Ngrayong Sandstones member of Middle Miocene Tawun Formation, is one of the main reservoir in the North East Java Basin. Currently depositional environment of Ngrayong Member is still being debated, because in Cepu area and surrounding sediments with relatively similar characteristics can be interpreted into two different environments are deltaic and deepwater turbidite. The difference being the object of interest to be discussed related to hydrocarbon reservoir potential.
The integrated research methods are surface geological mapping, measured stratigraphic column, granulometry analysis (spericity, roundness, composition, grain size and sorting variations), petrography, porosity and permeability measurements.
Litofacies unit in the mapping area are Facies of Shale with Sandstone Intercalation, Coal and Gypsum Fragments, Sandstone Facies, Siltstone Facies, Interbeding Sandstone – Siltstone Facies, and Red Sandstone Facies. Sedimentary structures such as bedding fisillity, bedding, bicurrent crossbedding, oscillation ripplemark, flaser, lenticular bedding, coarsening upward. Petrographic analysis of red sandstone shows types of sandstone such as Arenite Quartz, arkosic Wacke until Sub Arckose. Granulometri analysis shows sortation ranged from poorly – moderately, level of roundness and sphericity are low sphericity with a little clay content, showing a level of maturity from submature until mature which was deposited with medium energy. Porosity and permeability test of Sandstone facies result porosity 32% -44% with permeability-833 mD – 0.015 mD, and Porosity and permeability test of Red Sandstone facies result porosity 32% -41% with 90 – 404 mD permeability. Based on an integrated analysis of sedimentology known that many facies was deposited in Pro-delta environment and Delta Front. The most excellent reservoir quality are the Red Sandstone facies and sandstones facies with good porosity and permeability which are interpreted as part of mouth bar from deltaic system.
Keywords: Ngrayong, Delta, Sandstone
Authors : Bhaskara Aji, Prihatin Tri Setyobudi, Rizqi Syawal, Aveliansyah