Nontechnical Guide to Petroleum Geology Exploration

vii

ContentsPreface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vIntroduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xv

Conventional Oil and Gas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xvUnconventional Oil and Gas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xxvThe End of the Well . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xxvii

Chapter 1The Nature of Gas and Oil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Petroleum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1The Chemistry of Crude Oil and Natural Gas . . . . . . . . . . . . . . . . . . . . . . . . . . . 1Crude oil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Hydrocarbon Molecules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2° API Gravity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3Sulfur . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4Benchmark Crude Oils . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4Pour Point . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5Crude Streams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6Measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6Refining . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Natural Gas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9Composition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9Occurrence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10Condensate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11Measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Barrel of oil equivalent . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

Chapter 2The Earth’s Crust – Where We Find It . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Rocks and Minerals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13Types of Rocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Igneous Rocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14Sedimentary Rocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15Metamorphic Rocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Structure of the Earth’s Crust . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Chapter 3Identification of Common Rocks and Minerals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

Identification of Minerals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23Identification of Rocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26Rocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

324421PVX_DRILLING_CC2018_PC.indd 7 09/10/2019 15:41:48

viii Nontechnical Guide to Petroleum

Igneous Rocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27Metamorphic Rocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27Sedimentary Rocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

Chapter 4Geological Time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

Absolute Age Dating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31Relative Age Dating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

Fossils . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34Microfossils . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

The Geologic Time Scale . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39Earth History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

Chapter 5Deformation of Sedimentary Rocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

Weathering, Erosion, and Unconformities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45Anticlines and Synclines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50Domes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52Homoclines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53Fractures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

Joints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54Faults . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

Chapter 6Ocean Environment and Plate Tectonics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

Continental Margins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61Deep Water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66Ocean Sediments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68Earth’s Interior . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68Continental Drift . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69Seafloor Spreading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69Plate Tectonics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73Failed Arm Basins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74Middle East Oil Fields . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76

Chapter 7Sedimentary Rock Distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79

Basin Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79Sedimentary Rock Facies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85Subsurface Rock Layers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87

Chapter 8Mapping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91

Topographic Maps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91Geologic Maps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93


Contents ix

Base Maps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96Global Positioning System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96Subsurface Maps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97

Structural Map . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97Isopach Map . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99Percentage Map . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100

Chapter 9Source Rocks, Generation, Migration, and Accumulation of Petroleum . . . . . . . . . . . . . 103

Source Rocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103Generation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104Migration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106Accumulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107Age . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109Reservoir Rocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109Saturation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112

Chapter 10Reservoir Rocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115

Sandstone Reservoir Rocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115Dune Sandstones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116Shoreline Sandstones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118River Sandstones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119Delta Sandstones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121Granite Wash . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126

Carbonate Reservoir Rocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127Reefs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127Limestone Platforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131Karst Limestone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132Chalk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134Dolomite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135

Fractured Reservoirs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138

Chapter 11Petroleum Traps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141

Structural Traps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141Anticlines and Domes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141Growth Faults and Rollover Anticlines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144Drag Folds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149Tilted Fault Blocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152

Stratigraphic Traps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153Primary Stratigraphic Traps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153Secondary Stratigraphic Traps – Angular Unconformities . . . . . . . . . . . . 158

Combination Traps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160


x Nontechnical Guide to Petroleum

Bald-headed structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160Salt domes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163

Chapter 12Petroleum Exploration – Geological and Geochemical . . . . . . . . . . . . . . . . . . . . . . . . . . 169

Seeps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 169Geological Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171

Subsurface Maps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173Correlation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 175

Geochemical Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 180Plays and Trends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181

Chapter 13Petroleum Exploration – Geophysical . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183

Gravity and Magnetic Exploration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183Seismic Exploration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187

Acquisition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187Processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193Seismic Section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 195Interpretation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 197Time-to-Depth Conversion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 201Amplitude Versus Offset . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2013-D Seismic Exploration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2024-D and 4-C Seismic Exploration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 204Artificial Intelligence and Predictive Analytics . . . . . . . . . . . . . . . . . . . . . . . 205

