Subsurface Methods PDF

Methods of Contouring by Hand 1. Mechanical Contouring A 2 4 It is not a reliable method for geologic...

Methods of Contouring by Hand 1. Mechanical Contouring A 2 4 It is not a reliable method for geological maps in general, but it can be used in Geometry 5 these cases: A’ Beginning work in a new geographic area Geology Mature fields with sufficient amount of seismic or well control. Mechanical contouring method Litigation (Modified from Bishop 1960. Published by permission of author.) Equity determination This method is used when assuming the slope Unitization or angle of dip of the surface being contoured is uniform between points of control. Methods of Contouring by Hand 2. Parallel Contouring This method has several advantages over mechanical contouring: It allow some geologic license to draw a more realistic map, because there is no assumption of uniform dip. It is not as conservative as the true mechanical contouring. Therefore, it may reveal features that would not be represented on a mechanically contoured map This method does not assume uniformity of slope or angle of dip, but the contour lines are drawn parallel or nearly parallel to each other. Parallel contouring method (Modified from Bishop 1960. Published by permission of author.) Methods of Contouring by Hand 3. Equal-Spaced Contouring This method can be used in the early stages of mapping, because it can define the maximum number of structural highs and lows expected in the study area. The data used to establish the slope or rate of dip are not on opposite sides of a nose or on Equal-spaced contouring method opposite flanks of a fold. (Modified from Bishop 1960. Published by permission of author.) Methods of Contouring by Hand 4. Interpretive Contouring Geological Thinking No assumptions are made. Therefore, the geoscientist can use experience, imagination, ability to think in three dimensions, and an understanding of the structural and depositional style in the geologic region being worked to Interpretive contouring method (Modified from Bishop 1960. Published by permission of author.) develop a realistic interpretation. Methods of Contouring by Hand The specific method chosen for contouring may be dictated by such factors as the number of control points, the areal extent of these points, and the purpose of the map. It is essential to remember that no matter which method is used in making a subsurface map, the map is probably not perfectly correct, because no one can really develop a definite interpretation of the subsurface with the same accuracy as that of a topographic map. What is important is to develop the most reasonable and realistic interpretation of the subsurface with the available data, whether the maps are constructed by hand or with the use of a computer. Computer-Based Contouring Computers allow us to quickly create a map even without having to think about the surface that is being contoured. They give us the ability to generate a map to the point where the actual geology may be overlooked. Computers have made it easy to skip tried-and-tested techniques that ensure accurate maps because those techniques take too long or are not available in the computer program. This is the downside of computer mapping; the side that lacks the interpretive chemistry that occurs when a geoscientist draws contours by hand and thinks about a surface being contoured. That interpretive thought process in many instances has been replaced with concerns about transferring data from one program to another and learning which parameters will create a surface map. Computer-Based Contouring There is an upside to computer mapping, as well. With the speed and power of computers the geoscientist can quickly test many interpretations, easily check two surfaces to see if they cross, use colors to see if faults reverse direction along strike, and view in three dimensions the created surface to understand the reasonableness and validity of its form. With computers, just as with hand contouring, if the correct methods and proper quality control are not used, then the generated map will likely be wrong. Computer-Based Contouring Compute-based contouring in its simplest definition is an interpolation. An interpolation process calculates the value of a surface at locations where it is unknown, based upon the values at locations where it is known. Interpolation is an art in the sense that there is no limit to the number of mathematical formulae that may be conceived to make the estimates, and the choice of formulae includes subjective and aesthetic criteria:  Does the map look geologically reasonable?  