ABSTRACT: Evaluation of hydrocarbon reservoirs within coal-shale sequence of Geleki field: A case study

Sharma, S. C., B. Jhaldiyal, U. S. Roy, and V. K. Sud , KDM Institute of Petroleum Exploration, ONGC, Dehradun, India

The dominantly argillaceous part of upper Barail sediments (Oligocene Age) in Geleki field, known as Barail Coal Shale (BCS) sequence, has many lenticular discontinuous sands with limited aerial extent. Realistic evaluation of these sands has been major problem because of their low resistivity and silty nature. The present paper suggests methodology for evaluation of such reservoirs.

Integrated analysis of geo-scientific data suggests a depositional model of generalised distributory channels with clastic input from west and north-west direction. Significant variation in reservoir character of different sand units is inherent feature of such depositional environments. The dominant facies occurring in the sequence are (a) finning upward sequence with low resistivity at the top and (b) high resistivity silty sands. Water saturations computed from conventional interpretation techniques are inconsistent with testing data in such sands.

A 'Surface Conductivity' model which, takes into consideration the additional conductance due to surface charge density developed because of the formation of ionic double layer on the wet grain surface, has been utilised to evaluate the realistic fluid saturations. The surface conductance effect acquires significance in fining upward sequences and silty sands because of the increased surface area of silts and presence of fresh formation water. There is substantial improvement in hydrocarbon saturations when evaluated with the help of this model.

This model has been successfully used for evaluation of realistic water saturation and identification of new hydrocarbon bearing areas.

AAPG Search and Discovery Article #90913©2000 AAPG International Conference and Exhibition, Bali, Indonesia