Petroleum Exploration: Geology of petroleum reservoir rocks

Introduction: Because the oil and gas well construction industry is diverse and involves people from differing

technical and academic backgrounds, it will be necessary we explain basic geologic concepts and definitions with regards to the targets that the drilling of a well is supposed to reach. In the previous article, it was explained that the thermodynamic properties of petroleum is used to classify it into oil and gas. Oil then consist of higher order hydrocarbons with much smaller order of lower order hydrocarbons while gases consist of more of lower order hydrocarbons. Petroleum found at a condition above the bubble point pressure is all liquid or undersaturated. Below the bubble point, the petroleum is 2 phased or saturated. At considerable lower pressure and below the dew point pressure, petroleum are all in the gaseous phase. Petroleum is found mainly in sedimentary basins trapped by overlying impermeable closing rock. The aim of geological reconnaissance survey is to locate this subsurface closures using magnetic gravity survey and seismic velocity surveys. If drilling an exploration well in areas with limited data, location of a petroleum reservoir closure by interpreting 3D seismic data may be the only available insight into the nature of the subsurface rock formation. Seismic velocity data can also be used to identify and profile subsurface rock formations in a lithological section then locate fault lines and bed boundaries . At this level, take note that hydrocarbons are stored in porous and permeable sedimentary rock formations (sandstone,limestone, dolomite etc.). The nature of the geologic trap depends on the geologic changes that lead to the formation of the petroleum reservoir trap. It may be lateral convex or anticline trap. It may be a permeability or a pinchout trap.It may also be a fault or piercement trap  Because of wellbore stability and well control issues during actual drilling operations, the petroleum geophysicist may use rock mechanics models or offset well log data if available to predict pore pressure regimes of the formations that may be encountered prior to getting to the target payzone geologic formation. With the available basis of well design data from the subsurface geology team, well trajectory planning/casing design/drilling program and completion planning operations will be commenced by the well construction team.
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