Low Permeability Reservoir Rock Characterization Using Petrographic and Depositional Studies — Case Study: Optimizing Production for Low Permeability Bekasap Sandstones in the Central Sumatra, Indonesia

Prasetyo, Hadi¹
 Suparka, Emmy¹
 Darussalam, Noeradi D.¹

An integrated petrographic i.e. thin section, x-ray defraction (XRD) and scanning electron microscopy (SEM), and depositional environment study is one of many methods available, which is relatively cheap, and giving quick and accurate results to support reservoir characterization.

The objectives of this study are to identify Bekasap sandstone regarding production optimization. This stduy also docummented that geologic factors controlled on development strategy and production optimization for the Bekasap sandstone, which is deposited at an estuarine, tidal dominated delta system. According to lithofacies, the sandstone can be divided into upper and lower parts. The lower part consist of medium grained, conglomeratic cross-bedded and massive sandstone, slightly bioturbated sandstone, mainly arenite type that have horizontal permeability up to 1,900

On the other hand, the upper one is composed of fine-very fine grained, medium-highly bioturbated sandstone, greywacke type dominated, with horizontal permeability vary from tens up to 200

In general, reservoir quality development especially horizontal permeability is more controlled by depositional environment factor i.e: grain-size, bioturbatuion and mud matrix content rather than diagenetic process i.e: cementation and dissolution. But, at several depths, both permeability and porosity reduction are significantly controlled by cementation.

Permeability reduction is significantly controlled by decreasing grain size and sorting, and increasing both matrix and bioturbation content. On the other hand, decreasing both matrix and bioturbation volume, coarser grain size and better sortation are influenced on increasing permeability. Precipitation of dolomite cement is dramatically destroyed both permeability and porosity whereas dissolution is slightly increased both porosity and permeability.

Both technically and economically, horizontal drilling technology as an optimum strategy increasing production for low permeability, which is caused by depositional environment aspect.