to water, they may escape their source rock, seeping upwards through fractures in the water-filled rocks. Tis ‘migration’ takes the easiest route, and can cover considerable distances, both vertically and laterally, but where the hydrocarbons meet impermeable rocks (‘cap’ rocks), such as shale or salt, they are trapped in the underlying ‘reservoir’ rock and cannot migrate further. A reservoir rock is a porous sedimentary rock, such as sandstone, which is composed of sand grains about 0.1–1 mm diameter, or limestone that was deposited as shell beds or reefs. Sand grains are like spheres (although the actual shape of a grain might differ significantly from a perfect sphere), and in the pores – the spaces between grains – a lot of oil and gas can be stored. In limestones, there are pores between the shells and corals, in addition to fractures, both of which can store oil and gas. Te pores in the reservoir rock were originally filled with


Trap Gas Oil Gas Oil Migrating Hydrocarbons c e


Figure 1.5: A schematic cross-section of part of a petroleum-bearing basin identifying the key elements. Four phases of petroleum migration are considered during resource assessment: primary migration or expulsion from the source rock; secondary migration from source to reservoir along a simple or complex carrier system; tertiary migration, which is migration to the surface; and re-migration between reservoirs. Geologic studies have indicated that migration can continue over hundreds of kilometres. The mechanisms and physics of primary migration are still poorly understood and are a subject of debate in the scientific community.


sandstone mudstone


e Net Gross S s


Overburden


that we explore and drill, looking for areas where the geological conditions for commercial oil and gas deposits are met: i.e. the presence of source rock, reservoir rock, trap and seal. Petroleum geoscience concerns the disciplines of geology


salt water, which occurs naturally in subsurface sedimentary rocks. In the hydrocarbon trap, the fluids separate according to their density, with the gas moving to the top of the trap to form the free gas cap. Te oil stores itself in the middle to form the oil reservoir and water is forced to stay at the bottom of the reservoir. Te interfaces between the fluids are mostly flat; therefore, in seismic sections, geophysicists search for ‘flat spots’, which are reflections of gas-oil and oil-water contacts (when all fluids are present), or gas-water contacts (when there is no oil), or oil-water contacts (when there is no gas). Rocks are divided into igneous rocks, which, like granite


and basalt, are the result of volcanic or plutonic processes; metamorphic rocks, such as gneiss, which have undergone a physical change due to extreme heat and pressure; and sedimentary rocks composed of sediments which were deposited on the surface of the earth or the bottom of the sea. About 99% of the sedimentary rocks that make up the Earth’s crust are shales, sandstones and carbonates (limestones and dolomites). Sediments consist either of particles such as sand grains


formed from mechanical weathering debris, or seashells, or rock salt that precipitated from evaporating water, all of which are deposited over many years as sand along beaches, sand and mud on the seabed, or beds of seashells, for example. Tese ancient deposits, piled layer upon layer in sedimentary basins, today form the sedimentary rocks in the uppermost crust of the earth


and geophysics as applied to understanding the origin, distribution and properties of petroleum and petroleum- bearing rocks, and to the production of oil and gas. Over the years, geologists and geophysicists have steadily developed new exploration techniques to improve our understanding of petroleum geology and to increase the efficiency of exploration and the accuracy of well siting, to reduce costs, and to enhance the chance of success. An additional important task is to predict the likely recoverable volumes that may be present. When a field has been found in the rock formations, reservoir management, applying elements of geology, geophysics and petroleum engineering, is used to predict and manage the recovery of oil and gas. By definition, geophysics is the scientific study


of the whole Earth using methods of physics and mathematics, and so involves many disciplines, including seismic, electromagnetism, gravimetry, geomagnetism, geothermometry and tectonophysics. From measurements made mostly at its surface, we can ‘look into’ the Earth. Tese measurements are sensitive to the physical properties of the subsurface rocks and fluids and therefore we can describe the subsurface in physical terms such as acoustic velocities, density, electrical resistivity, magnetism, and so on. Petroleum geophysics has a more restricted meaning as it


applies methods of physics to understand how commercial accumulations of petroleum have developed and can be found within sedimentary basins, and how they can be produced.


