Data processing: measuring – calibrating – or testing – Measurement system in a specific environment – Earth science
Reexamination Certificate
2001-01-08
2002-11-05
McElheny, Jr., Donald E. (Department: 2862)
Data processing: measuring, calibrating, or testing
Measurement system in a specific environment
Earth science
Reexamination Certificate
active
06477469
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to the geological study of earth formations for the location and exploitation of mineral deposits using electrofacies analysis. More particularly, the present invention relates to a new method for identifying formations of mineral deposits using a user-friendly and reliable ordering technique to order the electrofacies for different sets of log data and interpretation rules.
2. Description of the Related Art
Mineral and hydrocarbon prospecting is based upon the geological study and observation of formations of the earth's crust. Correlations have long been established between geological phenomena and the formation of mineral and hydrocarbon deposits that are sufficiently dense to make their exploitation economically profitable.
The study of rock and soil facies encountered while prospecting for minerals takes on particular importance. As used herein, a facies is an assemblage of characteristics that distinguish a rock or stratified body from others. A facies results from the physical, chemical and biological conditions involved in the formation of a rock that distinguish it from other rocks or soil. This set of characteristics provides information on the origin of the deposits, their distribution channels and the environment within which they were produced. For example, sedimentary deposits can be classified according to their location (continental, shoreline or marine), according to their origin (fluviatile, lacustrine, eolian) and according to the environment within which they occurred (estuaries, deltas, marshes, etc.). This information in turn makes it possible to detect, for example, zones in which the probability of hydrocarbon accumulation is high.
The set of characteristics used to define a facies depends on the situation. For example, a lithofacies may be defined by the rock's petrographic and petrophysical characteristics. These are the composition, texture and structure of the rock. Examples of mineral composition are silicate, carbonate, evaporite, and so on. A rock's texture is determined by its grain size, sorting, morphology, degree of compaction, and degree of cementation. The rock structure includes the thickness of beds, their alternation, presence of stones, lenses, fractures, degree of parallelism of laminations, etc. All of these parameters are related to the macroscopic appearance of the rock.
For extraction of hydrocarbons from geologic formations, the particularly pertinent characteristics of the lithofacies are the porosity of the reservoir rocks and their permeability, as well as the fraction of the pore volume occupied by these hydrocarbons. These aid in estimating the nature, quantity, and producibility of the hydrocarbons contained in such strata.
There are various sources of information on formation lithofacies. Information may be gathered from subsurface observations such as, for example, by the study of core samples taken from rock formations during the drilling of a borehole for an oil well. Such information can also be provided by drill cuttings sent up to the surface from the bottom of a well by means of a fluid (generally drilling mud) injected near the drilling tool. It is not normally cost-effective to identify facies using these methods. Information on geological formations traversed by a borehole is more commonly gathered by a measurement sonde passing through the borehole. The gathered information as a function of the sonde's position along the borehole is then stored or “logged”.
Many downhole measurement techniques have been used in the past, including passive measurements such as measuring the natural emission of gamma rays; and active measurements such as emitting some form of energy into the formation and measuring the response. Common active measurements include using acoustic waves, electromagnetic waves, electric currents, and nuclear particles. The sonde measurements are designed to reflect the distinguishing characteristics of the rock facies. Multiple logs and sondes may be used to gather the measurements, which are then correlated and standardized to furnish measurements at discrete levels separated by equal depth intervals. The measurement standardization allows the automation of data interpretation in order to obtain estimates of the rock mineral composition, the porosity of the rocks encountered, the pore size distribution, texture and structure, the pore volume occupied by hydrocarbons, and the ease of flow of hydrocarbons out of the reservoirs in the case of petroleum prospecting. The set of measured formation characteristics values that distinguish the strata in a given borehole is herein termed the electrofacies.
Interpretation studies have demonstrated a strong correlation between the electrofacies and lithofacies, thereby making it possible to identify with confidence the lithofacies of the rocks traversed by boreholes based on the sonde measurements. It has been established that the sets of log measurements (i.e., sample points) which correspond to a given lithofacies form a “cluster” in “data space”. That is, when the measured characteristic values of a formation are graphed, the points generally fall into a continuous region that is distinguishable from the regions where points for other formations would fall.
Electrofacies allow geologists to present log data that describe the cored interval of a petroleum reservoir in a standardized format such as that shown in FIG.
1
. In this figure, the leftmost column
110
shows lithofacies of a core description coded using one set of standard interpretation rules (depositional environments are ordered according to a bathymetric profile) and the rightmost column
120
shows the electrofacies obtained by log clustering and after ordering over same depth interval. In terms of the clusters in data space, the electrofacies ordering shown in
FIG. 1
may be equated to drawing a path that connects each of the clusters. The sequence in which the path visits the nodes is the order of the nodes. When considered in this manner, the relationship between the clusters is thereby simplified to a single dimension. The electrofacies may then be easily compared to each other, and the vertical distribution of electrofacies in a well, when represented in this standardized format, may more easily be compared to known sedimentary sequences. The ordering of electrofacies also allows a geologist to draw inferences about the geological setting at the time of sediment deposition or to the diagenetic history of the sediments.
Rules to manually order electrofacies are empirical, based on the observation of shale content, cementation, sediment grain size and sorting. These parameters reflect the level of the sediment deposition area with respect to base sea level and reflect the energy of the deposition environment. Thus, the rules used to order electrofacies traditionally allow the geologist to survey the vertical and lateral variation of facies that define the porous sedimentary bodies' constituent of the petroleum reservoir.
An electrofacies ordering is initially obtained and continuously updated by calibration on core material and log interpretations pertaining to core intervals (log measurements of the formations from which the cores were taken). As discussed below, ordering of the electrofacies requires interpreting the relative positions of electrofacies kernel points in the log space and interpreting the geologic significance of core interval logs. No absolute rules exist for electrofacies ordering, only guidelines adapted for different geological settings that are calibrated in the early stages of the exploration process. Because of the complexity of the calibration process and for economic reasons, core descriptions are often taken as an absolute reference. If log interpretation methods to generate electrofacies do not match the physical core descriptions, then the log interpretation methods are blamed for the discrepancy.
Geologists generally begin electrofacies ordering
Rabiller Philippe J. Y. M.
Ye Shin-Ju
Conley & Rose & Tayon P.C.
McElheny Jr. Donald E.
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