Core Description

(adsbygoogle = window.adsbygoogle || []).push({}); Learning Objectives After completing this topic "Core Description", you will be able to: Summarize how a comprehensive core description contributes to, and supplements, the overall evaluation of a reservoir. Discuss the elements required for a comprehensive core description. State the useful data and information that each descriptive element adds to the understanding of the reservoir. Introduction to Core Description https://youtu.be/kLOO81eUK98 (adsbygoogle = window.adsbygoogle || []).push({}); Whenever full diameter cores are acquired, a thorough core description (Figure 1) should be made. Figure 1: Detailed core description example In addition to the routine analysis of core plugs (porosity, permeability, grain density and fluid saturation) and special core analysis (SCAL) measurements, important data and technical interpretations can result from a thorough visual study and full description of cores (Figure 2). Careful examination and the documentation from continuous, full diameter cores provide critical data for sedimentological studies, stratigraphic correlation, depositional environment interpretation, fracture and directional permeability studies as well as LWD/wireline well log calibration. Figure 2: Cores laid out for visual inspection Such studies are important for both conventional and unconventional hydrocarbon reservoirs. These observable characteristics may be subsequently computerized and analyzed statistically, such as is routinely done with fracture study data. They may also be carefully described and permanently recorded by the use of white light, ultraviolet light and color photographs (Figure 3). Figure 3: Core photographs (adsbygoogle = window.adsbygoogle || []).push({}); In laminated zones, the measured thickness of reservoir quality rock and shale zones (Figure 4) can form the basis for a net sand determination. The depositional environment can often be inferred from the occurrence of ripple marks and cross-bedding (Figure 5), and a coarsening or fining of grain size with depth (Figure 6), especially when confirmed by other data and studies. Figure 4: Shale core Figure 5: Core section with cross-bedding Figure 6: Coarsening upward sequence Core Description Format Summary Core descriptions typically include all, or a significant number, of the following descriptions, normally in the order which it is listed below. Rock type and lithological description Color Thickness of units Grain size, sorting and texture Major characteristics Minor characteristics Degree of formation consolidation Sedimentary structures Fossils and microfossils Oil staining and fluorescence Fracturing Porosity (adsbygoogle = window.adsbygoogle || []).push({}); Lithology, Rock Type, Color and Thickness of Units (adsbygoogle = window.adsbygoogle || []).push({}); Understanding the lithology of the rock cored is important because changes in the lithology indicate variations in the chemical and physical properties, as well as in the sediment's depositional environment (Figure 1). Changes in lithology give rise to changes in grain density, natural radioactivity, acoustic properties and resistivity, all of which are important in formation evaluation and, in particular, in understanding the downhole wireline and/or LWD well log responses. Figure 1: Depositional environments The lithological description of cores should follow a basic format similar to that used in the description of ditch cuttings. The American Association of Petroleum Geologists, among many others, have published detailed instructions on sample description methodology. If more than one lithology or interbedded lithologies occur within an interval, the percentage volume of each lithology should be estimated. The contacts between lithologies should be identified as gradational, interbedded or sharp, some of which will be erosional and may represent unconformity surfaces. Hydrocarbon reservoir rock types in conventional reservoirs usually break down into sandstones (clastic rocks), composed predominantly of siliceous grains (SiO2), carbonates, composed of limestone (CaCO3) and dolomite (CaMgCO3) or, in unconventional reservoirs, shales and coals. Coarse-grained sandstones tend to make high quality reservoirs, but with reducing grain size, eventually degrade into siltstones (Figure 2) which may also be of commercial reservoir quality but often with significantly reduced permeability. Figure 2: Sandstones and siltstones (adsbygoogle = window.adsbygoogle || []).push({}); Carbonates (Figure 3) are sedimentary rocks formed at, or near, the earth's surface by precipitation from solution at surface temperatures, or by the accumulation and lithification of fragments of pre-existing rocks or the remains of organisms. Figure 3: Carbonate rocks Shales (Figure 4) will also be encountered in cores, either as reservoirs in unconventional shale oil and gas plays or as non-reservoir intervals and seals in conventional hydrocarbon accumulations. Shale gas is natural gas stored in rocks that are rich in organic material, such as dark-colored shale. Gas shales are often both the source rocks and the reservoir for the natural gas, which is stored in three ways: Adsorbed onto insoluble organic matter, kerogen, that forms a molecular or atomic film Trapped in the pore spaces of the fine-grained sediments interbedded with shale, much like in conventional reservoirs Confined in fractures within the shale itself Figure 4: Shale gas reservoir rocks Organic-rich shales, which traditionally have been viewed as source rocks and cap rocks for conventional hydrocarbon accumulations, are also now viewed as unconventional reservoir rocks in certain situations. Unlike a conventional oil and gas reservoir, in which the trapping mechanism limits the extent of a hydrocarbon accumulation, shale oil and gas reservoirs can be a