JNTU B.Tech II PETROLEUM GEOLOGY Previous Question Paper (November/December - 2020 - R15)

Subsurface temperature and pressure vary spatially and temporally due to geological, tectonic, and fluid-dynamic factors. Temperature variations are primarily driven by the geothermal gradient, which is influenced by the thermal conductivity of rock layers, proximity to magmatic intrusions, and the depth of the basement. Spatially, regions with high heat flow, such as rift basins or volcanic arcs, exhibit higher temperatures compared to stable cratons. Temporally, temperature changes occur due to burial history, uplift, and erosion, which alter the depth of sedimentary units relative to the surface heat source.

Pressure variations are governed by the weight of the overlying rock column (lithostatic pressure) and the weight of the fluid column (hydrostatic pressure). Spatial variations arise from differences in rock density, porosity, and structural compartmentalization. Overpressure, or abnormal pressure, occurs when fluid escape is restricted by low-permeability seals, such as shales, preventing the dissipation of pressure during rapid burial. Temporally, pressure changes are induced by tectonic loading, hydrocarbon generation (which increases fluid volume), and the migration of fluids into or out of a reservoir, causing transient pressure fluctuations within the basin.

Temperature and pressure are critical drivers of hydrocarbon migration, influencing both the physical state of the hydrocarbons and the rock properties that facilitate flow.

Temperature primarily controls the viscosity and density of hydrocarbons. As temperature increases with depth, the viscosity of oil decreases, significantly enhancing its mobility through pore throats. Furthermore, thermal maturation processes generate hydrocarbons from kerogen; the resulting increase in fluid volume creates an internal pressure gradient that acts as a primary driving force for primary migration out of source rocks.

Pressure dictates the buoyancy and effective stress within the basin. Hydrocarbon migration is largely driven by buoyancy, where the density contrast between hydrocarbons and formation water causes them to migrate upward. High pressure, particularly overpressure, can induce micro-fracturing in seals, creating temporary pathways for secondary migration. Additionally, pressure affects the effective stress on rock grains; as fluid pressure increases, effective stress decreases, which can maintain or enhance porosity and permeability, thereby facilitating the movement of hydrocarbons through carrier beds toward structural or stratigraphic traps.