Subsurface Mapping Introduction

Introduction Objectives of subsurface mapping Methods Before you start

Objectives Most of the Earth’s structure and stratigraphy are hidden Structure/stratigraphy may host features of economic or academic importance Need to be able to map/interpret the subsurface

Objectives Petroleum industry Mining Groundwater Explore for and develop oil and natural gas reserves Mining Explore for and develop minerals and other economic deposits Groundwater Explore for and develop groundwater

Objectives Waste disposal Geotechnical engineering Academic studies Find suitable repositories for waste Environmental remediation Geotechnical engineering Locate and map the distribution of layers with specific properties Academic studies Structure, stratigraphy Etc.

Objectives This course: focus on petroleum Why? Methods applicable to other fields Best datasets

Objectives Develop the most reasonable subsurface interpretation(s) for the area being studied, even in areas where the data are sparse or absent Integration of different types of data Interpretations used to direct future exploration & development

Methods Wireline logs Core, cuttings (“samples”) Seismic data (2-D, 3-D) Potential field data (gravity, aeromag) Ground-penetrating radar Engineering data (fluids, pressures, etc.) Previous reports Etc.

Geophysical logs used in groundwater studies http://sofia.usgs.gov/publications/wri/94-4010/upperflaq.html

Geotechnical borehole logs http://www.geoprobe.com/products/tools/geotechnical/cptpro_log.htm

Tail buoy Streamer Air Guns Marine seismic vessels typically tow arrays of air guns and streamers containing hydrophones a few meters below the surface of the water.

Messinian Salt – Mediterranean Sea Courtesy TGS-Nopec

Vibroseis Operations - Algeria Courtesy François Gauthier

Mapping Faults Using Dip Panels in Thrust Regimes Waiting on 84wy314 * Dip panels are areas of continuous, (near) parallel dip * 'Smooth' changes in dip usually indicate axial planes * Discordant dip panels show fault locations * No data areas are usaully POORLY IMAGED steep dips, not faults Courtesy Anadarko

Slicing and Dicing to Extract Geologic Information Seismic Cube Timeslices

Methods Previous reports Paper copies of data (analog) Digital data: computer-based interpretations

Methods Caveat: Without understanding what you’re doing, you’ll get a wrong answer. Working with computers, you’ll get a wrong answer at light speed (but it may be good-looking!).

Methods Interpretations (maps, etc.) are working hypotheses, never “final” May need to update, revise as new data become available Easier with digital data Applies to ANY branch of (Earth) Science!!!

Collect Data Collect Data Interpret Data Interpret Data Drill Drill

Before you start… Need clear definition of objectives Structure? Stratigraphy? Exploration? Development? Etc. Without knowing where you’re going, you’re unlikely to get there…

Before you start… Need good understanding of structural geology, stratigraphy and other related fields The more geology one knows, the more reasonable the resulting interpretation Need good understanding of physical basis for any geophysical methods being used

Prestack Depth Migration Example Subsalt – Offshore GOM

Before you start… Use correct mapping techniques and methods Prepare reasonable subsurface maps and cross-sections “Maps and cross-sections are the primary vehicles used to organize, interpret and present available subsurface information” Digital products (e.g., volumes) becoming more common

http://www.kgs.ku.edu/Workshops/IVF2000/nissan-ivf/tocnav640.html

Geocellular model: Each cube is assigned initial properties (porosity, permeability, fluid saturation etc.)

3D display of georeferenced scanned images of collar locations, geological cross-sections, integrated with drillholes displaying assay concentrations, and magnetic and gravity modeled bodies. www.rockware.com

Before you start… Need to define fault behaviour in 3-D Faults may have important affects on location, movement of fluids Show location of faults on maps, but also map fault surfaces May need to do structural reconstruction to validate accuracy of fault interpretations

A 2-D expression of a fault

Faults are 3-D features Twiss and Moores, 1992

Fault visualization (Hart et al., 2001)

Fault visualization (Sagan and Hart, in press)

Courtesy Midland Valley

Before you start… All subsurface data need to be used to develop a reasonable and accurate subsurface interpretation Geologic (including paleontologic), geophysical, engineering Attention to resolution, accuracy of each type Integration of different types of data results in more robust interpretation

Before you start… Need to document the work done. Lots of data likely to be collected and/or generated – document in a format that may be referenced, used, revised People working on project may change, may want to “revisit” work many years after it is done Electronic media (PowerPoint files, hypertext documents, etc.) good vehicles – especially when working with digital data

Really great stuff Fred, but how did you generate it? Ummm…. Really great stuff Fred, but how did you generate it?

Summary Subsurface mapping of important economic & fundamental importance Push towards digital, computer-based interpretation Need to understand fundamentals – geology, geophysics Integration of different types of data results in more robust interpretation