Chapter 14Drilling Preliminaries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 207

Land and Leasing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 207Foreign Contracts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 209

Authority for Expenditure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 210Economic Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 210Drilling Contracts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211Joint Operating Agreements and Support Agreements . . . . . . . . . . . . . . . . . . 212Site Preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 212Types of Wells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 214Government Regulations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 215Cable Tool Rigs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 216

Chapter 15Drilling a Well – The Mechanics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 219

Power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 220Hoisting System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 221


Contents xi

Rotating System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 224Circulating System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 233Drilling Operations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 239Modern Rotary Drilling Rigs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 241

Chapter 16Drilling Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 245

Subsurface Conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 245Problems While Drilling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 246

Fishing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 246Stuck Pipe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 248Sloughing Shale . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 250Lost Circulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 250Formation Damage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 251Corrosive Gasses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 251Washout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 251Abnormal High Pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 251Stacking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 254

Dry Hole Risk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 254

Chapter 17Drilling Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 257

Straight Hole . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 257Directional Drilling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 258Well Depth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 266Air and Foam Drilling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 266

Chapter 18Testing a Well . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 269

Sample or Lithologic Log . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 269Drilling-Time Log . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 273Mud Log . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 274Wireline Well Logs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 276

Electrical log . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 280Induction and Guard Logs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 286Natural Gamma Ray Log . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 288Radioactive Logs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 289Neutron Porosity Log . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 289Formation Density or Gamma-Gamma Log . . . . . . . . . . . . . . . . . . . . . . . . . 290Gas Effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 292Sonic or Acoustic Velocity Log . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 293Caliper Log . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 295Dipmeter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 297


xii Nontechnical Guide to Petroleum

Borehole Image Log . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 298Nuclear Magnetic Resonance Log . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 299Computer-Generated Log . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 300Log Combinations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 300

Measurements-While-Drilling and Logging-While-Drilling . . . . . . . . . . . . . . 301Drillstem Test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 301Repeat Formation Tester . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 303

Chapter 19Completing a Well . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 305

Casing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 305Bottom-Hole Completions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 312Expandable Casing and Liner . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 314Tubing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 315Wellhead . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 316Chokes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 317Surface Equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 317Multiple Completions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 326Intelligent Wells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 327

Chapter 20Surface Treatment and Storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 329

Flowlines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 329Separators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 331Gas Treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 335Storage and Measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 337

Crude oil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 337Natural gas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 341

Payment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 342Modern Wells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 342

Chapter 21Offshore . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 345

Mineral Rights . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 346Offshore Drilling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 347Exploratory Drilling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 348Spudding an Offshore Exploratory Well . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 352Developmental Drilling and Production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 353Subsea Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 359Subsea Completions and Wells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 360Unstable Sea Bed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 362Decommissioning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 362Offshore Disasters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 363


Contents xiii

Chapter 22Well Intervention . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 365

Equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 365Preparing the Well . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 369Well Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 369

Sand Cleanout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 369Well Cleanout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 370Pulling Rods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 370Pulling and Repairing Tubing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 371Downhole Pump . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 371Casing Repair . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 371Secondary Cementing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 372Swabbing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 373Replacing Gas Lift Valves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 374Replacing Packers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 374Recompletion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 374

Well Intervention on Offshore Wells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 375

Chapter 23Reservoir Mechanics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 377

Reservoir Drives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 377Oil Reservoir Drives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 377Natural Gas Reservoir Drives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 382

Maximum Efficient Rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 383

Chapter 24Petroleum Production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 385

Employees . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 385Well and Reservoir Pressures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 385Well Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 386Production Log . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 388Decline Curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 388Bypassing and Coning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 390Cycling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 391Well Stimulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 391

Acidizing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 391Explosive Fracturing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 392Hydraulic Fracturing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 392

Produced Water and Solution Gas Disposal . . . . . . . . . . . . . . . . . . . . . . . . . . . . 395Surface Subsidence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 397Corrosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 398Production Maps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 399Stranded Gas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 399Modern production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 400


xiv Nontechnical Guide to Petroleum

Chapter 25Reserves and Resources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 403

