This chapter deals with applications and case studies in petroleum geology. It contains sections on the principles and practice of petroleum geology; on applications and on case studies of applications in petroleum exploration; on applications and on case studies of applications in reservoir exploitation; on applications and on case studies of applications in well-site operations; on unconventional petroleum geology; and on micropalaeontology and Health, Safety and Environmental (HSE) issues in the petroleum industry. The section on the principles and practice of petroleum geology includes sub-sections on play components; on petroleum exploration; on reservoir exploitation; and on well-site operations. The section on applications in petroleum exploration includes sub-sections on chronostratigraphy and palaeoenvironmental interpretation; and on operational biostratigraphy. The section on case studies of applications in petroleum exploration includes case studies from the Middle East; the Indian subcontinent; the central and northern North Sea; the South Atlantic salt basins; the eastern Venezuelan basin; and the Gulf of Mexico. The section on applications in reservoir exploitation includes a sub-section on integrated reservoir characterisation. The section on case studies of applications in reservoir exploitation includes case studies from the marginal to shallow marine, peri-deltaic clastic reservoirs in the Gullfaks, Snorre and Statfjord fields in the Norwegian sector, and in the Ninian and Thistle fields in the UK sector, in the North Sea; the marginal to shallow marine, peri-deltaic clastic reservoir in the Pedernales field in Venezuela in northern South America; the shallow marine, peri-reefal carbonate reservoirs in the Al Huwaisah, Dhulaima, Lekhwair and Yibal fields in Oman, and in Shaybah field in Saudi Arabia, in the Middle East; and the deep marine, submarine fan reservoir in Forties field in the UK sector in the North Sea. The section on applications in well-site operations includes a sub-section on ‘biosteering’.