Geochemical Analyses of Oils in the Goudron Field, Onshore Trinidad

Abstract

The Goudron Field is a mature oilfield onshore southeastern Trinidad. Originally discovered in 1927, most of the wells in the field had very light oil, lighter than many of the oils produced in neighbouring fields. In addition, in wells which produced from two different zones, some found lighter oil at shallower depths, others found shallow oils to be heavier than deeper oils. Most Trinidad oils have been found to be geochemically altered, many by evaporative fractionation, others by biodegradation. Thirteen oils were collected from the field, in different geographic areas and from different depths and different producing horizons. These range from the lower to middle Pliocene Gros Morne sandstone member of the Moruga Formation to the upper Pliocene Goudron Sandstone Member of the Mayaro formation. The latter includes oils from the upper Miocene Cruse formation and the Gros Morne sands. An oil was also collected from the upper Cretaceous fractured Naparima Hill formation in a well outside the field area because of the very heavy API gravity, at 15.0 degrees. In addition five sidewall samples were collected from a newly drilled well, from all of the known producing horizons within the field area. It was suspected from the API gravity characteristics that the Goudron oils had undergone various forms of alteration, as follows: The deeper oils were residual oils, the shallower lighter oils were evaporative and the heavier shallow oils were biodegraded. It was also believed that the Cretaceous oil was either a residual oil or biodegraded or both. The goals were to determine if the oils were indeed altered and if they were, what alterations had occurred, as this could point the way to additional prospects. For example if evaporative oils were found to be the deepest in any area, it would imply that deeper residual oils were present. Or if residual oils were found without encountering shallower evaporative oils, it would mean that the evaporative oils were either lost to the surface or trapped in offset shallow reservoirs. Results of the analyses, interpretations and exploration implications are discussed.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015