Recovery Factor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 403Shrinkage Factor and Formation Volume Factor . . . . . . . . . . . . . . . . . . . . . . . . 404Reserve Calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 405

Oil Reserves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 405Gas Reserves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 407Materials Balance Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 408

Types of Reserves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 408Resources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 410

Chapter 26Improved Oil Recovery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 411

Waterflood . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 411Enhanced Oil Recovery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 414

Miscible Gas Drive . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 414Pressure Maintenance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 414Chemical Flood . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 415Thermal Recovery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 416

Efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 419Unitization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 419Plug and Abandon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 419

Chapter 27Unconventional Resources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 421

Shale . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 422Tight Reservoirs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 430

Tight Gas Sands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 430Granite Wash . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 431Bakken Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 431

Coal Bed or Coal Seam Gas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 432Oil Shales . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 434Oil or Tar Sands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 435Gas Hydrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 437

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 439

World Records . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 445

Unit Conversions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 449

Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 451

Abbreviations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 471

Industry Terms & Locations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 499


14 Nontechnical Guide to Petroleum

Types of RocksThe earth’s crust is made up of three types of rock: igneous, sedimentary, and meta-

morphic . Igneous rocks have crystallized from a hot, molten liquid . Sedimentary rocks are composed of sediments, particles that were deposited on the surface of the ground or bottom of the ocean or from salts that precipitated out of water . Metamorphic rocks have been recrystallized from other rocks under high temperatures and pressures .

Igneous RocksIgneous rocks form when a molten melt has cooled . Two types of igneous rocks

are plutonic and volcanic, depending on where they formed . Plutonic igneous rocks crystallized and solidified while still below the surface of the earth . Because the solid rocks that surround the cooling plutonic rocks are good insulators, plutonic rocks often take thousands of years to solidify . When given a long time to crystallize, large mineral crystals grow . Plutonic igneous rocks are easy to identify because the mineral grains are all large enough to be seen by the naked eye (plate 2-1) . Plutonic igneous rocks formed as hot liquids that were injected into and displaced preexist-ing rocks in the subsurface (fig . 2-1) . Because of this, now solidified plutonic rock bodies are called intrusions . Volcanic igneous rocks crystallize on the surface of the earth as lava . As the lava flows out of a volcano, it immediately comes in contact with air or water and rapidly solidifies . The rapid crystallization only allows very small crystals to form that are difficult to distinguish with the naked eye . In gen-eral, igneous rocks are harder to drill than sedimentary rocks . Buried lava flows and intrusions are occasionally encountered when drilling through sedimentary rocks for gas and oil and slow the drilling down .

Fig. 2–1. Igneous rocks showing an intrusion of plutonic igneous rocks into sedimentary rock layers and volcanic igneous rocks on the surface along with microscopic views of coarse-grained plutonic and fine-grained volcanic rocks


Chapter 2 The Earth’s Crust – Where We Find It 15

Sedimentary RocksSedimentary rocks are composed of three types of sediments . Clastic sediments

are whole particles formed by the breakdown of rocks and were transported and deposited as whole particles . Boulders, sand grains, and mud particles are exam-ples . Organic sediments formed biologically such as seashells . Crystalline sediments are formed by precipitation of salt out of water . As these sediments are buried in the subsurface, they eventually become solid, sedimentary rocks . Sedimentary rocks are the rocks that are drilled to find gas and oil . They are the source and res-ervoir rocks for gas and oil .

Loose sediments (unconsolidated sediments) become relatively hard sedimen-tary rocks (consolidated sediments) in the subsurface by the processes of natural cementation and compaction . No matter how some sediments such as sand grains are packed together, there will be pore spaces between the grains (fig . 2–2) . Once the grains have been buried in the subsurface, the pore spaces are naturally filled with groundwater that can be very salty . Under higher subsurface temperatures and pressures, chemicals often precipitate out of the subsurface waters to coat the grains, and some eventually grow together to bridge the loose grains . This process, called natural cementation, bonds the loose grains into a solid sedimentary rock . The most common cement is the mineral calcite (CaCO3) . Also, as the sediments are buried deeper, the increasing weight of overlying rocks exerts more pressure on the grains . This compacts the sediments and also solidifies the rock .