Does it come close to how you would do it by hand? Is it pleasing? Computer-Based Contouring Two approaches to deal with random data distribution have emerged:  Indirect (Gridded)  Direct (Non-gridded) (Triangulation) Computer-Based Contouring  Indirect (Gridded) The main purpose is to simplify the contouring by making the geometry more manageable. + + + + + + + + + + + + + + + + + + B + + + + + + + + + A + + + + + + + + + + + + + + + + + + + + + + C+ + + + + + + + + + + + + + + +D + + + + + E+ + + + + + + + + + + Primary (Original) Data Points + Grid Nodes Computer-Based Contouring Steps Involved In Gridding Gridding has come to mean using the original data points to estimate values at the calculated points, or grid nodes. Gridding steps: 1. Select a grid size and origin. 2. Select neighboring data points to be used in calculating a value at each grid node. 3. Estimate the value at that grid node using values from the neighboring points. These last two steps are where numerous schemes have been developed to make maps that are aesthetically pleasing and that honor the data points as much as possible. Computer-Based Contouring Selecting Neighbors 1. Nearest “n” Neighbors 2. Natural Neighbors Computer-Based Contouring Estimating Values at Grid Nodes Once we have selected neighbors of a grid node, we proceed to use the Z values of these neighbors to estimate a value at the grid node. The following are some of the methods used 1- Weighted average 6- Minimum curvature 2- Least squares 7- Polynomial fit 3- Tangential 8- Double Fourier 4- Spline 9- Triangle plane 5- Hyperbolic Each of these methods (and other schemes) has its advocates and adversaries. Any of them works well if the data points are well distributed and well behaved. Each method has problems under certain circumstances Computer-Based Contouring Highlights about Indirect Techniques (Gridding) 1. Gridding can never guarantee maps that honor all of the data points. On the other hand, the non-honoring of data may be acceptable if the data are noisy or if the calculated value and the observed value at a data point location differ by an amount that is within the accuracy of the data. 2. Sparse data sets that contain clusters of closely spaced data can be troublesome for computer contouring systems, gridded or non-gridded. An example of such clustered data distribution is in oil and gas exploration areas, which include wildcat areas (sparse data) and some oil and gas fields (clustered data). Computer-Based Contouring  Direct (Non-gridded) (Triangulation) Triangulation is the most common of the direct contouring techniques that interpolate values along a pattern which need not be regular but which is derived from the pattern of the original data. The pattern includes the locations of the original data, which are kept throughout the subsequent processing, thus providing the opportunity that all contour lines will honor all the original data. Computer-Based Contouring  Direct (Non-gridded) (Triangulation) Computer-Based Contouring  Direct (Non-gridded) (Triangulation) Computer-Based Contouring  Direct (Non-gridded) (Triangulation) Computer-Based Contouring  Direct (Non-gridded) (Triangulation) Computer-Based Contouring Highlights about Triangulation 1. Triangulation always honors every data point because the original data points always remain in the data set. 2. The interpretation is essentially the same, regardless of the number of triangles or smoothing. 3. The user does not have to worry about data distribution. Leaving no gaps and creating no overlap.  DIRECTIONALLY DRILLED WELLS AND DIRECTIONAL SURVEYS Drilling a Well Drilling a Well Vertical Wells KB: Distance from Kellybushing to sea level MD: Measured Depth which is the measured distance along the path of a wellbore from the KB to TD (total depth of the well). TVD: True Vertical Depth which is the vertical distance from the KB to any point in the Subsurface. SSTVD: Sub-sea True Vertical Depth which is the vertical distance from sea level to any point in the subsurface. Vertical Wellbore: A well drilled 90 degree to a horizontal reference, usually sea level (also called a straight hole). Definition Of Directionally Drilled Wells A directionally drilled or deviated well is defined as a well drilled at an angle less than 90 degrees to the horizontal. Wells are normally deviated intentionally in response to a predetermined plan; however, straight holes often deviate from the vertical due to bit