Suban is a prolific gas field producing from the fractured Baturaja, Talang Akar and pre-Tertiary Formations. Throughout most of the field development, the wells were drilled using water based mud and these wells experienced drilling problems. In 2014, synthetic oil based mud (SOBM) was used to drill the Telisa section as a way to improve the drilling performance and reduce drilling cost. This new mud system resulted in a three times increase in the rate of penetration (ROP) without generating drilling problems. Although drilling performance was improved, the gas readings were very different than other nearby development wells drilled with water based mud. Since future development wells will use oil based mud, it is very important to understand the gas readings from these wells so that any future potential in this interval can be correctly evaluated.

The effect of SOBM on gas readings is not a welldocumented phenomenon. SOBM distorts the gas reading by absorption of the C2-C5 components. In addition, the longer the hydrocarbon chain the more absorption takes place. In order to determine what type of hydrocarbon fluid is indicated by gas readings in SOBM in the Telisa Formation of Suban Field, the mud gas data from 5 wells drilled with water based mud (WBM) and 3 wells drilled with SOBM were compared using the Pixler and Gas Ratio methods. The Pixler method compares interval value ratio of C1 to C2, C3, C4, and C5. Using this method, the gas readings in SOBM indicate a hydrocarbon which is lighter than actually encountered in the wells. The Gas Ratio method compares light to medium, light to heavy and heavy to medium isotopes to reveal the hydrocarbon type in the reservoir. This method results in a true estimate of the hydrocarbon type in the reservoir.