SV data to be slightly overcorrected, suggesting the presence of vertical transverse isotropy. An inside-trace mute eliminates the surface-wave noise cone. Dominant reflections in the stacked data are from the top and base of the Morrow clastic interval. Reflection signal quality is superior on the SH data.

Multicomponent seismic technology for imaging deep gas prospects

Across the Gulf of Mexico, operators are targeting deeper and deeper drilling objectives. For deep targets to be evaluated, seismic data require relatively long source-receiver offsets. Most shallow-water operators in the gulf consider 30 000 ft (9 km) to be the deepest target depth that will be drilled for the next several years. For geology at depths of 9 km to be imaged, seismic reflection data must be acquired with offsets of at least 9 km. We suggest in this paper that modern 4-C OBC data can provide good quality P-SV data to such depths and should be integrated into prospect evaluations. Long-offset surveys are difficult to achieve using towed-cable seismic technology in areas congested with production facilities, typical for many shallow-water blocks across the northern Gulf of Mexico shelf. Ocean-bottom-cable (OBC) and ocean-bottom-sensor (OBS) technologies are logical options for long-offset data acquisition in congested production ar-eas because ocean-floor sensors are immobile once deployed and can be positioned quite close to platforms, well heads, or other obstructions that interfere with towed-cable operations. An example illustrating the deployment of ocean-floor sensors through a congested platform complex in part of the area of study is illustrated in Figure 1. A 10-km diameter circle is positioned atop this map of production facilities to illustrate the difficulty of towing a 10-km cable across the area in any azimuth direction. In contrast, OBC lines AA, BB, and CC (actual profiles used in this study) pass within a few meters of several production platforms. Figure 1. 4-C OBC data acquisition across congested areas. An additional appeal of OBC seismic technology is that 4-C data can be acquired, allowing targeted reservoir intervals to be imaged with P-SV wavefields, as well as P-P wavefields. Once 4-C seafloor receivers are deployed, source boats can maneuver along a receiver line to …

Enhanced PS-wave images of deep-water, near-seafloor geology from 2-D 4-C OBC data in the Gulf of Mexico

Summary We present a method for constructing high-resolution converted PS-wave images of near-seafloor strata in deep water (~800m) Gulf of Mexico using data from a 2-D 4-C ocean bottom cable (OBC) survey. Images are constructed from radial component common- receiver gathers. New concepts presented here include the use of commonreceiver gathers as a proxy for common conversion point gathers in the near-ocean-bottom environment, and a method for reducing PP-wavefield effects on radial gathers to better estimate the upgoing PS-wavefield. The techniques reduce the effects of P-wave contamination in the image and allow for better interpretation of near-oceanbottom sediments. The resulting PS-images with dominant 90Hz energy are comparable to PP-images from highresolution data from a 2-8kHz source.

Surface seismic monitoring of an active water flood

This paper describes the result3 of a field experiment where repeated seismic surveys were used to monitor the progress of a waterflood’s movement through a -producing oil reservoir. Theoretical analvais of the reservoir and its fluids predicts-that the seismic method is sufficiently accurate to allow the observation of the expected changes of reflection amplitudes caused by saturating fluid variations,

Interpreting multicomponent seismic data in the Gulf of Mexico for shallow sedimentary properties: methodology and case history

We developed a methodology for manually establishing tie points of depth-equivalent surfaces in P-P and P-S seismic data volumes derived from a 4-C ocean bottom seismic survey using seismic attribute volumes viewed in time slices. These tie points were used as a basis for establishing interpretations of depth-equivalent surfaces throughout the volumes that were then used as a basis for depth registration of several 2-D sections throughout the volume. We examine the Vp/Vs ratios derived from this interpretation. While these ratios are physically reasonable, they are averaged over several stratigraphic sequences and do not provided enough detail to represent the true interval Vp/Vs values of a given sequence. However, we use these Vp/Vs ratios to correct the interpretation and perform an initial registration of the P-S volume to P-P two-way time. An analysis of the registered, or “warped”, P-S volume shows the limitations of this simple technique, and from this we infer what processes must be addressed for a robust method of registration. Introduction The work summarized here is part of a larger study undertaken to develop methods of multicomponent data analysis for the detection of gas hydrate prospects in the northern Gulf of Mexico. Methane gas hydrates deposits occur within a narrow window of near ocean-bottom strata extending no more than a few hundred meters below the seafloor, with size and extent dependent on local temperature and pressure conditions in the region. In many regions of North America, including the southern Gulf of Mexico (GOM), Alaska, and the Atlantic and Pacific coasts of the United States, known gas hydrate deposits can be identified on P-wave seismic surveys by a diagnostic bottom-simulating reflector (BSR) at the base of the frozen hydrates. In the northern GOM, gas hydrates do not exhibit the classic BSR. In this region, the known hydrate deposits have no known distinctive seismic signature, but they do tend to be associated with shallow “gas clouds” which show up as no-data or poor-data quality zones on traditional Pwave seismic surveys. Converted-mode shear waves have already been proven a useful tool for imaging through gas cloud regions (Thomsen et. al, 1997) and specifically in the northern GOM for the purpose of identifying deep targets of interest for oil and gas production (Cafarelli et al, 2000, Knapp et al, 2001). We consider the identification of shallow gas clouds for hydrate detection another possible application for this type of multicomponent seismic data. As part of this study, we introduced the use of a set of multicomponent seismic data designed for deep target exploration as a tool to aid in the identification of possible hydrate prospects and other possible engineering hazards in the upper 1000m of strata across a study region. It should be noted that this study area of the northern GOM shelf is not itself a candidate for hydrate deposits. Large regional 3-D multicomponent data sets of such environments in the deepwater GOM are not available to us at this time. However, the data provided to us for this study provide an opportunity to demonstrate the utility of multicomponent data for the purpose of analyzing nearseafloor sediments and elastic properties which can be extended to other deepwater environments. 4-C Seismic Data The data provided us were from a multi-client 4-C ocean bottom cable survey acquired over 46 OCS blocks of East Cameron South, offshore Louisiana in 1999 and 2000, covering approximately 1000km. The survey is located on the continental shelf in water depths of approximately 100m. The data were processed as a pure-mode P-wave (P-P) volume created from vertical geophone velocity and hydrophone pressure data, and as a radially rotated converted shear mode (P-S or C-wave) volume created from the horizontal velocity phones. For more details of the survey design and processing, refer to Knapp et. al (2001). We were given a P-P wave volume with an extent of 1s in two-way traveltime, and a P-S wave volume extending to 2s so as to allow us enough data to complete our shallow regional studies and protect the confidentiality of the deeper data currently in use for the location of commercial hydrocarbon prospects.

Study of Subtle Traps Using Horizontal Seismic Sections: ABSTRACT

A three-dimensional seismic survey, after proper design, data collection, and data processing, yields a three-dimensionally migrated data volume. Horizontal, or SeiscropTM, sections sliced from this data volume provide a direct horizontal view of the subsurface from which structural interpretation can be straightforward. In the absence of structure, Seiscrop sections display stratigraphic or paleogeomorphic features directly. However, structural deformation can be removed from the data by flattening. Horizon Seiscrop sections, sliced from the flattened volume, permit stratigraphic and other depositional features to be recognized and studied in detail without the confusion of structure. Using horizontal seismic sections primarily from the Gulf of Thailand, a variety of small and subtle traps have been identified. These include small fault traps, sand channels, and sandbars. The acoustic nature of these features has been further studied using seismic logs, derived by wave equation inversion. Reservoirs thicker than 30 ft (9 m) have proved mappable. End_of_Article - Last_Page 907------------