rotation and natural deviation tendencies related to rock types and structure. Application Of Directionally Drilled Wells There are a number of reasons to drill a directional well. The most common application is the drilling of offshore wells from a single platform location. The use of a single platform from which multiple wells are drilled improves economics and simplifies production facilities. Application Of Directionally Drilled Wells Onshore, wells are commonly deviated due to inaccessibility to the surface location directly over the subsurface target. Buildings, towns, cities, rivers, and mountains are the kinds of surface obstructions that require the drilling of a deviated well. Horizontal wells are a special type of directional well. One very important safety application of a deviated well is the drilling of a relief well to kill a well that has blown out. Application Of Directionally Drilled Wells From LeRoy and LeRoy 1977. Published by permission of the Colorado School of Mines. Applications of directional drilling (a) Multiple wells offshore or from artificial islands. (e) Stratigraphic trap. (b) Shoreline drilling. (f) Relief well control. (c) Fault control. (g) Straightening hole and side tracking. (d) Inaccessible surface location. (h, i, j) Salt dome drilling. Types Of Directionally Drilled Wells There are many complex factors that go into the design of a directionally drilled well; however, most deviated wells fall into one of three types. The most common type is a simple ramp well, sometimes called an “L” shape hole. These wells are drilled vertically to a predetermined depth and then deviated to a certain angle, which is usually held constant to total depth (TD) of the well Types Of Directionally Drilled Wells Many wells are drilled with an “S” shape design. For an “S” shape hole, the well begins as a vertical hole and then builds to a predetermined angle, maintains this angle to a designated depth, and then the angle is lowered again, often going back to vertical. Horizontal wells are configured by continuously building the deviation angle until the desired near-horizontal orientation is reached. Types Of Directionally Drilled Wells KOP Kick-off point. Depth of initial deviation from vertical measured as measured depth (MD), true vertical depth (TVD), or subsea true vertical depth (SSTVD). Build Rate Build angle. Rate at which the angle changes during deviation. It is usually expressed in degrees per 100 ft drilled. Example: 2 deg per 100 ft. Ramp Hole angle, drift angle, angle of deviation. Angle Angle from the vertical that a well maintains from the end of the build through the ramp segment of the well. BHL Bottom-hole location. Horizontal and vertical coordinates to the total depth point usually measured from the surface location. Diagrammatic cross section illustration of a simple ramp or “L” shape well. Published by permission of Tenneco Oil Company Types Of Directionally Drilled Wells KOP Kick-off point. Depth of initial deviation from vertical measured as measured depth (MD), true vertical depth (TVD), or subsea true vertical depth (SSTVD). Build Rate Build angle. Rate at which the angle changes during deviation. It is usually expressed in degrees per 100 ft drilled. Example: 2 deg per 100 ft. Ramp Hole angle, drift angle, angle of deviation. Angle Angle from the vertical that a well maintains from the end of the build through the ramp segment of the well. BHL Bottom-hole location. Horizontal and vertical coordinates to the total depth point usually measured from the surface location. Drop Rate Rate at which the ramp angle changes in degrees per 100 ft. Measured in “S” shape holes. Vertical The depth where the well is back to vertical, Point measured as MD, TVD, or SSTVD. Diagrammatic cross section illustration of a complicated “S” Shape well. Published by permission of Tenneco Oil Company Horizontal Wells Horizontal wells are typically considered to be wells with the borehole drilled within about 3 degrees of bed dip or wells drilled nearly horizontally. The purpose of drilling most horizontal wells is to improve the economics of a project by increasing production rates and shortening well life. Types Of Directionally Drilled Wells Diagrammatic cross section illustration of a horizontal well. Published by permission of J. Brewton Directional Well Plan A variety of data go into the design of a directionally drilled well, including the depth and distance from the surface location to each subsurface target, diameter of the target, KOP, build rate, platform