continuous layer of hydrocarbon-bearing rock, often pervasive over a large area. The color of the core varies from all shades of gray through brown, and brown through reds and yellows (Figure 5). The color may be uniform or it may be mottled, spotted or banded. In those areas where the most productive rocks exhibit essentially the same color, this property may occasionally be omitted from core analysis reports. Figure 5: Shades of rock colors Thickness of Units A physical examination and thorough description of the core allows the thicknesses of geological units to be described down to at least the centimeter scale and even to the millimeter scale, if required. Many authors suggest that the smallest feature which is realistic to show in core descriptions should be 0.05 inches in height. Such vertical resolution is significantly superior to even the most sophisticated, modern well logging tools which average their readings over the logged interval according to the vertical resolution of the specific wireline or LWD tool in question. (adsbygoogle = window.adsbygoogle || []).push({}); Grain Size, Sorting and Texture (adsbygoogle = window.adsbygoogle || []).push({}); Clastic grain sizes and carbonate rock types are documented as a continuous vertical curve, which reflects the variations in the depositional energy of the sediments (Figure 1). By convention, grain size increases to the left, copying the typical gamma ray response in a sand/shale sequence. Figure 1: Idealized graphic log The grain size is ascertained by comparison with a grain size comparator chart (Figure 2), or a visual pattern. A high quality hand lens magnifier and a reflecting light binocular microscope are essential for accurate grain size descriptions. Figure 2: Grain size comparator (adsbygoogle = window.adsbygoogle || []).push({}); Textural characteristics of the rock include its grain size, grain size distribution and the degree of shaliness. These characteristics influence the porosity, permeability and the interstitial water saturation. Another commonly used scale for assessing grain size in clastics is the Wentworth Scale (1922). This scale, shown in modified form in Figure 3, links medium, fine or other reported grain sizes to a specific grain diameter. Figure 3: Grain size comparator (after Wentworth) The detrital clay content and the sorting and roundness of grains are factors in the textural maturity of sandstones. Folk's (1974) model for textural maturity is well respected and followed by many in the oil industry (Figure 4). Figure 4: Folk's classification system (adsbygoogle = window.adsbygoogle || []).push({}); Major and Minor Characteristics and Degree of Formation Consolidation (adsbygoogle = window.adsbygoogle || []).push({}); Major Characteristics Major characteristics might include the fact that the rock is laminated (Figure 1) or shaly, as well as the minerals present (quartz, feldspar [Figure 1], clays, etc.). Figure 1: From left to right: rock laminations, quartz and feldspar Minor Characteristics Minor characteristics might indicate a minor percentage of accessory minerals such as mica, pyrite or glauconite (Figure 2). Other important sedimentary characteristics and structures, such as the presence of stylolites (serrated surfaces within a rock mass from which mineral material has been removed by pressure dissolution in a process that decreases the total volume of rock) (Figure 3), should be recorded when present. (adsbygoogle = window.adsbygoogle || []).push({}); Figure 2: From left to right: mica (muscovite), pyrite and glauconite Figure 3: Stylolite Although a hardness scale exists for pure minerals, it is not generally used for core descriptions. Instead, the degree of consolidation or unconsolidation, and whether it is friable or well cemented (Figure 4 ), are the characteristics that are noted in describing a core. Figure 4: Friable core section (left) and well-cemented core section (right) (adsbygoogle = window.adsbygoogle || []).push({}); Sedimentary Structures (adsbygoogle = window.adsbygoogle || []).push({}); Sedimentary structures are those structures formed during the deposition of the sediments. Sedimentary structures, such as cross-bedding and graded-bedding, are used in sedimentological and stratigraphic studies to indicate the original position of strata in geologically complex areas and, very importantly, to better understand the sediment's depositional environment. Sedimentary Structures Cross-Bedding Cross-bedding includes ripples, dunes and any cross-stratification caused by currents. There is an angle between flat bedding and the inclined bedding of the cross-bed, hence the term "cross-bedding" (Figure 1). Figure 1: Cross bedding Graded Bedding Graded bedding occurs when the current velocity reduces and grains are progressively dropped out (Figure 2). Figure 2: Graded bedding (adsbygoogle = window.adsbygoogle || []).push({}); Ripple Marks Ripple marks form in conditions with flowing water; they can be symmetrical when formed by a two-directional current, or asymmetrical when formed by a one-way current (Figure 3). Figure 3: Ripple marks Once the depositional environment is well understood, the highest and lowest quality areas of the reservoir can be inferred and, consequently, development well placement and spacing optimized appropriately. This applies to both conventional and unconventional reservoirs. The sedimentary structures are normally noted to the right of the grain size curve. (adsbygoogle = window.adsbygoogle || []).push({}); Fossils and Microfossils (adsbygoogle = window.adsbygoogle || []).push({}); The preserved remains or traces of animals, plants and other organisms (Figure 1) are often good indicators of the depositional environment and, where fossils have a limited existence range, the age of the formation. Fossil material encountered in cores has higher reliability than that obtained