Porosity preservation and Generation in deep clastic reservoir: a review

Abstract

A number of processes and specific geological situations act to control the preservation and generation of porosity in deep clastic reservoirs. The main factors contributing to preservation are: (1) late burial of the reservoir at its current depth; (2) development of abnormal fluid pressures; (3) the compositional stability of framework grains; (4) covering of the grains by clay and/or oxide coatings or rims; (5) early partial cementation by carbonates or sulfates; (6) early hydrocarbon saturation of the reservoir. The processes and solvents that produce subsurface porosity are: (1) deep infiltration of meteoric waters; (2) CO₂ generated by the thermal maturation of organic matter; (3) organic solvents (mainly carboxylic acids) released by organic matter; (4) acidic fluids generated by inorganic reactions with clay-minerals: (5) thermogenic sulfate reduction by hydrocarbons, which produces CO₂ and H₂S; (6) thermal convection of solvent fluids; (7) overlapping of source rocks associated to the same reservoir; (8) mixture of meteoric waters with sea or saline connate waters; (9) inorganic chloride complexes; (10) ammonia; and (11) juvenile waters containing CO₂ from hydrothermal or volcanic sources or from the metamorphism of limestones. An assessment of preservation mechanisms indicates that early hydrocarbon saturation is the most efficient, although late burial probably has more widespread influence. Among the mechanisms that generate subsurface porosity, organic solvents seem to play the most important role, but a number of other processes can also be quite influential and should be systematically evaluated as well.