location, lease lines, hole size, and total depth of the well. Once preliminary studies indicate the need for a deviated well, most companies rely on a directional drilling service company to prepare the final directional plan. Directional Well Plan Vertical section plan for a directional well Horizontal plan for the same directional well Published by permission of Gardes Directional Drilling Published by permission of Gardes Directional Drilling Directional Tools Used For Measurements Three features of a directional wellbore are measured at given points within the well: (1) Measured Depth, which is the distance from the surface to a given point, measured along the wellbore. (2) Angle of Inclination from the vertical (drift angle or deviation angle). (3) Drift Direction, or the directional path of the wellbore. These parameters are the basis for calculations of the position of each point in the subsurface, and all this information is included in a directional survey. Directional Tools Used For Measurements Drift angle and drift direction are measured by a survey tool conveyed by drill pipe or by wireline, and measured depth is determined by length of drill pipe or wireline. The various tools that are used fall into two categories magnetic and nonmagnetic. Wireline Logging WIRELINE LOGGING Record of the properties of rock and other subsurface measurements against depth. (e.g. Resistivity, Density, Porosity, Dipping , Pressure,…..etc.) Wireline Logging Wireline Logging Open Hole Logs include: SP -Spontaneous Potential GR –Gamma Ray Caliper Resistivity Density Porosity Sonic Dipmeter Cased Hole Logs include: Cement Bond Log (CBL) Production Logs Wireline Logging 0.2 Ohmm 2000 Shale Sand Shale Shallow Hydrocarbon Sand Deep Water Shale Applications of Wireline Logs in Defining Subsurface Structures Different outcrops showing angular unconformities ISOPACH Vs. ISOCHORE Cross Sections Stratigraphic X-Sections Structural X-Sections 1- Flattened at Stratigraphic Datum “GM” 1- Flattened at Sub-sea Depth 2- Show Stratigraphic Plays/ Facies 2- Show Structural Features 3- Reflects Old Stratigraphic Settings at 3- Reflects Present Day Settings Genetic markers 4- Important for Stratigraphic Models 4- Important for Potential Evaluation Fence Diagram Fence diagram, also called panel diagrams consists of a three dimensional network of cross section drawn in two dimension. They are designed to illustrate the areal relationship among several wells that are located in close proximity to each other. Fence diagrams can either be structural or stratigraphic. References Tearpock, D., & Bischke, R. (1991). Applied Subsurface Geological Mapping. Prentice-Hall PTR. Tearpock, D., & Bischke, R. (2002). Applied Subsurface Geological Mapping with Structural Methods. Prentice-Hall PTR. Philosophical Doctrine 1 2 3 4 5 6 7 8 9 10 The Philosophical doctrine for generating an accurate subsurface interpretation and mapping is the result of decades of research, observation, and analyses as the best-proven process for finding/developing hydrocarbons. It basically requires common sense, a certain technical background, experience, logic, and the application of proven scientific methods. Philosophical Doctrine 1 2 3 4 5 6 7 8 9 10 All subsurface interpretations must be geologically and geometrically valid in three dimensions. Remember… It is necessary to validate your interpretation prior to any investment decision. Philosophical Doctrine 1 2 3 4 5 6 7 8 9 10 An interpreter must have a fundamental, classic education in geology and a strong background in sedimentology, petroleum geology, and structural geology for the tectonic setting being worked. Philosophical Doctrine 1 2 3 4 5 6 7 8 9 10 Sufficient planning, time, and detail are required to generate reliable prospects. Haste makes waste. Philosophical Doctrine 1 2 3 4 5 6 7 8 9 10 All subsurface data must be used to develop a reasonable and accurate subsurface interpretation. Philosophical Doctrine 1 2 3 4 5 6 7 8 9 10 Accurate correlations (well log and seismic) are required for reliable geologic interpretations. Philosophical Doctrine 1 2 3 4 5 6 7 8 9 10 The use of correct mapping techniques and methods is essential to generate reasonable and correct subsurface interpretations. Philosophical Doctrine 1 2 3 4 5 6 7 8 9 10 All important and relevant geologic surfaces must be mapped and the maps integrated to arrive at a reasonable and accurate subsurface picture. Philosophical Doctrine 1 2 3 4 5 6 7 8 9 10 The mapping of multiple horizons is essential to develop reasonably