Fig. 2–2. Microscopic view of pores between sediment grains

Clastic sedimentary rocks often consist of three parts when examined under a microscope (fig . 2–3) . First, there are sediment grains such as sand . Second, there are natural cements that coat and bond the grains together . Third, there are spaces called pores . In the subsurface, these pores are filled with fluids, usually water, but sometimes gas or oil .

There is an enormous amount of water below the surface in sedimentary rock pores called groundwater (fig . 2–4) . Groundwater is described by salt content in parts per thousand (ppt) . Fresh water contains so little salt (0–1 ppt) that it can be used for drinking water . Brines are subsurface waters that contain more salt than



seawater (35–300 ppt) . Brackish waters are mixtures of fresh waters and brines (1–35 ppt) . Below the surface is a boundary called the water table separating the dry pores filled with air above and pores filled with groundwater below . The water table can be on the surface or very deep depending on how much rain falls in that area . Just below the water table, the groundwater is usually fresh because of rain-water that percolates down from the surface . Deep waters, that have occupied the rock pores for a very long time, are salty . In general, the deeper you drill, the more saline the water is . When a well is drilled, completed, and producing, near-surface fresh waters that are or can be used for drinking or irrigation are protected from pollution by law throughout the world .

The size of clastic grains that make up an ancient sedimentary rock is important . The rock is often classified according to the grain size . Sandstones are composed

Fig. 2–3. Microscopic view of clastic sedimentary rock

Fig. 2–4. Groundwater



of sand-sized grains, whereas shales are composed of fine-grained, clay-sized par-ticles . Also, the size of the grains controls the size of the pore spaces and the qual-ity of the oil or gas reservoir . Larger grains have larger pores between them . It is easier for fluids, such as gas and oil, to flow through larger pores and into a well . Clastic grains in sedimentary rocks are classified by their diameters in millimeters (fig . 2–5) . They are called boulder, cobble, pebble, granule, sand, silt, and clay-sized particles . The finest grains (sand, silt, and clay-sized) are the most common .

Fig. 2–5. Grain sizes in millimeters (1mm = 1/25th in.)

Sedimentary rocks are identified by their characteristic layering called strat-ification or bedding (plate 2–2) . As sediments are deposited, there are frequent variations in the amount and composition of sediment supply such as the rise and fall of sea level that cause the layering . All these layers are assumed to be originally deposited horizontal in water .

Plate. 2–2. Dolomite Mountains of Italy (courtesy of Robert Laffi, Paola Ronchi, and American Association of Petroleum Geologists)



Geologists can interpret how ancient sedimentary rocks were deposited by looking for clues . Lithology (rock composition) is an important clue . Sand grains, mud particles, and shell beds each form different sedimentary rocks, and each was originally deposited in a very different environment . Sedimentary structures such as ripple marks, mud cracks, and flow marks that are preserved in sedimentary rocks (plate 2-3) help visualize the environment in which the rock was deposited . Another aid to interpretation is fossils, preserved remains of plants and animals .

Plate. 2–3. Preserved ripples in an ancient, Colorado sandstone rock

Metamorphic RocksMetamorphic rocks are any rocks that have been chemically altered by high heat

and pressure . Marble (CaCO3), a metamorphic rock, is formed from the sedimen-tary rock limestone (CaCO3) . Since temperatures and pressures become greater with depth, a rock often becomes metamorphosed when buried deep in the earth .

Structure of the Earth’s CrustThe earth is estimated to be about 4 .5 billion years old . Even the sedimentary

rocks that generated and hold gas and oil are millions to hundreds of millions of years old . Where did these sedimentary rock layers come from? During that vast expanse of geological time, sea level has not been constant . Sea level has been rising and falling . During these fluctuations in sea level, sediments were depos-ited in layers . Sands and seashells were deposited along the ancient beaches . Mud was deposited in the seas offshore . These ancient sediments form the sedimen-tary rock layers that are drilled to find gas and oil . The rise and fall in sea level have occurred in numerous cycles (fig . 2–6) . The longest cycles occurred every few



hundred million years . There are shorter cycles within the large cycles and even shorter cycles within them . At least five orders of sea level rise and fall cycles have occurred, with the shortest occurring every few tens of thousands of years . The freezing and melting of glaciers caused the shortest cycles .

