One of the first steps in the facies analysis of a clastic reservoir is the description and interpretation of available conventional core.[4] An important result of core description is the subdivision of cores into lithofacies, defined as subdivisions of a sedimentary sequence based on lithology, grain size, physical and biogenic sedimentary structures, and stratification that bear a direct relationship to the depositional processes that produced them. Lithofacies and lithofacies associations (groups of related lithofacies) are the basic units for the interpretation of depositional environments.
Sedimentary Environments From Wireline Logs Pdf 21
Interpretation of the environment in which lithofacies were deposited from analysis of cored sequences involves relating the identified lithofacies to the physical and biological processes that produced them. This process-response relationship identifies the specific processes responsible for the sequence and, by inference, the depositional setting in which these processes occurred. The application of the process-response approach relies primarily on depositional models constructed through study of both modern and ancient analogs.
Wireline logs to be used for facies analysis should, whenever possible, always be calibrated by core. This calibration involves (1) shifting core to log depths (see Preprocessing of logging data and Core-log transformations and porosity-permeability relationships) and (2) establishing a relationship between lithofacies associations and curve shape. Core gamma scans, obtained by passing the core through a device that measures the natural radioactivity of the rock, are particularly useful for shifting cores to logs. The calibration of wireline log shape by core is particularly important for firmly establishing the log response and the identity of vertical sequences on these logs.
An alluvial fan is a wedge of clastic detritus that forms at the base of a mountain front as sediments eroding from the mountains are transported downslope by streams or debris flows and deposited at the base (Figure 3e). The fan-shaped body is generally characterized by a gradation from coarser sediments at the apex to finer sediments at the toe. Alluvial fans are commonly divided into proximal, mid-fan, and distal fan subenvironments.
Gamma ray, SP, and resistivity logs through braided channel complexes generally have a blocky character, whereas individual meandering channels have an upward-fining signature except where stacked and cross-cut, where they may exhibit more complex wireline log signatures.
In the subsurface, eolian sandstones generally comprise thickly bedded sequences with few major interstratified shales. The sequences tend to be uniform and lack discernible coarsening- or fining-upward trends and, thus, exhibit blocky to weakly serrated gamma ray, SP, and resistivity log profiles The well-bedded and high angle cross stratified nature of eolian sandstones promotes reliable results from dipmeter logs. Dune and interdune deposits can often be distinguished and paleowind directions inferred using correctly processed dipmeter data.[15]
Lacustrine rocks are generally the source rocks for hydrocarbons found in alluvial fan, fluvial, eolian, and deltaic rocks rather than the reservoirs. However, sandstone bars, beaches, turbidites, and fan deltas associated with lake margins can be reservoirs sourced by open lake deposits. The core and log response characteristics of these deposits are similar to those described from analogous marine environments.
Most marine sand bodies are upward coarsening with the best reservoir quality rocks at the top of the body. Gamma ray, SP, and resistivity logs have a corresponding upward-coarsening character. In the case of storm-deposited sheet sands either attached or detached from the shoreface, amalgamation of individual storm deposits at the top of the bodies produces the greatest permeability and porosity and the most laterally continuous units.[20][21] In the case of tidal- and storm-generated shelf sand ridges, best reservoir quality is also at the top in the form of several different types of large scale cross bedding.
Submarine fans may form at the base of slopes that have a delta-like appearance in plan view (Figure 3i). Internal facies vary from channelized sand and gravel bodies to sheet-like, thin, graded beds deposited by turbidity flows in distal parts of the fan. Vertical sequences through channelized portions of the fan typically show an upward-fining character accompanied by an upward-fining wireline log motif. Vertical sequences through more distal parts of the fan show an alternation between sandstone and mudstone beds, so that wireline logs are typically interdigitate and irregular. Reservoir quality varys accordingly. Many variations of morphologies and internal facies configurations occur in submarine fans as a function of sediment supply, sea level, type of continental margin, and local tectonic features.