correct, three dimensional interpretations of complexly faulted areas. Philosophical Doctrine 1 2 3 4 5 6 7 8 9 10 Balanced cross sections are required to prepare a reasonably correct interpretation of complexly deformed structures. Philosophical Doctrine 1 2 3 4 5 6 7 8 9 10 All work should be documented Types of Subsurface Maps The following list shows examples of contourable data and the associated contour map. Data Type of Map Depth Structure, Fault, Salt Thickness Isopach, Isochore Thickness of Pay Net Pay Pressure Isobar Temperature Isotherm Lithology Isolith Types of Subsurface Maps and Cross Sections Subsurface Maps Structure, porosity, fault surface, salt, net reservoir, net oil / gas, isochore, isopach, isolith, and facies maps Cross Sections Structural, stratigraphic, correlation sections, fence diagram Petroleum System Elements Hydrocarbons 1. Source Rock 3. Seal Rock 5. Migration 2. Reservoir Rock 4. Trap 6. Timing Source Rocks Reservoir Rocks Reservoir Rocks Reservoir Rocks Petroleum System E Youngest D C B A Oldest Original Horizontality Petroleum System E Youngest D C B A Oldest Original Horizontality Petroleum System Petroleum System Petroleum System Petroleum System Elements No Source Rock Petroleum System Elements No Reservoir Rock Petroleum System Elements No Seal Rock Petroleum System Elements No Trap Petroleum System Elements No Migration Petroleum System Elements Timing Petroleum System Elements Ideal Case Hydrocarbons Contouring Contouring Isometric view of dipping plane intersecting three horizontal planes. Isometric view of a curved surface intersecting a finite number of evenly (Modified from Appelbaum. Geological & Engineering Mapping of Subsurface: A workshop course by Robert Appelbaum. spaced horizontal planes. Published by permission of Prentice-Hall, Inc.) (Modified from Appelbaum. Geological & Engineering Mapping of Subsurface: A workshop course by Robert Appelbaum. Published by permission of Prentice-Hall, Inc.) Contouring The spacing of contour lines is a function of the shape and slope of the surface being contoured. Rules of Contouring A contour line cannot cross itself or any other contour except under special circumstances. Moreover, a contour line cannot merge with contours of the same value or different values. They may appear to merge or even cross where there is an overhang, overturned fold, or vertical surface. With these exceptions, the key word is appear. Consider a vertical cliff that is being mapped. In map view the contours appear to merge, but in three- dimensional space these lines are above each other. Rules of Contouring Using dashed contour lines on the underside of the structure is representing or illustrating a three- dimensional overhang or overturned fold. (From Tearpock and Harris 1987. Published by permission of Tenneco Oil Company.) Rules of Contouring Rules of Contouring Rules of Contouring 14 m 14 12 m 10 m Rules of Contouring Contour lines must pass between points whose values are lower and higher than its own value. A contour line of a given value is repeated to indicate reversal of slope direction. This figure illustrates the application of this rule across a structural high (anticline) and a structural low (syncline). A contour line must be repeated to show reversal of slope direction. (From Tearpock and Harris 1987. Published by permission of Tenneco Oil Company.) Rules of Contouring All contour maps should have a chosen reference to which the contour values are compared. A structure contour map, as an example, typically uses mean sea level as the chosen reference. Therefore, the elevations on the map can be referenced as being above or below mean sea level. A negative sign in front of a depth value means the elevation is below sea level (e.g., –7000 ft). Rules of Contouring The contour interval on a map should be constant because the distance between successive lines has a direct relationship to the steepness of slope. Steep slopes are represented by closely spaced contours and gentle slopes by widely spaced contours. If for some reason the contour interval is changed on a map, it should be clearly indicated. This can occur where a mapped surface contains both very steep and gentle slopes, such as those seen in areas of salt diapirs. Rules of Contouring Rules of Contouring The choice of a contour interval is an important decision. Several factors must be considered in making such a choice. These factors include the density of data, the practical limits of data accuracy, the steepness of slope, the scale of the map, and its