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Trenton-Black River: Seismic Imaging

maging of Near-Surface Reservoir Analogs

This project is being conducted in conjunction with a related project on the geology and geochemistry of the Ordovician dolostones. The seismic imaging phase of the work is being funded by the National Energy Technology Laboratory (NETL) of the U.S. Department of Energy. This additional work will help make the outcrop analogs more applicable to the subsurface Trenton-Black River gas reservoirs in New York. The shallow dolomite bodies will be imaged using reflection seismic techniques, and with ground-penetrating radar prior to drilling two research boreholes.

Geophysical Imaging of Dolomite Reservoir Analogs

Research at the Kentucky Geological Survey (KGS) is using outcrop exposures of Ordovician fault-controlled dolomites in central Kentucky to help understand similar subsurface natural gas reservoirs in the Trenton Limestone and Black River Group deeper in the Appalachian Basin. This work is funded by NYSERDA, with an industry contribution from Triana Energy, Charleston, W.Va. Triana will be coring two research boreholes through dolomite bodies to provide data on their three-dimensional characteristics.

The interpreted hydrothermal dolomites in Kentucky are localized along deep-seated faults, and appear to be good analogs to subsurface hydrothermal dolomite reservoirs in New York, Ohio, Michigan, and Ontario. The Kentucky dolomite bodies have been mapped at the surface, but their subsurface extent and three-dimensional geometry are unknown. To better understand the emplacement of the dolomite zones, we plan to image the dolomites using shallow, high-resolution seismic techniques and ground-penetrating radar. Seismic techniques will include P-wave and S-wave (shear) energy sources. Equipment for both of these imaging techniques is available at the University of Kentucky. Data acquisition will be supervised by Dr. Ed Woolery and graduate students at KGS.

Tasks

1. Commercial data evaluation and shallow seismic acquisition design. Commercially available seismic-reflection data from Clark County will be evaluated for use in the project. If any available data are suitable, they will be purchased for the project. Shallow, high-resolution seismic data acquisition will also be evaluated. This evaluation will include proposed location of seismic lines, acquisition methods and parameters, and anticipated problems.

2. Acquisition of seismic reflection data in Clark County. Seismic data will be collected, using techniques determined in task 1. Deliverables for this task will be the seismic-reflection profiles, after initial processing.

3. Data interpretation and final report. Final processing and geologic interpretation of the seismic data will be accomplished. Dolomite bodies imaged on the profiles will be interpreted. Processing techniques used to optimize dolomite resolution will be documented. Shallow seismic data will be compared to deeper data from producing fields.

Supplementary Information on Data Acquisition and Geophysical Equipment at the University of Kentucky

Shear-wave energy sources available for the near-surface profiling include vibratory and impact. The most effective source for very near-surface imaging is a section of steel H-pile struck horizontally with a 1.4 kg hammer. The hold-down weight of the H-pile is approximately 70 to 80 kg, including the weight of the hammer swinger and the H-pile section. The flanges of the H-pile are placed and struck perpendicular to the geophone spread for SH-mode generation. The H-pile is placed in prepared slit trenches to resist movement and improve the energy input into the ground. Polarity reversals and impacts of the sledgehammer on both sides of the energy source are recorded to ensure the correct identification of the SH-wave energy. The overall signal interpretation is corroborated using preliminary walkaway soundings. Geophones in the two inline-spreads will most likely be spaced at 2- or 4-m intervals, for a total spread length of 96 to 192 meters. In general, the hammer blows are stacked six to nine times per shotpoint.

The Geophysics Lab at the University of Kentucky is well equipped with state-of-the-art instrumentation to do the field studies and processing necessary to successfully complete the proposed study. Field equipment available for the proposed study includes: (1) trailer mounted IVI T-7000 series MiniVib (with SH-wave option), (2) vacuum-assisted weight drop, (3) 8 ga. Betsy SeisGun, (4) a 24-bit, 48-channel Geometrics StataVisor with extended memory, and (5) a wide range of horizontal and vertical geophones, takeout and extension cables, rollalong switches, etc. Most of the field equipment to be used in the study was purchased new in 1997. The MiniVib, a vibroseis unit, has a hold-down weight of 7,500 lb and is capable of generating a useable signal within the frequency range of 10 to 550 Hz, with linear, nonlinear, or segmented nonlinear sweeps. UK's MiniVib includes the shear-wave option, which consists of a rotating mass mechanism for positioning a shear-wave base plate to do inline and crossline production shear-wave operation.

The StrataVisor was purchased new in April 2000, and is equipped with an onboard correlator for performing realtime cross-correlation with a pilot and the input signal. Correlation between the pilot and input signals can be done for successive shots, or after each input sweep. For the seismic reflection data we are currently using 750 Mhz Pentium III PC's that run both Parallel Geosciences and VISTAWIN 2.5 (Seismic Image Software, 2000) processing software packages. We also have available the KINGDOM Suite+ (Seismic Micro-Technology) seismic modeling algorithms. For processing and interpreting the seismic refraction data we use SIPT2 V-4.1 (Rimrock Geophysical, Inc., 1995), and REFRACT32 (Socorro Scientific Software, 1998).

The ground-penetrating radar data will be acquired with a new PulseEkko 100 digital radar unit. The high-fidelity system has fully digital timing, as well as data acquisition. Bistatic antennae allow for various CMP acquisition arrangements. Various antenna frequencies are also available (e.g., 25 MHz to 200 MHz) to match site-specific dielectric conditions. The data are saved in SEG-Y format and can be processed using the standard digital signal-processing software packages described above.