Logging provides data needed to evaluate the quantity of hydrocarbon in the coating on the actual situation. Curves log provide data on the properties of rocks and fluids.
Rock properties that are important to analyze the logs are porosity, water saturation, permeability. The first two parameters can be calculated quantity of hydrocarbons in the layer, and the last parameter indicate where the hydrocarbon can be produced.
Porosity
Porosity is Part of the Total volume of porous rocks. In unconsolidated formation, number of porosity depends on grain size distribution. If all grain have a similar size, porosity will be able on number 0,35-0,4 (high). But, it will be lower if grains have variative size, because the small grain able to fill in space between larger grain.
Water Saturation
Water Saturation is Part of porous space filled with water (Sw). Residual part of water saturation named hydrocarbon saturation (Sh). Sh = 1 - Sw. The main purpose from logging operation is for this, make calculating saturation of water and hydrocarbon
Permeability
Permeability is ability of rocks to pass the fluids. Permeability depends on grain size of sedimentary rocks. Sediment with large grain size and large pore has high permeability number, and the opposite.
Hydrocarbon-Bearing Rocks
Generally consist of sandstone, limestone, and dolomit.
Sand can moved and deposited by water. The more swift water flow, more coarse grain. Therefore the sand tends to have a similar porosity between of it's grains.
Limestone isn't like sand, it will be deposited by seawater movement. A few part solute deposit, and the other part is heaps of organic remains. Porosity of limestone is less similar than sandstone.
Dolomit is built when water rich of magnesium flow on limestone, and change few calcium with magnesium. Dolomitization is important mechanism to avail pore space for hydrocarbon accumulation.
Clays and Shales
Clays are main component of sedimentary rock. There are montmorillonite, illite, chlorite, and kaolinite.
Shales are composed from clays and silts ( soft silicates) that deposited by low energy sedimentation.
The permeability of shales is 0. Shales and Clays make Hydrocarbon Formation analyzing be difficult. You have to know log interpretation on clean formation, then analyzing dirty formation.
Article Basic of Evaluation Concepts : Properties of Rock for Log Analyzing is free in thi blog.
Source : Harsono,Adi.1994. Pengantar Evaluasi Log.
