Abstract This chapter describes an integrated approach to reservoir characterization and three-dimensional (3-D) geologic modeling of the San Andres Formation at Vacuum field, New Mexico, United States. We present techniques to identify significant heterogeneities within a carbonate reservoir using stratigraphic, petrophysical, and 3-D multicomponent seismic data. This integrated approach provides a detailed static description of reservoir heterogeneity and improved delineation of the reservoir framework in terms of flow units. We use a petrophysics-based method to identify hydraulic flow units within a sequence-stratigraphic framework. Flow units are characterized within high-frequency carbonate sequences through analysis of the vertical variation of flow (kh) and storage capacity (ϕh) and pore-throat radius (R35) associated with successions of subtidal, intertidal, and supratidal rocks. Pore-throat radii from cored wells are used to modify the empirically derived Winland equation to estimate values of pore-throat radius in non-cored wells. Flow profiles, constructed from log porosities and neural-network permeabilities, are correlated and used to build a 3-D geologic-model framework. Characterization of both matrix and fracture properties within a reservoir is possible using 3-D multicomponent seismic data and wire-line logs. Compressional- and shear-wave amplitude attributes together provide more accurate porosity estimates than those determined from compressional-wave data alone. Shear-wave anisotropy measurements provide information about inferred fracture density and orientation that can be used to modify permeability models to account for regions with open fractures. Because of this study, reservoir-simulation models that incorporate modified permeability distributions more accurately account for unexpected early CO 2 -breakthrough times observed in the field. In addition, flow-simulation results indicate that the need to upscale the geologic model was significantly reduced or eliminated by describing flow units using the combined sequence-stratigraphic- and petrophysics-based method.

Abstract Muspac and Catedral are two of the most important gas- and condensate-producing fields in southern Mexico. They produce from Cretaceous fractured carbonates. The objective of this integrated study is to define the stratigraphic and structural controls that caused early water production in those fields. Open-hole log correlation of 45 wells served to define eight reservoir zones, based on petrophysical characteristics. Petrophysical properties were mapped using a volumetric parameter to analyze the anisotropy of the gas-storage capacity in the fields. Dipmeter and borehole image logs were interpreted in 14 wells using cumulative dip and vector plot techniques to define unconformities, flooding surfaces, and faults. Borehole images from five wells were extremely useful in detecting evidence of sedimentologic and structural features. Fracture density depends on petrophysical properties of the reservoir rocks. In-situ stress directions were determined in 18 wells using borehole breakouts to define the predominant northwest-southeast orientation of the open fractures. Two dominant fracture sets, determined from seismic attributes and borehole images, are parallel to seismically determined faults. According to this study, early water production is caused by coning through fractures, faults, and karstic zones. Some water-producing intervals depend on the location of perforations, especially when these are located in highly fractured rocks and close to the gas-water contact. To minimize early water production, the operator must avoid wells in fault zones, wells on the flanks close to the gas-water contact, and deviated wells drilled perpendicular to the direction of open fractures.

ABSTRACT Eolian reservoirs exhibit significant compartmentalization and directional permeability caused by the processes taking place during accumulation of sediments within an eolian system. The contrast in grain packing across erosional bounding surfaces is one of the primary controls of fluid-flow patterns within eolian reservoirs. Better prediction of the geometry of flow units bounded by erosional surfaces can be made by reconstructing the type of bedform that formed the accumulation. Subsurface study of the occurrence and the frequency of erosional bounding surfaces has been limited by the availability and quality of core data. However, using borehole images, specifically FMI and FMS logs, the orientation of stratification can be resolved, and the cross-cutting relationships produced by erosional bounding surfaces can be identified. Comparison of the stratification orientation above and below an erosional bounding surface makes it possible to classify the erosional bounding surface within a process-oriented hierarchy. Using the foreset and bounding surface orientations gathered from the FMI and FMS log data, and using computer simulation methods for bedform reconstruction, a bedform that reflects the observed variations in stratification can be constructed. An integrated study of FMS logs, FMI logs, and cores from the Tensleep Sandstone in the Oregon Basin Field, Bighorn Basin, Wyoming indicates that erosional bounding surfaces can be identified and classified. The FMI and FMS logs also allow delineation of eolian facies such as interdune accumulations.

