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Images in: Appendix A

Gulf of Mexico

Images - Appendix A

Gulf of Mexico Subsalt

Images - Appendix A

World Data

Images - Appendix A Courtesy of Dr. Ahr (TEXAS A&M UNIVERSITY)

World Data

Images - Appendix A Courtesy of Dr. Ahr (TEXAS A&M UNIVERSITY)

World Data

Images - Appendix A Courtesy of Dr. Ahr (TEXAS A&M UNIVERSITY)

World Data

Images - Appendix A Courtesy of Dr. Ahr (TEXAS A&M UNIVERSITY)

Rock Cycle

Images - Appendix A An illustration of some of the more common methods of rock formation and alteration.

Igneous rocks are formed from the cooling and crystallization of molten rock (or magma)below the crust. When magma cools slowly within the crust, it forms coarsely crystalline rocks, such as granite and gabbro. When magma erupts at the surface of the earth, it forms glassy and microcrystalline volcanic lavas, such as basalt and rhyolite.

Metamorphic rocks are formed by the modification of pre-existing rocks by high temperature and/or high pressure. Regional metamorphism occurs at great depths in the crust. Thermal or contact metamorphism occurs where magma intrudes shallow rocks, and it is characterized by high temperature but low pressure.

Igneous and metamorphic rocks both have crystalline fabrics and lack porosity. They form petroleum reservoirs only in rare instances and are commonly referred to as ``basement.''

Sedimentary rocks are formed from the breakdown of pre-existing rocks: igneous, metamorphic, and sedimentary. They are formed both by the direct deposition of fragments of pre-existing rock (detrital) as well as from the precipitation of their solutes (chemical sediments, such as evaporites).

Sedimentary rocks, by their very nature, tend to be deposited with voids between the sediment particles. Thus most petroleum reservoirs are found in the coarser sediments. Fine-grained sediments serve as impermeable seals to petroleum migration and also as petroleum source beds. Sediments are classified by both grain size (clay, claystone; silt, siltstone; sand, sandstone; gravel, conglomerate) and chemistry (shale is composed principally of clay minerals; sandstone is composed principally of quartz; limestone is composed principally of calcium carbonate; dolomite is composed of the mineral dolomite; evaporites are salts formed from the evaporation of sea water: gypsum, anhydrite, halite, etc.).

Sedimentary rocks are thus often the result of a long history involving weathering, erosion, transportation, deposition, and subsequent physical and chemical changes during burial (called diagenesis). These various processes are defined below.