DIAGNESIS AND FLUID FLOW IN RESPONSE TO UPLIFT AND EXHUMATION

Abstract

Uplift of sedimentary rocks is accompanied by a wide range of physical and chemical changes that contribute to diagenesis and modify fluid flow regimes. Topography becomes a major driving force behind fluid flow patterns, and meteoric water may penetrate to several kilometres below the surface. Typical diagenetic processes include alteration and leaching of feldspars and other unstable minerals, precipitation of iron oxides and kaolin, and leaching of carbonate and sulphate cements. Reservoired oil may be degraded by near-surface waters, but reservoir rocks may become more oil-wet. Brittle fracturing is enhanced near the surface, and fluid flow may become predominantly fracture-bound as fractures dilate. Uplift also causes tilting of fluid contacts and remigration of hydrocarbons. Exsolution and expansion of gas similarly causes remigration of oil to peripheral traps. Although basin uplift is generally regarded as being detrimental to hydrocarbon prospectivity, especially as a result of breaching of traps, there is also an enhanced potential for hydrocarbon plays based on reserves of exsolved gas, condensate dropout, peripheral traps and fractured reservoirs.