Chapter 3 - Geology of tight unconventional oil reservoirs

Abstract

Conventional petroleum geology is no longer suitable to describe the petroleum geology of unconventional reservoirs; hence, a developed unconventional petroleum geology discipline is used instead. The reason for this is that the traditional subsurface trap and the hydrocarbon generation and storage theory are not applicable for unconventional reservoirs. Hydrocarbons in such resources are generated during the thermal evolution of source rocks. Oil is generated in source rocks at, relatively, low maturity, and then, oil shale is developed to full maturity. Thermal maturity is a key process to organic matter transformation to developed hydrocarbons (oil and/or gas). Tight unconventional oils are distributed in basins through the sedimentary slopes. There are a number of conditions required for the formation of tight oil reservoirs. These conditions include the need for stable structural deposition, suitable conditions for near-source accumulation, and preserved conditions for total subsidence. Tight unconventional oil reservoirs can be of two types. One is the source accumulations where oil has been developed and remained in the source rock, and the other is the near-source accumulations where the oil has been developed in the source rock and then migrated to a very short distance close to the source rock and remained until the time of discovery. Unlike conventional reservoirs, tight unconventional oils can be found both fully and partially matured, and tight oil reservoirs result in primary- or secondary-based genesis.