purpose. If the contour interval chosen is too large, small closures with less relief than the contour interval may be overlooked. If the contour interval is too small, the map can become too chaotic and reflect inaccuracies of the basic data. Rules of Contouring All maps should include a graphic scale. Many people may eventually work with or review a map. A graphic scale provides an exact reference and gives the reviewer an idea of the areal extent of the map and the magnitude of the features shown. Also, it is not uncommon for a map to be reproduced. During this process, the map may be reduced or enlarged. Without a graphic scale, the values shown on the map may become useless. Rules of Contouring Every fifth contour should be thicker than the other contours, and it should be labeled with the value of the contour. This fifth contour is referred to as an index contour. Hachured lines should be used to indicate closed depressions Rules of Contouring Start contouring in areas with maximum number of control points Construct the contours in groups of several lines rather than one single contour at a time. This should save time and provide better visualization of the surface being contoured. Rules of Contouring Use a smooth rather than undulating style of contouring unless the data indicate otherwise. Choose the simplest contour solution that honors the control points and provides a realistic subsurface interpretation.  LOG CHARACTERS AND FACIES CORRELATION Retrogradation, Aggradation, and Progradation Parasequence Stacking Pattern Retrogradational Sequence Rate of deposition is less than rate of accommodation Aggradational Sequence Rate of deposition is equal to rate of accommodation Progradational Sequence Rate of deposition is more than rate of accommodation General Gamma Ray Response to Variations in Grain Size Aggrading Prograding Retrograding Prograding & Aggrading Retrograding General Gamma Ray Response to Variations in Facies and Lithology Log Response of Various Depositional Environments  FAULTS AND FAULT MAPPING Fault Terminology AB = Dip Slip is the term used to describe the actual relative displacement of a fault. It is defined as the measurement of the distance of the actual relative motion between two formerly adjacent points on opposite sides of a fault. AC = Fault Throw is the vertical component of dip slip. It is the difference in vertical depth between the fault intersection with a line or plane (as formation top) in one fault block and the fault intersection with the same line or plane in the opposing fault block, determined in a direction perpendicular to the strike of the fault. AE = Vertical Separation or “Missing Section” is the vertical component of bed displacement. It is measured as the vertical distance between a line or plane (as Formation top) projected from one fault block across a fault to a point where the projection is vertically over or under the same line or plane in the opposite fault block. BC = Fault Heave is the horizontal component of dip slip. It is determined in a direction perpendicular to the to the strike of the fault. Throw = AC = AB * Sin Ө Relation Between Fault Throw and Missing Section Missing section = Vertical Separation = Fault Throw Relation Between Fault Throw and Missing Section Fault Throw is Greater than Missing Section Relation Between Fault Throw and Missing Section Fault Throw is Less than Missing Section Types of Faults Normal Fault Hanging wall moves down relative to Foot wall due to extensional forces Reverse Fault Hanging wall moves up relative to Foot wall due to compressional forces Strike Slip Fault Hanging wall and Foot wall moves laterally relative to each others Attributes of Dip-Slip Faults Missing Stratigraphic Units Repeated Stratigraphic Units Extended Layering Contracted Layering Younger Rocks above Older Rocks Older Rocks above Younger Rocks Strike-Slip Faults Strike-Slip - Horizontal Motion Strike-Slip - Horizontal Motion Left Lateral “Sinistral”; when Right Lateral “Dextral”; when blocks are moving laterally anti- blocks are moving laterally clockwise direction clockwise direction anti-clockwise direction clockwise direction Strike-Slip Faults and Associated Structures Mountains, and folds structures Pull-apart Basins Strike slip displacement occurs along near-vertical faults that offset basement. Displacements along strike-slip faults are predominantly in the strike direction of the fault, and the vertical separations of the horizons along strike-slip faults may alternate between normal and reverse separation.