ABSTRACT Modified Fischer plots can be used in subsurface reservoirs for parasequence-scale correlations. Although the technique is applied to deep-marine sediments, it is not restricted to such sediments. The approach is based on high-resolution bedboundary detection using borehole-imaging logs such as FMS (Formation MicroScanner, Trademark of Schlumberger, Inc.), FMI (Formation Microlmager, Trademark of Schlumberger, Inc.), or any of the other electrical or acoustic dipmeter devices. The operator interactively selects dips at the top of every clay-sand and clay-chalk interbed while viewing images on a computer workstation. A spreadsheet is prepared which lists the depths to the top of each selected bed boundary. From all contiguous cycles, one calculates a mean-cycle thickness and the departure of each cycle thickness from the mean. Dimensionless cycle thickness is then calculated by dividing mean-cycle thickness into departure from mean-cycle thickness. Interpretations are made on a cross plot of cycle number (a function of depth) vs. cumulative dimensionless departure from mean cycle thickness. The curve that results has a distinctive shape which is based on changes in cycle thickness. This curve can be used like a well-log trace to correlate cycles between wells, look for missing section, and hypothesize about changes in sea level, channel migration, and lateral continuity of facies.

Abstract Porous dolomites are present below a distinctive stratigraphic marker within the lower Quintuco Formation (Lower Cretaceous, Berriasian-lower Valanginian) in the eastern Neuquén basin, Argentina. Dolomitized pack-stones and wackestones with moldic and sucrosic porosity provide the main reservoir facies in Rio Neuquén field and perhaps other oil fields in the area. Lower Quintuco carbonates are comprised of: (1) oolitic grainstones, (2) burrowed, dolomitized oolite-skeletal-peloid packstones/wackestones, (3) dolomudstones and bedded anhydrites, and (4) very fine-grained, superficially coated oolite grainstones. These sediments are commonly packaged into shoaling- and coarsening-upward parasequences. Reservoir-quality porosity and permeability exist almost exclusively in burrowed, dolomitized packs tones and wackestones. These strata are interpreted as off-bar facies deposited on the landward side of bar complexes, similar to modern facies analogs known in the Joulters Cay area of the Bahamas. In the lower Quintuco Formation, dolomite preferentially replaced carbonate mud. Below an inferred widespread paleo-exposure surface, ooid-skeletal-peloid grains were then dissolved to leave an open pore network with abundant moldic and intercrystalline porosity.

Abstract A mineralogical, geochemical, and rock-magnetic investigation of sediments deposited during the last 300 years in Lake Greifen, a hard-water lake with moderate sulfate concentrations (<250 μmol/L) and seasonal anoxia, shows that both authigenic single-domain biogenic magnetite and multidomain detrital titanomagnetite were preserved within the bioturbated marls deposited prior to the onset of anthropogenically induced eutrophication. Subsequently, in response to a gradual change from oxic to dysaerobic to anoxic bottom-waters, the deposition of organic carbon-rich varved sediments occurred and the degree of magnetite preservation decreased as altered diagenetic conditions resulted in the rapid dissolution and sulfidization of the biogenic and detrital magnetite. The occurrence of both biogenically produced magnetite and detrital titano-magnetite within the upper 4 cm of sediment indicates that (1) biogenic magnetite may be produced within the near surface organic carbon-rich sediments, probably on an annual basis when the overlying waters are oxygenated, and (2) detrital magnetite is continuously deposited. Changes in magnetic properties below this zone of surface magnetite production and microscopic examination of corroded fine-grained biogenic magnetite extracted from this interval indicate the rapid destruction of the most recently produced (or deposited) magnetite. Our findings demonstrate that (1) lacustrine sedimentary magnetic properties may reflect redox conditions, which are in the case of Lake Greifen determined by productivity, and (2) rapid destruction and sulfidization of fine-grained and coarse-grained magnetite can occur in lacustrine systems that are characterized by high productivity, low available lake-water sulfate, low concentrations of dissolved sulfide, and rapid sediment accumulation rates. These findings differ from marine studies in which magnetite dissolution and sulfidization is postulated to occur in systems characterized by high productivity, high concentrations of dissolved sulfide, and low sediment accumulation rates. Based on our observations, we propose that microbially mediated processes are contributing, either directly or indirectly, not only to the authigenesis of magnetite in the Lake Greifen sediments but also to its destruction.

Abstract Glacio-lacustrine varved clay of late Wisconsinan age in western New York has stable remanent magnetization and anisotropic magnetic susceptibility (AMS). Remanence is carried by interacting single-domain grains of magnetite, but coarse multidomain grains of magnetite are also present. Remanent inclination is anomalously shallow, given the latitude of the area of deposition and the existence of a geocentric dipole field at the time of deposition. The AMS consists of a foliation that is gently inclined to bedding and a weaker lineation in the plane of foliation. Independence of magnetic fabric and direction of remanence is demonstrated by comparison of remanence and AMS at closely spaced sites within individual clay laminae. Magnetic fabric resulted from alignment of multidomain grains by transient density currents at the time of deposition; alignment of single-domain grains by the geomagnetic field occurred later in a dilute slurry at rest on the surface of deposition to produce a post-depositional detrital remanent magnetization (pDRM). Rapid deposition and compaction resulted in anomalously low remanent inclination. Remanent declination and magnetic lineation were unaffected by compaction, but magnetic foliation may have been increased.

