Unusual high acidity oils from the Great Palogue Field, Melut Basin, Sudan

Abstract

Crude oils produced from the Great Palogue Field in the Melut Basin, Sudan display unusually high total acid number values (TAN, up to 10.4 mg KOH/g oil). This field was discovered in 2002 and produces oils of highly variable (15–31°) API gravity. The main production comes from the Paleogene Samma and Yabus Formations, with the Upper Cretaceous Melut Formation being a minor contributor. The effective hydrocarbon source rocks are most likely within Lower Cretaceous lacustrine shales deposited during the syn-rift stages of the basin’s evolution. This study addressed 20 crude oil samples from five exploration and appraisal wells in the Great Palogue Field and four oil samples from other oil fields in the basin to represent the various production fluids from the Upper Cretaceous and Paleogene pay zones. These oil samples were characterized using routine molecular geochemical techniques to determine the primary geological and geochemical controls on oil acidity. Based on source and maturity constraints, two genetic types of high acidity oils have been recognized from the Melut Basin. The first group of high acidity oils is likely primary high TAN oils with a TAN value of 0.5–0.99 mg KOH/g oil. They are buried deeper than 2000 m where the temperature is higher than 80 °C. These include the oils only in the Upper Cretaceous Melut Formation of the Anbar-1 and Palogue South-2 blocks. The second group of high acidity oils show higher TAN values (>1.0 mg KOH/g oil) and various levels of biodegradation. These oils represent the dominant oil type in the Melut Basin, occurring mainly in the Paleogene Yabus and Samma Formations. A multiple phase oil charging model is proposed to explain the horizontal and vertical variation in the oil TAN values across the Great Palogue Field.