Exploration, drilling, and oil production have been very rewarding with several offshore fields in the Norphlet Sandstone buried on the bottom of the northern part of the Gulf of Mexico southeast of New Orleans, Louisiana . The sandstone was originally described in 1922 from an outcrop on land in far south-ern Arkansas . It underlies thousands of square miles of Louisiana, Mississippi and Alabama on land and extends offshore into greater than 7,500 ft . (2286 m) water depth . It was originally deposited about 170 million years ago as extensive sand dunes when the ancestral Gulf of Mexico shoreline was located to the south of where it is today . Sea level rises and falls, mountains rise, and ocean bottoms sink . What was originally land millions of years ago can be under the ocean today and vice versa .

In a typical section of the earth’s crust such as at Tulsa, Oklahoma, about 3000 to 5000 ft . (900 to 1525 m) of well-layered sedimentary rocks is underlain by older metamorphic or igneous rocks (fig . 2–7) . There are about one hundred layers of sedimentary rocks . Sands form the rock sandstone . Muds form the rock shale . Seashells form the rock limestone . The unproductive rocks for gas and oil, usu-ally igneous and metamorphic rocks underlying the sedimentary rocks, are called basement rocks . When drilling a well encounters basement rock, drilling is usually stopped .

In some areas of the earth, there are no sedimentary rocks . They were orig-inally there but have been removed by natural forces, and the basement rock is

Fig. 2–6. Sea level cycles (Hyne, 1995)



now exposed on the surface . These areas are called shields, and there is no gas or oil there . Every continent of the world has at least one shield area (fig . 2–8) . A shield, such as the Canadian Shield in eastern Canada, tends to be a large, low relief area . Ore minerals such as iron, copper, lead, zinc, gold, and silver are mined from basement rock in shield areas . All the gas and oil in Canada on land is located to the west of the Canadian Shield where there are thick sedi-mentary rocks . The southwest portion of Saudi Arabia is a shield (fig . 2-9) . All the Saudi Arabian oil fields are located in sedimentary rocks to the northeast of the Arabian Shield .

Fig. 2–8. Map of world showing location of shields in black where unproductive rocks for gas and oil occur on the surface

Fig. 2–7. Cross section of earth’s crust



Fig. 2–9. Map of Arabian shield and oil fields (modified from Z.R. Beydoun, 1991)

In other areas called basins, the sedimentary rocks are very thick . Most basins have been filled in with sedimentary rocks and are dry land today . Some basins, however, are partially filled with sedimentary rocks and parts are still covered with water such as the Gulf of Mexico basin . The Caspian basin (Caspian Sea) has about 85,000 ft . (26,000 m) of sedimentary rock fill . However, 20,000 ft . to 40,000 ft . (6,000 m to 12,000 m) of sedimentary rocks is typical of many basins . Basins such as the Gulf of Mexico and the Anadarko basin of southwestern Oklahoma are large areas that are often more than 100 miles (160 km) across . Most gas and oil are found and produced from sedimentary rock basins . Because of the thick sedimentary rock fill, most basins have source rocks that have been buried deep enough in the geological past to generate gas and oil (fig . 2-10) . The deep part of the basin where the organic matter is heated (cooked) to form gas and oil is called the kitchen . After the gas and oil is generated, some of it migrates upward into the overlying rocks where it can be trapped . The trap, such as an anticline, is a relatively small feature compared to the basin . Numerous traps can occur along the flanks of the basin . There are about 500 sedimentary rock basins in the world . Of the well-explored and drilled basins, about 50% are very productive . Those productive basins have about 70,000 conventional producing fields (~60% oil and 40% gas fields) . About 70% of the world’s discovered conventional oil and


Documents

Nontechnical Guide to Petroleum Geology Exploration