FORMATION EVALUATION OF A FINELY LAMINATED RESERVOIR
Sunil Chaudhary and Narendra Vashist
Oil and Natural Gas Commission (ONGC), New Delhi, India
Abstract
Recently oil has been produced from a formation which is devoid of fossils and which exhibits a time-transgressive relationship with the overlying formation. As a consequence, the major problems of exploration are the fixing of boundaries and genetic sub-divisions of the formation. The depositional history, magnitude and the frequency of occurrence of various heterogeneities are also not known. There is little to suggest commercial quantities of hydrocarbons from conventional log analysis, because of the finely laminated nature of the reservoir.
Wireline techniques were used to define formation boundaries, facies variation and the depositional environment. Laminated sand analysis and pattern recognition techniques have been used to compute reservoir parameters. The results of each of these techniques are critically examined and compared.
The water saturations averaged over the 2 feet resolution of conventional openhole logs turned out to be as high as 75 percent and the reservoir was considered marginal. However our results show that sand-layers in the laminated sand/shale sequences have water saturations as low as 35 percent. The improved reserves estimate made on the basis of the revised formation evaluation results, and an areal estimate based on the anticlinal nature and dip of the structure are of the order of 2 million metric tonne.