In spite of the low oil price condition, Indonesia has been driven to prepare its future energy by exploring its unconventional hydrocarbon resources. The first drilled well in 2016 has been dedicated to discover organic-rich source rock which estimated filled in some huge oil and gas reservoirs in North Sumatra Basin and it certainly will be a precursor well for Indonesia's future shale development. In order to obtain the best zone interval to be developed (perforated and fractured), the petrophysical study has crucial roles in determining the interest zone (sweet spot). However, the high complexity of shale reservoir has been challenging to predict the shale gas petrophysical properties, organic-rich content, elastic, and classifying rock group by only using conventional log interpretation.
This paper presents techniques to determine the sweet spot interval and rock typing method in very tight shale reservoir by maximizing and integrating geological, geochemical, rock mechanical and engineering data available such as well logs, routine and special core analysis, petrography, total organic carbon (TOC), and brittleness index (BI) calculation. Integration of all data leads to establishing rock types that proposed to determine the best depth to be developed and future production plans along the shale formation of interest.
A well having complete data as mentioned above is used in this study. At first, the main conventional petrophysical properties such as clay content, porosity, and water saturation are interpreted by using triple combo logs and supported with sonic and photoelectric factor log. Equally important; the permeability, TOC, and elastic properties in the depth-by-depth are estimated by using empirical equation for permeability, the average between DLogR Passey and Carbolog for TOC, and BI are predicted by using minerals content of rocks and sonic log. All interpreted and estimated parameters are validated with laboratory data in order to persuade the accuracy of prediction. With those, the shale quality index (SQI) and rock types along the depth interval are established. In conclusion, the higher value of SQI describes the good quality potential of shale reservoir to be developed and vice versa.