A petrophysical study was conducted on the Jurassic "P" sandstone in an attempt to explain the causes of oil being found updip of gas in the Carrot Creek Field.
As a result of this study, the reasons for the anomalous occurrence were found to be:
Lack of reservoir continuity of the Jurassic "P" sands: The depositional environment of this reservoir appears to be siliclastic marinal bar deposits. Diagenesis has altered the original pore system, resulting in porosity and permeability pinchouts in segments of the sand bars.
Pore throat restrictions in hydrocarbon migration: Lateral variations in pore throat radii has affected the migration of oil and gas, resulting in oil accumulations updip of gas production.
In recent years, hydrocarbon productive reservoirs have been developed in the Lower Mannville and Jurassic sandstones in northern Alberta, Canada. It should be noted that the general oil field nomenclature for these Jurassic sands has been "Rock Creek". Amoco Canada, however, believes these sands to be younger in age and has referred to them in this paper as Jurassic "O" and "P" (consistent with the current Energy Resources Conservation Board pool designation in this area).
Both oil and gas pools have been encountered in these sands. In conjunction with the occurrence of oil wells, gas is often found updip. However, gas wells have also drilled downdip of oil production, and high GOR wells have been completed within oil pools. This leaves us with anomalies that are very difficult to understand, and the predictability of oil or gas production prior to drilling can often be quite difficult.
This paper is a segment of a more extensive petrophysical study of the Jurassic sundstones in Amoco Canada's Carrot Creek field (Figure 1). This study was undertaken to obtain a better' understanding of the reservoir complexities and the anomalous gas and oil production in the area (Figure 2). Although the study concentrated on the Jurassic "P" sandstone, it was necessary to consider the productive potential of the Jurassic oil and Lower Mannville "M" (Figure 3), since these horizons are also hydrocarbon bearing in parts of the Carrot Creek field and can often be in wellbore communication after completion. Evaluation of completion information along with petrophysical data showed the anomalous gas/oil production tests could not be exclusively attributed to hydrocarbon production from these different strata simultaneously. It became apparent that reservoir complexities do result in variable gas or oil potential within an individual horizontal.
A study of the depositional environment and diagenetic alteration of the Jurassic "P" sandstone reservoir showed that limited reservoir continuity exists in the area. This would indicate that the field may be made up of several individual hydrocarbon pools with limited or no pressure communication between them.
Interpretation of the depositional environment was done with reference to Reading (1978)1 and Walker (1979)2.
The Jurassic sandstone sequence is a shallow marine bar through coastal sediment complex deposited in an epicontinental seaway (Figure 4).
