A Feasibility Study of High-Resolution Reverse VSP and Interwell
Seismic Methods for Hydrocarbon Reservoir Characterization
Jorge O. Parra: Southwest Research Institute
Abstract: Reverse VSP and interwell seismic measurements together with log data have yielded information on the anisotropic characteristic of the shale and heterogeneities associated with the lateral changes in porosity within the Silurian Kankakee limestone formation in the Mount Sterling area. High-resolution seismic experiments were conducted to delineate geological structures and obtain a detailed knowledge of the rock physical properties between wells at the Western Kentucky Petroleum Buckhorn test site near Quincy, Illinois. Specifically, experiments were conducted to analyze seismic data for mapping geological structures and to extract rock physical parameters from seismic waveforms to characterize the Kankakee, which is a prolific oil producer in the area. A significant reduction of amplitude for waves traveling in the limestone formation infers a heterogeneity that correlates accurately with a low-velocity and high-porosity zone delineated with traveltime tomography and well-log information. These results indicate that the high attenuation affecting the seismic pulses is caused by the heterogeneous conditions of the porous and fractured partially saturated rock matrix. In addition, the agreement between synthetic and field data confirms the interpretation of well logs and seismic measurements recorded in the hydrocarbon-bearing formation at the Buckhorn test site. This suggests that reverse VSP (using several three-component detectors in shallow boreholes) and interwell seismic data integrated with logs and seismic modeling can delineate the hydrocarbon reservoir and geological structures at the test site.
Developments in the Mini-CHARTS System and Field Trial Results in
Measuring Excavation Damage in Granite
J. G. Hayles, K. M. Stevens, M. H. Serzu, and G.S. Lodha: Applied Geoscience
Branch, Whiteshell Laboratories, Pinawa, Manitoba
Abstract: As part of the Canadian Nuclear Fuel Waste Management Program, Atomic Energy of Canada Limited (AECL) is studying plutonic rock of the Canadian Shield as a potential medium for disposal of nuclear fuel waste. One aspect of the work is the assessment of crosshole seismic techniques for characterization of areas of less competent rock within a granite rock mass. A prototype crosshole seismic system for 0.5- to 60-m interborehole distances is being tested in the underground research laboratory (URL) near the Whiteshell Laboratories. The URL is located approximately 150 km northeast of Winnipeg, Manitoba, in the Lac Du Bonnet batholith within the Superior Province of the Canadian Shield. Physical characteristics of the instruments and field survey results from five areas in the URL are presented in terms of P- and S-wave velocity.
The surveys suggest that an area of microcracked and damaged rock exists within about 1 m of most of the excavation surfaces: the P- and S-wave velocity is reduced in this area by about 10% compared with the velocity in the more intact rock located farther back from the excavation wall. In an area where a stress breakout notch occurs, the pattern of educed velocity close to the excavation is not observed. This is thought to be an indication of the local stress field closing the microcrack array.
Statistical Integration of Log Data for Lithology Determination and
Formation Evaluation
Vladimir G. Ingerman
Abstract: Integrating data from varying logs in the same well is a complicated problem because of the differences in measured parameters. The absence of a simple general solution to this problem significantly decreases the efficiency of log data interpretation in areas of complicated geology.
Application of Wireline Logs in Characterization and Evaluation of
Generation Potential of Palaeocene-Lower Eocene Source Rocks in Parts of
Upper Assam Basin, India
R. K. Mallick and S. V. Raju: Oil India Limited, Duliajan, Assam, India
Abstract: Organic-rich carbonaceous shale units of Upper Palaeocene-Lower Eocene age represent major hydrocarbon source rocks in the Upper Assam basin. The individual shale beds are thin and often interbedded with thin sandstone layers, which are potential oil and/or gas producers in the basin. The overlying sediments are principally shale, siltstone, and calcareous sandstones (marls). They are sufficiently thick to act as a competent seal and have probably isolated these reservoir systems from the higher hydrocarbon pools.
Thin source rocks interbedded with reservoir rocks pose problems in their identification and, hence, determination of net source-rock thickness and their hydrocarbon generation potential. Well-log responses of these shales include high natural radioactivity (with high uranium content), high formation resistivity, and low bulk density. These wireline log characters, coupled with pyrolysis data, provide a basis for their identification and characterization. Crossplots of formation density versus resistivity (with gamma ray in Z-axis) help in effectively delineating these potential source rocks. In addition, total organic carbon (TOC) values have been derived from formation density logs using an equation developed for the basin; the equation is similar to that applied to Devonian shales of the western Appalachian basin, Woodford shales of the Anadarko basin, and Bakken shales of the Williston basin.
The Upper Palaeocene-Lower Eocene source rocks are present throughout the basin, albeit with varying thickness. Total average thickness of the source-rock units is about 15 m and TOC averages 6%. Though the individual shale units are limited in thickness ( 0.5 m to 2 m), these are expected to be laterally extensive as the tectono-sedimentary analysis for the basin indicates a shallow-marine/near-shore environment during the deposition of Palaeocene-Lower Eocene sediments. The maturity study indicates the vitrinite reflectance values range from 0.6% to >> 1%, suggesting that the source rocks are within early to main phase of oil generation over a significant portion of the study area.
A tentative quantitative estimation has shown that about 1,300 million tonnes of hydrocarbon might have been generated from these source rocks over an area of about 3,500 km2. Using an assumed and possibly conservative expulsion-migration-accumulation factor of 0.15, the order of magnitude of hydrocarbons that could be accumulated in the Palaeocene-Lower Eocene reservoir rocks works out to be 200 million tonnes.
Laboratory Conductivity Measurement Using a Contactless Coil-Type
Probe
Ming Huang, Ce Liu, Liang C. Shen, and David Shattuck: Well Logging Laboratory,
University of Houston
Abstract: A new apparatus has been developed to measure the conductivity of samples at 100 kHz. The probe consists of coil-type transmitters and receivers and requires no physical contact with the sample. The measurement is based on the induction principle. The conductivity of a sample as a function of its length can be measured with good resolution. The resolution, defined as the thickness of the conductivity inhomogeneity that can be resolved with 90% accuracy of the conductivity, is approximately 2.5 cm when the contrast in conductivity between adjacent layers is 2; it is 5 cm when the contrast is 20. An inversion technique using the maximum entropy criterion is developed to interpret the measured data. Data processing is accomplished by applying a constrained optimization algorithm with an automatically adjusted Lagrange multiplier. The measurement system is validated by using saline solutions and water-saturated sands of known conductivities. The apparatus is accurate when the sample resistivity is between 0.1 and 20 ohm-m.