Well Log Parameter Mapping:A Technique to Delineate Secondary Natural Gas ResourcesWithin Thin-Bed Reservoirs of Mature Fields
Mark J. Burn: Bureau of Economic Geology, The University of Texas at Austin
Jos‚ M. Vidal: ResTech
Abstract: Bypassed, incompletely drained, and untapped thin-bed (6-ft or 2-m) reservoirs of North McFaddin field (Tertiary Gulf Coast basin, Texas) form important economic targets for "secondary" natural gas recovery. Undrained reservoir occurrence is undoubtedly related to technological constraints associated with the vertical resolution of electric log (EL) and induction-electric log (IEL) tools used during primary development. High-resolution induction tools and computed log analysis, particularly enhanced porosity processing, are fundamental to thin-bed petrophysical characterization and resource identification. Subsequent to secondary resource identification, conventional EL and IEL suites (typical of mature fields) are used to delineate potentially productive reservoir limits. This involves superposing contour maps of resistivity, relative spontaneous potential (SP), net thickness, and structure, and integrating these maps with data from well tests, wireline-formation tests, and sidewall cores. When combined with engineering analyses, the technique enables resource quantification and facilitates development design. The informally termed 4,200-ft zone no. 5 reservoir serves as an illustrative example. Relative SP, reflecting SP as a fraction of static SP, represents a crudely normalized parameter. In the case of EL data, thin-bed true resistivity approximations were determined using the anomalous response of the lateral device. The deep induction curve was used in the case of IEL data. Well log parameters are interpreted as approximations of petrophysical properties. Resistivity reflects gas saturation, and relative SP corresponds to reservoir quality. By determining appropriate parameter cutoff values, the method is transportable and serves as a model to delineate and evaluate similar secondary thin-bed resources common to other mature gas fields, located perhaps farther afield than the Tertiary Gulf Coast.
Computer Note
Archiving Wireline Log Data
D. J. Camden: British Gas Exploration and Production, Reading, England
C. C. L. Vermeulen: Petrologic Ltd., Cambridge, England
Abstract: This paper details the development of a wireline log data archive. The chosen medium was CD ROM, but the methods employed to organize and access the data are equally applicable to any high-density random access storage medium. Data are stored as LIS structured disk files. LIS was chosen because of its flexibility and dominance in the log data industry. No attempt is made to organize the files on the storage media such as by tape or by well; this task is carried out through an indexing and database management system. Indexing the large volumes of data is automated and relatively error free because the data themselves are used to create the index. To simplify data identification, a generic logging tool descriptor was developed.
A primary objective of the project was to build an archive that would be independent of both hardware and software systems and be as future proof as possible. We hope that others may benefit from our experience and create some standards for our industry, which will improve the ease and efficiency of data movement within and between companies.
Technical Note
Pyrite Volume Estimation by Well Log Analysis and Petrophysical Studies
T. Klimentos: Research Institute / Petroleum Engineering, Division I King Fahd University of Petroleum and Minerals, Dhahran, Saudi Arabia
Abstract: Pyrite exhibits a very good electrical conductivity that is usually comparable to or even higher than the conductivity of the formation water. The electrical properties of porous rocks with pyrites are strongly dependent on the amount and distribution of pyrite and the frequency of the measuring current. Resistivity corrections for pyrite effects can be obtained from charts available in the literature. However, the pyritic volume must be known in order to use these charts.
This article is a case study of pyrite volume calculation from logs in conjunction with scanning electron microscopy (SEM), x-ray diffraction (XRD), thin-section, and core-analysis studies. The log-calculated results of pyritic volume were in good agreement with the measured (XRD) volumes on core samples. Hence, in similar case studies, such an approach may allow the geologist/engineer to calculate pyritic content and thereby correct resistivities for pyrite effect and calculate reliably water saturation and reserves.