In contrast to the above discussion, there is also a widely held assumption that hydrocarbon emplacement physically ``shuts off'' diagenetic reactions by excluding water from reaction sites and restricting fluid mobility. The result is higher porosity and permeability in hydrocarbon-bearing rocks than water-bearing rocks. Unfortunately, this has been a difficult assumption to test. In single wells, facies changes often occur over the same interval as the hydrocarbon-water contact, making it difficult to obtain data from any one facies type spanning both hydrocarbon- and water-bearing zones. However, recent development of North Sea fields has resulted in the drilling and coring of injector and producer wells in similar facies in both hydrocarbon and water zones, thus enabling a test of the assumption. The results of core porosity analyses suggest that the net effect of hydrocarbons on porosity may be on the order of 2-3% of the rock volume (Paxton et al., in prep; Pottorf and Summa, 1994), although other studies suggest that there is little to no effect (Walderhaug, 1994b; Ramm, 1994; and Giles et al., 1992). Where differences in porosity have been observed in core analysis data, they have been difficult to detect petrographically, which may account for the disparity in the conclusions regarding porosity preservation.
With regard to permeability, the effects are much more evident. In reservoirs prone to the precipitation of fibrous illite, porosities can be up to an order of magnitude lower in water-bearing zones relative to hydrocarbon zones (e.g. Pottorf and Summa, 1994; Giles et al., 1992; Kantorowicz et al., 1992). The timing of hydrocarbon migration into these reservoirs is clearly a key factor in determining whether the hydrocarbons will inhibit further diagenetic reactions. As such, there has been an increased emphasis on efforts to date hydrocarbon migration, along with other diagenetic events, as reported in the following section.
Other commonly proposed mechanisms of cement inhibition and porosity preservation include grain-coating clays, and fluid pressure. Little is known about whether high fluid pressures act to inhibit chemical reactions. However, several recent references attest to the ability of well-developed clay coats to preserve porosity by physically blocking the sites on which later cements may nucleate (e.g. Ehrenberg 1993; Pittman et al., 1992).