5


S


R o


l o o


e u


a v R


r e r


i c


r k


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60  |  Page 61  |  Page 62  |  Page 63  |  Page 64  |  Page 65  |  Page 66  |  Page 67  |  Page 68  |  Page 69  |  Page 70  |  Page 71  |  Page 72  |  Page 73  |  Page 74  |  Page 75  |  Page 76  |  Page 77  |  Page 78  |  Page 79  |  Page 80  |  Page 81  |  Page 82  |  Page 83  |  Page 84  |  Page 85  |  Page 86  |  Page 87  |  Page 88  |  Page 89  |  Page 90  |  Page 91  |  Page 92  |  Page 93  |  Page 94  |  Page 95  |  Page 96  |  Page 97  |  Page 98  |  Page 99  |  Page 100  |  Page 101  |  Page 102  |  Page 103  |  Page 104  |  Page 105  |  Page 106  |  Page 107  |  Page 108  |  Page 109  |  Page 110  |  Page 111  |  Page 112  |  Page 113  |  Page 114  |  Page 115  |  Page 116  |  Page 117  |  Page 118  |  Page 119  |  Page 120  |  Page 121  |  Page 122  |  Page 123  |  Page 124  |  Page 125  |  Page 126  |  Page 127  |  Page 128  |  Page 129  |  Page 130  |  Page 131  |  Page 132  |  Page 133  |  Page 134  |  Page 135  |  Page 136  |  Page 137  |  Page 138  |  Page 139  |  Page 140  |  Page 141  |  Page 142  |  Page 143  |  Page 144  |  Page 145  |  Page 146  |  Page 147  |  Page 148  |  Page 149  |  Page 150  |  Page 151  |  Page 152  |  Page 153  |  Page 154  |  Page 155  |  Page 156  |  Page 157  |  Page 158  |  Page 159  |  Page 160  |  Page 161  |  Page 162  |  Page 163  |  Page 164  |  Page 165  |  Page 166  |  Page 167  |  Page 168  |  Page 169  |  Page 170  |  Page 171  |  Page 172  |  Page 173  |  Page 174  |  Page 175  |  Page 176  |  Page 177  |  Page 178  |  Page 179  |  Page 180  |  Page 181  |  Page 182  |  Page 183  |  Page 184  |  Page 185  |  Page 186  |  Page 187  |  Page 188  |  Page 189  |  Page 190  |  Page 191  |  Page 192  |  Page 193  |  Page 194  |  Page 195  |  Page 196  |  Page 197  |  Page 198  |  Page 199  |  Page 200  |  Page 201  |  Page 202  |  Page 203  |  Page 204  |  Page 205  |  Page 206  |  Page 207  |  Page 208  |  Page 209  |  Page 210  |  Page 211  |  Page 212  |  Page 213  |  Page 214  |  Page 215  |  Page 216  |  Page 217  |  Page 218  |  Page 219  |  Page 220  |  Page 221  |  Page 222  |  Page 223  |  Page 224  |  Page 225  |  Page 226  |  Page 227  |  Page 228  |  Page 229  |  Page 230  |  Page 231  |  Page 232  |  Page 233  |  Page 234  |  Page 235  |  Page 236  |  Page 237  |  Page 238  |  Page 239  |  Page 240  |  Page 241  |  Page 242  |  Page 243  |  Page 244  |  Page 245  |  Page 246  |  Page 247  |  Page 248  |  Page 249  |  Page 250  |  Page 251  |  Page 252  |  Page 253  |  Page 254  |  Page 255  |  Page 256  |  Page 257  |  Page 258  |  Page 259  |  Page 260  |  Page 261  |  Page 262  |  Page 263  |  Page 264  |  Page 265  |  Page 266  |  Page 267  |  Page 268  |  Page 269  |  Page 270  |  Page 271  |  Page 272  |  Page 273  |  Page 274  |  Page 275  |  Page 276  |  Page 277  |  Page 278  |  Page 279  |  Page 280  |  Page 281  |  Page 282  |  Page 283  |  Page 284  |  Page 285  |  Page 286  |  Page 287  |  Page 288  |  Page 289  |  Page 290  |  Page 291  |  Page 292  |  Page 293  |  Page 294  |  Page 295  |  Page 296  |  Page 297  |  Page 298  |  Page 299  |  Page 300  |  Page 301  |  Page 302  |  Page 303  |  Page 304  |  Page 305  |  Page 306  |  Page 307  |  Page 308  |  Page 309  |  Page 310  |  Page 311  |  Page 312  |  Page 313  |  Page 314  |  Page 315  |  Page 316  |  Page 317  |  Page 318  |  Page 319  |  Page 320  |  Page 321  |  Page 322  |  Page 323  |  Page 324  |  Page 325  |  Page 326  |  Page 327  |  Page 328  |  Page 329  |  Page 330  |  Page 331  |  Page 332  |  Page 333  |  Page 334  |  Page 335  |  Page 336  |  Page 337  |  Page 338  |  Page 339  |  Page 340  |  Page 341  |  Page 342  |  Page 343  |  Page 344  |  Page 345  |  Page 346  |  Page 347  |  Page 348  |  Page 349  |  Page 350  |  Page 351  |  Page 352