The use of wire-line logs in subsurface studies is all too often restricted to the correlation of selected stratigraphic horizons. There is an increasing content of valuable geologic information in modern wire-line logs that can be extracted by simple computer processing. The resulting log transformations provide lengthy and continuous records of sections of interest. Examples of these methods, as applied to Cretaceous and Permian stratigraphic case studies, are described here. The log data can be incorporated in either forward- or reverse-modeling modes in the simulation and analysis of sedimentary sequences. In addition to their geologic information content, wire-line logs are quantitative, and so their data can be entered easily into numerical modeling programs. Analysis can be made in either the stratigraphic time or frequency domain. The power spectra of logs give key insights into the nature and scale of sedimentary depositional mechanisms.
The length of stratigraphic successions penetrated by the typical borehole easily surpasses that of outcrops, even those of exposures in deep canyon walls. Myriad boreholes have been drilled in the sedimentary basins of the world, and their records provide an extraordinary data resource for sedimentary modeling. However, the accessibility of deep stratigraphic units for detailed geologic observation is considerably more restricted than that of surface exposures. Core is costly to recover; drill cuttings are fragmentary, can become contaminated with cavings, and are sampled over coarse depth intervals. By contrast, wire-line logs are the standard reference material for subsurface stratigraphic correlation, but their geologic information content is generally underutilized by geologists. The correlation of stratigraphic units using logs from boreholes defines three-dimensional surfaces that express the large-scale geometry of sedimentation units. However, a set of correlation surfaces is purely a geometric skeleton framework because it is based entirely on depths and geographic coordinates. Explicit geologic information linked with the magnitudes of the log measurements can be used to fill in the body of the framework.
The traces of most logs reflect primarily shale content and pore volume and provide crude but effective indicators of gross geologic variation. For example, the waxing and waning of coarse clastic sediment supply is often indicated by spontaneous potential (SP) and gamma-ray log shapes and trends. These shapes can also aid in the recognition of specific sedimentary environments [see, for example, Selley (1976) and Garcia (1981)]. Porosity logs of carbonate sections can discriminate lithofacies whose history of genesis and diagenesis is reflected in pore volume characteristics (Doveton, 1986). These qualitative log features have been used by generations of petroleum geologists to assess potential plays and reservoir architectures. Geologic data and interpretations are commonly annotated on logs, which are widely used as the standard graphic base for subsurface cross sections.
The increasing demands on reservoir engineering have stimulated the development of new wire-line tools. Measurements obtained with these tools are sensitive to mineral and elemental compositions in clastic rocks, carbonate rocks, and shales. The economic benefits include better estimations of porosity and permeability of "complex" carbonates, recognition of clay mineral species in sandstone pore networks, and improved ability to analyze reservoir zones in exotic rock types. These nuclear tools are now commonly used, and their logs can be transformed into continuous and quantitative profiles of interpreted mineral composition and geochemistry. The log transforms are useful for studies of sequence stratigraphy but are particularly valuable for computer modeling because log data are both numerical and numerous. They can be analyzed statistically to extract trends and periods, which can be keyed to specific geologic properties rather than to vague log character. The results of such analyses should provide explicit input for realistic computer modeling. Alternatively, the output from such models can be used to evaluate the degree to which the data match the spatial distribution of diagnostic log features.
I discuss two case studies that apply natural spectral gamma-ray and lithodensity-neutron logs to the analysis of geochemistry and mineralogy. Both these log combinations have been run increasingly frequently since their introduction in the 1970's and can be found in boreholes from most basins of the world. They should be viewed as the forerunners of a new generation of logging tools that measure properties with a high geologic information content. One example is induced spectral gamma-ray sondes, which provide logs of a variety of elements and have been used extensively in the Deep Sea Drilling Program [e.g., Brewer et al. (1990)]. The case studies give some examples of transformations of nuclear logs to profiles of variables keyed to properties of sedimentary facies. The profiles can be thought of as templates to be matched critically with the results from computer-simulation modeling. In addition to their sedimentologic information content, the profiles have the particular advantages of being lengthy, continuous, and numerical and so can be readily compared with the output of simulation computer runs. 2ff7e9595c
Comments