Abstract We have been studying the effects of diagenesis on the paleomagnetic signal of lacustrine sediments by examining tephra layers found in unaltered and altered states in the same diagenetic environment. In this paper, we report rock magnetic data from seven such layers in six diagenetic environments ranging from saline-alkaline to mildly alkaline. The effects of diagenesis can be complicated and varied but certain patterns are evident. These patterns indicate that both the physical state of the magnetic grains and the geochemistry of the porewaters are important in determining the effects of diagenesis. In some cases, diagenesis simply reduces the intensity of the original magnetization, but in others it produces a new magnetization that completely overwhelms the original one. Measurements of various rock magnetic parameters have allowed us to relate the changes in intensity to changes in the particle-size distribution of the magnetic carriers. In particular, decreases in intensity appear to be associated with selective dissolution of the fine-grained magnetic carriers or with a general reduction in the quantity of magnetic carriers of all grain sizes; increases in intensity seem to result from a reduction in the numbers of coarse-grained magnetic carriers. Plots of the demagnetization behavior of natural and saturation isothermal remanent magnetizations may be useful in discriminating between unaltered and altered material, especially when used in conjunction with other rock magnetic information.

Abstract Eight short stratigraphic sections within the Moenkopi Formation of northeastern Arizona verify a network of magnetic polarity stratigraphy previously observed in that area. The magnetostratigraphic signature was used to test the time relationships of numerous vertebrate faunal occurrences and to test the synchroneity of a major change in facies over this part of the depositional basin. The magnetic polarity data show that the vertebrate occurrences are not all of the same age; two or three different ages of fauna are indicated. Parallelism of the changes of magnetic polarity and lithology indicate a relatively rapid spread of sand-laden streams across the depositional basin. The near time-synchroneity of this lithologic change over much of the area indicates that the revised definition of the boundary between the Moqui and Holbrook members of the Moenkopi Formation as the first occurrence of a persistent, ledge-forming sandstone (Purucker and others, 1980) is a well-founded definition. A paleopole position was calculated from those samples that exhibited demagnetization behavior that is univectorial to the origin of orthogonal axes plots; this early Middle Triassic (early Anisian) paleopole is located at 94.8°E, 58.5°N (alpha-95 = 3.4°). Global correlation of the magnetostratigraphic pattern of the Moenkopi Formation to the patterns of two marine sequences indicates that Moenkopi deposition began in the Early Triassic mid-Griesbachian Stage and continued until it probably was interrupted by a hiatus that represents much of the Smithian. Deposition resumed in late Smithian and continued until late Spathian. The latest Spathian is represented by a hiatus. Deposition resumed again in the early Middle Triassic (early Anisian), thus the Moenkopi Formation provides a heretofore unknown record of geomagnetic field polarity during this time interval. A widespread Smithian hiatus, i.e., a lowstand, is suggested by comparisons of observations in south China, the Moenkopi Formation, and the Chugwater Group to those of the Arctic stratotypes.

Abstract Magnetostratigraphic dating of sedimentary strata is often the most precise technique available for temporally constraining the evolution of and controls upon sedimentary basins over I Ma in age. Uncertainties in the absolute dates derived by this technique are often difficult to assess quantitatively, despite the desirability of specifying their precision. An explicit discrimination should be made between correlations of the local magneto-polarity stratigraphy (MPS) to the global geomagnetic polarity time scale (GPTS) based on independent biostratigraphic or radiometric time control and those based on the smoothest derived sediment-accumulation rates. Situations in which there is a single, compelling correlation and those in which the correlation is the most reasonable of several possibilities should also be explicitly distinguished. In the latter case, alternative feasible correlations should be illustrated in order to permit a qualitative assessment of the uncertainties involved. Two classes of uncertainties are associated with the temporal calibration of magnetostratigraphic sections: those related to the creation of the local MPS and those related to the GPTS. Imprecision in measured stratal thicknesses and in the position of magnetozone boundaries can produce significant (up to 50 percent) uncertainties both in magnetozone patterns and in derived rates of sediment accumulation. Uncertainties in the GPTS result from uncertainties in the radiometric calibration of magnetic anomaly patterns. Comparison of available GPTS’s indicates uncertainties of (1) as much as 100 percent for sediment accumulation rate calculations involving intervals of less than 1–2 my and (2) up to 3 my in absolute ages. An example drawn from the Late Cretaceous to Eocene Axhandle thrust-top Basin of central Utah illustrates these uncertainties.