No abstract

No abstract

Summary Shallow oil fields frequently contain oils that not only have low oil gravity and high viscosities, but are also highly variable in terms of these properties. It is thus crucial to gather sufficient oil-quality data to sample a statistical cross section of the oil population. This paper describes how oil gravity and dead-oil viscosity can be predicted from geochemical parameters that can be measured on core or sidewall core samples, thus allowing oil samples to be accessed with greater spatial coverage than could reasonably be achieved with conventional fluid sampling. This technique is demonstrated on cored wells from the large Schrader Bluff viscous oil accumulation in Milne Point, Alaska. An oil can inherit a particular oil gravity or viscosity for a variety of causes. Consequently, spatial variations in such bulk-oil properties can be difficult to map empirically simply by contouring data because more than one controlling factor may be operative. A major step forward in fluid mapping is to identify and quantify these controls and map the controls. In Schrader Bluff the controls were discovered to be the degree of biodegradation and the presence of an in-mixed second charge of (light) oil. Variations in these processes can be tracked with geochemical parameters and mapped with a much greater degree of certainty than the bulk-oil properties. The resulting individual maps may then be combined to predict oil gravity and viscosity. Conceptual models exist for these processes, opening the possibility of model-driven prediction of oil properties and sweetspots in areas away from well control. Introduction Many shallow oil accumulations contain oils that have been affected by microbial processes. This so-called biodegradation results in easily metabolized compounds being progressively stripped from the oil. The normal alkanes are the first to disappear, followed by more complex cyclic and aromatic compounds. In the most severe cases, all that remains is a tarry mass of chemically unresolvable high-molecular-weight compounds. Progressive biodegradation is accompanied by an increase in oil density and thus a decrease in oil gravity. Oil viscosity increases too, meaning that well flow rates can be a problem and that special stimulation methods may be needed to achieve commercial rates.