Due to relatively low hydrocarbon prices in recent years, making most out of each well has become crucial to increase the profit margins of oil and gas companies. One effective way to strengthen the economics of the oil and gas companies is through the delineation of new prospective zones using pre-existing wells. This study shows an example of a very successful formation evaluation case, which totally changed the economic perspective of a mature conventional oil and gas field. The re-evaluation of formations via utilization of low resistivity pay (LRP) approach led to the discovery of new gas bearing zones in a legacy gas well drilled in 2006 (Yesilgol-O1), which was on the verge of abandonment. The insight from Yesilgol-O1 also led to the opening of a new production horizon for a nearby legacy oil well (Yesilgol-1), drilled in 1982, which never produced at economic rates. The current saturation of the shallow gas reservoir in Yesilgol-1 was evaluated with a pulsed neutron logging operation. The interpretation of the logs was supported by five different data sources (seismic cross sections, lithology recordings, X-ray Diffraction (XRD) analysis of drill cuttings, gas readings in mud logging unit (MLU), sonic (DT) and deep resistivity (LLD) logs recorded in open hole). The re-evaluation led to the discovery of several new reservoir intervals with considerably high gas rates in the 36 years old Yesigol-1 well.
The Yesilgol Field used in this study is located north of the Thrace Basin, which is on the European side of Turkey. The Thrace Basin is of Tertiary age, deposited over magmatic and metamorphic rocks. Typically, the Osmancik, Mezardere, Ceylan, Sogucak and Hamitabat Formations have reservoir rock properties indicating good potential for hydrocarbon accumulations. The source rocks for the generated hydrocarbons are the Hamitabat and the Mezardere Formations (Cinar 1996).
The target reservoir for our study is within the Osmancik Formation, which has a gradual contact with the overlying and underlying formations. The Osmancik Formation is a regressive succession of a progressive front delta, with the principal lithologies made up of sandstone, shale, and a minor amount of limestone, tuff, and conglomerate (Figure 1; MTA 2006). Commercial oil and gas production dates back to the 1970's in the Thrace Region of Turkey. At the early stages of exploration, the license holder, National Oil Company of Turkey "TPAO" was focused to discover oil reservoirs within the lower zones of the Thrace Region. Unfortunately, due to a non-existent natural gas market in Turkey in the 1970's, natural gas was not seen a valuable commodity for the company. Hence, the potential natural gas zones within the upper formations were by-passed, and logging and testing efforts were minimal.