Abstract

The Cinta Field lies 95 kilometers northwest of Jakarta within the Sunda Basin area. Late Oligocene sandstones of the Talang Akar Formation provide an excellent reservoir in a structure defined by arching over a rotated fault block high that is the northern edge of a major half graben. The Cinta Field structure separates the Sunda Basin to the north from the Kitty-Nora Sub-Basin to the south. The field has produced 206 million barrels of oil (32.8×10⁶ m³) from fluvio-deltaic clastics of the Talang Akar Formation and overlying Baturaja Formation reefal carbonates.

Studies on cores cut from recently drilled wells show that the reservoir sandstones frequently contain large amounts of pore-filling clays, largely in the form of kaolinite. This kaolinite is believed to have resulted from the breakdown of potassium feldspars formerly within poorly-sorted volcanic detritus. Permeability within the reservoirs can be drastically reduced by the presence of diagenetic kaolinite, the amount of which depends on the extent of the alteration of the feldspars as well as the original feldspar content.

Permeability can be approximately determined from wireline log information. The critical factors determining permeability are porosity, which is dependent on grain size and sorting, and the amount of kaolinite present. In Cinta, the gamma ray log responds to kaolinite, probably due to the presence of small amounts of thorium associated with this clay. A mathematical fit of data from the Cinta B-6 and F-11 cores, shows a good correlation between gamma ray response and the density-derived porosity with the measured permeability. A series of hyperbolae give a best fit of data for determining permeability from wireline logs in uncored wells.

Whilst the method is at best an approximation, it is specific to Talang Akar sandstones within the field and can be used to assess reservoir quality in unperforated pay intervals and to predict low and high permeability reservoir sections. Some high permeability zones lack clay cements, and these are usually friable and prone to sanding in a producing well.