from ditch cuttings, which may have their depth integrity made uncertain by cavings and the drilling mud circulation lag time. Figure 1: Seaweed imprint in a core of the Miocene Santos Shale (adsbygoogle = window.adsbygoogle || []).push({}); Oil Staining and Fluorescence (adsbygoogle = window.adsbygoogle || []).push({}); The presence and color of any oil stain and oil fluorescence are both routinely recorded (Figure 1). Figure 1: Core photographs Core samples that contain oil will glow when placed under an ultraviolet light. It is often possible to prove the presence of hydrocarbons by observing this fluorescence, and to predict the expected hydrocarbon production phase from a zone. For example, gas condensate typically appears as a uniformly bright blue/white, whereas oil productive zones exhibit a uniform bright gold fluorescence. The absence of fluorescence indicates either that oil is absent, or, if oil is present, that the aromatic compounds have been leached from the oil into the core. Some minerals exhibit fluorescence under ultraviolet light, but mineral fluorescence is of no commercial significance. When it is not possible to differentiate between the oil fluorescence and mineral fluorescence, a small portion of the core is chipped off and placed in a clean, dry dish. A solvent such as chlorothene, is poured onto the chip. This dissolves the oil that is present, and the fluorescence may be observed in the solvent moving away from the core sample. This is referred to as “oil cut fluorescence,” and the nature and speed of the cut is noted. (adsbygoogle = window.adsbygoogle || []).push({}); Fracturing and Porosity (adsbygoogle = window.adsbygoogle || []).push({}); Fractures are significant to both reservoir capacity and performance, especially its permeability. The data needed to evaluate a fractured reservoir are commonly obtained from the detailed description of reservoir cores (Figure 1), including their associated well logs. Cores and core-calibrated well logs provide the one-dimensional depth and lithological variation in fracturing in one position in the reservoir. Figure 1: Measuring and evaluating core fractures (adsbygoogle = window.adsbygoogle || []).push({}); For clastic rocks, porosity falls into the following categories: Intergranular Intragranular Microporosity (difficult to recognize, even with a binocular microscope) Fracture Ideally, thin sections should be made from which determinations of the porosity types and percentages can be accurately made (Figure 2). A thin section is a laboratory preparation of the rock for use with a polarizing petrographic microscope, electron microscope and electron microprobe. A thin sliver of rock is cut from the core sample with a diamond saw and ground optically flat. These will supplement the core logging estimates. Figure 2: Thin section of carbonate rock For carbonates, a scheme such as that shown in Figure 3 is often used. Figure 3: Types of carbonate porosity (adsbygoogle = window.adsbygoogle || []).push({}); Core Description Example (adsbygoogle = window.adsbygoogle || []).push({}); All of the properties of a full diameter core described in this course are evaluated and integrated to form a summary core description, which might read as follows for one small interval: "Depth interval from 10,000 to 10,000.2 ft / 3,048 to 3,048.1 m: Sandstone, buff-yellow, 5 cm / 2 in. thick lamination, fine-grained, shaly and laminated, friable, minor secondary silica cementation, minor cross-bedding, one lagenida foraminifera fossil and minor bioturbation, blue/white uniform hydrocarbon fluorescence with a slow streaming cut, no visible fracturing, low intergranular porosity." Figure 1: Detailed core description example (adsbygoogle = window.adsbygoogle || []).push({}); Assignment (adsbygoogle = window.adsbygoogle || []).push({}); You are the supervisor for the core analysis program to be conducted on some full diameter core recently acquired by your company. Provide a list of the topics which you would like to see addressed in the core description report. Solution Ideally, the core description report should include descriptions of all of the following: Rock type Lithological description Color Thickness of units Grain size Sorting Texture Major characteristics Minor characteristics Degree of formation consolidation Sedimentary structures Fossils and microfossils Oil staining and fluorescence Porosity Assessment 1. Which of the following can generally be acquired with a core description? (Select all that apply.) A .Fossils Correct ✔B .Color Correct ✔C .Degree of formation consolidation Correct ✔D .Grain size distributionE .Permeability (adsbygoogle = window.adsbygoogle || []).push({}); 2. What types of porosity can occur in clastic rocks? (Select all that apply.) A .Intragranular Correct ✔B .Microporosity Correct ✔C .Fracture Correct ✔D .Intergranular Correct ✔E .Fenestral 3. What types of preserved remains are often good indicators of the depositional environment? (Select all that apply.) A .Animals Correct ✔B .Plants Correct ✔C .HumansD .Other organisms Correct ✔ 4. Variations in what properties often cause changes in a rock’s lithology? (Select all that apply.) A .Variations in the physical properties Correct ✔B .Variations in the sediment’s depositional environment Correct ✔C .Variations in the chemical properties Correct ✔D .Variations in the rock’s age 5. When the rock’s color is not uniform, how can it be described? (Select all that apply.) A .Mottled Correct ✔B .FadedC .Spotted Correct ✔D .Banded Correct ✔ 6. What color fluorescence do oil productive zones usually exhibit? A .WhiteB .SilverC .Gold Correct ✔D .Black Recommended for You Core Reports Permeability Measurement The Principles of Geosteering | Geosteering Best Practices Preparing the Geosteering Plan Geosteering Practices Case Study Geosteering (adsbygoogle = window.adsbygoogle || []).push({});