18 subsurfacemapping
- 1. 1 Geol 493 Subsurface Mapping Representing a 3D surface on a map Movie The arrow points at … A. A ridge. B. A valley. What is a contour line? Describe this surface
- 2. 2 Simple Contouring Rules • A contour line separates points that are higher from points that are lower • Contour lines cannot cross • Contour lines cannot merge • Contour lines must close on themselves or extend to the edge of the map • Contour interval must remain constant for the entire map How do you make a contour map? • A contour lines separate points that are higher from points that are lower • Space the contours between all the know points • Honor all the data Contour this data using 25m interval Datum = 0 m
- 3. 3 10m contour spacing Problem: How to interpolate between the known points? 100 125 150 175 15 Triangulation 16 Gridding Methods of interpolation • Least squares • Tangential • Spline • Weighted average • Minimum curvature • Polynomial • Hyperbolic • Kriging • Trend surface • Etc., etc., etc.! a. Inverse distance b. Kriging c. Minimum curvature d. Triangulation e. Polynomial regressiona. c. b. e.d.
- 4. 4 Advantages of Computer Contouring • Fast • Objective and unbiased • Easily updated • Ready for later processing • Only practical way if you have 1000s of data points Advantages of Hand Contouring • Allows you to impart your geologic knowledge to the interpretation Depth Structure Maps Sea level 132 (132) 511 (502) -362 530 (525) -385 362 (356) -211 Land surface Cross Sectional View Well 1 Well 2 Well 4 Well 3 Well 5 •Log Depth •(True Vertical Depth) •Subsea Depth Dealing with multiple layers •Isopach maps •Plot the elevation difference between two maps Mississippian Top PennsylvanianTop 1800 1800 1700
- 5. 5 Pennsylvanian-Mississippian Isopach 500 600 300 400 800 700 Pennsylvanian plus Isopach Which well would encounter the thickest sand? Isopach Map What about Unconformities? Cross section -8000 -8200 -7800 NESW A Unconformity Surface Map Top of Layer A
- 6. 6 Base of Layer A Isopach of Layer A 25 25 50 What about mapping other parameters? • Extensive Surfaces: – Tops and Bottoms of formations – Isopachs – Fault surfaces – Fluid contacts, i.e. Gas-Oil, Water-Oil • Intensive Surfaces: – Porosity – Fluid Saturation – Production • Black contours are structure • Colors are porosity • Circles are production Michigan Michigan Tech Porosity map from reservoir model Gaspar et al. 2013 Mapping faults
- 7. 7 Contouring Faulted Surfaces • Faults break the continuity of a surface Describe the Structure Faulted surfaces Normal Fault- North Sea Virtual Seismic Atlas Making a map of a faulted structure 1. Contour the surface without faults 2. Contour the fault plane 3. Find the intersection of the two maps 4. Correct the contours for the vertical separation across the fault 5. Clean up
- 8. 8 Faulted structure contour map 3 2 14 1 Faults from well data • A normal fault can remove a piece of the stratigraphy in a well • Missing section is a indicates faulting Cross SectionNW SE Reverse Faults- cross section Reverse Faulting Example UNION OOLITE THRUST MODEL WITH FRACTURE SWARM (after Nelson, 2001) no scale UNION OOLITE
- 9. 9 DEPI #5639 (047-055-00226) Gamma Ray log UNION OOLITE (Hanging wall) UNION OOLITE (Foot wall) 100’ PILOT KNOB THRUST FAULT 228’ vertical displacement Thrust faults cause repetitions in the stratigraphy Cross-section A - A’ A A’ 0 -500 -1000 -1500 PILOT KNOB ARISTA NW SE Scale: 500’ 500’ UNION PICKAWAY DENMAR LITTLE LIME PRICE Where is the highest point in the structure in this depth map? A. B. C. D. E. AD C B E